Type-II GaSb/GaAs Quantum Ring Intermediate Band Solar Cell

Montesdeoca Cardenes, Denise and Carrington, Peter and Krier, Anthony and Wagener, Magnus C. (2019) Type-II GaSb/GaAs Quantum Ring Intermediate Band Solar Cell. PhD thesis, Lancaster University.

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Abstract

There is considerable interest in the development of high efficiency cost-effective solar cells for renewable energy generation. Multi-junction cells based on III-V compound semiconductors currently hold a strong position because they are well suited to solar concentrator systems. However, high efficiency can be also be achieved by exploiting two photon photocurrent relying on the intermediate band concept. This thesis reports on an in-depth investigation of solar cells containing type-II GaSb/GaAs quantum ring (QR) nanostructures, systematically studying their electrical and optical performance under different test conditions. The aim is to understand the potential and limitations of this system, working as an intermediate band solar cell (IBSC), towards improving the efficiency of the conventional GaAs single junction solar cell. Two different approaches were developed and investigated to enhance solar cell performance; (i) increasing the number of QR layers and (ii) decreasing the overall thickness of the QR stack. High density stacks of type-II GaSb/GaAs QR of high crystalline quality were successfully grown and characterized as QRSC for the first time. The stacks consisted of up to 40 QR layers with a linear density of 0.17 nm-1 along the growth direction, which is the highest reported to date for a type-II IBSC. By increasing the number of QR layers from 10 to 40, both the efficiency and the electroluminescence of the WL and QR transitions were enhanced by a factor of 4. Electroluminescence and open-circuit voltage (VOC) reciprocity was demonstrated experimentally, showing that QR radiative recombination limits VOC. Hole transport through the intrinsic region of the cell was improved by decreasing the QR stack thickness from 400 to 60 nm, which resulted in an enhancement of 15% in short-circuit current and 30% in conversion efficiency. However, the open-circuit voltage drops when adding QR to the GaAs matrix preventing high efficiency from being obtained. Two-photon photocurrent was studied in type-II QRSC using two-colour spectroscopy at 17 K and trapping was identified as the mechanism controlling QR (hole) charging. Above 100 K thermionic emission of holes was found to dominate QR discharging over optical emission. The non-radiative and radiative decay times of the QR were measured for first time at 10 K to be 10-100 ns, equivalent to rates of 108-107 s-1, under a peak flux of 1017 cm-2s-1. As illumination increases SRH is overtaken by radiative band-to-band recombination in high density QR stack SC. Partial VOC recovery (up to 56%) under high solar concentration of 4000 suns was demonstrated in type-II QRSC at room temperature, analysing current versus voltage characteristic for first time. Full recovery of VOC is prevented by the thermal coupling between the QR intermediate band and the valence band. In addition, hydrostatic pressure was applied to IBSC for the first time, to characterize type-II GaSb/GaAs QRSC. Under 8 kbar at room temperature the bandgap increased by 100 meV without modifying the carrier thermal energy and resulting in a VOC increase of 15%.