Investigation on the Effect of Non-Fullerene Acceptor and Conjugated Polymer Donor on Organic Semiconductor Solar Cell

INTRODUCTION

Especially, the development of PSCs is always accompanied by photoactive material innovations. As the key component, photoactive materials are basically classified as a p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). Due to the unique advantages of strong electron-accepting and high electron-transport capabilities, fullerene derivatives were predominately used as the acceptor in PSCs in the past two decades, driving the power conversion efficiency (PCE) of PSCs to 11–12%. Nevertheless, fullerenes based acceptors show critical shortcomings of weak absorption and limited structural modification, hindering further improve photovoltaic performance of devices. To overcome these obstacles of fullerenes based acceptors, many efforts have been devoted to developing new kind of nonfullerene acceptor materials. Very recently, A-D-A conjugated fused-ring molecules based on indacenodithiophene (IDT) or DTIDT unit were reported as excellent nonfullerene acceptors for high performance PSCs, leading the PCE of device to over 13%. Unlike fullerene derivatives, fused-ring based nonfullerene acceptor materials offer many molecular design strategies to tune their optoelectronic properties and thus photovoltaic performance. In this review, we will provide some representative cases of molecular manipulation on IDT and DTIDT based nonfullerene acceptors to fine-tune the physicochemical and photovoltaic acceptors for efficient nonfullerene PSCs. IDT unit which features phenylene ring fused to thiophene was firstly reported by Wong in 2006. Such fused rings structure is beneficial to forming effective interchain π-π overlap and enhance the rigidity of the molecular backbone as well as the degree of conjugation. Zhan et al. innovatively used IDT as central core to develop an A-D-A (A = acceptor, D = donor) type acceptor material (NA1) with 2-(3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile as terminal acceptor unit. NA1 showed promising energy levels and absorption spectrum as acceptor material for PSCs. By using NA1 as acceptor and PDBT-T1 as donor to fabricate PSC device, a high PCE of 7.39% was obtained, with Voc = 0.92 V, Jsc = 13.39 mA cm−2, and FF = 0.60. Molecular optimization based on NA1 greatly affects the photovoltaic performance. In the following, we will discuss the molecular design strategies including extension of conjugated backbone, substituted side chains, and end-capped group were conducted on NA1.

ORGANIC SEMICONDUCTOR SOLAR CELL

The Voc of PSCs device is tightly correlated with the energy level difference between the HOMO of the donor and the LUMO of the acceptor. Therefore, high LUMO level of acceptor material is essential for achieving high Voc value. In D-A conjugated molecular system, the donor unit mainly determines the HOMO level. In other words, the optical bandgap (Eg) of D-A based molecules can be tuned by incorporating donor unit as conjugated extension while maintaining the similar LUMO level. For example, Zhan et al. employed one and two IDT units as conjugated extension block to develop two molecules NA2 and NA3. Due to the longer conjugated backbone, the absorption profiles of NA2 and NA3 are effectively red-shifted compared to NA1, and NA3 possesses the lowest Eg of 1.53 eV. On the other hand, NA2 and NA3 showed up-shifted HOMO levels while similar LUMO levels. Due to the weaker molecular π-π stacking compared to NA1, the NA2 based device exhibited a low PCE of 2.58%, while no photovoltaic response was observed from the NA3 based device. In comparison with NA1, NA4 with thiophene units as π-bridge for conjugated extension showed slightly red-shifted absorption spectrum (Eg = 1.55 eV), up-shifted HOMO level of −5.42 eV, and similar LUMO level of −3.85 eV. The device based on PBDTTT-C-T:NA4 exhibited a PCE of 3.93%, with a Voc of 0.90 V, Jsc of 8.33 mA bridge of NA1. By using IEIC as acceptor and PTB7-Th as donor to fabricate device, a promising PCE of 6.31% was obtained, with Voc = 0.97 V, Jsc = 13.55 mA cm−2, and FF = 0.48. To reduce the Eg of IEIC, Hou et al. replaced the 2-ethylhexyl chains of IEIC with alkoxy chains to develop a new acceptor IEICO . Attributing to the strong electron-donating ability of alkoxy chains, IEICO exhibited a lower Eg of 1.34 eV than IEIC. Relative to IEIC, the density functional theory calculation result suggests that the introduction of alkoxy chains effectively up-shifted the HOMO level (~0.19 eV) while maintaining the similar LUMO level (0.01 eV lower than IEIC). The PSCs using IEICO as acceptor yielded a high PCE of 8.4%, with a Voc of 0.82 V and a Jsc of 17.7 mA cm−2, while the control device with IEIC as acceptor exhibited a much lower PCE of 4.9%. In comparison with IEIC-based PSCs, the higher Jsc value of IEICO-based device should be resulted from the much broader photo-response spectrum with higher external quantum efficiency. Bo et al. used bis(alkoxy)-substituted or dialkyl-substituted benzene ring as π bridge for conjugated extension to develop two molecules NA5 and NA6. Benefiting from the non-covalent S···O interaction locks, NA6 exhibited better planarity and broader absorption spectrum than NA5, with a lower Eg of 1.63 eV. In addition, NA6 with locked conformation exhibited a higher quantum yield, which can effectively suppress the non-radiative energy loss and afford higher Voc for devices. The device based on NA6 realized a promising PCE of 9.60%, with Voc = 1.01 V, Jsc = 17.52 mA cm−2, and FF = 0.54, while the NA5 based device showed a much lower PCE of 2.3%. Since the LUMO distribution is mainly located at the acceptor unit in D-A conjugated molecular system, using acceptor unit as building block can simultaneously manipulate the LUMO level and Eg of the molecules. Zhan et al. develop a nonfullerene acceptor NA7 using benzothiadiazole as π-bridge. NA7 shows flat backbone configuration which is beneficial for charge transport, and a large dihedral angle between the hexylphenyl group and backbone plane which can prevent the over self-aggregation when blending with P3HT. Due to the relatively high LUMO level of NA7, the device based on P3HT:NA7 exhibited a high Voc of 0.84 V, with a high PCE of 5.12%. Relative to NA7 (Eg = 1.68 eV), NA8 exhibited a similar Eg of 1.67 eV, while NA9 showed a slightly larger Eg of 1.71 eV. The orientations of the fluorine

molecules.

The device based on PTzBI:NA8 exhibited a PCE of 7.44%, higher than that of PTzBI:NA9 based device (PCE = 5.28%). The photovoltaic performance of NA9 based device is poorer than that of NA8 based device, which should be resulted from the less optimal BHJ morphology. Zhou et al. replaced the benzothiadiazole units of NA7 by benzotriazole units to develop NA10. Due to the weaker electron-accepting ability of benzotriazole than benzothiadiazole, NA10 showed a higher LUMO level than NA7. Therefore, the J61:NA10 based device achieved an encouraging Voc of 1.24 V, with a PCE of 3.02%. In short, the extension of conjugated backbone with donor or acceptor units will generally broaden absorption spectra and reduce Eg of the resulting molecules, and the introduction of donor units as π-bridge is more effective to reduce the Eg than acceptor units. On the other hand, the LUMO levels will be up-shifted when using donor units as π-bridge, while the incorporation of acceptor units will lead to higher LUMO levels. The conjugated side chains substituents on the IDT unit will increase steric hindrance, reduce intermolecular interactions, and prevent over self-aggregation and large phase separation in blend film. Herein, the physicochemical and photovoltaic properties of IDT based acceptors can easily tune via side chains engineering in IDT unit. Zhan et al. developed an acceptor with non-conjugated alkyl chains in IDT unit. NA11 exhibited a nearly flat molecular backbone configuration, with a lower Eg, higher HOMO level, and higher electron mobility than NA7. The device based on PTB7-Th:NA11 yielded a higher PCE of 8.7% than that of PTB7-Th:NA7 based device. Furthermore, the NA11 based device exhibited better thermal stability and photo stability in comparison with NA7 based device. McCulloch et al. reported two alkyl chains substituted IDT based nonfullerene acceptors NA12 and NA 13. NA12 with linear alkyl chains showed a stronger crystallinity and a narrower Eg relative to NA13 with branched chains, resulting in higher Jsc and PCE values. In addition, the oxidative stability of these devices is superior to the benchmark P3HT:PC60BM device. intermolecular interactions and up-shifted the LUMO level of IDTO. The device based on PBDB-T:IDTC exhibited a PCE of 9.35%, with a Voc of 0.917 V, while the PBDB-T:IDTO based device showed a higher PCE of 10.02% and a higher Voc of 0.943 V. Hou et al. introduced fluorine atoms onto the end group of IEICO (IEICO-4F) to enhance the intramolecular charge transfer effect. IEICO-4F showed lower Eg of 1.24 eV and higher LUMO level of −4.19 eV than IEICO. Using IEICO-4F as acceptor, PBDTTT-EFT or J52 as donor, high Jsc values over 20 mA cm−2 were both recorded in the corresponding devices.

Due to the enhanced intermolecular interactions and molecular ordering, IDTN shows a better molecular planarity and higher electron mobility than NA1. Therefore, an outstanding PCE of 12.2% was achieved from the PBDB-TF:IDTN based device, which is significantly higher than that of the NA1 based device (PCE = 7.4%). Zhan et al. used 2-(benzo[c][1,2,5]- thiadiazol-4-ylmethylene)-malononitrile as end-capped groups to develop a new acceptor NA14. Relative to NA7, the stronger electron-withdrawing ability of end-capped units of NA14 leads to a lower Eg of 1.60 eV, lower LUMO of −3.8 eV, and lower HOMO of −5.6 eV. The PBDTTT-C-T:NA14 based device afforded a relatively high PCE of 4.26%. Zhou et al. systematically engineered the end-capped units of three nonfullerene acceptors to carefully tune the driving force for high Voc and Jsc values.

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Research Scholar, Pune University, Maharashtra