Photoelectrochemical Properties Of Cu₂ZnSnS₄Thin Films By Ultrasonic Spray Pyrolysis Method

With energy shortage and serious environmental issues raised, renewable energy is inevitable for sustainable development of society. Solar energy is abundant and clean renewable energy, which is a promise energy resource for future. Therefore, the PV cells and PEC cells are the two straight ways of using solar energy. For a PV cell and a PEC cell, their core parts are the semiconductor films, which shoud have a high efficient> a high absorption coefficient and suitable band positon. In this study, we have been working on a p-type Cu2ZnSnS4(CZTS) films without precious and toxic element, which is rising up as a promising material for a thin film solar cell and a photoelectrochemical cell.Concerning of the high cost of vacuum equipment, we fabricated CZTS films by low-cost ultrasonic spray pyrolysis(USP). Base on the XRD, Raman, SEM and EDS measurement, we can understand the growth kinetics of CZTS films deposited at different substrate temperatures. Photoelectrochemical properties of Cu2ZnSnS4thin films were also measured by the photoelctrochemical measurement. In the meantime, we measured the band offset in the CdS/CZTS heterojunction to know whether CdS is the ideal buffer layer of the CZTS solar cells. The research content mainly includes three parts:(1) We fabricated the CZTS thin films by low-cost ultrasonic spray pyrolysis(USP) and the crystallite size, about30nm,increases with increasing substrate temperatures. Moreover, a Stannite CZTS forms at relative lower temperature, which turns into a more stable Kesterite phase at high temperatures higher than450℃. We also found small amount of ZnS existed on the surface of the CZTS films at400℃-500℃.(2) According to the photoelectrochemical measurements of the CZTS films at different substrate temperatures, at a potential-0.7VSCE, the films at400℃exhibit the best photocurrent of0.77mA/cm2. The reduction of the photoelectrochemical properties may come from the Sn loss at high temperature. The CZTS films deposited at higher substrate temperatures at450℃and500℃become pore, only observed on the surface of the CZTS film.(3) In our experiments, it has been found that CdS/CZTS belongs to the type-Ⅱ semiconductor hetero-junction. Photo-generated minority can across from the conduction band of CZTS into the conduction band of CdS buffer layer, so carrier recombination from CZTS to CdS buffer layer is increased at the interface. This should be avoided in solar cell’s designment, so CdS should be replaced by a buffer layer material which has the higher band position to acquire high photoelectric conversion efficiency.