Thinning down the absorber layer of GaAs solar cells can reduce their cost and improve their radiation hardness, which is important for space applications. However, the lighttrapping schemes necessary to achieve high absorptance in these cells can be experimentally challenging or introduce various parasitic losses. In this work, a facile light‐trapping approach based on wet chemical etching is demonstrated. The rear‐side contact layer of ultrathin GaAs solar cells is wet‐chemically textured in between local Ohmic contact points using an NaOH‐based etchant. The resulting contact layer morphology is characterized using atomic force microscopy and scanning electron miscroscopy. High broadband diffuse reflectance and haze factors are measured on bare and Ag‐coated textured contact layers. The textured contact layer is successfully integrated as a diffusive rear mirror in thin‐film solar cells comprising a 300‐nm GaAs absorber and Ag rear contact. Consistent increases in short‐circuit current density (JSC) of approximately 3 mA cm−2 (15%) are achieved in the textured cells, while the open‐circuit voltages and fill factors do not suffer from the textured rear mirror. The best cell achieves a JSC of 24.8 mA cm−2 and a power conversion efficiency of 21.4%. The textured rear mirror enhances outcoupling of luminescence at open circuit, leading to a strong increase in the external luminescent efficiency.

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A thin, lightweight, flexible solar cell is developed that maximizes the power-to-mass ratio under AM0 illumination and has a competitive efficiency after typical high energy electron irradiation. The inverted metamorphic triple junction (IMM3J) solar cells with Ga 0.51 In 0.49 P/GaAs/Ga 0.73 In 0.27 As subcells are grown on GaAs substrates and have a total epitaxy thickness of about 10 μm. After epitaxial growth, the inverted layer stack is metallized, with the metal serving as back-contact, back reflector and support layer for the ultra-thin solar cells before the GaAs substrate is separated by an epitaxial lift-off (ELO) process. The nondestructive nature of the ELO process makes multiple reuses of the GaAs substrate possible. The solar cell structure is optimized for maximum EOL efficiency, that is, after 1-MeV electron irradiation with a fluence of 1 Â 10 15 cm À2 , by means of simulations that include the irradiation induced defects in the various semiconductor alloys. Assuming realistic charge carrier lifetime in the materials, we predict a near-term efficiency potential for the IMM3J ELO of 30.9% under AM0 illumination before and 26.7% after irradiation. Several IMM3J ELO solar cells with an area of approximately 20 cm 2 from different development stages were tested under AM0 illumination. The newest solar cells (generation III) with a mass density of only 13.2 mg/cm 2 reach conversion efficiencies of 30.2% at 25 C. The resulting power-to-mass ratio of 3.0 W/g for the bare solar cell is one of the highest published ratios. After irradiation, a conversion efficiency of 25.4% was measured for "generation II" devices under AM0 illumination, which corresponds to a power-to-mass ratio of 2.6 W/g. IMM3J ELO solar cells from "generation I" were also tested for mechanical stability as "de-risking" test of this new cell technology. No degradation of the cell performance was found after dipping the cell in liquid N 2 and then heating up to 25 C for five times, despite of strong deformation of the flexible cell during the temperature cycle.

The effects of electron irradiation on the performance of GaAs solar cells with a range of architectures is studied. Solar cells with shallow and deep junction designs processed on the native wafer as well as into a thin‐film were irradiated by 1‐MeV electrons with fluence up to 1×1015 e−/cm2. The degradation of the cell performance due to irradiation was studied experimentally and theoretically using model simulations, and a coherent set of minority carriers' lifetime damage constants was derived. The solar cell performance degradation primarily depends on the junction depth and the thickness of the active layers, whereas the material damage shows to be insensitive to the cell architecture and fabrication steps. The modeling study has pointed out that besides the reduction of carriers lifetime, the electron irradiation strongly affects the quality of hetero‐interfaces, an effect scarcely addressed in the literature. It is demonstrated that linear increase with the electron fluence of the surface recombination velocity at the front and rear hetero‐interfaces of the solar cell accurately describes the degradation of the spectral response and of the dark current characteristic upon irradiation. A shallow junction solar cell processed into a thin‐film device has the lowest sensitivity to electron radiation, showing an efficiency at the end of life equivalent to 82% of the beginning‐of‐life efficiency.

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