  Carrier thermalization in a superlattice solar cell made of polar
semiconductors is studied theoretically by considering a minimal model where
electron-phonon scattering is the principal channel of carrier energy loss.
Importantly, the effect of an intrinsic quantum mechanical property; the phonon
coherence, on carrier thermalization is investigated, within 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 solar cell. The resulting
thermalization power is indeed significantly reduced by the coherent phonons,
resulting in enhanced hot carrier effect, particularly for thin enough well
layer where carrier confinement is also strong. A recent experiment on
superlattice solar cell prototype is shown to manifest the coherent
phonons-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.
