(630d) Dielectric Materials for Low Operating Voltage Organic Thin-Film Transistors

Over the past few decades, interest in organic materials for electronic applications has been steadily increasing due to their distinct advantages for inexpensive, large area processing. While vast attention has been paid to organic materials as the active semiconductor layer, modest consideration has been given to the gate-dielectric material. Some of the earlier reports have shown great improvements in device performance using polymer blends as the dielectric layer. Furthermore, low operating voltage devices have been realized with thin dielectric layers consisting of cross-linked networks of polymers in an alkylsilane matrix. In our research, we show alternative materials and methods for cross-linking polymer layers integrated into organic thin-film transistors and continuing work toward low operating voltage devices.

Various strategies are investigated to achieve a cross-linked polymer dielectric layer. In the most general scheme, poly(vinylphenol) is mixed with a variety of cross-linking agents and additives and spin-coated onto either silicon or plastic substrates. The layers and then cured to promote cross-linking and remove any additional solvent. To ensure cross-linking with the polymer and not the formation of an interdigitated network, the cross-linking agents chosen for this study do not react with each other. After post bake, the degree of cross-linking is determined using Fourier transform infrared spectroscopy. Capacitance and thickness measurements are also performed to fully characterize the insulating layer over a range of thicknesses controlled by spin rate.

The utility of the cross-linked dielectric layer is evaluated using many common organic semiconductors, such as pentacene, perfluorinated copper phthalocyanine, and alkyl-substituted fluorene-thiophene oligomers. To gauge the electronic performance, top-contact devices are fabricated with thermally evaporated films with gold electrodes patterned through a shadow mask. The performance of the devices is referenced to those fabricated on OTS treated silicon oxide substrates. Devices fabricated with thin polymer dielectric layers demonstrate ideal electric characteristics for operating voltages below 1V for the semiconductor materials tested.