Mapping Interfacial Energetic Landscape in Organic Solar Cells Reveals Pathways to Reducing Nonradiative Losses

Abstract

Bulk heterojunction (BHJ) organic solar cells have made remarkable inroads towards 20% efficiency, yet nonradiative recombination losses (ΔVnr) remain high compared to silicon and perovskite photovoltaics. Interfaces buried within BHJ blends hold the key to recombination losses but access to their energetic landscape underpinning charge transfer (CT) states and their disorder remain elusive. Here, we reveal the energetic landscape and CT state manifold of modern BHJs with both spatial and energetic resolutions and link the offset between singlet (ES1) and CT energy (ES1-CT) and interfacial energetic disorder with ΔVnr. 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. A key new ability is the identification of the specific BHJ interfaces associated with the CT manifold, including where the lowest energy CT states reside. Moreover, 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-CT offset and interfacial energetic disorder on ΔVnr across morphologically varied BHJs. Our results show both factors influencing energy losses in different ways. We demonstrate that PM6:Y6 can achieve low ΔVnr by forming a nominally sharp D/A interface with exceptionally low interfacial disorder via judicious processing combined with a low S1 to CT offset. This provides a design rule to minimize ΔVnr for modern NFAs: sharp D/A interfaces with low S1 to CT offset exhibiting minimal interfacial disorder.