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

Jonathan L. Langentepe-Kong1, Manasi Pranav2, Safa Shoaee1,2,*, and Dieter Neher2,†

  • *Contact author: shoaee@pdi-berlin.de
  • †Contact author: neher@uni-potsdam.de
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PRX Energy 5, 013003 – Published 14 January, 2026

DOI: https://doi.org/10.1103/vsgd-ktnh

Abstract

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.

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Popular Summary

Organic solar cells (OSCs) have recently surpassed the long-sought power conversion efficiency threshold of 20%, yet further improvements remain limited without targeted strategies to enhance light harvesting or charge carrier extraction. In this work, the authors present a unified analytical framework that elucidates the fundamental photophysical mechanisms governing free charge generation and recombination in OSCs, with particular emphasis on the role of triplet states. This framework not only rationalizes the performance limitations observed in current material systems but also offers concrete design guidelines for next-generation non-fullerene acceptors aimed at pushing device efficiencies beyond current records. At the core of this approach is a comprehensive 5-state kinetic model that explicitly incorporates both singlet and triplet charge transfer (CT) states, as well as the formation, dissociation, and decay dynamics of local triplet excitons. This model simultaneously and accurately reproduces key photovoltaic observables such as the charge generation efficiency, the photoluminescence and electroluminescence quantum yields, and the Langevin reduction factor—highlighting the decisive influence of triplet energetics and kinetics on overall device performance. Importantly, the authors find that optimal binary blends are characterized by a moderate singlet-to-CT offset near 150 meV; a regime where triplet decay becomes the dominant recombination pathway. Beyond offering mechanistic insight into efficiency-limiting losses in prominent systems such as PM6:Y6, the presented model enables the extraction of critical physical parameters, such as the singlet-triplet energy gap. These insights inform several promising strategies to overcome the current ∼20% efficiency plateau, including reducing singlet-triplet energy offsets and reorganization energies or enhancing donor:acceptor coupling and optical absorption. Altogether, this work provides a predictive, experimentally validated foundation for the rational design of higher-performing OSC materials and architectures.

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PRX Energy 5, 013003– Published 14 January, 2026
  • Received 22 May 2025
  • Revised 7 November 2025
  • Accepted 8 December 2025
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