Pure sulfide wide gap CIGS on silicon for tandem applications by exploring versatile coevaporation of metallic films and sulfur annealing

Cu(In,Ga)(S,Se) 2 (CIGS) is a good candidate for tandem solar cell applications, thanks to its bandgap which can be tuned by changing the ratios In/Ga and Se/S. In particular, wide-gap CIGS is well suited to be implemented into tandem solar cells with silicon bottom cells, the CIGS acting as the top semi-transparent solar cell. Pure sulfide 1.55 eV CIGS already reached efficiencies of 16,9 % via a two-step route consisting of the deposition of metals followed by a reactive sulfur annealing [1], and a 14.2% efficient solar cell was recently reported by Barreau et al, for a bandgap of 1.6 eV based on co-evaporation [2]. In this work, we report on the investigation of two step CIGS deposition on silicon for tandem application. The CIGS absorber is deposited via a sequential method, where Cu, In and Ga are deposited by versatile co-evaporation process, followed by an annealing at 600°C in presence of sulfur powder. Optimization of deposition and annealing conditions led to the formation of a dense and adherent CIGS film on silicon. EDX mapping analysis show the formation of a two-layer structure which is suitable for high efficiency cells [2] with overall Cu(In+Ga) (CGI) of 1,0. XRD and PL analysis confirm the formation of qualitative wide gap CIGS material. This work shows the suitability of using this coevaporation method for exploring the synthesis of wide-gap pure sulfide CIGS on silicon. A further investigation on the addition of selenium during the evaporation process shows the possibility to tune the gallium grading in the final CIGSu(Se) layer.