Design of ultrathin metal-based transparent electrodes including the impact of interface roughness

Highlights: • Ultrathin metal-based TiOx/Au/AZO transparent electrodes were sputtered. • Simulations including the interface roughness were performed. • Excellent agreement between simulated and experimental spectra • Optimized transparent electrodes showed a maximum transmittance of 0.88. In this paper, an efficient model for the precise design and prediction of the optical properties of transparent electrodes that are composed of ultrathin metals embedded in dielectric layers, is introduced. Specifically, the investigated electrode employs an ultrathin Au layer on a TiOx-coated glass substrate and covered with Al-doped ZnO (AZO), deposited by sputtering. This structure was simulated with a transfer matrix algorithm and was optically characterized by measurements of direct and diffuse transmittance, as well as specular reflectance. The comparison of simulated and experimental spectra reveals significant differences, which is due to the increased absorption in the ultrathin metal. To achieve a more precise prediction of the optical properties, the roughness of the interfaces was included in the simulation through an effective medium approximation. The improved simulation model requires only one fitting parameter and was applied to optimize the electrode's performance through the variation of the dielectric layers' thickness. In agreement with the simulations, the deposited TiOx/Au/AZO electrodes showed a maximum transmittance of 0.88 at 550 nm (including the substrate) and average transmittance in the visible wavelength range of > 0.80, combined with a sheet resistance of 14 Ω/square.