Performance and stability studies of inverted polymer solar cells with TiO₂ film as a buffer layer

TiO₂ based inverted polymer solar cells (PSCs) with a structure of fluorine-doped tin oxide (FTO)/TiO₂/P3HT:PCBM/PEDOT:PSS/Ag presented excellent air stabilities,; the power conversion efficiency (PCE) of devices exhibited only 15 % decay as compared to the highest value while being exposed in air-condition for more than 20 days. Interestingly, an overall enhancement of PCE from 3.5 % to 3.9 % was observed while the PSCs were exposed in air-condition up to 3 days; the improvement of performance was attributed to the TiO₂ films' oxygen and water protection effect and the oxidation of Ag, which will benefit to form an effective work function match with the HOMO of P3HT leading to improved ohmic contact. However, the performance slowly decreased when the exposure time remains longer due to the physical adsorbed oxygen. UV-ozone treatment on the TiO₂ films' leads to the formation of a metal-deficient oxide that results in a decreased PCE for the devices. Finally, X-ray photo-emission spectroscopy (XPS) was used to analyze the compositional changes of the TiO₂ films while they were exposed in air-condition or treated by UV-ozone.