Understanding the influence of carboxylate substitution on the property of high-performance donor polymers in non-fullerene organic solar cells

Carboxylate substitution is a common approach to tune the energy level of donor polymers for organic solar cells. However, the influence of carboxylate substitution on the morphological and electronic properties of donor polymers is not well understood. In this paper, we study two pairs of structurally similar terthiophene or quarterthiophene donor polymers with partial or complete carboxylate substitution on the alkyl side chains. It is found that the carboxylate substitution can enhance the crystallinity of the donor polymers and introduce larger and purer domains. Moreover, the polymers with the carboxylate substitution exhibit reduced bimolecular recombination due to the improved morphology. For device efficiencies, the terthiophene-based polymer, P3TEA (with 50% carboxylate substitution), exhibits the best performance. The alkyl side chains on P3TEA provide a typical temperature-dependent aggregation property, allowing for effective morphology control, while the carboxylate substitution deepens the HOMO level and enhances the crystallinity of the polymer. These benefits yield a near optimal morphology and high Voc value, and thus the best device efficiency among the polymers studied.