Native oxidation of CuGaSe2 crystals and thin films studied by electron paramagnetic resonance and photoelectron spectroscopy -: art. no. 205321

The oxidation of CuGaSe2 crystals and polycrystalline thin films was investigated with electron paramagnetic resonance (EPR) and photoelectron spectroscopy (PES). An EPR signal assigned to Cu2+ evolves when the samples were stored under ambient conditions for a few months, independent of the morphology of the specimens. The formation of a native oxide layer consisting of Ga2O3 and SeO2 was observed in PES during the initial stage of oxidation. Only after long term oxidation for several months an additional copper hydroxide phase, Cu(OH)(2), was detected. When the Cu(OH)(2) phase was reduced by annealing in vacuum at 200 degreesC or removed by etching in KCN also the Cu2+ EPR signal disappeared. It is shown that the Cu2+ EPR signal originates from the Cu(OH)(2) phase and the activation energy for the thermal reduction of Cu2+ to nonparamagnetic Cu+ is E-a=0.29 eV. A model for the native oxidation of CuGaSe2 will be presented and implications for solar cell device performance discussed.