Additive-Induced Stabilization of the Energetic Landscape of PM6:Y12 Organic Solar Cells
Solvent additive engineering is a common strategy in organic photovoltaic (OPV) fabrication to improve film morphology and enhance device performance by controlling phase-separation kinetics and crystallinity. However, its effect on photostability, particularly with respect to the evolution of the energetic landscape under operational stress, remains unclear. This study investigates the impact of the additive 1-chloronaphthalene (1-CN) on the evolution of the device's energetic landscape in PM6:Y12 bulk heterojunction organic solar cells upon photoaging. Ultraviolet photoemission spectroscopy combined with argon gas cluster ion beam depth profiling is employed to probe the depth-resolved evolution of donor (PM6) and acceptor (Y12) energy levels before and after photodegradation. Our findings show that in additive-free devices, photodegradation leads to a significant 200 meV downward shift in the PM6 highest occupied molecular orbital (HOMO) level, reducing the donor-acceptor HOMO offset and impairing the driving force for hole transfer. As a consequence, the device experiences substantial efficiency loss. On the other hand, the incorporation of 1-CN effectively stabilizes the PM6 HOMO level, preserving adequate driving force for efficient exciton dissociation. Advanced X-ray diffraction characterization reveals more pronounced nanostructural degradation in blends without 1-CN than those with 1-CN upon photoaging. Collectively, these findings identify PM6 as the primary degradation pathway in PM6:Y12 blends and demonstrate that 1-CN enhances device stability by stabilizing PM6 energetics and preserving the nanostructural integrity upon photoaging.