1 Towards high performance Cd-free CZTSe solar cells with ZnS(O,OH) buffer layer: The influence of the thiourea concentration in the chemical bath deposition Markus Neuschitzer 1 , Karla Lienau 2 , Maxim Guc 1 , Lorenzo Calvo Barrio 3 , Stefan Haass 2 , Jose Marquez Prieto 4 , Yudania Sanchez 1 , Moises Espindola-Rodriguez 1 , Yaroslav Romanyuk 2 , Alejandro Perez-Rodriguez 1,5 , Victor Izquierdo-Roca 1 , and Edgardo Saucedo 1 1 Catalonia Institute for Energy Research- IREC, Jadins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain 2 Empa – Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Thin Films and Photovoltaics, 8600 Dübendorf, Switzerland 3 Centres Científics i Tecnològics CCiTUB, Universitat de Barcelona, C. Lluis Solé i Sabarés 1, 08028 Barcelona, Spain 4 Northumbria Photovoltaic Applications Group, Department of Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST 5 IN2UB, Departament d’Electrònica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona, Spain 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%). Keywords: thin film solar cell, buffer layer, chalcogenides, CZTSe, Cd-free, ZnS(O,OH), light soaking 1.Introduction In recent years kesterite Cu 2 ZnSn(S 1-x Se x ) 4 (CZTSSe) attracted much attention as possible alternative to more mature chalcopyrites (CuIn 1-x Ga x Se 2 – CIGS) p-type absorber material for thin film solar cells due to its composition of more abundant elements. [1] Different to most silicon solar cells where a homojunction is created by doping of the Si, chalcogenide thin film solar cells use a heterojunction architecture to create the p-n junction for the separation of light generated electron hole pairs. [2] This heterostructure consist of a polycrystalline