The Energy Structure of Spin States in Reducing the Nonradiative Voltage Loss in Organic Solar Cells

In organic solar cells (OSCs), the nonradiative voltage loss (ΔV_nr) has been identified as a critical factor for the relatively lower open-circuit voltage. Under open-circuit conditions, most of the charge recombination processes occur via the triplet exciton state, underscoring the importance of the energy structures concerning the local exciton (LE) and charge transfer (CT) spin states. In this Letter, we propose a five-state model to explore the spin state energy structures to reduce ΔV_nr. Our calculations reveal that, to minimize ΔV_nr, the spin singlet state for LE should possess a lower energy than the triplet state, E_S < E_T. In contrast, the energies of the CT spin states have a negligible effect on ΔV_nr. We identify the best energy structure as E_S < E_T ∼ E_CT. Moreover, our calculations demonstrate that strong couplings between these spin states, particularly involving spin flip, can effectively mitigate ΔV_nr. These findings present novel insights for the advancement of OSCs.