Reverse-Offset Printing of Metal-Nitrate-Based Metal Oxide Semiconductor Ink for Flexible TFTs

Reverse-offset printing (ROP) is a novel printing technique capable of forming electronics-industry-relevant linewidths (≈1 µm) with good thickness control and sharp edge definition. It is demonstrated that through a controlled oxygen-plasma treatment, the energy of the surfaces related to the process steps of ROP can be optimized to allow the patterning of metal-oxide semiconductor layers using a simple printing ink based on metal nitrates dissolved in an organic solvent. The steps of the ROP process are analyzed using surface-energy measurements and Fourier transform infrared spectra of the ink during drying. Thin-film transistors (TFTs) fabricated using a roll-to-plate ROP of In₂O₃ semiconductor and evaporated Al source/drain (S/D) contacts show, on average, mobilities of 3.1 and 3.5 cm² V⁻¹ s⁻¹, and ON/OFF-ratios up to 10⁸ and 10⁷ on a Si/SiO₂ substrate and on a flexible polyimide-type substrate, respectively. TFTs on the flexible substrate with also the S/D-contacts printed with ROP using Ag nanoparticle ink exhibit a 1.4 cm² V⁻¹ s⁻¹ mobility. To demonstrate the scalability of the process, continuous lines of In₂O₃ are printed using a roll-to-roll-compatible (R2R) ROP with linewidths down to ≈2 µm. This process is expected to lead to miniaturized metal-oxide circuits as required by flexible high-resolution sensor arrays and displays.