Enhancing electrical and thermoelectrical properties of $\mathrm{WO}_3$ thin films by spray pyrolysis technique

Abstract

Tungsten trioxide WO 3 thin films have been deposited on glass substrates by spray pyrolysis technique with three different rates R 1 = 1 ml/min (sample A 1 ), R 2 = 3 ml/min (sample A 2 ) and R 3 = 7 ml/min (sample A 3 ). We have then annealed all samples for 2 h at 500 • C in air. Structural, morphological, optical and electrical properties of WO 3 thin films are investigated in detail. According to the FESEM images, all layers are formed in various circular strings with different diameters. The diameter of each strain decreases when the deposition rate increases. The XRD structural analysis indicates that before annealing, except the lowest deposition rate sample R 1 with a polycrystalline nature and mixed tetragonal and hexagonal phases, other samples are grown in an amorphous phase. After annealing, sample A 2 (with rate R 2 = 3 ml/min) remains in an amorphous phase, while samples A 1 and A 3 (with the corresponding rates R 1 = 1 ml/min and R 3 = 7 ml/min respectively) are polycrystalline. Moreover, from UV-Vis. spectra analysis we found that by increasing spray rate the band gap of the layers monotonically decreases. This phenomenon is the result of attenuation of the quantum confined effect. The thermoelectric properties confirm being n-type conductivity of the layers, also variation in the current-voltage characteristic is in accordance with the oxygen vacancies sites. We found that the sample A 3 with the highest rate has lowest resistivity.