Effect of copper-deficiency on multi-stage co-evaporated Cu(In,Ga)S2 absorber layers and solar cells

Solar cell absorber films of Cu(In,Ga)S2 have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In] + [Ga]) = 0.93-0.99 and [Ga]/([In] + [Ga]) = 0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn5S8 thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn5S8 formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738 mV, short-circuit current 19.3 mA/cm2, fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In] + [Ga]) = 0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04 eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S2.