Room-Temperature Postannealing Reduction via Aqueous Sodium Borohydride and Composition Optimization of Fully Solution-Processed Indium Tin Oxide Films

Solution-processed transparent conductive oxides offer the advantages of low-cost, high-throughput fabrication of electronic devices compared to the specific requirements of vacuum deposition techniques. However, adapting the current state of the art to ink deposition calls for optimization of the precursor ink composition and the postdeposition process. Solution processing of indium tin oxide films can be accomplished at reduced temperatures (250-400 °C) by annealing soluble precursor metal salts together with a fuel/oxidizer, causing an exothermic reaction with elevated local temperatures. Following layer-by-layer cycles of deposition and annealing, a postprocessing step is required via heating (300 °C) under a 5% H₂ reducing atmosphere. To address the discrepancy between the versatility of ink deposition and the limitations of controlled atmosphere postprocessing, here we investigate the effects of postprocess dipping in aqueous sodium borohydride at room temperature as an alternative, which allows for a completely solution-based process from ink to film. In addition to postprocessing, the solution composition was also optimized by removing the fuel additive and by adjusting the In/Sn content. Indium tin oxide (ITO) films were spin-coated and annealed in air at 250, 300, and 400 °C and characterized by UV/vis spectroscopy to obtain optical transmittance, atomic force microscopy to obtain film thickness and surface morphology, and a Hall effect system for electrical parameters. Additional data from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) indicate that crystallinity is affected by the reducing environment. Results revealed an order-of-magnitude improvement of the Haacke figure of merit (FOM) from 4.3 × 10^-4 Ω^-1, 382 Ω/□ sheet resistance (R_s), and 84% transmittance (%T) for the traditional 9:1 In/Sn precursor ink with fuel additive followed by 300 °C of 5% H₂-furnace post-treatment compared to that of the optimized fully solution-processed 8.5:1.5 In/Sn ink without fuel followed by an ambient air at 25 °C dipping in aqueous sodium borohydride, leading to 3.0 × 10^-3 Ω^-1 FOM, 84.5 Ω/□ R_s, and 87%T including the glass substrate.