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

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 C₆₀ 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 S₀→S₁ transition makes this excitation sensitive to the polarizable environment, with dielectric stabilization red-shifting S₁ and bringing LE and CT configurations into near resonance. This polarization-driven LE-CT hybridization contrasts with the situation in ITIC and C₆₀, where S₁ remains LE (Frenkel)-like and CT states lie energetically higher. Also, reports of intrinsic free-charge photogeneration in neat Y6 and C₆₀ 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.