Enhancement of Hot Carrier Effects and Signatures of Confinement in Terms of Thermalization Power in Quantum Well Solar Cells

TL;DR

This paper develops a theoretical model to analyze hot carrier thermalization in quantum well solar cells, revealing confinement effects that reduce thermalization power and offering insights for high-efficiency photovoltaic device design.

Contribution

A novel theoretical model quantifies hot carrier thermalization in quantum wells, highlighting confinement effects and the influence of phonons and screening on thermalization power.

Findings

Thermalization power density is reduced in 2D systems compared to bulk.

Thermalization power increases with quantum well thickness.

Intra-subband dominates at small thickness; inter-subband dominates in bulk limit.

Abstract

A theoretical model using electron-phonon scattering rate equations is developed for assessing carrier thermalization under steady-state conditions in two-dimensional systems. The model is applied to investigate the hot carrier effect in III-V hot-carrier solar cells with a quantum well absorber. The question underlying the proposed investigation is: what is the power required to maintain two populations of electron and hole carriers in a quasi-equilibrium state at fixed temperatures and quasi-Fermi level splitting? The obtained answer is that the thermalization power density is reduced in two-dimensional systems compared to their bulk counterpart, which demonstrates a confinement-induced enhancement of the hot carrier effect in quantum wells. This power overall increases with the well thickness, and it is moreover shown that the intra-subband contribution dominates at small thicknesses while the inter-subband contribution increases with thickness and dominates in the bulk limit. Finally, the effects of the thermodynamic state of phonons and screening are clarified. In particular, the two-dimensional thermalization power density exhibits a non-monotonic dependence on the thickness of the quantum well layer, when both out-of-equilibrium longitudinal optical phonons and screening effects are taken into account. Our theoretical and numerical results provide tracks to interpret intriguing experimental observations in quantum well physics. They will also offer guidelines to increase the yield of photovoltaic effect based on the hot carrier effect using quantum well heterostructures, a result critical to the research toward high-efficiency solar cell devices.