Stoichiometry-Dependent Charge Carrier Generation Mechanisms in PM6:Y6 Blends

Abstract

Understanding how donor–acceptor stoichiometry governs charge carrier separation in non-fullerene bulk heterojunction active layers remains incomplete. By combining transient absorption and time-resolved photoluminescence spectroscopy with selective donor excitation, we elucidate the composition-dependent charge carrier generation pathways in PM6:Y6 blends. Donor excitations evolve via two competing parallel channels: direct charge transfer (CT) and energy transfer to the acceptor followed by subsequent hole transfer. We find that direct ultrafast CT state formation dominates in near-optimal blends, resulting in efficient carrier generation. Under moderate compositional imbalance, CT state formation and subsequent charge separation are only weakly perturbed, and the overall dynamics remain largely unchanged. In contrast, strong compositional imbalance introduces distinct limiting mechanisms. In strongly donor-rich blends, a significant fraction of donor excitons decay within the donor phase before reaching the donor-acceptor heterojunction. In strongly acceptor-rich blends, the energy transfer pathway is favored, accompanied by relaxation losses of acceptor excitons within the acceptor phase. Furthermore, both non-optimal compositions hinder efficient CT state splitting into separated charges, likely resulting from trapping of one of the charges within less favorable donor or acceptor domains. This work clarifies how efficient charge separation emerges in low-offset non-fullerene PM6:Y6 blends and identifies stoichiometry as a key parameter governing charge generation efficiency.