Variation in chemical bath pH and the corresponding precursor concentration for optimizing the optical, structural and morphological properties of ZnO thin films

In the present study, ZnO thin films were deposited by chemical bath deposition carried out by selective correlation of varying (i) pH values at fixed concentration and (ii) concentration of the precursors at fixed pH. The selective correlations were done by using the characterization tools like X-ray diffraction, scanning electron microscopy, transmittance, refractive index, dielectric constant, Fourier-transform infrared spectroscopy and IV measurements. Transmittance was found to increase from 57 to 87% on varying the pH from basic side (10.8) to acidic side (pH 6.8) with a blue shift in band gap. The nature and morphology of the deposited films were found to be dependent on pH as well as concentration. Acidic pH 5.0 was found to be most suitable for deposition of highly transparent film with low absorption coefficient, refractive index and dielectric constant. On the other hand, nearly complete coverage of the substrate and high purity was observed in the ZnO thin films which was deposited by taking equal 100 mM concentration of Zn(NO₃) and HMTA precursors at a fixed pH 5.0 as desired, sheet resistance was also found to increase on the acidic pH side which is useful in case of buffer layer solar cell application. These studies lay a foundation stone for understanding the optical and morphological parameters by selectively correlating the pH and concentration variation at the same time.