Tungsten oxide (WO3) is a transition metal oxide with a number of interesting properties and is being extensively investigated for various applications such as electrochromic devices and gas sensors. Thin films of WO3 have been prepared by many techniques, including evaporation, sputtering, chemical vapor deposition and spray pyrolysis.
In this work, we present an optoelectronic characterization of this material deposited by flame spray pyrolysis for the synthesis and direct deposition of pure and Si-doped WO3 nanoparticle films onto glass substrates featuring a set of interdigitated Pt electrodes
Characterization has been performed by measuring the current-voltage (IV) characteristics in dark conditions and by monitoring the time evolution of photocurrent under monochromatic radiation. These optoelectronic properties are particularly important when WO3 is used as an active material in photoresistor and/or light emission devices.
The IV measurements show a resistive behavior of the material. Furthermore, the presence of hysteresis in the IV curves in dark conditions measured in forward and backward directions, between -0.5 and 0.5 Volts, indicates a capacitive behavior of the device due to trapping and de-trapping of carriers from defects inside the energy gap.
The presence of defects is confirmed by the time evolution of the photocurrent, measured under monochromatic radiation at 420nm. These data show an exponential increase with a time constant equal to 1600s independently on the applied voltage. This behavior can be ascribed to the presence of defects lying at 1.1eV below the conduction band. The responsivity at the steady state regime is equal to 2.5 mA/W at 0.5V.
When the radiation is turned off, the photocurrent shows an exponential decrease, once again independently on the applied voltage, with a time constant equal to 1000s. The defect level corresponding to this relaxation process corresponds to 0.92eV.
Currently we are performing experiments based on the current evolution at different temperatures and under sinusoidal signal in order to better understand the defect position and influence on the device performance. At the same time, an electrical model, based on the experimental results, is under development.