The Phase Separation Control in All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) are of interest owing to their unique advantages, including remarkably improved device stability and exceptional mechanical stretchability. Over recent years, there has been a notable increase in the power conversion efficiency (PCE) of all-PSCs, largely attributed to advancements in the morphology control of the active layer. Notably, the domain size is of paramount importance as it impacts critical factors such as exciton dissociation, charge transport, and collection. However, the low glass transition temperature of conjugated polymers, coupled with a minimal change in mixing entropy, often results in an excessive degree of phase separation. Consequently, it is essential to comprehend the evolution of phase separation and develop strategies to regulate the domain size. In this review, we elucidate the key parameters that contribute to the enhancement of phase separation and present qualitative and quantitative characterization techniques for domain size. Building on this foundation, we introduce the strategies and principles for regulating domain sizes, encompassing factors such as crystallinity, miscibility, and molecular conformation from a thermodynamic perspective, as well as the film-forming kinetics and the crystallization sequence from a kinetic perspective. Lastly, we offer insights into the current challenges and potential future prospects for the evolution of all-PSCs.