Hot carrier solar cells

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.