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Solar photons possessing energy less than the band-gap of a single-junctionsolar cell can be utilized via the up-conversion (UC) of two or more photons,resulting in the emission of a single above-bandgap photon. Due to thenon-linear nature of UC, highly concentrated light is required, which istypically much greater than the practical concentration limits of a solar cell.It has been proposed that concentrating up-conversion solar cells (UC-SC) withoptical elements integrated into the device could help realize the high solarirradiance required. To avoid scattering problems arising from common UCmaterials based on micro-crystalline powders, in this work concentrators areinvestigated with mono-crystalline up-converters in silicon-based tandemdevices. An external quantum efficiency (EQE) of 6% with 1493 nm infraredillumination at $876 W/m^2$ was obtained in upconverter device with concaveintegrated optics. At an irradiance higher than $90 W/m^2$ (equivalent to 2.95xin the 1450-1600 nm range), the non-concentrating UC-SC exhibited 1.5x higherEQE than the UC-SC with CPC, while below $90 W/m^2$ the CPC UC-SC exhibited1.95x higher EQE than the non-concentrating reference device. Due to thenegligible scattering of the UC layer, the distribution of localized irradianceis revealed along with its effect on the performance of devices. It is foundthat irradiance is accumulated within the first 1 mm of the UC layer with peaksat variable depths according to the concentrating scheme. These results suggestample space for improved up-conversion devices by using integrated optics.

Single crystal monolithic up-converter solar cell device tandems with integrated optics