Ultraviolet-ozone anode surface treatment and its effect on organic solar cells

The effect of ultraviolet (UV)-ozone-treated indium-tin oxide (ITO, anode) on the initial parameters and degradation in small-molecule organic solar cells (OSCs) with the structure of ITO/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (cathode buffer)/Ag (cathode) was experimentally investigated. A UV-ozone exposure with a UV intensity of 13.5 mW/cm2 on the ITO surface was examined. The results indicated that compared with OSCs with no UV exposure, the initial short-circuit current density and power-conversion efficiency ηp increased in the OSC by 20% and 13%, respectively, after 1 min of UV-ozone exposure. A repeated illumination stress comprising 3 s of illumination and 12 s of darkness was imposed on OSCs in air. The UV-ozone-treated OSCs showed excellent durability under the repeated illumination stress. After 10 min of UV-ozone exposure, the OSCs exhibited only an 8% decrease in ηp after 100 illumination cycles. However, with no UV exposure, ηp decreased by approximately 65%, and the OSCs developed strong S-shaped kinks in their current-voltage characteristics, suggesting an increase in series resistance at the ITO-CuPc interface. The UV-ozone treatment much reduces the carbon contaminants on the ITO surface and increases the clean surface with polar components of OH functional groups and negatively charged oxygen. This generates attractive force between ITO and CuPc film. The attractive force prevents further CuPc crystal disorder and void formation at the ITO/CuPc interface, and achieves the durability against the illumination stress.