P. Aliberti

Investigation of theoretical efficiency limit of hot carriers solar cells with a bulk indium nitride absorber

Theoretical efficiencies of a hot carrier solar cell considering indium nitride as the absorber material have been calculated in this work. In a hot carrier solar cell highly energetic carriers are extracted from the device before thermalisation, allowing higher efficiencies in comparison to conventional solar cells. Previous reports on efficiency calculations approached the problem using two different theoretical frameworks, the particle conservation (PC) model or the impact ionization model, which are only valid in particular extreme conditions. In addition an ideal absorber material with the approximation of parabolic bands has always been considered in the past. Such assumptions give an overestimation of the efficiency limits and results can only be considered indicative. In this report the real properties of wurtzite bulk InN absorber have been taken into account for the calculation, including the actual dispersion relation and absorbance. A new hybrid model that considers particle balance and energy...

Non-ideal energy selective contacts and their effect on the performance of a hot carrier solar cell with an indium nitride absorber

The hot carrier solar cell is a third generation photovoltaic device that extracts photo-generated carriers before they thermalise. In this work, the efficiency of a hot carrier solar cell with a 50 nm indium nitride (InN) absorber layer has been calculated, taking into account the realistic transport properties of energy selective contacts. The cell performance has been modeled considering the carrier extraction through contacts as ballistic. A potential practical implementation of a hot carrier solar cell, with contacts based on an InXGa1−XN/InN/InXGa1−XN quantum well structure, has been proposed, with calculated maximum efficiency of 37.15% under 1000 suns.

Practical Factors Lowering Conversion Efficiency of Hot Carrier Solar Cells

We have evaluated the influence of practical factors on the conversion efficiency of hot carrier solar cells, from which photogenerated carriers are extracted before being completely thermalized. Equilibration and thermalization of the carriers, and energy dissipation associated with hot carrier extraction were involved in a thermodynamic modeling. Among them, thermalization has been found to have the greatest impact. Even though a 1 ns thermalization time could be realized, the conversion efficiency is close to the Shockley–Queisser limit (34%) under 1 sun irradiation, and lower than the limiting values of triple-junction cells (around 60%) at 1000 sun.

Effects of non-ideal energy selective contacts and experimental carrier cooling rate on the performance of an indium nitride based hot carrier solar cell

The performance of an InN based hot carrier solar cell with a bulk InN absorber has been evaluated using an innovative approach that takes into account absorber energy-momentum dispersion relations, energy conservation, Auger recombination and impact ionization mechanisms simultaneously. The non ideality of the energy selective filters has also been included in the model. In order to obtain practical achievable values of conversion efficiency, the actual thermalisation velocity of hot carriers in InN has been measured using time resolved photoluminescence. Results of the computations shown limiting efficiencies of 24% for 1000 suns and 36.2% for maximal concentration.

Hot carrier solar cells: Challenges and recent progress

The limiting efficiency on the conversion efficiency of terrestrial global sunlight is not circa 31%, as commonly assumed, but 74%. To reach the lowest possible costs and hence to attain its intrinsic potential as a major source of future sustainable energy supplies, it would appear photovoltaics has to evolve to devices targeting the latter efficiency rather than the former. The hot carrier solar cell, although presenting substantial device challenges, is arguably the highest efficiency photovoltaic device concept yet suggested and hence worthy of efforts to investigate its practicality. Challenges in the implementation of hot carrier cells are identified and progress in overcoming these are discussed.

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.

Synthesis and structural properties of Ge nanocrystals in multilayer superlattice structure

Ge nanocrystals (Ge NCs) were grown in a multilayered superlattice using magnetron co-sputtering and subsequent thermal annealing. The purpose is to produce a material in which the band gap can be controlled by controlling the Ge NC size and to investigate the potential of this material for use in tandem solar cells. The presence of size-controlled Ge NCs was revealed by Raman spectroscopy, glancing incidence X-ray diffraction (GIXRD) and Transmission Electron Microscope (TEM), and this was supplemented by the observation of blue shifts in the absorption and photoluminescence (PL) properties. Raman spectra showed Ge-Ge active phonon modes at around 300 cm-1 implying the formation of high quality Ge NCs. With increasing annealing temperature and duration, more Ge precipitate changed from a non-crystalline phase to a crystalline phase. However, calculation of degree of crystallinity indicated that a considerable amount of non-crystalline Ge remained at our chosen annealing conditions. GIXRD measurements exhibited three Bragg peaks associated with crystalline Ge. TEM images showed direct evidence of the crystal lattice of the Ge NCs. The size of nanocrystals increased with annealing duration indicating nanocrystal growth by diffusion. The growth of nanocrystals was found to be confined by the GeO2/SiO2 spacing layers, and the average crystallite size was determined by the thickness of the GeRO layers. However, enhanced interdiffusion at elevated annealing temperature weakened the size confinement effect of the multilayer structure. Hence an optimum annealing condition is needed to produce high quality and reproducible Ge NCs. Our preliminary work indicates that it may be promising to use Ge NCs as absorber materials in tandem solar cells..

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.

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.