Swaminathan Parasuraman

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% transparency at 550 nm and a low sheet resistance of 28.7 Ω sq-1, 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 photo-generated electrons in ZnO from moving to the NWs, while the photo-generated holes recombine with the electrons flowing in the NWs leading to a resistance increase. The PD exhibits an improved photoresponsivity of 35 mA W-1, at a relatively low biasing voltage of 1 V, compared to a pure ZnO absorber layer with a responsivity of 14 mA W-1 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.

In this work, we describe the preparation of a zinc oxide-ethylene glycol nanoparticle ink and the parameters that control the printing using a custom-built direct writer system. The ink (nanoparticle dispersion) was prepared using a two-step wet synthesis method, without using any surfactant. Its viscosity was found to be in the suitable range for printing and straight lines were printed on cleaned glass substrates. The influence of various printing parameters, such as total dispersed volume, number of printed layers, substrate temperature, drying temperature and time, and particle loading, on the morphology of the printed patterns was investigated. In-situ and post-printing drying of the printed pattern, at the same temperature, produced different morphologies, which can be attributed to the direction of heat transfer and solvent removal. Optimization of these printing parameters enabled us to obtain a continuous printed pattern with uniform morphology using a direct writing system, which can be extended to a variety of nanoparticle based inks.

Zinc oxide (ZnO) is an intrinsically n-type semiconductor, which forms Schottky junctions with various metals. Understanding the origin and modulating the rectifying behavior of these junctions has been a challenge, especially for polycrystalline ZnO. This work shows that adding p-type nickel oxide (NiO) to a polycrystalline ZnO-silver (Ag) Schottky junction can enhance the current–voltage response. The maximum improvement is seen at around 10 mol.% NiO, beyond which the ohmic behavior of the NiO-Ag interface dominates and the rectification is lowered. The ZnO-NiO composites were prepared by solid-state synthesis, and electrical contacts were made using silver epoxy. X-ray diffraction studies revealed limited solid solubility in both oxide lattices while scanning electron micrographs revealed a non-homogeneous composite microstructure. Photoluminescence spectroscopies characterized defects in the system. Finally, the current–voltage behavior depends on the composite composition and can be explained based on a combination of Schottky and built-in pn-junctions.