Planar Transparent Conductive Oxide/Ag Rear Contacts for High Efficiency III-V Photovoltaics

Abstract

Photon recycling in photovoltaic devices can be attained by using highly reflective back surfaces. Some of the highest reflectance surfaces are composed of a plane of dielectric material deposited on a highly reflective metal such as Ag or Au. Although optically effective, the use of a planar dielectric layer complicates electrical contact to the device, leading to approaches such as point contacts. A simple solution-that may not result in significant optical or resistive losses-is to use a planar transparent conductive oxide (TCO) layer instead of a dielectric layer. This work investigates the viability of a such a contact. It is observed that contact resistivities of the TCO/Ag stack on a highly doped AlGaAs or GaAs contact layer are below 0.1 Ω cm2 for TCO doping concentrations on the order of 1019 cm-3. The contact resistivity can be reduced further by increasing the doping in the semiconductor layer. Internal hemispheric reflectances of the proposed contacts are expected to reach up to 98% at the wavelength of interest, facilitating photon recycling. The performance of this contact structure suggests use in technologies such as photonic power converters and thermophotovoltaics.