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Opto-electronic properties of solution-processed zinc oxide thin films: role of solvents and doping

Undoped zinc oxide (ZnO) and nitrogen-doped zinc oxide (NZO) thin films were prepared on transparent conducting oxide-coated glass substrates by employing sol-gel technique. The effect of different solvents and nitrogen doping on the optical, structural, and electrical properties was investigated by UV-visible absorption spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), profilometry, and Hall effect studies. ZnO films yielded transmittance above 85% and the bandgap of ZnO thin films decreased with doping. XRD pattern confirmed hexagonal wurtzite structure of ZnO. NZO thin films were found to be in the nano-thin film phase with thickness of 40 nm. Hall effect studies yielded carrier concentration of 1.2 x 10(15)cm(-3)and 2.03 x 10(14)cm(-3), respectively, for undoped and doped ZnO thin films. The changes in vibrational modes of ZnO due to nitrogen doping were detected using Fourier transform infrared (FTIR) analysis. It was found that p-type doping, leading to an improved surface morphology, led to a reduction in optical bandgap and an increased charge carrier mobility. The choice of the solvent was found to have a profound influence on the surface morphology, optical bandgap, tail states distribution, and charge carrier mobility.