Polymer Thin-film Transistors Research Articles

Phosphonic acid self-assembled monolayer and amorphous hafnium oxide hybrid dielectric for high performance polymer thin film transistors on plastic substrates

A vacuum-free solution processed hybrid dielectric composed of an n-octadecyl-phosphonic acid self-assembled monolayer on amorphous sol-gel processed hafnium oxide (HfOx) is demonstrated for low-voltage polymer semiconductor-based thin film transistors (TFTs). The phosphonic acid/HfOx hybrid dielectric provides high capacitance (0.41 μF/cm2), low leakage current (5×10−8 A/cm2), and is compatible with plastic substrates. The utility of this dielectric is demonstrated by fabricating high performance polymer TFTs based on a spin coated thiophene-thiazolothiazole copolymer with operating voltages under −2 V, negligible hysteresis, subthreshold slopes as low as 100 mV/dec, and hole mobilities up to 0.11 cm2 V s.

Stability of PMMA on P3HT PTFTs under stress

In this paper we study the stability under stress of polymeric thin film transistors, PTFTs, fabricated with poly(methyl methacrylate) PMMA on poly(3-hexylthiophene) P3HT as dielectric and semiconductor layers, respectively. Devices were tested during and after gate bias stress combined with drain bias. When the stress is done at low drain bias, V T shift in linear transfer curves remained below 1 V. When both high gate and drain bias are applied during stress, linear transfer curves shifted less than 5 V. In both cases hysteresis was less than 1 V. Transfer curves for V DS = −30 V after stress showed deformation in addition to shift. Variation of I DS at V DS = −30 V during both types of stress remained always below 6%. Mechanisms responsible for the observed behavior are discussed.

Ion Gel-Gated Polymer Thin-Film Transistors: Operating Mechanism and Characterization of Gate Dielectric Capacitance, Switching Speed, and Stability

We report comprehensive characterization of electrolyte-gated polymer thin-film transistors (TFTs) incorporating solution processable polymer semiconductors and high capacitance “ion gel” gate dielectrics. The ion gel dielectrics comprise self-assembled networks of triblock copolymers such as poly(styrene-b-methylmethacrylate-b-styrene) [PS-PMMA-PS] that are swollen with ionic liquids, e.g., (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The capacitance of the gels is exceptionally large (>10 μF/cm2 at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gel-gated organic TFTs (GEL-OTFTs) at very low voltages (< 2.5 V). Gate-induced hole densities in GEL-OTFTs employing different polythiophene semiconductors in the channel are on the order of 1014 carriers/cm2, with associated saturation hole mobilities that are also remarkably large, ∼1 cm2/(V s), likely because of the large gate-induced carrier densities. Examination of the ...

All-polymer thin film transistors on patterned elastomeric substrates

A patterned soft elastomeric substrate is utilized for fabricating an all-polymer thin film transistor (TFT). With a polymer solution for the source/drain electrodes, it is difficult to form a well defined narrow channel. The problem is resolved with the aid of a micromolding technique and the patterned substrate. When the all polymer TFT is subjected to bending, the polymer gate dielectric is the layer that experiences the most stress. This stress problem, which can lead to device failure, can be relieved to an extent by introducing a small fraction of ethyl cellulose to the polymer dielectric. The effects of the ethyl cellulose content on the device performance are not significant.

Effect of HAuCl[sub 4] Doping on the Contact Properties of Polymer Thin-Film Transistors

We show that the electrical properties of polymer thin-film transistors can be enhanced by doping poly(3-hexylthiophene) (P3HT) with HAuCl 4 . Specifically, the addition of HAuCl 4 causes an increase in the two-dimensional molecular ordering of P3HT and a remarkable reduction in the contact resistance at the electrode/semiconductor interface with no pre- or post-treatment process. This phenomenon is understood in terms of broadening of the transport manifold in the organic semiconductor, induced by HAuCl 4 , which results in a reduction in the hole-injection barrier and an enhancement of the interfacial stability at the contact between the printed electrode and the semiconductor layers.

A comparative study of plasma-enhanced chemical vapor gate dielectrics for solution-processed polymer thin-film transistor circuit integration

This paper considers plasma-enhanced chemical vapor deposited (PECVD) silicon nitride (SiNx) and silicon oxide (SiOx) as gate dielectrics for organic thin-film transistors (OTFTs), with solution-processed poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12) as the active semiconductor layer. We examine transistors with SiNx films of varying composition deposited at 300 °C as well as 150 °C for plastic compatibility. The transistors show over 100% (two times) improvement in field-effect mobility as the silicon content in SiNx increases, with mobility (μFE) up to 0.14 cm2/V s and on/off current ratio (ION/IOFF) of 108. With PECVD SiOx gate dielectric, preliminary devices exhibit a μFE of 0.4 cm2/V s and ION/IOFF of 108. PQT-12 OTFTs with PECVD SiNx and SiOx gate dielectrics on flexible plastic substrates are also presented. These results demonstrate the viability of using PECVD SiNx and SiOx as gate dielectrics for OTFT circuit integration, where the low temperature and large area deposition capabilities of PECVD films are highly amenable to integration of OTFT circuits targeted for flexible and lightweight applications.

Improved upper contacts PMMA on P3HT PTFTS using photolithographic processes

In this work we present a new polymeric thin film transistor (PTFT), fabrication process where standard photolithographic and dry etching techniques are used to place gate contact on the upper surface of the device, and to access in an easy and reliable way the buried bottom drain and source contacts from the upper surface of the device. This fabrication technique benefits of all the advantages that photolithographic processes report, as device isolation, patterning of small dimensions and possibility of device integration on both rigid and flexible substrates. We also show that, since PMMA is placed as the outermost polymeric layer, it serves both as dielectric and as protective layer, allowing devices to work in ambient conditions with significantly less degradation than other PTFTs structures previously reported. Electrical characterization of the devices showed that this technological approach provides mobility values an order of magnitude higher than those previously reported by us, using the same polymeric layers, but with buried drain and source contacts.

Inkjet-printed polymer thin-film transistors: Enhancing performances by contact resistances engineering

In this paper, we demonstrate how to enhance polymer thin-film transistors (PTFTs) performances made by low-cost inkjet printing technique. Indeed, in PTFTs, contact resistances between semiconducting conjugated polymers (SCPs) and Source and Drain (S&D) contacts may dominate the transport properties of such electronic devices. Here, we report measurements of these parasitic resistances for several couples of (i) SCPs, as active material, and (ii) electrodes, as S&D contacts, in bottom-contact inkjetted PTFTs. The differences in PTFT performances are discussed upon these contact resistance. For this, we evaluate the performances of several inkjetted couples of SCP/S&D compared to devices with evaporated metal-based S&D. By this way, we show that inkjet printing is a suitable low-cost technique to dispense polymers and inorganic nanoparticles for direct-writing of PTFTs. A significant reduction in the contact resistance R C was achieved when inkjetted Pedot:Pss-based S&D electrodes are used instead of evaporated metal-based S&D electrodes. The improved efficiency of charge carrier injection is assumed to be due to the formation of a p-doped interfacial layer at the interface between the SCP and the S&D electrodes. All these results pave the way towards flexible electronics applications by using inkjetted polymers both for electrodes and semiconducting active layer on flexible plastic substrate.

Polymer thin film transistor without surface pretreatment on silicon nitride gate dielectric

It is well known that surface modification of the gate dielectric in organic thin film transistors (TFTs) plays an important role in device performance, often giving rise to severalfold improvements in field-effect mobility. This paper reports on solution-processed polymer TFTs with mobilities comparable to high performance counterparts despite the absence of dielectric surface pretreatment. An effective mobility of 0.1cm2∕Vs was obtained with poly(2,5-bis(3-dodecylthiophene-2-yl)thieno[3,2-b]thiophene) transistors on silicon nitride gate dielectric. The results indicate that by judicious preparation of the device layers, one can mitigate the need for dielectric surface pretreatment, thereby reducing fabrication complexity without compromising TFT performance.

Contact effects and extraction of intrinsic parameters in poly(3-alkylthiophene) thin film field-effect transistors

We report on contact effects in polymeric thin film transistors based on poly(3-octylthiophene) and poly(3-hexadecylthiophene) with gold contact electrodes and in the bottom contact configuration. A method to extract the intrinsic channel mobility from the measured extrinsic mobility over a broad range of gate voltage is presented. This method uses the I-V characteristics of the transistor in its reverse mode operation. The results show that the intrinsic mobility in the channel is gate voltage dependent and increases almost linearly with voltages at biases above the threshold voltage. By applying a model based on the theory of space-charge-limited conduction, the dependence of the threshold voltage on the contacts and the shifts observed in this parameter with different polymer film thicknesses are explained. We also apply this model to explain the effects of light in reducing the contact effects and changing the device parameters from extrinsic in the dark to intrinsic under illumination.

Polymer Thin-Film Transistors with Chemically Modified Dielectric Interfaces by Atmospheric-Pressure Plasma Technology

Poly-3-hexylthiophene (P3HT) is coated on a chemically modified SiO2 gate dielectric layer as an active layer to fabricate organic thin-film transistors (OTFTs). This study uses a new atmospheric-pressure plasma (APP) approach to treat SiO2 surfaces, and investigates its effect on the electric characteristics of OTFTs. The APP process can be performed at low temperature (<100ºC) and atmospheric pressure, so it is very suitable for use on a plastic substrate. After the SiO2 surface has been modified by the APP process with hexamethyldisilazane (HMDS), it exhibits typical I-V characteristics of TFTs. Calculations reveal that its field effect mobility can reach 0.02-0.03 cm2/Vs, which is about 15 times higher than that before the modification, and the threshold voltage is below -10V. The performance is even better than that obtained following the usual surface treatment of the SiO2 surface by spin-coating or evaporation.

Characterization of semiconducting polymers for thin film transistors

A review of characterization methods used to study semiconducting polymers in thin film transistors is presented. The basic operation of polymeric thin film transistors (TFTs) is described. Methods for microstructural characterization of thin films of semiconducting polymers such as x-ray scattering are discussed. Examples of how these methods have been applied to study structural ordering in semicrystalline polymers such as poly(3-hexylthiophene) are presented. The electronic structure of these materials is important for understanding the operation of TFTs and has been studied using ultraviolet photoelectronic spectroscopy, optical spectroscopy, and electrochemical methods. Instabilities of the operation of TFTs have been examined using time-dependent current-voltage measurements, charge modulated spectroscopies, and scanning probe methods. These methods have revealed the nature of charge carriers and trap states in semiconducting polymers.

High work function materials for source/drain contacts in printed polymer thin film transistors

Studies of materials for source-drain electrodes in ink-jet printed polymer-based thin film transistors (TFTs) are reported. Two systems are studied: a blend of Ag nanoparticles with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and an ethylene glycol-doped PEDOT:PSS solution (modified-PEDOT). The semiconductor used is the polythiophene derivative poly [5,5′-bis(3-dodecyl-2-thienyl)-2,2,2′-bithiophene]. PEDOT:Ag blends and modified-PEDOT yield TFTs with mobilities around 10−2 and 10−3cm2∕Vs, respectively, subthreshold slopes around 1.6V/decade and on-to-off current ratios of 106–107. Both systems show considerable improvement over printed TFTs with Ag nanoparticle source-drain electrodes. Results on film resistivity and morphology are discussed along with device characteristic analysis.

Erratum: Downscaling of self-aligned, all-printed polymer thin-film transistors

Nature Nanotechnology 2, 784–789 (2007). In figure 2d, the units for the drain current were incorrectly given in amperes (A). The correct units should be microamperes (μA).

Photo-response of polymer thin-film transistors based on poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene)

Polymer thin-film transistors (PTFT) and resistors based on poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) are fabricated by spin-coating process, and photo-sensing characteristics of the devices are investigated in visible light. Upon illumination, a significant increase in the drain current of the PTFT is observed with a maximum photosensitivity of 198 in the subthreshold operation mode at an illumination intensity of 1200 lux. On the other hand, the photosensitivity of the thin-film resistor is only 18.5 under the same illumination intensity for an applied voltage of 2 V, indicating that there is a photocurrent amplification effect in the PTFT. It is found that the illumination effectively decreases the threshold voltage of the PTFT, but hardly affects its field-effect mobility. The mechanisms for the photocurrent formation in the thin-film resistor and for the photocurrent amplification in the PTFT are discussed.

Downscaling of self-aligned, all-printed polymer thin-film transistors

Printing is an emerging approach for low-cost, large-area manufacturing of electronic circuits, but it has the disadvantages of poor resolution, large overlap capacitances, and film thickness limitations, resulting in slow circuit speeds and high operating voltages. Here, we demonstrate a self-aligned printing approach that allows downscaling of printed organic thin-film transistors to channel lengths of 100-400 nm. The use of a crosslinkable polymer gate dielectric with 30-50 nm thickness ensures that basic scaling requirements are fulfilled and that operating voltages are below 5 V. The device architecture minimizes contact resistance effects, enabling clean scaling of transistor current with channel length. A self-aligned gate configuration minimizes parasitic overlap capacitance to values as low as 0.2-0.6 pF mm(-1), and allows transition frequencies of fT = 1.6 MHz to be reached. Our self-aligned process provides a way to improve the performance of printed organic transistor circuits by downscaling, while remaining compatible with the requirements of large-area, flexible electronics manufacturing.

Solution Based Self‐Assembly of an Array of Polymeric Thin‐Film Transistors

Spontaneous self-organization of different materials from solution into an array of isolated and encapsulated thin-film-transistors (TFTs) is induced. We demonstrate the self-assembly of a 64 polymer TFT array with a yield higher than 95%. This work constitutes a first step towards self-assembly of multi-layer, multi-component circuits out of solution.

Synthesis and characterization of poly(benzodithiophene) derivative for organic thin film transistors

AbstractPoly{2,6‐bis(3‐dodecylthiophen‐2‐yl) benzo[1,2‐b;4,5‐b′]dithiophene} (PTBT) was synthesized, via oxidative polymerization by oxidative agent (FeCl3). The mole ratio of FeCl3 and monomer (3.5:1), and keeping low temperature during the dropping of diluted catalyst were very important for the polymerization without crosslinking. The PTBT was confirmed by 1H NMR, FTIR spectra, and elemental analysis. The PTBT has very good solubility in organic solvents such as chloroform, tetrahydrofuran, etc, and good thermal stability with Tg of 164 °C. The PTBT shows UV‐optical absorption at 406 nm and photoluminescence (PL) spectroscopy at 504 nm in a film. The highest occupied molecular orbital (HOMO) energy of the polymer is −5.71 eV by measuring cyclic voltammetry (CV). A solution‐processed polymer thin film transistor device shows a mobility of 3 × 10−5 – 8 × 10−5 cm2 V−1 s−1, and an on/off current ratio of 104. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5277–5284, 2007

Stability of polymeric thin film transistors for x-ray imaging applications

The effects of x-ray irradiation on the electrical performance of organic thin film transistors (OTFTs) have been examined up to equivalent lifetime doses for medical digital x-ray imaging backplanes. Transistor performance metrics have been assessed before and after exposures up to 500Gy for bottom-gate, bottom-contact OTFTs of p-channel polytriarylamine and polyfluorene semiconductors. There is no discernible degradation in device performance for either material and no indication of the creation of additional trap states. In situ measurements indicate that x-ray exposure tends to reverse undesirable shifts in the threshold voltage due to prolonged atmospheric exposure and bias stress.

Polymer Thin-Film Transistors with Chemically Modified Dielectric Interfaces by Atmospheric-Pressure Plasma Technology.

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