THz Study on the Role of ZnO Crystallinity in ZnO/AgNW/ZnO Composite Electrodes

We present the conducting mechanism of silver nanowire (AgNW) networks embedded between zinc oxide (ZnO) bilayers studied by terahertz (THz) reflection spectroscopy. Percolatively connected AgNWs have been considered as one of the most promising alternatives for indium-tin-oxide transparent conductive electrodes (TCEs). Although AgNWs can be spin-coated on nearly any kind of substrate, they leave a large open area between randomly scattered NWs. In ZnO/AgNW/ZnO composite electrodes, the conductive ZnO underlayer has the advantage in charge transport through electrodes and another ZnO layer covering the AgNW/ZnO layer can suppress oxidation of NW surface and improve the thermal stability. The performance of AgNW-based TCEs is highly correlated to the connectivity of NWs at junctions, which can be realized by percolational transition near a critical NW density. Terahertz reflection spectroscopic measurement shows that the photoconductivity of ZnO/AgNW/ZnO composite electrodes is strongly influenced by the ZnO crystallinity. The percolational behavior of the confinement factor in THz spectroscopic analysis reveals that single-crystalline ZnO layers in ZnO/AgNW/ZnO increase the photoconductivity by forming wider effective charge collection area around AgNWs and filling the gap between AgNWs to improve the ohmic contact.