Heat-Assisted Inkjet Printing of Tungsten Oxide for High-Performance Ultraviolet Photodetectors.

An ammonium metatungstate precursor (WO3Pr) ink was printed for tungsten oxide (WO3) UV detectors on SiO2/Si wafers with prefabricated Au electrodes. A systematic study was carried out on the printing parameters including substrate temperatures in the range of 22-80 °C, WO3Pr molar concentrations of 0.01, 0.02, and 0.03 M, and printing scan numbers up to 7 to understand their effects on the resulted WO3 film morphology and optoelectronic properties. It has been found that the printing parameters can sensitively affect the WO3 film morphology, which in turn impacts the WO3 photodetector performance. In particular, the printed films experienced a systematic change from discontinuous droplets at below 40 °C to continuous films at 40-60 °C of the substrate temperature. At higher temperatures, the excessive heat from the substrate not only caused drastic evaporation of the printed ink, resulting in highly nonuniform films, but also detrimental heating of the ink in the printer nozzle in proximity of the substrate, preventing continuous printing operation. An optimal printing window of the substrate temperature of 45-55 °C at a molar concentration of 0.02 M of ammonium metatungstate and three printing scans was obtained for the best UV detector performance. A large on/off ratio of 3538 and a high responsivity up to 2.70 A/W at 5 V bias (0.54 A/W·V) represent a significant improvement over the best report of ∼0.28 μA/W·V on WOX photodetectors, which indicates that the printed WO3 films are promising for various applications of optoelectronics and sensors.