ZnSe thin films grown by chemical vapour deposition for application as buffer layer in CIGSS solar cells

Polycrystalline ZnSe films were grown by an open-tube process using a ZnSe powder source and iodine enriched hydrogen as transport agent. The experimental transport rates could be reproduced by thermochemical equilibrium calculations giving an insight into the different transport mechanisms depending on the process parameters. For fabrication of Cu(In,Ga)(Se,S)2/buffer/ZnO solar cells ZnSe layers were grown on polycrystalline Cu(In,Ga)(Se,S)2 films at temperatures below 300°C for less than 3 min and covered by ZnO:Ga sputtered films. The solar cells achieved efficiencies up to 9.6% so far. Unfortunately, long-term degradation of the open circuit voltage and the fill factor was observed. This behaviour may be explained by the incorporation of iodine in the ZnSe layer detected by X-ray analysis for process temperatures below 400°C.