Origin, Nature, and Location of Defects in PM6:Y6 Organic Solar Cells

Targeted strategies to overcome defects in organic semiconductors require insight into their identity and origin. Here the formation, nature, and location of defects is studied in PM6:Y6 organic solar cells by sensitive EQE measurements. Exposure of the active layer to ambient atmosphere and H₂O-saturated compressed air indicates that a trace constituent in ambient air causes the formation of defects. By exposing the active layer to O₃-enriched air, O₃ is identified as the species creating defects in PM6:Y6 blends. Aging of complete inverted (n–i–p) configuration solar cells in H₂O-saturated compressed air also increases the defect response. This is attributed to a reduced band bending at the PM6:Y6 | MoO₃ hole-collecting contact, caused by a change in work function of MoO₃ interacting with the H₂O, which allows more defect states to be filled and available for photoexcitation. By measuring energy resolved-electrochemical impedance spectroscopy and by fabricating semitransparent cells, regular architecture cells, and semitransparent cells with an optical spacer−mirror stack it is found that defects originate predominantly from PM6 and are located near the top electrode, independent of device polarity. Because O₃ is omnipresent in ambient atmosphere, albeit in small amounts, it likely causes defects in many organic semiconductors exposed to ambient air.