High work function materials for source/drain electrodes in printed polymer TFTs
Part of the challenge of fully printing polymer-based TFT arrays is finding suitable inks for the source and drain electrodes. The source and drain are in direct contact with the semiconducting polymer and a high work function ink is required for charge injection and minimal contact resistance at that interface. In addition to this, the ink must be homogeneous, compatible with the printer nozzle, and highly conductive. Gold nanoparticle inks satisfy these requirements; however, lower cost alternates are preferred. In this study we consider the use of Pedot:PSS (a high work-function polymer) in combination with Ag nanoparticles in bilayer structures and blends as an alternative material for source and drain of polymer-based TFTs. We also tested the use of a Pedot:PSS solution modified by the addition of conductivity enhancers such as Ethylene Glycol and poly(vinyl) pyrrolidone. These materials were tested in a TFT structure with (poly[5,5 -bis(3-dodecyl-2-thienyl)-2,2 -bithiophene] (PQT-12) as the semiconducting layer. TFTs with Ag:Pedot bilayers and the modified Pedot as source and drain showed very low contact resistance and transistor characteristics comparable to those of Au electrode devices. TFT mobilities of 4x10-3cm2/Vs, ON/OFF ratios of 106, and subtreshold slopes of 2.7V/decade were achieved at drain voltages of 30V. Although yielding good transistor characteristics (with mobilities of 10-2cm2/Vs and ON/OFF ratios of 107), full sintering of the Ag nanoparticles in the blends was hindered by the PSS in the Pedot:PSS solution. We were not able to achieve the high conductivity of pure Ag nanoparticles films when using the blend. Film morphology and printing conditions will also be discussed.
Sholin, V. ; Street, R. A. ; Endicott, F .; Arias, A. C. High work function materials for source/drain electrodes in printed polymer TFTs. 2007 MRS Spring Meeting; 2007 April 9-13; San Francisco; CA.