Semitransparent solar cells with over 12% efficiency based on a new low bandgap fluorinated small molecule acceptor†

Mei Luo‡ ^a, Chunyan Zhao‡ ^b, Jun Yuan ^a, Jiefeng Hai ^c, Fangfang Cai ^a, Yunbin Hu ^a, Hongjian Peng ^a, Yiming Bai ^bd, Zhan'ao Tan *^bd and Yingping Zou *^a
aCollege of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China. E-mail: yingpingzou@csu.edu.cn
^bBeijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing 102206, China
^cGuangxi Key Laboratory of Electrochemical and Magneto-Chemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, China
^dBeijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: tanzhanao@mail.buct.edu.cn

First published on 20th September 2019

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1 Introduction

Organic solar cells (OSCs) have attracted considerable attention over the past several years due to some potential advantages, such as lightweight, flexibility, semitransparency and being a renewable source for future energy supply. In particular, the comprehensive utilization of semitransparent organic solar cells (ST-OSCs) with transparent facilities, such as automobile glass and building windows, is of great interest. The most successful OSCs adopt a bulk-heterojunction (BHJ) structure, comprising a blend film of a p-type electron donor and an n-type electron acceptor working as the photoactive layer.^1–7 So far, the power conversion efficiencies (PCEs) of single-junction OSCs based on fullerene derivatives and non-fullerene small molecules as electron acceptors have been boosted to 11%⁸ and 16%,^9–12 respectively. This progress has been mainly driven by the extensive efforts devoted to developing new active layer materials,^13,14 optimizing the device fabrication,^15,16 and interface engineering.^17–19

For a long time, fullerene derivatives such as[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) have been the predominant n-type electron acceptors for high-efficiency OSCs due to their high electron affinity, electron mobilities, and isotropic electron transport.^8,20–22 However, fullerene derivatives also have several intrinsic drawbacks such as limited regulation of energy levels, weak absorption in the visible and near-infrared regions, and morphologically thermal instability. In the past few years, huge efforts have been devoted to developing new non-fullerene acceptors (NFAs) with acceptor–donor–acceptor (A–D–A) structures, which achieved over 15% efficiency based on single-junction OSCs.²³ The well-known NFA of an A–D–A structure, named 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indacen[1,2-b:5,6-b′]dithiophene (ITIC), was developed by Zhan et al.²⁴ ITIC possesses limited absorption spectrum (the full width at half maximum of 95 nm) with a strong absorption band from 620 to 780 nm, which is attributed to the intramolecular charge transfer (ICT) effects from the D unit to the A unit.^25,26 Hence, the PCEs of the devices based on ITIC and its derivatives have a bottleneck. Stronger electron-rich cores^27–29 or electron-donating π bridges^30,31 have been introduced to extend the spectral absorption range to long wavelengths, improve the ICT effects and thus improve photovoltaic performance. Recently, our group has synthesized some new efficient NFAs with an A–DA′D–A-type structure.^32–34 We attached electron-deficient units (benzotriazole or benzodithiazole) onto the thienopyrrole fused-ring structure and obtained A–DA′D–A-type electron acceptors. The DA′D fused core adopts a relatively planar backbone to enhance the delocalization of π-electrons, which facilitates good intermolecular packing via strong π–π interactions to increase the charge-carrier mobility.³⁴ In addition, these NFAs with broad absorption in the near-infrared region (NIR) were demonstrated to be potential candidates as the active layer in ST-OSCs with balanced PCE and AVT.³⁵

In this study, an A–DA′D–A type NIR non-fullerene acceptor is designed and synthesized, that is benzo[b][1,2,3]triazole fused with two 3-undecyl-thieno[2′,3′:4,5]thieno[3,2-b]pyrrole subunits and end-capped with fluorinated 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (FIC), namely Y14 (Fig. 1). Branched side-chains are introduced into the central core of Y14 aiming to enhance the solubility and prevent strong self-aggregation of the aromatic fused-rings, which is known for A–D–A type small molecule acceptors.³⁶

Electron transport properties are related to stacking of electron-deficient end groups (EGs) providing the main electron transport channels, in this type of electron acceptor. Formed π–π interactions based on EGs and polymer donors or adjacent acceptors can facilitate efficient electron transfer and transport.³⁷ It is noteworthy that the EGs of electron acceptors with strong electron-pulling effects are also suitable for other types of acceptors, which can improve the ICT effects, thus leading to lower energy levels, red-shifted absorption and strong extinction coefficient.^38,39 We choose FIC⁴⁰ as EGs of the A–DA′D–A type electron acceptor for judicious modulation of the ICT effects and to broaden absorption, which are favorable for charge transport and thus to improve the PCEs. High efficiency and stability are concerned aspects for the industrial application of photovoltaic devices in the future. Hence, developing a photoactive layer material and fabricating a desirable device structure have become the most common and successful strategies to yield an excellent performance.¹⁷

Recently, semitransparent OSCs have shown great potential for use in no-visual-obstacle building-integrated photovoltaics. However, the development of semitransparent OSCs is largely lagging behind opaque OSCs. To address this issue, an effective strategy has been developed by using near-infrared absorbing materials to minimize the visible-light absorption. Among them, NFAs with strong NIR absorption have attracted attention to obtain efficient semitransparent OSCs.⁴³ And ST-OSCs based on NFAs have achieved good results.^44–46

Here, we mainly explored inverted OSC devices. Compared to a conventional OSC device structure, the inverted OSC device has better stability and is structured as cathode(ITO)/electron transport layer/active layer/hole transport layer (HTL)/anode.⁴¹ Hence, Y14 was firstly incorporated into an inverted single-junction OSC consisting of different medium bandgap polymer donors (PCE10, PM6 and PBDB-T). We optimized inverted devices by changing the electron transport layers and thermal annealing (TA) temperature to facilitate the charge extraction and transportation. Inverted OSCs with a MoO₃/Al top electrode and SnO₂ electron transport layer, fabricated by PCE10:Y14, PM6:Y14 and PBDB-T:Y14, yielded PCEs of 10.44% with TA 100 °C, 8.49% with TA 150 °C and 13.88% with TA 150 °C, respectively. Inverted OSCs with a titanium(diisopropoxide)bis(2,4-pentanedionate)(TIPD)⁴² electrontransport layer, fabricated by PCE10:Y14, PM6:Y14 and PBDB-T:Y14, yield PCEs of 10.01% with TA 100 °C, 8.10% with TA 150 °C and 14.25% with TA 150 °C, respectively. These results show that inverted devices based on PBDB-T:Y14 blend films have promising potential applications.

We further investigated the photovoltaic performance of the semitransparent inverted OSCs based on the PBDB-T:Y14 blend (SnO₂ as electron transport layer), thus achieving a high PCE of 12.67% with an AVT of 23.69%, which is one of the highest PCEs for STOSCs reported to date. As a result, this BTA-core-based non-fullerene acceptor is a promising candidate that is not only suitable for high efficiency OSCs but also has great potential as an acceptor material in a semitransparent device.

2 Results and discussion

The synthetic route of Y14 is shown in Scheme 1 and Scheme S1 (ESI†). The detailed synthetic procedure, ¹H NMR and ¹³C NMR of the intermediates are shown in the ESI.† The target molecule was analyzed via¹H NMR and, ¹³C NMR, mass and elemental analysis. Y14 exhibits good solubility in common organic solvents, such as dichloromethane (DCM), chloroform (CF), and chlorobenzene (CB) at room temperature.

Ultraviolet-visible (UV-vis) absorption spectra of Y14 in chloroform solution and in thin film are shown in Fig. 1(a). In solution, Y14 exhibits a maximum absorption peak at 759 nm. In the solid thin film, Y14 displays broad absorption ranging from 600 to 920 nm with a maximum absorption peak located at 853 nm. Y14 exhibits a narrow optical bandgap (E^opt_g) of 1.30 eV calculated from the film absorption onset.⁴⁷ It is worth noting that large red shift of 94 nm is observed for Y14, upon shifting from solution to the film state, indicating stronger intermolecular interactions of Y14 in the solid state. The red-shifted absorption enables effective utilization of the NIR photons and significantly minimizes the absorption overlapping with PBDB-T. Hence, Y14 could be an ideal candidate to fabricate semitransparent inverted OSCs. Compared with Y9 in our previous work,⁴⁸ Y14 possesses a wider absorption spectrum, indicating that fluorination can lead to a red-shift of the λ_max and absorption onset, resulting in a lower optical bandgap.⁴⁹

The electrochemical curve of Y14 was measured by cyclic voltammetry (CV). The energy levels estimated from the CV oxidation and reduction potentials are shown in Fig. S2(b) (ESI†).⁵⁰ Y14 shows a highest occupied molecular orbital (HOMO) of −5.56 eV and a lowest unoccupied molecular orbital (LUMO) of −4.01 eV. The lower LUMO energy level is attributed to the electron-withdrawing effects of fluorine in the EGs, compared to those of counterpart Y9, and UV-vis absorption spectra and energy level spectra of the materials are shown in Fig. 1(b) and (c).

To investigate the photovoltaic characteristics of Y14-based single-junction opaque OSCs, we fabricated inverted devices with the sandwich architecture of glass/ITO/SnO₂ or TIPD/active layer/MoO₃/Al. Furthermore, Y9, PCE10, PBDB-T and PM6 (as shown in Fig. 2(a)) were used as the p-type polymers to blend with Y14. We optimized the morphology of the active layer, charge extraction and transportation by adjusting the temperatures of TA and using different electron transport layers. The current density–voltage (J–V) curves and detailed corresponding device parameters are shown in Fig. 2(b) and (c) and Table 1, respectively.

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The inverted OSC devices used SnO₂ as an electron transport layer, where the TA temperature was increased from 100 °C to 150 °C. The PCEs of PM6:Y14 and PBDB-T:Y14-based devices were improved from 7.47% and 13.00% to 8.49% and 13.88%, respectively. However, the PCE of the PCE10:Y14-based device was reduced from 10.44% to 8.30%. When the TA temperature is increased from 100 °C to 150 °C, these inverted OSC devices use TIPD as an electron transport layer. The PCEs of PM6:Y14 and PBDB-T:Y14-based devices were increased from 7.95% and 13.33% to 8.10% and 14.25%, respectively. However, the PCE of the PCE10:Y14-based device was reduced from 10.01% to 8.46%. After TA treatment, the PBDB-T:Y14-based inverted devices (TIPD as an electron transport layer) show better photovoltaic performance, compared with other devices. Then, the performance of the PBDB-T:Y14-based device was further optimized by adding 1% CN. The champion PCE of 14.92% (a V_oc of 0.798 V, a J_sc of 26.15 mA cm⁻², an FF of 71.48%) was obtained, compared with all PBDB-T:Y14-based devices, as shown in Fig. 2f. The external quantum efficiency (EQE) curves of the corresponding devices are displayed in Fig. 2(d), (e) and (g). All devices based on PBDB-T:Y14 possess high and broad photoresponse in the range of 300–900 nm, and the highest EQE value of the corresponding devices is even over 80%. The integrated current densities of these devices from EQE spectra are consistent with the values obtained from the J–V measurements. When the TA temperature was 110–140 °C, the J–V and EQE curves of the PBDB-T:Y14 devices are shown in Fig. S3(a) and (b) (ESI†), and the detailed data are listed in Table S1 (ESI†). Compared to the PBDB-T:Y9-based device,⁴² the higher FF and J_sc of Y14-based devices suggests that the effect of fluorinated EGs could enhance the π–π stacking of Y14 in the solid state, it can be beneficial for charge transport.

To understand the exciton dissociation and charge collection properties in the optimal device, their photocurrent (J_ph) versus the effective applied voltage (V_eff) were investigated, as shown in Fig. 3(a). The charge dissociation probability (P_diss = J_ph/J_sat) values can indicate the exciton dissociation and charge collection efficiency, where J_sat is the saturation photocurrent density. At V_eff ≥ 2.0 V, J_ph is saturated, and the high internal electric field cause the minimized recombination in the cell. At V_eff = 2.0 V, P_diss of 90.9% was obtained for the optimized device. The higher P_diss for the single-junction device suggests the combination of a medium bandgap polymer PBDB-T and NIR non-fullerene acceptor Y14 is beneficial for exciton dissociation and charge transportation, which is consistent with the enhanced J_sc and good FF. To obtain more insight into the extent of the bimolecular recombination process of the OSC device, we determined the dependence of J_sc on light-intensity (P) measurement. The relationship between J_sc and P_light can be defined as J_sc ∝ P^α_light, where the α value is close to 1, and the extent of bimolecular recombination can be negligible in the OSC devices. As seen in Fig. 3(b), the value of α for the device with TA and 1% CN treatment is 0.97. These results imply that the optimal device possesses a highly efficient photoelectron conversion process and negligible bimolecular recombination, leading to higher J_sc and FF.

The electron-only devices (Glass/ITO/TIPD/Active layer/Ca/Al) and hole-only devices (Glass/ITO/PEDOT:PSS/Active layer/MoO₃/Ag) were fabricated to investigate the hole and electron mobilities using the space-charge limited current (SCLC) model.⁵¹ As seen in Fig. 4, the hole-only (μ_h) and electron-only (μ_e) mobilities were calculated to be 4.35 × 10⁻⁵ and 1.00 × 10⁻⁴ cm² V⁻¹ s⁻¹ for the PBDB-T:Y14 blend, respectively. The balanced hole and electron transport (μ_h/μ_e = 0.43) of the PBDB-T:Y14 blend could effectively prevent the charge accumulation and recombination, contributing to better molecular packing and charge transport. The high hole and electron mobilities of PBDB-T:Y14 based devices are in good agreement with the high J_sc and FF of PBDB-T:Y14 based devices.

The surface morphologies of the inverted binary PBDB-T:Y14 blend were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As seen in Fig. 5(a) and (b), the PBDB-T:Y14-based blend film exhibited a relatively smooth top surface with a root-mean-square (RMS) roughness of 1.41 nm, which would benefit the effective charge extraction at the interface between the active layer and cathode. The images of the blend film displayed an optimized phase separation with nanoscale fibril-like interpenetrating net frameworks throughout the whole blend film, which is supposed to be positive to enhance both the charge transport and collection, contributing to improving J_sc and FF.

The strong NIR absorbing characteristics of Y14 endow it with potential in ST-OSCs. By replacing Al with 15 nm Ag as the transparent electrode in the PBDB-T:Y14-based devices, we fabricated semitransparent inverted OSC devices with a sandwich architecture of ITO/SnO₂/PBDB-T:Y14/MoO₃/Ag to evaluate the performances of the ST-OSCs. In order to explore the effects of additive treatment on the photovoltaic performance of the PBDB-T:Y14 blend, we added 0.5% CN and 1% CN to the PBDB-T:Y14 blend system, respectively. The J–V curves of the opaque and semitransparent inverted devices are shown in Fig. 6(a), and the detailed photovoltaic parameters are listed in Table 2. We could observe that the additive has less effect on device performance. The semitransparent inverted devices showed an overall decreased J_sc compared with their opaque counterparts, the binary complementary absorption characteristic was still maintained, while V_oc and FF were only slightly changed. Hence, the semitransparent inverted devices achieved a PCE of over 12%. The transmission spectrum of the semitransparent inverted device is displayed in Fig. 6(b), and the AVT of the PBDB-T:Y14-based device was 23.69%. These results suggested that these devices have potential for use in flexible semitransparent device applications. The EQE curves of the corresponding devices are displayed in Fig. 6(c).

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3. Conclusions

In summary, we designed and synthesized a new fused dithienothiophen[3,2-b]-pyrrolobenzotriazole (BTA-core)-based low bandgap electron acceptor, namely Y14. High performance non-fullerene OSCs based on Y14 can be obtained by optimizing the device fabrication process. As a result, an outstanding PCE of 14.92% based on the PBDB-T:Y14 device was demonstrated, due to efficient charge separation, high and balanced carrier mobilities, and nanoscale phase separation morphology. Moreover, the semitransparent inverted OSCs based on PBDB-T:Y14 yielded a PCE of 12.67% with AVT of 23.69%. These results demonstrate the efficiency of our molecular design and show that the narrow bandgap BTA-core-based non-fullerene acceptors are promising candidates toward high performance semitransparent inverted OSCs.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Y. Zou acknowledges the National Key Research & Development Projects of China (2017YFA0206600), the National Natural Science Foundation of China (21875286), and the Science Fund for Distinguished Young Scholars of Hunan Province (2017JJ1029). Z. Tan acknowledges the National Natural Science Foundation of China (51573042, 51873007, 21835006 and 5181101540).

References

1. J.-H. Kim, C. Schaefer and T. Ma, et al., The critical impact of material and process compatibility on the active layer morphology and performance of organic ternary solar cells, Adv. Energy Mater., 2019, 9(2), 1802293 CrossRef .
2. Y. Li, J. D. Lin and X. Che, et al., High efficiency near-infrared and semitransparent non-fullerene acceptor organic photovoltaic cells, J. Am. Chem. Soc., 2017, 139(47), 17114–17119 CrossRef CAS PubMed .
3. W. Zhao, S. Li and H. Yao, et al., Molecular optimization enables over 13% efficiency in organic solar cells, J. Am. Chem. Soc., 2017, 139(21), 7148–7151 CrossRef CAS PubMed .
4. Q. An, W. Gao and F. Zhang, et al., Energy level modulation of non-fullerene acceptors enables efficient organic solar cells with small energy loss, J. Mater. Chem. A, 2018, 6(6), 2468–2475 RSC .
5. W. Gao, Q. An and R. Ming, et al., Side group engineering of small molecular acceptors for high-performance fullerene-free polymer solar cells: thiophene being superior to selenophene, Adv. Funct. Mater., 2017, 27(34), 1702194 CrossRef .
6. K. Gao, S. B. Jo and X. Shi, et al., Over 12% efficiency nonfullerene all-small-molecule organic solar cells with sequentially evolved multilength scale morphologies, Adv. Mater., 2019, 31(12), e1807842 CrossRef PubMed .
7. K. Gao, J. Miao and L. Xiao, et al., Multi-length-scale morphologies driven by mixed additives in porphyrin-based organic photovoltaics, Adv. Mater., 2016, 28(23), 4727–4733 CrossRef CAS PubMed .
8. J. Zhao, Y. Li and G. Yang, et al., Efficient organic solar cells processed from hydrocarbon solvents, Nat. Energy, 2016, 1(2), 15027 CrossRef CAS .
9. J. Yuan, Y. Zhang and L. Zhou, et al., Single-junction organic solar cell with over 15% efficiency using fused-ring acceptor with electron-deficient core, Joule, 2019, 3, 1140–1151 CrossRef CAS .
10. X. Xu, K. Feng and Z. Bi, et al., Single-junction polymer solar cells with 16.35% efficiency enabled by a platinum(ii) complexation strategy, Adv. Mater., 2019, 1901872 CrossRef PubMed .
11. Y. Cui, H. Yao and J. Zhang, et al., Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages, Nat. Commun., 2019, 10(1), 2515 CrossRef PubMed .
12. K. Li, Y. Wu and Y. Tang, et al., Ternary blended fullerene-free polymer solar cells with 16.5% efficiency enabled with a higher-lumo-level acceptor to improve film morphology, Adv. Energy Mater., 2019, 1901728 CrossRef .
13. H. Yao, Y. Cui and R. Yu, et al., Design, synthesis, and photovoltaic characterization of a small molecular acceptor with an ultra-narrow band gap, Angew. Chem., Int. Ed., 2017, 56(11), 3045–3049 CrossRef CAS PubMed .
14. B. Kan, H. Feng and H. Yao, et al., A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss, Sci. China: Chem., 2018, 61(10), 1307–1313 CrossRef CAS .
15. Y. Chen, P. Ye and Z. G. Zhu, et al., Achieving high-performance ternary organic solar cells through tuning acceptor alloy, Adv. Mater., 2017, 29(6), 1603154 CrossRef PubMed .
16. Y. Qin, Y. Chen and Y. Cui, et al., Achieving 12.8% efficiency by simultaneously improving open-circuit voltage and short-circuit current density in tandem organic solar cells, Adv. Mater., 2017, 29(24), 1606340 CrossRef PubMed .
17. L. Hu, F. Wu and C. Li, et al., Alcohol-soluble n-type conjugated polyelectrolyte as electron transport layer for polymer solar cells, Macromolecules, 2015, 48(16), 5578–5586 CrossRef CAS .
18. N. Zheng, Z. Wang and K. Zhang, et al., High-performance inverted polymer solar cells without an electron extraction layer via a one-step coating of cathode buffer and active layer, J. Mater. Chem. A, 2019, 7(4), 1429–1434 RSC .
19. C. Sun, Z. Wu and Z. Hu, et al., Interface design for high-efficiency non-fullerene polymer solar cells, Energy Environ. Sci., 2017, 10(8), 1784–1791 RSC .
20. M. Li, K. Gao and X. Wan, et al., Solution-processed organic tandem solar cells with power conversion efficiencies >12%, Nat. Photonics, 2017, 11(2), 85–90 CrossRef CAS .
21. H. Zhou, Y. Zhang and C. K. Mai, et al., Polymer homo-tandem solar cells with best efficiency of 11.3%, Adv. Mater., 2015, 27(10), 1767–1773 CrossRef CAS PubMed .
22. Z. He, B. Xiao and F. Liu, et al., Single-junction polymer solar cells with high efficiency and photovoltage, Nat. Photonics, 2015, 9(3), 174–179 CrossRef CAS .
23. Y. Cui, H. Yao and L. Hong, et al., Achieving over 15% efficiency in organic photovoltaic cells via copolymer design, Adv. Mater., 2019, 1808356 CrossRef PubMed .
24. Y. Lin, J. Wang and Z. G. Zhang, et al., An electron acceptor challenging fullerenes for efficient polymer solar cells, Adv. Mater., 2015, 27(7), 1170–1174 CrossRef CAS PubMed .
25. J. Miao, B. Meng and J. Liu, et al., An a–d–a′–d–a type small molecule acceptor with a broad absorption spectrum for organic solar cells, Chem. Commun., 2018, 54(3), 303–306 RSC .
26. P. M. Beaujuge, C. M. Amb and R. J. Reynolds, Spectral engineering in π-conjugated polymers with intramolecular donor–acceptor interactions, Acc. Chem. Res., 2010, 43(11), 1396–1407 CrossRef CAS PubMed .
27. Y. Liu, M. Li and X. Zhou, et al., Nonfullerene acceptors with enhanced solubility and ordered packing for high-efficiency polymer solar cells, ACS Energy Lett., 2018, 3(8), 1832–1839 CrossRef CAS .
28. C. Li, J. Song and L. Ye, et al., High-performance eight-membered indacenodithiophene-based asymmetric a–d–a type non-fullerene acceptors, Sol. RRL, 2019, 3(1), 1800246 CrossRef .
29. K. L. L. Gao and T. Lai, et al., Deep absorbing porphyrin small molecule for high-performance organic solar cells with very low energy losses, J. Am. Chem. Soc., 2015, 137(23), 7282–7285 CrossRef CAS PubMed .
30. Y. Liu, Z. Zhang and S. Feng, et al., Exploiting noncovalently conformational locking as a design strategy for high performance fused-ring electron acceptor used in polymer solar cells, J. Am. Chem. Soc., 2017, 139(9), 3356–3359 CrossRef CAS PubMed .
31. W. Zhong, J. Cui and B. Fan, et al., Enhanced photovoltaic performance of ternary polymer solar cells by incorporation of a narrow-bandgap nonfullerene acceptor, Chem. Mater., 2017, 29(19), 8177–8186 CrossRef CAS .
32. J. Yuan, T. Huang and P. Cheng, et al., Enabling low voltage losses and high photocurrent in fullerene-free organic photovoltaics, Nat. Commun., 2019, 10(1), 570 CrossRef PubMed .
33. L. Feng, J. Yuan and Z. Zhang, et al., Thieno[3,2-b]pyrrolo-fused pentacyclic benzotriazole-based acceptor for efficient organic photovoltaics, ACS Appl. Mater. Interfaces, 2017, 9(37), 31985–31992 CrossRef CAS PubMed .
34. J. Yuan, Y. Zhang and L. Zhou, et al., Fused benzothiadiazole: a building block for n-type organic acceptor to achieve high-performance organic solar cells, Adv. Mater., 2019, 31(17), e1807577 CrossRef PubMed .
35. V. V. Brus, J. Lee and B. Luginbuhl, et al., Solution-processed semitransparent organic photovoltaics: from molecular design to device performance, Adv. Mater., 2019, 1900904 CrossRef PubMed .
36. H. Yao, L. Ye and J. Hou, et al., Achieving highly efficient nonfullerene organic solar cells with improved intermolecular interaction and open-circuit voltage, Adv. Mater., 2017, 29(21), 1700254 CrossRef PubMed .
37. J. Hou, O. Inganas and R. H. Friend, et al., Organic solar cells based on non-fullerene acceptors, Nat. Mater., 2018, 17(2), 119–128 CrossRef CAS PubMed .
38. S. S. Wan, C. Chang and J. L. Wang, et al., Effects of the number of bromine substitution on photovoltaic efficiency and energy loss of benzo[1,2-b:4,5-b′]diselenophene-based narrow-bandgap multibrominated nonfullerene acceptors, Sol. RRL, 2018, 1800250 CrossRef .
39. H. Zhang, H. Yao and J. Hou, et al., Over 14% efficiency in organic solar cells enabled by chlorinated nonfullerene small-molecule acceptors, Adv. Mater., 2018, 30(28), e1800613 CrossRef PubMed .
40. S. Dai, F. Zhao and Q. Zhang, et al., Fused nonacyclic electron acceptors for efficient polymer solar cells, J. Am. Chem. Soc., 2017, 139(3), 1336–1343 CrossRef CAS PubMed .
41. D. J. Lee, D. K. Heo, C. Yun, Y. H. Kim and M. H. Kang, Solution-processed semitransparent inverted organic solar cells from a transparent conductive polymer electrode, ECS J. Solid State Sci. Technol., 2019, 8(2), Q32–Q37 CrossRef CAS .
42. Z. Tan, C. Yang and E. Zhou, et al., Performance improvement of polymer solar cells by using a solution processible titanium chelate as cathode buffer layer, Appl. Phys. Lett., 2007, 91(2), 023509 CrossRef .
43. Y. Cui, C. Yang and H. Yao, et al., Efficient semitransparent organic solar cells with tunable color enabled by an ultralow-bandgap nonfullerene acceptor, Adv. Mater., 2017, 29(43), 1703080 CrossRef PubMed .
44. R. Xia, H. Gu and S. Liu, et al., Optical analysis for semitransparent organic solar cells, Sol. RRL, 2019, 3(1), 1800270 CrossRef .
45. S. Dai and X. Zhan, Nonfullerene acceptors for semitransparent organic solar cells, Adv. Energy Mater., 2018, 8(21), 1800002 CrossRef .
46. W. Wang, C. Yan and T. K. Lau, et al., Fused hexacyclic nonfullerene acceptor with strong near-infrared absorption for semitransparent organic solar cells with 9.77% efficiency, Adv. Mater., 2017, 29(31), 1701308 CrossRef PubMed .
47. M. Chang, Y. Wang and Y.-Q.-Q. Yi, et al., Fine-tuning the side-chains of non-fullerene small molecule acceptors to match with appropriate polymer donors, J. Mater. Chem. A, 2018, 6(18), 8586–8594 RSC .
48. M. Luo, L. Zhou and J. Yuan, et al., A new non-fullerene acceptor based on the heptacyclic benzotriazole unit for efficient organic solar cells, J. Energy Chem., 2020, 42, 169–173 CrossRef .
49. W. Zhong, B. Fan and J. Cui, et al., Regioisomeric non-fullerene acceptors containing fluorobenzo[c][1,2,5]thiadiazole unit for polymer solar cells, ACS Appl. Mater. Interfaces, 2017, 9(42), 37087–37093 CrossRef CAS PubMed .
50. F. Liu, Z. Zhou and C. Zhang, et al., A thieno[3,4-b]thiophene-based non-fullerene electron acceptor for high-performance bulk-heterojunction organic solar cells, J. Am. Chem. Soc., 2016, 138(48), 15523–15526 CrossRef CAS PubMed .
51. T. Wang, L. Feng and J. Yuan, et al., Side-chain fluorination on the pyrido[3,4-b]pyrazine unit towards efficient photovoltaic polymers, Sci. China: Chem., 2017, 61(2), 206–214 CrossRef .

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qm00499h

‡ Mei Luo and Chunyan Zhao contributed equally to this work.

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