Optimized Phase Separation and Reduced Geminate Recombination in High Fill Factor Small-Molecule Organic Solar Cells

In recent years, a rapid increase in the power conversion efficiency above 10% in small-molecule-based organic solar cells (SM-OSCs) has been made possible. However, one of the key device parameters, fill factor (FF), which is mainly limited by comprehensive courses, including charge generation, recombination, transport, and extraction, still remains moderate. Here we demonstrate a record high FF of 78.35% in SM-OSCs obtained through dichloromethane solvent vapor annealing, which provides optimized phase-separation morphology for efficient charge generation and facilitates charge transport and extraction at the same time. We use a combined charge dynamic measurement and current-voltage characteristic reconstruction to identify that geminate recombination loss that resulted from undesired film morphology is mainly responsible for low FF in the pristine devices. Even higher FF that is comparable with that of crystal silicon solar cells in organic solar cells is very likely with the presence of charge mobility around 5 × 10^-3 cm² V^-1 s^-1 and proper film morphology.