Interplay of electron trapping by defect midgap state and quantum confinement to optimize the hot-carrier effect in a nanowire structure

The 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 the hot-carrier effect via the spatial confinement, as occurs in a nanowire structure. The recent experimental study by Esmaielpour et al. [ACS Appl. Nano Mater. 7, 2817 (2024)] reveals a fascinating nonmonotonic 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 nonmonotonic behavior can be explained by a simple model for electron energy loss that involves two principal mechanisms. First, electron-phonon scattering, that increases with the nanowire diameter, leading to the 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 a 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 the hot-carrier effect in nanowire solar cells and ultimately their efficiency by adjusting the dimensions and microstructural properties of nanowires.