Hot carrier exploitation strategies and model for efficient solar cell applications

Abstract

Hot carriers are electrons or holes that are created in semiconductors upon the absorption of photons with energies greater than the fundamental bandgap. The excess energy of the hot carrier cools to the lattice temperature via carrier–phonon scattering and wasted as heat in [the] picoseconds timescale. The hot-carrier cooling represents a severe loss in the solar cells that have significantly limits their power conversion efficiencies. Hot carrier solar cells aim to mitigate this optical limitation by effective utilization of carriers at elevated energies. However, exploitation of hot carrier energy is extremely challenging as hot carriers rapidly lose their excess energy in phonon emission and therefore requires a substantial delay of carrier cooling in absorber material. In this paper a simple model was formulated to study the kinetic energies and hence the energy levels of the photo excited carriers in the quantum dots (QDs) whereas Schaller model was used to investigate the threshold energies of considered QDs. Results strongly indicate low threshold photon energies within the energy conservation limit for PbSe, PbTe, PbS, InAs, and InAs QDs. These materials seem to be good candidates for efficient carrier multiplication. It is found also that PbSe, PbTe, PbS, InAs, ZnS and InAs QDs exhibit promising potential for possible hot carrier absorber due to their widely spaced energy levels predicted to offer a large phononic gap between the optical and acoustic branches in the phonon dispersion. This in principle enhances phonon bottleneck effect that dramatically slows down hot carrier cooling leading to retention of hot carriers long enough to enable their exploitation. Two novel strategies were employed for the conversion of hot carriers into usable energies. The first approach involves the extraction of the energetic hot carriers while they are ‘hot’ to create higher photo voltage while the second approach uses the hot carrier to produce more carriers through impact ionization to create higher photo current. These mechanisms theoretically give rise to high overall conversion efficiencies of hot carrier energy well above Shockley and Queisser limit of conventional solar cells.