Role of donor fluorination in voltage losses of organic solar cells

Organic solar cells have exhibited high power conversion efficiency of up to 20 % with the rapid development of non-fullerene solar cells in recent years. However, further materials and device optimizations are still necessary due to the considerably large voltage losses compared to traditional inorganic counterparts such as Si or GaAs. In this study, we employed various characterization methods to present a systematic investigation based on fluorinated donors and nine different acceptors to determine the role of donor fluorination in the performance of organic solar cells, especially the voltage losses. We found that the devices with fluorinated PM6 as donor exhibit significantly lower voltage losses (including radiative and non-radiative) than the ones with non-fluorinate PBDB-T donor when paired with different typical non-fullerene and fullerene acceptors, which can be attributed to the reduced charge transfer state reorganization energies, lower Urbach energy, and enhanced radiative decay rate. In addition, the PM6-based devices exhibit generally larger deep trap energy than the PBDB-T ones, but it is not the determinant to the voltage losses. These results enable us to identify the key features for minimizing the voltage losses and suggest that molecular design strategies focusing on the reduction of reorganization energy along with Urbach energy and increase of radiative decay rate is crucial for further improving the performance of organic solar cells. • Fluorination of donor shows lower voltage losses than the non-fluorinated one. • Reduced voltage losses are due to lower reorganization energy and Urbach energy. • CT state reorganization energy is positively correlated with non-radiative losses. • Donor fluorination can significantly enhance the radiative decay rate. • Designing materials with low reorganization energy and Urbach energy is crucial.