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

Solar photons possessing energy less than the band-gap of a single-junction solar 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 the non-linear nature of UC, highly concentrated light is required, which is typically much greater than the practical concentration limits of a solar cell. It has been proposed that concentrating up-conversion solar cells (UC-SC) with optical elements integrated into the device could help realize the high solar irradiance required. To avoid scattering problems arising from common UC materials based on micro-crystalline powders, in this work concentrators are investigated with mono-crystalline up-converters in silicon-based tandem devices. An external quantum efficiency (EQE) of 6% with 1493 nm infrared illumination at $876 W/m^2$ was obtained in upconverter device with concave integrated optics. At an irradiance higher than $90 W/m^2$ (equivalent to 2.95x in the 1450-1600 nm range), the non-concentrating UC-SC exhibited 1.5x higher EQE than the UC-SC with CPC, while below $90 W/m^2$ the CPC UC-SC exhibited 1.95x higher EQE than the non-concentrating reference device. Due to the negligible scattering of the UC layer, the distribution of localized irradiance is revealed along with its effect on the performance of devices. It is found that irradiance is accumulated within the first 1 mm of the UC layer with peaks at variable depths according to the concentrating scheme. These results suggest ample space for improved up-conversion devices by using integrated optics.