Hot Carrier Solar Cells: Choice of Materials for Efficiency and Abundance

An ideal Hot Carrier cell would absorb a wide range of photon energies and extract a large fraction of the energy to give very high efficiencies, (up to 65% under 1 sun) by extracting ’hot’ carriers before they thermalise to the band edges [. The requirements for the absorber material for such a cell are challenging, but not mutually exclusive. Identification of the optimum materials for absorbers will increase the potential for implementation of real devices. The key property for a hot carrier absorber is to slow the rate of carrier cooling from the picosecond timescale to at least 100s of ps. Slowed cooling has been observed in some III-V compounds in which there is a large difference in atomic mass. The large gap between optical and acoustic modes in the phonon dispersion prevents the decay of optical phonons by the emission of acoustic phonons with half the energy (the Klemens mechanism). The resultant non-equilibrium or ‘hot’ optical phonons scatter their energy back to the carriers, thus slowing overall carrier cooling. III-V bulk materials with a large anion:cation mass ratio have a large gap between acoustic and optical modes sufficient to block this Klemens’ decay. In particular the In compounds have the largest gaps and have been shown experimentally to have slower carrier cooling. As such they are good model materials to test the concepts, but the very low abundance of In requires a search for alternatives. Analogues of InN are investigated with reference to the periodic table and for compounds with similarly large mass ratios to InN. Some of the group IIIA and IVA nitrides and the compounds of group IV elements look most attractive in terms of mass ratio, abundance of elements and low electronic band gaps. Nanostructures also may be applicable in tuning the properties of some of these potential hot carrier absorber materials.