Publication date: 11th March 2026
Ultrafast spectroscopic signatures of charge generation in non-fullerene organic photovoltaics provide critical insight into the mechanisms enabling high device performance. Here, we investigate how donor–acceptor stoichiometry modulates the temporal evolution of excited states in PM6:Y6 bulk heterojunctions using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By resolving exciton decay, charge transfer state formation, and the emergence of long-lived charge-separated species across a wide compositional range, we directly link blend composition to the dominant microscopic relaxation pathways.
Near-balanced PM6:Y6 blends exhibit rapid quenching of donor excitons accompanied by the prompt appearance of interfacial charge-transfer signatures within a few picoseconds. These states subsequently evolve into stabilized charge-separated populations, evidenced by the growth of acceptor anion photoinduced absorption on sub-100 ps timescales. In donor-rich films, spectroscopic dynamics reveal slowed exciton diffusion and incomplete quenching, indicating reduced interfacial access and inefficient charge separation. Conversely, acceptor-rich blends display pronounced energy transfer to Y6 followed by spectral relaxation into low-energy acceptor states, which competes with and ultimately suppresses charge-transfer formation.
The results demonstrate that stoichiometry dictates not only the yield but also the kinetic hierarchy of charge generation pathways. Efficient free carrier production emerges only when compositional balance preserves both ultrafast interfacial coupling and controlled energetic relaxation within the acceptor phase. These findings establish stoichiometry-resolved ultrafast spectroscopy as a powerful tool for rational design of high-performance non-fullerene organic solar cells.
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