Abstract:
Cadmium sulfide (Cds) is a commonly used buffer material for Cu(In,Ga)Se2based thin films photovoltaic cells. Numerous properties such as high optical transmittance and low resistivity are required for CdS to act as appropriate buffer material. CdS buffer layer allows a favorable conduction-band alignment at the CdS/CIGS hetero junction and compatible with CIGS lattice structure. Cadmium sulfide (CdS) thin films were deposited on microscopy glass substrate using well known chemical bath deposition technique. A systematic parametric study was performed in order to evaluate the CdS films with optimum electrical and optical properties. The effect of molar ratio of precursors, the deposition time and temperature and annealing treatments was investigated. X-ray diffractrometer (XRD) and field emission scanning electron microscopy (FESEM) analysis were used to investigate the crystallinity and surface morphology of the films. Van de Pauw method was used to find resistivity, carrier mobility and carrier density of deposited films. Optical transmittance and thickness of films was measured using spectrophotometer and interferometer. Minimum resistivity of 1.03 × 102 with optical transmittance as high as 70 % was obtained for the CdS films deposited at 85 oC for 40 minutes and annealed at 400 oC for 30 minutes. CdS thin films by CBD method in large scale solar cell production could raise a serious environmental problem because of the great amount of Cd-containing waste which results in the deposition process. Environmental issues have inspired research in developing Cd-free buffer layers. ZnS can be considered as an alternate buffer layer in CuInSe2 based devices. ZnS with its wide band gap is an important buffer material to be used in thin film solar cells. ZnS thin films were deposited in order to have a comparison in terms of electrical and optical properties with CdS films. Minimum resistivity of 1.54 × 103 with optical transmittance as high as 70 % was obtained for the ZnS films deposited at 85 oC for 30 minutes and annealed at 500 oC for 30 minutes.
