Heterojunctions based on Cu2ZnSnS4 and Cu2ZnSnSe4 thin films
The preparation of thin films of Cu2ZnSnS4 and Cu2ZnSnSe4 by thermal evaporation of the elements or chalcogenide binaries in high vacuum is described. Such films are interesting alternatives for absorber layers in photovoltaic devices because all components are readily available on the earth's surface. The methods used for the material characterization are scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffractometry. The study addresses film growth and incorporation of components from the gas phase. The quaternary compounds Cu2ZnSnS4 and Cu2ZnSnSe4 form in the kesterite-type structure. The binary phases CuS, CuSe, ZnS, ZnSe, and ternary copper-tin chalcogenide compounds are present as secondary phases. Cu, Zn and S or Se, however, form separate CuS(Se) and ZnS(Se) crystallites. Nearly all quaternary films have p-type conductivity. The sulfide films have a band gap aroud 1.5 eV; the selenide films around 0.8 eV. Devices completed with a standard CdS/ZnO window demonstrate up to 2.3% conversion efficiency for Cu2ZnSnS4 and up to 0.6% for Cu2ZnSnSe4. The highest open-circuit voltage so far is 570 mV with a Cu2ZnSnS4 device. These device results are the first reported for Cu2ZnSnSe4 and the best reported so far for Cu2ZnSnS4.