Role of hexamethylenetetramine/zinc nitrate hexahydrate molar ratio in controlling structural, morphological and optical properties of ZnO thin films synthesized by CBD

The present study examines the role of the hexamethylenetetramine (HMTA)-to-zinc nitrate hexahydrate (ZNH) molar ratio in controlling the physical properties of zinc oxide (ZnO) thin films obtained by chemical bath deposition (CBD) for potential applications in dye-sensitized solar cells (DSSCs). ZnO thin films were synthesized on glass substrates using aqueous solutions with different HMTA-to-ZNH molar ratios of 0, 1, 2, 3, 4, 5, and 6. The results show that this parameter strongly influences the crystallinity, morphology, and optical characteristics of the films. The X-ray diffraction analysis shows that all the ZnO thin films synthesized are polycrystalline and crystallize in a hexagonal wurtzite structure, with a fluctuating variation in crystallite size (9.5 to 43.7 nm), indicating crystal growth modulated by HMTA. The presence of characteristic functional groups and chemical bonds in ZnO thin films was confirmed by Fourier transform infrared spectroscopy. Scanning electron microscope images revealed that the molar ratio of HMTA/ZNH in the bath solution affected the morphology of the films through grain shape. The films became more compact as molar ratio increased, showing the presence of agglomerated nanotubes. UV-visible analysis showed that films obtained with HMTA/ZNH molar ratio of 4 exhibited the highest transmittance (85%) while those elaborated without HMTA exhibited the lowest transmittance (60%) indicating the impact of HMTA on physical properties of ZnO thin films. Based on structural, morphological, and optical properties, ZnO thin films obtained with HMTA/ZNH molar ratio of 4 are more suitable to be used as a compact electron transport layer in DSSCs to limit charges recombination.