Studies on the Disorder in DC Magnetron Sputtered Cu2ZnSnS4 (CZTS) Thin Films Grown in Sulfide Plasma

Cu2ZnSnS4 (CZTS) is a widely studied material in thin film photovoltaics. The desirable properties of copper-poor and zinc-rich stoichiometry in CZTS thin films introduce several point defects and secondary and tertiary phases irrespective of growth techniques and growth conditions. These defects and the phases introduce disorder in the lattice. In the present study, the disorder in direct current (DC) magnetron-sputtered CZTS thin films prepared at 520 K in the presence of hydrogen sulfide plasma is studied. The CZTS thin films have been prepared at 30 W and 40 W magnetron powers. X-ray diffraction and Raman spectroscopy show that the CZTS films are polycrystalline and have a partially disordered kesterite (PDKS) phase. The CZTS films grown are copper-poor and zinc-rich. The optical band gap of the films is 1.56 eV. The disorder of PDKS CZTS films has been evaluated by low-temperature (300-20 K) electrical resistivity and Hall effect measurements. The transport of charge carriers in disordered p-type CZTS takes place via hopping mechanisms. CZTS films prepared with 30 W magnetron power have more disordered structures due to the copper vacancy (V-cu). In the case of the CZTS films prepared with 40 W magnetron power, V-cu defects are suppressed due to the formation of a complex defect [V-cu(-)+Zn-cu(+)] that is favored in Zn-rich growth conditions. The Kelvin probe measurement also illustrates that the Fermi level shifts toward the valence band edge in the case of CZTS films prepared at 40 W magnetron power. This is due to the passivation of the V-cu defect by the complex defect [V-cu(-)+Zn-cu(+)]. The passivation of the V-cu defect lowers the recombination centers, which can be beneficial to the high-efficiency solar cell device. The average surface work function, measured by a Kelvin probe for CZTS thin films prepared with 30 W and 40 W magnetron power, are 4.74 eV and 4.76 eV respectively.