Solar technology is crucial in generating power for space exploration and simultaneously has the potential to satisfy a large percentage of projected world energy demands. The high cost per watt has prevented terrestrial solar from becoming a cost-effective alternative to fossil fuels. In order to make solar cell technology more competitive on Earth and to also meet the ever-increasing power demands of space applications, we need to both increase efficiency and lower the cost per area. Current commercially available silicon solar cells are approaching the theoretical Shockley-Queisser efficiency limit of ~30%, motivating the use of III-V multijunction cells. Such devices have reached 46.0% efficiency under concentration by using an optimized combination of materials with different bandgaps to collect a broad portion of the solar spectrum and minimize thermalization losses. The increased efficiency and stacked design of multijunction solar cells also offer the benefit of a high specific power relative to other currently available solar technologies, making multijunction cells particularly attractive for space applications. While III-V cells have traditionally been the most efficient, they suffer from high cost. In this thesis, I describe four highly complementary research paths that aim to address efficiency, specific power, and cost issues through improved wide-bandgap top junctions and the integration of III-V multijunction cells onto silicon.
The first project aims to improve efficiency through the introduction of a widebandgap top junction to better collect the higher energy photons in the solar spectrum. The two materials we study here are GaP and metamorphic InA1P. While GaP benefits from being Al-free and a simple binary system, the InAIP alloy has the widest direct bandgap of all non-nitride III-Vs. Both of these materials remain largely unstudied, the former due to its indirect bandgap, and the latter because it is not lattice-matched to common substrates which necessitates complex graded buffers to minimize strain-induced crystalline defects (threading dislocations) that can harm solar cell performance via increased non-radiative recombination. Recent advancements in graded buffer growth techniques have enabled investigation of this promising material. In this thesis, I performed the first study of the effect of growth conditions and device structures of GaP and metamorphic InA1P solar cells.
The second project focuses on reducing cost through the integration of III-V cells onto Si for its low-cost and established manufacturing infrastructure. One method to accomplish this is via metamorphic growth of GaAs0.75P 0.25 on Si. Previous studies on GaAso.75Po.25 solar cells grown on GaP/Si templates indicate challenges with extended crystalline defect nucleation upon relaxation of GaP on Si. In particular, we investigate both the glide and nucleation of threading dislocations during graded buffer growth to minimize the final threading dislocation density which will help improve cell characteristics. Through dislocation engineering and enhancements to device design, we achieved a new record for GaAsP solar cells grown on GaP/Si.
The third project targets an integral component of multijunction devices: tunnel junctions. While these are widely used to electrically and monolithically connect two pnjunctions in series, the Lee research group has never before grown tunnels on a GaAs0.75P0.25 lattice constant, necessary for the metamorphic III-V on Si pathway. The larger bandgap of GaAs0.75 P0.25 reduces tunneling probability, which makes it more difficult to pass current with minimal voltage loss in a multijunction structure. This is confounded further by inevitable thermal damage to tunnel junction performance during growth of the top cell. We explored the use of strained GaAs tunnels and delta doping to bolster tunneling current enough to maintain functionality after the top cell growth.
The fourth project aims to accomplish low-cost III-V on Si using the alternative approach of selective area growth of GaAs on Si via nanoimprint lithography. By growing largely lattice-mismatched materials selectively in nanopatterns with large enough aspect ratios, threading dislocations can be moderately controlled. Techniques to control other III-V/Si interfacial defects, such as anti-phase domains, are also described. Selective area growth could offer a lower-cost approach than thick metamorphic buffers since it could enable thinner buffers to achieve a low threading dislocation density, while implementing low-cost nanoimprint lithography for patterning. In this work, we explore GaAs solar cells grown on Si via selective area growth, showing improvements to device design that enable significantly enhanced performance. Overall, the results presented in this thesis on the integration of wide-bandgap top cells and III-V materials onto low-cost Si substrates provide a promising pathway towards reducing costs and improving efficiency of both space and terrestrial photovoltaics.
|Advisor:||Lee, Minjoo Larry, Reed, Mark|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 79/12(E), Dissertation Abstracts International|
|Subjects:||Middle Eastern Studies, Nanoscience, Materials science|
|Keywords:||III-V on Si, Metamorphic, Photovoltaics, Selective area, Solar Cell, Solar Energy|
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