Thermophotovoltaic (TPV) devices based on GaInAsSb lattice matched to GaSb (100) substrates have demonstrated high external quantum efficiencies (EQEs) in the mid-infrared spectral range, making them ...promising candidates for waste heat recovery from high temperature “blackbody” sources. In this work, the GaInAsSb alloy has been integrated onto more cost-effective GaAs (100) substrates by using advanced metamorphic buffer layer techniques in molecular beam epitaxy (MBE), which included an interfacial misfit (IMF) array at the GaSb/GaAs interface followed by GaInSb/GaSb dislocation filtering layers. The threading dislocations in the GaInAsSb region can be efficiently supressed, resulting in high quality materials for TPV applications. To determine the performance of the GaInAsSb TPV on GaAs it was compared with a cell grown lattice matched onto GaSb substrate having the same structure. The resulting TPV on GaAs exhibited similar dark current-voltage characteristics with that on GaSb. Under illumination from an 800 °C silicon nitride emitter, the short circuit current density (Jsc) from the GaInAsSb TPVs on GaAs reached more than 90% of the control cell on GaSb, and the open circuit voltage (Voc) exceeded 80% of the cell on GaSb. The EQE from the TPV on GaAs reached around 62%, the highest value reported from this type of TPV on GaAs. With improved TPV structure design, large area GaInAsSb TPV panels on GaAs substrates can be realized in the future for waste heat energy recovery applications.
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•GaInAsSb material was integrated onto GaAs substrates with advanced buffer layer, resulting in low dislocation densities.•GaInSb/GaSb dislocation filtering layers can effectively reduce the threading dislocations.•The GaInAsSb thermophotovoltaic (TPV) on GaAs achieved similar performance as the one on lattice matched GaSb substrate.•Highest external quantum efficiency of 62% was reported from the GaInAsSb TPV on GaAs.
The capture cross-section, intersubband optical cross-section and non-radiative emission rates related to localized hole states are obtained for p-i-n solar cells containing GaSb/GaAs quantum rings ...embedded within the i-region of the device. The technique developed uses the intraband photoemission current to probe the charge state of the nanostructures during two-color excitation. Analysis of the excitation power dependence revealed a non-radiative hole capture lifetime of 12 ns under low excitation conditions, with high injection leading to the saturation of the hole occupancy within the quantum-rings. The decay characteristics of the optical hole emission current has also been exploited to determine the spectral and temperature dependence of the radiative and non-radiative hole escape mechanisms from the quantum-rings.
•Power dependence of intraband photocurrent reveals quantum dot dynamics.•Two-color photoresponse correlated to capture/emission efficiency of nanostructures.•Optical and non-radiative carrier emission determined for type-II quantum structures.
A method to produce photonic glasses (disordered dielectric structures built from monodisperse spheres) composed of silica is discussed. This method is capable of dispensing samples in a few minutes ...with the help of a uniaxial press. An analytical model that accounts for the most relevant features and explains how Mie scattering is the leading phenomenon is developed.
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-2 s -1 . As illumination increases SRH is overtaken by radiative band-toband 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%.
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