ABSORPTION THRESHOLD EXTENDED TO 1.15 eV USING InGaAs/GaAsP QUANTUM WELLS FOR OVER-50-EFFICIENT LATTICE-MATCHED QUAD-JUNCTION SOLAR CELLS

摘要

Bandgap engineering of strain-balanced InGaAs/GaAsP multiple quantum wells (MQWs) allows high-quality materials with an absorption edge beyond GaAs to be epitaxially grown in Ge/GaAs-based multijunction solar cells. We demonstrate MQW solar cells with effective bandgaps ranging from 1.30 eV to as low as 1.15 eV. The bandgap-voltage-offset of MQWs is found to be independent of effective bandgaps and superior to a bulk reference by approximately 0.1 V. This implies the merit of high photovoltage as compared with bulk cells with the same bandgap in addition to their widely bandgap-tunable property. Towards the realization of fully lattice-matched quad-junction devices, we demonstrate a 70-period, 1.15-eV bandgap MQW cell as a promising material in 0.66/1.15/1.51/1.99-eV quad-junction cells, whose practical efficiency has a potential to achieve over 50%. With such a high number of MQWs the reverse-biased external quantum efficiency reaches an average of over 60% in the spectral region corresponding to a 1.15-eV subcell; this is achieved with only a-few-percent drop at short-circuit condition. The device presented here reaches the target open-circuit voltage and over 75% of the current density required for realizing a 1.15-eV subcell in a 50%-efficient quad-junction solar cell without any light-trapping structures.