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Cite this: Chem. Commun., 2014, 50, 1591

Rational molecular engineering towards efficient non-fullerene small molecule acceptors for inverted bulk heterojunction organic solar cells†

Received 24th September 2013, Accepted 19th November 2013

Yu-Qing Zheng, Ya-Zhong Dai, Yan Zhou, Jie-Yu Wang* and Jian Pei*

DOI: 10.1039/c3cc47289b www.rsc.org/chemcomm

Two non-fullerene small molecules based on fluoranthene-fused imide were developed as acceptors for solution-processed inverted organic bulk heterojunction (BHJ) solar cells, which showed good power conversion efficiency and high open-circuit voltage.

As one of the most promising renewable energy resources, organic photovoltaics (OPVs) have attracted great interest because of low cost, flexible substrates, and easy fabrication.1 Much effort has been put into improving the power conversion efficiency (PCE) of BHJ organic solar cells over 10%.2 To realize higher PCEs, p-type and n-type organic semiconductors are mixed to form interpenetrating phase separations in the active layer, which effectively increases the donor (D)/acceptor (A) interface and reduces the recombination of the excitons.3 Among all the acceptor materials, fullerene derivatives, such as [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), are the dominant acceptors in OPVs due to high electron mobility, large electron affinity, and desirable phase separation in the active layer.4 However, fullerene derivatives show several disadvantages, such as negligible absorption contribution of PC61BM in the visible and near-IR range, low open circuit voltage (Voc) resulting from high electron affinity, and complex and expensive synthesis and purification. Recently, some fullerene-free molecules were reported as acceptors in conventional BHJ OPVs, and a few materials had PCEs approaching or over 2%.5 Our group developed a small molecule acceptor for OPVs based on fluoranthene-fused imide (FFI) which exhibited a relatively high PCE of 1.8% using a conventional photovoltaic structure, indicating that the FFI skeleton can serve as a promising segment for acceptors in organic solar cells.6 Therefore, detailed studies on structure modification were conducted to improve the device performance and to find a correlation between the structure and device performance. It is common sense when designing acceptors that introducing Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: [email protected], [email protected]; Tel: +86-10-62758145 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cc47289b

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electron-withdrawing groups could effectively lower the LUMO levels of the desired molecules, thus facilitating electron transport.7 Herein, we incorporate a bulky electron-withdrawing group, p-formylphenyl, to the FFI skeleton, and yield a new acceptor Th–PhCHO. Such a bulky group might influence the molecular configuration of Th–PhCHO, and also greatly affect the morphology of the active layer when blended with the P3HT donor. For comparison, a methyl-ester group is also introduced to modify the FFI skeleton, affording Th–COOMe, which is estimated to exhibit similar energy levels to Th–PhCHO, while having a distinct molecular configuration. Fig. 1 illustrates the structures of the two acceptors, Th–PhCHO and Th–COOMe. After blending with P3HT, the Voc of both OPV devices using Th–PhCHO and Th–COOMe as acceptors are quite similar, while a large difference in Jsc is achieved. Such a difference causes a large change in the PCEs: 1.6% for Th–COOMe and 2.4% for Th–PhCHO, which are among the highest PCEs for non-fullerene small molecule acceptors. Our results demonstrate that the chemical properties, energy levels and molecule configurations of non-fullerene small molecule acceptors can be easily tuned to achieve better device performance, and subtle changes in their structures may greatly influence the device performance, which motivates us to keep on studying the correlation between structure and properties. Since the energy offset controlled by the LUMO level of the acceptors and HOMO level of the donors plays a vital role in

Fig. 1

Chemical structures of the two acceptors.

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determining the open circuit voltage (Voc) of the device, incorporating suitable electron-withdrawing groups is of great importance in designing effective acceptor materials. Density functional theory (DFT) calculations were carried out to investigate the effect of the electron-withdrawing groups on the HOMO/LUMO levels, the orbital distributions and the molecule configurations, which provide effective theoretical guidance to molecular engineering (calculated orbitals are shown in Fig. S1, ESI†). The theoretical calculation revealed that the HOMOs of both molecules mainly delocalized over the entire backbones except the imide groups, while their LUMOs mainly delocalized over the entire backbones other than the thiophenes. The two molecules with different electron-withdrawing groups showed similar LUMO/HOMO levels ( 2.72/ 6.18 eV for Th–COOMe and 2.67/ 6.12 eV for Th–PhCHO), both lower than that of the core structure FFI. In addition, there is a rotation angle between the p-formylphenyl and the FFI core due to the repulsion between two hydrogen atoms, which is estimated to be 48.91, whereas the ester group is coplanar with the core structure. These results indicate that the structures of the electron-withdrawing groups not only influence the HOMO/LUMO levels of the molecules, but also greatly affect the molecular configuration and the latter has been proved to be closely related to the molecular packing mode in active layers. DFT calculations proved the rationality of our molecular design, and we synthesized the two acceptors following the reported procedures.8 The structures and purity of both acceptors were characterized and verified by 1H and 13C NMR, MS, and elemental analysis (see ESI†). The HOMO/LUMO levels of both acceptors were measured using cyclic voltammetry (CV) (Fig. 2b). Th–PhCHO and Th–COOMe were dissolved in dichloromethane containing

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0.1 M n-Bu4NPF6 as a supporting electrolyte at a concentration of 10 3 M. Ag/AgCl was used as a reference electrode, while ferrocene was used as a standard compound. The LUMO levels of Th–PhCHO and Th–COOMe were estimated from the onset of reduction waves and their HOMO levels were calculated by the corresponding LUMO levels and optical gaps obtained from the onset of absorptions. As methyl ester and p-formylphenyl have comparable electron-withdrawing ability, the LUMO levels of the two compounds are close to each other, 3.41 eV for Th–COOMe and 3.30 eV for Th–PhCHO. Theoretically, the open circuit voltage (Voc) is controlled by the difference between the LUMO level of the acceptor and the HOMO level of the donor. These two non-fullerene acceptors should lead to much higher Voc than that of PC61BM. The investigation of the absorption features indicates that the absorption maximum lmax in thin films peaked at 422 nm for Th–PhCHO and 411 nm for Th–COOMe (see Fig. S2, ESI†). Fig. 2a shows the UV-vis features of the active layers on ZnO-modified ITOcoated glass, in which the ratio of P3HT/acceptor (D/A, w/w) is 1 : 1. For both films, the absorption peaks at around 400–430 nm were attributed to our acceptor molecules, while the doublet band at 555 nm and the shoulder peak at 600 nm were attributed to the aggregation of regioregular P3HT, suggesting that P3HT formed an ordered phase within the active layers. The absorptions of the active layers covered most of the solar spectrum thanks to the absorption of the visible light from the acceptors, indicating that our nonfullerene acceptors improve the utilization of sunlight. Very weak emissions were observed from these active layers (Fig. S3, ESI†). Inverted solar cells are believed to improve the device stability by using high-work-function metals, such as Ag and Au, thus simplifying the manufacturing.9 Therefore, inverted solar cells are considered to

Fig. 2 (a) UV-vis absorption spectra of Th–PhCHO and Th–COOMe blended with P3HT; (b) Cyclic voltammograms of Th–PhCHO and Th–COOMe dissolved in CH2Cl2; (c) J–V curves and (d) IPCE spectra of devices with the structure ITO/ZnO/acceptor:P3HT (1 : 1, w/w)/MoO3/Ag.

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Table 1

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Device performance based on acceptor:P3HT (1 : 1, w/w)

Acceptor

Voc/V

Jsc/mA cm

Th–PhCHO Th–COOMe

1.03 1.02

5.14 3.50

2

FF

PCE (%)

0.45 0.45

2.40 1.61

be promising device structures for future applications. However, fullerene-free acceptors have seldom been explored in inverted solar cells so far.8 In order to test the device performance of Th–PhCHO and Th–COOMe in OPVs, both molecules as acceptors and P3HT as the donor were blended with various weight ratios to form interpenetrating networks with an inverted structure of ITO/ZnO/acceptor: P3HT/MoO3/Ag. After careful optimization, a weight ratio of 1 : 1 (w/w) for D/A was adopted and all the devices underwent preannealing at 100 1C for 15 min. The device performance for the two acceptors is summarized in Table 1. Both materials showed much higher Voc (over 1 V) than that of PC61BM as expected from their higher LUMO levels. The improved open circuit voltages prove that non-fullerene acceptors have more potential for diverse regulation and control in order to realize better device performance. The Jsc of the Th–PhCHO-based device increases by ca. 47% in comparison with that of the Th–COOMe-based device, contributing to the great difference in PCEs of the two devices containing different acceptors, Th–PhCHO with 2.4% and Th–COOMe with 1.6%. Typically, for realizing high PCE, the most important issue is to improve Jsc while retaining high Voc. Our acceptor Th–PhCHO, compared with Th–COOMe, showed much higher Jsc, while Voc was unchanged. This result indicates that through structure design, Jsc and Voc can be raised at the same time. To find the reason for this phenomenon, we tested the external quantum efficiency (EQE), morphology of the active layers and the electron mobilities of these two acceptors. The incident photon to current conversion efficiency (IPCE) spectra for Th–PhCHO and Th–COOMe are shown in Fig. 2d. The EQE tracks the trend of Jsc for the two acceptors, where Th–PhCHO with a higher Jsc showed a much higher quantum efficiency. Devices based on both acceptors showed an EQE contribution at wavelengths from 400 to 700 nm, which indicates that both acceptors and the donor absorbed photons to generate excitons. Devices based on Th–PhCHO showed relatively high EQE (35–40%) throughout the range of 400–700 nm, whereas those based on Th–COOMe showed moderate EQE (20–25%). The EQE observed for Th–PhCHO is higher than that for Th–COOMe by more than 35% throughout the absorption spectrum, suggesting the more efficient separation of photogenerated excitons in Th–PhCHO:P3HT than in Th–COOMe:P3HT. In BHJ solar cells, the morphology of the active layer plays a decisive role in the device performance. Atomic force microscopy (AFM) was performed to investigate the morphology of the active layers. The AFM images in Fig. 3 show that the film of Th–PhCHO/ P3HT is continuous with several fibrous structures on it, while the film of Th–COOMe/P3HT shows large discontinuous domains, indicating that the phase separation in these two active layers is quite different. The roughness of the film of Th–PhCHO/P3HT is 7.9 nm, smaller than that of film of Th–COOMe/P3HT (9.1 nm). Such a smoother and continuous active layer without large domains suggests the formation of a better interpenetrating network in the bulk phase, which is conducive to higher Jsc and thus higher PCE.10

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Fig. 3 (a) AFM height image and (b) phase image of Th–PhCHO/P3HT blended films; (c) AFM height image and (d) phase image of Th–COOMe/ P3HT blended films.

The large difference in phase separation may come from the different structures of the two acceptors. Th–PhCHO, with a larger torsional angle and more twisted structure, may have better miscibility with P3HT, thus forming a fine and uniform active layer. The two acceptors exhibit generally similar Voc due to the similar energy levels, and significant changes in Jsc probably mainly result from the different molecule packing, which is largely controlled by intrinsic molecular structures. In addition, a smoother active layer also guarantees better contact with the electrodes, thus facilitating effective electron collection.11 Charge transport properties of organic semiconductors also play an important role in obtaining high Jsc. The electron mobilities of our acceptors were measured using the space charge limited current (SCLC) method with a device structure of ITO/ZnO/active layer/LiF/Al.12 Electron mobilities of 4.8  10 8 cm2 V 1 s 1 for Th–PhCHO and 2.0  10 8 cm2 V 1 s 1 for Th–COOMe were obtained. Th–PhCHO showed more than twice the electron mobility than that of Th–COOMe, which contributed to its much higher Jsc. Furthermore, the SCLC method also revealed that in the device using Th–PhCHO:P3HT more balanced hole and electron mobilities were achieved, since the sub-linear region can be clearly pointed out in the SCLC curve of Th–COOMe:P3HT (see Fig. S4, ESI†).12 We have successfully developed two FFI derivatives, Th–PhCHO and Th–COOMe, which serve as non-fullerene acceptors for inverted organic BHJ solar cells. Both of them show higher LUMO levels ( 3.41 eV for Th–COOMe and 3.30 eV for Th–PhCHO) than that of PC61BM, which is beneficial for a higher Voc of the devices with the molecules as acceptors. When blended with P3HT in a weight ratio of 1 : 1, both acceptors show high PCEs of up to 2.4%. Furthermore, the device with Th–PhCHO as the acceptor represents one of the best device performances of the inverted BHJ solar cells with non-fullerene small molecule acceptors, which indicates that electron-withdrawing groups effectively affected both the HOMO/LUMO levels and molecular packing mode in the blends, thus yielding distinct device performances. This work further proves that FFI derivatives could serve as efficient acceptors for solution-processed OPVs and show great possibility to improve device performance.

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This work was supported by the Major State Basic Research Development Program (no. 2009CB623601 and 2013CB933501) from the Ministry of Science and Technology, and National Natural Science Foundation of China. 6

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Rational molecular engineering towards efficient non-fullerene small molecule acceptors for inverted bulk heterojunction organic solar cells.

Two non-fullerene small molecules based on fluoranthene-fused imide were developed as acceptors for solution-processed inverted organic bulk heterojun...
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