Photovoltages exceeding the band gap observed with WSe2/I? solar cells

Abstract

Photovoltages ranging between 1.2 and 2.1 V have been observed with single-crystal WSe2, grown without transport agent, in contact with I−/I3−3 solutions. This effect is found only with surfaces which have not been renewed by peeling. They show negligible dark currents and both cathodic and anodic photocurrents. The abnormally high photovoltage sometimes disappeared without evident reason but it could nevertheless be used to drive a stable solar cell with an efficiency between 1.5 and 5% for a period of five days. Optical and electrochemical studies show that the phenomenon is due to the presence of two different types of material which coexist at the same surface with different band gaps and flat band potentials. One type shows a compensated behaviour, the other n-type behaviour. High photovoltages are obtained when anodic and cathodic photocurrents, generated simultaneously but at different wavelengths, compensate each other, thus shifting the zero-current point towards more negative potential values. Large photovoltages are therefore only possible while photocurrents are circulating through the two interfaces, thus dissipating energy. The observed phenomenon, though at present fragmentarily understood, could be of some value for the development of semiconductor electrodes with high photovoltages for the decomposition of water.