Gate Dielectrics In Organic Thin-film Transistors Research Articles

Low-temperature crosslinked soluble polyimide as a dielectric for organic thin-film transistors: enhanced electrical stability and performance

We have prepared a low-temperature cross-linked soluble polyimide (SPI) as a dielectric material for organic thin-film transistors (OTFTs) to improve their electrical stability. Two types of SPIs (DOCDA/6FHAB and 6FDA/6FHAB) were synthesized by a one-step polymerization process using 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and hexafluoroisopropylidene diphthalic anhydride (6FDA) as the dianhydrides and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FHAB) as a diamine. To further enhance the electrical performance, the SPI thin films were crosslinked with methylated/ethylated (hydroxymethyl)benzoguanamine (HMBG) through a low temperature process at 160 °C. Crosslinking considerably improved the insulating properties, resulting in a substantial reduction in leakage current from 10−7 A cm−2 to 10−9 A cm−2 at 2.0 MV cm−1. When crosslinked SPIs were used as gate dielectrics in OTFTs, device stability and reliability, as measured by the off-current, threshold voltage, and hysteresis, improved significantly. Our results demonstrate the potential of crosslinked SPIs as effective gate dielectric materials for advanced organic thin-film transistors.

Towards flexible organic thin film transistors (OTFTs) for biosensing.

We have studied parylene-N and parylene-C for their use as substrates and gate dielectrics in OTFTs. Parylene-N films with a thickness of 300 nm show the required dielectric properties, as verified by breakthrough-voltage measurements. The surface roughness measured for 300 nm thick parylene-N films is 4-5 nm. However, initial growth of parylene depends on the subjacent surface. This results in different thicknesses on Au electrodes and substrate materials for thin films. Capping of micro-patterned Au-electrodes with a thin Al layer via lift-off results in homogenous parylene film thickness on the whole sample surface. OTFTs are fabricated on glass with parylene-N as a gate dielectric and pentacene as a semiconductor. The electrodes are patterned by photolithography enabling micrometer sized features. The contact resistance is extracted by variation of the channel length. Modification of the parylene dielectric layer surface by plasma treatment with oxygen after deposition allows shifting of the threshold voltage to more positive values, however at the cost of increasing hysteresis. OTFTs fabricated on thin parylene-C films can be peeled off and could result in flexible devices employing parylene-C foil as a substrate. For a foil thickness of 3-4 μm, operational devices can be bent down to radii less than 1 mm, e.g. in the range of cannulas. Operation of such OTFTs with parylene-C as a gate dielectric in liquids is demonstrated. The OTFT current can be modulated by the potential in the electrolyte as well as by the bottom gate potential. This allows for application of such OTFTs as sensors in medical devices.

OTFTs using hybrid films as gate dielectrics prepared via UV curing and sol–gel reaction

The work presented here focuses on the preparation and characterization of gate dielectrics in organic thin-film transistors (OTFTs), fabricated by the sol–gel process. Hybrid dielectrics were prepared with acryl UV resin, titanium n-butoxide, catalytic HCl, and acetylacetone by sol–gel process and patterned by UV cross-linking below 120 °C. Leakage currents of dielectric layers remained below 10 −9 A under operating voltage and dielectric constants were measured to be ∼6.5 at 10 kHz. The field effect mobility and on–off ratio were ∼0.86 cm 2/V s and ∼10 4, respectively. These results demonstrate that sol–gel hybrid systems are suitable for gate dielectrics in OTFTs.