A Comprehensive Study of Polymer Donor Aggregation Effects in Two Different Types of Non-Fullerene Organic Solar Cells

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. Three BDT-ttTPD-based polymer donors with different aggregation behaviors are designed and synthesized, which are employed to P(NDI2OD-T2)-based all-PSC and Y6-based NFSMA-PSC systems. We demonstrate 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 distinct 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.