The Mott transition and optimal performance of transparent conducting oxides in thin-film solar cells

Abstract

The performance of thin-film solar cells is critically dependent upon the effective operation of transparent conducting oxide (TCO) layers, which play a significant role in both optical and electrical power transmission through these photovoltaic devices. In this article, we model the optical and electrical power transmission through TCO layers in thin-film solar cells as a function of both the electron carrier density, n, and its mobility, μ. The electrical and optical properties of the TCO layer are described by the simple Drude model of the degenerate free electron gas and the concomitant electromagnetic absorption due to skin-depth effects is thereby calculated. Above the critical carrier density for the composition-induced Mott Insulator-Conductor Transition, TCOs exhibit metallic-type conduction. However, with increasing electron (carrier) density above the transition, the optical transparency of the layer is significantly decreased. Importantly, in order to achieve high electrical conductivity whilst preserving high optical transparency of the TCO layer, electron mobilities need to be increased in preference to increasing electron densities. To reach higher carrier mobilities in any given TCO system, we propose that one should move the material close to the Mott transition. A model of ionized impurity scattering in indium tin oxide (ITO) at high carrier densities allows direct comparison of the μ-n relationship in real TCO layers to the total power absorption in such layers in thin-film solar devices. We determine that decreasing the electron density from 2.6 × 1021 cm−3 to 2 × 1021 cm−3 in such an ITO layer above the Mott critical density can decrease the total power absorption in the layer by a large amount (around 8% relative to the minimum theoretical absorption).