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Abstract
Current paradigms suggest that silicon-based cells will dominate the photovoltaic landscape for decades to come, if not longer. Yet the application of certain, current, dominant paradigms would also appear to lead to the extinction of the human race, and other events beyond the scope of the current work.
Indeed, the assumption that all energy of incoming photons in excess of the acceptance threshold of the solar cell material must be lost to (undesirable) heating of the structure creates a conflict between two issues: the need for a high material threshold-energy—electronic bandgap—and the wide range of energies in the dominant solar spectrum. The higher the threshold, the fewer photons will be accepted; the lower the threshold, the greater the thermalisation losses of power delivered from high energy photons. If, however, it is possible to tap the “excess” energy of charge carriers before they are able to thermalize, then not only can the high energy photons be utilized efficiently, but the bandgap can be designed significantly lower to allow for the acceptance of a greater number of photons.
The hot carrier solar cell (HCSC) would challenge a fundamental assumption of the Shockley-Queisser analysis: that all energy of incoming photons in excess of the acceptance threshold of the cell material is lost as heat. In the ideal case, if “excess” energy charge carriers are tapped before they thermalize, theoretical cell efficiency approaches that of a hypothetical infinite junction cell (67% without concentration), or double the theoretical maximum efficiency of any single junction cell. Two principal tasks, however, await the development of even a rudimentary hot carrier cell: actual retardation of carrier thermalisation, and extraction of the carriers via devices known as “Energy Selective Contacts”, which withdraw only carriers possessing a narrow range of energies.
This dissertation proposes construction of a hot carrier cell absorber utilizing elemental group III-V compounds. Key to this is establishment of a phonon bottleneck to re-supply carriers with energy until collection. To inform and adjust prior models of phonon dispersions in both such materials, Raman spectroscopy and inelastic X-ray scattering are utilized. Indium nitride, with its small electronic band gap, is able to absorb a large portion of the solar spectrum. It also possesses a large phononic band gap, and thus might form a promising hot carrier cell absorber due to blockage of Klemens decay of zone-centre optical phonons. Deposition of highly crystalline InN and complementary InGaN material systems is difficult, but may be possible to achieve via still-developing epitaxial techniques. InGaAs alloy multiple quantum well superlattices have been investigated, and slowed carrier relaxation to the band edge has been demonstrated via femtosecond-resolved photoluminescence.
Constriction of the carriers and optical phonons is instigated within the thin well structures, for it is this constriction which is thought to lead to a phonon bottleneck, and therefore a sustained hot carrier population. Slowed cooling may also be observed when an optical phonon dispersion mismatch is maintained between well and barrier, suggesting the desirability of disjoint frequency ranges in adjacent materials. This work attempts to isolate the advantages of carrier diffusion impediment vs. phonon diffusion blocking. The former should vary directly with barrier thickness and the latter inversely. To distinguish both from the phonon dispersion mismatch effect, MQW structures of varying composition were here employed, geared to affect dispersion mismatch but not carrier diffusion.