3D integration of printed organic dual-gate FETs on a flexible substrate

Abstract

The direct printing of organic thin-film transistors(TFTs) gives us a facile and fast route to fabrication of freeform electronicapplications on a flexible substrate. However, despite the significantimprovement in the performances of organic semiconductors, electronics industrystill has not adopted the printed organic TFTs for manufacturing electronic applications.The low resolution, large feature size, and variation of inkjet printing havelimited the implementation of organic integrated circuits with reasonabletransistor densities and uniformity. To overcome these problems, we have developeda robust inkjet design rule optimized for high-yield circuit patterning, and fabricatea three-dimensional (3D) printed dual-gate (DG) organic TFT where n- and p-typeTFTs are stacked in transistor-on-transistor manner halving the transistor footprint. In this work, all the functional layers including electrodes andsemiconductors are fully printed except a parylene dielectric that is formed bychemical vapor deposition. The design rule has been developed via a systematic study on the influenceof inkjet printing parameters such as substrate temperature, drop spacing) on theminimum width, smoothness, and reproducibility of vertically or horizontally printedmetal lines. The stacked DG TFTs showed average carrier mobilities of around 0.2and 0.7 cm2V-1s-1 for the n-type and p-type, respectively,showing exceptional uniformity and long-term stability at 5 V operation voltage.We successfully demonstrated 7-stage ring-oscillator on a flexible PEN film withthe maximum frequency of 200 Hz and a gate delay of 340 ms. Our 3D printing approach providesa path for achieving high transistor density, high yield, high uniformity, andlong-term stability, which are critical for the realization of printed organic electronicsapplications.