Integration of single layer graphene into CZTS thin film solar cells

Abstract

In this study, CZTS samples were produced on Mo and graphene/Mo coated glass substrates using quaternary target. The CZTS thin films deposited by RF magnetron sputtering were annealed using rapid thermal processing (RTP) method in sulphur atmosphere at 500, 525, and 550 °C so as to obtain glass/Mo/CZTS and glass/Mo/graphene/CZTS (g-CZTS) structures. The obtained CZTS and g-CZTS thin films were then characterized by several methods such as EDX, XRD, Raman spectroscopy, SEM etc. The EDX data demonstrated that all CZTS thin films had Cu poor composition regardless of the sulfurization temperature and increasing the temperature led to Sn loss from the films. Diffraction peaks of kesterite CZTS phase were observed in all the samples; additionally, SnS and CuS secondary phases were also observed in CZTS samples annealed at 500 °C. The crystallite size of the CZTS thin films were found to be increasing with both increasing annealing temperature and use of graphene film as an inter-layer. The creation of kesterite phase with a very small CTS phase in all the samples were verified by the Raman spectroscopy measurement. The SEM images of the samples indicated that using graphene improves the crystalline quality of the CZTS films and contributes to forming more compact, homogenous and larger crystal structure. The determined optical band gap values varied from 1.41 to 1.44 eV depending on the Sn-content of the samples. The produced solar cells selected from the more promising absorber layers showed that implementing graphene in CZTS cell structure enhanced the conversion efficiency from 2.40% to 3.52% due to improvement of crystalline quality of the absorber layer.