Elucidating Important Roles of Polymer Donor Aggregation in Blend Morphology and Device Performance of Different Types of Non-Fullerene Polymer Solar Cells
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We report the first comparative study of polymer donor (PD) aggregation effects in all-polymer solar cells (all-PSCs) and non-fullerene small molecule acceptor (NFSMA)-PSCs based on BDT-ttTPD-based polymer donor series with different aggregation behaviors. It is observed that PD aggregation is a decisive factor in both systems, but its impact is more significant in all-PSCs. The P-Si PD with the strongest aggregation yields severe phase separation in the all-polymer blend, resulting in low power conversion efficiencies (PCEs) of corresponding all-PSCs. In contrast, the P-Si based NFSMA-PSC shows a well-mixed blend morphology, leading to the highest PCE of 12%. These different roles of PD aggregation originate from different entropic contributions to the mixing of two materials, which is inversely related to the molecular size of the acceptor. Thus, our work provides a comprehensive understanding of the PD aggregation-blend morphology relationship in two non-fullerene PSC systems.