Bis(thieno[3,2-b]thieno)cyclopentafluorene-Based Acceptor with Efficient and Comparable Photovoltaic Performance under Various Processing Conditions.

The morphology of a bulk heterojunction (BHJ) blend within a polymer solar cell (PSC) device plays a crucial role in its performance. The ideal morphology is generally achieved through molecular engineering and optimization under film processing conditions. Under different processing conditions, the deviation of the resulted morphology characteristics from the ideal one leads to the dispersion of device performance. For a specific donor/acceptor BHJ blend, it is of great challenge to maintain an efficient and comparable photovoltaic performance under various processing conditions. The solution to this challenge would be of great value in offering more choices for a suitable processing technology in practical applications. Based on the acceptor BTTFIC with the core of bis(thieno[3,2-b]thieno)cyclopentafluorene (BTTF) in our previous work, we chemically modified BTTFIC by fluorination of the end groups of 1,1-dicyanomethylene-3-indanones (IC) and the switching part of octyls in BTTF with 4-hexylphenyls to offer a novel acceptor (BTTFIC4F-Ar). The inverted PBDB-T-2Cl:BTTFIC4F-Ar blend device provided an average power conversion efficiency (PCE) of 10.61, 11.08, and 11.55% when processed under solvent annealing (SA), thermal annealing (TA), and additive treatment with 1,8-diodooctane (DIO), respectively. Different from the reported discrete performance under various processing conditions for a specific donor/acceptor BHJ blend, a low mean absolute performance deviation of 3% was attained. This slight enhancement trend was unexceptionally reflected on charge generation, transportation, and recombination within the blend films from SA, TA, and DIO conditions. A slightly improved ordering of BTTFIC4F-Ar within the DIO blend was observed. Meanwhile, very similar molecular packings as well as a close amorphous domain size of the mixture of PBDB-T-2Cl and BTTFIC4F-Ar within the three blends were observed. These morphological characteristics are in good agreement with the photoelectrical conversion performance of the blends under the three processing conditions. Furthermore, similar attenuation behaviors in performance were also observed. This investigation may provide new guidance on the molecular engineering of nonfullerene acceptors to achieve an efficient BHJ blend with more options for a suitable and cost-effective processing method in practical applications.