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Background: Non-contrast computed tomography of the brain (NCCTB) is commonly used in clinical practice to detect intracranial pathology but is subject to interpretation errors. Machine learning is capable of augmenting clinical decision making and there is an opportunity to apply deep learning to improve the clinical interpretation of NCCTB scans. This retrospective detection accuracy study assessed the performance changes of radiologists assisted by a deep learning model designed to identify many NCCTB clinical findings and also compared the standalone performance of the model with that of unassisted radiologists. Methods: A deep learning model was trained on 212,484 CT scan images of the brain. Thirty-two radiologists each reviewed 2,848 NCCTB cases in a test dataset with and without the assistance of the deep learning model. The consensus of three subspecialist neuroradiologists with access to reports and clinical history was used as a ground truth baseline for comparison. Performance metrics including area under the receiver operating characteristic curve (AUC) were calculated for the unassisted and assisted radiologists. Average assisted and unassisted radiologist performance was also compared to that of the model for each clinical finding. Findings: Use of the deep learning model by radiologists significantly improved interpretation performance. Assisted and unassisted radiologists demonstrated an average AUC of 0.79 and 0.73 across the 22 grouped parent findings and 0.72 and 0.68 across all 189 child findings combined, respectively. When the model was used as an assistant, change in radiologist AUC was positive and significant for 91 child findings and 158 findings were clinically non-inferior. AUC decrements were identified for 17 findings. The model alone demonstrated an average AUC of 0.93 across all 144 model findings. Interpretation: The assistance of a comprehensive NCCTB deep learning model in a non-clinical setting significantly improved radiologist detection accuracy across a wide range of clinical findings. This study demonstrated the potential of the evaluated model to improve NCCTB interpretation.

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials., Slow cooling of hot charge carriers in lead halide perovskite could be used in photovoltaics devices. Here, Yang et al. study hot carrier dynamics by transient absorption spectroscopy. They relate the phonon bottleneck to the up-conversion of low-energy phonons, facilitated by the presence of organic cations.

The hot carrier solar cell is a promising concept for high efficiency photovoltaics. Obtaining the dynamic temperature information for the hot carriers in an absorber is of crucial importance to this concept. In this paper, we present a dynamical hot carrier theory that extracts the carrier temperature and the instantaneous carrier occupation from transient absorption spectra. We have applied this model to two transition metal nitride materials, HfN and ZrN. The analysis showed that for HfN the initial carrier temperature rose to 360 K immediately after a femtosecond pulse excitation and then dropped rapidly to 330 K within 0.13 ps. After this initial process, the carrier temperature still remained relatively elevated at above 320 K for as long as 2 ns. The initial fast process was attributed to the strong carrier–carrier scattering in the material while the slow decay of temperature might be due to unusually weak electron–phonon interactions. A shorter cooling time and overall lower hot carrier temperatures were observed in ZrN.

The hot carrier solar cell is an advanced concept photovoltaic device that is predicted to deliver efficiencies in excess of conventional single bandgap devices. The design requires the ability to concurrently have extended carrier thermalization times within an absorber material, giving a hot carrier population, and the ability to efficiently collect the hot carriers at an energy above the bandgap of the absorber material. In order to achieve this, we require an absorber material with a long-lived hot carrier population. We investigate the carrier thermalization rates of In 0.17Ga0.83As/GaAs0.80P0.20 multiple quantum well samples with different barrier thicknesses. For a 40 quantum well strain-balanced structure, the cooling lifetime is found to be 1.23 ± 0.07 ns, but in samples which are not strain-balanced, defect-assisted carrier cooling increases the thermalization rate. Immediately following an ultrafast excitation, the initial carrier temperature is greater in samples with wider barriers. However, any gain in carrier temperature from utilizing wide barriers is negated by an increased thermalization rate as one deviates from strain-balanced conditions. We conclude that strain balancing is required for multiple quantum well hot carrier absorbers.

Hot carrier solar cells is an attractive technology with the potential of reaching high energy conversion efficiencies approaching the thermodynamic limit of infinitely stacked multi-junction solar cells: 65% under one sun and 86% under maximally concentrated. The hot carrier solar cell is conceptually simple consisting of two key components: absorber and energy selective contacts. High efficiencies are achieved by minimising the energy lost to thermalisaton of hot photo-generated carriers while absorbing majority of the solar spectrum. For this to be achieved, energy selective contacts are required to allow the extraction of carriers fast enough at an energy level above the electronic band edge. It is critical for the absorber to be able to maintain a hot carrier population for a sufficiently long time period for the extraction of carriers while they are ‘hot’. Bulk materials with a large gap between acoustic and optical branches in the phonon dispersion are predicted to exhibit slow hot carrier thermalisation rates. Hafnium nitride is such a material with a large gap in its phonon dispersion and is identified as a potential material to be used as a hot carrier absorber. Hafnium nitride has been deposited using reactive sputtering and characterised to investigate material properties and carrier cooling rates.

Intrigued by the high demand of fast design and development of nanoscale electronic devices, electronic transport across atomic dimensions becomes an important theoretical and computational problem. In this paper we present a tight-binding based model specially tailed for calculating realistic tunneling structures with scattering region dimensions of several nanometers. The proposed model allows for a proper treatment of the electron–phonon coupling effects in a tractable manner. By greatly reducing the complexity of the phonon-involved problem down to a quadratic level, transmission calculation for large-scale systems, including both planar structures and quantum wire structures, becomes practically feasible.

Nano-crystalline Ge x C 1 − x is a potential third generation solar cell absorber material due to its favourable opto-electronic properties and relatively high abundance of elements. The ability to grow nano-crystalline Ge x C 1 − x in large areas by an industry-friendly process can enhance its scope as a photovoltaic absorber. In this work nano-crystalline Ge x C 1 − x thin films have been grown on Si (100) substrate using radio frequency magnetron sputtering. The crystallinity, composition, structure and optical properties of the films were determined by, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, atomic force microscopy, transmission electron microscopy (TEM) and ultra-violet visible infrared spectroscopy. From TEM results it was found that Ge x C 1 − x crystals were scattered in the film with d-spacing of 3.4 nm between the fringes (calculated a = 5.53 A), but that a small number of nanoparticles of GeC were present. The Raman signature of the local Ge–C mode is identified near 530 cm − 1 in Ge x C 1 − x film grown at 350 °C. The band gap energy value was estimated to be 0.90 eV from optical reflectance spectra. Maximum 15.5% of Ge x C 1 − x is found in the film deposited at 350 °C using XPS fitting.

The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalise to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 66% for unconcentrated sunlight and 85% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. Required absorber material properties have been identified, relating to phononic and electronic properties as well as to practical considerations. Candidate materials evaluated in terms of these prerequisites include large mass anion, transition metal nitrides, Group IV compounds and nanostructures. Promising candidate materials include InN, HfN and SnSi. Initial measurements indicate slowed carrier cooling in III–Vs with large phonon band gaps in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.

The application of silicon quantum dot (Si QD) based material is regarded as a promising approach for the realization of high efficiency solar cells. When silicon nanocrystals are made very small (within the vicinity of the exciton Bohr radius of bulk Si), they behave as quantum dots due to the three-dimensional quantum confinement, which could cause the material's effective optical band gap to increase. The optical band gap can be deduced from the absorption coefficient. In this paper, we analyze optical absorption and emission processes in Si QD and attempt to simulate the band-edge absorption features based on the photoluminescence spectrum. We also investigate the application of ellipsometry in the study of optical properties of Si QD thin films. Based on WVASE32 modeling tool, a homogeneous mixture model is developed to extract the absorption coefficient of this material. From these results, we extract the effective optical band gap and analyze optical properties of Si QDs materials. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High performance bismuth-based semiconductors are attractive large atom containing candidates needing further development. Theoretical investigations of defect mechanisms in NaBiS2 have been performed using PBE GGA functionals. Effective masses are estimated at the band extrema and found to be 0.36m0or less. High levels of oxygen substitution and significant sulfur deficiency lead to a loss of the fundamental bandgap. Lattice parameters for the defective materials have been derived and provide an experimental signature for high levels of these defects. The work informs the development of promising but little explored lower symmetry bismuth compounds for solar energy applications.

Photolithography is used as an alternative method to overcome the challenge of making anode and cathode contacts on a Si nanocrystal solar cell deposited on non-conductive substrates instead of reactive ion etching (RIE). The advantages of this method include better control of isolation mesa fabrication and the avoidance of device exposure to highly energetic particles which may cause unpredictable damage. The photovoltaic device fabricated shows an open-circuit voltage (VOC) and a short-circuit current density (JSC) of 270 mV and 0.124 mA/cm2 respectively at room temperature under one-sun illumination. Current–voltage measurements were performed at temperatures (T) from 77 K to 300 K. A model that includes recombination-generation current in the depletion region is considered to explain the observed current behaviour of the device. An ideality factor very close to 2 was calculated based on Suns-VOC measurement, which indicates that the device is limited by recombination in the depletion region. A discrepancy was observed between the peaks (1.47 eV) in the photoluminescence spectrum and maximum VOC (0.81 V) extrapolated from the VOC–T relation at 0 K. This discrepancy has been attributed to the temperature dependence of the carrier lifetime in the depletion region characterized by an activation energy later defined in this article.

The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalize to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 65% under 1 sun and 86% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. A number of materials and structures are being investigated with these properties and test structures are being fabricated. Initial measurements indicate slowed carrier cooling in III-Vs with large phonon band gaps and in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.

Hot carrier solar cells aims to circumvent fundamental loss mechanisms by extracting carriers before thermalization of hot carriers occurs while also minimizing sub-bandgap losses. The absorber must be able to maintain a hot carrier for a sufficiently long time. Hafnium nitride owing to its phononic properties to slow carrier thermalization is a potential absorber for the hot carrier solar cell. The phonon properties have been measured by Raman spectroscopy. The carrier dynamics in hafnium nitride has been studied by ultrafast pump-probe transient absorption where nanosecond long lifetimes have been observed. The carrier temperature has been extracted from the transient absorption spectra to show the carrier temperature remained above 350 K for a long time with a decay time constant of 1.2 ns.

Hot carrier solar cell (HCSC) is a promising third generation photovoltaic device which can potentially able to overcome the Shockley-Queisser limit and achieve much higher power efficiency than present single junction solar cells. The theoretical efficiency for an ideal HCSC is predicted to be 65% under 1 sun solar radiation or 85% under maximal concentration. A critical requirement of the absorber is to slow down the cooling rate of photoexcited hot carriers, from few ps for typical semiconductors to hundreds of ps or longer. Transition nitrides HfN and ZrN theoretically have large phononic bandgaps and thus much slowed cooling time may be expected in these materials. In this investigation ultrafast transient absorption (TA) was used to investigate HfN and ZrN thin films. Experiments reveal three featured TA bands in the visible and near infrared, ascribed to the excited state absorption and bleaching. Cooling time of hot carriers as long as 3 ns in HfN is attributed to the significantly suppressed Klemens' decay. While hundreds of ps cooling time were observed in ZrN. The slowed thermalization process in HfN and ZrN suggests these bulk materials are a promising absorber candidate for hot carrier solar cells.

The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalize to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 65% under 1 sun and 86% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. A number of materials and structures are being investigated with these properties and test structures are being fabricated. Initial measurements indicate slowed carrier cooling in III-Vs with large phonon band gaps and in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.

Inelastic X-ray scattering (IXS) on an In0.17 Ga0.83As/ GaAs0.8 P0.2 multiple quantum well (MQW) superlattice has been conducted to investigate the potential for phonon bottlenecks in low dimensional materials. This work shows that the measured spectra are in good agreement with an adiabatic bond charge model prediction and back-folded phonon modes make large contributions to the broadening of peaks observed in the spectra. The high-lying mode at 45 meV in the MQW is attributed to vibrations of Ga and P and confirmed by both experiment and theory. The acoustic phonons have a dominant contribution from the Ga and As components, and the contribution from InAs is small and only visible at around 29.7 meV. Low energy optical modes resulting from back-folding might be a key to increased electron-phonon coupling in the material system. The suitability of utilizing IXS as a means to investigate phonon modes in low dimensional materials is also discussed.

The Hot Carrier (HC) solar cell aims to tackle a major loss in conventional solar cells by collecting the hot carriers before they thermalise. The calculated efficiency of the HC solar cell is very close to the limiting efficiency for an infinite tandem cell. The HC solar cell requires an absorber with a low electronic band gap so that it can absorb a large fraction of the solar spectrum. Importantly the absorber must sufficiently slow down the rate of carrier cooling so that adequate time is available to collect the hot carriers. In this work the main mechanisms of carrier cooling and possible approaches to restrict these mechanisms will be discussed. Hafnium nitride and zirconium nitride are presented as potential absorber materials for HC solar cells. Besides a large “phononic band gap” suitable to block the main carrier cooling mechanism, these materials have reasonable abundance to allow large scale implementation. Recent work on the fabrication of these materials at UNSW will also be presented.

The behavior of phosphine-doped Si nanocrystal material is studied in this work from the perspectives of crystallization, photoluminescence, carrier density and conductivity. Phosphine was incorporated in the material during the sputter process. It was observed that the phosphine helped to reduce the defects in the material. This was evident from the reduction of the photoluminescence of a possible defect-related energy level located at 1.30 eV, which prevailed at low temperature when phosphine was absent. Temperature dependent Hall measurement showed the carrier densities in Si nanocrystal material remained around 1019 and 1020 cm−3 from 80 K to 340 K for samples doped with 3 sccm and 9 sccm phosphine respectively during sputtering. Such metallic behavior can be due to degenerate doping in the material.

The Hot Carrier solar cell is a third generation photovoltaic concept which has the potential to achieve high efficiencies, exceeding the Shockley-Queisser limit for a conventional p-n junction solar cell. The theoretical efficiencies achievable for the Hot Carrier solar cell is 65% for non-concentrated solar radiation and 85% for maximally concentrated light, very close to the limits of an infinite tandem solar cell. The approach of the Hot Carrier solar cell is to extract carriers generated before thermalisation to the bandgap edge occurs when their excess energy is lost to the environment as heat. To achieve this, the rate of carrier cooling in the absorber must be slowed down sufficiently enough to allow carriers to be collected while they are hot. This work investigates using hafnium nitride as such an absorber to restrict mechanisms of carrier cooling. Hafnium nitride’s phononic properties, where a large ‘phononic band gap’ exist can reduce the carrier cooling rate by means of a phonon bottleneck such that optical phonons cannot decay into acoustic phonons by means of the Klemens’ mechanism. Optical phonon-electron scattering can maintain a hot electron population while acoustic phonons are irrecoverable and lost as heat. The electronic and phononic properties of hafnium nitride are evaluated for their suitability to be used in a Hot Carrier solar cell absorber. Recent work on the fabrication of hafnium nitride at UNSW is presented.

To circumvent Shockley-Queisser Limit whilst utilizing thin film deposition, we intend construction of a hot carrier solar cell (HCSC). This device would challenge a fundamental assumption of Shockley-Queisser: that all energy of incoming photons in excess of the acceptance threshold of the cell material is lost as heat. If “excess” energy charge carriers are tapped before they thermalize with the matrix, theoretical cell efficiency (66%) under one sun is twice that of a single-junction silicon cell. In this pursuit, two principal tasks await: actual retardation of carrier thermalization by preventing the decay of accompanying optical phonons, and collection of the carriers via devices known as “Energy Selective Contacts” (ESCs), which withdraw only carriers possessing a narrow range of energies to prevent entropic losses. We propose construction of a Hot Carrier Solar Cell utilizing elemental group III Nitrides for ESC and absorber. Indium Nitride, with its large phononic band gap and small electronic band gap, can provide a suitable absorber, whereas alloys of In(x)Ga(1-x)N can form complementary and lattice-matched ESCs.

Optical constants of silicon quantum dots (Si QDs) in SiO2/Si3N4 hybrid matrix on quartz substrates have been determined in the wavelength range of 350–2500 nm. Forouhi–Bloomer (F-B) dispersion model is applied to describe the optical properties of silicon nanocrystals and the mismatch due to global fitting is fixed point by point. To extract the specific optical bandgap of silicon nanocrystal samples, Tauc-plots with γ = 1/2 and γ = 1/3 are used and discussed for the spectral range of 2 eV–3 eV. The effect of quantum dot size on the optical bandgap is explored. The results from both Tauc-plot and photoluminescence show the trend that the optical bandgap increases with decreasing quantum dot size, which agrees with the prediction of quantum confinement.

Hot carrier solar cell (HCSC) requires a slow cooling rate of carriers in the absorber, which can potentially be fullled by semiconductor superlattices. In this paper the energy relaxation time of electrons in InN InxGa1-xN superlattices are computed with Monte Carlo simulations considering the multi-stage energy loss of electrons. As a result the effect of each stage in the relaxation process is revealed for superlattice absorbers. The energy relaxation rate figures are obtained for different material systems of the absorber, i.e. for different combinations of Indium compositions and the thicknesses of well and barrier layers in the superlattices. The optimum material system for the absorber has been suggested, with the potential to realize HCSCs with high efficiency.

In this work, the ground-state properties of the solution processable semiconductor M2 SnBr6 (M = K, Rb, Cs) have been computed using density functional theory. Similarities in the band structures are observed among these three materials and are shown to result from minimal contributions of the cation to electronic states near the Fermi level. A fundamental bandgap of 1.2 eV is predicted for the materials, which is close to the ideal bandgap for single-junction photovoltaic applications. However, in reality, a larger bandgap is expected because DFT calculations with the PBE functional underestimate the gap. Material optical properties including dielectric constants, reflective indices, reflectance and absorption coefficients are shown to be competitive for solar-energy harvesting. The ionization energies are 6 eV below the vacuum level, while effective masses are relatively small around 0.3, with light hole masses comparable to those of electrons.

Silicon nanocrystals (Si NCs) embedded in a dielectric matrix is regarded as one of the most promising materials for the third generation photovoltaics, owing to their tunable bandgap that allows fabrication of optimized tandem devices. Previous work has demonstrated fabrication of Si NCs based tandem solar cells by sputter-annealing of thin multi-layers of silicon rich oxide and SiO2. However, these device efficiencies were much lower than expected given that their theoretical values are much higher. Thus, it is necessary to understand the practical conversion efficiency limits for these devices. In this article, practical efficiency limits of Si NC based double junction tandem cells determined by fundamental material properties such as minority carrier, mobility, and lifetime are investigated. The practical conversion efficiency limits for these devices are significantly different from the reported efficiency limits which use Shockley-Queisser assumptions. Results show that the practical efficiency limi...

We have investigated the ultrafast carrier dynamics in a 1 μm bulk In0.265Ga0.735N thin film grown using energetic neutral atom-beam lithography/epitaxy molecular beam epitaxy. Cathodoluminescence and X-ray diffraction experiments are used to observe the existence of indium-rich domains in the sample. These domains give rise to a second carrier population and bi-exponential carrier cooling is observed with characteristic lifetimes of 1.6 and 14 ps at a carrier density of 1.3 × 1016 cm−3. A combination of band-filling, screening, and hot-phonon effects gives rise to a two-fold enhanced mono-exponential cooling rate of 28 ps at a carrier density of 8.4 × 1018 cm−3. This is the longest carrier thermalization time observed in bulk InGaN alloys to date.

The Hot Carrier solar cell is a Third Generation device that aims to tackle the carrier thermalisation loss after absorption of above band-gap photons. It is theoretically capable of efficiencies very close to the maximum thermodynamic limit. It relies on slowing the rate of carrier cooling in the absorber from ps to ns. This challenge can be addressed through nanostructures and modulation of phonon dispersions. The mechanisms of carrier cooling are discussed and methods to interrupt this process investigated to give a list of properties required of an absorber material. Quantum well or nano-well structures and large mass difference compounds with phonon band gaps are discussed in the context of enhancing phonon bottleneck and hence slowing carrier cooling. Materials for these structures are discussed and potential combined structures to maximize phonon bottleneck and slow carrier cooling are suggested.

It has been proposed that the rate of hot carrier thermalisation can be slowed down if there is a sufficiently large gap in the phonon dispersion for a bulk material. This phenomenon is critical for the development of high efficiency hot carrier solar cells to minimise energy loss to thermalisation. A gap where the minimum of the optical branches is at least twice that of the maximum of the acoustic branches can prevent the primary pathway where optical phonons loses energy, the Klemens' decay mechanism. The large gap in the phonon dispersion eliminates the Klemens' decay pathway due to energy and momentum conservation laws. This enables the electron population to remain hot by allowing sufficient time for optical phonons to re-scatter its energy to electrons. Binary compounds with a large mass difference between the two constituent atoms and high level of crystal symmetry such as zirconium nitride and hafnium nitride (HfN) have such a gap in their phonon dispersion. HfN thin films have been sputtered on silicon and quartz substrates. Characterisation of hot electron lifetimes in HfN films have been performed using ultrafast transient absorption spectroscopy. Preliminary analysis of transient absorption data, both spectra and time evolution has indicated high carrier temperatures with a nanosecond long decay time. It is postulated the long hot carrier lifetime is due to the large phononic gap.

Anharmonic decay of high frequency phonons into low frequency vibrations is a significant energy loss mechanism in semiconductors. In the field of Hot Carrier Solar Cells (HCSC), preventing this decay is of great importance as it helps increase hot carriers lifetime. Phonon decay in nitride compounds as well as their consisting nano-crystals like quantum dots and multiple quantum wells (MQWs) has not been extensively studied in the literature. In this work, the decay channels of the A1 and the high-lying E2 optical phonons in an InN/GaN MQW are analysed. We find that the no Klemens decay is present in A1 mode whereas on the contrary E2 is dominated by this process. We also observe that the linewidth for A1 is enlarged a few times than the bulk counterpart while that for E2 remains similar, which is attributed to different vibration nature.

The rates of charge carrier relaxation by phonon emission are of substantial importance in the field of hot carrier solar cell, primarily in investigation of mechanisms to slow down hot carrier cooling. In this work, energy and momentum resolved deformation potentials relevant to electron-phonon scattering are computed for wurtzite InN and GaN as well as an InN/GaN multiple quantum well (MQW) superlattice using ab-initio methods. These deformation potentials reveal important features such as discontinuities across the electronic bandgap of the materials and variations over tens of eV. The energy dependence of the deformation potential is found to be very similar for wurtzite nitrides despite differences between the In and Ga pseudopotentials and their corresponding electronic band structures. Charge carrier relaxation by this mechanism is expected to be minimal for electrons within a few eV of the conduction band edge. However, hole scattering at energies more accessible to excitation by solar radiation is possible between heavy and light hole states. Moderate reductions in overall scattering rates are observed in MQW relative to the bulk nitride materials.

In this study, the phonon dispersion of a wurtzite InN-GaN multiple quantum well superlattice was produced using a modified adiabatic bond charge model. Bulk binary compound parameters obtained from ab initio calculations were used as input parameters for the modified bond charge model. The phononic bandgap, visualization of the vibrational modes, specific heat, and thermal conductivity in the quantum confined direction for the superlattice material are reported. The size of the phononic bandgap is predicted to be between that of the two bulk materials in all symmetry directions and insufficient to prevent first order phonon decay. Surprisingly, the A1-like mode has a greater vibration frequency than the E1-like mode, which is opposite to what was believed in the bulk counterparts. The specific heat of the superlattice is similar to that of GaN, despite similar volumetric fractions of the materials. The low thermal conductivity of InN limits the thermal conduction in the confined direction.

28th European Photovoltaic Solar Energy Conference and Exhibition; 123-126, The Hot Carrier solar cell reduces carrier thermalisation loss. It relies on slowing the rate of carrier cooling in the absorber from ps to ns. This is addressed through nanostructures and modulation of phonon dispersions.