Effects of alkyl chains of benzothiadiazole-based conjugated polymers on the photovoltaic performance of non-fullerene organic solar cells

Abstract

Alkyl chains are commonly used as solubilizer to guarantee the solubility of conjugated polymers. More importantly, the position, length, and type (linear/branched) of alkyl chains in conjugated polymers also have a substantial influence on the molecular configuration, intermolecular interaction, and associated stacking/packing modes. In this work, two donor polymers based on a benzothiadiazole (BT) unit, PBDT-T12BT with linear side chains and PBDT-TEhBT with branched side chains, were synthesized for application in organic solar cells (OSCs). It was found that the introduction of branched alkyl chains could not only improve the aggregation of polymer backbones both in solutions and thin films, but also enhanced the crystallinity in blended films. The higher aggregation/crystallinity of PBDT-TEhBT provides optimal morphology, enhanced charge-transporting ability, and suppressed charge recombination in blended films. Hence, PBDT-TEhBT-based OSCs exhibited a high power conversion efficiency (PCE) of 15.5% compared to 5.7% for cells based on PBDT-T12BT, presenting the highest performance based on BT-polymers as donor in OSCs.