Organic Single‐component Photovoltaics: The Critical Role of the Interplay between Local‐exciton and Charge‐transfer Electronic States

Abstract

ABSTRACT The emergence of non‐fullerene acceptors (NFAs), particularly the Y‐type series, has reshaped organic photovoltaics, nowadays enabling ∼21% efficient solar cells with high charge generation and low voltage loss. Yet, the origin of these properties is still not entirely understood. Here, we describe a unified picture of the lowest electronic excited states in Y6 films and contrast them with those prevalent in ITIC and C 60 films. Y6 supports hybrid local‐exciton (LE)–charge‐transfer (CT) states stabilized via intermolecular electronic couplings and short π–π contacts, which result in excimer‐like states delocalized over aggregates. The large change in dipole moment for the S 0 →S 1 transition makes this excitation sensitive to the polarizable environment, with dielectric stabilization red‐shifting S 1 and bringing LE and CT configurations into near resonance. This polarization‐driven LE–CT hybridization contrasts with the situation in ITIC and C 60 , where S 1 remains LE (Frenkel)‐like and CT states lie energetically higher. Also, reports of intrinsic free‐charge photogeneration in neat Y6 and C 60 films are discussed; devices are found to deliver efficiencies <1% unless aided by transport layers or donor additives. These insights define design rules for NFAs—favoring dipolar transitions, co‐facial packing, and near‐resonant LE‐CT energetics—to realize single‐component photovoltaics with built‐in charge separation.