Investigations on rf magnetron sputtered cu2znsns4 czts absorber layer for thin film solar cells

Abstract

Thin film solar cells which belong to the second generation of photovoltaic devices are made up of amorphous silicon or non-silicon materials such as Cadmium Telluride (CdTe), Gallium Arsenide (GaAs), Indium Phosphide (InP), Copper Indium Selenide (CIS), Copper Indium Gallium Selenide (CIGS) and Copper Zinc Tin Sulphide (CZTS). These solar cells are made up of layers which are of few microns thick. CZTS, a quaternary semiconducting material has been investigated for photovoltaic applications for the last two decades. Researchers replaced Indium and Gallium with Zinc and Tin in the CIGS structure and formed CZTS exhibiting Kesterite structure. Moreover, CZTS has advantages like high absorption coefficient of 104 cm-1 and energy band gap of 1.45 eV, which makes it a suitable absorber layer material for thin film solar cells. CZTS absorber layer thin films were deposited using RF magnetron sputtering on soda lime glass substrates (SLG) using CuS, SnS and ZnS sputtering targets (99.99% pure, Cathay Advanced Materials, China) in six different stacking sequences. Initially, deposition of individual binary layers viz., CuS, ZnS and SnS onto soda lime glass substrates was carried out using the respective sulphur based binary targets. Each and every layer has been studied separately to ascertain their feasibility of stacking to form a single absorber layer. Subsequently, the three-sulphur based binary layers were stacked in six sequences as follows, CuS/ ZnS/ SnS, ZnS/ CuS/ SnS, CuS/ SnS/ ZnS, ZnS/ SnS/ CuS, SnS/ ZnS/ CuS and SnS/ CuS/ ZnS. CZTS thin films prepared using this arrangement were studied for composition and structure using XPS, XRD and Raman, respectively. From the XPS results, it was observed that the films prepared with the stacking sequence SLG/ CuS/ ZnS/ SnS was near to stoichiometry and the XRD measurements carried out revealed the formation of kesterite structure. Subsequent experiments and trials were carried out using this optimized stacking order. In order to promote the sulphur content in the films, post-deposition sulphurisation was carried out at 350°C on a quartz boat placed inside a tubular furnace, which was evacuated to 1 x 10-3 mbar using a rotary pump. Hydrogen sulphide (H2S) gas was purged into the chamber for 1 hr and the furnace temperature was raised to 350°C. The tubular furnace was then cooled and vented with N2 gas to remove the toxic residues. The properties of the sulphurized CZTS thin films were studied in detail. X-Ray diffraction results revealed the formation of CZTS kesterite phase lt112gt orientation with other secondary peaks. Further, on sulphurization, the intensity of CZTS kesterite peak was observed to be prominent and the presence of secondary phase SnS peak was also found. To substantiate the formation of CZTS, the samples were analysed using Raman spectroscopy. The low energy vibrational modes of CZTS were noticed at 330 cm-1 and 337 cm-1, with the presence of SnS secondary phases. The Raman spectra of the sulphurised CZTS films showed formation of kesterite CZTS phase with ternary CuSnS3 (CTS) in monoclinic and cubic phases. newline