Vapor Transport Deposition and Characterization of Polycrystalline CdTe Solar Absorbers
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Vapor Transport Deposition and Characterization of Polycrystalline CdTe Solar Absorbers 1
James M. Kestner, 1Sarah McElvain, 1Colin A. Wolden, 2Stephen Kelly, 2Tim R. Ohno, 3 Lawrence M. Woods, and 3Rosine Ribelin Departments of 1Chemical Engineering and 2Physics, Colorado School of Mines, Golden, CO 80401, U.S.A. 3 ITN Energy Systems, 8130 Shaffer Parkway, Littleton, CO 80127-4107, U.S.A. ABSTRACT Vapor transport deposition is being developed for high-rate synthesis of CdTe thin films. Films have been deposited at rates in excess of 20 µm/min. The growth rate dependence on source temperature yielded an apparent activation energy of 42 kcal/mol, in good agreement with the theoretical value for CdTe sublimation (45.7 kcal/mol). For substrate temperatures greater than 400ºC the rate limiting step was resublimation. This phenomenon had a dramatic influence on morphology, although x-ray diffraction of all films indicated a strong (111) orientation. A preliminary device optimization investigating the effect of CdTe deposition temperature, postdeposition CdCl2 anneal parameters, alternative back contacts, and high-resistance buffer layers yielded a best cell with efficiency of 9.8% (704 mV Voc, 21.0 mA/cm2 Jsc, 66% FF). INTRODUCTION High throughput processing will be essential for the success of CdTe/CdS thin film technology. To date, record cells of 16% efficiency have all been fabricated using CdTe films grown by close space sublimation (CSS) [1, 2]. While this process works well for laboratory scale studies, it has inherent limitations for large-scale manufacturing. Alternative deposition technologies that are more amenable to high-throughput processing are therefore of interest. The low-pressure vapor transport deposition (VTD) process employs the proven reaction chemistry of CSS. The major difference is that mass transfer in VTD is controlled by convection, whereas CSS is limited by diffusion. The use of a carrier gas in VTD allows for a decoupling of the source and substrate environments. This allows for much higher growth rates than in CSS, as well as greater flexibility in engineering a system amenable to continuous manufacturing. The manufacturability of this process has been demonstrated by First Solar [3]. However, there remains a paucity of experimental characterization data on VTD material compared to say, CSS.
VAPOR TRANSPORT DEPOSITION REACTOR DESIGN A schematic of the VTD reactor described here is shown in Figure 1. The reactor consists of a standard six-way-cross stainless steel vacuum chamber with 8” Conflat flanges. The entire system is evacuated using a mechanical pump that achieves a base pressure of approximately 10 mTorr. CdTe source material is held in a quartz tube heated by nichrome ribbon wrapped around
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the source tube. Glass substrates co
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