Investigation of the copper concentration on photocurrent collection of CuInSe₂ solar cells

In this study, CuInSe₂ (CISe) thin films were prepared from thermally evaporated Cu/In precursors, having various Cu/In atomic ratio, under the same selenization conditions. The precursors were converted into CISe absorber by annealing in a quartz tube furnace in the selenium vapours at substrate temperature of 500 °C. We developed four CISe films with Cu/In atomic ratio of 0.81–1.19, denoted as Cu-very rich, Cu-rich, Cu-poor, and Cu-very poor CISe thin films respectively. The effects of Cu/In atomic ratio on grain size, surface morphology, micro-structure and defect formation of the resulting CISe films were examined. It has been found that the photovoltaic properties were strongly related to Cu concentration, as well as carrier transport mechanism. Defects at the surface and in the bulk of CISe thin films were observed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy and scanning electron microscopy. Moreover, XRD revealed that the CISe film surface had a preferred orientation along the (112) plane. The XRD intensity and full width at half maximum of the (112) plane of CISe varied according to the Cu/In atomic ratio. Our experimental results show that the Cu-rich solar cell achieves conversion efficiency of 4.55% and exhibits an exceptional high short-circuit current density.