  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.
