Publication date: 15th December 2025
The power conversion efficiency of organic solar cells has recently surpassed 20%, yet a unified framework that captures the underlying photophysical mechanisms remains elusive. These devices operate through a complex excited-state choreography arising at the donor-acceptor interface where excitons, charge transfer (CT) and charge-separated states continuously interconvert according to the materials’ energetic landscape.
In this talk, we share our five-state kinetic model that, for the first time, explicitly incorporates the formation and re-splitting of local triplet excitons. Fully parameterized by the interfacial energy offset, this unified framework reproduces key photovoltaic observables – such as the charge generation efficiency, photoluminescence, electroluminescence and Langevin reduction factor. Our results indicate that the triplet state dynamics govern device performance across a wide range of energy offsets. In systems with moderate offset, triplet decay emerges as the dominant recombination pathway, reconciling long-standing experimental findings, including those in benchmark systems like PM6:Y6. The model further offers a mechanistic explanation for the empirically observed link between energy offset and non-Langevin recombination, and accurately predicts the device efficiency across different material systems. Notably, it identifies a singlet-CT offset of ~150 meV as optimal for efficient charge separation while suppressing loss pathways.
By connecting excited-state kinetics with macroscopic device metrics, our work offers a unified mechanistic picture of the photophysics in organic semiconductors, in particular the effects of energy offsets and disorder on the generation and recombination pathways of free charges. The insights gathered provide guidance for material and interface design strategies, aimed at overcoming the apparent efficiency ceiling in state-of-the-art organic solar cells.
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