Abstract
Cadmium sulfide (CdS) is an indispensable layer of thin film solar cell such as CIGS and CdTe solar cell, there are many methods to synthesize CdS films. Chemical Bath Deposition (CBD) was used in this study. Ammonia is used as complexing agent in classical CBD, but we use an ammonia-free process to synthesize CdS films. Sodium citrate is substituted for ammonia as complexing agent Cd(Ac)2 dissociates to form Cd2+. Thiourea is applied as S2 source. KOH is used to adjust the pH value. The process used in our laboratory is improved in this study. The influence of pH value, deposition temperature, and deposition time are studied. We get the most appropriate process parameters through experiment. There is no CdS thin film deposited on substrate when pH=13. CdS films deposited in process pH=11.5 is a continuous film on substrate and has better preferred crystal orientation than pH=10. The CdS films in process pH=10 is a porous and discontinuous film. When process temperature rises, the crystal intensity slightly increases with no obvious influence on the morphology and energy band gap. The pH value has a bigger impact on process than the parameter temperature. Theresults of different deposition time provide information of CdS formation process. As deposition time increase, the film thickness and energy band gap increase. The crystal orientation changes gradually and the SEM surface mophology changes from sphere-like to strip-like. According to the X ray diffraction pattern, the band gap calculated from results of optical spectrophotometer and a published literature, we could indicate that the CdS film deposited with the best process pH=11.5, 70°C has a hexagonal structure. It needs only 10 min to form CdS crystalline film with preferred orienttation. EDS analysis of elemental composition ratio at different deposition time provides information on the reaction during process. The CdS deposited is an n-type semiconductor. The carrier concentration is 1.64×1010 cm-3 and the mobility is 1714 cm2/Vs by Hall measurement. The CdS film with 53nm thickness prepared with pH=11.5, 70°C, and deposition time 10 min is appropriate to be a buffer layer in CIGS thin film solar cell.