Enhanced Photovoltaic Performance of Amorphous Copolymers Based on Dithienosilole and Dioxocycloalkene-annelated Thiophene

Organic photovoltaics (OPVs) have attracted considerable attention due to their potential for generating renewable energy. The power conversion efficiency (PCE) of the OPVs largely depends on the organic semiconducting materials. Thus, the elucidation of structureproperty OPV performance relationships is important for the rational improvement of OPVs. Here, low-bandgap copolymers comprising dithieno[3,2-b:2',3'-d]silole as a donor unit and dialkyl-substituted naphtho[2,3-c]thiophene-4,9-dione as an acceptor unit were synthesized to investigate the influence of the polymer molecular weight and the alkyl chain length in the acceptor unit on the polymer properties and photovoltaic performance. All the prepared copolymers are amorphous in the solid state. Both the increase of polymer molecular weight and variation of the alkyl side chains in the acceptor unit subtly affected molecular properties. However, these structural modifications showed significant impact on the photovoltaic performance in bulk heterojunction (BHJ) solar cells based on copolymer/[6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM), with PCEs that range between 2.35 and 5.21%. Furthermore, the optimization of thin-film fabrication by use of a ternary solvent system led to the appearance of improved morphology accompanied by subtly ordered states of the copolymer in the BHJ films and, hence, improved carrier mobility and charge-separation efficiency. Consequently, the BHJ solar cell can achieve a PCE of 7.85%, which is the highest performance among the amorphous copolymers in the conventional device structure. This result highlights the importance of fine-tuning both the molecular structure and device fabrication in the construction of high-performance organic photovoltaics based on amorphous copolymers and PC71BM.