Experimental study of Cu2ZnSn(Se,S)4 thin films for solar cell applications

Abstract: Cu2ZnSn(Se,S)4 (CZT(Se,S)) has recently been shown to be a promising material to use in thin film solar cells. It has a band gap of between 1eV (CZTSe) and 1.5eV (CZTS). CZT(Se,S) solar cells have reached 10% efficiency.   The aim of this project was to find an effective way to selenise metallic Cu, Zn, Sn films in order to produce Cu2ZnSnSe4 (CZTSe) films, without destroying the metallic molybdenum (Mo) back contact. Another aim was to make films containing both Se and S and to study the possibility to achieve a controlled S/Se ratio in the films. Selenisation and sulfurisation reactions were carried out in quartz ampoules and samples were characterized using scanning electron microscopy, x-ray diffraction and energy dispersive microscopy. Since Mo reacts rapidly with Se, experiments with selenisation and sulfurisation of plain Mo films were performed. The time and temperature dependence of the thickness of the reacted layer was studied, and it was found that Se reacts much faster with Mo than S does. Pre-sulfurisation of Mo appeared to hinder Se from reacting with the Mo.   The next step was to react precursor films to make CZTSe and CZT(Se,S) absorber layers. For mixed Se and S samples, the result is the same as for plain Mo samples; pre-sulfurisation, or inclusion of S in the precursor layer, can prevent selenisation of the Mo. The S/Se ratio was controlled with reaction time.    The best efficiency of 3.2% was obtained for a CZTSe sample while the best CZT(Se,S) sample gave 2.3%.