Photoresponse of a printed transparent silver nanowire-zinc oxide nanocomposite

Zinc oxide (ZnO) is widely used as an absorber layer in photodetectors (PDs). Here, we demonstrate a printed transparent PD with an absorber layer based on a nanocomposite of ZnO with silver nanowires (Ag NWs). The nanocomposite shows good optoelectronic properties; a single pass film exhibits 93 percentage transparency at 550 nm and a low sheet resistance of 28.7 Ω/sq., which allows for the device to operate at low voltages. Its formulation as printable ink with a low annealing temperature of 100 °C makes it suitable for a single step patterned deposition and compatible with flexible substrates that cannot withstand higher temperature. Unlike conventional PDs, the photoconductivity decreases under illumination, i.e., the resistance of the nanocomposite is higher than in dark condition. This anomalous behavior can be explained based on the band alignment between ZnO and Ag in the nanocomposite. The Schottky barrier between ZnO and Ag prevents photogenerated electrons in ZnO from moving to the NWs, while the photogenerated holes recombine with the electrons flowing in the NWs leading to a resistance increase. The PD exhibits an improved photoresponsivity of 35 mA/W, at a relatively low biasing voltage of 1 V, compared to a pure ZnO absorber layer with a responsivity of 14 mA/W at 5 V bias for an illumination at 365 nm. The properties of the nanocomposite make it suitable for single layer, low cost, and large area transparent PDs. The anomalous resistance change can also be extended to fabricating other kinds of sensors, such as gas or humidity sensors, with this nanocomposite.