Modelling of hot carrier solar cell absorbers

Abstract

Hot Carrier cells aim to tackle the carrier thermalisation loss after absorption of above band gap photons by separating and collecting carriers before they thermalise. Such slowing of carrier cooling may be achieved by modulation of the phonon decay mechanisms in nanostructures. 3D force constant modelling of quantum dot nanostructures indicates that complete mini-gaps in the phonon dispersion can be achieved across reciprocal space for very small (1 nm) close packed quantum dots with a large mass difference between quantum dot (QD) and matrix. This work uses force constants from the literature for bulk materials. A 3D model using the more accurate ab-initio calculation of force constants indicates that only very small mini-gaps in reciprocal space exist for larger quantum dots with a small mass difference, although thus far the model has not been able to simulate these small, high mass difference, closely packed QD systems. For such small QD systems it is indicated that if correctly engineered the mini-gaps could prevent the major Klemens’-type decay mechanism of a longitudinal/transverse optical (LTO) phonon decaying to two longitudinal acoustic (LA) phonons of half the energy and equal and opposite momenta. As this is the primary decay mechanism of non-equilibrium ‘hot’ phonons emitted by hot electrons, its prevention can create a ‘hot phonon bottleneck’ which will re-heat the electron gas and thus slow the rate of carrier cooling.