Towards Sustainable, Scalable, Low-Cost InP Space Solar Power

The world needs clean, sustainable, and consistent sources of power. Solar energy is an attractive source of renewable power, but energy storage is a challenge for intermittent sources like solar and wind. The massive reduction in space launch costs now makes the prospect of space solar power as a 24/7, consistent baseline source of clean power more of a viable possibility than before. Solar cells designed to provide power for Earth while in space must not only be low cost and efficient but also light and maintain their performance in harsh space environments, especially in extreme temperature conditions while being subject to damaging space radiation. This thesis explores two different approaches to achieving radiation hardness: using nanowire solar cells instead of planar cells and using thin-film diffusion-doped cells. It does so with cells made of indium phosphide (InP), which is inherently more resilient to damage from space radiation than materials like silicon (Si) and gallium arsenide (GaAs). The performance of diffusion-doped planar cells is compared to epitaxially-grown solar cells as a baseline. Nanowire cells were simulated in Lumerical to understand the light-trapping properties compared to planar cells with and without surface texturing that also enhances light absorption. The nanowires were simulated with and without hemispherical nanoparticles to enhance light absorption while maintaining a transparent conducting top layer to adequately transport the generated electricity. Nanowire and planar cells were further simulated in Sentaurus to predict their electrical performance. Nanowire fabrication was attempted as well, but reliable, consistent fabrication was challenging. Given the cost and scalability challenges of this approach, the rest of the work pivots to planar cells. Epitaxially-grown InP cells optimized using Sentaurus were fabricated as a baseline and to work out fabrication challenges adjacent to the p-n junction formation itself. Then, diffusion-doped InP cells were fabricated using Cd₃P₂ and Zn₃P₂ as p-type dopants on undoped InP substrates. Preliminary cell performance optimzation was conducted by adjusting diffusion temperatures and times as well as thinning the emitter layer. Efficiencies of up to 4.94% were achieved in Cd-doped cells and up to 3.85% were achieved in Zn-doped cells without anti-reflection coatings, with a maximum JSC of 11.33 mA/cm² and a maximum VOC of 778.1 mV in 100 nm thinned Zn-doped cells.