Wide Band Gap and Highly Conjugated Copolymers Incorporating 2-(Triisopropylsilylethynyl)thiophene-Substituted Benzodithiophene for Efficient Non-Fullerene Organic Solar Cells

Abstract

Recent years have seen a rapid progress in the power conversion efficiencies (PCEs) of non-fullerene polymer solar cells (NF PSCs). However, the donor materials accordingly used are typical low or medium band gap polymers, some of which possess badly overlapped absorption spectra relative to the low band gap n-type acceptors, for example, 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-indaceno[1,2-b:5,6-b']dithiophene) (ITIC). To obtain polymers simultaneously owning a wide band gap, a highly extended π-conjugation system, and a low-lying highest occupied molecular orbital (HOMO), a polymer (PBDTSi-TA) incorporating 2-(triisopropylsilylethynyl)thiophene substituted benzodithiophene (BDTSi) and fluorinated benzotriazole (FTAZ) units was designed and synthesized. PBDTSi-TA (Egopt = 1.92 eV) exhibits strong molecular aggregation properties and a lower-lying HOMO energy level compared to its structural analogues. When blended with ITIC and after device optimization with solvent vapor annealing in combination with a developed PDIN/BCP/Ag cathode structure, PSCs yielded a PCE of 7.51%, with Voc = 0.96 V. Moreover, a rather small energy loss (Eloss) of 0.6-0.63 eV was determined. For comparison, another polymer (PBDTSi-Qx) with a more-electron-deficient quinoxaline-based acceptor unit was also synthesized and applied to NF PSCs. Charge generation rate, exciton dissociation probabilities, dark leakage current, nanoscale morphology, and charge carrier mobilities have been evaluated to probe the reasons for the differentiated performances. The results suggest that PBDTSi-TA is a promising donor material for NF PSCs, and the molecular design strategy demonstrated here would be helpful for pursuing high-performance polymers for PSCs.