Towards high performance Cd-free CZTSe solar cells with a ZnS(O,OH) buffer layer: the influence of thiourea concentration on chemical bath deposition cells

Abstract

High performance kesterite (CZTSe) based solar cell devices usually employ an absorber/buffer heterostructure using toxic CdS deposited by chemical bath deposition (CBD)as buffer layer. This is due to the favourable spike like conduction band alignment of CdS buffer and CZTSe absorber. ZnS(O,OH) buffer layers provide a promising nontoxic alternative. Here, a variation of the thiourea concentration in the CBD of ZnS(O,OH) buffer layers and its influence on device performances of pure selenide kesterite heterostructure solar cells is presented. Furthermore, the influence of buffer layer deposition conditions on light induces metastabilities are discussed. ZnS(O,OH) buffer deposited with high thiourea concentration leads to distorted illuminated JV curves as expected for devices with unfavourable high spike like conduction band alignment between buffer and CZTSe absorber. By adjusting the thiourea concentration JV curve distortions can be reduced. An optimized CBD process leads to device efficiency of up to 6.5% after light soaking which is comparable to the efficiency of a reference device employing CdS as buffer layer (6.9%).