Optoelectronics properties of tungsten oxide nanoparticle networks deposited by flame spray pyrolysis
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.495
Optoelectronics properties of tungsten oxide nanoparticle networks deposited by flame spray pyrolysis Domenico Caputo1, Shaul Ajò1, Giampiero de Cesare1, Alessio Buzzin1, Renheng Bo2 and Antonio Tricoli2
1
Department of Information Engineering, Electronics and Telecommunications, University of Rome Sapienza, Rome, Italy. 2
Nanotechnology Research Laboratory, Research School of Engineering, Australian National University, Canberra, ACT, Australia.
Abstract
In this work, we present the optoelectronic characterization of pure tungsten oxide nanoparticle networks synthesized and self-assembled by flame spray pyrolysis. Currentvoltage measurements performed in dark conditions indicate the presence of trapping and detrapping phenomena from defects inside the energy gap. The presence of defects is confirmed by the time evolution of the photocurrent, measured under monochromatic radiation at 420 nm. After illuminating the WO3 films with light, the current increases exponentially with a time constant independent on the applied voltage. This behavior is ascribed to the presence of defects lying at 1.1 eV below the conduction band of WO3 (bandgap of ca. 2.9 eV). When the illumination is terminated, the photocurrent shows an exponential decrease, once again independently on the applied voltage. The defect level corresponding to this relaxation process corresponds to 0.92eV below the conduction band.
INTRODUCTION Tungsten oxide (WO3) is a transition metal oxide with interesting optoelectronic properties and [1] is being extensively investigated for various applications such as electrochromic devices [2] and gas sensors. In particular, thin films of WO3 have shown their sensitivity to O2, NO2, H2S, ethanol and acetone [3, 4]. WO3
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can be grown by many techniques, including evaporation [5], sputtering [6] and chemical vapor deposition [7]. Due to its low-cost processing method and easy-of-use, spray pyrolysis is a very attractive technique for preparing metal oxide nanoparticles suitable for different applications. Indeed, both dense and porous materials, thin metal oxide and chalcogenide films deposited with this technique have been utilized in solar cells, anti-reflection coatings and gas sensors [8]. In addition to previous works [9], recently different research groups have been focused on the spray pyrolysis deposition of WO 3 thin films. In particular, Aly et al. [10] have presented the effect of spray pyrolysis parameters (deposition temperature and post annealing temperature) on the microstructural and dark electrical properties of WO 3 films, while Ji et al. [11] proposed the use of WO 3 deposited by low-temperature ultrasonic spray pyrolysis method as an anode buffer layer for organic solar cells. In
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