Process structure property relationships in inkjet-printed ZnO thin films: decoupling the effects of substrate temperature and layer number

This study systematically investigates the influence of substrate temperature (ranging from 50 to 90°C) and the number of layers (5, 10, and 15 passes) of inkjet-printed ZnO thin films deposited onto glass substrates using a high-precision 12-nozzle DC Samba printhead (LP-50 platform). The purpose of this work is to evaluate the impact of substrate temperature and number of layers on structural, morphological, and optoelectronic properties of ZnO thin films after a fixed annealing treatment at 420°C for 1 hour. X-ray diffraction analysis of all samples showed characteristic reflections consistent with hexagonal wurtzite ZnO (JCPDS 36-1451) within the measured 2θ range. The estimated crystallite size increased from approximately 5-9 nm for the 5-layer films to approximately 11-13 nm for the 10- and 15-layer films, indicating that the main increase occurred between 5 and 10 printed layers. The optical properties of all samples, analyzed by UV-Vis spectroscopy, showed high optical transmittance above 94% in the visible spectrum, with a fixed direct optical band gap of 3.22-3.25 eV. These values are consistent with the optical response of ZnO thin films. SEM analysis of ZnO thin films revealed a significant change in morphology with increasing substrate temperature. The ZnO thin film deposited at 80°C for 15 layers showed 86.5% surface coverage with a fully coalesced quasi-continuous ZnO network, compared to 62% for a film deposited at 50°C. The present study shows that, among all samples, 15 printed layers at 80 °C yield the highest surface coverage on the substrate, as observed from SEM images, while also maintaining high substrate-referenced visible transmittance. These results provide valuable insight into the processing of inkjet-printed ZnO thin films. However, before assessing their suitability for applications such as transparent conductive oxide layers, transparent electrodes, or thin-film transistor channels, electrical characterization is required.