Studies of the Zn2SnO4 and InxSy as Alternative Buffer Layers Deposited via R.F. Sputtering for Chalcogenide Photovoltaics

Thin film solar cells based on Cu(In,Ga)Se2 (CIGS) yield, up to now, a maximum photovoltaic conversion efficiency if CdS deposited by chemical bath deposition is used as buffer layer. However, due to the disadvantages of CdS (like the toxicity classification of Cd, the non-vacuum nature of the chemical bath deposition and the absorption of blue light) an extensive research was employed in the last years on finding new alternative buffer layers. Thereby, the aim of this work follows the technical progress of the research which focuses on the development of Cd-free buffer layers for chalcogenide photovoltaics. Zinc tin oxide, (Zn2SnO4) and indium sulfide (InxSy) thin films were evaluated as alternative buffer layer for chalcogenide-based PV. The investigated thin films buffer layer have been deposited both on soda lime glass substrates and on Cu(In,Ga)(Se,S)2, in order to prepare the solar cells, by using r.f. sputtering. Some of the technological problems of the Cd-free buffer layers like the influence of the growth conditions, homogeneity, short deposition times, and post thermal treatments are investigated. The performances of the CIGS-based solar cells fabricated using ZTO and InxSy as buffer layers were compared to the standard CdS solar cells deposited by chemical bath deposition (CBD). Since the alternative buffer layers allowed more or less half of the efficiency reached using CBD-CdS further optimizations of the investigated buffer layers are currently in progress.