Influence of Cl and H2O in spray-ILGAR® solutions on the voltage gain of Cu(In,Ga)(S,Se)2 solar cells with In2S3 buffer

In2S3 thin layers can be prepared by Spray-ILGAR® (Ion Layer Gas Reaction) from indium trichloride or from indium triacetylacetonate solutions. In the first case, the film contains a controllable amount of Cl. The concentration of H2O or Cl was varied systematically in the precursor solutions that were used for the deposition of In2S3 buffer layers on commercially available Cu(Ga,In)(S,Se)2 absorbers. The layers were characterized by ultraviolet photo electron spectroscopy, optical spectroscopy, modulated surface photovoltage spectroscopy and X-ray photoelectron spectroscopy, combined with a sequential etching procedure. The latter allowed to estimate an element-specific depth profile. Experimental findings were correlated with the performance of the corresponding solar cells. The presence of H2O or Cl in the precursor solutions has a strong influence on the diffusion of Cu+ and Na+ cations from the absorber into the buffer layer. The open circuit voltage of solar cells with In2S3 buffer layers prepared from Cl-free, H2O-containing precursor solutions was higher by up to 100 mV than those from Cl-containing, but H2O-free solutions. For this reason, the average solar energy conversion efficiency of cells of the first buffer type was higher (14.7%) than for solar cells of the second type (12.1%).