In${}_2$S${}_3$ alternative buffer layers for Cu(In,Ga)Se${}_2$ solar cells deposited by RF magnetron sputtering

A thin semiconducting material of thickness ∼50 nm known as a buffer layer is the key for obtaining high efficiencies in Cu(In,Ga)Se${}_2$ (CIGSe) thin film solar cells. A thin buffer layer improves the photo-response of a solar cell which significantly enhances the efficiency. Cadmium sulfide (CdS) buffer layer deposited by chemical bath deposition (CBD) is used commercially for large-area high-efficiency CIGSe solar cells. However, toxicity of Cadmium (Cd) and the CBD deposition process makes CdS unsuitable for large-scale production. So-called 'Cd-free' or 'alternative' buffer layers to CdS, deposited by vacuum-based methods are extensively studied by the photovoltaic (PV) community. Therefore, this thesis primarily explores the potential of In${}_2$S${}_3$ as a performant buffer layer material when deposited by RF magnetron sputtering. RF magnetron sputtered In${}_2$S${}_3$ buffer layers were deposited by two approaches: (i) at "low sputter pressure" using Ar-ion sputtering and (ii) at "higher sputter pressure" using H${}_2$S/Ar reactive sputtering. The Ar-ion sputtered In${}_2$S${}_3$ buffer layers reached photovoltaic conversion efficiencies of 13.6% with fill factor (FF) of 53%. However, absorber surface damage and non-uniform buffer layer thickness were the primary limitations impeding cell efficiency. The extent of induced sputter damage and annealing-induced intermixing at the In${}_2$S${}_3$/CIGSe heterointerface was estimated using atom probe tomography.In${}_2$S${}_3$ buffer layers deposited by H${}_2$S/Ar reactive sputtering at HSP had lower absorber surface damage. Moreover, crystalline In${}_2$S${}_3$ thin films with smaller average crystallite size were obtained by reactive sputtering as compared to the amorphous In${}_2$S${}_3$ thin films by Ar-ion sputtering. This improved optoelectronic performance with lower interfacial recombination and higher photon collection, giving a higher fill factor of 65% and a normalized efficiency of 16.33%. Elemental intermixing at the In${}_2$S${}_3$/CIGSe heterointerface for different annealing temperatures was estimated using atom probe tomography. The effect of Cu-self doping of In${}_2$S${}_3$ from CIGSe, Cu depletion from CIGSe surface and the segregation of Na at In${}_2$S${}_3$/CIGSe heterointerface suppress the detrimental defect sites at the interface. The passivation of the defect sites results in an effective charge carrier collection, lowered recombination, and thus better cell performance. With this work, the deposition of In${}_2$S${}_3$ as a buffer layer material by rf magnetron sputtering is optimized to obtain high-efficiency buffer layer material for CIGSe solar cells. Additionally, a detailed study of the chemistry of buried In${}_2$S${}_3$/CIGSe heterointerface and its effect (beneficial/detrimental) on electrical properties and cell performance is also presented.