Formation of cadmium sulfide thin films on different substrates using electrochemical atomic layer deposition (E-ALD)

This dissertation discussed the layer-by-layer growth of CdS thin film using electrochemical atomic layer deposition (E-ALD) as well as the investigations into the possible fabrication of both substrate and superstrate configurations of CdTe/CdS solar cells. E-ALD is the electrochemical equivalence of gas phase atomic layer deposition (ALD). Similar to ALD, E-ALD allows for the layer-by-layer deposition of materials using surface-limited reaction. Underpotential deposition (UPD) is usually used in E-ALD, allowing for atomically thick layers of materials to be deposited on a second material at a potential before the potential needed for the element to deposit on itself. In this report, CdS thin films with different thicknesses were grown using E-ALD. Results indicated that the E-ALD grown CdS on Au was stoichiometric, crystalline, uniform, and is N-type. Formation of CdS on Ag substrates and on ITO was also studied.Preliminary studies on the fabrication of CdS/CdTe photovoltaic device with both superstrate and substrate configuration were also performed. In the substrate configuration, CdTe was first deposited onto Au substrates using 15 minutes of codeposition at -700 mV, followed by electrochemical atomic layer deposition (E-ALD) of CdS on CdTe/Au. In the superstrate configuration, the ITO substrate was first patterned with 1x1 mm2 squares using photolithography, and then treated with electrochemical methods to form Cu nucleation sites on the ITO surface. CdS E-ALD was performed on the treated ITO substrate. CdTe thin films were deposited onto CdS/ITO using pulse plating atomic layer deposition (PP-ALD). A layer of Au was deposited electrochemically on CdTe/CdS/ITO as the back contact. Photoelectrochemical measurements on the substrate configuration showed good P-type response and clean dark current. XRD showed CdTe zinc blend <111> and CdS zinc blend <111> structure. SEM images of the CdTe/CdS/ITO showed compact deposits covering the whole surface. Work on collecting more information on cell performance is on the way.