The double-edged function of UV light in polymer solar cells with an inverted structure

Inverted polymer solar cells with a configuration of ITO/ZnO/PBDTTT-C:PC₇₁BM/MoO₃/Ag were fabricated and a power conversion efficiency of 6.64% was achieved under AM 1.5G irradiation (89 mW/cm²). The function of UV light was systematically investigated by tracking the changes of the device performance for 212 min under continuous illumination with or without UV light. We found that UV light plays a decisive role not only in the photo-annealing step, leading to a remarkable increase of PCE by increasing ZnO conductance, but also in the photo-bleaching step, causing slow degradation of solar cells through photo-oxidation of the conjugated polymer.