ZnSe buffer prepared by iodine-enhanced chemical vapour deposition for Cu(In,Ga)(Se,S)2-based solar cells

Cu(In,Ga)(Se,S)2-based solar cells with an alternative ZnSe buffer prepared by an iodine-enhanced low-pressure chemical vapour deposition (CVD) from a powder source were investigated. The application of iodine allowed acceptable transport rates even at substrate temperatures below 300°C, which is essential to avoid damage to the predeposited absorber. So far, the most efficient cells provided with a CVD ZnSe buffer yielded efficiencies of more than 80% of reference cells with CdS buffer from a chemical bath. The losses are due to a reduced open circuit voltage and fill factor whereas the short circuit current density was improved as a consequence of less absorption losses in the buffer. The photocurrent was observed to be voltage dependent, which affects open circuit voltage and fill factor. In addition, measurements of the voltage-dependent quantum efficiency as well as the junction capacitance revealed a reduced doping density and an enlarged space charge region of the Cu(In,Ga)(Se,S)2 absorber in solar cells with a ZnSe buffer prepared by CVD.