Sulphur treated single step electrodeposited Cu2ZnSnS4

Abstract

Solar cells, directly converting sunlight into electricity through the photovoltaic (PV) effect, is the best alternative for the global energy crisis. Among the various solar energy materials, Cu2ZnSnS4 (CZTS) is a promising material for solar cell applications due to its unique optoelectronic properties. This study studied the possibility of the growth of quaternary CZTS thin films using a single-step electrodeposition technique for applications in PV devices. CZTS thin films were potentiostatically electrodeposited at - 0.89 V vs Ag/AgCl for 4 minutes on Titanium (Ti) substrate in a three-electrode electrochemical cell containing, 0.02 M copper (II) sulphate pentahydrate (CuSO4.5H2O),0.01M zinc sulphate heptahydrate (ZnSO4 .7H2O), 0.02 M tin sulphate (SnSO4) and 0.02 M sodium thiosulphate (Na2S2O3) at room temperature. 0.2 M tri-sodium citrate (C6H5Na3O7:Na3 - citrate) was used as a complexing agent and tartaric acid (C4H6O6) was used as pH control solution. pH of the bath was maintained at 5. The counter and reference electrodes were Pt plate and Ag/AgCl respectively. Prior to the CZTS deposition, Ti substrates were initially polished with sandpaper and then cleaned with detergent, diluted HCl, and finally cleaned ultrasonically in distilled water for 15 min. Two sets of samples were prepared by annealing as grown CZTS thin films at 550 °C for 30 minutes in N2 and H2S atmospheres respectively. As grown, annealed in N2, and annealed in H2S, CZTS films were characterized and compared using dark and light Current-Voltage (I-V) and Capacitance-Voltage (Mott-Schottky) measurements in a photoelectrochemical cell (PEC) containing a 0.1 M sodium acetate aqueous electrolyte. Grown CZTS thin films did not show any photoactive properties. However, as revealed by the I-V characteristics, films annealed in N2 produced n-type photoconductivity having Voc of 204 mV and Jsc of 20 µAcm-2 in PEC while films annealing in H2S produced p-type photoconductivity having Voc of ~ 250 mV and Jsc of ~ 110 µAcm-2 in the same PEC. This finding was further studied using Mott-Schottky characteristics. Results revealed that films annealed in N2 and H2S attribute n-type and p-type photoconductivity respectively. Further, flat band potential (VFB) values of -0.066V and +0.594V vs Ag/AgCl in the same PEC exhibited for the films annealed in N2 and H2S respectively indicating the formation of a better interface between CZTS and electrolyte for the samples annealed in H2S. In conclusion, significant photoactive enhancement in single step electrodeposited CZTS can be achieved with H2S treatment.