Crosslinking Organic–Inorganic Hybrid Dielectric with Azide-functionalized Ligand for High-Performance Solution-Processed Oxide Thin Film Transistors

Owing to the global development in IoT technology, interest in metal-oxide semiconductor-based circuits with low standby power consumption, particularly in thin-film transistor (TFT) materials capable of low-cost solution processing, has been rapidly increasing. Notably, most of the high electron mobility oxide TFTs were based on high-<i>k</i> inorganic gate dielectrics. Compared with conventional SiO₂, high-<i>k</i> oxides such as HfO₂, ZrO₂, and Al₂O₃ are ideal candidates for realizing a high-capacitance dielectric layer capable of low-voltage driving and high density of charge accumulation. However, these high-k inorganic dielectric layers are hindered by 1) expensive deposition equipment and high process temperature for vacuum deposition, and 2) imperfect inorganic purity and porous morphology for solution deposition. Therefore, the development of a solution-processed high-<i>k</i> dielectric layer with a high dielectric strength is urgently required. A possible strategy to strengthen the dielectric properties of solution-processed high-<i>k</i> oxides is to realize an organic-inorganic hybrid dielectric layer, which typically comprises nanocomposites of polymer and high-<i>k</i> oxide nanoparticles (NPs) combining high permittivity of the inorganic NPs and high breakdown strength, mechanical flexibility, and easy processability of the polymer dielectrics. However, existing hybrid dielectric layers have not shown this synergistic effect and yielded only marginal performances, particularly for oxide TFTs.<br/>The aforementioned synergetic effects can be achieved only when the complementary organic and inorganic constituents are well mixed. In reality, owing to the different surface energies of organic and inorganic inclusions, the thin-film morphology of hybrid dielectrics often has the limitations of air voids and low thin-film density, thereby resulting in a high leakage current and consequently a low on/off ratio of the resulting TFT. Furthermore, owing to the thermodynamic instability of the mixed binary phase, the hybrid dielectric layer can lack long-term operational stability. Therefore, it is challenging to simultaneously realize a high-<i>k</i> and high dielectric strength from a hybrid dielectric layer.<br/>In this study, we attempted to achieve an organic–inorganic hybrid dielectric layer with a covalently networked morphology between organic and inorganic inclusions for high-performance solution-processed oxide TFTs. We propose a method for chemically crosslinking zirconia NPs and PMMA with functionalized azide ligands, which show higher crosslinking efficiency with a minimized amount. However, simply mixing azide molecules with ZrO₂ NPs and PMMA cannot guarantee crosslinking between inorganic and organic phases; nitrene generated from azide can react with alkyl CH or <i>π</i>-conjugated aromatic groups; therefore, crosslinking is limited only between PMMAs and not between the ZrO₂ NPs and PMMA. Therefore, we newly synthesized azide-functionalized acetylacetonate, functioning as a 1) ligand of ZrO₂ NP in post sol-gel synthesis and 2) crosslinking agent between ZrO₂ NP and PMMA. From the optimized processing conditions, we obtained an excellent dielectric strength of over 4.0 MV cm^-1, a high-<i>k</i> of ~14, and a low surface energy of 38 mN m^-1. We demonstrated the fabrication of exceptionally high-performance, hysteresis-free n-type solution-processed oxide TFTs comprising an In₂O₃/ZnO double layer as an active channel with an electron mobility of over 50 cm²V⁻¹s⁻¹, on/off ratio of ~10⁷, subthreshold swing of 108 mV dec^-1, and outstanding bias stability. From temperature-dependent <i>I</i>–<i>V</i> analyses combined with charge transport mechanism analyses, we demonstrated that the proposed hybrid dielectric layer provides percolation-limited charge transport for the In₂O₃/ZnO double layer under field-effect conditions.