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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.

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.

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.

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.

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.

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.

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.