Organic solar cells: Evolution of the morphological and electronic properties as a function of steric hindrance from the acceptor side groups

Abstract

High non-radiative recombination losses are currently limiting the efficiency of organic solar cells (OSCs). Attaching sterically bulky side groups to acceptors is a strategy that has been recently developed to reduce these losses. However, it remains unclear how these side groups impact the nanoscale morphology and electronic properties in neat acceptor films and donor:acceptor blends as a function of their steric hindrance due to the experimental challenge in accessing these aspects, which hinders the further development of more efficient acceptors. Here, taking acceptors with benzene-, trimethylbenzene-, and triisopropylbenzene-functionalized groups on Y6 core shoulders (i.e., BTP-B, BTP-Bme, and BTP-Biso acceptors) and PB2 polymer donor as representative systems, we characterize these aspects via a tight combination of all-atom molecular dynamics simulations and long-range corrected density functional theory calculations. As steric hindrance increases, excessive acceptor aggregation is suppressed; acceptor_acceptor packing distances increase, reducing electronic couplings and intermolecular electron-transfer rates (ke). Interestingly, PB2_PB2 packing distances also increase; however, its packing pattern changes, enhancing electronic couplings and interchain hole-transfer rates (kh). This brings the PB2:BTP-Bme blend more balanced ke/kh ratio. Also, PB2_acceptor packing distances increase, which elevates the charge-transfer states and decreases electronic couplings between these states and the ground state, thereby lowering related non-radiative recombination rates and losses. These together bring the lowest non-radiative recombination losses in the PB2:BTP-Bme-based OSCs. Overall, we draw a comprehensive picture that describes how such strategy improves nanoscale morphology and electronic properties in OSCs, which can be beneficial for further design of highly efficient acceptors.