Neeti Gupta

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.

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.

Hot carrier solar cell absorbers: materials, mechanisms and nanostructures

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.

Potential of hafnium nitride for the hot carrier solar cell

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.

Potential of HfN, ZrN, and TiH as hot carrier absorber and Al2O3/Ge quantum well/Al2O3 and Al2O3/PbS quantum dots/Al2O3 as energy selective contacts

The hot carrier (HC) solar cell is one of the most promising advanced photovoltaic concepts. It aims to minimise two major losses in single junction solar cells due to sub-band gap loss and thermalisation of above band gap photons by using a small bandgap absorber, and, importantly, collecting the photo-generated carriers before they thermalise. In this paper we will present recent development of the two critical components of the HC solar cell, i.e., the absorber and energy selective contacts (ESCs). For absorber, fabrication and carrier cooling rates in potential bulk materials — hafnium nitride, zirconium nitride, and titanium hydride are presented. Results of ESCs employing double barrier resonant tunneling structures Al2O3/Ge quantum well (QW)/Al2O3 and Al2O3/PbS quantum dots (QDs)/Al2O3 are also presented. These results are expected to guide further development of practical HC solar cell devices.

Carrier dynamics and phonon properties of hafnium nitride: Potential hot carrier solar cell absorber

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 absorbers: investigation of carrier cooling properties of candidate materials

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 from group III-V quantum well structures

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.

Growth and characterization of germanium carbide films for hot carrier solar cell absorber

The Hot carrier (HC) solar cell is a promising third generation photovoltaic device with potentially very high efficiency. It aims to tackle the major loss in conventional solar cells by collecting carriers with energies above the band gap (“hot” carriers) before they thermalise. This demands an absorber with a sufficiently slow carrier cooling rate to allow collection of these carriers. In this work, GeC has been investigated as a potential HC absorber material. GeC films have been deposited by RF magnetron sputtering at different temperature and using different targets. Preliminary measurements show the formation of GeC, consistent with previous observations.