PHONONIC ENGINEERING FOR SLOWEDCARRIER COOLING IN HOT CARRIER SOLAR CELL ABSORBERS (talk)

An important property of a hot carrier cell is to slow the rate of carrier cooling to allow hot carriers to be extracted and do useful work in the external circuit. Carrier thermalisation in semiconductors occurs principally through scattering of hot electrons with zone centre optical phonons to build up a hot phonon population. This population can be prolonged – and hence carrier cooling slowed – by prevention of the dominant decay mechanisms of optical phonons into 2LA phonons (Klemens) and also possibly of the next most dominant mechanism of decay into a TO and a LA phonon (Ridley). Large gaps between optical and acoustic modes in some III-Vs (notably InN, BiN, GaN, AlSb & SnO) or some other heavy metal compounds are discussed as a means for suppressing one or both of these mechanisms with evidence for reduced cooling cited. An extension of this suppression mechanism in the mini-gaps in the folded phonon dispersions of QW and QD superlattices is modelled using a nearest neighbour force constant model. In addition the important role of interface modes is discussed, and it is shown that very ‘soft’ interface modes have the potential to reduce the fine superlattice tuning required, or to widen the range of material combinations, to give these reduced carrier cooling effects. Furthermore for either very soft or very stiff interface modes, it seems that a more direct mimicry of the wide phononic gaps in some bulk semiconducotrs may be possible.