Achieving ultra-narrow bandgap non-halogenated non-fullerene acceptors via vinylene π-bridges for efficient organic solar cells

We report a series of NFAs (namely BTP-1V, BTP-2V, BTP-3V, and BTP-4V) by inserting vinylene π-bridges between the Y-series central fused core and non-halogenated end group IC.


Introduction
Thanks to their light weight, low-cost solution processing, flexibility, semi-transparent outlook and compatibility with high throughput rollto-roll printing methods, organic solar cells (OSCs) have received extensive research attention. 1,2 With superior photoinduced electron transfer and suitable phase-separation property, fullerene derivates have been widely used as electron acceptors in OSCs for many years. 3 Due to the intrinsic disadvantages of fullerene acceptors (such as weak photon harvesting ability in the long wavelength range and the limited regulation on their molecular energy levels), the improvement of fullerene-based OSCs faces great challenges. 4 Compared with fullerene acceptors, non-fullerene electron acceptors (NFAs) can realize faster carrier separation with negligible driving force, hence achieving lower energy loss (E loss ) and greatly improved efficiencies. 5 With the tremendous opportunities to tune their chemical structures, NFAs with acceptor-donor-acceptor (A-D-A) configuration provide a wealth of materials choice for potentially achieving efficient OSCs. 6,7 Recently, researchers designed acceptor-donor-acceptor'-donoracceptor (A-DA'D-A) structure NFAs namely Y6 reaching marvellous efficiencies of 15.7%. 8 Y6 is a sensational NFA material for pioneering in breaking the power conversion efficiency (PCE) bottleneck of 15% for binary OSCs. The impressive efficiencies in Yseries-based OSCs have been attributed to the wide and high photoresponsivity with a relatively low E loss . Therefore, plenty of works related to this innovative Y-series family have been carried out together with extraordinary PCE surpassing 17% for binary singlejunction OSCs, showing great potential in commercial application. [9][10][11][12][13] However, the modification of the Y-series NFAs is primarily limited to side chain and terminal group engineering, which limits the improvement of their absorption region and PCE. [14][15][16][17][18] Therefore, it is necessary to develop ultra-narrow bandgap (ultra-NBG) NFAs with near-infrared region (NIR) absorption for light harvesting. 19 Recent studies have suggested that the introduction of π-bridges between central fused core and terminal electron-withdrawing units is an effective strategy to broaden the spectrum for NFA-based organic solar cells. The extensively used π-bridge units include alkoxyl or alkylthio thiophene, 20, 21 selenophene, 22 thiazole, 23 benzene, 24 thieno[3,2-b]-thiophene, 22 and cyclopentadithiophene. 25 These conjugated units with different substituent groups usually weaken the planarity of the whole conjugated molecule to some extent. 26 The carbon-carbon double bond group (vinylene) is used to construct D-A narrow bandgap polymers, [27][28][29][30] but was rarely applied in A-DA'D-A type NFAs based PSCs. [31][32][33] So far, there has been no report about gradually introduction of multi vinylene units into the backbone of NFAs to regulate their optoelectronic properties. With these concerns, we envisioned a general modification strategy for Y6, aiming to broaden absorption nature of Y6 while preserving its other excellent structural features. Therefore, we reported a general synthetic approach toward a series of the Y-series NFAs, featuring ultra-NBG (the optical bandgap ≤ 1.28 eV) and various electron accepting abilities. The four Y-series NFAs namely BTP-1V, BTP-2V, BTP-3V, and BTP-4V were designed by introducing different numbers of vinylene units between the central electron-deficient fused benzothiadiazole core and terminal electron-deficient 2-(3-oxo-2,3dihydro-1H-inden-1-ylidene)malononitrile (IC) end-capped unit (Fig.  1). When the number of vinylene was one or three，two asymmetrical Y-series BTP-1V and BTP-3V were obtained. Compared with Y5 34 and Y6 8 , asymmetric vinylene modification of the Y-series NFAs simultaneously extends the conjugation length of molecules to enhance the electron-donating properties, generates a large natural dipole moment, and alters the molecular stacking/packing form in the solid state to some extent. 35,36 The 2-butyloctyl side chain on the nitrogen atom of the centre fused deficient core provides good solubility and crystallinity. These four Y-series NFAs bear the broader absorption range (550~1000 nm), the narrower optical bandgap (E g opt ) (1.21~1.28 eV), and the higher occupied molecular orbital (HOMO) energy levels (-5.54~-5.34 eV), compared to that of the Y6 counterpart. The OSC devices based on these Y-series NFAs blended with the donor polymer PBDB-T or PCE10 were systematically studied. As a result, the best device based on PBDB-T/BTP-1V showed a V oc of 0.84 V, a J sc of 20.86 mA cm -2 , an FF of 0.63 and a PCE of 11.03%, whereas devices based on PCE10/BTP-2V, PCE10/BTP-3V and PCE10/BTP-4V achieved PCE of 7.87%, 2.04%, 1.04%, respectively. BTP-1V-based device demonstrated the best PCE in these ultra-NBG NFAs-based devices. The lower HOMO energy level of BTP-1V, the more balanced charge transport and the better phase separation morphology in PBDB-T/BTP-1V blend was found to be major reasons for the superior performance of BTP-1Vbased devices, compared to that of other BTP-V-based devices. To the best of our knowledge, the PCE of over 11% is the highest value of binary OSCs based on non-halogenated NFA with a bandgap below 1.28 eV. Our work suggests that introducing multiple vinylene πbridges is an alternative strategy to fine-tune the energy level and absorption range, and maintain planarity of whole conjugated molecule, which endows these Y-series NFAs with the huge potential to construct efficient OSCs based on ultra-NBG NFAs.

Optical and electrochemical properties
To better understand the effect of vinylene π-bridges on the optical properties of the four Y-series NFAs, the ultraviolet-visible (UV-Vis) absorption spectra of the four Y-series NFAs in dilute chloroform solution and in film are exhibited in Fig. 2. In dilute chloroform solutions, the absorption edges of these Y-series NFAs shows a gradual and significant red shift along with increasing numbers of vinylene units. These NFAs show strong and broad absorption in the range of 500~900 nm. The maximum absorption values for these NFAs are observed at 737, 756, 768, 760 nm for BTP-1V, BTP-2V, BTP-3V, and BTP-4V, respectively. In films, all four Y-series NFAs exhibit an obvious redshifts absorption and vibration shoulder peaks comparison to that of their solutions, which could be attributed to their strong intermolecular packing in the solid state. The absorption onsets in films for these NFAs are observed at 963, 968, 1020 and 1024 nm, with E g opt of 1.28, 1.28, 1.22 and 1.21 eV for BTP-1V, BTP-2V, BTP-3V and BTP-4V, respectively. BTP-4V shows a very similar absorption profile with BTP-3V, indicating that further extending π Please do not adjust margins Please do not adjust margins conjugated backbone has little pronounced effect on broadening the NIR absorption. Compared with the absorption edge of Y6 film, a large bathochromic shift of 31~87 nm is observed for these BTP-Vbased Y-series NFAs, which indicates that vinylene π-bridges could gradually decrease the bandgap and broaden absorption range. To the best of our knowledge, these BTP-V-based NFAs are the first example of the Y-series NFAs attaching IC end group with a bandgap below 1.28 eV. 8,13,[37][38][39][40][41][42][43][44][45] Generally, fluorine substituted IC and chlorine substituted IC are used as terminal groups to build efficient OSCs based on ultra-NBG NFAs. 18,[42][43][44][45][46][47][48][49][50] It should be noted that BTP-4V has an absorption onset of 1020 nm, providing one of the smallest bandgaps (1.21 eV) among four ultra-NBG Y-series NFAs, despite of attaching non-halogenated IC end group.   The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the four NFAs were measured by electrochemical cyclic voltammetry (CV) measurements with Ag/AgCl as the reference electrode and the Fc + /Fc couple used as the internal standard at a scanning rate of 20 mV s -1 , with CV curves shown in the SI. The onset oxidation/reduction potentials (φ ox/red ) can be obtained from the cyclic voltammograms. According to the equations of E HOMO/LUMO = -e (φ ox/red -φ Fc + /Fc + 4.8) (eV). The E HOMO /E LUMO values of BTP-1V, BTP-2V, BTP-3V, and BTP-4V are estimated to be -5.54 eV/-3.89 eV, -5.43 eV/-3.91 eV, -5.38 eV/-3.90 eV, and -5.34 eV/-3.94 eV, respectively. The electrochemical data of BTP-V-based NFAs are summarized in Table  1. Having obtained the energy level of the four Y-series NFAs, we then compare them in an energy diagram displayed in Fig. 2c. It is worth noting that extending π-conjugated length of the molecular backbone obviously elevates the HOMO energy levels of the BTP-Vbased NFAs, and meanwhile slightly lowers the LUMO energy levels, where it effectively reduces the bandgap of these NFAs due to the electron-donating properties of vinylene π-bridge. Compared with the HOMO value of -5.55 eV and LUMO value of -3.87 eV for Y5, 34

Theoretical Calculation
For rational design of ultra-NBG NFAs, density functional theory (DFT) calculations were employed to screen possible candidates of the Y-series NFAs with rigid backbone, and proper HOMO/LUMO energy levels. Compared with multi-fused ring on central core to enhance electron-donating ability, we adapted for increasing conjugation length of Y5 to narrow its bandgap. The DFT calculations of Y5, BTP-1V, BTP-2V, BTP-3V and BTP-4V were performed with Gaussian at the B3LYP/6-31G (d,p) level to elucidate the influence of inserting vinylene onto backbone of the fused conjugated molecule on molecular geometries and energy level of molecules, where the long alkyl side chains were simplified into methyl groups (Fig. 3). Seen from optimized conformation of five molecules, similar distributions of HOMO and LUMO were seen from Y5 to four BTP-V-based NFAs. The acceptor consists of two planar units, with a twist in the nitrogen atom of centre core attached alkyl groups. The dihedral angle of N-C-C-N were 10.1, 10.0, 9.9, 9.9 and 9.9°C for Y5, BTP-1V, BTP-2V, BTP-3V and BTP-4V, respectively, indicating perfect coplanarity for these four Y-series NFAs. The conjugated skeleton length of these NFAs were designed to have different amounts of vinylene π-bridges, with the aim to subtly regulate the conjugation length of the acceptor while promoting the π-electron delocalization, and thereby facilitating photon absorption and charge transportation in the solid state. The intramolecular H … O conformational lock offered four NFAs a planar structure to promote π-π stacking in the solid state. The HOMO/LUMO energy levels of Y5, BTP-1V, BTP-2V, BTP-3V and BTP-4V were calculated to be -5.74/-3.70, -5.69/-3.70, -5.64/-3.70, -5.58/-3.70, and -5.52/-3.70 eV, respectively, which were consistent with the CV measurements and UV-vis spectra. Increasing in π-conjugated length of backbone of the Y-series NFAs clearly upshifted the HOMO level and basically maintained the LUMO level. According to theory calculation, the HOMO orbital was mainly located at the electron-donating core, while the LUMO orbital was delocalized across the entire molecule with more focus on the electron-withdrawing end groups, facilitating charge transport within and across molecules.

Photovoltaic properties
To investigate the photovoltaic performance of the four Y-series NFAs, single-junction OSCs with a conventional architecture of ITO/PEDOT:PSS/active layer/PNDIT-F3N/Ag (Fig. 2d) were fabricated. The details of device photovoltaic parameters are listed in Table 2. The PEDOT:PSS and PNDIT-F3N 46 were selected as the hole and electron transport layer, respectively. Considering matched energy and complementary absorption, the polymer PBDB-T was selected as the donor material for BTP-1V (PCE10 for BTP-2V, BTP-3V and BTP-4V). The details of device preparation can be found in the ESI † . The current density-voltage (J-V) curves of the optimal BTP-V-based OSC devices are plotted in Fig. 4a, and the detailed photovoltaic parameters are summarized in Table 2. The PCE10/BTP-4V-and PCE10/BTP-3V-based device showed the low PCE of 1~2%, where the values of J sc (3.61 mA cm -2 for BTP-4V, 6.46 mA cm -2 for BTP-3V), V oc of (0.67 V for BTP-4V, 0.72 V for BTP-3V) and FF (0.42 for BTP-4V, 0.44 for BTP-3V) are very poor. When the numbers of the vinyl π-bridges of the BTP-V-based NFAs gradually reduced, the photovoltaic parameters of OSCs dramatically increased. Compared to the PCE10/BTP-3V-based device, the BTP-2V-based device exhibited a much higher PCE of 7.78% due to the increased J sc (19.80 mA cm -2 ), V oc (0.77 V), and FF (0.51). Meanwhile, changing PCE10 to PBDB-T, PBDB/BTP-1V-based device showed a slightly enhanced J sc (20.86 mA cm -2 ), significantly increased V oc (0.84 V) and FF (0.63), and thus a highest PCE of 11.03%. To our knowledge, the PCE of 11% is the highest values for binary single-junction OSCs based on ultra-NBG (E g ≤ 1.28 eV) nonhalogenated NFAs with IC ending groups to date. 34,[47][48][49][50] The V oc is decreased from BTP-1V (0.84 V), BTP-2V (0.77 V), BTP-3V (0.72 V) to BTP-4V (0.67 V), which is consistent with energy level alignment.
The current densities of all best-performance devices were calibrated with the corresponding EQE data. The EQE spectra of the four BTP-V-based devices are shown in Fig. 4b. The integrated J sc values from EQE curves of PBDB-T/BTP-1V-and PCE10/BTP-2Vbased devices are 20.53 and 18.92 mA cm -2 , respectively, matching well with the J sc values from the J-V curves (less than 5% mismatch). Generally, PCE10/BTP-2V-based device exhibits a more broaden EQE response from 300 to 960 nm compared to that of the device based on PBDB-T/BTP-1V. This result is consistent with their UVvisible NIR absorption spectra in thin film. However, PBDB-T/BTP-1V-based device showed much higher average EQE values compared to that of the PCE10/BTP-2V-based device. The better stacking in PBDB-T/BTP-1V films is proposed to be the reason for the higher EQE despite their weaker absorption intensity. A discussion on film morphologies will be covered in later sections to support this point. The higher average EQE values is the main reason for the improved J sc value for the PBDB-T/BTP-1V-based device. As a result, in combination with a large V oc and J sc , the PBDB-T/BTP-1V-based device exhibited a much higher PCE of 11.03% compared to that of the PCE10/BTP-2V-based device. The integrated currents from the EQE spectra of PCE10/BTP-3V-and PCE10/BTP-4V-based devices are relatively low. It is revealed that both devices show low integrated currents due to inefficiently harvesting photons in a broad absorption range.
It is noteworthy that the corresponding energy loss (E loss ) for BTP-1V-based device is calculated to be 0.58 eV. Generally, E loss is defined by E loss = E g -qV oc , where E g refers to the optical bandgap determined by the smaller value of donor or acceptor. 51,52 It is relatively small value among the NIR absorption Y-series NFAs. 8,19,53 The value of E loss is important to overcome the trade-off between V oc and J sc , which could boost the two parameters simultaneously for high efficiency OSCs based on NFAs. Meanwhile, such the small E loss of BTP-1Vbased device is achieved from the Y-series NFAs with an E g opt of 1.28 eV, indicating a brighter future of efficient OSCs based on ultra-NBG Y-series NFAs.

Charge recombination and carrier mobility
The exciton dissociation and charge extraction behaviours of the respective devices were further studied by plotting photo-generated current density (J ph ) against effective voltage (V eff ) (Fig. S27). J ph is defined as J L -J D , V eff is calculated as V o -V a , where J L and J D represent the current densities under the illumination of AM 1.5 G 100 mW/cm 2 and in the dark, V o is the voltage when J ph is zero and V a is the voltage applied voltage externally. The value of J ph /J sat (J sat where is saturation photo-generated current density) is defined as a charge exciton dissociation probability (P diss ). BTP-1V-based device exhibited the highest P diss (93.9%) in the four BTP-V-based devices (82.3% for BTP-2V-based device, 53.4% for BTP-3V-based device, and 50.8% for BTP-4V-based device). It implies that the most  The light intensity (P light ) dependence of J sc was determined to evaluate the charge carrier recombination behaviour of these four OSC devices. The plots of log J sc versus log P light is shown in Fig.  S28. Theoretically, the correlation between J sc and P light α is written as J sc ∝P light α , where α is the bimolecular recombination index. 54 When α value gets close to 1, it implies that all dissociated charges are collected with minimal bimolecular recombination. In our cases, the α values are 0.93, 0.86, 0.83 and 0.85 for BTP-1V-, BTP-2V-, BTP-3V-and BTP-4V-based device, respectively, indicating the most effective carrier collection and the weakest bimolecular recombination in BTP-1V-based device, thus leading to the highest FF. Fig. S29 shows the plots of V oc versus ln P light of the four optimized BTP-V-based OSC devices. For bimolecular recombination in BHJ OSCs, the semilogarithmic plot of V oc as a function of the light intensity should a linear relationship with a slope of 1*kT/q, where k is the Boltzmann constant, T is room temperature (298 K), and q is the elementary charge. 55 In contrast, a slope of 2*kT/q implies that the trap-assisted or monomolecular recombination is the dominating mechanism. 56 The slopes of the four BTP-V-based device curves are 1.20 kT/q, 1.71 kT/q, 2.53 kT/q and 4.01 kT/q for BTP-1V-, BTP-2V-, BTP-3V-and BTP-4V-based devices respectively, signifying the dominant bimolecular recombination for BTP-1V-based device, whereas BTP-2V-, BTP-3V-and BTP-4Vbased device affected by monomolecular or trap-assisted recombination. The results collectively suggest that charge recombination is effectively suppressed in the BTP-1V-based device. The existence of monomolecular or trap-assisted recombination indicates that domain sizes in the PCE10/BTP-2V, PCE10/BTP-3V and PCE10/BTP-4V blends is large to some extent, whereas the PBDB-T/BTP-1V blends may have more appropriate domain sizes.
Additionally, the photoluminescence (PL) measurements of the neat and blend films were carried out to investigate the exciton dissociation and photo-induced charge transfer between polymer donor and Y-series NFAs. As shown in Fig. S25, PL intensities of the pure acceptors decrease dramatically upon incorporating the other component in the corresponding blend films which suggests that the exciton dissociation and charge transfer between polymer and the four acceptors are generally efficient. PBDB-T/BTP-1V and PCE10/BTP-2V blend films exhibit sufficiently quenching efficiencies (91% for BTP-1V and 87% for BTP-2V) in PL experiments, implying efficient exciton dissociation and charge transfer in blend layers. Meanwhile, PCE10/BTP-3V and PCE10/BTP-4V blend films exhibit deficiently quenching efficiencies (71% for BTP-3V and 66% for BTP-4V) in PL quenching experiments, which indicates that less free charge can be generated in the blends and results in a low external quantum efficiency (EQE) of BTP-3V-and BTP-4V-based devices. The charge-transfer efficiency between PBDB-T and BTP-1V is the highest in four blend films, suggesting that the hole transfer from BTP-1V to the donor polymer PBDB-T is the most effective among the four BTP-V-based blend films.

Morphology
Atomic force microscopy (AFM) was utilized to investigate the surface morphology of active layers (Fig. 5). In AFM height images, all four blends show smooth surface morphologies, and the root mean square roughness (R q ) for PBDB-T/BTP-1V, PCE10/BTP-2V, PCE10/BTP-3V and PCE10/BTP-4V are 1.27, 1.19, 0.94 and 0.81 nm, respectively. As shown in AFM phase images, in compassion to the BTP-2V, BTP-3V and BTP-4V-based blends, the BTP-1V-based blend exhibits the smallest domains, which is beneficial to construct bi-continuous interpenetrating networks in the blend film. The PBDB-T/BTP-1V blend film displays a more well-ordered morphology of BTP-1V and PBDB-T interlayer in the blend film, which facilitates the efficient exciton dissociation and charge transport, resulting in an improved parameter of OSC device.
To investigate the molecular stacking and crystallization of the four BTP-V-based blend films, two-dimensional grazing incidence wideangle X-ray scattering (GIWAXS) measurements were performed on pure and blend films. As shown in the Fig. 6, the 2D GIWAXS patterns and the corresponding 1D line-cuts in the corresponding 1D line-cuts in the out-of-plane (OOP) and in-plane (IP) directions of the four Y-series NFAs suggest that four blend films adopt a preferential face-on orientation with respect to the substrate, as indicated by the strong (010) π-π stacking peaks in the OOP direction and the (100) lamellar stacking peaks in the IP direction, which is beneficial for charge transport in the vertical direction across the blend films. The charge trend of π-π stacking distances of the four blends are similar to that of the corresponding neat acceptor films. The PBDB-T/BTP-1V, PCE10/BTP-2V, PCE10/BTP-3V, and PCE10/BTP-4V blend films showed obvious (010) diffraction peaks at q z of 1.73 Å -1 , 1.71 Å -1 , 1.72 Å -1 , and 1.70 Å -1 , respectively, and the corresponding π-π stacking distances are calculated to be 3.63 Å, 3.67 Å, 3.65 Å and 3.69 Å. The PBDB-T/BTP-1V blend film exhibits the shortest π-π stacking distance in the four blend films. It is envisaged that the closest π-π stacking distance of PBDB-T/BTP-1V blend film facilitates the charge transport and restrains charge recombination, thus accounting for the best J sc , FF and PCE values for PBDB-T/BTP-1V-based device. The GIWAXS results generally is consistent with the results of charge recombination studies.

Conclusions
In summary, we initially designed and synthesized a series of the Yseries BTP-V-based NFAs through a general versatile synthetic approach, featuring ultra-NBG and various electron accepting abilities. Compared with sidechain and terminal group engineering, this synthetic approach of the Y-series NFAs simplified extending conjugated backbone of NFAs by introducing vinylene π-bridges. More importantly, this work opens the door to a great variety Y6 family. These changes of molecular structures have a huge influence on the optical, electronic, and photovoltaic properties of a series of the related BTP-V-based NFAs. The PBDB-T/BTP-1V blend film shows the highest electron mobility, the best balance charge transport, and the least charge recombination among four BTP-V-based NFAs. To the best of our knowledge, this is the first time that the PCE of over 11% has been realized using a non-halogenated Y-series NFA with a bandgap below 1.28 eV. The result not only demonstrates that the fine-tuning extent of NFAs conjugated system is an effective method to adjust their energy levels, optical absorption, charge transport, and morphology, but provides a useful guideline to alter the electrochemical properties of the Y-series NFAs for highperformance ultra-NBG NFAs-based OSCs.