Improvement of electrical characteristics of flexible AZO/Ag/Cu/AZO transparent conductive films by Cu deposition

Flexible electrodes are prepared by deposition on polyethylene naphthalate substrates, which melt at elevated temperatures, and are, therefore, generally unsuitable for deposition at high temperatures. However, only limited improvement in the conductivity can be achieved for Al-doped ZnO (AZO) films formed at low temperatures. Multilayer transparent conductive films (AZO/Ag/AZO), in which a conductive metal such as Ag is sandwiched between AZO, exhibit superior resistivity and electrical stability against bending compared to AZO films and have attracted considerable attention. In this study, AZO transparent conducting films were investigated as alternatives to indium tin oxide. The electrical characteristics of AZO/Ag/AZO films are not optimal at low temperatures owing to oxidation of Ag and its diffusion into the AZO layer. We, therefore, developed transparent conductive films with an AZO/Ag/Cu/AZO structure in which an intermediate Cu layer suppresses the oxidation of Ag and inhibits its diffusion into the substrate-side AZO layer, changing the deposition conditions of Cu. The optimal characteristics were obtained at a Cu deposition rate of 0.08 nm/s. A further increase in Cu layer thickness suppresses the oxidation of the Ag layer and its diffusion into the substrate-side AZO layer, thereby improving resistivity. Notably, a 5 nm thick Cu layer exhibited exclusive Cu regions, which further prevented the oxidation of Ag and its diffusion into the substrate side of the AZO layer, with a resistivity of 5.12 × 10 −5 Ω cm. This resistivity is comparable to that of existing transparent conducting films used in practical applications; however, the transmittance of the AZO/Ag/Cu/AZO film was reduced owing to the low transparency of Cu.