Highly Transparent and Low-Resistance Indium-Free ZnO/Ag/ZnO Multilayer Electrodes for Organic Photovoltaic Devices

We investigated the effect of ZnO layer thickness on the optical and electrical properties of ZnO/Ag/ZnO multilayer films deposited on glass substrates. The transmission window became wider and shifted toward the lower energy side with increasing ZnO thickness. The ZnO/Ag/ZnO (40 nm/18.8 nm/40 nm) multilayer sample showed transmittance of ~96% at 550nm. As the ZnO thickness was increased from 8 nm to 80 nm, the carrier concentration gradually decreased from 1.74 × 10²² cm⁻³ to 4.33 × 10²¹ cm⁻³, while the charge mobility varied from 23.8 cm²/V-s to 24.8 cm²/V-s. With increasing ZnO thickness, the samples exhibited similar sheet resistances of 3.6 Ω/sq to 3.9 Ω/sq, but the resistivity increased by a factor of 4.58. The samples showed smooth surfaces with root-mean-square roughness in the range of 0.47 nm to 0.94 nm. Haacke’s figure of merit (FOM) was calculated for all the samples; the ZnO (40 nm)/Ag (18.8 nm)/ZnO (40 nm) multilayer produced the highest FOM of 148.9 × 10⁻³ Ω⁻¹.