Coherent-Phonon-Driven Hot-Carrier Effect in a Superlattice Solar Cell

Carrier thermalization in a superlattice solar cell made of polar semiconductors is studied theoreti-cally by considering a minimal model where electron-phonon scattering is the principal channel of carrier energy loss. Of note, the effect of an intrinsic quantum mechanical property, the phonon coherence, on carrier thermalization is investigated, within a semiclassical picture in terms of phonon wave packet. It turns out that coherent longitudinal optical (LO) phonons weaken the effective electron-phonon coupling, thus supposedly lowering the carrier-energy-loss rate in the solar cell. The resulting thermalization power is indeed significantly reduced by the coherent phonons, resulting in an enhanced hot-carrier effect, par-ticularly for thin enough well layer where carrier confinement is also strong. A recent experiment on a superlattice solar-cell prototype is shown to manifest the coherent phonon-driven phenomenon. Our results demonstrate the practical implications of the fundamental quantum coherence property of phonons in semiconductors for improving superlattice solar-cell performance, via hot-carrier effect.