Dark Side of Organic Solar Cells: How Triplet Excitons Shape Charge Recombination and Efficiency Limits

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 via a complex interplay between multiple excited states—excitons, charge transfer (CT), and charge-separated states—arising at the donor-acceptor heterojunction. Here, we present a comprehensive 5-state kinetic model that, for the first time, integrates both singlet and triplet CT states and explicitly incorporates the formation and resplitting of local triplet excitons, fully parametrized by the interfacial energy offset. This model not only reproduces essential photovoltaic observables—such as charge generation efficiency, photoluminescence, electroluminescence, and the Langevin reduction factor—but also reveals that triplet-mediated recombination governs device performance across a wide range of energy offsets. Strikingly, we find that in systems with moderate offset, triplet decay emerges as the dominant recombination pathway, reconciling long-standing experimental findings, including those in benchmark systems such as PM6:Y6. The model 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. These results advance the fundamental understanding of exciton and charge dynamics in organic photovoltaics, and provide predictive guidance for overcoming the apparent efficiency ceiling of ∼20% for binary donor-acceptor blends, paving the way for next-generation high-performance devices.