Part of the reason may be that a staement of 70+% rectenna efficiency is based on microwave frequencies at which the antenna portion of the rectenna can be made very efficient over the bandwidth of the incident microwave energy. For optical rectenna's in addition to the difficulty of making small scale antenna elements, there are the additional considerations that the solar spectrum is spread across a wide bandwidth, and that material properties will not provide the same results as metals are much less conductive at optical freqeunceis than at microwave frequencies.
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Part of the reason may be that a staement of 70+% rectenna efficiency is based on microwave frequencies at which the antenna portion of the rectenna can be made very efficient over the bandwidth of the incident microwave energy. For optical rectenna's in addition to the difficulty of making small scale antenna elements, there are the additional considerations that the solar spectrum is spread across a wide bandwidth, and that material properties will not provide the same results as metals are much less conductive at optical freqeunceis than at microwave frequencies.