Design, experiment, and analysis of a photovoltaic absorbing medium with intermediate levels

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Levy, Michael Yehuda
Honsberg, Christiana
Citrin, David S.
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The absorption of the sun's radiation and its efficient conversion to useful work by a photovoltaic solar cell is of interest to the community at large. Scientists and engineers are particularly interested in approaches that exceed the Shockley-Queisser limit of photovoltaic solar-energy conversion. The abstract notion of increasing the efficiency of photovoltaic solar cells by constructing a three-transition solar cell via an absorber with intermediate levels is well-established. Until now, proposed approaches to realize the three-transition solar cell do not render the efficiency gains that are theorized; therefore, researchers are experimenting to ascertain where the faults lie. In my opinion, it is unclear if the abstract efficiency gains are obtainable. Furthermore, it is difficult to determine whether three-transition absorbers are even incorporated in the existing three-transition solar cell prototypes. I assert that there are material systems derived from the technologically important compound semiconductors and their ternary alloys that more clearly determine the suitability of employing nanostructured absorbers to realize a three-transition solar cell. The author reports on a nanostructured absorber composed of InAs quantum dots completely enveloped in a GaAsSb matrix that is grown by molecular beam epitaxy. The material system, InAs/GaAs$_{0.88}$Sb$_{0.12}$, is identified as an absorber for a three transition solar cell. This material system will more easily determine the suitability of employing nanostructured absorbers because its quantum-dot heterojunctions have negligible valence-band discontinuities, which abate the difficulty of interpreting optical experimental results. A key tool used to identify the GaAs$_{1-x}$Sb$_{x}$ ($xapprox 0.12$) is a maximum-power iso-efficiency contour plot. This contour plot is only obtainable by first having analyzed the impact of both finite intermediate-band width and spectral selectivity on the optimized detailed-balance conversion efficiencies of the three-transition solar cell. Obtaining the contour plot is facilitated by employing a rapid and precise method to calculate particle flux (Appendix~ ef{ch:Rapid-Precise}). The author largely determines the electronic structure of the InAs/GaAs$_{1-x}$Sb$_{x}$ ($xapprox 0.12$) absorber that is grown by molecular beam epitaxy from optical experimental methods and in particular, from photoluminescent spectroscopy. The interpretation of the experimental photoluminescent spectrum is facilitated by having first studied the theoretical photoluminescent spectra of idealized three-transition absorbers.
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