Investigations on RF-magnetron sputtered Co₃O₄thin films regarding the solar energy conversion properties

Abstract

Co3O4 samples have been deposited using RF-magnetron sputtering from an oxide target. In-situ X-Ray and Ultraviolet Photoelectron Spectroscopy and X-Ray Diffraction have been carried out to identify the phase and composition of the films. The work function of Co3O4 films showed variation of about 1 eV depending on the oxygen content of the sputter gas while the Fermi level position in the band gap was invariant towards different deposition parameters. Optical transmission spectroscopy in the UV/VIS/NIR regime showed two optical transitions at 0.8 eV and 1.7 eV. However, the optical absorption from these transition does not seem to aid in the conduction of charge carriers as has been revealed by conductivity measurements in a linear 4-point-setup. Diodes were prepared in a glass|FTO|TiO2|Co3O4|NiO|Au-stacking geometry. They show poor photovoltaic behaviour with a short-circuit current of 0.33mAcm−1 and an open-circuit voltage of 0.15V, resulting in an overall efficiency of n = 0.01%. The limitation of Co3O4 as an absorber in an All-Oxide solar cell can be related to poor transport properties combined with defect states in the band gap and Fermi level pinning at interfaces.