Broader contextPhotovoltaics (PVs) harvest electrical energy directly from sunlight and are potentially a major component of the solution to the growing energy challenge. Current PV technology is dominated by silicon (Si) and the performance has improved steadily as the technology has matured. However, the cost remains uncompetitive with traditional sources of power and there is a clear need for new inexpensive PV technologies. Devices based on organic semiconductors (OPVs) are therefore attracting a great deal of attention. They can be fabricated on large area, lightweight, flexible substrates and they offer a path to cost competitiveness with fossil fuel power generation. However, the performance of OPVs remains poor when compared to Si-based devices and it is widely recognised that significant improvements will be required for the technology to develop. In this communication, we focus on one of the most important issues in OPV device development, namely the interface between the transparent conducting electrode (indium tin oxide, ITO) and the organic donor layer (chloroaluminium phthalocyanine, ClAlPc). Specifically, we show how insertion of an ultra-thin molybdenum oxide layer at the ITO/ClAlPc interface greatly improves both the efficiency and stability of ClAlPc/C60devices. Our results highlight the importance of understanding and controlling interface properties.