Phase development, microstructure and optical properties of Cu₂ZnSnSe₄ thin films modified with Pb and Ti

For the first time we report the growth of Cu₂ZnSnSe₄ (CZTSe) thin films modified with Pb and Ti, and their structure, morphology and optical properties for possible photovoltaic applications. The polycrystalline films with stannite structure were grown by pulsed laser deposition from the homegrown dense targets. While a small amount of Pb was soluble in CZTSe, a higher concentration of Pb led to the distinct formation of secondary phase of PbSe. For Ti modification, however, no Ti-related secondary phase could be identified and only a minor trace of Cu₂Se was detected at a high concentration of Ti. Grain growth and surface morphology of the films were affected in a different manner by the substitution of Pb and Ti. For both Pb and Ti, an initial decrease followed by a large increase in the band gap of the films was observed. The band gap tunability in the range of about 1.2 to 2.1eV by the substitutions is believed to be advantageous for photovoltaic applications.