Three-Dimensional Nanoscale Mapping of Porosity in Solution-Processed ITO Multilayer Thin Films for Patternable Transparent Electrodes

Indium tin oxide (ITO) films constitute components of many layered heterostructures used for emergent technologies beyond conventional optoelectronics. Compositional and morphological changes have a direct impact on the device's performance. Hence control over the morphology with advanced multimodal characterization approaches is required to evaluate the devices. Herein multilayer ITO films deposited by spin-coating were quantified in nanoscale detail in three dimensions by combining results from depth-sensitive neutron reflectometry (NR), noncontact topographic AFM images, and cross-sectional SEM images. Films with a different number of deposited layers were visually transparent even though the topmost layer was as high as 60% porous, with porosity gradually decreasing as the number of the underneath sublayers increased. Surface and interfacial roughness through the total film and individual layer thickness were obtained. NR data also furnished quantitative depth information on the films' chemical composition and layer-by-layer bulk density, which has never been obtained before, providing a way to monitor and ultimately control the sheet resistivity via the pore network. When the same formulation is used for inkjet printing patterns, the larger pores disappear, and the optical properties are improved to >90% transmittance at all visible wavelengths. All 5L films achieved sheet resistivities as low as 10^-2 ω cm and can therefore be used as patternable transparent electrodes for many devices including liquid crystal displays.