Critical Role of Polymer Aggregation and Miscibility in Nonfullerene-Based Organic Photovoltaics

Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature-dependent aggregation, namely PffBT4T-2OD [poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3 "'-di(2-octyldodecyl)-2,2 ';5 ',2 ";5 ",2 "'-quaterthiophen-5,5-diyl)], also known as PCE-11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O-IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O-IDTBR are a strong function of the chemical composition, leading to different donor-acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid-liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor-acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor-acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends.