Light-soaking free organic photovoltaic devices with sol–gel deposited ZnO and AZO electron transport layers

This paper investigates light-soaking effects in inverted organic photovoltaic (OPV) devices with zinc oxide (ZnO) and aluminum doped ZnO (AZO) electron transport layers (ETL), which is important for the development of low-cost and stable solar cells. The samples demonstrated high solar harvesting properties with power conversion efficiency up to 3.9%. Air-stability tests of up to 150 days were performed on devices with different Al doping levels. The devices maintained higher than 60% of the initial PCE after 50 days of open-air exposure. The light-soaking mechanism was investigated with experiments and simulations and shown to be eliminated when the Al fraction of the AZO is higher than 4%. The simulated band diagram of the OPV devices indicates that the low carrier density in the ZnO layer by virtue of depletion is the main reason of the light-soaking effect. Doping the ZnO layer as well as exposing the devices under UV irradiation will introduce additional free carriers into the ETL and reduce the width of the depletion region at both sides of the ETL.