Interplay of Electron Trapping by Defect Midgap State and Quantum Confinement to Optimize Hot Carrier Effect in a Nanowire Structure

TL;DR

This paper explains the non-monotonic dependence of hot carrier effects in nanowire solar cells by modeling electron-phonon scattering and defect trapping, providing guidelines for optimizing nanowire dimensions.

Contribution

It introduces a simple model combining two mechanisms to explain experimental observations of hot carrier behavior in nanowires, aiding optimization strategies.

Findings

Optimal nanowire diameter balances electron-phonon scattering and defect trapping effects.

Non-monotonic hot carrier effect observed experimentally is explained by the model.

Guidelines for nanowire design to maximize hot carrier effects in solar cells.

Abstract

Hot carrier effect, a phenomenon where charge carriers generated by photon absorption remain energetic by not losing much energy, has been one of the leading strategies in increasing solar cell efficiency. Nanostructuring offers an effective approach to enhance hot carrier effect via the spatial confinement, as occurring in a nanowire structure. The recent experimental study by Esmaielpour et al. [ACS Applied Nano Materials 7, 2817 (2024)] reveals a fascinating non-monotonic dependence of the hot carrier effect in nanowire array on the diameter of the nanowire, contrary to what might be expected from quantum confinement alone. We show that this non-monotonic behavior can be explained by a simple model for electron energy loss that involves two principal mechanisms. First, electron-phonon scattering, that increases with nanowire diameter, leading to hot carrier effect that decreases with increasing diameter. Second, electron capture by a defect level within band gap, that is, a midgap state, that decreases with nanowire diameter, leading to hot carrier effect that increases with increasing diameter. The two mechanisms balance at a certain diameter corresponding to optimal hot carrier effect. Our result offers a guideline to optimize hot carrier effect in nanowire solar cells and ultimately their efficiency by adjusting the dimensions and micro-structural properties of nanowires.