Indium-free transparent TiO_x/Ag/WO₃ stacked composite electrode with improved moisture resistance

Amorphous TiO_x, WO₃ and Ag mid-layer films were deposited on polyethersulfone (PES) and soda-lime glass via electron beam evaporation at room temperature. The crystallinity, microstructure and surface morphology of asymmetric TiO_x/Ag/WO₃ multilayers (TAW) and symmetric WO₃/Ag/WO₃ (WAW) multilayers were investigated. The polygrains oriented along the (111) planes in the inserted Ag thin films were adopted to supply carriers into both of the dielectric layers and reduce the sheet resistance of the multilayer. The TAW multilayer with a sufficiently large Ag thickness (>15 nm) exhibited low resistance, and highly visible transmittance, low infra-red transmittance, to serve as a transparent conductive electrode and a good heat mirror. The dependence of the Ag and TiO_x thickness on the optical and electrical properties of TAW multilayers was also explored. A figure of merit (FOM) was used to find an optimal layer structure for a TAW multilayer with superior conductivity and highly visible transparency. A high FOM of 1.2 × 10⁻¹ (Ω⁻¹) was achieved at the visible wavelength of 550 nm for a asymmetric TAW stacked layer on PES with a 15-nm-thick Ag layer, a 40-nm-thick TiO_x and a 40 nm-thick WO₃ layer. The moisture resistances of the TAW stacking layer showed more robustness than that of the WAW sample after damp heat exposure.