Efficient Solar Cells Based on a Polymer Donor with β-Branching in Trialkylsilyl Side Chains

Side-chain engineering is an important strategy in designing novel polymer semiconductor materials for high-efficient organic solar cells. The use of trialkylsilyl side chains can improve the photovoltaic efficiency by decreasing the energy of the highest occupied molecular orbital (HOMO) of the polymer and improving its crystallinity and hole mobility. Compared to simple linear derivatives, α-branching in the alkyl groups of trialkylsilyl side chains causes strong aggregation and excessive phase separation in the photoactive layer, leading to poor device performance. β-branching of the alkyl groups has not yet been used in trialkylsilyl side chains. Herein, we describe a new polymer (J77) with triisobutylsilyl side chains to investigate the effect of β-branching on the molecular aggregation, optical properties, energy levels, and photovoltaic properties. We find that compared to α-branching, β-branching of alkyl groups in trialkylsilyl side chains significantly reduces aggregation. This enables J77 to form blend morphologies in films that provide high-efficient solar cells in combination with different non-fullerene acceptors. Moreover β-branching of the alkyl groups in trialkylsilyl side chains lowers the HOMO energy level of J77 and increases the open-circuit voltage of J77-based solar cells without sacrificing short-circuit current density or fill facto