Organoboron molecules and polymers for organic solar cell applications

Abstract

Organic solar cells (OSCs) are emerging as a new photovoltaic technology with the great advantages of low cost, light-weight, flexibility and semi-transparency. They are promising for portable energy-conversion products and building-integrated photovoltaics. Organoboron chemistry offers an important toolbox to design novel organic/polymer optoelectronic materials and to tune their optoelectronic properties for OSC applications. At present, organoboron small molecules and polymers have become an important class of organic photovoltaic materials. Power conversion efficiencies (PCEs) of 16% and 14% have been realized with organoboron polymer electron donors and electron acceptors, respectively. In this review, we summarize the research progress in various kinds of organoboron photovoltaic materials for OSC applications, including organoboron small molecular electron donors, organoboron small molecular electron acceptors, organoboron polymer electron donors and organoboron polymer electron acceptors. This review also discusses how to tune their opto-electronic properties and active layer morphology for enhancing OSC device performance. We also offer our insight into the opportunities and challenges in improving the OSC device performance of organoboron photovoltaic materials.

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Junhui Miao

Junhui Miao received his PhD degree from Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences (CAS), in 2020. Now he is an assistant researcher in Professor Jun Liu's group in State Key Laboratory of Polymer Physics and Chemistry at CIAC. His research interest is focused on organic photovoltaic materials.

Yinghui Wang

Yinghui Wang received her BS degree from Zhengzhou University (China) in 2018. She is now a PhD student under the supervision of Prof. Jun Liu, in CIAC, CAS. Her research topic is organoboron polymer for organic solar cell applications.

Jun Liu

Jun Liu received his PhD degree in Polymer Physics and Chemistry in 2007 from CIAC, CAS. After postdoctoral training at the University of Würzburg (Germany), University of California, Los Angeles (USA), and Case Western Reserve University (USA) for five years, he joined CIAC as a principal investigator in 2013. His main research interests are organic/polymer opto-electronic materials and polymer solar cells.

Lixiang Wang

Lixiang Wang received his PhD degree from CIAC, CAS, in 1989. During 1994–1997, he was a postdoctoral researcher at Max Planck Institute for Polymer Research (Germany) and University of Massachusetts Amherst (USA). Since 1997, he has been a principal investigator in CIAC. His main research interests focus on solution-processable dendrimers/polymers for organic light-emitting diodes, chemical sensors and polymer solar cells.

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1. Introduction

Solar cells are an important kind of renewable clean energy technology to address the energy and environmental issues mankind is facing today.^1–4 Among various kinds of solar cells, organic solar cells (OSCs) have the advantages of flexibility, light-weight, semi-transparency and low cost.^5–14 These advantages make OSCs promising in portable, wearable power supplies and building integrated photovoltaics.^15–23 Recently, great progress has been made in the field of OSCs. Small-area single-junction OSC devices with a power conversion efficiency (PCE) exceeding 18% or with a lifetime of over twenty years have been demonstrated in labs.^24–27 Large-area OSC modules have also been fabricated by vacuum-deposition or roll-to-roll processing. Provided that three key factors, efficiency, stability and cost, can be synergistically addressed, OSCs will be used in commercial applications.

OSCs use a pair of small molecular/polymer electron donors and electron acceptors as photoactive layers. The development of electron donor and electron acceptor materials plays a key role in the OSC device performance improvement. In 1986, Tang et al. reported the pioneering work on OSCs which used double layers (Fig. 1a) of an electron donor and an electron acceptor and gave a PCE of ∼1%.²⁸ In 1995, Heeger et al. developed bulk heterojunction (BHJ) OSCs (Fig. 1b) using a mixture of an electron donor and an electron acceptor as the photoactive layer.²⁹ Since then, the widely used electron acceptors have been fullerene derivatives.^30,31 Materials chemists have made attempts to develop small molecular electron donors or polymer electron donors to match fullerene electron acceptors to enhance the PCE of OSCs.^32–41 From 2015 to date, owing to the emergence and optimization of non-fullerene electron acceptors, the PCE of OSCs has been enhanced from ca. 10% to more than 18%. Many high-efficiency non-fullerene electron acceptors have been developed, including small molecular electron acceptors and polymer electron acceptors.^42–62

Fig. 1 Schematic illustration of the device structure of (a) two-layer heterojunction and (b) bulk heterojunction OSCs. (c) Schematic illustration of the empirical working principle of OSCs. (d) A typical current density–voltage (J–V) curve of OSC devices and the meaning of J_SC, V_OC, FF and PCE.

Organoboron chemistry offers a toolbox not only to design new organic opto-electronic materials, but also to tune their optoelectronic properties towards excellent device performance.^63–74 In the past decade, a large amount of organoboron small molecule or polymer electron donors/acceptors have been developed and used in OSC devices.^75–78 High PCEs and excellent OSC device stability have been demonstrated with these organoboron photovoltaic materials. Therefore, organoboron molecules and polymers have become an important class of materials for OSCs.

The aim of this review is to discuss various kinds of organoboron photovoltaic materials for OSC applications, including organoboron small molecular electron donors, organoboron small molecular electron acceptors, organoboron polymer electron donors and organoboron polymer electron acceptors. We discuss not only how to design novel organoboron photovoltaic materials, but also how to tune their opto-electronic properties and active layer morphology for enhanced OSC device performance. In this review, we start with the research background, i.e. introduction to OSC devices, requirement of electron donor/acceptor materials, and principle of organoboron chemistry to tune opto-electronic properties. Then, we discuss the various kinds of organoboron photovoltaic materials. Finally, we provide an outlook on the opportunities and challenges in improving the OSC device performance of organoboron photovoltaic materials.

2. Research background

2.1 A brief introduction to OSCs

Fig. 1a and b show the schematic illustration of bilayer and BHJ OSC device structures, respectively. A pair of electron donors and electron acceptors with a bilayer structure or single layer structure are used as active layers, which are sandwiched between the anode and the cathode. Fig. 1c shows a schematic illustration of the empirical working principle of OSCs. Under solar light irradiation, the active layer absorbs photons and generates excitons (electron/hole pairs). The excitons diffuse to the donor/acceptor interfaces and dissociate to separated electrons and holes. The electrons and holes are transported in the active layer and are collected by the cathode and the anode, respectively. With these processes of light absorption, exciton diffusion/dissociation, electron/hole transportation and electron/hole collection, solar light energy can be converted to electrical energy.

The device performance of OSCs can be expressed by the current density–voltage (J–V) curve under simulated solar light irradiation (Fig. 1d). The main parameter for evaluating photovoltaic performance is power conversion efficiency (PCE), which is defined as the ratio between the maximum power output density (P_m) and the incident light input power (P_in). The P_m of the cell is determined as the product of open-circuit voltage (V_OC), short-circuit current (J_SC), and fill factor (FF). Therefore, the PCE can be written as the following equation: PCE = V_OC × J_SC × FF/P_in. For OSCs, V_OC mainly depends on the difference between the highest occupied molecular orbital (HOMO) energy level of the electron donor and the lowest unoccupied molecular orbital (LUMO) energy level of the electron acceptor. In addition, charge carrier recombination and electronic coupling between the donor and the acceptor also affect V_OC. J_SC is determined by light absorption of the active layer, diffusion and dissociation of excitons, transport and collection of charge carriers, etc. FF reflects the ratio of charge extraction and charge recombination and is also affected by multiple factors.

2.2 Requirements of electron donor/acceptor materials

According to the working principle of OSC devices, to obtain excellent OSC device performance, electron donor and electron acceptor materials should meet multiple requirements. At first, to effectively harvest sunlight, electron donor materials and electron acceptor materials in active layers should have wide absorption spectra and large absorption coefficients. Secondly, HOMO and LUMO energy levels of electron donor materials and electron acceptor materials greatly affect the exciton dissociation process and to a large extent determine the voltage output of OSCs. Therefore, donor and acceptor materials should have appropriate LUMO/HOMO energy level alignment. Thirdly, to improve charge transport in active layers, the donor and acceptor materials should have high hole mobilities and high electron mobilities, respectively. Fourthly, to improve exciton dissociation at the donor/acceptor interfaces and provide pathways for electron/hole transportation, donor materials and acceptor materials should form bis-continuous network phase separation morphology with a phase domain size of ca. 10–30 nm. It is challenging to develop an electron donor or acceptor material that fulfills all these requirements. Materials chemists always have to carefully optimize the chemical structures of electron donor or acceptor materials to fulfill all these requirements for high OSC device efficiency.

2.3 Organoboron chemistry as a toolbox for molecular design

The boron element, neighboring the carbon element in the periodic table of elements, has the atomic number of 5. The boron atom has a vacant 2p orbital, leading to strong Lewis acidity and high propensity to coordinate with Lewis bases (Fig. 2). In organic molecules and polymers, the boron atom adopts either sp² hybridization (tri-coordination) or sp³ hybridization (tetra-coordination) (Fig. 2). In tri-coordinated organoboron π-conjugated compounds, the empty 2p orbital of the boron atom forms p–π* conjugation with the π system, leading to a slightly downshifted LUMO energy level.^68,73 Therefore, tri-coordinated organoboron molecules or building blocks are always used in electron donors with optimized LUMO/HOMO energy levels. For tetra-coordinated organoboron π-conjugated compounds, the empty 2p orbital of the boron atom (Lewis acidic) coordinates with the lone electron pairs of some atoms, such as oxygen atom and nitrogen atom (Lewis basic).^79–82 Since coordination bonds of B ← N and B ← O have strong electron withdrawing capability, many tetra-coordinated organoboron compounds or polymers exhibit very low-lying LUMO energy levels. As a result, the majority of tetra-coordinated organoboron molecules or building blocks are used in electron acceptors with deep LUMO/HOMO energy levels.

Fig. 2 Illustration of a tri-coordinated organoboron compound and a tetra-coordinated organoboron compound.

Many tri-coordinated and tetra-coordinated organoboron small molecules and polymers with excellent opto-electronic properties are used for organic opto-electronic device applications.^72,83–91 Generally, two paradigms have been adopted to develop organoboron photovoltaic materials. The first one is to modify the chemical structures of some well-known organoboron compounds, e.g. 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) and boron subphthalocyanine (SubPc), for optimized opto-electronic properties. BODIPY and SubPc derivatives have already been intensively studied before OSCs attracted wide-spread attention. Materials chemists utilize molecular design to tune their opto-electronic properties to meet the requirement of electron donors or electron acceptors for excellent OSC device performance. The other paradigm is to design brand-new organoboron molecules or building blocks for OSC application. A typical example is double B ← N bridged bipyridine (BNBP), which is used as an electron-deficient building block to design polymer electron acceptors and small molecular electron acceptors.⁷⁶ BNBP was firstly developed aiming at OSC application by Liu et al. in 2016.

An important issue for organoboron molecules or polymers for OSC application is their chemical stability. Both tri-coordinated organoboron compounds and tetra-coordinated organoboron compounds are highly labile upon Lewis base. For tri-coordinated organoboron compounds, the vacant 2p orbital of the boron atom may be attacked by some molecules containing oxygen or nitrogen atoms with lone electron pairs, e.g. H₂O, alcohol, triethylamine, etc. Normally, large steric hindrance or a rigid π-conjugated skeleton can be used to substantially enhance the chemical stability of tri-coordinated organoboron compounds. For tetra-coordinated organoboron compounds, coordination bonds of B ← N and B ← O are sometimes inadequately strong. For example, the intermolecular B ← N bond is reversible, and the intramolecular B ← N bond is irreversible but the bond energy is only ca. 30 kJ mol⁻¹. A portion of tetra-coordinated organoboron compounds is not stable against moisture or polar solvents. Therefore, for both tri-coordinated and tetra-coordinated organoboron compounds to be used in OSCs, their chemical structures must be carefully designed to ensure adequate chemical stability.

3. Organoboron small molecules for OSCs

Both organic small molecules and conjugated polymers can be used as electron donors or electron acceptors in OSCs. The small molecules have the merits of explicit chemical structures, high purity of materials and easy tuning of phase separation morphology. In comparison, the conjugated polymers have the advantages of excellent mechanical properties and morphological stability. Empirically, the combination of small molecular electron acceptors and polymer electron donors affords optimal OSC device efficiency, and the combination of polymer electron donors and polymer acceptors gives the best OSC device stability. In this part, we will introduce organoboron small molecular electron donors and electron acceptors. The general strategy to design small molecular organoboron photovoltaic materials is to modify the chemical structures of already-known organoboron compounds, e.g. BODIPY and SubPc, to tune their opto-electronic properties.

3.1 BODIPY-based small molecular electron donors

BODIPY (Fig. 3a) is a well-known dye with strong and tunable absorption in the visible and near infrared (NIR) region, a high molar extinction coefficient and a high fluorescence quantum yield, as well as excellent photostability and chemical stability.^92–101 BODIPY was discovered by Kreuzer and Treibs in 1968.^78,102–108 In 2009, Roncali and coworkers pioneered the use of BODIPY derivatives as electron donors in OSCs.¹⁰⁹ In the past decade, a large family of BODIPY derivatives have been synthesized for application in OSCs.^92,93 In this review, due to the limited space, we are not able to discuss all the BODIPY-based organic photovoltaic materials. For more comprehensive and more detailed information, the readers are recommended to refer to some recent excellent reviews on BODIPY derivatives for organic opto-electronic device applications.^{78,92,93,106,108}

Fig. 3 (a) Chemical structure of unmodified BODIPY. (b) Functionalization of BODIPY at the α-, β- or meso-positions. (c) Fusing the BODIPY core with aromatic rings. (d) Substitution of the meso-carbon atom by a nitrogen atom.

Unmodified BODIPY shows an absorption spectrum peaking at ca. 503 nm with a bandgap of ca. 2.4 eV. The opto-electronic properties of BODIPY must be tuned by chemical modification to meet the requirements of electron donors for high-performance OSCs. As shown in Fig. 3, in view of the chemical structure of BODIPY, three strategies can be used to functionalize BODIPY:^110–112 (i) functionalization of BODIPY at the α-, β- or meso-positions; (ii) fusing the BODIPY core with aromatic rings to extend π-conjugation; and (iii) substitution of the meso-carbon atom by a nitrogen atom to afford aza-BODIPYs. Following the three strategies and aiming at optimized opto-electronic properties, BODIPY has been functionalized with π-conjugated units, other dye building blocks or electron-donating/withdrawing substituents. The resulting BODIPY-based small molecular electron donors exhibit excellent photovoltaic performance in OSC devices.^113,114

3.1.1 Substitution of BODIPY at the α-, β- or meso-positions. BODIPY can be facilely substituted at the meso-, α-, or β-positions (see Fig. 3). The substitution position plays an important role in the opto-electronic properties of the resulting BODIPY-based organic photovoltaic materials.^115–125 Substituents at the meso-position are poorly conjugated with the BODIPY skeleton and just slightly affect the absorption spectra and LUMO/HOMO energy levels of BODIPY. However, because of the steric hindrance effect, substituents at the meso-position greatly influence molecular packing in solid state, which obviously affects the active layer morphology of OSC devices. Linkage of substituents through the α-position can greatly extend the effective π-conjugation of BODIPY, resulting in a much reduced bandgap. Therefore, functionalization at the α-position can be used to design BODIPY-based small electron donors with narrow bandgap and strong NIR absorption. Substituents at the β-position are conjugated with BODIPY to a limited extent. β-Position functionalization can be used to develop small molecular electron donors with wide absorption spectra, i.e. those with panchromatic property.

Sobral et al. reported a series of BODIPY derivatives (M1–M6) with various aromatic groups at the meso-positions (Fig. 4a).¹²⁶ The meso-substitution aromatic groups were nearly perpendicular to the BODIPY plane and consequently were not effectively conjugated with the BODIPY skeleton (Fig. 4b). Therefore, M1–M6 exhibited similar LUMO/HOMO energy levels and absorption spectra (Fig. 4c and d). Solution-processed OSCs were fabricated using M1–M6 as the electron donors and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as the electron acceptor Table 1. The device based on M6 exhibited a low PCE of 0.47%, which might result from the non-optimal active layer morphology and the low charge carrier mobility. An effective approach to alleviate the steric hindrance effect of meso-substitution is to use π-linkers between the BODIPY skeleton and meso-substitution. Sharma et al. employed ethynyl as the π-linker and attached the bulky 2,7-tert-butyl-carbazole moiety to the meso-position of BODIPY to synthesize M7 (Fig. 4e).¹²⁷ Due to the acetylene connection between BODIPY and the carbazole substituent, M7 showed a strong donor–acceptor (D–A) character. The absorption spectrum of M7 in solution covered from 450 nm to 650 nm, and a shoulder peak appeared at around 590 nm. From solution to film, the absorption spectrum became broad and redshifted, which may be due to the strong intermolecular interaction. M7 exhibited a narrow bandgap with the onset absorption wavelength exceeding 700 nm. Moreover, this compound adopted a highly planar configuration and showed high crystallinity in thin film, which improved the hole mobility. The optimized OSC devices based on M7:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) showed a PCE of 5.05% with a V_OC of 0.90 V, a J_SC of 10.20 mA cm⁻², and an FF of 0.55.

Fig. 4 (a) Chemical structures of BODIPY dyes M1–M6 functionalized at meso-positions. (b) Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawing of the molecular structure with ellipsoids drawn at the 50% probability level of the M5 structure. (c) Electronic distribution of LUMO and HOMO of M5. (d) Normalized absorption spectra of M1–M6 (BDP1–BDP6) in solution. (e) Chemical structure of M7. Panels (b–d) were reproduced from ref. 126 with permission from Elsevier Ltd, copyright 2019.

Table 1 Optoelectronic properties and OSC device performance of BODIPY-based small molecules

Small molecules	HOMO/LUMO (eV/eV)	E ^optg (eV)	Device structure	V OC (V)	J SC (mA cm^−2)	FF	PCE^a (%)	Ref.
a The maximum PCE value of the corresponding device.
M1	−5.35/−3.08	2.28	ITO/PEDOT:PSS/M1:PC61BM/LiF/Al	0.54	1.93	0.26	0.31	126
M2	−5.41/−3.12	2.25	—	—	—	—	—	126
M3	−5.50/−3.29	2.19	ITO/PEDOT:PSS/M3:PC61BM/LiF/Al	0.42	0.09	0.26	0.01	126
M4	−5.27/−2.99	2.25	ITO/PEDOT:PSS/M4:PC61BM/LiF/Al	0.53	0.49	0.30	0.09	126
M5	−5.39/−3.10	2.26	ITO/PEDOT:PSS/M5:PC61BM/LiF/Al	0.62	1.55	0.27	0.30	126
M6	−5.35/−3.08	2.27	ITO/PEDOT:PSS/M6:PC61BM/LiF/Al	0.67	2.27	0.27	0.47	126
M7	−5.48/−3.44	1.72	ITO/PEDOT:PSS/M7:PC71BM/Al	0.90	10.20	0.55	5.05	127
M8	−5.56/−3.75	1.70	ITO/PEDOT:PSS/M8:PC71BM/Al	0.75	4.14	0.44	1.34	109
M9	−5.69/−3.66	1.95	ITO/PEDOT:PSS/M9:PC71BM/Al	0.80	4.43	0.34	1.17	109
M10	−5.32/−3.86	1.45	ITO/PEDOT:PSS/M10:PC61BM/Al	0.7	14.3	0.47	4.7	129
M11	−5.06/−3.73	1.41	ITO/PEDOT:PSS/M11:PC71BM/Ca/Al	0.74	11.28	0.38	3.13	140
M12	−5.02/−3.64	1.40	ITO/PEDOT:PSS/M12:PC71BM/Ca/Al	0.67	6.80	0.34	1.56	140
M13	−5.23/−3.72	1.60	ITO/PEDOT:PSS/M13:PC71BM/PFN/Al	0.97	10.55	0.47	4.75	141
M14	−5.06/−3.60	1.46	ITO/PEDOT:PSS/M14:PC71BM/Ca/Al	0.7	12.98	0.62	5.61	142
M15	−4.93/−3.28	—	ITO/PEDOT:PSS/M15:PC71BM/Ca/Al	0.77	13.79	0.67	7.2	143
M16	−5.36/−3.79	1.44	ITO/PEDOT:PSS/M16:PC71BM/PFN/Al	0.95	14.32	0.67	8.98	144
M17	−5.62/−3.52	1.84	ITO/PEDOT:PSS/M17:PC71BM/Al	0.90	10.48	0.56	5.29	145
M18	−4.95/−3.50	—	ITO/PEDOT:PSS/P3HT:IC70BM:M18/Ca/Al	0.87	7.0	0.71	4.3	152
M19	−5.45/−3.87	1.37	ITO/MH250:W2(hpp)4/C70/M19:C60/BPAPF/BPAPF:NDP9/NDP9/Al	0.89	6.1	0.45	2.5	153
M20	−5.36/−3.80	1.35	ITO/MH250:W2(hpp)4/C70/M20:C60/BPAPF/BPAPF:NDP9/NDP9/Al	0.85	9.9	0.54	4.6	153
M21	−5.23/−3.78	1.32	ITO/MH250:W2(hpp)4/C70/M21:C60/BPAPF/BPAPF:NDP9/NDP9/Al	0.73	13.3	0.63	6.1	153
M22	−5.68/−4.01	1.62	ITO/C60/M22/PV-TPD/p-PV-TPD/p-ZnPc/Au	0.92	3.0	0.33	0.8	157
M23	−5.22/−3.65	1.48	ITO/C60/M23/PV-TPD/p-PV-TPD/p-ZnPc/Au	0.65	2.4	0.65	1.1	157
M24	−5.13/−3.40	—	ITO/MH250:W2(hpp)4/C60/M24:C60/M24/BF-DPB/BF-DPB:NDP9/NDP9/Al	0.77	10.4	0.57	4.5	162
M25	−5.40/−3.79	1.73	ITO/ZnO/P3HT:M25/MoO3/Ag	0.65	3.09	0.60	1.21	163
M26	−5.16/−3.82	1.54	ITO/ZnO/P3HT:M26/MoO3/Ag	0.62	3.90	0.63	1.51	163
M27	−5.14/−3.74	1.47	ITO/ZnO/P3HT:M27/MoO3/Ag	0.57	3.28	0.63	1.18	163
M28	−5.36/−3.79	1.50	ITO/PEDOT:PSS/PTB7-Th:p-DTS(FBTTh2)2:M28/Ca/Al	0.76	7.03	0.53	2.84	164

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α-Functionalization of BODIPY is used to design small molecular electron donors with small bandgap and NIR absorption. In the first report of BODIPY-based small molecular electron donors by Roncali et al., the BODIPY core was functionalized with phenylene-vinyl at the two α-positions (M8 and M9 in Fig. 5a). Preliminary OSC device results show a PCE of 1.34% for M8 and 1.17% for M9.^109,128 Ziessel and coworkers developed M10 by attaching two vinyl-bithiophene substituents to the α-positions of BODIPY (Fig. 5b).¹²⁹ The single-crystal structure of M10 indicated that the vinyl-bithiophene skeletons were coplanar with the BODIPY core (Fig. 5c). Moreover, the vinyl-bithiophene skeletons are well conjugated with the BODIPY core, leading to a narrow bandgap of 1.45 eV with an absorption maximum wavelength of 780 nm. In solid state, M10 molecules organized in three dimensions with strong intermolecular interactions, favoring short distances between neighboring molecules (Fig. 5d). As a result, M10 showed excellent charge transporting property, i.e. both hole and electron mobilities in the order of 10⁻³ cm² V⁻¹ s⁻¹ in ambipolar organic field-effect transistors (OFETs). The OSC device with the M10:PC₆₁BM blend as the active layer showed a broad external quantum efficiency (EQE) response covering from 350 nm to ca. 900 nm. The device exhibited a V_OC of 0.7 V, a J_SC of 14.3 mA cm⁻², and an FF of 0.47, leading to a PCE of 4.7%.

Fig. 5 (a) Chemical structures of BODIPY derivatives M8 and M9 functionalized at α-positions. (b) Chemical structure of BODIPY derivative M10. (c) ORTEP view of the single crystal of compound M10. (d) Packing view down the ac direction of M10. Panels (c) and (d) were reproduced from ref. 129 with permission from the American Chemical Society, copyright 2012.

α-Functionalized BODIPY can be used as the A terminals in acceptor–donor–acceptor (A–D–A) type small molecular electron donors.^130–139 This molecular design motif can be used to tune the active layer morphology of OSCs for improved device performance. Zhan et al. synthesized M11 (Fig. 6),¹⁴⁰ in which two α-functionalized BODIPY units were connected to a terthiophene unit. M11 showed a similar absorption spectrum and LUMO/HOMO energy levels to those of its monomeric derivative M12. When blended with PC₇₁BM in thin film, the monomeric M12 lost its crystalline structure but the dimeric M11 could maintain its ordered packing. This was due to the expanded π-conjugated backbone and the steric-pairing effect of the twisted configuration of M11. The OSC device based on the blend of M11 and PC₇₁BM exhibited a PCE of 3.13% and a high hole/electron mobility of 1.16 × 10⁻³ cm² V⁻¹ s⁻¹/0.90 × 10⁻³ cm² V⁻¹ s⁻¹. Zhu et al. synthesized M13 by end-capping a dialkylthienyl flanked benzo[1,2-b:4,5-b′]dithiophene (BDT) core with two BODIPY units (Fig. 6).¹⁴¹M13 exhibited a HOMO energy level of −5.23 eV, a LUMO energy level of −3.72 eV and an onset absorption wavelength at 774 nm. With a blend of M13 and PC₇₁BM as the active layer, the OSC device achieved a PCE of 4.75%. The good OSC device performance of M13 was assigned to the optimal phase separation morphology with a small domain size in the active layer.

Fig. 6 Chemical structures of M11–M13 functionalized at the α-positions of BODIPY.

β-Funtionalized BODIPY can be used to develop small molecular electron donors with panchromatic properties, i.e. those that have ultra-wide absorption spectra. Xu et al. developed M14 by functionalizing a BODIPY core at the β-position with two BDT terminals via two alkynyl linkages (Fig. 7a).¹⁴² Although the BDT units were co-planar with the BODIPY core, the BDT units were not well conjugated with the BODIPY core. This was evidenced by the delocalized HOMO over the whole skeleton and the localized LUMO on the BODIPY core unit of M14. An interesting property of M14 was its ultra-wide absorption spectrum in thin film, which effectively covered from 300 nm to 800 nm. This wide absorption spectrum indicated strong sunlight harvesting capability and was desirable for solar cell applications. Using M14 as the electron donor and PC₇₁BM as the electron acceptor, the authors demonstrated OSC devices with a PCE of 5.61%. Another example of BODIPY-based electron donors with wide absorption spectra is M15, which is reported by Singh and coworkers.¹⁴³ In M15, BODIPY acted as the core and was functionalized at the β-position, two dithiafulvalene acted as the terminals, and the furan unit acted as the bridging unit (Fig. 7b). M15 showed a wide absorption spectrum in film ranging from 300 nm to 800 nm with three absorption peaks at 376 nm, 458 nm and 627 nm. The optimized OSC device based on the M15:PC₇₁BM blend showed a high PCE of 7.2% together with a high EQE value of 88.1% at 371 nm.

Fig. 7 Chemical structures of M14–M17 functionalized at the β-positions of BODIPY.

An interesting approach to design small molecular electron donors is to link BODIPY and other low-bandgap dyes for wide absorption spectra. Gros et al. designed and synthesized M16 by introducing two porphyrin units at the α-positions and attaching two dithienyl-diketopyrrolopyrrole (Th-DPP) units to the β-positions of BODIPY (Fig. 7c).¹⁴⁴ The absorption spectra of BODIPY, porphyrin and Th-DPP were complementary, leading to the broad absorption spectrum of M16 covering from 350 nm to 850 nm. The OSC device based on the M16:PC₇₁BM active layer showed a remarkable PCE of 8.98% with a J_SC of 14.32 mA cm⁻², a V_OC of 0.95 V and an FF of 0.67. Moreover, the device showed a small photon energy loss of 0.5 eV due to the small LUMO offset between M16 and PC₇₁BM. Sharma et al. designed a triad of porphyrin and BODIPY bridged by 1,3,5-triazine (M17) (Fig. 7d).¹⁴⁵ Its absorption spectrum showed five peaks at 422 nm, 552 nm, 596 nm, 652 nm and 502 nm. The first four peaks were assigned to porphyrin and the last one peak was attributed to BODIPY. The OSC device based on M17 and PC₇₁BM showed a PCE of 5.29% with a J_SC of 10.48 mA cm⁻².

3.1.2 Annulation of the BODIPY core. Annulation, i.e. fusing aromatic rings to the BODIPY skeleton, is an effective strategy to redshift the absorption and fluorescence spectra of BODIPY derivatives. Compared to the aforementioned substitution approaches, the annulation strategy maintains the rigidity and planarity of BODIPY skeletons, and thus facilitates effective π-conjugation and ordered π–π stacking in solid state. BODIPY compounds annulated at the [a]-bond, [b]-bond or at the boron atom (Fig. 3c) have been synthesized and utilized in OSCs.^146–150 Noteworthily, the annulation position greatly affects the opto-electronic properties of the resulting BODIPY derivatives. As reported by Yamaguchi et al.,¹⁵¹ when annulated at the [a]-bond, the resulting BODIPY derivatives showed slightly upshifted HOMO and LUMO energy levels as well as decreased bandgaps. When BODIPY skeletons were annulated at the [b]-bond, their HOMO energy levels upshifted while their LUMO energy levels downshifted, leading to significantly decreased bandgaps.

Annulated BODIPYs always show a small bandgap, redshifted absorption spectra and strong sunlight harvesting capability. For example, M18,¹⁵² in which a BODIPY core was fused with two benzene rings at the [a]-bond (Fig. 8a), showed a narrow bandgap with the absorption spectrum peaking at 733 nm. M18 could be used as the third component to be added to the poly(3-hexylthiophene) (P3HT)/indene-C₇₀ bisadduct (IC₇₀BA) active layer to improve the sunlight absorption of the OSC devices. As a result, upon the addiction of M18, the J_SC of the device was increased from 6.3 mA cm⁻² to 7.0 mA cm⁻² and the PCE was improved from 3.7% to 4.3%. Leo et al. developed a series of furan-fused BODIPY derivatives (M19–M21) with trifluoromethyl (CF₃) on the meso-carbon atom and various substituents at the peripheral phenyl groups (Fig. 8a).¹⁵³ The substituents on the phenyl groups greatly affected molecular packing in solid state of these furan-fused BODIPY derivatives (Fig. 8b). M19 showed a slipped face-to-face stacking with a mean plane distance of 3.8 Å. In M20 with the methyl substituents, two parallel molecules were arranged as a repeating unit in a tilted ladder-type arrangement. M21 with the methoxy substituents exhibited a brickwork-type arrangement. It seemed that M19 and M21 adopted J-aggregation and M20 adopted H-aggregation. Their absorption spectra in thin film spanned from 500 nm to 900 nm, and the redshift and blueshift of absorption spectra might be attributed to the J-/H-aggregation of the annulated BODIPY in solid state (Fig. 8c). From M19 to M21, HOMO energy levels continuously upshifted from −5.45 eV to −5.23 eV, and LUMO energy levels upshifted from −3.87 eV to −3.78 eV. The upshifted HOMO/LUMO energy levels were due to the electron-donating properties of the methyl and methoxyl groups. Vacuum-deposited OSC devices were fabricated using M19–M21 as the electron donors and C₆₀ as the electron acceptor. The PCEs of the single-junction OSC devices were 2.5% for M19, 4.6% for M20 and 6.1% for M21 (Fig. 8d). The poor device performance of the M19-based OSC was mainly due to the low-lying LUMO energy level of M19, which could not provide sufficient driving force for photo-induced charge separation. The multi-junction OSC device with M21 as the NIR absorber showed a PCE as high as 9.9%.

Fig. 8 (a) Chemical structures of BODIPY derivatives M18–M21 annulated at the [a]-bond or the [b]-bond. (b) ORTEP plots (left) and packing configurations (right) of M19–M21; hydrogen atoms are omitted. (c) Absorption spectra of M19–M21 in film. (d) J–V curves in the dark and under AM 1.5G illumination of the single-junction OSCs using M19–M21 as the electron donors. Panels (b)–(d) were reproduced from ref. 153 with permission from the American Chemical Society, copyright 2017.

3.1.3 Aza-BODIPY as electron donors. Replacing the meso-carbon atom of BODIPY with a nitrogen atom afforded aza-borondipyrromethene (aza-BODIPY), which was initially designed and synthesized by O’Shea et al.¹⁵⁴ Compared to BODIPY, aza-BODIPY showed a red-shifted absorption spectrum, which was beneficial for sunlight absorption of OSCs. Mueller and coworkers pioneered the use of aza-BODIPYs as electron donor materials in OSCs.^155–157 They synthesized and compared the phenyl-substituted aza-BODIPY (M22) and benzannulated aza-BODIPY (M23) (Fig. 9).¹⁵⁷ The absorption spectrum of M23 in thin film was redshifted by ca. 70 nm compared to that of M22. The vacuum-deposited OSC device based on M23:C₆₀ showed a PCE of 1.1%. Later a PCE of 3.8% was demonstrated with M23 in a n–i–p type OSC device.¹⁵⁸ The NIR absorption property of M23 was utilized to develop OSCs with enhanced PCE or with additional functions. For example, Brabec et al. used M23 as the third component to improve the light absorption of P3HT:PC₆₁BM based OSCs in the NIR region.¹⁵⁹ Riede et al. used M23 to fabricate semi-transparent OSC devices¹⁶⁰ and Leo et al. used M23 to construct triple-junction OSC devices with a PCE of 10.4%.¹⁶¹ To enhance the thermal stability of aza-BODIPY for multilayer OSC device fabrication by vacuum-deposition, Leo et al. replaced the BF₂ unit in aza-BODIPY (M23) by borafluorene and synthesized M24 (Fig. 9).¹⁶²M24 could not only be synthesized at high yield, but could also be purified by sublimation at high yield. Its planar heterojunction (PHJ) OSC device exhibited an FF higher than 70% and its BHJ OSC device gave a PCE of 4.5%.

Fig. 9 Chemical structures of aza-BODIPY derivatives M22–M24.

In summary, the rich chemistry and facile synthesis enable the development of a large family of BODIPY-based small molecular electron donors. After careful chemical structure optimization and OSC device optimization, a PCE of about 10% has been demonstrated with these BODIPY-based small molecular electron donors. A striking feature of this class of organic photovoltaic materials is their strong sunlight harvesting capability. Both narrow bandgap with strong NIR absorption and panchromatic property with wide absorption spectra have been demonstrated with BODIPY-based small molecular electron donors. It is worth noting that most of the above OSC device results are obtained with fullerene derivatives as the electron acceptors. Great efforts should be devoted to the matching of BODIPY-based small molecular electron donors and non-fullerene electron acceptors. In view of the excellent opto-electronic properties of BODIPY-based small molecules, large room for PCE enhancement of BODIPY-based small molecular electron donors can be anticipated, provided that high-efficiency non-fullerene electron acceptors are used in these OSC devices.

3.2 BODIPY-based small molecular electron acceptors

In view of the low LUMO energy levels of BODIPY, materials chemists have attempted to design BODIPY-based small molecular electron acceptors for OSCs. These compounds always use some π-conjugated linkers to connect two BODIPY units as the terminals. Fig. 10 shows several examples. However, their device performance is not satisfactory possibly due to the unfavorable charge transport and the non-optimal active layer morphology. Thayumanavan et al. reported a series of A–D–A type compounds (M25–M27), in which BODIPY as the A unit was linked at the meso-position by various electron donating units (BDT, cyclopenta-[2,1-b:3,4-b]dithiophene (CPDT) and dithieno [3,2-b:2′,3′-d]pyrrole (DTP)) (Fig. 10).¹⁶³ These compounds exhibited strong visible absorption with the extinction coefficient on the order of 10⁴ M⁻¹ cm⁻¹ and low-lying LUMO energy levels of lower than −3.7 eV. They showed pure n-type characteristics with electron mobilities on the order of 10⁻⁵ cm² V⁻¹ s⁻¹. The low electron mobilities of M25–M27 might be due to the limited intermolecular interactions, caused by the skeletal twist between the thiophene and the meso-position linked BODIPY. Inverted OSC devices with M25, M26 or M27 as the electron acceptor and P3HT as the electron donor showed a PCE of 1.51%. The limited device performance may be attributed to the noneffective charge transport. Zhan et al. used the DPP unit as the core and two BODIPY units as the endcapper, and linked them with two ethynyl bridges to design M28 (Fig. 10).¹⁶⁴ The ethynyl bridges not only enhanced the planarity, but also played an important role in π-electron delocalization of the molecule.

Fig. 10 Chemical structures of BODIPY-based electron acceptors M25–M28.

M28 showed a wide absorption spectrum covering the ultraviolet-visible-near infrared (UV-vis-NIR) region with a low bandgap of 1.5 eV. The OSC device based on M28 as the electron acceptor showed a PCE of 2.84% (Fig. 17).

3.3 SubPc-based small molecular electron donors

Boron subphthalocyanine (SubPc, see Fig. 11) is a 14 π-electron heteroaromatic molecule, in which a tetracoordinated boron atom is surrounded by three N-fused diiminoisoindole units.^77,165–167 SubPc can be synthesized with high yield by the cyclotrimerization reaction of phthalonitrile precursors with boron trichloride (BCl₃). Unmodified SubPc has strong absorption in the wavelength range of 450–620 nm with a bandgap of ca. 2.1 eV. Its opto-electronic properties can be tuned by functionalization at the axial position and peripheral position. SubPc has a C₃-symmetric cone-shaped geometry. The nonplanar configuration of SubPc is beneficial not only for vacuum sublimation deposition, but also for good phase separation morphology when blended with other organic opto-electronic materials and processed by spin-coating. SubPc and its derivatives are always used as electron donors in OSCs, while carefully modified SubPc derivatives can also work as electron acceptors Table 2. Unmodified SubPc is always used in vacuum-deposited OSC devices, while modified SubPc with enhanced solubility can be used to fabricate solution-processed OSC devices.

Fig. 11 Chemical structures, functionalized positions and 3D-geometry of SubPc. The 3D-geometry of SubPc was reproduced from ref. 165 with permission from the American Chemical Society, copyright 2012.

Table 2 Optoelectronic properties and OSC device performance of SubPc-based small molecules

Small molecules	HOMO/LUMO (eV/eV)	E ^optg (eV)	Device structure	V OC (V)	J SC (mA cm^−2)	FF	PCE^a (%)	Ref.
a The maximum PCE value of the corresponding device.
SubPc	−5.6/−3.6	2.07	ITO/SubPc/C60/BCP/Al	0.97	3.36	0.57	2.1	168
			ITO/mCP/SubPc/C60/BCP/Al	1.09	7.87	0.59	5.08	171
			ITO/MoO3/SubPc:C70/BCP/Al	1.02	12.90	0.41	5.4	172
			ITO/MoOx/Tc/SubPc/BCP/Al	1.24	4.01	0.58	2.89	191
SubNc	−5.4/−3.6	1.71	ITO/PEDOT:PSS/SubNc/C60/BCP/Al	0.55	5.59	0.49	1.47	182
			ITO/MoO3/SubNc:PC71BM/BCP/Al	0.92	10.10	0.43	4.0	183
			ITO/PEDOT:PSS/α-6T/SubNc/BCP/Al	0.94	12.04	0.54	6.02	199
			ITO/PEDOT:PSS/α-6T/SubNc/SubPc/BCP/Al	0.96	14.55	0.61	8.40	199
M29	−5.06/—	—	ITO/PEDOT:PSS/M29/C60/BCP/Al	0.73	3.72	0.51	1.39	184
M30	−4.81/—	—	ITO/PEDOT:PSS/M30/C60/BCP/Al	0.63	3.30	0.34	0.70	184
M31	−5.6/−3.4	2.22	ITO/MoOx/SubPc/M31/BCP/Al	1.10	0.83	0.22	0.19	192
M32	−5.8/−3.6	2.24	ITO/MoOx/SubPc/M32/BCP/Al	1.22	1.56	0.43	0.80	192
M33	−5.8/−3.7	2.17	ITO/MoOx/SubPc/M33/BCP/Al	1.31	3.53	0.58	2.68	192
			ITO/ZnO/PTB7-Th:M33/MoOx/Ag	0.77	10.7	0.48	4.0	201
M34	—/−3.54	—	ITO/MoO3/SubNc/M34/BCP:C60/Ag	1.08	4.83	0.48	2.44	193
M35	—/−3.71	—	ITO/MoO3/SubNc/M35/BCP:C60/Ag	0.89	6.18	0.43	2.36	193
M36	—/−3.85	—	ITO/PEDOT:PSS/DIP/SubNc/M36/BCP:Yb	0.75	8.55	0.53	3.31	193
M37	−5.7/−3.6	—	ITO/MoO3/SubNc/M37/BCP/Al	1.06	2.06	0.43	0.94	195
M38	—	—	ITO/PEDOT:PSS/α-6T/M38/BCP/Ag	1.17	4.83	0.49	2.77	198
M39	—	—	ITO/PEDOT:PSS/α-6T/M39/BCP/Ag	1.21	4.40	0.47	2.51	198
M40	−6.00/−3.84	2.16	ITO/ZnO/PTB7-Th:M40/MoOx/Ag	0.74	3.6	0.40	1.1	201
M41	−6.02/−3.86	2.16	ITO/ZnO/PTB7-Th:M41/MoOx/Ag	0.81	5.3	0.41	1.8	201
M42	−6.08/−3.93	2.15	ITO/ZnO/PTB7-Th:M42/MoOx/Ag	0.50	2.1	0.45	0.5	201
M43	−5.69/−3.98	1.82	ITO/ZnO/PBDB-T:M43/MoO3/Al	0.74	5.18	0.46	1.78	202
M44	−5.71/−3.83	1.88	ITO/ZnO/PBDB-T:M44/MoO3/Al	0.89	9.18	0.55	4.53	202
M45	−5.60/−3.90	1.68	ITO/PEDOT:PSS/PBDB-T:M45/Ca/Al	0.87	10.06	0.54	4.69	203
M46	−5.63/−4.06	1.66	ITO/PEDOT:PSS/PBDB-T:M46/Ca/Al	0.77	10.51	0.47	3.80	203
M47	−5.82/−3.91	1.93	ITO/ZnO/PM6:M47/MoO3/Ag	0.84	9.69	0.60	4.92	204
M48	−5.39/−3.81	1.74	ITO/ZnO/PM6:M48/MoO3/Ag	0.97	13.97	0.55	7.41	205

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In 2006, Thompson and co-workers pioneered the development of OSCs based on SubPc.¹⁶⁸ They used SubPc as the electron donor and fullerene (C₆₀) as the electron acceptor to fabricate bilayer OSC devices by vacuum deposition (Fig. 12a and b). Because of the deep HOMO energy level (−5.6 eV) of SubPc, the SubPc/C₆₀-based OSC device exhibited a high V_OC of 0.97 V, a J_SC of 3.36 mA cm⁻² and a PCE of 2.1%. This vacuum deposited SubPc/C₆₀ bilayer OSC device allows in-depth investigation of the fundamental physics of organic photovoltaics. Device physicists have used SubPc/C₆₀ bilayer OSC devices as a test-bed to study the energy transfer mechanism, photoinduced charge transfer, interface energetics, charge recombination dynamics, charge collection efficiency, etc. of OSC devices. For a comprehensive review of the physics of SubPc/C₆₀ bilayer OSC devices, the readers are recommended to refer to some excellent reviews.^77,165,167

Fig. 12 (a) Chemical structures of the electron donor SubPc and the electron acceptors C₆₀ and C₇₀. (b) Device architectures of vacuum-deposited planar heterojunction (PHJ) OSCs. (c) J–V curves of the devices based on the active layer of SubPc:C₆₀ with different anode interfacial layer (mCP) thickness. (d) J–V curves of the single device and tandem device based on the active layer of SubPc:C₇₀. Panel (c) was reproduced from ref. 171 with permission from Elsevier, copyright 2013. Panel (d) was reproduced from ref. 173 with permission from Elsevier, copyright 2014.

Great efforts have been made to improve the PCE of SubPc-based vacuum-deposited OSC devices.^169,170 The deep HOMO energy level of SubPc leads to an energy barrier for hole extraction from the active layers to the indium tin oxide (ITO) anode. Therefore, the device performance of SubPc/C₆₀ can be improved by modification of the ITO surface to minimize the energy barrier. Lin et al. introduced a thin layer of N,N-dicarbazolyl-3,5-benzene (mCP) between the ITO anode and the SubPc layer.¹⁷¹ The mCP layer not only improved hole extraction, but also acted as an electron blocking layer and reduced the leakage current of the OSC device. As a result, all the three key parameters, V_OC, J_SC and FF, of the device increased, leading to a greatly enhanced PCE of 5.1% (Fig. 12c). A drawback of C₆₀ is the weak light absorption capability, i.e. a small extinction coefficient in the visible range. C₇₀ has a larger extinction coefficient and a red-shifted absorption spectrum compared to those of C₆₀, thereby has stronger sunlight absorption capability. Holmes et al. utilized C₇₀ as the electron acceptor and SubPc as the electron donor, and fabricated a vacuum-deposited OSC with a continuously graded heterojunction layer.¹⁷² The device exhibited a significantly improved J_SC of 12.90 mA cm⁻², which was due to the improved sunlight harvesting capability and the increased donor/acceptor interfacial area in the active layers. A PCE of 5.4% was obtained, which was much higher than that of the C₆₀-based device. Furthermore, by stacking two SubPc:C₇₀ layers with an interconnecting layer inserted, a tandem OSC device was made and a high PCE of 7.66% was demonstrated (Fig. 12d).¹⁷³ These results indicate that unmodified SubPc is an excellent electron donor for vacuum-deposited OSCs.^174–178

Compared with vacuum deposition, solution processing techniques such as. spin-coating and blade-coating are low-cost processing techniques to fabricate OSCs. The cone-shaped geometry of SubPc derivatives prevents compact molecular stacking and makes them readily soluble in organic solvents. Therefore, SubPc derivatives can also be solution-processed to fabricate OSCs. A typical example is chloroboron(III) subnaphthalocyanine (SubNc), which is comprised of three N-fused benzoisoindole units and an axial ligand around a boron core (Fig. 13a).^179,180 Compared with SubPc, SubNc has extended π-conjugation and consequently shows an upshifted HOMO energy level and a red-shifted absorption spectrum.¹⁸¹ SubNc exhibits strong light absorption spanning from 550 nm to 750 nm. Ma et al. initially adopted solution processing to fabricate bilayer OSC devices based on SubNc. The SubNc layer was spin-coated with the chlorobenzene solution and the C₆₀ layer was vacuum-deposited.¹⁸² Hong et al. reported OSC devices using a blend of SubNc and PC₇₁BM as the active layer, which was spin-coated with their blend solution in 1,2-dichlorobenzene (Fig. 13b).¹⁸³ The device exhibited a high J_SC of 10.1 mA cm⁻² and a PCE of 4.0%.

Fig. 13 (a) Chemical structures of the electron donor SubNc and the electron acceptor PC₇₁BM. (b) Device architecture of the SubNc:PC₇₁BM bulk heterojunction (BHJ) device for solution-processed OSCs. (c) Chemical structures of M29 and M30. (d) View of the crystal packing of M29 (left) and M30 (right). (e) Atomic force microscopy (AFM) images of M29 (left) and M30 (right). Panels (d) and (e) were reproduced from ref. 184 with permission from the American Chemical Society, copyright 2010.

An interesting approach to develop a solution-processed SubPc-based electron donor is to introduce bithiophene or quaterthiophene as the axial ligand in SubPc with phenoxy as the linker. The chemical structures of the resulting two compounds, M29 and M30, are shown in Fig. 13c and their molecular packing in crystals is provided in Fig. 13d.¹⁸⁴ The oligothiophene axial ligands of M29 and M30 provided the driving force for ordered molecular stacking and thus enhanced crystallization. Moreover, the longer the oligothiophene ligand of the linkage, the stronger the crystallization tendency of the compounds. In the films, M30 with quaterthiophene as the axial ligand was more crystalline than M29 with bithiophene as the axial ligand. The atomic force microscopy (AFM) height images of the two films are shown in Fig. 13e. The surface of the M29 film was smooth with a root-mean-square (RMS) roughness of about 0.5 nm. The surface of the M30 film displayed some aggregates with a relatively high RMS roughness of 1.4 nm, which could be attributed to the crystallization of M30. Bilayer OSCs were fabricated with the spin-coated M29 or M30 layer and the vacuum-evaporated C₆₀ layer and the preliminary PCE was 1.39%.

3.4 SubPc-based small molecular electron acceptors

The LUMO/HOMO energy levels of unsubstituted SubPc are about −3.6 eV/−5.6 eV and it is initially used as the electron donor.¹⁶⁸ When the peripheral or axial positions of SubPc are modified with electron-deficient groups, the resulting SubPc derivatives will show downshifted HOMO/LUMO energy levels and can be used as electron acceptors in OSCs.^185–190 That is, SubPc derivatives can be converted from electron donors to electron acceptors by substitution with electron-deficient groups.

The feasibility of SubPc as an electron acceptor for OSCs was firstly demonstrated by Jones et al. They fabricated a vacuum-deposited OSC with tetracene (Tc) as the electron donor and SubPc as the electron acceptor.¹⁹¹ The device showed a high V_OC of 1.24 V and a PCE of 2.89%. Although SubPc was typically used as an electron donor, the compound could also work as an electron acceptor provided that the offset between the donor and the acceptor was sufficient to dissociate charges efficiently. The authors introduced different types and numbers of halogen to the peripheral positions of SubPc and obtained three halogenated-SubPc (M31–M33, see Fig. 14).¹⁹² Compared with M31 with three F substituents at the peripheral position, the increase in the number of F atoms in M32 significantly reduced the LUMO energy level from −3.4 eV to −3.6 eV. The replacement of F atoms in M32 with Cl atoms further reduced the LUMO energy level to −3.7 eV of M33, the LUMO energy level was further reduced to −3.7 eV, which was derived from the stronger electron-withdrawing property of the Cl atom compared to the F atom. The low-lying LUMO energy levels made the halogenated-SubPc compounds work as electron acceptors to match unsubstituted SubPc as the electron donor. Among the three halogenated-SubPc compounds, M33 with the lowest LUMO energy level of −3.7 eV exhibited the best OSC device performance. The device based on the SubPc:M33 active layer showed a PCE of 2.68% with a V_OC of 1.31 V, a J_SC of 3.53 mA cm⁻² and an FF of 0.58.

Fig. 14 Chemical structures of SubPc derivatives with substitution at the peripheral and axial positions.

A main reason for the low J_SC of the SubPc:halogenated-SubPc device was the overlap of the absorption spectra between the electron donor and the electron acceptor. Device efficiency enhancement was achieved by selecting electron donors with complementary absorption spectra. Cnops et al. used four peripherally substituted SubPc derivatives (M33–M36) as the electron acceptors.¹⁹³ The electron-deficient character of the substituents (F, Cl and cyano group) could lower the frontier orbital energies of M33–M36 in different degrees, resulting in LUMO energy levels ranging from −3.54 eV to −3.85 eV. They chose small-bandgap donor materials and fabricated vacuum-evaporated PHJ OSC devices. The devices gave a large photocurrent because of the improved sunlight absorption. After elegant device optimization, the OSC device based on SubNc:M33 showed a PCE of 6.86% with a V_OC over 1.0 V and a J_SC of 10.1 mA cm⁻². The high V_OC of SubPc-based OSC devices can be used to construct tandem OSCs with extremely high V_OC to power a typical battery for energy storage. Jones et al. selected two sub-cell D/A pairs of SubPc/C₆₀ (V_OC = 1.1 V) and SubPc/M33 (V_OC = 1.3 V) to fabricate multi-junction OSC devices.¹⁹⁴ The resulting single-stack tandem OSC device showed a PCE of 3.4% and a V_OC of 4.54 V. Moreover, an ultra-high V_OC of over 7 V was achieved with 6-heterojunction. The devices could be used to charge a micro-energy battery under low intensity white light or monochromatic LED illumination.

An alternative approach to design SubPc-based electron acceptors is axial substitution with electron-withdrawing groups. Bender et al. developed a SubPc derivative (M37) with pentafluorophenoxy functionalization in the axial position.¹⁹⁵ The pentafluorophenoxy functionalization led to downshift of both HOMO and LUMO energy levels by about 0.1 eV. M37 could be used as either an electron donor or an electron acceptor in PHJ OSC devices. The authors also investigated the outdoor performance and stability of axially functionalized SubPc derivatives as electron acceptors.^196,197 They used phenoxy, phenyl and chloro-functionalized SubPcs (M38, M39 and SubPc) as electron acceptors and α-sexithiophene (α-6T) as an electron donor to construct PHJ OSC devices,¹⁹⁸ which were encapsulated and tested under outdoor conditions. The axial functionalization groups greatly affected the device lifetime. The OSC device based on SubPc with the chlorine substituent exhibited the highest J_SC and PCE under passive load and took far longer to degrade than the device based on M38 with the phenoxy substituent and the device based on M39 with the phenyl substituent. The T₈₀ (the operation time at which the OPV reaches 80% of its original PCE) of SubPc is more than 80 h of irradiation, while the T₅₀ (the operation time at which the OPV reaches 50% of its original PCE) of M38 and M39 is only 39 h and 22 h, respectively. Compared with M38 and M39, chloro-SubPc showed the highest outdoor device stability, which is attributed to the best intrinsic stability and the slowest degradation rate of chloro-SubPc. A possible reason for the faster degradation rates of M38 and M39 is that they are more susceptible to hydrolysis, compared with chloro-SubPc. These studies have shown that the functional groups at the axial positions of SubPcs have an important impact on their stability, and minor chemical modification may greatly affect their stability.

SubNc with extended π-conjugation and a redshifted absorption spectrum can also work as an electron acceptor in OSCs. Cnops et al. used SubNc as the electron acceptor and α-6T as the electron donor to make a PHJ OSC device, which showed a high PCE of 6.0% with a V_OC of 0.94 V, a J_SC of 12.04 mA cm⁻² and an FF of 0.54.¹⁹⁹ To further improve the sunlight absorption of the OSC device, the authors developed a three-layer PHJ architecture comprising both SubNc and SubPc as separated acceptor layers and α-6T as the donor layer (Fig. 15a).¹⁹⁹ In this device, excitons generated in the SubPc acceptor (outer layer) could be transferred to the smaller-bandgap SubNc acceptor (middle layer) by long-range Förster energy transfer, and then dissociated at the α-6T/SubNc interface (Fig. 15b). SubNc functioned not only as an energy acceptor for excitons generated in the SubPc layer, but also as a charge acceptor at the interface of α-6T/SubNc. SubNc, SubPc and α-6T all contributed to the generation of photocurrent, leading to the increase from 12.04 mA cm⁻² for the α-6T/SubNc-based device to 14.55 mA cm⁻² for the α-6T/SubNc/SubPc-based device. Surprisingly, this three-layer device exhibited a record-high PCE of 8.4% with a high V_OC close to 1 V and a wide EQE spectrum covering from 400 nm to 720 nm (Fig. 15c–e).

Fig. 15 (a) The device structure of the three-layer PHJ OSC based on α-6T, SubPc and SubNc. (b) Illustration of the two-step exciton dissociation process of the active layer. (c) The absorption spectra of α-6T, SubPc and SubNc. (d) The external quantum efficiency (EQE) and internal quantum efficiency (IQE) spectra of the two-layer or three-layer PHJ OSC devices. (e) J–V curves of the two-layer or three-layer PHJ OSC devices. Panels (c–e) were reproduced from ref. 199 with permission from Springer Nature, copyright 2014.

SubPc derivatives with enhanced solubility had been used in solution-processed OSCs and excellent photovoltaic performance had been demonstrated.²⁰⁰ Janssen et al. reported a series of chlorinated SubPcs (M33, M40–M42) bearing different axial substituents, including chlorine and substituted phenoxy groups (Fig. 14 and 16).²⁰¹ Modification of axial substituents had little effect on the absorption spectra and LUMO/HOMO energy levels of the compounds. The optical bandgaps of these compounds were almost identical, and the LUMO/HOMO energy levels only spanned from −3.84 eV/−6.00 eV for M40 to −3.93 eV/−6.08 eV for M42. Although the axial substituents did not obviously affect the opto-electronic properties of the compounds, they significantly affected the aggregation and charge transporting properties of the molecules. The axial substituents could act as a steric hindrance to prevent stacking of molecular backbones. As the chlorine atom in M33 was smaller than the phenoxy groups in M40–M42, M33 showed more tight molecular stacking than others. This could be verified by the electron mobilities in blend films. Compared with M40, M41 or M42 with the phenoxy substituent, M33 with a small chlorine substituent exhibited higher electron mobility in the PTB7-Th:M33 blend film. Benefiting from the more balanced charge transport, the solution-processed OSC device based on the PTB7-Th:M33 active layer exhibited a higher PCE of 4.0%.

Fig. 16 Chemical structures of SubPc derivatives as electron acceptors for solution-processed OSCs.

Wang et al. introduced three perylene diimide (PDI) units to the periphery of SubPc to develop star-shaped molecules, M43 and M44 (Fig. 16).²⁰² The PDIs as wing groups could not only broaden the absorption spectrum but also lower the LUMO/HOMO energy levels of SubPcs. Compared with the acetylene bridge linked M43, the single bond linked M44 showed a more twisted molecular geometry, which reduced the intermolecular aggregation interaction, leading to appropriate phase separation and more balanced charge transport in the active layers of solution-processed OSCs. As a result, the device based on M44 exhibited remarkably higher V_OC, J_SC and FF than those of the M43-based device. The authors also designed two star-shaped SubPc-based electron acceptors, M45 and M46, in which 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) and fluorinated IC were introduced with thiophene bridges, respectively (Fig. 16).²⁰³ Due to the strong intramolecular charge transfer (ICT) between the SubPc core and the peripheral IC groups as well as the extended conjugation, M45 and M46 exhibited wide absorption spectra with the full width at half maximum (FWHM) of over 260 nm together with narrow bandgaps of 1.68 eV and 1.66 eV, respectively. Compared with M46 with fluorinated IC, M45 with IC showed a higher LUMO energy level of −3.90 eV, which could lead to a higher V_OC value in OSCs. The solution-processed OSCs with M45 or M46 as the acceptor and PBDB-T as the donor showed a PCE of 4.69% with a V_OC of 0.87 V or 3.80% with a V_OC of 0.77 V, respectively. These devices exhibited a wide photoresponse in the range of 300–750 nm.

An alternative approach to design solution-processable SubPc-based electron acceptors is to fuse electron-withdrawing peripheral aromatic rings, e.g. imide²⁰⁴ and PDI,²⁰⁵ with SubPc cores. Fig. 16 shows two examples of these compounds, M47 and M48. In M48, a SubPc core is fused with three PDI units. The molecule had a cone-shaped and fully conjugated molecular geometry. The dihedral angle between PDI wings and the cone base was ∼25°. The introduction of the PDI wings significantly broadened the absorption spectrum with a red-shifted absorption peak at 680 nm and a high extinction coefficient of 1.25 × 10⁵ M⁻¹ cm⁻¹. The π-conjugated and fully fused connection between the SubPc core and the electron-withdrawing PDI units obviously reduced the LUMO energy level (−3.81 eV) of M48, which was close to that of the PDI unit itself. The low-lying energy level of M48 made it to match with common electron donors in OSCs. Solution-processed OSCs with M48 as the electron acceptor and PM6 as the electron donor exhibited a PCE of 7.53% with a J_SC of 13.97 mA cm⁻², a V_OC of 0.97 V, and an FF of 0.55. This was the best photovoltaic performance for solution-processed OSCs based on SubPc compounds.

Fig. 17 Chemical structures of the electron donor/acceptor materials discussed in this manuscript.

In summary, SubPc derivatives have been intensively used in vacuum-deposited and solution-processed OSCs. PCEs of 7–8% have been demonstrated. A bottleneck limiting the photovoltaic performance of SubPc derivatives is their low charge carrier mobility, which may stem from their cone-shaped configurations. A rational molecular design of SubPc derivatives needs to be adapted to improve their charge transporting properties and their microstructure in thin film towards enhanced OSC device performance. SubPc derivatives have tunable LUMO/HOMO energy levels and they can be used as either electron donors and electron acceptors in OSCs. Noteworthily, in cyclic voltammetry (CV), SubPc derivatives often show irreversible oxidation waves but sometimes show reversible reduction waves. This means that SubPc derivatives are more stable as radical anions than as radical cations and that SubPc derivatives may be more suitable as n-type materials than as p-type materials. Therefore, in OSC applications, we think that SubPc derivatives are more suitable as electron acceptors than as electron donors.

3.5 BNBP-based small molecular electron acceptors

Double B ← N bridged bipyridine (BNBP) is an electron-deficient building block with planar configuration and it was firstly reported by Liu et al. in 2016. BNBP is always used to design n-type polymer semiconductors for application in OSCs, OFETs and organic thermoelectrics.^76,206–208 High-efficiency small molecular electron acceptors always have an A–D–A structure, in which an electron-donating core unit is endcapped by two electron-accepting terminals. In 2018, Liu et al. used the electron-deficient BNBP as the core unit to develop small molecular electron acceptors with wide absorption spectra.²⁰⁹ As shown in Fig. 18a, the BNBP-based small molecular electron acceptor, M49, consisted of BNBP as the core unit, two thiophenes as the bridging units and two strong electron-withdrawing IC as the endcapping groups.²⁰⁹ Four bulky phenyl substituents were attached to the BNBP core unit. They acted as a steric hindrance, preventing over-aggregation of the molecules and large-size phase separation in the active layers of OSCs. Typical A–D–A type small molecular acceptors exhibited only one strong absorption band in the long-wavelength region. In comparison, the BNBP-based small molecular acceptor M49 showed two strong absorption bands in the long-wavelength region (λ = ca. 700 nm) and the short wavelength region (λ = ca. 500 nm). The wide absorption spectrum suggested strong sunlight harvesting capability, which was very desirable for photovoltaic applications.²¹⁰ This wide absorption spectrum was due to the delocalized LUMO and localized HOMO (Fig. 18b), which partially suppressed the HOMO → LUMO electronic transition. M49 exhibited LUMO/HOMO energy levels of −3.93 eV/−5.34 eV, which matched well with a commercially available polymer electron donor, PTB7-Th. The OSC device based on the PTB7-Th:M49 active layer exhibited a PCE of 7.06% and showed a wide photoresponse wavelength extending to 900 nm (Fig. 18d and e) Table 3.

Fig. 18 (a) Chemical structure of M49. (b) Kohn–Sham LUMO and HOMO based on density functional theory (DFT) calculations for the model molecule of M49. (c) UV-vis absorption spectra of M49 in solution and in thin film. (d) J–V curve and (e) EQE curve of the device based on the active layer of PTB7-Th:M49. (f) Chemical structures of M50 and M51. Panels (b–e) were reproduced from ref. 209 with permission from the Royal Society of Chemistry, copyright 2017.

Table 3 Optoelectronic properties and OSC device performance of organoboron small molecules (M49–M63)

Small molecules	HOMO/LUMO (eV/eV)	E ^optg (eV)	Device structure	V OC (V)	J SC (mA cm^−2)	FF	PCE^a (%)	Ref.
a The maximum PCE value of the corresponding device.
M49	−5.34/−3.93	1.40	ITO/PEDOT:PSS/PTB7-Th:M49/Ca/Al	0.78	14.62	0.62	7.06	209
M50	−5.39/−3.94	1.47	ITO/PEDOT:PSS/PTB7-Th:M50/Ca/Al	0.73	11.40	0.60	4.96	211
M51	−5.35/−4.02	1.33	ITO/PEDOT:PSS/PTB7-Th:M51/Ca/Al	0.64	8.80	0.44	2.49	211
M52	−5.73/−3.61	2.16	ITO/PEDOT:PSS/D-OEG:M52/Ca/Al	1.08	2.83	0.34	1.03	216
M53	−5.92/−3.97	1.91	ITO/ZnO/PTB7-Th:M53/MoO3/Al	0.80	8.97	0.44	3.12	217
M54	−5.87/−3.91	1.86	ITO/ZnO/PTB7-Th:M54/MoO3/Al	0.87	10.30	0.42	3.75	217
M55	−5.63/−3.97	1.71	ITO/ZnO/PTB7-Th:M55/MoO3/Al	0.84	10.27	0.47	4.07	217
M56	−5.82/−3.62	2.13	ITO/ZnO/TTFQX-T1:M56/MoOx/Ag	1.04	9.51	0.37	3.64	218
M57	—	—	ITO/ZnO/M57:PC71BM/MoO3/Ag	0.66	4.60	0.42	1.27	219
M58	−4.89/−3.68	1.21	ITO/ZnO/M58:PC71BM/MoOx/Ag	0.67	11.7	0.50	3.88	220
M59	−5.57/−3.65	1.82	ITO/ZnO/J61:M59/MoO3/Al	0.77	4.10	0.47	1.49	221
M60	−5.47/−3.72	1.59	ITO/ZnO/J52:M60/MoO3/Al	0.78	10.58	0.52	4.31	221
M61	−6.24/−3.88	2.27	ITO/ZnO/PTB7-Th:M61/MoO3/Ag	0.87	5.4	0.40	1.9	222
M63	—/−4.08	—	ITO/ZnO/PTB7-Th:M63/MoO3/Al	0.87	8.44	0.40	2.9	223

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In order to tune the LUMO/HOMO energy levels of M49, Liu et al. introduced four electron-withdrawing fluorine substituents to either the organoboron core unit or the endcapping groups and developed M50 or M51 (Fig. 18f).²¹¹ Compared to M49, M50 with fluorine substituents at the organoboron core unit exhibited a downshifted HOMO energy level and a blueshifted absorption spectrum. In comparison, M51 with fluorine substituents at the endcapping groups showed a downshifted LUMO energy level and a redshifted absorption spectrum. The difference came from the unique electronic structures of BNBP-based small molecular electron acceptors, i.e. delocalized LUMO over the whole π-conjugated backbone and localized HOMO on the core units. The OSC devices based on M50 or M51 as electron acceptors exhibited a moderate PCE of 2% to 5% probably due to the non-optimal phase separation morphology of the active layers. The two absorption bands of M49 can be tuned by rational molecular design towards panchromatic property and superior sunlight harvesting capability. Liu et al. further developed a series of BNBP derivatives by changing the chemical structures of the core unit, the π-bridging units or the end-capping groups of M49.²¹⁰ The modification of their chemical structures led to finely tuned energy levels and absorption spectra. A panchromatic absorption spectrum with a FWHM of 280 nm was realized.

A bottleneck limiting the OSC device performance of BNBP-based small molecular electron acceptors is their localized HOMO on the core unit and the large steric hindrance on the core unit. This is detrimental for the electronic interactions between the electron donor and the electron acceptor. Consequently, this would inhibit the photoinduced hole transfer from the BNBP-based small molecular electron acceptors to electron donors. Although the localized HOMO of BNBP-based small molecular electron acceptors leads to wide absorption spectra and excellent sunlight harvesting capability, it may also limit OSC device efficiency.

3.6 Triarylborane-based small molecular electron acceptors

As tri-coordinated boron compounds, triarylboranes possess moderate electron-deficient property because of the p–π* conjugation between the vacant 2p orbital of the boron atom and the π* orbital of the aryl units.^67,212–214 Many triarylborane compounds are not adequately stable under ambient conditions because the vacant 2p orbital of the boron atom may be attacked by moisture. The stability issue prevents triarylborane compounds from being used in OSC applications. In 2014, Marder and Jäkle et al. reported two air-stable triarylborane compounds,²¹⁵ di(thiophen-2-yl)(2,4,6-tri-tert-butylphenyl)borane (BDT) and di(thiophen-2-yl)(2,4,6-tris(trifluoromethyl)phenyl)borane (FBDT), in which the vacant 2p orbital of the boron atom was protected by bulky 2,4,6-tri-tert-butylphenyl (Mes*) and 2,4,6-tris(trifluoromethyl)-phenyl (^FMes) groups for excellent air-stability. Moreover, the bulky Mes* or ^FMes group leads to non-planar configurations of the molecules and inhibits over-crystallization of the molecules in blend films. Therefore, triarylboranes can be used as the core units to design small molecular electron acceptors. Their OSC devices exhibit moderate PCEs of 3–4% with relatively low FF. This may be due to the low electron mobilities and non-optimized phase separation morphology of the triarylborane-based small molecular electron acceptors. However, a great advantage of triarylborane-based small molecular electron acceptors is their excellent solubility, which enables eco-friendly processing of OSCs.

The active layers of OSCs are always processed with halogenated or aromatic solvents, which are harmful for the environment and human health. In the mass production of OSCs, the active layers must be processed with eco-friendly solvents, such as alcohol. However, typical electron donor and electron acceptor materials are insoluble in alcohol solvents. Jäkle and Liu et al.²¹⁶ reported an alcohol-soluble small molecular electron acceptor based on triarylborane in the absence of polar side chains. Fig. 19a shows the chemical structure of the compound, M52. Mes*-substituted dithienylborane was used as the core unit, thiophene was used as the bridging unit and 2-(1,1-dicyanomethylene)rhodanine was used as the endcapping group. M52 exhibited low-lying HOMO/LUMO energy levels of −3.61 eV/−5.73 eV and a good electron mobility of 1.37 × 10⁻⁵ cm² V⁻¹ s⁻¹. The backbone of M52 possessed a large dipole moment (Fig. 19a) and enhanced flexibility (Fig. 19b) because of the embedding of the boron atom, leading to the solubility of M52 in alcohol. OSC devices with M52 as the electron acceptor were fabricated using 1-hexanol as the processing solvent and a PCE of 1.03% was demonstrated. Although this PCE was moderate, this work provided the first alcohol-processed non-fullerene electron acceptor, which was in strong demand for environment-friendly processing of OSCs.

Fig. 19 (a) Chemical structure of small molecular acceptor M52, and the optimized configuration and natural dipole moment based on DFT calculations for the model molecule of M52. (b) Potential energy surface scan of the model molecule M52 for dihedral angles (θ) between the two thienyls of the triarylborane unit ranging from −180° to 30°; the red dashed line denotes k_BT ≈ 0.6 kcal mol⁻¹ at 298 K. Chemical structures of small molecular acceptors (c) M53, M54, and M55 and (d) M56. Panels (a) and (b) were reproduced from ref. 216 with permission from the Royal Society of Chemistry, copyright 2019.

Liu's group also developed a series of small molecules with triaryborane as the core unit, thiophene as the bridging unit and IC as the endcapping groups (M53, M54 and M55, see Fig. 19c).²¹⁷ Butyl or butoxy groups were introduced to the thiophene bridging units to tune the optoelectronic properties of the compounds. Compared with M53 and M54, M55 containing butoxy groups showed almost an identical LUMO energy level but an obviously upshifted HOMO energy level, which was ascribed to the electron-donating effect of the alkoxy lateral chains in thiophene units. Consequently, the bandgap of M55 was greatly reduced and the absorption spectrum was redshifted. The introduction of butoxy groups in M55 could form intramolecular S⋯O interactions and improve backbone planarity, which favored molecular stacking in the film state. The ordered stacking in thin film was beneficial for charge transport in OSCs. OSC devices based on these triaryborane-based small molecular electron acceptors showed encouraging PCEs of 3% to 4%. Welch and Jäkle et al. designed M56 by combining the ^FMes-substituted dithienylborane core with highly electron-deficient PDI units (Fig. 19d).²¹⁸ Compared to the control compound with unfunctionalized bithiophene as the core unit, M56 with boron functionalized bithiophene had a more twisted molecular geometry, which was expected to prevent the aggregation of the molecules for small-size phase separation morphology. Owing to the p–π* conjugation, M56 exhibited HOMO/LUMO energy levels of −5.82 eV/−3.62 eV, which were lower by 0.20 eV/0.03 eV than those of the control compound, respectively. The OSC device using M56 as the electron acceptor gave a PCE of 3.64%.

3.7 Other organoboron compounds as electron donors or electron acceptors

Besides organoboron small molecules based on BODIPY, SubPc, BNBP and triarylborane, there are some other organoboron small molecules to be used as electron donors or electron acceptors in OSCs. Shimizu et al. synthesized a series of pyrrolopyrrole aza-BODIPY (PPAB) derivatives via titanium tetrachloride-mediated Schiff-base formation reaction of DPP.²¹⁹ These compounds had planar molecular frameworks and exhibited strong absorption in the Vis/NIR region. Some of them had been tested as electron donors in OSC devices. Fig. 20 shows the chemical structures of two examples of monomeric PPAB (M57) and dimeric PPAB (M58). M58 was an A–D–A type small molecular electron donor with two electron-accepting PPAB terminals connected by an electron-donating cyclopentadithiophene (CPDT) unit. The CPDT unit bearing alkyl side chains not only enhanced the solubility but also improved the interchromophore interactions. Compared to the control molecule with DPP as the terminals, M58 exhibited greatly downshifted LUMO energy levels and much smaller bandgap. As a result, M58 exhibited a maximum absorption wavelength of 845 nm in thin film and an optical bandgap of only 1.21 eV.²²⁰ The OSC device with M58 as the electron donor and PC₇₁BM as the electron acceptor showed a PCE of 3.88% with a J_SC of 11.7 mA cm⁻², a V_OC of 0.67 V, and an FF of 0.50.

Fig. 20 Chemical structures of small molecular donors (M57, M58) and small molecular acceptors (M59–M63).

Yam et al. reported a series of A–D–A type small molecular acceptors, M59 and M60, using the strong electron-deficient difluoroboron-substituted β-diketonate (BF₂bdk) moieties as the A units (Fig. 20).²²¹ The single crystal structure of M59 suggested a highly coplanar backbone with small torsion angles (<6°) between the dioxaborine ring and the adjacent thiophene rings. This planar configuration was favorable for molecular packing and charge transport. M59 and M60 exhibited low-lying LUMO energy levels of −3.65 eV and −3.72 eV, respectively, and absorption peak wavelengths of 553 nm and 695 nm, respectively. OSC devices with these BF₂bdk-based electron acceptors exhibited a PCE of 4.31%. These results indicate that BF₂bdk is an alternative electron-deficient unit to design small molecular electron acceptors.

Piers and Welch et al. reported a family of air and water stable B ← N-doped dihydroindeno[1,2-b]fluorenes through transmetallation via diarylzinc reagents.²²² By modifying the substituents on either the boron atom or the flanking aryl rings, the absorption spectra of these molecules could be tuned from the visible region to NIR region. Among these molecules, M61 with 2,4,6-trifluorophenyl substituents on the boron atoms and tert-butyl groups on the flanking aryl rings exhibited low-lying HOMO/LUMO energy levels of −6.24 eV/−3.88 eV. The OSC device with M61 as the electron acceptor and PTB7-Th as the electron donor exhibited a PCE of 1.9%.

Würthner et al. synthesized ambient-stable core- and periphery-tuned boron-doped polycyclic aromatic hydrocarbons (PAHs) by a facile one-pot hydroboration electrophilic borylation cascade/dehygrogenation approach.²²³Fig. 20 shows the chemical structures of two examples (M62 and M63). The mesityl substitution at the boron center ensured air-stability of the compounds. By modifying the chemical structures of the boron-doped PAHs, their LUMO energy levels could be finely tuned in a wide range from −3.17 eV to −4.08 eV and their absorption spectra could be tuned within the whole visible region. Among them, M63 exhibited a low LUMO energy level of −4.08 eV and a high electron mobility of 3 × 10⁻³ cm² V⁻¹ s⁻¹ in OFETs. The OSC device with M63 as the electron acceptor exhibited a PCE of 2.9%.

4. Organoboron polymers for OSCs

Efficient polymer electron donors and polymer electron acceptors are always D–A type conjugated polymers, in which electron-donating units (D) and electron-accepting units (A) are alternatively copolymerized. LUMO and HOMO energy levels of D–A type conjugated polymers can be rationally tuned by modifying the chemical structures of A units and D units, respectively. Moreover, carefully designed D–A type polymers can also have narrow bandgaps, wide absorption spectra, and high hole/electron mobilities. Thereby, D–A type conjugated polymers can fulfill the multiple requirements of polymer electron donors or polymer electron acceptors, and consequently can be used to obtain high-efficiency OSCs. Most of tri-coordinated and tetra-coordinated organoboron building blocks are electron-accepting. Therefore, organoboron building blocks are always used as A building blocks to design D–A type conjugated polymers for OSC applications. The widely used organoboron building blocks include BODIPY derivatives, B ← N bridged thienylthiazole (BNTT), BNBP, BNIDT, BNT, FBDT (triarylborane) (Fig. 21), etc. Noteworthily, excellent OSC device performance has been demonstrated with organoboron polymers. Organoboron polymer electron donors with a PCE of 16% and organoboron polymer electron acceptors with a PCE of 14% have been reported by Duan et al. and Liu et al., respectively.

Fig. 21 Chemical structures of the building blocks for organoboron polymers in OSCs.

4.1 BODIPY-based polymer donors

With electron-accepting property and rigid planar configuration, BODIPY derivatives have been used as A building blocks to design D–A type polymer electron donors. Most of high-efficiency polymer electron donors show medium bandgaps of 1.7–1.9 eV with absorption spectra in the visible range. In comparison, BODIPY-based polymer donors always possess small bandgaps of around 1.5 eV and show strong light absorption in the NIR region.^224–229 The strong sunlight absorbing capability suggests that BODIPY-based polymer donors are an interesting and important class of organic photovoltaic materials.

The first BODIPY-based polymer donor was reported by Fréchet et al. in 2010.²³⁰ The chemical structures of the two polymer donors, P1 and P2, are shown in Fig. 22. Both polymers exhibited low bandgaps of ca. 1.6 eV and optimal LUMO/HOMO energy level alignment for charge separation when paired with PC₆₁BM. Using PC₆₁BM as the electron acceptor, OSC devices based on P1 and P2 exhibited PCEs of 1.3% and 2.0%, respectively Table 4. Although the OSC device efficiencies were moderate, this work triggered a lot of research on BODIPY-based polymer donors. For example, Qin et al. developed a polymer donor by copolymerizing BODIPY with an organometallic Pt-based building block.²³¹ Skabara et al. reported a polymer donor based on alternating BODIPY unit and bis(3,4-ethylenedioxythiophene) unit.²³² Employing α,β-unsubstituted BODIPY as a building block, Chochos et al. designed and synthesized a polymer donor with strong NIR absorption and an onset absorption wavelength of ca. 1100 nm.²³³ Ramamurthy et al. studied the effect of linking positions of BODIPY building blocks (via α-position or β-position) on the opto-electronic properties of BODIPY-based polymer donors.²³⁴ Gros et al. prepared a polymer donor consisting of alternating BODIPY and diethynylthiophene as the backbone and Zn(II)-porphyrin as the pendent group (P3, see Fig. 23a).²³⁵ Its absorption spectrum was the sum of absorption spectrum of pendant porphyrin groups and the polymer backbone (Fig. 23b). P3 exhibited an optical bandgap of 1.59 eV, a HOMO energy level of −5.32 eV and a LUMO energy level of −3.73 eV. The optimized OSC device based on the P3:PC₇₁BM active layer showed a PCE of 8.79% with a J_SC of 14.48 mA cm⁻², a V_OC of 0.92 V and an FF of 0.66. The photovoltaic performance of P3 was better than that of the control polymer donor (P(BdP-DEHT)) without pendant porphyrin groups (Fig. 23c). This was related to the broader absorption spectrum and better charge transport of P3 compared to the control polymer donor. The excellent photovoltaic performance of P3 in fullerene-based OSC devices demonstrated the great potential of the BODIPY unit in the molecular design of polymer donors.

Fig. 22 Chemical structures of BODIPY-based polymers P1 and P2.

Fig. 23 (a) Chemical structure of BODIPY dye P3 (P(BdP/Por-DEHT)). (b) Absorption spectra of P3 (black trace) in solution. (c) J–V curves of the as-cast and solvent vapor annealing (SVA)-treated devices based on the active layers of P3:PC₇₁BM and P(BdP-DEHT):PC₇₁BM. Panels (b) and (c) were reproduced from ref. 235 with permission from the American Chemical Society, copyright 2017.

Table 4 Optoelectronic properties and OSC device performance of organoboron polymers (P1–P33)

Polymers	HOMO/LUMO (eV/eV)	E ^optg (eV)	Device structure	V OC (V)	J SC (mA cm^−2)	FF	PCE^a (%)	Ref.
a The maximum PCE value of the corresponding device.
P1	−5.58/−3.73	1.61	ITO/PEDOT:PSS/P1:PC61BM/Al	0.76	4.00	0.43	1.3	230
P2	−5.45/−3.71	1.65	ITO/PEDOT:PSS/P2:PC61BM/Al	0.80	4.82	0.51	2.0	230
P3	−5.32/−3.73	1.59	ITO/PEDOT:PSS/P3:PC71BM/PFN/Al	0.92	14.48	0.66	8.79	235
P4	−5.45/−3.69	1.73	ITO/PEDOT:PSS/P4:PC71BM/PFN/Al	0.91	12.16	0.67	7.41	236
			ITO/PEDOT:PSS/P4:SMDPP/PFN/Al	1.06	13.48	0.65	9.29	236
P5	−5.36/−4.05	1.31	ITO/PEDOT:PSS/P5:N2200/PDINO/Al	0.69	1.52	0.31	0.32	237
P5-M	−5.31/−3.90	1.41	ITO/PEDOT:PSS/P5-M:N2200/PDINO/Al	0.83	13.90	0.50	5.80	237
			ITO/ZnO/P5-M:BTP-2Cl/MoO3/Ag	0.77	21.44	0.60	9.86	229
P6-CC	−5.37/−3.11	1.69	ITO/PEDOT:PSS/P6-CC:P6/LiF/Al	1.08	0.35	0.23	0.085	240
P6	−5.90/−3.76	1.88	ITO/PEDOT:PSS/P3HT:P6/LiF/Al	0.78	0.47	0.38	0.14	240
P7	−5.84/−3.80	1.63	ITO/PEDOT:PSS/PTB7-Th:P7/Ca/Al	0.92	11.37	0.48	5.04	243
P8	−5.81/−3.71	1.72	ITO/PEDOT:PSS/PTB7-Th:P8/Ca/Al	0.89	14.24	0.52	6.60	244
P9	−5.77/−3.50	1.92	ITO/PEDOT:PSS/PTB7:P9/LiF/Al	1.09	7.09	0.44	3.38	206
			ITO/PEDOT:PSS/PCDTBT:P9/Ca/Al	1.30	5.41	0.46	3.20	258
P10	−5.74/−3.59	1.93	ITO/PEDOT:PSS/J61:P10/Ca/Al	1.01	6.42	0.47	3.04	253
P11	−5.45/−3.87	1.56	ITO/PEDOT:PSS/PTB7:P11/LiF/Al	0.88	7.54	0.41	2.69	254
P12	−5.84/−3.66	1.87	ITO/PEDOT:PSS/PTB7-Th:P12/Ca/Al	1.03	10.02	0.42	4.26	255
P13-OMe	−5.37/−3.39	1.72	ITO/PEDOT:PSS/P13-OMe:PC71BM/Ca/Al	0.70	2.02	0.42	0.59	256
P13-Me	−5.81/−3.32	1.83	—	—	—	—	—	256
P13-H	−5.92/−3.43	1.91	—	—	—	—	—	256
P13-F	−6.04/−3.62	1.90	ITO/PEDOT:PSS/PTB7-Th:P13-F/Ca/Al	1.09	10.13	0.47	5.16	256
P14-OMe	−5.46/−3.48	1.66	ITO/PEDOT:PSS/P14-OMe:PC71BM/Ca/Al	0.76	7.59	0.51	2.92	256
P14-Me	−5.84/−3.42	1.81	—	—	—	—	—	256
P14-H	−5.77/−3.51	1.85	—	—	—	—	—	256
P14-F	−5.85/−3.71	1.84	ITO/PEDOT:PSS/PTB7-Th:P14-F/Ca/Al	1.12	7.33	0.45	3.70	256
P15-1T	−5.86/−3.75	1.94	ITO/PEDOT:PSS/J61:P15-1T/Ca/Al	0.89	4.57	0.30	1.23	257
P15-2T	−5.93/−3.54	1.90	ITO/PEDOT:PSS/J61:P15-2T/Ca/Al	1.06	7.26	0.46	3.57	257
P15-3T	−5.75/−3.48	1.89	ITO/PEDOT:PSS/J61:P15-3T/Ca/Al	1.22	11.15	0.48	6.52	257
P15-4T	−5.55/−3.33	1.83	ITO/PEDOT:PSS/P15-4T:PC71BM/Ca/Al	0.77	2.07	0.48	0.77	257
P15-5T	−5.45/−3.11	1.74	ITO/PEDOT:PSS/P15-5T:PC71BM/Ca/Al	0.81	11.62	0.62	5.79	257
P16	−5.87/−3.62	1.86	ITO/PEDOT:PSS/PTB7-Th:P16/LiF/Al	1.07	12.69	0.47	6.26	259
P17	−5.74/−3.72	1.97	ITO/PEDOT:PSS/PTB7-Th:P17/Ca/Al	1.11	7.58	0.45	3.77	260
P18	−5.52/−3.45	1.78	ITO/PEDOT:PSS/CD1:P18/LiF/Al	1.17	13.39	0.64	10.07	261
P18-C28	−5.58/−3.53	1.77	ITO/ZnO/BD3T:P18-C28/MoO3/Al	1.12	7.16	0.63	5.06	270
P19	−5.81/−3.42	1.95	ITO/PEDOT:PSS/J61:P19/Ca/Al	1.24	10.12	0.57	7.09	262
			ITO/PEDOT:PSS/CD1:P19/Ca/Al	1.29	10.10	0.61	7.93	264
P20	−5.96/−3.46	1.93	ITO/PEDOT:PSS/DR3TBDTC:P20/Ca/Al	1.11	11.18	0.65	8.01	268
P21	−5.78/−3.65	1.82	ITO/ZnO/BD3T:P21/MoO3/Al	1.04	13.82	0.66	9.51	270
P22	−5.43/−3.85	1.51	ITO/PEDOT:PSS/PBDB-T:P22/PNDIT-F3N/Al	0.93	8.02	0.49	4.03	273
P23	−5.71/−3.77	1.61	ITO/PEDOT:PSS/PBDB-T:P23/PNDIT-F3N/Al	0.93	13.01	0.70	8.78	273
P24	−5.46/−3.79	1.57	ITO/PEDOT:PSS/PBDB-T:P24/PDINO/Al	0.98	4.15	0.38	1.60	275
P25	−5.71/−3.77	1.60	ITO/PEDOT:PSS/PBDB-T:P25/PDINO/Al	0.96	8.75	0.44	3.71	275
P26	−5.74/−3.76	1.62	ITO/PEDOT:PSS/PBDB-T:P26/PDINO/Al	0.96	9.20	0.47	4.23	275
P27	−5.46/−3.63	1.38	ITO/PEDOT:PSS/PTB1:P27/PDINO/Al	0.80	16.90	0.54	7.25	276
P28	−5.52/−3.45	2.07	ITO/PEDOT:PSS/P28:PC71BM/Ca/Al	1.00	8.31	0.45	3.82	278
P29	−5.52/−3.46	1.86	ITO/PEDOT:PSS/P29:Y6-BO/PFNBr/Ag	0.88	25.4	0.72	16.1	279
P30	−5.88/−3.72	1.80	ITO/PEDOT:PSS/PTB7-Th:P30/Ca/Al	1.00	6.51	0.43	2.83	280
P31	−5.76/−3.58	1.83	ITO/PEDOT:PSS/PTB7-Th:P31/Ca/Al	1.08	4.47	0.32	1.56	280
P32	—/−3.75	2.16	ITO/PFN-P1/PTB7:P32/MoO3/Ag	0.40	0.94	0.31	0.12	281
P33	—/−4.35	2.07	ITO/PFN-P1/PTB7:P33/MoO3/Ag	0.40	1.50	0.36	0.23	281

---

BODIPY-based polymer donors also match well with non-fullerene small molecular acceptors to give excellent OSC device performance. Gros et al. reported a BODIPY-based polymer donor P4 (Fig. 24a).²³⁶ While the device based on P4:PC₇₁BM showed a PCE of 7.41%, the OSC device with P4 and the small molecular acceptor SMDPP exhibited a PCE of 9.29% together with the photon energy loss (E_loss) of only 0.48 eV. This E_loss value was among the lowest values for OSCs. The improved PCE of the P4:SMDPP device was attributed to enhanced J_SC and V_OC, which was caused by improved light absorption efficiency of the P4:SMDPP active layer and the higher LUMO energy level of SMDPP, respectively. Alkylthienyl-benzodithiophene (BDT) is a well-known electron-donating building block to design polymer donors. Li et al. copolymerized this BDT unit and BODIPY units to get two D–A type polymer donors, P5 and P5-M (Fig. 24a).²³⁷ The methyl substituents on the α-positions of BODIPY in P5-M significantly affected the optoelectronic properties of the polymer donors. Compared with the control polymer P5 without methyl substituents, P5-M with methyl substituents showed higher LUMO/HOMO energy levels, an increased absorption coefficient, higher crystallinity, and dramatically enhanced hole mobility (Fig. 24b). The authors selected a widely used polymer acceptor N2200 to blend with P5 or P5-M to construct all-polymer solar cells (all-PSCs). While the all-PSC device of P5 showed a PCE of only 0.32%, the device of P5-M showed a significantly enhanced PCE of 5.8% with a broad photoresponse from 300 to 900 nm (Fig. 24c). All-PSCs based on polymer acceptor N2200 suffered from low photoresponse in the NIR region. The utilization of polymer donor P5-M with strong NIR absorption in all-PSCs was expected to solve this problem and give high PCE. Xu and Li et al. further blended polymer donor P5-M with small molecular acceptor Y6-2Cl to fabricate OSC devices.²²⁹ The devices showed a PCE of 9.86%, which was the highest value reported so far for BODIPY-based polymer donors (Fig. 24d).

Fig. 24 (a) Chemical structures of BODIPY-based polymer donors P4, P5 and P5-M. (b) 2D-GIWAXS patterns of the donor polymers P5 (PBBDT) and P5-M (PMBBDT). (c) J–V curves of the devices based on P5:N2200 and P5-M:N2200 in the dark (dashed line) and under AM1.5G illumination (solid line). (d) J–V curves of the devices based on P5-M:ITIC-2Cl and P5-M:Y6-2Cl in the dark (dashed line) and under AM1.5G illumination (solid line). Panels (b) and (c) were reproduced from ref. 237 with permission from the American Chemical Society, copyright 2019. Panel (d) was reproduced from ref. 229 with permission from the Royal Society of Chemistry, copyright 2020.

Liu et al. designed an annulated BODIPY as a new building block to construct polymer donors with NIR absorption.²³⁸ In the chemical structure of the building block, a BODIPY core was fused with two benzene rings for a reduced bandgap and was attached with two alkylthienyl groups as conjugated side chains. The resulting polymers exhibited LUMO/HOMO energy levels of ca. −5.0 eV/−3.5 eV, a hole mobility of ca. 3 × 10⁻³ cm² V⁻¹ s⁻¹, and an optical bandgap of ca. 1.3 eV with the maximum absorption wavelength in film exceeding 800 nm. OSC devices of these polymer donors showed a PCE of 2.6%. The unsatisfactory OSC device performance was probably due to the large-size phase separation of the active layers.

At present, the highest PCE for BODIPY-based polymer donors is 9.8%, suggesting that BODIPY derivatives are useful building blocks to design polymer donors. Indeed, the opto-electronic properties of the reported BODIPY-based polymer donors, including LUMO/HOMO energy levels, absorption spectra and hole mobilities, can fulfill the multiple requirements of high-efficiency polymer donors. For further PCE enhancement, great attention should be paid to phase separation morphology of BODIPY-based polymer donors in OSCs. Noteworthily, BODIPY-based polymer donors always exhibit absorption mainly in the far-red or near-infrared region. This property is beneficial for semi-transparent OSCs. However, the application of BODIPY-based polymer donors in semi-transparent OSCs has not been reported until now.

4.2 BNTT-based polymer acceptors

Conjugated polymers containing B ← N units are an important class of polymer electron acceptors for all polymer solar cells (all-PSCs). All-PSCs, which use blends of polymer electron donors and polymer electron acceptors as active layers, have the great advantages of excellent morphology stability and superior mechanical properties. In the past decade, all-PSCs have received great attention and their PCEs have been greatly enhanced together with excellent stability. Since there are far less polymer acceptors than polymer donors, the lack of polymer acceptors limits the development of all-PSCs. The key requirement of polymer acceptors is low-lying LUMO and HOMO energy levels. As B ← N is able to greatly downshift LUMO/HOMO energy levels of π-conjugated systems, polymer acceptors can be designed with B ← N.

B ← N bridged thienylthiazole (BNTT, containing one B ← N unit) derivatives were initially reported by Yamaguchi et al. in 2006²³⁹ and were adopted to design polymer acceptors by Liu et al. in 2015.^240,241 As the BNTT unit is asymmetrical, BNTT-based polymer acceptors are always region-random. Moreover, the large phenyl substituents on the BNTT unit prevent close stacking of polymer backbones. Therefore, BNTT-based polymer acceptors are always amorphous or poorly crystalline. This property is different from typical polymer acceptors, which are semi-crystalline. PCEs of 6–7% have been demonstrated with all-PSCs of BNTT-based polymer acceptors.^242,243

B ← N is iso-electronic to the carbon–carbon single bond (C–C). As shown in Fig. 25a, P6-CC is a typical polymer donor with high-lying LUMO and HOMO energy levels. After replacing a C–C unit of P6-CC with a B ← N unit in P6, the LUMO and HOMO energy levels of the resulting polymer were lowered by 0.65 eV and 0.53 eV, respectively (Fig. 25b).²⁴⁰ The LUMO and HOMO energy levels of P6 were −3.76 eV and −5.90 eV, respectively, which were close to those of typical electron acceptors, e.g. PC₆₁BM. This indicated that the replacement of the C–C unit with the B ← N unit converted the polymer from electron donor to electron acceptor. An all-PSC device was fabricated using P6 as the polymer acceptor and P3HT as the polymer donor. This device exhibited a PCE of 0.14%. Although the PCE was low, this preliminary all-PSC device performance experimentally proved that the B ← N unit could be used to design polymer acceptors.

Fig. 25 (a) Chemical structures of the polymer P6-CC containing a C–C unit and the polymer P6 containing a B ← N unit. (b) HOMO/LUMO energy levels of P6-CC and P6.

The poor all-PSC device performance of P6 was preliminarily due to its low electron mobility (μ_e = 3.4 × 10⁻⁷ cm² V⁻¹ s⁻¹). This was attributed to the large steric hindrance of the phenyl groups on the boron atoms, which inhibited the tight packing of polymer backbones in thin film and hindered the transport of charge carriers between the polymer backbones. To improve the electron mobility of BNTT-based polymer acceptors, Liu et al. extended the length of the repeating units to alleviate the steric hindrance effect of bulky pendant substituents.²⁴³ As shown in Fig. 26a, the polymer acceptor P7 was developed by replacing the thieno[3,4-c]pyrrole-4,6-dione-1,3-diyl (TPD) unit in P6 with the long planar isoindigo (IID) unit. According to the optimized configuration obtained by DFT calculations, the length of the repeating unit of P7 was 1.95 nm, which was much longer than that (1.24 nm) of P6. As a result, the π–π stacking distance was reduced from 0.46 nm for P6 to 0.38 nm for P7, and the electron mobility was significantly enhanced by nearly two orders of magnitude from 3.4 × 10⁻⁷ cm² V⁻¹ s⁻¹ for P6 to 2.8 × 10⁻⁵ cm² V⁻¹ s⁻¹ for P7. The PCE of the resulting all-PSC device was improved from 0.12% for P6 to 5.04% for P7 (Fig. 26b).

Fig. 26 (a) Chemical structures and optimized molecular configurations of the repeating units of P6 and P7. (b) J–V curves of the devices based on the active layers of PTB7-Th:P6 and PTB7-Th:P7. (c) Chemical structure of P8. (d) 2D-grazing incidence wide-angle X-ray scattering (2D-GIWAXS) pattern of the P8 film. (e) AFM height images of P8-based blend films with different polymer donors (PTB7-Th, J71 and PffBT4T-2OD). Panels (a) and (b) were reproduced from ref. 243 with permission from the American Chemical Society, copyright 2020. Panels (d) and (e) were reproduced from ref. 242 with permission from the American Chemical Society, copyright 2019.

Liu et al. further developed the polymer acceptor, P8, which was an alternating copolymer of BNTT unit and pyridine-flanked diketopyrrolopyrrole (PyDPP) unit (Fig. 26c).²⁴²P8 showed low-lying HOMO/LUMO energy levels of −5.81 eV/−3.71 eV and a high electron mobility of 2.2 × 10⁻⁴ cm² V⁻¹ s⁻¹. Due to its regiorandom structure and large steric hindrance, P8 was amorphous in solid state (Fig. 26d). P8 could match with various polymer donors to give all-PSC devices with high PCEs of 5.2–6.6%.²⁴⁴ When the amorphous P8 was blended with these various polymer donors, the crystallization of polymer donors governed the film-forming process and dominated the phase separation morphology (Fig. 26e). All the polymer donor/P8 blends showed good phase separation morphology and consequently exhibited high PCEs. Recently, Liu et al. developed a new polymer acceptor by copolymerizing a Y-series small molecular acceptor with a BNTT unit.²⁴⁵ Compared with the control polymer acceptor without B ← N, the incorporation of B ← N in the polymer downshifted the LUMO/HOMO energy levels of the polymer by ca. 0.1 eV, and thus promoted efficient photo-induced hole transfer from the polymer acceptor to polymer donor. The OSC device based on this polymer containing the BNTT unit exhibited a PCE of 14.36% with a V_OC of 0.89 V, a J_SC of 22.62 mA cm⁻² and an FF of 0.72. The high device performance demonstrated that BNTT as a building block had potential to design high-performance polymer acceptors.

4.3 BNBP-based polymer acceptors

BNBP,^{206,246–252} an electron-withdrawing building block based on B ← N, has been intensively studied in the molecular design of polymer acceptors for OSC applications because of four reasons.²⁰⁷ Firstly, owing to the two B ← N units, BNBP exhibits low LUMO/HOMO energy levels. BNBP-based conjugated polymers consequently show low LUMO/HOMO energy levels. Secondly, the two B ← N units lead to the redshifted absorption spectrum of BNBP. This improves the sunlight harvesting capability of BNBP-based conjugated polymers. Thirdly, the two B ← N units fix the planar configuration of BNBP. Therefore, BNBP-based conjugated polymers may show close π-stacking in thin film and possess high electron mobility. Finally, LUMO energy levels of BNBP-based conjugated polymers can be readily tuned, leading to high V_OC of the resulting OSC devices. BNBP-based polymer acceptors have become an important class of high-efficiency electron acceptors for OSCs. Their all-polymer photovoltaic devices can afford a PCE of 10% under simulated solar light irradiation and a PCE of 27% under indoor artificial light illumination.

The first BNBP-based polymer acceptor, P9 (see Fig. 27a), was an alternating copolymer of BNBP and thiophene.²⁰⁶P9 showed the absorption peak at 622 nm in film, low-lying HOMO/LUMO energy levels of −5.77 eV/−3.50 eV and an electron mobility of 6.9 × 10⁻⁵ cm² V⁻¹ s⁻¹. The all-PSC device based on P9 as the polymer acceptor and PTB7 as the polymer donor exhibited a PCE of 3.38% (Fig. 27b). A homopolymer of BNBP, P10, was synthesized by Yamamoto polycondensation of the di-bromo monomer of BNBP (Fig. 27a).²⁵³P10 exhibited an ultra-narrow absorption spectrum in thin film with the FWHM of only 13 nm. The HOMO/LUMO energy levels of P10 were −5.74 eV and −3.59 eV, respectively. P10 could match various polymer donors in all-PSC devices and gave PCEs of 2.44–3.04%. These early works demonstrated the feasibility of designing polymer acceptors using BNBP.

Fig. 27 (a) Chemical structures of BNBP-based polymer acceptors, P9 and P10. (b) J–V curve and EQE spectrum of the device based on the PTB7:P9 blend. Panel (b) was reproduced from ref. 206 with permission from John Wiley and Sons, copyright 2016.

To improve the all-PSC device performance of BNBP-based polymer acceptors, Liu et al. modified their chemical structures to systematically tune their absorption spectra, energy levels and electron mobilities. For example, a polymer acceptor with narrow bandgap and wide absorption spectrum (P11, see Fig. 28a) was developed by copolymerizing BNBP with Th-DPP.²⁵⁴ While P9 showed an optical bandgap of 1.92 eV and an onset absorption wavelength of 646 nm, P11 exhibited an optical bandgap of 1.56 eV and an onset absorption wavelength of 796 nm, indicating much improved sunlight harvesting capability (Fig. 28b). As a result, all-PSC devices based on P11 exhibited broad EQE spectra covering the range of 350–800 nm (Fig. 28c).

Fig. 28 (a) Chemical structure of BNBP-based polymer acceptor P11. (b) The absorption spectra of P9 (P-BNBP-T) and P11 (P-BNBP-DPP) in solution and in film. (c) EQE spectra of the devices based on PTB7:P9 and PTB7:P11 blends. Panels (b) and (c) were reproduced from ref. 254 with permission from the Royal Society of Chemistry, copyright 2016.

LUMO energy levels of BNBP-based based polymer acceptors greatly affect their all-PSC device performance. For example, P12 was designed by replacing the thiophene copolymerizing unit of P9 by selenophene (Fig. 29a).²⁵⁵ This replacement resulted in downshifting of LUMO energy levels from −3.50 eV for P9 to −3.66 eV for P12. Because of the low-lying LUMO energy level, P12 matched well with the benchmark polymer donor, PTB7-Th. The all-PSC device based on PTB7-Th:P12 showed a PCE of 4.26%, which was much higher than that of the PTB7-Th:P9 device (PCE = 2.27%) (Fig. 29b). The downshifted LUMO energy level of P12 also led to slightly decreased V_OC of the all-PSC device.

Fig. 29 (a) Chemical structures of P9 and P12, and Kohn–Sham LUMOs of the model compounds of P9 and P12. (b) HOMO/LUMO energy levels of P9 and P12, and J–V curves of the devices based on the PTB7-Th:P9 and PTB7-Th:P12 blends. (c) Chemical structures of the P13 series and P14 series and LUMO energy level alignment for these polymers. (d) Chemical structures of the P15 series and HOMO/LUMO energy level alignment for these polymers. Panel (b) was reproduced from ref. 76 with permission from the American Chemical Society, copyright 2020.

LUMO energy levels of polymer acceptors should be carefully optimized to ensure photo-induced charge separation and to maximize V_OC of all-PSC devices. An important feature of BNBP-based conjugated polymers is their tunable LUMO energy levels. Their LUMOs are delocalized over the whole polymer backbones.²⁵⁵ Therefore, their LUMO energy levels can be tuned by modifying the chemical structures of either the BNBP unit or the copolymerizing units. Indeed, LUMO energy levels both as high as −3.2 eV and as low as −4.4 eV have been realized for BNBP-based conjugated polymers. Optimal LUMO energy levels of about −3.6 eV for polymer acceptors can also be readily achieved by molecular design of BNBP-based conjugated polymers.

Liu et al. had finely tuned the LUMO energy levels of BNBP-based polymers. As shown in Fig. 29c, they synthesized a series of polymers with the same backbone of alternating BNBP unit and bithiophene unit.²⁵⁶ By changing the substituents on the BNBP unit or the bithiophene unit, the LUMO energy levels of these polymers could be finely tuned in the range from −3.3 eV to −3.7 eV. Among these polymers, P13-F and P14-F with fluoro substituents on the bithiophene unit showed low LUMO energy levels of below −3.6 eV and could be used as polymer acceptors in all-PSCs. P13-OMe and P14-OMe with methyloxy substituents on the bithiophene unit showed relatively high LUMO/HOMO energy levels and could be used as polymer donors in OSCs. Liu et al. also studied the five polymers (P15 series, see Fig. 29d) consisting of alternating BNBP unit and oligothiophene units of various length.²⁵⁷ With the increasing length of the electron-donating oligothiophene units, the LUMO energy levels of the polymers gradually upshifted from −3.75 eV to −3.11 eV. Among these polymers, P15-3T, which was the alternating polymer of BNBP unit and tri-thiophene unit, showed the most optimal LUMO energy level for polymer acceptor application. The all-PSC device with P15-3T as the electron acceptor exhibited a high PCE of 6.52%. P15-5T, the alternating polymer of BNBP unit and penta-thiophene unit, showed the highest LUMO energy level of −3.11 eV and could work as an electron donor in OSCs. OSC devices with P15-5T as the polymer donor and PC₇₁BM as the electron acceptor showed a PCE of 5.79%.

The V_OC of OSCs is strongly related to the LUMO energy levels of electron acceptor materials. The tunable LUMO energy levels of BNBP-based conjugated polymers indicate an opportunity to develop OSCs with remarkably high V_OC. For example, an all-PSC device was fabricated by using P9 with a high-lying LUMO energy level as the polymer acceptor and PCDTBT with a low-lying HOMO energy level as the polymer donor.²⁵⁸ An extraordinarily high V_OC of 1.3 V was obtained. This V_OC was higher by ca. 0.3–0.4 V than the V_OC of the corresponding devices based on traditional electron acceptors, such as PC₆₁BM or N2200. The photon energy loss (E_loss) of OSCs, which is the difference between the lowest optical bandgap of donor/acceptor materials and eV_OC of the device, is a key factor limiting the PCE of OSCs. OSC devices always have a large E_loss of 0.7–1.0 eV. Due to the tunable LUMO energy levels of the BNBP-based polymer acceptors and the high V_OC of the corresponding OSC devices, these devices exhibit a small E_loss of about 0.5–0.6 eV. For example, based on the blend of P16 (Fig. 30a) as the polymer acceptor and PTB7-Th as the polymer donor, an all-PSC device exhibited a small E_loss of 0.51 eV with a PCE of 6.26%.²⁵⁹ The large V_OC and the small E_loss of BNBP-based polymer acceptors suggested that there was large room for PCE enhancement.

Fig. 30 Chemical structures of (a) P16 and P17 and (b) P13 and P18. (c) 2D-GIWAXS patterns of P13 and P18 films. (d) J–V curves and EQE spectra of the devices based on the CD1:P13 and CD1:P18 blends. Panels (c) and (d) were reproduced from ref. 261 with permission from the American Chemical Society, copyright 2020.

BNBP-based conjugated polymers possess excellent electron transporting property, which is proved by their electron mobility of ca. 0.2 cm² V⁻¹ in OFETs.²⁴⁶ Liu et al. modified the chemical structures of BNBP-based polymer acceptors for enhanced electron mobility towards all-PSC applications. The “conjugated side chains” strategy is widely used to enhance the hole mobility of polymer donors. This strategy was also adopted to improve the electron mobility of BNBP-based polymer acceptors.²⁶⁰ As shown in Fig. 30a, P17 had the same polymer backbone as P9 but with the conjugated alkyloxyphenyl side chains on the BNBP unit. P17 with conjugated alkyloxyphenyl side chains exhibited a smaller π–π stacking distance (d_π–π = 0.36 nm) than that of P9 with alkyl side chains (d_π–π = 0.38 nm). Moreover, the conjugated side chains in P17 could also promote intermolecular π-orbital overlap, which is beneficial for charge transport between polymer chains. As a result, the electron mobility was enhanced from 7.16 × 10⁻⁵ cm² V⁻¹ s⁻¹ for P9 to 3.24 × 10⁻⁴ cm² V⁻¹ s⁻¹ for P17. The PCE of all-PSC devices was also increased from 2.3% for P9 to 3.8% for P17.

A noteworthy BNBP-based polymer acceptor was P18, which was an alternating copolymer of BNBP unit and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole unit (Fig. 30b).²⁶¹P18 showed appropriate LUMO/HOMO energy levels of −3.45 eV/−5.52 eV and a high electron mobility of 1.68 × 10⁻³ cm² V⁻¹ s⁻¹. In the 2D-GIWAXS pattern of P18 (Fig. 30c), strong (100) and (200) lamellar peaks in the out-of-plane direction and extra (010) π–π stacking peaks in both in-plane and out-of-plane directions could be observed, suggesting ordered packing and mixed face-on/edge-on orientation of the polymer chains. The all-PSC device with P18 as the electron acceptor and CD1 as the electron donor exhibited a PCE of 10.07% with a V_OC of 1.17 V, a J_SC of 13.39 mA cm⁻² and an FF of 0.64 (Fig. 30d). This is among the highest photovoltaic efficiencies reported so far for organoboron polymer acceptors. The incorporation of benzo[c][1,2,5]-thiadiazole into the polymer backbone of P18 greatly affected its opto-electronic properties and thin film morphology, and played an important role in the excellent all-PSC device performance.

Liu et al. have also optimized the phase separation morphology of BNBP-based polymer acceptors in all-PSCs. In the film-forming process of polymer/polymer blends, aggregation of polymer chains in solution can be partially preserved in the resulting film because polymer chains diffuse very slowly in solution. Therefore, the key to optimize the phase separation morphology of polymer/polymer blends in all-PSCs is to control the aggregation of polymer chains in solution. This can be realized by changing the solution preparation method for device fabrication. The traditional method to prepare the solution is to dissolve the polymer donor and polymer acceptor together to get a well-intermixed blend solution (DT method). With this method, the aggregated tendency of polymer chains in solution may be partially disturbed, affecting the polymer stacking in the thin film after spin-coating. To improve polymer ordering and enhance domain purity in thin film, Liu et al. developed a method by dissolving the polymer donor and polymer acceptor individually and then blending them immediately before spin-coating (DI method).²⁶² With J61 as the polymer donor and P19 (Fig. 31a) as the polymer acceptor, while the all-PSC device prepared with the DT method showed a PCE of 5.36%, the device prepared with the DI method exhibited an enhanced PCE of 7.09%. Another approach to control aggregation of polymer chains in solution is to optimize the solution temperature for spin-coating.²⁶³ For the all-PSC device based on the CD1:P20 blend, as the solution temperature increased from 30 °C to 90 °C, the polymer chains became more dis-aggregated in solution and were able to diffuse more quickly in the film-forming process. As a result, compared to that with the solution temperature of 30 °C, the all-PSC device prepared with the solution temperature of 90 °C exhibited smaller phase-separation size and a more optimal crystallization degree of the polymers, which led to an improved PCE from 7.7% to 10.0%.

Fig. 31 (a) Chemical structure of P19. (b) The AM 1.5G solar spectrum and emission spectra of a white LED and FL. (c) Absorption spectra of CD1 and P19. (d) EQE curves of the photovoltaic cells based on CD1:P19 and CD1:ITIC. (e) The IPV efficiency and V_OC distributions of different types of photovoltaic cells. Panels (b–e) were reproduced from ref. 264 with permission from the Royal Society of Chemistry, copyright 2019.

BNBP-based polymer acceptors with bandgaps of ca. 1.9–2.0 eV are suitable for application in indoor photovoltaics (IPVs). The rapid development of Internet of Things (IoT) needs numerous wireless sensors to collect and communicate data. These wireless sensors have power consumption on the microwatt to milliwatt scale. IPVs, which harvests light energy from the indoor environment and generates electricity, is an ideal option to power these sensor nodes for practical applications. BNBP-based polymer acceptors, e.g.P19, have the absorption band mainly in the visible range from 450 nm to 650 nm, which overlaps well with the emission spectra of indoor artificial lighting sources, such as a fluorescent lamp (FL) and light emitting diodes (LEDs). Liu et al. used a blend of CD1 and P19 as the active layer to fabricate an all-polymer photovoltaic device.²⁶⁴ The photo-response of the device was in the wavelength range of 390–630 nm, which lies wholly in the visible region (Fig. 31b–d). Under simulated solar light irradiation, the device showed a PCE of 7.9% with a V_OC of 1.29 V and an FF of 0.61. Under fluorescent lamp illumination at the brightness of 2000 lux, the device exhibited a PCE of 27.4% with a V_OC of 1.16 V and an FF of 0.67. This was the first report of all-polymer indoor photovoltaics. This PCE was fairly comparable to those of other indoor photovoltaic technologies based on organic materials or inorganic materials (Fig. 31e).

High-efficiency OSCs always use blends of polymer donors and small molecular acceptors as active layers. Small molecular donor/polymer acceptor type (M_D/P_A-type) OSCs have the merit of excellent thermal stability but have not received widespread attention. This is due to their low PCEs, which come from the poor phase separation morphology of M_D/P_A blends. BNBP-based polymer acceptors have been used in M_D/P_A-type OSCs with optimized phase separation morphology and enhanced PCE together with excellent thermal stability.^265,266

Liu et al. initially used a commercially available small molecular donor, (p-DTS(FBTTh₂)₂), to blend with P9 as the polymer acceptor to fabricate M_D/P_A-type OSC devices.²⁶⁷ The device with the as-cast p-DTS(FBTTh₂)₂:P9 active layer exhibited a low PCE of 1.00% because of the large-size phase separation morphology. After using a solvent additive and thermal annealing, the PCE of the M_D/P_A-type OSC device was enhanced to 3.50%. The problem of large size phase separation in M_D/P_A-type OSC devices can be solved by using small molecular donors with suppressed π-stacking or polymer acceptors with stronger aggregation tendency in solution. As shown in Fig. 32a, DR3TBDTC was a small molecular donor bearing bulky carbazolyl substituents, which acted as steric hindrance groups and suppressed π–π stacking between molecular backbones. A BNBP-based polymer acceptor, P20, was used to pair with DR3TBDTC to fabricate M_D/P_A-type OSC devices.²⁶⁸ In the as-cast film of DR3TBDTC:P20, P20 formed ordered aggregates and DR3TBDTC was amorphous. No crystallization of DR3TBDTC was observed in the blend film, indicating that the aggregation of P20 governed the film-forming process and dominated the film morphology. After thermal annealing, DR3TBDTC crystallized into nano-sized domains and formed interconnected networks in the active layer (Fig. 32b). As a result, the M_D/P_A-type OSC device exhibited optimal phase separation morphology and showed a much enhanced PCE of 8.01% (Fig. 32d). The bulky carbazolyl substituents on the small molecular donor played an important role in the excellent active layer morphology of the M_D/P_A-type OSC devices (Fig. 32c). In addition, the device of DR3TBDTC:P20 showed excellent thermal stability, i.e. maintaining 89% of its initial efficiency after thermal annealing the active layer at 180 °C for 7 days. In contrast, the polymer donor/small molecular acceptor type device of PTB7-Th:EH-IDTBR exhibited a large PCE decrease and retained only 45% of its initial efficiency after 7 days. The high thermal stability of the DR3TBDTC:P20 blend indicated prominent morphological stability, which was mainly attributed to the excellent phase stability of the two materials themselves and the high crystallinity of the blend film. This result further demonstrated the superior intrinsic stability of BNBP-based polymer acceptor materials.

Fig. 32 (a) Chemical structures of small molecular donor DR3TBDTC and polymer acceptor P20. (b) 2D-GIWAXS patterns of the as-cast and thermally annealed DR3TBDTC:P20 blend film. (c) AFM height images of DR3TBDTC:P20 and DR3TBDTT:P20 blend films. (d) J–V curves and EQE spectra of the devices based on the DR3TBDTC:P20 and DR3TBDTT:P20 blends. Panels (b–d) were reproduced from ref. 268 with permission from Springer Nature, copyright 2019.

Molecular weights of polymer acceptors affect the aggregation tendency of polymer chains in solution, and thus affect the phase separation morphology of M_D/P_A blends. For OSC devices based on the DR3TBDTC:P16 blend, by increasing the molecular weight of polymer acceptor P16, the aggregation and crystallization of small molecular donor DR3TBDTC could be suppressed, and small-size phase separated domains of the blends could be obtained.²⁶⁹ The OSC device based on P16 with a number average molecular weight of 117.3 kg mol⁻¹ exhibited an optimized active layer morphology and showed a PCE of 6.4%. The aggregation tendency of polymer acceptors in solution can also be tuned by modifying the chemical structures of the polymers. Fig. 33a shows the chemical structures of two polymer acceptors, P18-C28 and P21.²⁷⁰ The introduction of two extra F substituents on the 2,1,3-benzothiadiazole (BT) unit increased the aggregation tendency of P21 in solution. For P18-C28 with weak aggregation, the active layer of the BD3T:P18-C28 blend exhibited over-crystallization of BD3T, which led to large-size phase separation. In contrast, in the BD3T:P21 blend, the aggregation of P21 was dominant and the crystallization of BD3T was restricted, leading to small-size phase separation in the active layer. While the device of BD3T:P18-C28 exhibited a PCE of 5.06%, the device of BD3T:P21 showed a much enhanced PCE of 9.51% (Fig. 33b). This was the most efficient M_D/P_A-type OSC device reported so far, indicating the versatility of BNBP-based polymer acceptors. Noteworthily, the M_D/P_A-type OSC device based on BD3T:P21 exhibited excellent thermal stability. After thermal treatment at 150 °C for 72 hours, the device could retain 84% of its initial PCE value (Fig. 33c and d). This superior device stability at high temperature was attributed to the high phase transition temperatures of the two materials and the excellent thermal stability of morphology in the M_D/P_A-type active layer.

Fig. 33 (a) Chemical structures of small molecular donor BD3T and polymer acceptors P18-C28 and P21. (b) J–V curves and EQE spectra of the devices based on the BD3T:P18-C28 and BD3T:P21 blends. (c) The inverted device structure of the OSC operating at 150 °C. (d) The normalized PCE for the OSC devices based on BD3T:P21 and PTB7-Th:EH-IDTBR blends with thermal treatment of the devices at 150 °C for different time periods. Panels (b–d) were reproduced from ref. 270 with permission from the Royal Society of Chemistry, copyright 2020.

In summary, aiming at excellent OSC device performance, the opto-electronic properties of BNBP-based polymer acceptors have been systematically tuned by molecular design. Active layer phase separation morphologies of BNBP-based polymer acceptors have also been optimized by various approaches. In OSC devices, BNBP-based polymer acceptors can give a PCE of 10% when blended with polymer donors or a PCE of 9.5% when blended with small molecular donors. Owing to the medium optical bandgap and the high V_OC of most of BNBP-based polymer acceptors, these polymers are very suitable for indoor photovoltaic application and a remarkable PCE of 27% has been demonstrated. In the future, research on BNBP-based polymer acceptors should focus on phase separation morphology optimization and indoor photovoltaic application.

4.4 BNIDT-based polymer acceptors

B ← N brigded indacenodithiophene, BNIDT, was used by Huang et al. to design polymer acceptors with excellent all-PSC device performance. In 2016, Fang et al. initially reported the BNIDT skeleton with four bromo substituents on the two boron atoms.²⁷¹ The compound was not stable under ambient conditions. Later, Huang et al. added four bulky phenyl groups to the two boron atoms of BNIDT and greatly improved its stability.²⁷² Besides, the synthesis of the BNIDT unit was simple and convenient, only involving a one-pot reaction from 2,5-di(thiophen-2-yl)pyrazine (Pz-T). From X-ray diffraction crystallography, the BNIDT unit exhibited a large layer-to-layer distance of 7.59 Å, which was attributed to the steric effect of the vertical phenyl substituents. Although the BNIDT unit had a large layer-to-layer distance, it had a small backbone distance of 3.58 Å due to the presence of the tilted molecular layer, which could facilitate charge transport between the backbones. The excellent stability, facile synthesis and favorable packing structure of the BNIDT unit made it to be used as a new electron-accepting building block to develop functional materials. An important feature of BNIDT-based polymer acceptors is their low crystallinity with excellent electron transporting property. The four phenyl groups on the two boron atoms act as a steric hindrance, inhibit aggregation of polymer chains in solution and prevent close π-stacking of polymer chains in thin film. Despite their low crystallinity, BNIDT-based polymer acceptors exhibit excellent electron transporting property. Their electron and hole mobilities in OFETs are on the order of 10⁻³–10⁻² cm² V⁻¹ s⁻¹ and their electron mobilities in OSCs are on the order of 10⁻⁵ cm² V⁻¹ s⁻¹. All-PSC devices of BNIDT-based polymer acceptors show an FF higher than 0.7 and a PCE of 8.78%.²⁷³ Recently, a ternary OSC containing BNIDT-based polymer acceptors gave a PCE of 16%.²⁷⁴

In the first report of BNIDT-based polymer acceptors, Huang et al. copolymerized BNIDT with thiophene or 3,4-difluorothiophene to synthesize two polymers, P22 and P23 (Fig. 34a).²⁷³ In spite of the weak crystallinity, both P22 and P23 exhibited high charge mobilities of 10⁻³–10⁻² cm² V⁻¹ s⁻¹, which benefited from the close backbone stacking of the BNIDT unit and the coplanar backbone conformation of the polymers. P23 exhibited low-lying HOMO/LUMO energy levels of −5.71 eV/−3.77 eV and a strong sunlight absorption ability with an onset absorption wavelength of 770 nm (Fig. 34b and c). With PBDB-T as the polymer donor, all-PSCs based on P22 and P23 as the polymer acceptors afforded a moderate PCE of 4.03% and a high PCE of 8.78%, respectively (Fig. 34d and e). The enhanced performance in the latter case was ascribed to the favorable film morphology, the high and balanced hole/electron mobilities, depressed bimolecular recombination, efficient exciton dissociation, and charge collection.

Fig. 34 (a) Chemical structures of P22 and P23 containing the BNIDT unit. (b) HOMO/LUMO energy levels of P22 and P23. (c) The absorption spectra of P22 and P23 in solution and in film. (d) J–V curves and (e) EQE spectra of the devices based on the PBDB-T:P22 and PBDB-T:P23 blends. Panels (c–e) were reproduced from ref. 273 with permission from John Wiley and Sons, copyright 2019.

Huang et al. systematically studied the effect of halogenation on the all-PSC device performance of BNIDT-based polymer acceptors.²⁷⁵ They prepared three polymer acceptors (P24–P26) based on alternating BNIDT and BDT units as the backbones but with or without fluorine or chlorine substituents on the BDT units (Fig. 35). The fluorine or chlorine substituents did not affect the LUMO energy levels but decreased the HOMO energy levels of the polymer acceptors. In all-PSC devices, halogenation on the polymer acceptors led to enhanced hole-transfer driving forces and better donor/acceptor miscibility, which further gave rise to higher and more balanced hole/electron mobilities and efficient physical processes.

Fig. 35 Chemical structures of polymer acceptors P24–P27 containing the BNIDT unit.

To improve the light absorption capacity of B ← N-containing polymer acceptors and increase the photocurrent of the corresponding devices, Huang et al. developed a polymer acceptor P27 by copolymerizing BNIDT and vinyl units (Fig. 35).²⁷⁶P27 exhibited a strong absorption in the 600–900 nm region with the absorption onset wavelength at 900 nm, corresponding to the optical bandgap of 1.38 eV. An all-PSC device was fabricated using PTB1 as the donor and P27 as the acceptor. The EQE spectrum of this device showed wide responses from 300 to 900 nm, which corresponds to the high J_SC of 16.90 mA cm⁻². The J_SC value is the highest value among the J_SC's of reported B ← N-containing polymer acceptors.

4.5 Polymer donors containing the B–N unit

The boron–nitrogen covalent bond (B–N) is isoelectronic to the carbon–carbon double bond (C=C).²⁷⁷ While the C=C bond has a dipole of zero, the B–N bond has a dipole of ca. 1.7 Debye. Therefore, B–N can be used to replace C=C in π-conjugated organic molecules with interesting opto-electronic properties. New building blocks based on B–N have been designed to construct polymer electron donors for OSC applications.

Yu et al. developed a wide bandgap polymer donor, P28, by copolymerizing pendant-borylated carbazole with BDT (Fig. 36a).²⁷⁸ The B–N bond endowed P28 with increased electron affinity. P28 showed a deep HOMO energy level of −5.52 eV and a wide bandgap of 2.07 eV. The OSC device with the P28:PC₇₁BM active layer exhibited a PCE of 3.82% with a high V_OC of 1.0 V.

Fig. 36 Chemical structures of organoboron polymer donors (a) P28 and (b) P29 with boron–nitrogen bonds. (c) The principle of the opposite resonance effect caused by boron and nitrogen atoms. (d) The Jablonski diagram of the electronic states in P29 (ground states (S₀), singlet states (S₁), triplet states (T₁), singlet–triplet energy gap (ΔE_ST), charge transfer states (CT) and V_OC of the OSCs). (e) J–V curves and EQE spectra of the devices based on the active layers of P29 (PBNT-BDD):Y6-BO and PM6:Y6-BO. Panels (c–e) were reproduced from ref. 279 with permission from John Wiley and Sons, copyright 2021.

Recently, Duan et al. reported a new building block BNT featuring B–N bonds and developed a polymer donor, P29.²⁷⁹ An advantage of BNT was the facile synthesis, which was important for low cost OSCs. The di-bromo monomer of BNT could be synthesized by four steps with high overall yields. Compared to the widespread halogenated BDT unit, the easy synthesis of the BNT unit results in lower synthesis cost of the corresponding polymer. According to the single crystal structure of BNT, the boron and the nitrogen atoms formed p–π conjugation with the conjugated backbone. The π–π distance of the adjacent BNT units is 3.59 Å, indicating a weak intermolecular interaction between the units. The polymer donor, P29, was an alternating copolymer of BNT unit and thiophene-flanked benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione (BDD) unit (Fig. 36b). In the test of differential scanning calorimetry (DSC), no melting or crystalline peak was observed in P29. According to 2D-GIWAXS, P29 did not exhibit clear (100) lamellar diffraction signals. These results indicated that P29 has weak crystallinity, which may be due to the weak packing of the BNT unit. Due to the opposite resonance effect of the boron atom and the nitrogen atom in the B–N covalent bond, P29 exhibited a small singlet–triplet gap (ΔE_ST) between the singlet state (S₁) and the triple state (T₁) (Fig. 36c). This was beneficial to suppress the recombination loss from charge transfer (CT) state to T₁ (Fig. 36d). The OSC device with P29 as the polymer donor and Y6-BO as the small molecular acceptor showed a PCE of 16.1%, which was comparable to that of the device of the well-known polymer donor PM6 with the same acceptor (Fig. 36e). More importantly, owing to the high T₁ energy level and the small ΔE_ST of P29, the OSC device exhibited a non-radiative energy loss of 0.19 eV, which was among the lowest values reported to date for OSCs. The facile synthesis, interesting opto-electronic properties and excellent OSC device performance of P29 proved the great potential of organoboron polymer donors.

4.6 Other organoboron polymers as electron acceptors

Most of conjugated polymers rely on π-conjugation, in which the carbon–carbon single bond and double bond alternate in the main chains. The p–π* conjugation provides a new opportunity to tune the electronic structures of conjugated polymers and substantially expand the scope of conjugated polymers. Liu, Jäkle and coworkers reported two p–π* conjugated polymer acceptors, P30 and P31, by copolymerizing a triarylborane unit (FBDT) with IID or pyridine-flanked DPP units, respectively (Fig. 37a).²⁸⁰ Because of the highly electron-deficient character of the triarylborane unit and IID/PyDPP units, P30 and P31 exhibited low-lying LUMO energy levels of −3.72 eV and −3.58 eV, respectively (Fig. 37b). The electron mobilities of the two polymers were estimated to be ca. 1 × 10⁻⁵ cm² V⁻¹ s⁻¹. The low-lying LUMO energy levels and high electron mobilities of the two triarylborane-based polymers indicated that they could be used as electron acceptors. The all-PSC device with P30 as the polymer acceptor exhibited a PCE of 2.83% (Fig. 37c), which was higher than that of the devices (0.02%) using the control polymers without the boron atom. These results ambiguously proved the photovoltaic applications of p–π* conjugated polymers based on triarylborane.

Fig. 37 (a) Chemical structures of polymer acceptors, P30–P33. (b) HOMO/LUMO energy levels of P30 and P31. (c) J–V curves of the devices based on the PTB7-Th:P30 and PTB7-Th:P31 blends. Panel (c) was reproduced from ref. 280 with permission from John Wiley and Sons, copyright 2018.

In addition to preparing copolymers by Stille- or Suzuki-type cross-coupling polycondensation, postfunctionalization of polymers is also a way to obtain conjugated polymers to adjust their electronic and structural properties. Pammer, Hauff and coworkers reported a series of ladder polymers featuring intramolecular B ← N bonds via postfunctionalization by hydroboration with different hydroboranes.²⁸¹Fig. 37a shows two examples of the ladder polymers, P32 and P33, in which alkenyl-functionalized poly(biphenylene-pyrazinylene) (PPz) was postmodified by R₂B-H (9H-BBN) and (C₆F₅)2B-H (BPF-H), respectively. Compared with the substrate polymer PPz (−2.80 eV), postmodification by hydroboration could greatly reduce the LUMO energy levels of the polymers (−3.75 eV for P32 and −4.35 eV for P33). With the decrease of the LUMO energy levels, the bandgaps of P32 and P33 were significantly reduced to 2.16 eV and 2.07 eV, respectively, and the absorption spectra had significant red shift. These polymers exhibited ambipolar charge transport with hole and electron mobilities in the order of 2 × 10⁻⁵ cm² V⁻¹ s⁻¹. The low-lying LUMO energy levels of P32 and P33 enabled them to be used as acceptors in all-PSCs. This result demonstrated the potential of postfunctionalization of polymers to effectively adjust the opto-electronic properties of polymers.

5. Summary and outlook

Organoboron chemistry offers a powerful toolbox to design small molecules or polymers with tunable and excellent opto-electronic properties for OSC applications. In the past decade, a large family of organoboron small molecular donors/acceptors and organoboron polymer donors/acceptors have been developed. They have become an important class of organic photovoltaic materials. At the early stage, scientists have modified the chemical structures of classic organoboron compounds, such as BODIPY and SubPc, towards OSC applications. Later, novel organoboron building blocks, e.g. BNBP, BNIDT and BNT, have been designed and synthesized to develop organic photovoltaic materials. Based on these novel organoboron building blocks, a high PCE and excellent stability have been realized with the resulting OSC devices. At present, PCEs of 16% and 14% have been demonstrated with organoboron electron donors and organoboron electron acceptors, respectively.

Despite the great research progress, the OSC device performance of organoboron small molecules and polymers still lags behind that of the best organic photovoltaic materials. To pave the practical application of organoboron photovoltaic materials, great attention should be paid to the following aspects.

(1) New organoboron building blocks. Although plenty of organoboron compounds have been reported, only very few of them have been used to design electron donor or electron acceptor materials in OSCs. In the past several years, the development of organoboron photovoltaic materials has strongly relied on the utilization of some new organoboron building blocks, for example, BNBP, BNIDT, BNT, etc. Indeed, the resulting organoboron photovoltaic materials exhibit some unique properties and show great potential. Great attention should be paid to the molecular design of unknown organoboron building blocks and the utilization of known organoboron compounds.

(2) PCE enhancement. OSC devices based on organoboron materials suffer from low J_SC and low FF. EQE higher than 0.70 and FF greater than 0.70 are rarely revealed with organoboron electron donors or acceptors. To enhance the PCE of the organoboron photovoltaic materials, both their opto-electronic properties and their active layer morphology need to be optimized. This is expected to lead to synergistically enhanced J_SC and FF without the sacrifice of V_OC. So far, only opto-electronic properties of BNBP-based polymer acceptors have been intensively studied. The phase separation morphology optimization of organoboron photovoltaic materials is just in infancy. Provided the opto-electronic property optimization and phase separation morphology optimization are achieved, we believe that there is large room for PCE enhancement of organoboron small molecule and polymer donors/acceptors.

(3) Stability issue. The excellent thermal stability of OSC devices based on organoboron polymer acceptors has already been demonstrated by Liu et al. However, great attention should be paid to the chemical stability of the organoboron photovoltaic materials. Owing to the Lewis acidity or the vacant 2p orbital of the boron atom, organoboron compounds should be volatile to moisture. Indeed, only carefully designed organoboron compounds can possess chemical stability under ambient conditions. The long-term stability of organoboron photovoltaic materials under ambient conditions, e.g. moisture air, need to be evaluated. In-depth studies on the stability issue of organoboron photovoltaic materials should be carried out.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors are grateful for the financial support by the National Key Research and Development Program of China (No. 2019YFA0705900) funded by MOST and the National Natural Science Foundation of China (No. 21625403, 21875244).

Notes and references

1. S. Chu and A. Majumdar, Nature, 2012, 488, 294–303.
2. M. Hosenuzzaman, N. A. Rahim, J. Selvaraj, M. Hasanuzzaman, A. B. M. A. Malek and A. Nahar, Renewable Sustainable Energy Rev., 2015, 41, 284–297.
3. E. Kabir, P. Kumar, S. Kumar, A. A. Adelodun and K.-H. Kim, Renewable Sustainable Energy Rev., 2018, 82, 894–900.
4. F. Creutzig, P. Agoston, J. C. Goldschmidt, G. Luderer, G. Nemet and R. C. Pietzcker, Nat. Energy, 2017, 2, 17140.
5. Y. Li, G. Xu, C. Cui and Y. Li, Adv. Energy Mater., 2018, 8, 1701791.
6. V. V. Brus, J. Lee, B. R. Luginbuhl, S.-J. Ko, G. C. Bazan and T.-Q. Nguyen, Adv. Mater., 2019, 31, 1900904.
7. G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4, 864–868.
8. L. Meng, Y. Zhang, X. Wan, C. Li, X. Zhang, Y. Wang, X. Ke, Z. Xiao, L. Ding, R. Xia, H.-L. Yip, Y. Cao and Y. Chen, Science, 2018, 361, 1094–1098.
9. Q. Wang, M. Li, Z. Peng, N. Kirby, Y. Deng, L. Ye and Y. Geng, Sci. China: Chem., 2021, 64, 478–487.
10. Y. Liu, Z. Zhang, S. Feng, M. Li, L. Wu, R. Hou, X. Xu, X. Chen and Z. Bo, J. Am. Chem. Soc., 2017, 139, 3356–3359.
11. W. Song, R. Peng, L. Huang, C. Liu, B. Fanady, T. Lei, L. Hong, J. Ge, A. Facchetti and Z. Ge, iScience, 2020, 23, 100981.
12. R. Sun, Q. Wu, J. Guo, T. Wang, Y. Wu, B. Qiu, Z. Luo, W. Yang, Z. Hu, J. Guo, M. Shi, C. Yang, F. Huang, Y. Li and J. Min, Joule, 2020, 4, 407–419.
13. M. Ren, G. Zhang, Z. Chen, J. Xiao, X. Jiao, Y. Zou, H.-L. Yip and Y. Cao, ACS Appl. Mater. Interfaces, 2020, 12, 13077–13086.
14. W. Liu, J. Zhang, Z. Zhou, D. Zhang, Y. Zhang, S. Xu and X. Zhu, Adv. Mater., 2018, 30, 1800403.
15. F.-C. Chen, Adv. Opt. Mater., 2019, 7, 1800662.
16. S. A. Hashemi, S. Ramakrishna and A. G. Aberle, Energy Environ. Sci., 2020, 13, 685–743.
17. S. E. Root, S. Savagatrup, A. D. Printz, D. Rodriquez and D. J. Lipomi, Chem. Rev., 2017, 117, 6467–6499.
18. Y. Qian, X. Zhang, L. Xie, D. Qi, B. K. Chandran, X. Chen and W. Huang, Adv. Mater., 2016, 28, 9243–9265.
19. M. R. Lee, R. D. Eckert, K. Forberich, G. Dennler, C. J. Brabec and R. A. Gaudiana, Science, 2009, 324, 232–235.
20. K. Forberich, F. Guo, C. Bronnbauer and C. J. Brabec, Energy Technol., 2015, 3, 1051–1058.
21. M. Li, F. Igbari, Z.-K. Wang and L.-S. Liao, Adv. Energy Mater., 2020, 10, 2000641.
22. K. Feng, J. Huang, X. Zhang, Z. Wu, S. Shi, L. Thomsen, Y. Tian, H. Y. Woo, C. R. McNeill and X. Guo, Adv. Mater., 2020, 32, 2001476.
23. K. Li, Y. Wu, X. Li, H. Fu and C. Zhan, Sci. China: Chem., 2020, 63, 490–496.
24. Y. Cui, H. Yao, J. Zhang, K. Xian, T. Zhang, L. Hong, Y. Wang, Y. Xu, K. Ma, C. An, C. He, Z. Wei, F. Gao and J. Hou, Adv. Mater., 2020, 32, 1908205.
25. Q. Liu, Y. Jiang, K. Jin, J. Qin, J. Xu, W. Li, J. Xiong, J. Liu, Z. Xiao, K. Sun, S. Yang, X. Zhang and L. Ding, Sci. Bull., 2020, 65, 272–275.
26. J. Qin, L. Zhang, C. Zuo, Z. Xiao, Y. Yuan, S. Yang, F. Hao, M. Cheng, K. Sun, Q. Bao, Z. Bin, Z. Jin and L. Ding, J. Semicond., 2021, 42, 010501.
27. M. Zhang, L. Zhu, G. Zhou, T. Hao, C. Qiu, Z. Zhao, Q. Hu, B. W. Larson, H. Zhu, Z. Ma, Z. Tang, W. Feng, Y. Zhang, T. P. Russell and F. Liu, Nat. Commun., 2021, 12, 309.
28. C. W. Tang, Appl. Phys. Lett., 1986, 48, 183–185.
29. G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789.
30. Y. He and Y. Li, Phys. Chem. Chem. Phys., 2011, 13, 1970–1983.
31. Y. He, H.-Y. Chen, J. Hou and Y. Li, J. Am. Chem. Soc., 2010, 132, 1377–1382.
32. T. S. van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646–3649.
33. K. Sun, Z. Xiao, S. Lu, W. Zajaczkowski, W. Pisula, E. Hanssen, J. M. White, R. M. Williamson, J. Subbiah, J. Ouyang, A. B. Holmes, W. W. H. Wong and D. J. Jones, Nat. Commun., 2015, 6, 6013.
34. J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135, 8484–8487.
35. Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. McCulloch, C.-S. Ha and M. Ree, Nat. Mater., 2006, 5, 197–203.
36. S.-H. Liao, H.-J. Jhuo, Y.-S. Cheng and S.-A. Chen, Adv. Mater., 2013, 25, 4766–4771.
37. H. Hu, P. C. Y. Chow, G. Zhang, T. Ma, J. Liu, G. Yang and H. Yan, Acc. Chem. Res., 2017, 50, 2519–2528.
38. J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma and H. Yan, Nat. Energy, 2016, 1, 15027.
39. D. Deng, Y. Zhang, J. Zhang, Z. Wang, L. Zhu, J. Fang, B. Xia, Z. Wang, K. Lu, W. Ma and Z. Wei, Nat. Commun., 2016, 7, 13740.
40. J. Wan, X. Xu, G. Zhang, Y. Li, K. Feng and Q. Peng, Energy Environ. Sci., 2017, 10, 1739–1745.
41. S. Holliday, R. S. Ashraf, A. Wadsworth, D. Baran, S. A. Yousaf, C. B. Nielsen, C.-H. Tan, S. D. Dimitrov, Z. Shang, N. Gasparini, M. Alamoudi, F. Laquai, C. J. Brabec, A. Salleo, J. R. Durrant and I. McCulloch, Nat. Commun., 2016, 7, 11585.
42. Y. Lin, J. Wang, Z.-G. Zhang, H. Bai, Y. Li, D. Zhu and X. Zhan, Adv. Mater., 2015, 27, 1170–1174.
43. Q. Yue, W. Liu and X. Zhu, J. Am. Chem. Soc., 2020, 142, 11613–11628.
44. G. Zhang, J. Zhao, P. C. Y. Chow, K. Jiang, J. Zhang, Z. Zhu, J. Zhang, F. Huang and H. Yan, Chem. Rev., 2018, 118, 3447–3507.
45. X. Wan, C. Li, M. Zhang and Y. Chen, Chem. Soc. Rev., 2020, 49, 2828–2842.
46. Q. Wei, W. Liu, M. Leclerc, J. Yuan, H. Chen and Y. Zou, Sci. China: Chem., 2020, 63, 1352–1366.
47. S. Li, C.-Z. Li, M. Shi and H. Chen, ACS Energy Lett., 2020, 5, 1554–1567.
48. Z. Fei, F. D. Eisner, X. Jiao, M. Azzouzi, J. A. Röhr, Y. Han, M. Shahid, A. S. R. Chesman, C. D. Easton, C. R. McNeill, T. D. Anthopoulos, J. Nelson and M. Heeney, Adv. Mater., 2018, 30, 1705209.
49. C. Huang, X. Liao, K. Gao, L. Zuo, F. Lin, X. Shi, C.-Z. Li, H. Liu, X. Li, F. Liu, Y. Chen, H. Chen and A. K. Y. Jen, Chem. Mater., 2018, 30, 5429–5434.
50. J. Sun, X. Ma, Z. Zhang, J. Yu, J. Zhou, X. Yin, L. Yang, R. Geng, R. Zhu, F. Zhang and W. Tang, Adv. Mater., 2018, 30, 1707150.
51. Z. Zheng, Q. Hu, S. Zhang, D. Zhang, J. Wang, S. Xie, R. Wang, Y. Qin, W. Li, L. Hong, N. Liang, F. Liu, Y. Zhang, Z. Wei, Z. Tang, T. P. Russell, J. Hou and H. Zhou, Adv. Mater., 2018, 30, 1801801.
52. T. Jia, J. Zhang, W. Zhong, Y. Liang, K. Zhang, S. Dong, L. Ying, F. Liu, X. Wang, F. Huang and Y. Cao, Nano Energy, 2020, 72, 104718.
53. D. Hu, Q. Yang, H. Chen, F. Wobben, V. M. Le Corre, R. Singh, T. Liu, R. Ma, H. Tang, L. J. A. Koster, T. Duan, H. Yan, Z. Kan, Z. Xiao and S. Lu, Energy Environ. Sci., 2020, 13, 2134–2141.
54. J. Yuan, Y. Zhang, L. Zhou, G. Zhang, H.-L. Yip, T.-K. Lau, X. Lu, C. Zhu, H. Peng, P. A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. Li and Y. Zou, Joule, 2019, 3, 1140–1151.
55. Y.-J. Hwang, H. Li, B. A. E. Courtright, S. Subramaniyan and S. A. Jenekhe, Adv. Mater., 2016, 28, 124–131.
56. X. Meng, L. Zhang, Y. Xie, X. Hu, Z. Xing, Z. Huang, C. Liu, L. Tan, W. Zhou, Y. Sun, W. Ma and Y. Chen, Adv. Mater., 2019, 31, 1903649.
57. Z. Wang, H. Jiang, X. Liu, J. Liang, L. Zhang, L. Qing, Q. Wang, W. Zhang, Y. Cao and J. Chen, J. Mater. Chem. A, 2020, 8, 7765–7774.
58. B. Xiao, A. Tang, J. Zhang, A. Mahmood, Z. Wei and E. Zhou, Adv. Energy Mater., 2017, 7, 1602269.
59. Y. Ma, D. Cai, S. Wan, P. Wang, J. Wang and Q. Zheng, Angew. Chem., Int. Ed., 2020, 59, 21627–21633.
60. J. Song, C. Li, L. Zhu, J. Guo, J. Xu, X. Zhang, K. Weng, K. Zhang, J. Min, X. Hao, Y. Zhang, F. Liu and Y. Sun, Adv. Mater., 2019, 31, 1905645.
61. Y. Wu, S. Schneider, C. Walter, A. H. Chowdhury, B. Bahrami, H.-C. Wu, Q. Qiao, M. F. Toney and Z. Bao, J. Am. Chem. Soc., 2020, 142, 392–406.
62. Y. Tong, Z. Xiao, X. Du, C. Zuo, Y. Li, M. Lv, Y. Yuan, C. Yi, F. Hao, Y. Hua, T. Lei, Q. Lin, K. Sun, D. Zhao, C. Duan, X. Shao, W. Li, H.-L. Yip, Z. Xiao, B. Zhang, Q. Bian, Y. Cheng, S. Liu, M. Cheng, Z. Jin, S. Yang and L. Ding, Sci. China: Chem., 2020, 63, 758–765.
63. A. Wakamiya and S. Yamaguchi, Bull. Chem. Soc. Jpn., 2015, 88, 1357–1377.
64. S. K. Mellerup and S. Wang, Chem. Soc. Rev., 2019, 48, 3537–3549.
65. D. Li, H. Zhang and Y. Wang, Chem. Soc. Rev., 2013, 42, 8416–8433.
66. C. D. Entwistle and T. B. Marder, Angew. Chem., Int. Ed., 2002, 41, 2927–2931.
67. L. Ji, S. Griesbeck and T. B. Marder, Chem. Sci., 2017, 8, 846–863.
68. S.-Y. Li, Z.-B. Sun and C.-H. Zhao, Inorg. Chem., 2017, 56, 8705–8717.
69. E. von Grotthuss, A. John, T. Kaese and M. Wagner, Asian J. Org. Chem., 2018, 7, 37–53.
70. F. Jäkle, Chem. Rev., 2010, 110, 3985–4022.
71. S. K. Mellerup and S. Wang, Trends Chem., 2019, 1, 77–89.
72. N. Ikeda, S. Oda, R. Matsumoto, M. Yoshioka, D. Fukushima, K. Yoshiura, N. Yasuda and T. Hatakeyama, Adv. Mater., 2020, 32, 2004072.
73. X. Yin, J. Liu and F. Jäkle, Chem. – Eur. J., 2021, 27, 2973–2986.
74. H. Lu, T. Nakamuro, K. Yamashita, H. Yanagisawa, O. Nureki, M. Kikkawa, H. Gao, J. Tian, R. Shang and E. Nakamura, J. Am. Chem. Soc., 2020, 142, 18990–18996.
75. J. Huang and Y. Li, Front. Chem., 2018, 6, 341.
76. R. Zhao, J. Liu and L. Wang, Acc. Chem. Res., 2020, 53, 1557–1567.
77. G. de la Torre, G. Bottari and T. Torres, Adv. Energy Mater., 2017, 7, 1601700.
78. D. Ho, R. Ozdemir, H. Kim, T. Earmme, H. Usta and C. Kim, ChemPlusChem, 2019, 84, 18–37.
79. Y.-L. Rao and S. Wang, Inorg. Chem., 2011, 50, 12263–12274.
80. J. Huang, X. Wang, Y. Xiang, L. Guo and G. Chen, Adv. Energy Sustainability Res., 2021, 2100016.
81. D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2015, 54, 6400–6441.
82. C. Dou, S. Saito and S. Yamaguchi, J. Am. Chem. Soc., 2013, 135, 9346–9349.
83. X. Y. Wang, J. Y. Wang and J. Pei, Chem. – Eur. J., 2015, 21, 3528–3539.
84. Z. Huang, S. Wang, R. D. Dewhurst, N. V. Ignat'ev, M. Finze and H. Braunschweig, Angew. Chem., Int. Ed., 2020, 59, 8800–8816.
85. J. B. Gilroy and E. Otten, Chem. Soc. Rev., 2020, 49, 85–113.
86. H. Helten, Chem. – Eur. J., 2016, 22, 12972–12982.
87. A. N. Bismillah and I. Aprahamian, Chem. Soc. Rev., 2021, 50, 5631–5649.
88. C. R. Wade, A. E. J. Broomsgrove, S. Aldridge and F. P. Gabbaï, Chem. Rev., 2010, 110, 3958–3984.
89. Y. Min, C. Dou, D. Liu, H. Dong and J. Liu, J. Am. Chem. Soc., 2019, 141, 17015–17021.
90. Y. Min, C. Dou, H. Tian, Y. Geng, J. Liu and L. Wang, Angew. Chem., Int. Ed., 2018, 57, 2000–2004.
91. X. Shao, C. Dou, J. Liu and L. Wang, Sci. China: Chem., 2019, 62, 1387–1392.
92. A. Bessette and G. S. Hanan, Chem. Soc. Rev., 2014, 43, 3342–3405.
93. M. Poddar and R. Misra, Coord. Chem. Rev., 2020, 421, 213462.
94. S. Cherumukkil, B. Vedhanarayanan, G. Das, V. K. Praveen and A. Ajayaghosh, Bull. Chem. Soc. Jpn., 2017, 91, 100–120.
95. R. Ziessel, G. Ulrich and A. Harriman, New J. Chem., 2007, 31, 496–501.
96. H. Lu, J. Mack, T. Nyokong, N. Kobayashi and Z. Shen, Coord. Chem. Rev., 2016, 318, 1–15.
97. E. V. Antina and N. A. Bumagina, Chem. Heterocycl. Compd., 2017, 53, 39–41.
98. D. Chen, Z. Zhong, Q. Ma, J. Shao, W. Huang and X. Dong, ACS Appl. Mater. Interfaces, 2020, 12, 26914–26925.
99. Z. Shi, X. Han, W. Hu, H. Bai, B. Peng, L. Ji, Q. Fan, L. Li and W. Huang, Chem. Soc. Rev., 2020, 49, 7533–7567.
100. X. Jiang, S. Li, J. Guan, T. Fang, X. Liu and L. Xiao, Curr. Org. Chem., 2016, 20, 1736–1744.
101. Q. Wu, Z. Kang, Q. Gong, X. Guo, H. Wang, D. Wang, L. Jiao and E. Hao, Org. Lett., 2020, 22, 7513–7517.
102. A. Treibs and F.-H. Kreuzer, Justus Liebigs Ann. Chem., 1968, 718, 208–223.
103. S. Kolemen and E. U. Akkaya, Coord. Chem. Rev., 2018, 354, 121–134.
104. L. Bucher, N. Desbois, P. D. Harvey, G. D. Sharma and C. P. Gros, Sol. RRL, 2017, 1, 1700127.
105. B. M. Squeo, V. G. Gregoriou, A. Avgeropoulos, S. Baysec, S. Allard, U. Scherf and C. L. Chochos, Prog. Polym. Sci., 2017, 71, 26–52.
106. B. M. Squeo and M. Pasini, Supramol. Chem., 2020, 32, 56–70.
107. A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung and K. Burgess, Chem. Soc. Rev., 2013, 42, 77–88.
108. H. Klfout, A. Stewart, M. Elkhalifa and H. He, ACS Appl. Mater. Interfaces, 2017, 9, 39873–39889.
109. T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel and J. Roncali, Chem. Commun., 2009, 1673–1675.
110. S. Suzuki, M. Kozaki, K. Nozaki and K. Okada, J. Photochem. Photobiol., C, 2011, 12, 269–292.
111. N. Boens, B. Verbelen, M. J. Ortiz, L. Jiao and W. Dehaen, Coord. Chem. Rev., 2019, 399, 213024.
112. E. Bodio and C. Goze, Dyes Pigm., 2019, 160, 700–710.
113. L. Jean-Gérard, W. Vasseur, F. Scherninski and B. Andrioletti, Chem. Commun., 2018, 54, 12914–12929.
114. J. Wang, N. Boens, L. Jiao and E. Hao, Org. Biomol. Chem., 2020, 18, 4135–4156.
115. S. Kolemen, Y. Cakmak, T. Ozdemir, S. Erten-Ela, M. Buyuktemiz, Y. Dede and E. U. Akkaya, Tetrahedron, 2014, 70, 6229–6234.
116. J. Marques dos Santos, L. K. Jagadamma, N. M. Latif, A. Ruseckas, I. D. W. Samuel and G. Cooke, RSC Adv., 2019, 9, 15410–15423.
117. T. Rousseau, A. Cravino, E. Ripaud, P. Leriche, S. Rihn, A. De Nicola, R. Ziessel and J. Roncali, Chem. Commun., 2010, 46, 5082–5084.
118. J. Liao, Y. Xu, H. Zhao, Y. Wang, W. Zhang, F. Peng, S. Xie and X. Yang, RSC Adv., 2015, 5, 86453–86462.
119. J. Yang, C. H. Devillers, P. Fleurat-Lessard, H. Jiang, S. Wang, C. P. Gros, G. Gupta, G. D. Sharma and H. Xu, Dalton Trans., 2020, 49, 5606–5617.
120. A. Mirloup, N. Leclerc, S. Rihn, T. Bura, R. Bechara, A. Hébraud, P. Lévêque, T. Heiser and R. Ziessel, New J. Chem., 2014, 38, 3644–3653.
121. A. Aguiar, J. Farinhas, W. da Silva, M. E. Ghica, C. M. A. Brett, J. Morgado and A. J. F. N. Sobral, Dyes Pigm., 2019, 168, 103–110.
122. A. Sutter, P. Retailleau, W.-C. Huang, H.-W. Lin and R. Ziessel, New J. Chem., 2014, 38, 1701–1710.
123. J. Liao, H. Zhao, Y. Xu, Z. Cai, Z. Peng, W. Zhang, W. Zhou, B. Li, Q. Zong and X. Yang, Dyes Pigm., 2016, 128, 131–140.
124. H.-Y. Lin, W.-C. Huang, Y.-C. Chen, H.-H. Chou, C.-Y. Hsu, J. T. Lin and H.-W. Lin, Chem. Commun., 2012, 48, 8913–8915.
125. X. Zhang, Y. Zhang, L. Chen and Y. Xiao, RSC Adv., 2015, 5, 32283–32289.
126. A. Aguiar, J. Farinhas, W. da Silva, M. Susano, M. R. Silva, L. Alcácer, S. Kumar, C. M. A. Brett, J. Morgado and A. J. F. N. Sobral, Dyes Pigm., 2020, 172, 107842.
127. T. Jadhav, R. Misra, S. Biswas and G. D. Sharma, Phys. Chem. Chem. Phys., 2015, 17, 26580–26588.
128. T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel and J. Roncali, J. Mater. Chem., 2009, 19, 2298–2300.
129. T. Bura, N. Leclerc, S. Fall, P. Lévêque, T. Heiser, P. Retailleau, S. Rihn, A. Mirloup and R. Ziessel, J. Am. Chem. Soc., 2012, 134, 17404–17407.
130. C.-C. Chi, Y.-J. Huang and C.-T. Chen, J. Chin. Chem. Soc., 2012, 59, 305–316.
131. W. Liu, J. Yao and C. Zhan, RSC Adv., 2015, 5, 74238–74241.
132. L. Zou, S. Guan, L. Li and L. Zhao, Chem. Res. Chin. Univ., 2015, 31, 801–808.
133. R. Mishra, B. Basumatary, R. Singhal, G. D. Sharma and J. Sankar, ACS Appl. Mater. Interfaces, 2018, 10, 31462–31471.
134. G. Thumuganti, V. Gupta and S. P. Singh, New J. Chem., 2019, 43, 8735–8740.
135. İ. Ömeroğlu, A. Şenocak, H. Yetkin, H. Y. Güney, E. Demirbaş and M. Durmuş, J. Porphyrins Phthalocyanines, 2019, 23, 1132–1143.
136. P. Erwin, S. M. Conron, J. H. Golden, K. Allen and M. E. Thompson, Chem. Mater., 2015, 27, 5386–5392.
137. J. Liao, Y. Xu, H. Zhao, Q. Zong and Y. Fang, Org. Electron., 2017, 49, 321–333.
138. W. Liu, J. Yao and C. Zhan, Chin. Chem. Lett., 2017, 28, 875–880.
139. G. Tarafdar, J. C. Johnson, B. W. Larson and P. C. Ramamurthy, Dyes Pigm., 2020, 177, 108289.
140. W. Liu, A. Tang, J. Chen, Y. Wu, C. Zhan and J. Yao, ACS Appl. Mater. Interfaces, 2014, 6, 22496–22505.
141. L. Xiao, H. Wang, K. Gao, L. Li, C. Liu, X. Peng, W.-Y. Wong, W.-K. Wong and X. Zhu, Chem. – Asian J., 2015, 10, 1513–1518.
142. J. Liao, H. Zhao, Z. Cai, Y. Xu, F. G. F. Qin, Q. Zong, F. Peng and Y. Fang, Org. Electron., 2018, 61, 215–222.
143. R. Srinivasa Rao, A. Bagui, G. Hanumantha Rao, V. Gupta and S. P. Singh, Chem. Commun., 2017, 53, 6953–6956.
144. L. Bucher, N. Desbois, E. N. Koukaras, C. H. Devillers, S. Biswas, G. D. Sharma and C. P. Gros, J. Mater. Chem. A, 2018, 6, 8449–8461.
145. G. D. Sharma, S. A. Siddiqui, A. Nikiforou, G. E. Zervaki, I. Georgakaki, K. Ladomenou and A. G. Coutsolelos, J. Mater. Chem. C, 2015, 3, 6209–6217.
146. J. J. Chen, S. M. Conron, P. Erwin, M. Dimitriou, K. McAlahney and M. E. Thompson, ACS Appl. Mater. Interfaces, 2015, 7, 662–669.
147. K. Umezawa, Y. Nakamura, H. Makino, D. Citterio and K. Suzuki, J. Am. Chem. Soc., 2008, 130, 1550–1551.
148. Z.-W. Zhao, Q.-Q. Pan, Y.-C. Duan, Y. Wu, Y. Geng, S.-X. Wu and Z.-M. Su, J. Phys. Chem. C, 2019, 123, 6407–6415.
149. T.-y. Li, J. Benduhn, Z. Qiao, Y. Liu, Y. Li, R. Shivhare, F. Jaiser, P. Wang, J. Ma, O. Zeika, D. Neher, S. C. B. Mannsfeld, Z. Ma, K. Vandewal and K. Leo, J. Phys. Chem. Lett., 2019, 10, 2684–2691.
150. T.-y. Li, J. Benduhn, Y. Li, F. Jaiser, D. Spoltore, O. Zeika, Z. Ma, D. Neher, K. Vandewal and K. Leo, J. Mater. Chem. A, 2018, 6, 18583–18591.
151. A. Wakamiya, T. Murakami and S. Yamaguchi, Chem. Sci., 2013, 4, 1002–1007.
152. Y. Kubo, K. Watanabe, R. Nishiyabu, R. Hata, A. Murakami, T. Shoda and H. Ota, Org. Lett., 2011, 13, 4574–4577.
153. T.-y. Li, T. Meyer, Z. Ma, J. Benduhn, C. Körner, O. Zeika, K. Vandewal and K. Leo, J. Am. Chem. Soc., 2017, 139, 13636–13639.
154. J. Killoran, L. Allen, J. F. Gallagher, W. M. Gallagher and D. F. O′Shea, Chem. Commun., 2002, 1862–1863.
155. X. Zhang, H. Yu and Y. Xiao, J. Org. Chem., 2012, 77, 669–673.
156. T.-y. Li, T. Meyer, R. Meerheim, M. Höppner, C. Körner, K. Vandewal, O. Zeika and K. Leo, J. Mater. Chem. A, 2017, 5, 10696–10703.
157. T. Mueller, R. Gresser, K. Leo and M. Riede, Sol. Energy Mater. Sol. Cells, 2012, 99, 176–181.
158. S. Kraner, J. Widmer, J. Benduhn, E. Hieckmann, T. Jägeler-Hoheisel, S. Ullbrich, D. Schütze, K. Sebastian Radke, G. Cuniberti, F. Ortmann, M. Lorenz-Rothe, R. Meerheim, D. Spoltore, K. Vandewal, C. Koerner and K. Leo, Phys. Status Solidi A, 2015, 212, 2747–2753.
159. J. Min, T. Ameri, R. Gresser, M. Lorenz-Rothe, D. Baran, A. Troeger, V. Sgobba, K. Leo, M. Riede, D. M. Guldi and C. J. Brabec, ACS Appl. Mater. Interfaces, 2013, 5, 5609–5616.
160. J. Meiss, F. Holzmueller, R. Gresser, K. Leo and M. Riede, Appl. Phys. Lett., 2011, 99, 193307.
161. R. Meerheim, C. Körner, B. Oesen and K. Leo, Appl. Phys. Lett., 2016, 108, 103302.
162. M. Lorenz-Rothe, K. S. Schellhammer, T. Jägeler-Hoheisel, R. Meerheim, S. Kraner, M. P. Hein, C. Schünemann, M. L. Tietze, M. Hummert, F. Ortmann, G. Cuniberti, C. Körner and K. Leo, Adv. Electron. Mater., 2016, 2, 1600152.
163. A. M. Poe, A. M. Della Pelle, A. V. Subrahmanyam, W. White, G. Wantz and S. Thayumanavan, Chem. Commun., 2014, 50, 2913–2915.
164. W. Liu, J. Yao and C. Zhan, Chin. J. Chem., 2017, 35, 1813–1823.
165. G. E. Morse and T. P. Bender, ACS Appl. Mater. Interfaces, 2012, 4, 5055–5068.
166. C. G. Claessens, D. González-Rodríguez and T. Torres, Chem. Rev., 2002, 102, 835–854.
167. C. G. Claessens, D. González-Rodríguez, M. S. Rodríguez-Morgade, A. Medina and T. Torres, Chem. Rev., 2014, 114, 2192–2277.
168. K. L. Mutolo, E. I. Mayo, B. P. Rand, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2006, 128, 8108–8109.
169. C.-T. Chou, W.-L. Tang, Y. Tai, C.-H. Lin, C.-H. J. Liu, L.-C. Chen and K.-H. Chen, Thin Solid Films, 2012, 520, 2289–2292.
170. N. Beaumont, I. Hancox, P. Sullivan, R. A. Hatton and T. S. Jones, Energy Environ. Sci., 2011, 4, 1708–1711.
171. C.-F. Lin, V. M. Nichols, Y.-C. Cheng, C. J. Bardeen, M.-K. Wei, S.-W. Liu, C.-C. Lee, W.-C. Su, T.-L. Chiu, H.-C. Han, L.-C. Chen, C.-T. Chen and J.-H. Lee, Sol. Energy Mater. Sol. Cells, 2014, 122, 264–270.
172. R. Pandey, Y. Zou and R. J. Holmes, Appl. Phys. Lett., 2012, 101, 033308.
173. F. Jin, B. Chu, W. Li, Z. Su, X. Yan, J. Wang, R. Li, B. Zhao, T. Zhang, Y. Gao, C. S. Lee, H. Wu, F. Hou, T. Lin and Q. Song, Org. Electron., 2014, 15, 3756–3760.
174. H. Gommans, D. Cheyns, T. Aernouts, C. Girotto, J. Poortmans and P. Heremans, Adv. Funct. Mater., 2007, 17, 2653–2658.
175. R. Pandey, A. A. Gunawan, K. A. Mkhoyan and R. J. Holmes, Adv. Funct. Mater., 2012, 22, 617–624.
176. B. E. Lassiter, J. D. Zimmerman, A. Panda, X. Xiao and S. R. Forrest, Appl. Phys. Lett., 2012, 101, 063303.
177. I. J. Curtin and R. J. Holmes, Adv. Energy Mater., 2018, 8, 1702339.
178. S. M. Menke, W. A. Luhman and R. J. Holmes, Nat. Mater., 2013, 12, 152–157.
179. B. Verreet, S. Schols, D. Cheyns, B. P. Rand, H. Gommans, T. Aernouts, P. Heremans and J. Genoe, J. Mater. Chem., 2009, 19, 5295–5297.
180. J. D. Dang, D. S. Josey, A. J. Lough, Y. Li, A. Sifate, Z.-H. Lu and T. P. Bender, J. Mater. Chem. A, 2016, 4, 9566–9577.
181. D. Cheyns, B. P. Rand and P. Heremans, Appl. Phys. Lett., 2010, 97, 033301.
182. B. Ma, C. H. Woo, Y. Miyamoto and J. M. J. Fréchet, Chem. Mater., 2009, 21, 1413–1417.
183. G. Chen, H. Sasabe, T. Sano, X.-F. Wang, Z. Hong, J. Kido and Y. Yang, Nanotechnology, 2013, 24, 484007.
184. C. E. Mauldin, C. Piliego, D. Poulsen, D. A. Unruh, C. Woo, B. Ma, J. L. Mynar and J. M. J. Fréchet, ACS Appl. Mater. Interfaces, 2010, 2, 2833–2838.
185. J. Endres, I. Pelczer, B. P. Rand and A. Kahn, Chem. Mater., 2016, 28, 794–801.
186. B. Verreet, K. Cnops, D. Cheyns, P. Heremans, A. Stesmans, G. Zango, C. G. Claessens, T. Torres and B. P. Rand, Adv. Energy Mater., 2014, 4, 1301413.
187. H. Gommans, T. Aernouts, B. Verreet, P. Heremans, A. Medina, C. G. Claessens and T. Torres, Adv. Funct. Mater., 2009, 19, 3435–3439.
188. N. Beaumont, J. S. Castrucci, P. Sullivan, G. E. Morse, A. S. Paton, Z.-H. Lu, T. P. Bender and T. S. Jones, J. Phys. Chem. C, 2014, 118, 14813–14823.
189. B. Verreet, B. P. Rand, D. Cheyns, A. Hadipour, T. Aernouts, P. Heremans, A. Medina, C. G. Claessens and T. Torres, Adv. Energy Mater., 2011, 1, 565–568.
190. K. L. Sampson, G. E. Morse and T. P. Bender, ACS Appl. Energy Mater., 2018, 1, 2490–2501.
191. N. Beaumont, S. W. Cho, P. Sullivan, D. Newby, K. E. Smith and T. S. Jones, Adv. Funct. Mater., 2012, 22, 561–566.
192. P. Sullivan, A. Duraud, L. Hancox, N. Beaumont, G. Mirri, J. H. R. Tucker, R. A. Hatton, M. Shipman and T. S. Jones, Adv. Energy Mater., 2011, 1, 352–355.
193. K. Cnops, G. Zango, J. Genoe, P. Heremans, M. V. Martinez-Diaz, T. Torres and D. Cheyns, J. Am. Chem. Soc., 2015, 137, 8991–8997.
194. P. Sullivan, S. Schumann, R. Da Campo, T. Howells, A. Duraud, M. Shipman, R. A. Hatton and T. S. Jones, Adv. Energy Mater., 2013, 3, 239–244.
195. G. E. Morse, J. L. Gantz, K. X. Steirer, N. R. Armstrong and T. P. Bender, ACS Appl. Mater. Interfaces, 2014, 6, 1515–1524.
196. D. S. Josey, G. L. Ingram, R. K. Garner, J. M. Wang, G. J. Evans, Z.-H. Lu and T. P. Bender, ACS Appl. Energy Mater., 2019, 2, 979–986.
197. R. K. Garner, D. S. Josey, S. R. Nyikos, A. Dovijarski, J. M. Wang, G. J. Evans and T. P. Bender, Sol. Energy Mater. Sol. Cells, 2018, 176, 331–335.
198. D. S. Josey, S. R. Nyikos, R. K. Garner, A. Dovijarski, J. S. Castrucci, J. M. Wang, G. J. Evans and T. P. Bender, ACS Energy Lett., 2017, 2, 726–732.
199. K. Cnops, B. P. Rand, D. Cheyns, B. Verreet, M. A. Empl and P. Heremans, Nat. Commun., 2014, 5, 3406.
200. B. Ebenhoch, N. B. A. Prasetya, V. M. Rotello, G. Cooke and I. D. W. Samuel, J. Mater. Chem. A, 2015, 3, 7345–7352.
201. C. Duan, G. Zango, M. García Iglesias, F. J. M. Colberts, M. M. Wienk, M. V. Martínez-Díaz, R. A. J. Janssen and T. Torres, Angew. Chem., Int. Ed., 2017, 56, 148–152.
202. H. Hang, Z. Zhang, X. Wu, Y. Chen, H. Li, W. Wang, H. Tong and L. Wang, J. Mater. Chem. C, 2018, 6, 7141–7148.
203. H. Hang, X. Wu, Q. Xu, Y. Chen, H. Li, W. Wang, H. Tong and L. Wang, Dyes Pigm., 2019, 160, 243–251.
204. X. Huang, M. Hu, X. Zhao, C. Li, Z. Yuan, X. Liu, C. Cai, Y. Zhang, Y. Hu and Y. Chen, Org. Lett., 2019, 21, 3382–3386.
205. T. Huang, H. Chen, J. Feng, A. Zhang, W. Jiang, F. He and Z. Wang, ACS Mater. Lett., 2019, 1, 404–409.
206. C. Dou, X. Long, Z. Ding, Z. Xie, J. Liu and L. Wang, Angew. Chem., Int. Ed., 2016, 55, 1436–1440.
207. C. Dou, J. Liu and L. Wang, Sci. China: Chem., 2017, 60, 450–459.
208. F. Liu, J. Liu and L. Wang, Acta Phys. – Chim. Sin., 2019, 35, 251–256.
209. F. Liu, Z. Ding, J. Liu and L. Wang, Chem. Commun., 2017, 53, 12213–12216.
210. F. Liu, J. Liu and L. Wang, Chem. – Asian J., 2020, 15, 3314–3320.
211. F. Liu, J. Liu and L. Wang, Org. Chem. Front., 2019, 6, 1996–2003.
212. N. Matsumi, K. Naka and Y. Chujo, J. Am. Chem. Soc., 1998, 120, 5112–5113.
213. C. D. Entwistle and T. B. Marder, Chem. Mater., 2004, 16, 4574–4585.
214. N. Baser-Kirazli, R. A. Lalancette and F. Jäkle, Angew. Chem., Int. Ed., 2020, 59, 8689–8697.
215. X. Yin, J. Chen, R. A. Lalancette, T. B. Marder and F. Jäkle, Angew. Chem., Int. Ed., 2014, 53, 9761–9765.
216. Y. Yu, C. Dong, A. F. Alahmadi, B. Meng, J. Liu, F. Jäkle and L. Wang, J. Mater. Chem. C, 2019, 7, 7427–7432.
217. Y. Yu, B. Meng, F. Jäkle, J. Liu and L. Wang, Chem. – Eur. J., 2020, 26, 873–880.
218. T. A. Welsh, A. Laventure, A. F. Alahmadi, G. Zhang, T. Baumgartner, Y. Zou, F. Jäkle and G. C. Welch, ACS Appl. Energy Mater., 2019, 2, 1229–1240.
219. S. Shimizu, T. Iino, A. Saeki, S. Seki and N. Kobayashi, Chem. – Eur. J., 2015, 21, 2893–2904.
220. R. Feng, N. Sato, T. Yasuda, H. Furuta and S. Shimizu, Chem. Commun., 2020, 56, 2975–2978.
221. P. Li, Q. Liang, E. Y.-H. Hong, C.-Y. Chan, Y.-H. Cheng, M.-Y. Leung, M.-Y. Chan, K.-H. Low, H. Wu and V. W.-W. Yam, Chem. Sci., 2020, 11, 11601–11612.
222. M. M. Morgan, M. Nazari, T. Pickl, J. M. Rautiainen, H. M. Tuononen, W. E. Piers, G. C. Welch and B. S. Gelfand, Chem. Commun., 2019, 55, 11095–11098.
223. J. M. Farrell, C. Mützel, D. Bialas, M. Rudolf, K. Menekse, A.-M. Krause, M. Stolte and F. Würthner, J. Am. Chem. Soc., 2019, 141, 9096–9104.
224. A. He, Y. Qin, W. Dai and X. Luo, Dyes Pigm., 2019, 162, 671–679.
225. S. P. Economopoulos, C. L. Chochos, H. A. Ioannidou, M. Neophytou, C. Charilaou, G. A. Zissimou, J. M. Frost, T. Sachetan, M. Shahid, J. Nelson, M. Heeney, D. D. C. Bradley, G. Itskos, P. A. Koutentis and S. A. Choulis, RSC Adv., 2013, 3, 10221–10229.
226. D. Baran, S. Tuladhar, S. P. Economopoulos, M. Neophytou, A. Savva, G. Itskos, A. Othonos, D. D. C. Bradley, C. J. Brabec, J. Nelson and S. A. Choulis, Synth. Met., 2017, 226, 25–30.
227. B. C. Popere, A. M. Della Pelle and S. Thayumanavan, Macromolecules, 2011, 44, 4767–4776.
228. M. Ozdemir, S. W. Kim, H. Kim, M.-G. Kim, B. J. Kim, C. Kim and H. Usta, Adv. Electron. Mater., 2018, 4, 1700354.
229. B. Liu, Z. Ma, Y. Xu, Y. Guo, F. Yang, D. Xia, C. Li, Z. Tang and W. Li, J. Mater. Chem. C, 2020, 8, 2232–2237.
230. B. Kim, B. Ma, V. R. Donuru, H. Liu and J. M. J. Fréchet, Chem. Commun., 2010, 46, 4148–4150.
231. W. He, Y. Jiang and Y. Qin, Polym. Chem., 2014, 5, 1298–1304.
232. D. Cortizo-Lacalle, C. T. Howells, S. Gambino, F. Vilela, Z. Vobecka, N. J. Findlay, A. R. Inigo, S. A. J. Thomson, P. J. Skabara and I. D. W. Samuel, J. Mater. Chem., 2012, 22, 14119–14126.
233. B. M. Squeo, N. Gasparini, T. Ameri, A. Palma-Cando, S. Allard, V. G. Gregoriou, C. J. Brabec, U. Scherf and C. L. Chochos, J. Mater. Chem. A, 2015, 3, 16279–16286.
234. G. Tarafdar, U. K. Pandey, S. Sengupta and P. C. Ramamurthy, Sol. Energy, 2019, 186, 215–224.
235. L. Bucher, N. Desbois, P. D. Harvey, C. P. Gros and G. D. Sharma, ACS Appl. Mater. Interfaces, 2018, 10, 992–1004.
236. L. Bucher, N. Desbois, P. D. Harvey, C. P. Gros, R. Misra and G. D. Sharma, ACS Appl. Energy Mater., 2018, 1, 3359–3368.
237. Y. Guo, D. Xia, B. Liu, H. Wu, C. Li, Z. Tang, C. Xiao and W. Li, Macromolecules, 2019, 52, 8367–8373.
238. Y. Wang, J. Miao, C. Dou, J. Liu and L. Wang, Polym. Chem., 2020, 11, 5750–5756.
239. A. Wakamiya, T. Taniguchi and S. Yamaguchi, Angew. Chem., Int. Ed., 2006, 45, 3170–3173.
240. C. Dou, Z. Ding, Z. Zhang, Z. Xie, J. Liu and L. Wang, Angew. Chem., Int. Ed., 2015, 54, 3648–3652.
241. Z. Zhang, Z. Ding, C. Dou, J. Liu and L. Wang, Polym. Chem., 2015, 6, 8029–8035.
242. R. Zhao, B. Lin, J. Feng, C. Dou, Z. Ding, W. Ma, J. Liu and L. Wang, Macromolecules, 2019, 52, 7081–7088.
243. R. Zhao, C. Dou, Z. Xie, J. Liu and L. Wang, Angew. Chem., Int. Ed., 2016, 55, 5313–5317.
244. N. Wang, S. Zhang, R. Zhao, J. Feng, Z. Ding, W. Ma, J. Hu and J. Liu, ACS Appl. Electron. Mater., 2020, 2, 2274–2281.
245. Y. Wang, N. Wang, Q. Yang, J. Zhang, J. Liu and L. Wang, J. Mater. Chem. A, 2021, 9, 21071–21077.
246. R. Zhao, Y. Min, C. Dou, B. Lin, W. Ma, J. Liu and L. Wang, ACS Appl. Polym. Mater., 2020, 2, 19–25.
247. C. Dong, S. Deng, B. Meng, J. Liu and L. Wang, Angew. Chem., Int. Ed., 2021, 60, 16184–16190.
248. R. Zhao, C. Dou, J. Liu and L. Wang, Chin. J. Polym. Sci., 2017, 35, 198–206.
249. R. Zhao, Y. Min, C. Dou, J. Liu and L. Wang, Chem. – Eur. J., 2017, 23, 9486–9490.
250. T. Wang, C. Dou, J. Liu and L. Wang, Chem. – Eur. J., 2018, 24, 13043–13048.
251. X. Long, J. Yao, F. Cheng, C. Dou and Y. Xia, Mater. Chem. Front., 2019, 3, 70–77.
252. X. Long, D. Li, B. Wang, Z. Jiang, W. Xu, B. Wang, D. Yang and Y. Xia, Angew. Chem., Int. Ed., 2019, 58, 11369–11373.
253. X. Long, C. Dou, J. Liu and L. Wang, Chin. Chem. Lett., 2018, 29, 1343–1346.
254. X. Long, N. Wang, Z. Ding, C. Dou, J. Liu and L. Wang, J. Mater. Chem. C, 2016, 4, 9961–9967.
255. Z. Ding, X. Long, C. Dou, J. Liu and L. Wang, Chem. Sci., 2016, 7, 6197–6202.
256. X. Long, C. Dou, J. Liu and L. Wang, Macromolecules, 2017, 50, 8521–8528.
257. J. Miao, H. Li, T. Wang, Y. Han, J. Liu and L. Wang, J. Mater. Chem. A, 2020, 8, 20998–21006.
258. Z. Ding, X. Long, B. Meng, K. Bai, C. Dou, J. Liu and L. Wang, Nano Energy, 2017, 32, 216–224.
259. X. Long, Z. Ding, C. Dou, J. Zhang, J. Liu and L. Wang, Adv. Mater., 2016, 28, 6504–6508.
260. R. Zhao, Z. Bi, C. Dou, W. Ma, Y. Han, J. Liu and L. Wang, Macromolecules, 2017, 50, 3171–3178.
261. R. Zhao, N. Wang, Y. Yu and J. Liu, Chem. Mater., 2020, 32, 1308–1314.
262. N. Wang, X. Long, Z. Ding, J. Feng, B. Lin, W. Ma, C. Dou, J. Liu and L. Wang, Macromolecules, 2019, 52, 2402–2410.
263. N. Wang, Y. Yu, R. Zhao, Z. Ding, J. Liu and L. Wang, Macromolecules, 2020, 53, 3325–3331.
264. Z. Ding, R. Zhao, Y. Yu and J. Liu, J. Mater. Chem. A, 2019, 7, 26533–26539.
265. Z. Zhang, Z. Ding, D. J. Jones, W. W. H. Wong, B. Kan, Z. Bi, X. Wan, W. Ma, Y. Chen, X. Long, C. Dou, J. Liu and L. Wang, Sci. China: Chem., 2018, 61, 1025–1033.
266. Z. Zhang, Z. Ding, J. Miao, J. Xin, W. Ma, C. Dou, J. Liu and L. Wang, J. Mater. Chem. C, 2019, 7, 10521–10529.
267. Z. Zhang, Z. Ding, X. Long, C. Dou, J. Liu and L. Wang, J. Mater. Chem. C, 2017, 5, 6812–6819.
268. Z. Zhang, J. Miao, Z. Ding, B. Kan, B. Lin, X. Wan, W. Ma, Y. Chen, X. Long, C. Dou, J. Zhang, J. Liu and L. Wang, Nat. Commun., 2019, 10, 3271.
269. Z. Zhang, T. Wang, Z. Ding, J. Miao, J. Wang, C. Dou, B. Meng, J. Liu and L. Wang, Macromolecules, 2019, 52, 8682–8689.
270. J. Miao, B. Meng, Z. Ding, J. Liu and L. Wang, J. Mater. Chem. A, 2020, 8, 10983–10988.
271. C. Zhu, Z.-H. Guo, A. U. Mu, Y. Liu, S. E. Wheeler and L. Fang, J. Org. Chem., 2016, 81, 4347–4352.
272. Y. Li, B. Pang, H. Meng, Y. Xiang, Y. Li and J. Huang, Tetrahedron Lett., 2019, 60, 151286.
273. Y. Li, H. Meng, T. Liu, Y. Xiao, Z. Tang, B. Pang, Y. Li, Y. Xiang, G. Zhang, X. Lu, G. Yu, H. Yan, C. Zhan, J. Huang and J. Yao, Adv. Mater., 2019, 31, 1904585.
274. T. Liu, T. Yang, R. Ma, L. Zhan, Z. Luo, G. Zhang, Y. Li, K. Gao, Y. Xiao, J. Yu, X. Zou, H. Sun, M. Zhang, T. A. Dela Peña, Z. Xing, H. Liu, X. Li, G. Li, J. Huang, C. Duan, K. S. Wong, X. Lu, X. Guo, F. Gao, H. Chen, F. Huang, Y. Li, Y. Li, Y. Cao, B. Tang and H. Yan, Joule, 2021, 5, 914–930.
275. H. Meng, Y. Li, B. Pang, Y. Li, Y. Xiang, L. Guo, X. Li, C. Zhan and J. Huang, ACS Appl. Mater. Interfaces, 2020, 12, 2733–2742.
276. Y. Xiang, H. Meng, Q. Yao, Y. Chang, H. Yu, L. Guo, Q. Xue, C. Zhan, J. Huang and G. Chen, Macromolecules, 2020, 53, 9529–9538.
277. P. G. Campbell, A. J. V. Marwitz and S.-Y. Liu, Angew. Chem., Int. Ed., 2012, 51, 6074–6092.
278. R. G. Brandt, S. G. Sveegaard, M. Xiao, W. Yue, Z. Du, M. Qiu, D. Angmo, T. R. Andersen, F. C. Krebs, R. Yang and D. Yu, J. Mater. Chem. C, 2016, 4, 4393–4401.
279. S. Pang, Z. Wang, X. Yuan, L. Pan, W. Deng, H. Tang, H. Wu, S. Chen, C. Duan, F. Huang and Y. Cao, Angew. Chem., Int. Ed., 2021, 60, 8813–8817.
280. B. Meng, Y. Ren, J. Liu, F. Jäkle and L. Wang, Angew. Chem., Int. Ed., 2018, 57, 2183–2187.
281. M. Grandl, J. Schepper, S. Maity, A. Peukert, E. von Hauff and F. Pammer, Macromolecules, 2019, 52, 1013–1024.

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