Single crystal monolithic upconverter solar cell device tandems with integrated optics

Abstract

Solar photons possessing energy less than the bandgap of a single-junction solar cell can be utilized via the upconversion (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 upconversion 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 upconverters 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 2.95 × in the 1450–1600 nm range), the non-concentrating UC-SC exhibited 1 . 5 × higher EQE than the UC-SC with a compound parabolic concentrator (CPC), while below 90 W / m 2 the CPC UC-SC exhibited 1.95 × 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 UC devices by using integrated optics.