Influence of Grain Boundaries on Nanoscale Charge Transport Properties of Transparent Conductive ZnO-Based Electrodes

The presence of defect states, such as grain boundaries (GBs), can interfere with the charge transport properties of various semiconductor oxides. In this research, electrostatic force microscopy (EFM) and conducting atomic force microscopy (c-AFM) techniques were used to explore the nanoscale surface electrical properties of zinc oxide (ZnO) thin films deposited on a conductive fluorine-doped tin oxide (FTO) substrate. Films like these are often used as anode materials in photovoltaic and other optoelectronic devices. EFM measurements revealed the presence of charge trapping within the grain boundary region, suggesting localized band-bending effects. Furthermore, a current map obtained through c-AFM indicated that the grain regions exhibited higher conductivity, validating the observations made with EFM. By combination of c-AFM and Kelvin probe force microscopy (KPFM), it was possible to obtain experimental confirmation of band bending at grain boundaries. Data extracted from current–voltage (I–V) curves allowed the quantification of local saturation currents of 1.29 and 0.75 nA at the grain and GB. It was also possible to calculate the difference in potential barrier height between grain and GB as 50.40 meV. Urbach energy calculations identified the existence of defect states within the band gap. These defect states shifted the Fermi level toward the conduction band, reducing the local work functions to 3.93 and 3.89 eV for the grain and GB. These findings align with the thermionic emission (TE) model and Schottky–Mott theory, contributing to a deeper understanding of nanoscale charge transport within ZnO-based anodes and paving the way for the development of transparent conductive oxide-based optoelectronic devices and other applications.