Rapid Thermal Annealing on Cu(In,Ga)Se 2 Films and Solar Cells
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Rapid Thermal Annealing on Cu(In,Ga)Se2 Films and Solar Cells Xuege Wanga, Sheng. S. Lia, V. Craciunc, S. Yoonb, J. M. Howardc, S. Easwarand, O. Manasrehd, O. D. Crisalleb and T. J. Andersonb a. Dept. of Electrical and Computer Engineering, Univ. of Florida, Gainesville, FL 32611 b. Dept. of Chemical Engineering, Univ. of Florida, Gainesville, FL 32611 c. Dept. of Material Science and Engineering, Univ. of Florida, Gainesville, FL 32611 d. Dept. of Electrical Engineering, Univ. of Arkansas, Fayetteville, AR 72701 ABSTRACT Rapid thermal annealing (RTA), with fast ramp up and down rates, was performed on several Cu(In,Ga)Se2 (CIGS) films and solar cells under various peak annealing temperatures and holding times. The XRD, SEM, Hall- effect, photo J-V, and quantum efficiency (Q-E) measurements were made on CIGS films and cells before and after RTA treatments to study the effects of RTA on the CIGS film properties and cell performance. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties (resistivity, carrier concentration, and mobility) of CIGS films and cell performance while preserving the film composition and microstructure morphology. INTRODUCTION Rapid thermal annealing (RTA) at higher temperatures with a low thermal budget has been widely used [1, 2] in thin film manufacturing industry to activate doping impurities while at the same time minimizes the doping impurity diffusion that occurs during thermal processing. As a powerful annealing technique, the RTA process offers several advantages such as short cycle time, reduced thermal exposure and lot size flexibility compared to conventional furnaces. In general, RTA improves the temperature uniformity for large diameter wafers over the conventional furnace heating. Strong demand in thermal budget reduction and cycle time reduction makes RTA treatment a very popular thermal processing method in recent years. The RTA technique has been successfully applied to the fabrication of low-cost silicon solar cells. The motivation of using this technique on Cu(In,Ga)Se2 (CIGS) films and solar cells is to reduce damages and defect densities in the near surface junction region, which tends to reduce the losses due to recombination via defect centers in the junction space charge region and thus improves the cell performance. EXPERIMENTAL DETAILS The CIGS samples grown on the Mo- coated soda-lime glass (SLG) substrates were prepared by the National Renewable Energy Lab (NREL) and Shell Solar Industry (SSI), which were used in this work to study the effects of RTA treatments on CIGS film properties and cell performance. For the Hall-effect measurements, however, the CIGS films were grown directly on the insulating SLG substrates. To study the effects of RTA treatment on CIGS device performance, a thin (50 nm) CdS buffer layer was deposited on the CIGS samples by chemical bath deposition (CBD). ZnO window layer was then deposited on the CdS/CIGS/Mo/SLG samples by RF sputtering. Subsequent
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