Fine Tuning Miscibility of Donor/Acceptor through Solid Additives Enables All‐Polymer Solar Cells with 15.6% Efficiency

Optimization of the photovoltaic performance of all-polymer solar cells (all-PSCs) includes delicate control of the film morphology of the light-harvesting layer. Although miscibility of polymer donors and polymer acceptors plays a critical role in the description of film morphology of all-PSCs, the mixing thermodynamics is unrevealed. Herein, we demonstrate that by incorporating 1% weight ratio of PC71BM as the solid additive into the blends of electron-donating polymer PTzBI-oF and electron-accepting polymer PFA1, the miscibility of donor/acceptor can be improved by virtue of forming a favorable phase separation, which leads to an increased charge carrier transport and simultaneously enhanced fill factor. The maximum power conversion efficiency is thereby improved from 14.6% to 15.6%. The miscibility of two components in the photoactive layer can be quantitatively described using the Flory−Huggins interaction parameter (χ). In particular, a correlation between the Flory−Huggins parameters of the two components, in terms of phase separation morphology and device performance of all-PSCs, is established and the mechanism by which PC71BM is added to this system is explored. This study establishes guidelines for the selection of solid additives when optimizing the efficiency of all-PSCs and promotes the integration and development of polymer physics and organic photovoltaics.