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Conibeer G.,University of New South Wales | Guillemoles J.-F.,Institute Of Recherche Et Developpement Of Lenergie Photovoltaique | Yu F.,University of New South Wales | Levard H.,Institute Of Recherche Et Developpement Of Lenergie Photovoltaique
RSC Energy and Environment Series | Year: 2014

The hot carrier solar cell has the potential to achieve very high efficiencies in a device that is essentially a single junction. Detailed balance calculations indicate limiting efficiencies as high as 65% under 1 sun and 85% under maximum concentration. However a series of modelling developments has shown that as real material parameters are introduced the predicted efficiencies decrease. What emerges is that maximization of the thermalization time constant for hot carriers is critical to improved efficiency. The carrier cooling mechanisms are investigated and depend primarily on emission of optical phonons by cooling carriers, predominantly electrons. Under some circumstances these optical phonons can be produced at such a high density that they cannot decay away fast enough and a 'phonon bottleneck' is formed that allows the phonon energy to scatter back with the electron ensemble thus re-heating it. Creating the conditions for this phonon bottleneck seems the most fruitful route for significantly increasing the thermalization time constant. Quantum well nanostructures exhibit such phonon confinement with significantly hot carrier temperatures. The reasons for this are not completely clear but are affected by the restriction of hot carriers diffusing in the direction perpendicular to the wells and by confinement of phonons in the wells. Prevention of decay of optical phonons into acoustic phonons is another method for maximizing phonon bottleneck. Materials with a large difference in acoustic and optical phonon energies can block this Klemens route for phonon decay. A range of materials are identified as having these properties with the principle requirement that there is a large mass difference between their constituent atoms. Some of the most promising are IIInitrides, especially InN, and their analogues, which include transition metal nitrides (of which HfN and ZrN are most interesting) and group IV compounds (of which SnSi has the most impressive modelled properties). Experimental demonstrations of these effects are very limited at present although there are encouraging signs that these properties will soon be demonstrated in several material groups. Contacting to hot carrier cells requires specific contacts which only allow transmission of a narrow range of energies. This is so that cold carriers in the contacts do not cool carriers in the absorber. The most promising route to such contacts at present is the double barrier resonant tunnelling structure which can be tuned to specific energies. Such structures in high quality have been made in III-Vs and demonstration of resonant tunnelling achieved. Thin film structures involving silicon and oxides have also shown promising proof of concept. An alternative to electrically contacting is to allow the hot carrier absorber to stay at open circuit and re-radiate photons from hot carriers recombining. Such an approach requires an optically selective filter to illuminate a high efficiency conventional solar cell and has the advantage that optical and electrical properties can be optimized in separate structures. Combination of absorbers and contacts in full devices has yet to be realized. But there are now a number of designs for such combinations and the next few years should see their fabrication and demonstration of full proof of concept of these challenging but highly promising hot carrier devices. © The Royal Society of Chemistry 2014.

Chane-Ching J.-Y.,National Polytechnic Institute of Toulouse | Foncrose V.,National Polytechnic Institute of Toulouse | Zaberca O.,National Polytechnic Institute of Toulouse | Lagarde D.,INSA Toulouse | And 6 more authors.
Solar Energy Materials and Solar Cells | Year: 2015

A higherature gaslating strategy is proposed to synthesize Cu2ZnSnS4 (CZTS) nanocrystals for all-aqueous solar inks. Our gas templating process route involves the in-situ generation and stabilization of nanosized gas bubbles into a molten KSCN-based reaction mixture at 400°C. Chemical insights of the templating gas process are provided such as the simultaneous formation of gas bubbles and CZTS nuclei highlighting the crucial role of the nucleation stage on the sponge and resulting nanocrystals properties. The high porosity displayed by the resulting CZTS nanocrystals facilitates their further post-fragmentation, yielding individualized nanocrystals. The advantages of our high temperature gas templating route are illustrated by the following: (i) the low defect concentration displayed by the highly crystalline nanocrystals, (ii) the synthesis of CZTS nanocrystals displaying S2- polar surfaces after ligand exchange. The good photoluminescence properties recorded on the pure CZTS nanocrystals reveal potential for exploration of new complex chalcogenide nanocrystals useful for various applications including photovoltaics and water splitting. Here we demonstrate that using these building blocks, a CZTS solar cell can be successfully fabricated from an environment-friendly all-aqueous ink. © 2015 Elsevier B.V. All rights reserved.

Jutteau S.,Électricité de France | Jutteau S.,Institute Of Recherche Et Developpement Of Lenergie Photovoltaique | Paire M.,Électricité de France | Paire M.,Institute Of Recherche Et Developpement Of Lenergie Photovoltaique | And 6 more authors.
2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 | Year: 2015

In this work we look at a micro-concentrating system adapted to a new type of solar concentrator photovoltaic material, well known for flate-plate applications, Cu(In,Ga)Se2. Cu(In,Ga)Se2 solar cells are polycrystalline thin film devices that can be deposited by a variety of techniques. We proposed to use a microcell architecture [1], [2], with lateral dimensions varying from a few μm to hundreds of μm, to adapt the film cell to concentration conditions. A 5% absolute efficiency increase on Cu(In,Ga)Se2 microcells at 475 suns has been observed for a final efficiency of 21.3% [3]. We study micro-concentrating systems adapted to the low and middle concentration range ([4], [5]), where thin film concentrator cells will lead to substrate fabrication simplification and costs savings. Our study includes optical design, fabrication and experimental tests of prototypes. © 2015 IEEE.

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