Role of Electronic States and Their Coupling on Radiative Losses of Open-Circuit Voltage in Organic Photovoltaics

Voltage losses (ΔV_OC) are a crucial limitation for the performance of excitonic organic solar cells (OSCs) and can be estimated by two approaches─the radiative limit and the Marcus charge-transfer (MCT) model. In this work, we show that combining the radiative limit and MCT models for voltage loss calculations provides useful insights into the physics of emerging efficient OSCs. We studied nine different donor-acceptor systems, wherein the power conversion efficiency ranges from 4.4 to 14.1% and ΔV_OC varies from 0.55 to 0.95 V. For these state-of-the-art devices, we calculated the ΔV_OC using the radiative limit and the MCT model. Furthermore, we combined both models to derive new insights on the origin of radiative voltage losses (ΔV_rad) in OSCs. We quantified the contribution in ΔV_rad due to the bulk intramolecular (S₁) disorder and interfacial intermolecular (CT) disorder by revisiting the spectral regions of interest for OSCs. Our findings are in agreement with the expected relationship of V_OC with Urbach energy (E_U), which suggests that the low E_U is beneficial for reduced losses. However, unprecedentedly, we also identify a universal, almost linear relationship between the interfacial disorder (λ) and ΔV_rad. We believe that these results can be exploited by the organic photovoltaic (OPV) community for the design of new molecules and a combination of donor-acceptors to further improve OSCs.