Researchers at the University of Minnesota successfully demonstrated that ion-gel could be used as printable, high-capacitance gate insulator in organic thin-film transistors (OTFT). This proves to be a highly promising approach to one of the still existing issues in flexible electronics, e.g. printed flexible circuitry, sensors and displays.
Although, several functional inks based on metallic, semiconducting and insulating materials have already been demonstrated, enhanced printability and performance, especially for dielectrics, are still objectives for further research and development.
The researchers led by Daniel Frisbie and Timothy Lodge demonstrated transistors and unipolar inverters fully printed on plastic foil using Optomec's aerosol jet printing technique. Operating frequencies of up to 10 kHz were achieved. Thus, earlier reported 100-1,000 Hz operation for GEL-OTFTs on rigid substrate using shadow-masking and spin-coating was significantly improved.
Fig. 1: A GEL-OTFT schematic diagram in cross-section (left) and an optical image of a GEL-OTFT (right).
Because they are ionic conductors, but electronic insulators, dielectrics are mainly used for energy storage purposes in capacitors and for carrier accumulation in the semiconductor of a transistor.
The ion-gel films used were formed by gelation of a self-assembling triblock copolymer in an ionic liquid. Using this combination the gelation occurs at very low polymer weight fractions - as low as 4%. This results in a high ionic conductivity in the resulting ion-gel.
The material's advantages in terms of flexible electronics are printability and a very high specific capacitance, (in excess of 10 µF cm-2), which arises from the mobile ions in the gel. The higher capacitance of the gate insulator results in lower OTFT operating voltages, less than 2 V - an amount that can be supplied by a conventional single thin-film battery.
Additionally, both components of the ion-gel are soluble in a number of solvents. Thus, the dielectrics are likely to be compatible with other printing methods. The high polarizability of the material also enables simplified TFT architectures, where the requirements of the gate electrode alignment are relaxed.
Objectives for further development are the currently very high off currents of the order of 10 nA for typical devices. Apart from that, ion-gel dielectrics are extremely promising for flexible electronic applications due to their high capacitance, printability and suitable frequency response.
Top image: Optical image of an aerosol-printed GEL-OTFT array on flexible substrate with channel lengths of 20 µm and channel width of 1,400 µm.
For more attend Printed Electronics Asia 2009 or Printed Electronics USA 2009.
