A Unified Thermodynamic Framework for Photocurrent Generation and Voltage Losses in Organic Photovoltaics.

Abstract

Minimizing voltage losses while maintaining efficient charge generation remains a central challenge in organic photovoltaics (OPVs). Although interfacial energetic disorder intrinsically broadens the density of states (DOS), establishing macroscopic relationships between charge-transfer (CT) energetics and device performance is essential for rational material design. Here, we propose a unified scaling framework across a broad library of wide-bandgap OPVs using an effective emissive CT energy to represent the characteristic energy of the emissive CT manifold within the broadened DOS. We demonstrate that the free-carrier energy (ECS) provides a robust thermodynamic boundary for sustaining photocurrent under near-threshold conditions, highlighting the importance of free-carrier energetics in near-threshold operation. Importantly, the robustness of this macroscopic scaling is supported by the strong correspondence and internal consistency between independently evaluated CT energetics and radiative voltage-loss analysis. Within this framework, donor molecular rigidity is associated with reduced CT relaxation/broadening and effective CT exciton binding energies below 0.1 eV, thereby enabling efficient photocurrent generation near minimal energetic offsets. This scaling framework provides a practical guideline for navigating voltage-loss trade-offs and approaching low-voltage-loss operation near the thermodynamic limit.