Control of the film structure of polymer solar cells by manipulating the crystallization of conjugated polymers

Conjugated polymers are a promising material for polymer solar cells (PSCs) due to their intrinsic advantages, including flexibility, solution processability, light weight, and low cost. Conjugated polymer blend films possess an aggregated structure that is a bulk heterojunction (BHJ) morphology formed through a combination of crystallization and phase separation, and is crucial for device performance. The current reviews mainly focus on strategies for tuning the film structure and examining the structure-performance relationship, but the crystallization mechanisms behind them have not been discussed in depth. This review provides a systematic understanding of how to control the BHJ structure in terms of polymer crystallization. We first discuss the possible routes to form BHJ morphologies and conclude that prior crystallization of conjugated polymers is desired to control the film structure. Then, the crystallization mechanism of conjugated polymers is reviewed. Due to the rapid crystallization rate in solution processing, self-seeding via controlled pre-aggregation is crucial for the occurrence of prior crystallization of conjugated polymers. Methods for controlling pre-aggregation of conjugated polymers are reviewed and discussed. Finally, a concise summary is provided, followed by several current challenges.