Absorption threshold extended to 1.15 eV using InGaAs/GaAsP quantum wells for over‐50%‐efficient lattice‐matched quad‐junction solar cells

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 1.31 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 large period 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. We believe that future devices which exploit light‐trapping structures and enhanced carrier extraction will be able to reach the desired target.