Micron-Resolution Photocurrent of CdTe Solar Cells Using Multiple Wavelengths
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Micron-Resolution Photocurrent of CdTe Solar Cells Using Multiple Wavelengths Jason F. Hiltner1 and James R. Sites Department of Physics, Colorado State University Fort Collins, CO 80523-1875 1 current address: Corning Inc., Sullivan Park Research Center, Corning, NY 14831 ABSTRACT An apparatus developed recently at Colorado State University utilizes a diffractionlimited optics system and a high-resolution translation system to measure the laser-induced photocurrent at resolutions of 1 µm and equivalent incident laser intensities of 1 sun (100 mW/cm2). Multiple lasers in the 635-830 nm range can be easily selected by changing the fiberoptic connectors. The spot profile and location are unchanged when different lasers are selected. In addition, a laser temperature tuned through the 825-857 nm range allows measurement of local variations in the quantum efficiency near the CdTe band gap, which track intermixing of CdS/CdTe. This capability extends the previous analysis, which includes the separation of series resistance and shunting effects. The effect of post-deposition processing and elevated temperature stress on local variations in electrical and optical parameters, especially using the near-bandgap wavelengths, are examined using a series of samples fabricated at NREL. INTRODUCTION Polycrystalline thin-film solar cells are poised to provide power to terrestrial markets in the near future. The use of these materials necessitates uniformity considerations not generally employed in single-crystalline material. Investigation of the spatial uniformity of photocurrent collection has recently been aided by the development of an apparatus which features micron spatial resolution with near-solar incident intensities. The details of this technique are available elsewhere [1,2,3]. Observations provided by the instrument include the overall uniformity of collection, as well as more detailed characterization of the cause of local reductions in response. A recent addition to the system allows multiple wavelengths with energies near and slightly below the CdTe band gap (1.5 eV) to be used. Temperature tuning of a solid state laser allows smooth adjustment of photon wavelengths through the 825-857 nm range, corresponding to photon energies of 1.50-1.45 eV. This addition was stimulated by photoluminescence observations of bandgap lowering in this material system, which has been attributed to diffusion of sulfur from the CdS window layer into the CdTe absorber [4,5,6]. The CdTe1-xSx system exhibits band-bowing; small amounts of sulfur in CdTe will reduce the bandgap approximately 10 meV/at.%. [7]. EXPERIMENTAL The system used in this study (see figure 1) utilizes a laser beam focused by a microscope objective onto the sample, with the light incident perpendicular to the junction. The laser power density used is controlled to be very near solar densities, so that generation of electron-hole pairs takes place under conditions similar to standard operation of the solar cell. The collected
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