Microscopic Characterization of Polycrystalline APCVD CdTe Thin Film PV Devices
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Microscopic Characterization of Polycrystalline APCVD CdTe Thin Film PV Devices
Tim R. Ohno, Eli Sutter, James M. Kestner, A.S. Gilmore, Victor Kaydanov, Colin A. Wolden, Peter V. Meyers†*, Lawrence Woods†, Manuel J. Romero‡, M.M. Al-Jassim‡, Steve Johnston‡ Colorado School of Mines, Golden, CO 80401 † ITN Energy Systems, 8130 Shaffer Parkway, Littleton, CO 80127 ‡ National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401 * presently at First Solar LLC, 12900 Eckel Junction Road, Perrysburg, OH 43551 ABSTRACT Atmospheric pressure chemical vapor deposition (APCVD) is being studied as an alternative for large-area manufacturing of CdTe thin films. High efficiency research cells have been constructed, but the fundamental materials properties and limitations have not been fully explored. APCVD material is examined with several techniques and compared with close-space sublimation (CSS). Transmission and scanning electron microscopy studies show a similar morphology to CSS CdTe. However high resolution TEM scans show the formation of a disordered layer between the CdTe and CdS, and the removal of defects within some grain structures upon annealing. Cathodoluminescence shows electronic defect states localized to grain boundaries. A large concentration of trap states was also observed with deep-level transient spectroscopy that may correspond to hole traps found in lower amounts in other materials. The presence of traps was also indicated in impedance spectroscopy measurements. The latter studies indicate a high grain boundary resistance contributes to transport. INTRODUCTION Various methods of depositing thin films of CdTe for photovoltaic applications have been successfully demonstrated, and several groups have demonstrated deposition of high quality films over large areas. In APCVD chunks of CdTe are heated to 750 °C, and the reactants are transported to a slightly cooler substrate for deposition by a nitrogen stream [1]. The APCVD process shares many similarities with CSS, which has yielded record device efficiencies of greater than 16%, while recent best efficiencies for APCVD materials are ~12%. The major difference is that convection of Cd and Te2 gases formed by sublimed CdTe controls mass transfer in APCVD, whereas diffusion of these gases control the low pressure CSS process. Precise control of mass transfer with pressure and temperature gradients may allow for the rapid deposition of uniform films over large-area substrates. In addition since the source material is separated from the substrate, independent control of source and substrate process environments is possible. The system may also be simplified in the absence of vacuum requirements. These potential engineering advantages of APCVD as an alternative CdTe large-scale deposition technology to CSS are only valuable if CdTe films with comparable properties can be obtained. The objective of this study is to probe beyond standard photovoltaic cell characterization to understand limiting factors for devices made of the initial material produce
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