Balanced electron and hole transfer behavior enabled approaching 19% efficiency in thick-film organic solar cells with improved fill factor

The development of thick-film organic solar cells (OSCs) is crucial for enhancing reproducibility in large-area industrial fabrication. Unfortunately, the film thicknesses of several hundred nanometers can exacerbate the imbalance in charge transfer between donor and acceptor owing to differences in exciton diffusion length (L_D), leading to severe charge recombination and a marked decline in the fill factor (FF) compared with standard devices. In this work, we systematically investigated how charge transfer mechanisms influence device performance by modulating the active layer configuration in thick-film OSCs. Our findings revealed that balancing electron transfer from donor to acceptor and hole transfer from acceptor to donor, following exciton dissociation at the interface, was a critical factor for achieving a high FF in thick-film devices. This result was further supported by employing a ternary strategy, which facilitated a more balanced charge transfer efficiency, yielding a record high efficiency of 18.92% with a high FF of 76.8% at a film thickness of 300 nm. This study demonstrates broad applicability across other thick-film systems and provides a standardized approach for fabricating high-efficiency devices.