ZnS|Ag|TiO2 multilayer electrodes with broadband transparency for thin film solar cells

Abstract

A ZnS|Ag|TiO₂ (ZAT) multilayer architecture is proposed as an alternative to symmetric TiO₂|Ag|TiO₂ (TAT) electrodes. The TAT electrodes were proved to be among the best ITO-free transparent conductive electrodes. It is demonstrated that choosing ZnS instead of TiO₂ as a substrate for Ag allows better optical and electrical performances. The complex refractive indices of both dielectric materials are determined by spectroscopic ellipsometry and implemented into a transfer matrix algorithm to optimize the optical transmittance of ZAT and TAT multilayers in the visible part of the spectrum. It is shown that both types of electrodes are equivalent in terms of optical behavior. Manufactured electrodes with symmetric 40 nm dielectric thicknesses are then fabricated on glass substrates by e-beam evaporation and the effect of the silver layer thickness on performances is studied. It is found that ZAT multilayer performances are systematically better, and on a broader transmittance range, when the Ag layer is very thin than TAT stacks. A state of the art 90.23% maximum transmittance is reached at λ = 460 nm for a ZnS(36 nm)|Ag(12.7 nm)|TiO₂(37 nm) multilayer, with a sheet resistance R_S of 5 Ω Sq⁻¹. Over 80% transmittance is achieved in the [380–855] nm wavelength range for a ZnS(36 nm)|Ag(7 nm)|TiO₂(40 nm) multilayer, with a R_S of 11.3 Ω Sq⁻¹. Scanning electron microscopy (SEM) reveals continuous silver films grown on ZnS as opposed to those grown on TiO₂, thus justifying the better performances of the ZAT.