Synthesis and photovoltaic properties of a 2D-conjugated copolymer based on benzodithiophene with alkylthio-selenophene side chain

Abstract

A new 2D-conjugated copolymer (PBDTSe-S-TT) based on alkylthio-selenophene substituted benzodithiophene (BDTSe-S) and 2-ethylhexyl 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT) was designed and synthesized for application as a donor material in polymer solar cells (PSCs). PBDTSe-S-TT shows a broad absorption in the wavelength range from 300 to 800 nm, and a lower highest occupied molecular orbital (HOMO) energy level of −5.33 eV. The PSCs based on PBDTSe-S-TT as donor and [6, 6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as acceptor with 3% DIO as additive exhibited a power conversion efficiency (PCE) of 7.57%, under the illumination of AM 1.5 G, 100 mW cm⁻². These results indicate that attaching an alkylthio-selenophene side chain in 2D-conjugated polymers could be an alternative method to enhance the V_oc and PCE of the PSCs.

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1. Introduction

In the past few years, polymer solar cells (PSCs) have been a hot research topic in the photoelectric conversion field because of their advantages of light weight, low cost and the possibility to fabricate flexible large-area devices.^1–4 A conjugated polymer donor is the key photovoltaic material for high performance PSCs. Therefore, many conjugated polymers have been designed and synthesized for application as donor materials in PSCs.^5–10 Among the conjugated polymer donor materials, the two-dimension (2D)-conjugated copolymers based on bithienyl-benzodithiophene (BDTT) have attracted tremendous interests because of their broader absorption, higher hole mobility and outstanding photovoltaic properties.^11–15 In order to further tune the electronic energy levels of the 2D-conjugated polymers, the conjugated side chain of phenylene with alkyl and/or fluorine substituents was introduced on BDT unit to lower HOMO energy levels of the polymers and thus to realize higher open-circuit voltage (V_oc) and higher PCE of PSCs.^16,17 Recently, Cui et al. introduced alkylthio substituent on the thiophene conjugated side chains of the 2D-conjugated polymer and synthesized PBDTT-S-TT.¹⁸ In comparison with the corresponding polymer PBDTT-TT with alkyl substituent on thiophene conjugated side chain, PBDTT-S-TT possesses a slightly red-shifted absorption and down-shifted HOMO energy level, so that the PSCs with PBDTT-S-TT as donor demonstrated a higher open-circuit voltage (V_oc) and improved power conversion efficiency (PCE). The lower HOMO energy level of PBDTT-S-TT is mainly because of the π-acceptor capability of sulfur atom in the alkylthio substituent due to the formation of p_π(C)–d_π(S) orbital overlap where divalent sulfur accepts π-electron from the π-orbital of the carbon–carbon double bond in thiophene into its empty 3d-orbitals.^18,19

On the other hand, over the past few years, heterocycles such as furan and selenophene were also introduced into the conjugated polymer donor materials for improving their photovoltaic performance.^15c,20–22 Sulfur and selenium have similar atomic radius and electronegativity, and the conjugated five-membered heterocycles of thiophene and selenophene have similar dipole moment and aromaticity.²³ The relatively lower aromaticity of selenophene increases the ground-state quinoid resonance character of its resulting polymers, leading to improved planarity, increased effective conjugation length, and lower band-gap energy.^24,25 In addition, the larger polarizable radius of selenium leads to stronger intermolecular interactions and higher degree of rigidity. In 2014, Hou et al. reported three conjugated polymers with furan, thiophene and selenophene as side groups and investigated the effect of side groups on the photovoltaic properties of these polymers.^21d The polymers with thiophene and selenophene as side groups show similar photovoltaic performance with outstanding PCE of 9.0% and 8.78%, respectively. Recently, Jiang and Wei et al. found that the selenophene substitution on BDT results in a down-shifted HOMO energy level.^21c

Based on the effect of alkylthio substituent and selenophene conjugated side chain on the HOMO energy level and photovoltaic performance of the 2D-conjugated polymers mentioned above, here we introduced alkylthio-selenophene conjugated side chain on BDT unit (PBDTSe-S) and synthesized a new 2D-conjugated polymer PBDTSe-S-TT copolymerized with 2-ethylhexyl 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT), as shown in Scheme 1, in order to further improve photovoltaic performance of the polymer and increase V_oc of the corresponding devices for possible application in tandem PSCs. PBDTSe-S-TT shows a broad absorption in the wavelength range from 300 to 800 nm and a lower HOMO energy level of −5.33 eV. The PCE of the PSCs based on PBDTSe-S-TT/PC₇₁BM reached 7.57% with a higher V_oc of 0.81 V.

Scheme 1 Molecular structure and synthetic route of PBDTSe-S-TT.

2. Experimental section

2.1. Chemicals

All chemicals and solvents were reagent grades and purchased from J&K, Alfa Aesar, and TCI Chemical, respectively. Monomer TT was purchased from Suna Tech Inc. company.

The detailed fabrication and characterization of the PSC devices were described in ESI.†

2.2. Synthesis of the monomer and polymer

Compound (1). Selenophene (6.55 g, 50 mmol, 2.25 eq.) was dissolved into 50 mL dry THF, and cooled to 0 °C under argon protection. Then 20 mL n-BuLi (2.5 M in THF, 50 mmol) was added dropwise over 15 min. After the mixture was stirred at 0 °C for 1 h, sulfur powder (1.6 g, 50 mmol) was added, and then the resulting suspension was stirred at 0 °C for 2 h. Subsequently, 2-ethylhexylbromide (9.7 g, 50 mmol) was added dropwise. The reaction mixture was stirred overnight at room temperature. Then, ice-water containing NH₄Cl was added to the reaction, and the mixture was extracted with dichloromethane, washed with water, and dried over MgSO₄. After the removal of solvent, purification was carried out by silica gel column chromatography using petroleum ether as the eluent and compound 1 (12.4 g, yield 90%) was obtained as a light-yellow oil. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.99–7.98 (d, 1H), 7.23–7.22 (d, 1H), 7.17–7.14 (m, 1H), 2.87–2.85 (d, 2H), 1.59–1.57 (m, 1H), 1.48–1.23 (m, 8H), 0.91–0.85 (m, 6H). ¹³C NMR (400 MHz, CDCl₃), δ (ppm): 142.72, 134.03, 133.64, 129.88, 44.62, 39.11, 32.29, 28.93, 25.52, 23.16, 14.30, 10.92.

Compound (2). 17.6 mL of n-BuLi (44 mmol, 2.5 M in THF) was added dropwise into compound 1 (11 g, 40 mmol) in 100 mL of dry THF at 0 °C under argon protection. The mixture was then warmed to room temperature and stirred for 1 hour. Subsequently, compound benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (2.2 g, 10 mmol) was added to the reaction mixture, and then stirred at 50 °C for 2 h. After cooling to room temperature, a mixture of SnCl₂·2H₂O (17.9 g, 80 mmol) in 10% HCl (30 mL) was added and the mixture was stirred for an additional 2 h. After that, the reaction mixture was poured into ice-water and extracted with diethyl ether. The organic layer was washed with water and then dried over MgSO₄. After removal of the solvent, the crude product was purified by column chromatography on a silica gel using hexane as the eluent to afford compound 2 (3.3 g, yield 45%) as a yellow solid. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.63–7.61 (d, 2H), 7.47–7.46 (d, 2H), 7.43–7.42 (d, 2H), 7.34–7.33 (d, 2H), 2.98–2.97 (d, 4H), 1.72–1.66 (m, 2H), 1.53–1.26 (m, 16H), 0.94–0.91 (m, 12H). ¹³C NMR (400 MHz, CDCl₃), δ (ppm): 147.32, 144.89, 138.98, 136.43, 133.63, 130.80, 128.02, 126.07, 123.47, 44.42, 39.39, 32.45, 29.03, 25.68, 23.18, 14.35, 11.04.

Compound (3). Compound 2 (2.2 g, 30 mmol) and 40 mL dry THF were added into a 250 mL two-necked round-bottom flask under argon protection. The solution was cooled to −78 °C and then 30 mL of n-BuLi (75 mmol, 2.5 M in THF) was added dropwise. The mixture was then stirred at this temperature for 1 h. Subsequently, 90 mL trimethyltinchloride (1.0 M in hexane, 90 mmol) was added in several portions and the mixture was stirred for 3 h at ambient temperature. Then, the mixture was extracted with diethyl ether, the combined organic phase was washed with water, and dried over MgSO₄ for overnight. Then filter the MgSO₄ with Buchner funnel, After removal the solvent by rotary evaporators, the crude product was purified by recrystallization using ethanol to obtain the compound 3 as a yellow solid (2.5 g, yield 79%). ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.67 (s, 2H), 7.45–7.44 (d, 2H), 7.35–7.34 (d, 2H), 2.99–2.98 (d, 4H), 1.72–1.69 (m, 2H), 1.53–1.22 (m, 16H), 0.93–0.91 (m, 12H), 0.48–0.33 (t, 18H). ¹³C NMR (400 MHz, CDCl₃), δ (ppm): 148.28, 144.33, 143.16, 137.24, 136.72, 133.67, 131.15, 130.61, 124.39, 44.37, 39.43, 32.47, 29.03, 25.71, 23.18, 14.36, 11.08, −8.08. MS (MALDI:TOF) m/z: calcd for C₄₀H₅₈S₄Se₂Sn₂ [M], 1062.49; found, 1062.003.

2.3. PBDTSe-S-TT

Compound 3 (0.318 g, 0.3 mmol) and monomer TT (0.142 g, 0.3 mmol) were dissolved in 10 mL toluene and 2 mL DMF in a 50 mL double-neck round-bottom flask. The reaction container was purged with argon for 10 min to remove O₂, and then Pd(PPh₃)₄ (21 mg) was added. After another flushing with argon for 10 min, the reactant was heated to 110 °C for 12 h. The reactant was cooled down to room temperature and poured into methanol (100 mL), then filtered through a Soxhlet thimble, which was then subjected to Soxhlet extraction with methanol, hexane, and chloroform. The polymer was recovered as solid from the chloroform fraction by precipitation from methanol. The solid was dried under vacuum. Yield: 238 mg (76%). GPC: M_w = 126.03 K; M_n = 40.50 K; PDI = 3.11. Anal. calcd for C₄₉H₅₉FO₂S₆Se₂ (%): C, 56.09; H, 5.67; found (%): C, 56.93; H, 6.04.

3. Results and discussion

3.1. Synthesis and characterization of the polymer

The synthetic route of PBDTSe-S-TT was shown in Scheme 1. The polymer PBDTSe-S-TT was synthesized by the Stille coupling reaction under the action of Pd(PPh₃)₄, and was purified in turn by extraction with methanol, hexane and chloroform. PBDTSe-S-TT possesses good solubility in chloroform, chlorobenzene, and o-dichlorobenzene. The molecular weight of PBDTSe-S-TT was estimated by high temperature gel-permeation chromatography (GPC) using 1,2,4-trichlorobenzene as the eluent at 160 °C. The number-average molecular weight (M_n) of PBDTSe-S-TT was 40.50 kDa, with polydispersity (PDI) of 3.11. TGA analysis reveals that the onset temperature of 5% weight-loss of PBDTSe-S-TT is 357 °C (see Fig. S1 in ESI†) which indicates that the thermal stability of PBDTSe-S-TT is good enough for its application in PSCs. The hole mobility of PBDTSe-S-TT is 2.09 × 10⁻⁴ cm² V⁻¹ s⁻¹, which was measured by the space-charge-limited current (SCLC) method with the device structure of ITO/PEDOT:PSS/PBDTSe-S-TT/Au (see Fig. S2 in ESI†).

3.2. Absorption spectra

The absorption spectra of PBDTSe-S-TT in dilute o-dichlorobenzene solution and thin solid film are shown in Fig. 1a. The polymer PBDTSe-S-TT showed broad and strong absorption in the wavelength region from 300 nm to 800 nm. The polymer in solution exhibited an absorption peak at 653 nm. The maximum absorption of PBDTSe-S-TT film appeared at 663 nm; a slightly redshift of 10 nm relative to that in solution. Additionally, a strong vibronic shoulder peak at 697 nm implies strong intermolecular interaction in solid state. The absorption onset of PBDTSe-S-TT film locates at 800 nm, corresponding to an optical bandgap of 1.55 eV. The maximum absorption coefficient of PBDTSe-S-TT film at 649 nm is 0.46 × 10⁵ cm⁻¹, which is greater than that of other similar copolymers.^18,21d The high absorption coefficient of PBDTSe-S-TT in the film is beneficial for the improvement of J_sc.

Fig. 1 (a) UV-vis absorption spectra of PBDTSe-S-TT in o-dichlorobenzene solution and film; (b) cyclic voltammogram of PBDTSe-S-TT film on a glassy carbon electrode measured in a 0.1 mol L⁻¹ Bu₄NPF₆ acetonitrile solution at a scan rate of 50 mV s⁻¹.

3.3. Electrochemical properties

Electrochemical cyclic voltammetry (CV) was performed to determine the HOMO and LUMO energy levels of PBDTSe-S-TT. Fig. 1b shows the cyclic voltammogram of PBDTSe-S-TT film on a glassy carbon electrode in a 0.1 mol L⁻¹ Bu₄NPF₆ acetonitrile solution at a scan rate of 50 mV s⁻¹. The HOMO and LUMO energy levels are estimated from the onset potentials of oxidation and reduction waves. The CV results indicate that the HOMO and LUMO energy levels of PBDTSe-S-TT are −5.33 eV and −3.46 eV, respectively, corresponding to an electrochemical bandgap of 1.87 eV. The electrochemical band gap is slightly higher than optical band gap (1.55 eV), because the electron transfer for the oxidation and reduction needs to overcome the energy barrier at the electrode/solution interface which results in higher electrochemical bandgap, whereas the optical band gap is the energetic difference between the ground and the excited states.²⁶ It's worth noting that the HOMO energy levels of PBDTSe-S-TT with an alkylthio selenophene side chain is slightly lower than other analogue polymers,^18,27 which is beneficial for achieving a higher V_oc in PSCs.

3.4. Theoretical calculations

Theoretical calculations were performed by using the density functional theory (DFT) at the B3LYP/6-31G(d,p) basis set to predict the electronic properties and energy levels of PBDTSe-S-TT. To make computation easier, we chose three BDT-TT repeating units as a simplified model of the molecule. In addition, all alkyl side chains in the molecule were replaced by methyl group in the calculation to avoid excessive computation demand. The calculated orbital distribution of HOMO and LUMO and optimized molecular geometries of PBDTSe-S-TT are shown in Fig. 2. The calculated HOMO and LUMO energy levels of PBDTSe-S-TT are −4.95 eV and −2.82 eV, respectively. The molecular orbital distributions in Fig. 2 indicate that the HOMO of PBDTSe-S-TT is delocalized along the whole π-conjugated backbone of the molecule while its LUMO is mainly localized on the TT-based acceptor segment of PBDTSe-S-TT.

Fig. 2 The frontier molecular orbital of PBDTSe-S-TT trimer obtained from DFT calculations at the B3LYP/6-31G (d,p) level.

It should be mentioned that the calculated HOMO and LUMO energy levels of the conjugated polymers are usually higher for some extent than those values measured by experiments, because the simplified molecular structure was used and the interchain interaction in the polymer film was not considered in the theoretical calculation. Actually, the theoretically calculated HOMO and LUMO energy levels of the polymers can only be used for comparison with the calculated energy levels of the polymers with similar molecular structure.

3.5. Photovoltaic properties

In order to investigate the photovoltaic properties of PBDTSe-S-TT, PSCs were fabricated and characterized with PBDTSe-S-TT as donor and PC₇₁BM as acceptor with a conventional device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS)/active layers/Mg (20 nm)/Al (80 nm). The active layers were treated with methanol before deposition of the metal top electrode, as methanol treatment on the active layer of the PSCs can effectively improve the photovoltaic performance of the devices. The active layers were spin-coated from o-DCB blend solution of PBDTSe-S-TT and PC₇₁BM.

The effect of the donor/acceptor (D/A) weight ratios was studied for the optimization of the PSCs. Fig. S2a in ESI† shows the current density–voltage (J–V) characteristics of PSCs based on PBDTSe-S-TT:PC₇₁BM blend film with different D/A weight ratios of 1 : 1.5, 1 : 1.75 and 1 : 2, and the photovoltaic performance data of the devices were listed in Table 1. The power conversion efficiency (PCE) of the PSCs with the D/A weight ratio of 1 : 1.5, 1 : 1.75 and 1 : 2 reached 7.01%, 7.13% and 6.76%, respectively. The relatively higher V_oc of 0.81–0.84 V than other analogue polymers with the low bandgap of 1.55 eV (ref. 18 and 21d) should be benefitted from the lower HOMO energy level (−5.33 eV) of PBDTSe-S-TT. Obviously, among the devices with different D/A weight ratios, the device with the D/A weight ratio of 1 : 1.75 demonstrated the best photovoltaic performance. Therefore, we used the D/A weight ratio of 1 : 1.75 in the following studies.

Table 1 Photovoltaic parameters of the PSCs based on PBDTSe-S-TT:PC₇₁BM with different blend composition ratios and optimization process under illumination of AM 1.5 G, 100 mW cm⁻²

D : A (w/w)	DIO	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)	Active layer thickness (nm)	Hole mobility^b (cm^2 V^−1 s^−1)
a Integrated from EQE.b measured with the device structure of ITO/PEDOT:PSS/active layer/MoO3/Al.
1 : 1.5	—	0.84	14.74 (13.65)^a	56.6	7.01	70	—
1 : 1.75	—	0.82	15.51 (14.40)^a	56.0	7.13	80	4.12 × 10^−6
1 : 2	—	0.81	15.41 (14.23)^a	54.1	6.76	80	—
1 : 1.75	1%	0.81	14.80 (13.23)^a	60.1	7.20	72	7.50 × 10^−6
1 : 1.75	3%	0.81	16.84 (14.60)^a	55.5	7.57	80	1.91 × 10^−5
1 : 1.75	5%	0.82	15.53 (13.68)^a	57.0	7.26	80	8.24 × 10^−6

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In order to improve the morphology of active layer and further enhance the PCE value of the PSCs, 1,8-diiodooctane (DIO) was selected as solvent additive according to the reported literatures with the similar blend systems.^21d The J–V curves of the devices optimized with different ratios of DIO are shown in Fig. 3a, and the photovoltaic performance data of the PSCs were also listed in Table 1. The maximum PCE of the PSCs based on PBDTSe-S-TT:PC₇₁BM (1 : 1.75, w/w) with 3% DIO additive treatment reached 7.57% with a V_oc of 0.81 V, J_sc of 16.84 mA cm⁻², and a FF of 55.5%. Fig. 3b shows the external quantum efficiency (EQE) curves of the PSCs with different device fabrication conditions. It can be seen from the EQE curves, the device based on PBDTSe-S-TT:PC₇₁BM (1 : 1.75, w/w) with 3% DIO treatment exhibits a response range from 320 to 800 nm, with a maximum EQE value of 70% at 475 nm. The enhanced J_sc and improved PCE of the PSCs with 3% DIO additive treatment should result from the improvement of the morphology in active layer.²¹

Fig. 3 (a) J–V characteristics and (b) EQE curves of devices based on PBDTSe-S-TT:PC₇₁BM blend film with different device fabrication conditions.

In order to explore the effect of the additive on the active layer morphology, the surface and bulk morphology of the blend film of PBDTSe-S-TT and PC₇₁BM (1 : 1.75, w/w) without or with 3% DIO treatment were measured by AFM and TEM, as shown in Fig. 4. In the TEM images (Fig. 4a), alternately dark and bright domains can be distinguished, indicating that the networks of PBDTSe-S-TT and PC₇₁BM were formed. When 3% DIO was added as the additive, a fibrillar interpenetrating network is clearly observed with size of ca. 10−20 nm (Fig. 4b), which is beneficial for charge separation and transport. Furthermore, as shown in AFM images (Fig. 4c and d), the root-mean-square (RMS) values are 0.68 and 1.06 nm for the blends films without and with 3% DIO, respectively. The AFM results indicate that the blend film with 3% DIO exhibited a more legible phase separation compared with that of the blend without DIO. The TEM and AFM results are consistent with the optimized photovoltaic performance.

Fig. 4 TEM (left) and tapping-mode AFM topography (right) images of PBDTSe-S-TT:PC₇₁BM (1 : 1.75, w/w), (a, c) without DIO and (b, d) with 3% DIO.

We measured the hole mobility of the blend active layer by SCLC method for studying the effect of the DIO additive treatment on the hole mobility. The device structure for the hole mobility measurement is ITO/PEDOT:PSS/active layer/MoO₃/Al, and the measurement results are shown in Fig. S3 in ESI† and the measured hole mobilities were listed in Table 1. The hole mobility of the blend film of PBDTSe-S-TT:PC₇₁BM (1 : 1.75, w/w) without DIO additive treatment is 4.12 × 10⁻⁶ cm² V⁻¹ s⁻¹. While, the hole mobility of the blend film with 3% DIO treatment increased to 1.91 × 10⁻⁵ cm² V⁻¹ s⁻¹. The increased hole mobility could result in the higher J_sc and PCE for the PSCs with the DIO additive treatment.

It is also worth noting that the higher PCE of 7.57% was obtained for the devices with the thin active layer thickness of ca. 80 nm, which should be attributed to the large absorption coefficient (0.46 × 10⁵ cm⁻¹) of the donor polymer.

4. Conclusions

A new 2D-conjugated copolymer, PBDTSe-S-TT, was designed and synthesized for the application as donor material in PSCs. The polymer with alkylthio-selenophene side chains shows a broad absorption band, a high absorption coefficient and a lower HOMO energy level. The PSCs based on PBDTSe-S-TT:PC₇₁BM (1 : 1.75, w : w) with 3% DIO additive and methanol treatment achieved a promising PCE of 7.57%, under the illumination of AM 1.5 G, 100 mW cm⁻². These results indicate that attaching alkylthio-selenophene side chain in the 2D-conjugated polymer donor should be an alternative method to enhance the V_oc and PCE of the PSCs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (91333204, 91433117, 51203168, 51422306, 21502134), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12030200), the Postdoctoral research start-up funding of Soochow University (32317366, 32317400), and the Jiangsu Postdoctoral Grant (7131707314).

References

1. (a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789–1791; (b) S. Gunes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107, 1324–1338; (c) A. J. Heeger, Chem. Soc. Rev., 2010, 39, 2354–2371; (d) L. Lu, T. Zheng, Q. Wu, A. M. Schneider, D. Zhao and L. Yu, Chem. Rev., 2015, 115, 12666–12731; (e) L. Dou, Y. Liu, Z. Hong, G. Li and Y. Yang, Chem. Rev., 2015, 115, 12633–12665.
2. (a) Y. F. Li, Acc. Chem. Res., 2012, 45, 723–733; (b) Y. Huang, E. J. Kramer, A. J. Heeger and G. C. Bazan, Chem. Rev., 2014, 114, 7006–7043; (c) K. R. Graham, C. Cabanetos, J. P. Jahnke, M. N. Idso, A. E. Labban, G. O. N. Ndjawa, T. Heumueller, K. Vandewal, A. Salleo, B. F. Chmelka, A. Amassian, P. M. Beaujuge and M. D. McGehee, J. Am. Chem. Soc., 2014, 136, 9608–9618; (d) K. A. Mazzio and C. K. Luscombe, Chem. Soc. Rev., 2015, 44, 78–90.
3. (a) G. Li, R. Zhu and Y. Yang, Nat. Photonics, 2012, 6, 153–161; (b) Z. He, H. Wu and Y. Cao, Adv. Mater., 2014, 26, 1006–1024; (c) L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street and Y. Yang, Adv. Mater., 2013, 25, 4962–4965; (d) G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009, 21, 1323–1338.
4. (a) F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394–412; (b) F. C. Krebs, N. Espinosa, M. Hösel, P. R. Søndergaard and M. Jørgensen, Adv. Mater., 2013, 26, 29–39; (c) A. Rao, P. C. Y. Chow, S. Gélinas, C. W. Schlenker, C.-Z. Li, H.-L. Yip, A. K.-Y. Jen, D. S. Ginger and R. H. Friend, Nature, 2013, 500, 435–439; (d) S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297–302.
5. (a) H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649–653; (b) Z.-G. Zhang and Y. Li, Sci. China: Chem., 2015, 58, 192–209; (c) E. Wang, W. Mammo and M. R. Andersson, Adv. Mater., 2014, 26, 1801–1826; (d) M. Zhang, X. Guo, X. Wang, H. Wang and Y. Li, Chem. Mater., 2011, 23, 4264–4270; (e) P. M. Beaujuge and J. M. J. Fréchet, J. Am. Chem. Soc., 2011, 133, 20009–20029.
6. (a) L. Huo, T. Liu, X. Sun, Y. Cai, A. J. Heeger and Y. Sun, Adv. Mater., 2015, 27, 2938–2944; (b) Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade and H. Yan, Nat. Commun., 2014, 5, 5293; (c) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li and Y. Yang, Nat. Commun., 2013, 4, 1446; (d) J. Subbiah, B. Purushothaman, M. Chen, T. Qin, M. Gao, D. Vak, F. H. Scholes, X. Chen, S. E. Watkins, G. J. Wilson, A. B. Holmes, W. W. H. Wong and D. J. Jones, Adv. Mater., 2014, 27, 702–705; (e) J. H. Kim, S. A. Shin, J. B. Park, C. E. Song, W. S. Shin, H. Yang, Y. F. Li and D. H. Hwang, Macromolecules, 2014, 47, 1613–1622; (f) E. D. Głowacki, G. Voss and N. S. Sariciftci, Adv. Mater., 2013, 25, 6783–6800.
7. (a) X. Guo, N. Zhou, S. J. Lou, J. Smith, D. B. Tice, J. W. Hennek, R. P. Ortiz, J. T. L. Navarrete, S. Li, J. Strzalka, L. X. Chen, R. P. H. Chang, A. Facchetti and T. J. Marks, Nat. Photonics, 2013, 7, 825–833; (b) K. Li, Z. Li, K. Feng, X. Xu, L. Wang and Q. Peng, J. Am. Chem. Soc., 2013, 135, 13549–13557; (c) D. Qian, W. Ma, Z. Li, X. Guo, S. Zhang, L. Ye, H. Ade, Z. Tan and J. Hou, J. Am. Chem. Soc., 2013, 135, 8464–8467.
8. (a) C. Cabanetos, A. E. Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Fréchet, M. D. McGehee and P. M. Beaujuge, J. Am. Chem. Soc., 2013, 135, 4656–4659; (b) J. Cao, Q. Liao, X. Du, J. Chen, Z. Xiao, Q. Zuo and L. Ding, Energy Environ. Sci., 2013, 6, 3224–3228; (c) W. Li, K. H. Hendriks, W. S. C. Roelofs, Y. Kim, M. M. Wienk and R. A. J. Janssen, Adv. Mater., 2013, 25, 3182–3186; (d) Y. Deng, J. Liu, J. Wang, L. Liu, W. Li, H. Tian, X. Zhang, Z. Xie, Y. Geng and F. Wang, Adv. Mater., 2014, 26, 471–476.
9. (a) X. Guo, M. Zhang, J. Tan, S. Zhang, L. Huo, W. Hu, Y. Li and J. Hou, Adv. Mater., 2012, 24, 6536–6541; (b) D. Gendron and M. Leclerc, Energy Environ. Sci., 2011, 4, 1225–1237; (c) S. C. Price, A. C. Stuart, L. Yang, H. Zhou and W. You, J. Am. Chem. Soc., 2011, 133, 4625–4631; (d) Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray and L. Yu, J. Am. Chem. Soc., 2009, 131, 7792–7799; (e) C. M. Amb, S. Chen, K. R. Graham, J. Subbiah, C. E. Small, F. So and J. R. Reynolds, J. Am. Chem. Soc., 2011, 133, 10062–10065.
10. (a) H. Zhou, L. Yang, S. C. Price, K. J. Knight and W. You, Angew. Chem., Int. Ed., 2010, 49, 7992–7995; (b) E. Wang, L. Hou, Z. Wang, S. Hellström, F. Zhang, O. Inganäs and M. R. Andersson, Adv. Mater., 2010, 22, 5240–5244; (c) E. Wang, Z. Ma, Z. Zhang, K. Vandewal, P. Henriksson, O. Inganäs, F. Zhang and M. R. Andersson, J. Am. Chem. Soc., 2011, 133, 14244–14247; (d) M. Zhang, X. Guo, W. Ma, H. Ade and J. Hou, Adv. Mater., 2014, 26, 5880–5885.
11. (a) L. Ye, S. Zhang, L. Huo, M. Zhang and J. Hou, Acc. Chem. Res., 2014, 47, 1595–1603; (b) X. Guo, M. Zhang, W. Ma, L. Ye, S. Zhang, S. Liu, H. Ade, F. Huang and J. Hou, Adv. Mater., 2014, 26, 4043–4049; (c) S. Q. Zhang, L. Ye, W. C. Zhao, B. Yang, Q. Wang and J. H. Hou, Sci. China: Chem., 2015, 58, 248–256; (d) J. D. Chen, C. H. Cui, Y. Q. Li, L. Zhou, Q. D. Ou, C. Li, Y. F. Li and J. X. Tang, Adv. Mater., 2015, 27, 1035–1041.
12. (a) L. Huo and J. Hou, Polym. Chem., 2011, 2, 2453–2461; (b) L. Huo, J. Hou, S. Zhang, H.-Y. Chen and Y. Yang, Angew. Chem., Int. Ed., 2010, 49, 1500–1503; (c) L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li and J. Hou, Angew. Chem., Int. Ed., 2011, 50, 9697–9702; (d) W. Li, Q. D. Li, S. J. Liu, C. H. Duan, L. Ying, F. Huang and Y. Cao, Sci. China: Chem., 2015, 58, 257–266; (e) Z. L. Liu, J. M. Sun, Y. X. Zhu, P. Liu, L. J. Zhang, J. W. Chen, F. Huang and Y. Cao, Sci. China: Chem., 2015, 58, 267–275.
13. K. Sun, Z. Xiao, S. Lu, W. Zajaczkowski, W. Pisula, E. Hanssen, J. M. White, R. M. Williamson, J. Subbiah, J. Y. Ouyang, A. B. Holmes, W. W. H. Wong and D. J. Jones, Nat. Commun., 2015, 6, 6013.
14. (a) C. Duan, A. Furlan, J. J. Franeker, R. E. M. Willems, M. M. Wienk and R. A. J. Janssen, Adv. Mater., 2015, 27, 4461–4468; (b) R. Po, G. Bianchi, C. Carbonera and A. Pellegrino, Macromolecules, 2015, 48, 453–461; (c) M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang and Y. Cao, J. Am. Chem. Soc., 2011, 133, 9638–9641; (d) Y. Li, X. P. Xu, Z. J. Li, T. Yu and Q. Peng, Sci. China: Chem., 2015, 58, 276–285.
15. (a) M. Zhang, X. Guo, S. Zhang and J. Hou, Adv. Mater., 2013, 26, 1118–1123; (b) N. Wang, Z. Chen, W. Wei and Z. Jiang, J. Am. Chem. Soc., 2013, 135, 17060–17068; (c) L. Dou, W.-H. Chang, J. Gao, C.-C. Chen, J. You and Y. Yang, Adv. Mater., 2013, 25, 825–831.
16. M. Zhang, X. Guo, W. Ma, S. Zhang, L. Huo, H. Ade and J. Hou, Adv. Mater., 2014, 26, 2089–2095.
17. (a) M. Zhang, Y. Gu, X. Guo, F. Liu, S. Zhang, L. Huo, T. P. Russell and J. Hou, Adv. Mater., 2013, 25, 4944–4949; (b) M. Zhang, X. Guo, W. Ma, H. Ade and J. Hou, Adv. Mater., 2015, 27, 4655–4660.
18. C. H. Cui, W.-Y. Wong and Y. F. Li, Energy Environ. Sci., 2014, 7, 2276–2284.
19. Y.-J. Cheng, J. Luo, S. Huang, X. Zhou, Z. Shi, T.-D. Kim, D. H. Bale, S. Takahashi, A. Yick, B. M. Polishak, S.-H. Jang, L. R. Dalton, P. J. Reid, W. H. Steier and A. K.-Y. Jen, Chem. Mater., 2008, 20, 5047–5054.
20. (a) R. S. Ashraf, I. Meager, M. Nikolka, M. Kirkus, M. Planells, B. C. Schroeder, S. Holliday, M. Hurhangee, C. B. Nielsen, H. Sirringhaus and I. McCulloch, J. Am. Chem. Soc., 2015, 137, 1314–1321; (b) L. Dou, J. Gao, E. Richard, J. You, C.-C. Chen, K. C. Cha, Y. He, G. Li and Y. Yang, J. Am. Chem. Soc., 2012, 134, 10071–10079; (c) C.-C. Chen, L. Dou, J. Gao, W.-H. Chang, G. Li and Y. Yang, Energy Environ. Sci., 2013, 6, 2714–2720.
21. (a) T. Earmme, Y.-J. Hwang, N. M. Murari, S. Subramaniyan and S. A. Jenekhe, J. Am. Chem. Soc., 2013, 135, 14960–14963; (b) T. Earmme, Y.-J. Hwang, S. Subramaniyan and S. A. Jenekhe, Adv. Mater., 2014, 26, 6080–6085; (c) J.-M. Jiang, P. Raghunath, H.-K. Lin, Y.-C. Lin, M. C. Lin and K.-H. Wei, Macromolecules, 2014, 47, 7070–7080; (d) S. Zhang, L. Ye, W. Zhao, D. Liu, H. Yao and J. Hou, Macromolecules, 2014, 47, 4653–4659; (e) J. Warnan, A. E. Labban, C. Cabanetos, E. T. Hoke, P. K. Shukla, C. Risko, J.-L. Brédas, M. D. McGehee and P. M. Beaujuge, Chem. Mater., 2014, 26, 2299–2306.
22. (a) E. H. Jung, S. Bae, T. W. Yoo and W. H. Jo, Polym. Chem., 2014, 5, 6545–6550; (b) J.-S. Wu, J.-F. Jheng, J.-Y. Chang, Y.-Y. Lai, K.-Y. Wu, C.-L. Wang and C.-S. Hsu, Polym. Chem., 2014, 5, 6472–6479; (c) R. L. Uy, L. Yan, W. Li and W. You, Macromolecules, 2014, 47, 2289–2295; (d) W. Zhuang, H. Zhen, R. Kroon, Z. Tang, S. Hellström, L. Hou, E. Wang, D. Gedefaw, O. Inganäs, F. Zhang and M. R. Andersson, J. Mater. Chem. A, 2013, 1, 13422–13425; (e) Y. R. Cheon, Y. J. Kim, J.-J. Ha, M.-J. Kim, C. E. Park and Y.-H. Kim, Macromolecules, 2014, 47, 8570–8577.
23. M. Jeffries-EL, B. M. Kobilka and B. J. Hale, Macromolecules, 2014, 47, 7253–7271.
24. (a) A. Patra and M. Bendikov, J. Mater. Chem., 2010, 20, 422–433; (b) S. S. Zade, N. Zamoshchik and M. Bendikov, Chem.–Eur. J., 2009, 15, 8613–8624.
25. (a) I. Kang, H.-J. Yun, D. S. Chung, S.-K. Kwon and Y.-H. Kim, J. Am. Chem. Soc., 2013, 135, 14896–14899; (b) D. Gao, J. Hollinger and D. S. Seferos, ACS Nano, 2012, 6, 7114–7121; (c) K.-H. Kim, S. Park, H. Yu, H. Kang, I. Song, J. H. Oh and B. J. Kim, Chem. Mater., 2014, 26, 6963–6970.
26. H. J. Son, W. Wagn, T. Xu, Y. Liang, Y. Wu, G. Li and L. Yu, J. Am. Chem. Soc., 2011, 133, 1885–1894.
27. L. Ye, S. Zhang, W. Zhao, H. Yao and J. Hou, Chem. Mater., 2014, 26, 3603–3605.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27596b

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