Optimized circuit design for flexible 8-bit RFID transponders with active layer of ink-jet printed small molecule semiconductors

We ink-jet print a blend of 6,13-bis(triisopropyl-silylethynyl)pentacene and polystyrene as the active layer for flexible circuits. The discrete ink-jet printed transistors exhibit a saturation mobility of 0.5 cm2 V -1 s-1. The relative spread in transistor characteristics can be very large. This spread is due to the morphology of the TIPS-PEN crystals, and is difficult to control using inkjet printing. Here we apply a novel circuitry design to minimize the probability of differences between the transistors. We include up to four transistors under the same ink-jet printed deposit. With the new design, we demonstrate unipolar circuitry building blocks such as inverters, NANDs and 19-stage ring-oscillators. Additionally, we integrate almost 300 TFTs, on a surface area of 34 mm2 of plastic foil, for 8-bit RFID transponder circuits. © 2012 Elsevier B.V. All rights reserved

A common gate thin film transistor on poly(ethylene naphthalate) foil using step-and-flash imprint lithography

In this paper the fabrication of flexible thin film transistors (TFTs) on poly(ethylene naphthalate) foil is reported, with the source-drain layer patterned by step-and-flash imprint lithography (SFIL) as a first step towards fully UV-imprinted TFTs. The semiconductor was deposited by inkjet printing of a blend of TIPS-pentacene/polystyrene. The bottom contact, bottom gate TFTs were fabricated with the foil reversibly glued to a carrier, enhancing the dimensional stability and flatness of the foil to result in a thinner and more homogeneously distributed residual layer thickness. The obtained performance of the TFT devices, showing a mobility of μ = 0.56 cm2 V-1 s-1 with an on/off ratio of >107 and near-zero threshold voltage, was found to be in good agreement with similar, photolithographically patterned state-of-the-art devices recently reported in literature. The results presented here show the feasibility of SFIL as a roll-to-roll compatible and down scalable patterning technique on flexible PEN foil for the fabrication of bottom-gate, bottom-contact flexible high-quality TFTs

Electric field confinement effect on charge transport in organic field-effect transistors

While it is known that the charge-carrier mobility in organic semiconductors is only weakly dependent on the electric field at low fields, the experimental mobility in organic field-effect transistors using silylethynyl-substituted pentacene is found to be surprisingly field dependent at low source-drain fields. Corroborated by scanning Kelvin probe measurements, we explain this observation by the severe difference between local conductivities within grains and at grain boundaries. Redistribution of accumulated charges creates very strong local lateral fields in the latter regions. We further confirm this picture by verifying that the charge mobility in channels having no grain boundaries, made from the same organic semiconductor, is not significantly field dependent. We show that our model allows us to quantitatively model the source-drain field dependence of the mobility in polycrystalline organic transistors