Semiconductor solar cells have improved over the years but are subject to some fundamental limitations. Long wavelength light is not absorbed, and short wavelength light is only partially used, leading to maximum conversion efficiencies below approximately 30%. Multi-layer cells can improve upon that, but with additional complexity and cost. Semiconductor solar cells also require costly refined materials and transparent conducting layers. These layers are required to make electrical contact with the illuminated side of most solar cells and typically incorporate indium, a limited and expensive resource. Antenna-coupled diode solar cells, also called rectenna solar cells, work on an entirely different principle, much like a crystal radio receiver but for light. Incoming solar radiation (electromagnetic waves) is received by sub-micron-size antennas, which convert it to ultra-high-frequency alternating current (AC). This current passes through a nanometer-scale, ultra-high-frequency diode, which converts the AC to direct current (DC) and provides usable power (see Figure 1). A solar cell would incorporate a large array of millions of these elements in tandem deposited onto a glass or plastic substrate. Fabrication costs can be low, with devices processed cheaply in a roll-to-roll process.1 In principle, the conversion efficiency for rectenna solar cells can be very high, limited to 93% by the entropy of the photon gas,2 but other constraints limit the efficiency to well below this number. The demands placed on the diode are extreme. First, it must operate efficiently at extremely high frequencies—close to a petahertz (1015Hz) for visible light—orders of magnitude higher than the fastest electronics. Second, it must couple electrical power efficiently from the antenna. To do so, the impedance (a measure of the ratio of the voltage magnitude and phase to that of the current in an electronic element) of the diode must match the low impedance of the antenna. Unfortunately, the two requirements conflict with each other. An additional challenge Figure 1. Rectenna solar cell. The antenna converts incoming solar radiation to a petahertz alternating current through the diode, which rectifies it. The resulting direct-current (DC) power is available at the output leads.
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