Tuning the Solubility Parameters of Conjugated Polymers via Side-Chain Engineering for All-Polymer Solar Cells

Bulk heterojunction (BHJ) all-polymer solar cells (all-PSCs) have garnered significant attention as a promising solution for harnessing clean, renewable solar energy due to their merits of mechanical flexibility, morphological stability, and compatibility with large-scale solution-based manufacturing processes. Extensive efforts have been devoted toward controlling and optimizing the miscibility and morphology of polymer blends to enhance the performance of all-PSCs. Herein, we systematically explore molecular structure-miscibility-device efficiency relationships in all-PSCs. Specifically, the Hansen solubility parameters (HSPs) of representative donor and acceptor polymers were broken down into their backbone and side chain contributions. The effects of nonpolar or polar side chains on the resulting solubility parameters are correlated to empirically and experimentally determined HSPs as the different side chains were shown to significantly influence the resulting HSPs of the conjugated polymers. Furthermore, the HSP framework enabled miscibility predictions between polymer blends, which were verified by experimental determinations of miscibility via contact angle and Atomic Force Microscopy (AFM) measurements. Differences in photovoltaic performance between the four blend systems can be directly attributed to alterations in the side chains. This work establishes coherent structure-solubility parameter correlations by showing that as guided by the HSP framework, the miscibility between donor and acceptor polymers can be predictively manipulated via side-chain engineering, creating opportunities to pursue improvements in device efficiency and stability.