CdS/CdTe Solar Cells Made by High-Rate Magnetron Sputtering
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CdS/CdTe Solar Cells Made by High-Rate Magnetron Sputtering Victor Plotnikov, and Alvin Compaan Physics and Astronomy, The University of Toledo, Toledo, OH, 43606 ABSTRACT The dependence of sputtering deposition rate of CdTe film on deposition pressure and RF power has been studied. The optimum argon pressure was found to be 5 mTorr while the growth rate as a function of power is slightly superlinear. A deposition rate of 140 nm/min has been achieved which is five times faster than for our conventional deposition conditions. Films were characterized by means of X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). Pressure has been found to have a greater effect on film morphology than magnetron sputter power. Working devices have been made with 2-micron layers of CdTe deposited at different powers and 10 mTorr of argon pressure. The effect of different temperatures of CdCl2 postdeposition treatment on cell performance was studied. Efficiency of 11.8% with a 2-micron CdTe layer deposited at 133 nm/min was achieved. INTRODUCTION Magnetron sputtering is one of the variety of techniques used for successful CdS/CdTe solar cell fabrication. Unlike many other deposition techniques, sputtering allows growth of a high-quality polycrystalline film on relatively cool (150-250 °C) substrates. This means that the temperature of a typical growth is well below the softening point of the soda-lime glass conventionally used as a substrate material. Similarly, low deposition temperature makes sputtering suitable for cell fabrication on polymer substrates that have even smaller tolerance to high temperature than glass. This, in conjunction with a high radiation hardness of CdTe opens up an opportunity for CdS/CdTe sputtered cells on polymer substrates in space applications [1]. In addition the ability to keep a low deposition temperature prevents interdiffusion and structural changes of underlying layers in case of a multilayer structure typical for conventional and especially multijunction polycrystalline solar cells. Power savings arise from the use of low temperature too. In this work we investigate enhanced growth rates for magnetron sputtering deposition of CdTe which can be achieved with a conventional target and planar magnetron gun geometry. The deposition rate is studied as a function of sputtering gas pressure as well as RF power applied to the target. The effect of these parameters on CdTe film morphology and cell efficiency is also studied. EXPERIMENT Devices were fabricated at The University of Toledo, by magnetron sputtering of CdS/CdTe layers on a conventionally available soda-lime glass with a transparent conductive coating Tec-7 glass manufactured by Pilkington. Cells were completed with Cu/Au back contact made by thermal evaporation followed by 45 minutes of diffusion activation treatment at 150 °C in air. The thickness of the CdS layer was about 0.1 micron while for CdTe it was kept close to 2 micron independently of deposition conditions.
The design of the vacuum chamber used for sputtering pro
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