Synergistic Design of Imidazole-Based Polymer Donors for Enhanced Organic Solar Cell Efficiency

Within the realm of organic solar cells (OSCs), designing new high-efficiency polymer donors remains a significant challenge. Achieving the right balance in polymer backbone planarity is crucial: excessive planarity can lead to undesirable aggregation, while insufficient planarity can hinder the charge transport efficiency. In this study, we designed and synthesized an imidazole-based acceptor (A) unit for the first time and then investigated the impact of backbone planarity on charge transport capacity and power conversion efficiency (PCE). Backbone planarity was precisely tuned by incorporating isomeric alkyl chains on the thiophene π-bridge, resulting in four distinct polymer donors: MZC8-F, MZC8-Cl, MZEH-F, and MZEH-Cl. The results showed that the steric hindrance from the EH-branched alkyl chain induced backbone distortion and caused a blue-shift in the absorption spectrum. MZEH-Cl, with its poor planarity and excessively low HOMO energy level, achieved a PCE of just 7.6%. Through careful modulation, MZC8-Cl emerged as the most efficient, with a remarkable PCE of 17.3%, setting a new benchmark for imidazole-based polymer donors. This study not only deepens the understanding of the role of polymer backbone planarity in photovoltaic performance but also lays the groundwork for developing high-efficiency polymer donors.