An ultrathin glass layer gives flexible triple-junction solar cells both chemical and radiation resistance, opening a single design to space power and solar fuel production.
(Nanowerk Spotlight) Space solar arrays and solar-driven fuel production seem like unrelated technologies. Both, however, rely on III–V semiconductor photovoltaics, cells that stack crystalline layers of compounds such as indium gallium phosphide, gallium arsenide, and indium gallium arsenide to harvest different portions of the solar spectrum. Yet the two applications have historically demanded separate device designs. Space arrays need radiation shielding and lightweight flexibility. Solar fuel systems need chemical durability in corrosive electrolytes. Designing for one environment has typically meant leaving the device vulnerable in the other.
The core difficulty is encapsulation. Rigid coverglass blocks radiation and resists chemical attack, but it makes devices stiff and heavy. Thin polymer barriers preserve flexibility but dissolve or degrade in alkaline solutions and stop few energetic particles. A protective layer that bends with the cell while repelling both corrosive liquids and high-energy radiation has remained elusive.
Epitaxial characterization of the InGaP/GaAs/InGaAs IMM triple-junction solar cell. (a) Schematic of the triple-junction cell structure grown on a GaAs wafer. (b) AFM image (10 × 10 µm2) of the topmost epitaxial surface. (c) Cross-sectional bright-field TEM image of the triple-junction epitaxial stack, with magnified images highlighting the InGaAs base layer and the InAlGaAs metamorphicbufferlayer. (Image: Reproduced from DOI:style=”color:#0000FF” target=”_blank”, CC BY) (click on image to enlarge)
To build the platform, the research team grew an indium gallium phosphide, gallium arsenide, and indium gallium arsenide triple-junction stack in an inverted metamorphic configuration on a gallium arsenide wafer, then transferred the active layers to a 25 µm plastic substrate and laminated the ultrathin glass on top.
Growing the three subcells monolithically required managing a roughly 2% lattice mismatch between the gallium arsenide middle layer and the indium gallium arsenide bottom layer. A step-graded buffer whose composition shifts gradually confined threading dislocations, linear crystalline defects that degrade electronic performance, within the buffer itself. X-ray mapping confirmed the bottom subcell reached about 98% strain relaxation, and dislocation density remained very low across the active layers.
Under standard terrestrial illumination, a representative 1 × 1 cm² cell achieved 33.4% power conversion efficiency, with the best cell reaching 35.8%. Under unfiltered space sunlight the bare device delivered 31.7%. Bending tests found no measurable performance loss at a bending radius of 1.5 cm, and 1,000 repeated cycles at that curvature caused no degradation.
Laminating the ultrathin glass reduced terrestrial efficiency modestly to 31.7%, mainly because of optical absorption in the glass. The chemical protection it provided was substantial. Bare cells immersed in 70 °C deionized water lost most of their output within 17 days as oxide coatings dissolved and contacts oxidized. In 1 M potassium hydroxide at 30 °C, unprotected devices degraded severely within three hours. Glass-encapsulated cells retained nearly all performance under both conditions, losing less than 4% efficiency after 24 hours in the alkaline solution.
With the encapsulated cells surviving prolonged alkaline exposure, the team coupled them to copper catalyst electrodes in a carbon dioxide saturated potassium bicarbonate electrolyte to test bias-free electrocatalysis. The cells generated enough photovoltage to drive reduction reactions without any external power supply. Nuclear magnetic resonance spectroscopy identified formate as the dominant liquid product over a five-hour run, while ethanol and acetate remained minor byproducts. Faradaic efficiency reached roughly 30% and solar-to-fuel efficiency about 6%, values limited by the simple copper catalyst rather than the photovoltaic module itself.
Radiation tests exposed a sharp divide between bare and encapsulated devices. Unshielded cells bombarded with 1 MeV protons lost essentially all output at the highest fluence tested as displacement damage filled the semiconductor lattice with recombination centers. The ultrathin glass blocked both protons and electrons effectively. Encapsulated cells kept full efficiency after the highest proton dose and lost only 7% under equivalent electron bombardment, outperforming previously reported space photovoltaic modules.
Space agencies seeking lightweight, rollable arrays that survive prolonged orbital radiation now have a flexible platform with proven hardness at doses that destroyed unshielded devices. Researchers pursuing solar-driven chemical synthesis gain a photovoltaic module that tolerates the corrosive electrolytes their catalysts require.
Replacing the simple copper electrodes with more active and selective catalysts could substantially raise both Faradaic and solar-to-fuel efficiencies, while the high voltage output of the triple-junction design leaves headroom for more demanding electrochemical reactions. The platform removes a persistent obstacle to deploying III–V multijunction cells in environments where efficiency alone was never enough.
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