A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high-efficiency devices on tin oxide-coated soda-lime-glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short-circuit current. Meanwhile, fill factor is maintained, and open-circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%-efficient cell on 3.2-mm soda-lime glass. This window material was shown to be compatible with SnO 2 -based transparent conducting oxide and high resistance transparent coated substrates using in-line compatible processes.

The aim of this investigation is to apply advanced microstructural characterization techniques to study the effect of the cadmium chloride treatment on the physical properties of cadmium telluride solar cells deposited via close-spaced sublimation (CSS) and relate these to cell performance. A range of techniques have been used to observe the microstructural changes as well as the chemical changes before and after cadmium chloride treatment. Electrical measurements that link the device performance with the microstructural properties of the cells have also been undertaken. Transmission Electron Microscopy (TEM) has revealed high densities of stacking faults in the as-grown CdTe samples. Further, it has been observed that these stacking faults are removed during the cadmium chloride treatment. These observations show that the presence of chlorine plays an important role in the removal of these defects and the subsequent production of high efficiency thin film CdTe solar cells. Elemental analysis in the TEM indicates chlorine rich regions appearing at the CdTe/CdS interface as well as at grain boundaries after the treatment.

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A continuous, in-line process suitable for large volume manufacturing of CdSICdTe photovoltaic (PV) devices has been demonstrated at the pilot scale level. The pilot scale system incorporates the steps of glass heating, all semiconductor depositions, chloride heat treatment and ohmic contact formation in one chamber operating at modest vacuum. The cycle time is 2 minutes. The process is scaleable, uniform and reproducible. Utilizing this process, devices with efficiencies greater than 12% (verified at NREL) are repeatedly fabricated on mmodified Pilkington TEC 15 substrates without anti-reflection coatings. The stability of the devices is very promising. Many devices are being tested outdoors for stability. Outdoor results are compared to indoor stress testing. Detailed analyses of the novel copper based ohmic contact using X-Ray photoelectron spectroscopy (XPS) and glancing angle X-Ray diffraction (GAXRD) are presented. 0-7803-7471-1/02/$17.00 02002 IEEE

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