Mapping the interfacial energetic landscape in organic solar cells reveals pathways to reducing non-radiative losses

Abstract

Bulk heterojunction (BHJ) organic solar cells have made remarkable inroads toward 20% power conversion efficiency, yet non-radiative recombination losses (ΔV_nr) remain high. Here, we spatially map the energetic landscape of BHJs and ascribe charge transfer (CT) states to each interface, revealing where non-radiative recombination losses occur. We do so by locally mapping the energy distributions of modern PM6-based BHJs using scanning tunneling microscopy (STM) in combination with sensitive external quantum efficiency (s-EQE) analysis. The non-radiative energy losses are dictated by a combination of the singlet (S₁) to CT energy offset (ΔE_S1-CT) and the interfacial energetic disorder. PM6:Y6 achieves low ΔV_nr by forming a sharp donor/acceptor (D/A) interface with low interfacial disorder that can be tuned by judicious formulation and processing of the BHJ. The emerging design rule for low ΔV_nr in modern non-fullerene acceptors (NFAs) is to achieve sharp D/A interfaces with minimized ΔE_S1-CT and low interfacial electronic disorder of both D and A components.