Thermally‐Induced Degradation in PM6:Y6‐Based Bulk Heterojunction Organic Solar Cells

Thermally induced degradation of organic photovoltaic devices hinders the commercialization of this emerging PV technology. Thus, a precise understanding of the origin of thermal device instability, as well as identifying strategies to circumvent degradation is of utmost importance. Here, it investigates thermally‐induced degradation of state‐of‐the‐art PBDB‐T‐2F (PM6):BTP (Y6) bulk heterojunction solar cells at different temperatures and reveal changes of their optical properties, photophysics, and morphology. The open‐circuit voltage and fill factor of thermally degraded devices are limited by dissociation and charge collection efficiency differences, while the short‐circuit current density is only slightly affected. Energy‐resolved electrochemical impedance spectroscopy measurements reveal that thermally degraded samples exhibit a higher energy barrier for the charge‐transfer state to charge‐separated state conversion. Furthermore, the field dependence of charge generation, recombination, and extraction are studied by time‐delayed collection field and transient photocurrent and photovoltage experiments, indicating significant bimolecular recombination limits device performance. Finally, coupled optical‐electrical device simulations are conducted to fit the devices’ current‐voltage characteristics, enabling us to find useful correlations between optical and electrical properties of the active layers and device performance parameters. State‐of‐the‐art photoactive blend PM6:Y6‐based organic solar cells are studied upon thermal annealing at elevated temperatures. Annealing above 140 °C, energetic, structural, and nanoscale morphological modifications considerably reduce charge transfer and extraction, lowering open circuit voltage and fill factor due to the energy barrier at the interface and field‐dependent charge generation.