We fabricate and measure graded-index “black silicon” surfaces and find the underlying scaling law governing reflectance. Wet etching (100) silicon in HAuCl4, HF, and H2O2 produces Au nanoparticles that catalyze formation of a network of [100]-oriented nanopores. This network grades the near-surface optical constants and reduces reflectance to below 2% at wavelengths from 300 to 1000 nm. As the density-grade depth increases, reflectance decreases exponentially with a characteristic grade depth of about 1/8 the vacuum wavelength or half the wavelength in Si. Observation of Au nanoparticles at the ends of cylindrical nanopores confirms local catalytic action of moving Au nanoparticles.

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In this paper, we present an update and review on the progress made in the development of low-bandgap CulnSez (CIS) and wide-bandgap CuGaSez (CGS) solar cells. Our research project is primarily concerned with the optimization of the bottom and top cells of the tandem solar cell. This past year, we established new record totalarea efficiencies of 15.0% and 10.2% for CIS and surfacemodified CGS solar cells, respectively. These achievements were possible by modifying the growth process for CIS and CGS absorbers. We attempt to modify the surface region of the CGS absorber to be CIGS-like in composition. We also have designed a mechanical-stacked tandem solar cell where the 15% CIS cell serves as the bottom cell and transparent CdTe cell serves as the top cell. The NREL-confirmed totabarea efficiency for this CdTe/CIS tandem device is 15.31 %.

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