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Ultra-thin GaAs Photovoltaics for Space Applications



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Sayre, Larkin 


Ultra-thin photovoltaics (<100 nm) have shown an intrinsic tolerance to radiation-induced damage which makes them a potentially advantageous power source for spacecraft which need to withstand harsh environments outside Earth’s atmosphere. In the ultra-thin regime, high transmission losses can be mitigated by integrating light management structures with nanoscale features. A new type of ultra-thin single-junction GaAs solar cell was designed using drift-diffusion simulations with an 80 nm absorber layer thickness and optimised passivation layers. In particular, the use of InGaP as the front surface passivation layer, instead of the more widely used AlGaAs, produced optimal front surface passivation and performance despite being a direct band-gap semiconductor. The annealed n-type contact was optimised using a transmission line measurement study to minimise series resistance at the metal-semiconductor interface while avoiding excess diffusion of Au into the active layers of the device which degrades shunt resistance. Periodic metal-dielectric nanostructures were simulated and optimised for light management in 80 nm devices using rigorous coupled-wave analysis. Displacement Talbot lithography (DTL) was used for the first time in a photovoltaic application to produce these nanostructures. DTL is a non-contact, wafer-scale interference lithography technique that produces periodic features with excellent uniformity over significant topography in a single exposure. A hexagonal array of Ag pillars in a SiN layer was patterned on the back surface of the ultra-thin devices to increase the optical path length of photons through the active layers. A wafer lift-off process using an epoxy bond and substrate etch back technique was developed to remove the devices from their growth wafers. This lifted-off design produced an AM0 short circuit current of 15.35 mA/cm² and an AM0 efficiency of 9.08%, a 68% increase over the planar on-wafer equivalent. Optical simulations confirmed the contributions of Fabry-Perot and waveguide modes to this current increase. Simulated fabrication and design improvements showed a feasible pathway to 16% AM0 efficiency. Planar on and off-wafer 80 nm ultra-thin devices were then exposed to 68 MeV and 3 MeV proton radiation to test their resilience in the space environment. Irradiation results for on-wafer devices have shown boosted absorption of light compared to previous 80 nm onwafer ultra-thin designs in the literature. Maximum power values for off-wafer devices with integrated back surface planar mirror also exceeded cells that are two orders of magnitude thicker from 3×10¹¹ p⁺/cm², the lowest 3 MeV proton fluence that was tested. Devices with 3500 nm thickness produced just 53% of pre-exposure short circuit current at an equivalent fluence of 7.21×10¹² p⁺/cm². However, there was no degradation in short-circuit current for 80 nm devices up to 2×10¹⁴ p⁺/cm² . Time-resolved cathodoluminescence analysis was carried out on radiation damaged devices and was used to correlate the onset of short circuit current degradation with the point when extrapolated carrier lifetime drops below the calculated time for carriers to traverse the junction. This is the first evidence in the literature that suggests the intrinsic radiation tolerance of ultra-thin cells is due to carrier lifetimes remaining long in relation to junction traverse time even after radiation-induced defects are introduced.





Hirst, Louise


Photovoltaics, Ultra-thin, GaAs, III-V, Radiation tolerance, Light management


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Engineering and Physical Sciences Research Council (2104603)
EPSRC (2104603)