Nonradiative Energy Loss in Organic Solar Cells—Linking ΔV_nr to Morphology and Photostability

Bulk heterojunction (BHJ) organic solar cells have made remarkable inroads towards 20% power conversion efficiency, yet nonradiative voltage losses (ΔV_nr) remain the highest as compared to silicon and perovskite photovoltaics. Recent work has revealed nonradiative voltage losses are primarily due to back charge transfer (BCT) through the charge transfer (CT) state resulting in the formation of triplets within acceptor domains. Other work have revealed the CT state disorder and CT-S1 offset both influence nonradiative voltage losses (ΔV_nr). However, the CT manifold associated with interfaces buried within the BHJ blend, holds the key to these recombination losses and insights into the energetic landscape underpinning CT states and their disorder remain elusive. In this talk, we reveal the energetic landscape and CT state manifold of modern BHJs with both spatial and energetic resolutions and link the offset between singlet (S1) and CT energy (S1-CT) and interfacial energetic disorder with ΔV_nr. We do so by locally mapping the energy distributions of modern PM6-based BHJs with IT4F, Y6 and PC₇₁BM acceptors and combine it, for the first time, with sensitive EQE measurements, to visualize and quantify donor (D) and acceptor (A) energetics at interfaces and associated them with CT states within a modified Marcus framework. We quantify energy levels and electronic disorders directly at these and other interfaces and connect these contributions to the energy losses. We delineate the influences of S₁ to CT offset and interfacial energetic disorder on the ΔV_nr across multiple morphologically varied BHJs. Our results clearly show both factors influencing energy losses and that changing the interfacial disorder affects non-radiative voltage losses in systems with comparable S₁ to CT offset. We go on to connect these observations of interfacial energetics with ΔV_nr as well as with expected changes in the triplet quantum yield using complementary optical spectroscopies. Finally, we go on to demonstrate how modulating the triplet population in the BHJ through the CT state manifold influences photobleaching rate of the BHJ.