The optical influence of Na on Cu 2 ZnSnSe 4 films deposited with Na-containing sol–gel precursor

A novel Na-containing precursor solution is presented to investigate the sodium influences on Cu 2 ZnSnSe 4 (CZTSe) absorbers grown on Na-free substrates. With higher sodium incorporation, some delicate change in secondary phase Cu 2 SnSe 3 was detected and its influence on the surface valence states was given by X-ray photoelectron spectroscopy analyses. It is found that the band gap significantly increased with nominal addition of Na, while the impurity introduced by excess incorporation would restrain further broadening. This method may probably assist to form a suitable CZTSe film directly on flexible substrates other than soda-lime glass.