III-V semiconductors make for highly efficient solar cells, but are expensive to manufacture. However, there are many mechanisms for improving III-V photovoltaics in order to make them more competitive with other photovoltaic (PV) technologies. One possible method is to design cells for high efficiency under concentrated sunlight, effectively trading expensive III-V material for cheaper materials such as glass lenses. Another approach is to reduce the amount of III-V material necessary for the same power output, which can be achieved by removing the substrate and installing a reflector on the back of the cell, while also adding quantum structures to the cell to permit absorption of a greater portion of the solar spectrum.
Regarding the first approach, this dissertation focused on the development of an InAlAsSb material for a mulitjunction design with the potential of achieving 52.8% efficiency under 500 suns. First, development of a single-junction InAlAs cell lattice-matched to InP was executed as a preliminary step. The InAlAs cell design was optimized via simulation, then grown via metal organic vapor phase epitaxy (MOVPE) and fabricated resulting in 17.9% efficiency under 1-sun AM1.5, which was unprecedented for the InAlAs material. Identical InAlAs cells were grown using alternative MOVPE precursors to study the effects of necessary precursors for InAlAsSb. Fits to experimental device results showed longer lifetimes when grown with the alternative aluminum precursor. InAlAsSb grown using these alternative precursors targeted a 1.8 eV bandgap required for the multijunction design. Ultimately, InAlAsSb material with the desired bandgap was confirmed by photoreflectance spectroscopy.
For the second approach, this dissertation studied the integration of InAs quantum dots (QDs) in a GaAs solar cell in conjunction a back surface reflector (BSR). A quantum dot solar cell (QDSC) with a BSR has the potential to increase short-circuit current by 2.5 mA/cm2 and also increase open-circuit voltage due to photon recycling. In this study, multiple textured BSRs were fabricated by growing inverted QDSCs on epitaxial lift-off templates and then texturing the rear surface before removing the device from the substrate. Identical cells with a flat BSR served as controls. Optimization of inverted QDSC growth conditions was also performed via a cell design study. Device results showed increased open-circuit voltage with increasing optical path length, and the greatest improvement in sub-band current over a flat BSR control device was 40%.
In the final chapter, a life cycle assessment (LCA) of these technologies was performed to identify the hypothetical optimum at which energy investments in cell performance (such as the two described above) no longer correspond to improvements in the overall life cycle performance of the PV system. Four cell designs with sequentially increasing efficiencies were compared using a functional unit of 1 kWp. The first is a commercially available and has been studied in previous LCAs. The second is the design containing InAlAsSb mentioned above. The third represents the most material-intensive option, which bonds two substrates to create a five-junction cell. The fourth is a six-junction cell that uses a metamorphic grade between subcells and represents the most energy-intensive option. A thorough literature review of existing LCAs of high-concentration photovoltaic (HCPV) systems was performed, which obviated the need for data on the manufacture of MOVPE precursors and substrates. LCAs for the most common III-V substrate (GaAs) and precursors were executed prior to conducting the HCPV system LCA, due to the absence of detailed information on the life cycle impacts of these compounds in literature. Ultimately, both the cumulative energy demand and greenhouse gas emissions of the HCPV system decreased proportionally with increasing cell efficiency, even for the most energy and material-intensive cell designs. It was found that the substrates and precursors corresponded to less than 2% of system impacts. This implies that current mechanisms to increase cell efficiency are environmentally viable in HCPV applications without the need for material reduction, and would make III-V HCPV more environmentally competitive with dominant silicon PV technologies
