Efficiency enhancement in a lensed nanowire solar cell.

We investigate microlenses that selectively focus the light on only a small fraction of all nanowires within an arrayed InP nanowire solar cell. The nano-concentration improves both the short-circuit current (J s c ) and the open-circuit voltage (V o c ) of the solar cell. For this purpose, polymethyl methacrylate microlenses with 6 μm diameter were randomly positioned on top of an arrayed nanowire solar cell with 500 nm pitch. The microlenses were fabricated by first patterning cylindrical micropillars, which were subsequently shaped as lenses by using a thermal reflow process. The quality of the microlenses was experimentally assessed by Fourier microscopy showing strong collimation of the emitted photoluminescence. By analyzing the slope of the integrated photoluminescence vs excitation density, we deduce a substantial enhancement of the external radiative efficiency of a nanowire array by adding microlenses. The enhanced radiative efficiency of the lensed nanowire array results in a clear enhancement of the open-circuit voltage for a subset of our solar cells. The microlenses finally also allow to increase the short-circuit current of our relatively short nanowires, providing a route to significantly reduce the amount of expensive semiconductor material. [ABSTRACT FROM AUTHOR]