Organic photovoltaic cells utilising ZnO electron extraction layers produced through thermal conversion of ZnSe

Abstract

In this work, a thin ZnSe layer was deposited in a vacuum and then thermally annealed in air to provide an efficient electron extraction layer for an inverted organic photovoltaic (OPV) cell. Annealing the ZnSe film at 450 °C (ZnSe_{(450 °C)}) increased the device performance and gave an efficiency of 2.83%. X-ray photoelectron spectroscopy (XPS) measurements show that the increased device performance upon annealing at 450 °C is due to the thermal conversion of ZnSe to ZnO. ZnO has a wider band gap than ZnSe, which allows for more light to reach the photoactive layer. The electronic structures of the treated ZnSe films were explored by ultraviolet photoemission spectroscopy (UPS) which showed that the ZnSe_{(450 °C)} films had a Fermi level close to the conduction band edge, allowing for efficient electron extraction compared to the energetic barrier for extraction formed at the ZnSe_(RT)/organic interface.