PHONONIC ENGINEERING FOR HOT CARRIER SOLAR CELLS INCLUDING INTERFACE MODELLING (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) or failing that into a TO and a LA phonon (Ridley). Large gaps between optical and acoustic modes in some III-Vs (notably InN, GaN, AlSb & InP) 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 an elastic continuum model. In addition the important role of interface modes using nearest neighbour force constant models is discussed with the conclusion that very ‘soft’ interface modes have the potential to reduce the fine superlattice tuning required to give these reduced carrier cooling effects.
Keywords: Hot Carrier – 1; Quantum dot – 2; Phonon decay – 3; Carrier cooling – 4; Quantum well - 5