III/V semiconductor solar cells feature the highest photon conversion efficiencies (PCE), but they are still too expensive for terrestrial application. Conventional nanowire (NW) solar cells already partially resolve this issue since they can be grown on a silicon substrate and feature a low filling factor (the ratio of the 180 nm NW diameter to 500 nm NW pitch, squared). We take the next step by depositing PMMA micro-lenses with a diameter of 6 ΞΌm on top of a NW-array with the same 6 μm pitch, allowing to reduce the material consumption by more than 3 orders of magnitude. According to our FDTD simulations, the material consumption can even be further decreased by reducing the NW length with a factor of 2 down to 1 μm, since the lens is focusing the solar radiation near the top of the nanowires. We also expect a significantly increased Voc due to an increased internal radiative efficiency (IRE) at a higher excitation power. Preliminary measurement show an increase in Voc of at least 50 mV for randomly positioned microlenses on top of a dense NW array with 0.5 μm pitch.
Our previously reported 17.8 % efficiency InP nanowire solar cell1 showed a short-circuit current πΌπ π of 29.3 ππ΄ ππ2, which is not far from the theoretical maximum πΌπ π = 34.6 ππ΄ ππ2, but the loss in the open circuit voltage with respect to the radiative limit still amounted to 272 mV. To avoid this loss and reach the radiative limit we have to increase both the internal radiative efficiency πint PL and the photon escape probability πππ π towards unity, as shown by the last term in Eq. 1. πOC = πoc Ultimate β πBπ π |ππ πin πout |β πBπ π |ππ(πint PLπππ π)| (1) We report top-down etched InP nanowires intended to both optimize the amount of light outcoupling as well as the directionality of the emitted light. The photon entropy loss is governed by the ππ πin πout term, which is responsible for a 300 mV loss in the open circuit voltage. To circumvent this loss, we need to redirect all the emitted photoluminescence from the cell back to the sun (εππ = εππ’π‘), For this purpose, we have fabricated PMMA microlenses by using a reflow process, which can be precisely positioned with respect to the InP nanowires.
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