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  1. We performed detailed balance analysis using rigorous coupled-wave analysis (RCWA) on vertical GaAs nanowire (NW) arrays. Both freestanding NW arrays as well as NW arrays on a perfect back reflector are assessed. Both types of vertical NW arrays demonstrate efficiencies that exceed the Shockley Queisser (SQ) or radiative efficiency limit when the NWs are sufficiently long. The use of a back reflector enhances the efficiency of NW solar cells by increasing solar absorption and suppressing emission from the backside of the solar cell. We study the light trapping and material reduction advantages of NWs. Furthermore, we compare simulations that evaluate detailed balance efficiency with ultimate efficiency and show that ultimate efficiency studies can determine near-optimal solar cells while vastly reducing the number of simulations that need to be performed. While open circuit voltages above the radiative limit can be achieved, tradeoffs with short circuit current must be carefully considered. We also compare our simulation results to other claims in the literature that NWs are capable of exceeding the SQ limit.

     
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  2. Abstract

    Metal nanomeshes are demonstrated as flexible transparent conductors with performance comparable to indium tin oxide. However, it is not known what the performance limits of these structures are in terms of transparency and sheet resistance. More importantly, the haze, which describes how much incident light is scattered by these structures, has not been studied. In this paper, the transmission, sheet resistance, and haze of metal nanomeshes are comprehensively studied to determine their fundamental performance limits as transparent conductors through simulations and experiments. Numerical simulations and analytical calculations are used to evaluate the tradeoffs and correlations between these three figures of merit. A strong correlation is found between haze and transmission, where structures with high transmission tend to have low haze and vice versa. Structures with a pitch above 1000 nm are beneficial for achieving transmission over 80% and larger thickness is favorable in reducing sheet resistance without significantly affecting transmission. Furthermore, metal nanomeshes are fabricated to verify simulation results. The haze may be primarily explained by Fraunhofer diffraction, but the spectral dependence of haze requires analysis with Mie scattering theory. The results should apply to all metal grid or gratingā€like structures. The fundamental performance limits evaluated here are helpful for guiding engineering design and research prioritization.

     
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