The role of oxygen in CuInSe2 thin films and CdS/CuInSe2 devices

Abstract The solar cell device CdS/CuInSe2 has been shown to require a post-treatment in air at about 200 °C to reach its state-of-the-art efficiency of close to 11%. We have shown that treating the device in a solution of chemical oxidants has the same effect as the annealing in air. The results of oxidation can be reversed by treating the device in a solution of a reducing agent (hydrazine). Several experimental techniques were employed to show that the oxidation process, whether it is chemical or air annealing, serves to introduce oxygen into the CuInSe2 whereas the reduction process serves to remove it. The acceptance of oxygen depends on the stoichiometry of CuInSe2, and in turn affects the electrical properties. It is this interdependence between stoichiometry and the incorporation of oxygen which determines the extent of improvement in the CdS/CuInSe2 performance. We have also shown that the oxygen in the CuInSe2 can be dislodged from the CuInSe2 material upon exposure to an electron beam. We present electron beam induced current measurements on the CdS/CuInSe2 and argue that the oxygen is being partially removed from the CuInSe2 layer by the electron beam and hence show a buried homojunction rather than the expected heterojunction. Finally, we demonstrate how we can use the role of oxygen in this material to improve the electrical properties of the CuInSe2 thin film and the performance of the CdS/CuInSe2 device.