Binesh Puthen Veettil

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.

Pulsed KrF excimer laser dopant activation in nanocrystal silicon in a silicon dioxide matrix

We demonstrate that a pulsed KrF excimer laser (λ = 248 nm, τ = 22 ns) can be used as a post-furnace annealing method to greatly increase the electrically active doping concentration in nanocrystal silicon (ncSi) embedded in SiO2. The application of a single laser pulse of 202 mJ/cm2 improves the electrically active doping concentration by more than one order of magnitude while also improving the conductivity. It is confirmed that there is no film ablation or significant change in ncSi structure by atomic force microscopy and micro-Raman spectroscopy. We propose that the increase in free-carrier concentration is the result of interstitial P/B dopant activation, which are initially inside the Si crystallites. Evidence of mobility limited carrier transport and degenerate doping in the ncSi are measured with temperature-dependent conductivity.

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.

Characterisation of active dopants in boron-doped self-assembled silicon nanostructures

Doping of silicon nanocrystals has become an important topic due to its potential to enable the fabrication of environmentally friendly and cost-effective optoelectronic and photovoltaic devices. However, doping of silicon nanocrystals has been proven difficult and most of the structural and electronic properties are still not well understood. In this work, the intrinsic and boron-doped self-assembled silicon nanocrystals were prepared and mainly characterised by the transient current method to study the behaviour of charge carriers in these materials. Our experiments quantified the amount of electrically active boron dopants that contributed to charge transport. From this, the boron doping efficiency in the nanocrystal superlattice was estimated.

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.

Optimized resonant tunnelling structures with high conductivity and selectivity

An improved design for double-barrier resonant tunnelling structures using silicon quantum dots (QDs) was quantitatively analyzed using a multi-mode scatter matrix method. Multilayer metal-barrier-QD/matrix-barrier-metal stacks that maximize both electron transport and confined energy are sought. Si QDs grown in silicon dioxide with silicon carbide barriers were the most advantageous combination for single QD layer double-barrier structures (DBSs). Lateral SiO2 barriers provided greater confinement, especially in smaller dots and also caused increased splitting between resonant levels. These structures are excellent candidates for use as energy selective contacts (ESCs) and as layers in all-silicon tandem cells.

Morphology effects on the bandgap of silicon nanocrystals—Numerically modelled by a full multi-grid method

Silicon nanocrystals embedded in a dielectric matrix have been considered a potential candidate for many optoelectronic and photovoltaic applications and have been under vigorous study in recent years. One of the main properties of interest in this application is the absorption bandgap, which is determined by the quantum confinement of silicon nanocrystals. The ability to predict the absorption bandgap is a key step in designing an optimum solar cell using this material. Although several higher level algorithms are available to predict the electronic confinement in these nanocrystals, most of them make regular-shape assumptions for the ease of computation. In this work, we present a model for the accurate prediction of the quantum confinement in silicon nanocrystals of non-regular shape by employing an efficient, self-consistent Full-Multi-Grid method. Confined energies in spherical, elongated, and arbitrarily shaped nanocrystals are calculated. The excited level calculations quantify the wavefunction cou...

Phonon decay in nanostructures: Computational study

Phonon dispersion relations and in core-shell quantum dot superlattices and periodic arrays of gas-filled nanopores have been computed numerically in the harmonic approximation. Core-shell quantum dot superlattices show potential to inhibit first order phonon decay. Outcomes for nanoporous systems depend strongly on surface and gas-phase interactions. Phonon decay is inhibited in idealized cases. A light absorbing material with low probability for phonon decay is critical to achieving the high efficiencies expected from the hot carrier solar cell.