Hongze Xia

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

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.

Extended hot carrier lifetimes observed in bulk In0.265±0.02Ga0.735N under high-density photoexcitation

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.

Hot Carrier solar cell absorbers: Superstructures, materials and mechanisms for slowed carrier cooling

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.

Ab initio calculation of halide ligand passivation on PbSe quantum dot facets

The passivation and charge compensation provided by inorganic halide ligands on low index facets of lead selenide (PbSe) nanocrystals has been studied using density functional theory to produce projected densities of states (PDOS), bond lengths and to perform Bader analysis. The calculations were made using a grid-based planar augmented wave code with a localized double zeta potential basis and the generalized gradient approximation. Surface energies of halide ligands bonded onto surface Pb atoms show trends that are consistent with the increased electronegativity of the species, with iodine having the lowest binding energy of the halides investigated. Different densities of iodine ligands lead to different levels of passivation with a continuous widening of the bandgap on particular facets for increasing levels of coverage. In particular, the (111) plane shows a clear recovery of surface layer back to bulk property and widening of bandgap when the ligands cover most Pb atoms on the surfaces. Additionally, a possible increase of carrier conductance along with the increase of ligand density has been found using Bader analysis. Relative increases in the conductance for large halide atoms stem from the measurable increases to electronic states near the top of the valence band in these p-type semiconductors. The passivation is observed to increase along with the s-type character of the electron density at the surface, suggesting that a higher degree of symmetry in the electron density accompanies the reduction in defect levels.

All-silicon tandem solar cells: Practical limits for energy conversion and possible routes for improvement

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...

Evaluation of hafnium nitride and zirconium nitride as Hot Carrier absorber

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.

Theoretical calculation of the vibrational and thermal properties of wurtzite InN-GaN multiple quantum well superlattice

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.

Analysis on carrier energy relaxation in superlattices and its implications on the design of hot carrier solar cell absorbers

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.

Inelastic X-ray scattering measurements of III–V multiple quantum wells

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.

Hot Carrier Cooling in In 0.17 Ga 0.83 As/GaAs 0.80 P 0.20 Multiple Quantum Wells: The Effect of Barrier Thickness

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 InIn0.17Ga0.83As/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.