Flexible Organic Thin-film Transistors Research Articles

Megahertz operation of flexible low-voltage organic thin-film transistors

Bottom-gate, top-contact (inverted staggered) organic thin-film transistors with a channel length of 1μm have been fabricated on flexible plastic substrates using the vacuum-deposited small-molecule semiconductor 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT). The transistors have an effective field-effect mobility of 1.2cm2/Vs, an on/off ratio of 107, a width-normalized transconductance of 1.2S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420ns per stage at a supply voltage of 3V. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10V.

Low-voltage flexible pentacene thin film transistors with a solution-processed dielectric and modified copper source–drain electrodes

Low-voltage, flexibility and low-cost are essential prerequisites for large scale application of organic thin film transistors (OTFTs) in future low-end electronics. Here, we demonstrate a low-voltage flexible OTFT by using a low-temperature, solution-processed gate dielectric. Such a dielectric can be well integrated with an Au coated polyimide film, and exhibits a low leakage current density of less than 10−6 A cm−2 and a high capacitance density of 180 nF cm−2. Pentacene films deposited onto the solution-processed dielectric show a highly ordered “thin film phase”. The source–drain (S/D) electrodes are made of in situ modified Cu encapsulated by Au (Au/M-Cu). The obtained flexible OTFT exhibits outstanding electrical characteristics under a gate voltage of only −2 V, which include an on/off ratio of 2 × 104, a mobility (μ) of 1.5 cm2 V−1 s−1, a threshold voltage (VT) of −0.3 V and a subthreshold slope (SS) of 161 mV dec−1. The obtained mobility value is among the highest achieved in flexible pentacene OTFTs. The mechanical flexibility and reliability of the OTFTs are also studied and discussed in detail, and the observed degradation of the device performance under strains is attributed to the damage induced in the electrodes giving rise to increased contact resistance and the phase transition from the thin film phase to bulk phase of the pentacene films.

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.

Flexible spray-coated TIPS-pentacene organic thin-film transistors as ammonia gas sensors

Flexible ammonia (NH3) gas sensors based on solution-processable organic thin-film transistors (OTFTs) are fabricated using a TIPS-pentacene active layer/PMMA dielectric layer on glass and plastic substrates. These OTFT sensors exhibit outstanding NH3 gas response and recovery characteristics under multiple exposure/evacuation cycles at controlled NH3 concentrations.

Inkjet-printed organic thin film transistors based on TIPS pentacene with insulating polymers

The blending of the crystalline organic semiconductor, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), with amorphous polymers exhibits not only excellent solution processability, but also superior performance characteristics in organic thin film transistors (OTFTs). To understand the inkjet-printing behavior of TIPS pentacene/polymer blends, we use amorphous polycarbonate (APC), which is structurally beneficial to the facile phase separation of TIPS pentacene crystals due to the strong segregation strength estimated by the Flory–Huggins interaction parameter. The various inkjet-printing behaviors of TIPS pentacene/APC inks, which depend on the TIPS pentacene/APC compositions, ink viscosities, and different solvent mixtures, are investigated. These behaviors can ultimately determine the phase separation, morphology, shape, and orientation of the TIPS pentacene crystals in OTFT films. Flory–Huggins phase separation theory is applied, and various analytical methods, such as polarized optical microscopy, 3D surface profile, and time-of-flight secondary ion mass spectroscopy (TOF-SIMS), are utilized to explain these relationships. By controlling these inkjet-printing conditions, it is possible to easily regulate the optimal inkjet-printing process for TIPS-pentacene/polymer systems, which can derive the desirable stripe-shaped and vertically phase-separated TIPS pentacene crystals with the proper orientation and enhanced surface morphology. The resultant inkjet-printed films from the TIPS pentacene with APC show excellent device stability and an average mobility of 0.53 cm2 V−1 s−1. Furthermore, the inkjet-printed flexible OTFT array with an average mobility of 0.27 cm2 V−1 s−1 sustains the application of TIPS pentacene/APC in the field of flexible printed electronics.

Enhanced Stability of All Solution-Processed Organic Thin-Film Transistors Using Highly Conductive Modified Polymer Electrodes

Enhanced stability of all solution-processed organic thin-film transistors (OTFTs) has been achieved by replacing metallic electrodes with glycerol-modified poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) polymer electrodes. The modified PEDOT:PSS showed a substantially low electrical resistivity of 6.4×10-3 Ω cm with improved environmental stability and water-resistant characteristics, which are crucial for highly reliable applications. Additionally, the modified PEDOT:PSS electrodes were highly stable under intense mechanical stress, allowing their application to flexible electronics. Particularly, all solution-processed flexible and transparent OTFTs with the modified PEDOT:PSS electrodes showed a field-effect mobility decrease of only 2.7% after a tensile mode mechanical fatigue test, while OTFTs with metallic electrodes showed a mobility decrease of 56.6% under identical test conditions.

Morphology transformations of pentacene on an UV-patternable polymer insulator and their effect on performance of flexible organic thin-film transistors

This work presents a low temperature with high resolution capability UV-patternable polymer, i.e. mr-UVCur06, for use as a gate insulator in OTFTs, by investigating the morphology transformation of pentacene deposited on the mr-UVCur06. The device structure is polyethylene terephthalate (PET)/indium-tin oxide (ITO)/mr-UVCur06/pentacene/Au (source/drain). In addition to its solution-processable capability, mr-UVCur06 is directly patterned by UV light in a low-temperature process. UV/ozone post-treatment of the patternable mr-UVCur06 can illuminate organic contaminants from its surface and increases surface energy. Experimental results indicate that a high surface energy existing at the mr-UVCur06 surface has produced in a larger ratio of Ithin film phase/Itriclinic bulk phase in pentacene. Then, the distance of pentacene molecular crystal structure, which is arranged in the thin film phase, is shorter than that in triclinic bulk phase. The performance of pentacene-based OTFTs can be enhanced with few contaminants and a high surface energy on the UV-patternable gate insulator. By performing UV/ozone post-treatment on the mr-UVCur06 insulator surface for 60s, the OTFTs demonstrate a mobility, on/off drain current ratio, and VT of 0.34cm2/Vs, 5.5×104, and 2.5V, respectively. Furthermore, the low-temperature photopatternable polymer dielectric paves the way for a relatively easy and low-cost fabrication of an OTFT array without expensive and complicated photolithography and dry etching.Morphology transformations of pentacene on an UV-patternable polymer insulator and their effect on performance of flexible organic thin-film transistors

Flexible Low-Voltage Polymer Thin-Film Transistors Using Supercritical CO2-Deposited ZrO2 Dielectrics

The fabrication of low-voltage flexible organic thin film transistors using zirconia (ZrO(2)) dielectric layers prepared via supercritical fluid deposition was studied. Continuous, single-phase films of approximately 30 nm thick ZrO(2) were grown on polyimide (PI)/aluminum (Al) substrates at 250 °C via hydrolysis of tetrakis(2,2,6,6-tetramethyl-3,5-heptane-dionato) zirconium in supercritical carbon dioxide. This dielectric layer showed a high areal capacitance of 317 nF cm(-2) at 1 kHz and a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3 V. By using poly(3-hexylthiophene) (P3HT) as a semiconductor, we have fabricated flexible thin film transistors operating at V(DS) = -0.5 V and V(G) in a range from 0.5 V to -4 V, with on/off ratios on the order of 1 × 10(3) and mobility values higher than 0.1 cm(2)/(V s).

Low power flexible organic thin film transistors with amorphous Ba0.7Sr0.3TiO3 gate dielectric grown by pulsed laser deposition at low temperature

We deposited amorphous Ba0.7Sr0.3TiO3 (BST) on silicon and plastic substrate under 110°C by pulsed laser deposition (PLD) and use it as the dielectric of the organic transistor. Depends on the thickness of BST layer, the highest mobility of the devices can achieve 1.24cm2V−1s−1 and 1.01cm2V−1s−1 on the silicon and polyethylene naphthalate (PEN) substrate, respectively. We also studied the upward and downward bending tests on the transistors and the dielectric thin films. We found that the BST dielectric pentacene transistor can maintain the mobility at 0.5cm2V−1s−1 or higher while the bending radius is around 3mm in both upward and downward bending. Our finding demonstrates the potential application of PLD growth high-k dielectric in the large area organic electronics devices.

Polypyrrole top-contact electrodes patterned by inkjet printing assisted vapor deposition polymerization in flexible organic thin-film transistors

Highly conductive polymer, polypyrrole (PPy) was successfully patterned as source and drain (S/D) electrodes for flexible pentacene thin film transistors in top-contact structure by combining inkjet printing and vapor deposition polymerization. Facile inkjet printing of initiator and subsequent exposure of pyrrole monomers resulted in selective absorption and polymerization of pyrrole monomers on the patterned initiator region. Pentacene transistors based on printed PPy electrodes exhibited higher electrical characteristics than that of the devices with thermally evaporated Au electrodes. Improved performance of the devices based on PPy electrodes could be attributed to the reduction of contact resistance at the interface between polymer and organic semiconductor. For the replacement of metal electrodes, vapor deposition polymerization assisted inkjet printing technique can provide a versatile method to utilize highly conductive polymer as a functional electrode of flexible organic electronic devices.

The application of a high-k polymer in flexible low-voltage organic thin-film transistors

Although much progress has been made in the study of high-mobility organic semiconductors in recent years, one major challenge for organic thin film transistors (OTFTs) in real applications is the relatively high operating voltage, which is mainly related to the gate dielectric of the OTFT. Here we show the application of a high-k poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) as the gate dielectric of flexible low-voltage pentacene OTFTs for the first time. The performance of the OTFTs is optimized by modifying the surface of P(VDF-TrFE-CFE) films with various thin polymer films by a solution process. The flexible OTFTs show excellent performance at the operating voltage of 4 V and good stability after 1000 bending tests. It is expected that P(VDF-TrFE-CFE) is a suitable gate dielectric for low-voltage OTFTs based on various small-molecule organic semiconductors because P(VDF-TrFE-CFE) terpolymer films can be easily modified with different thin polymer films by a solution process.

Polymer–nanoparticle hybrid dielectrics for flexible transistors and inverters

We report solution processed high-k dielectric films with excellent insulating quality (leakage current densities ∼10−8 A cm−2 at 2 MV cm−1), tunable dielectric constant (4.7–6.7) and high flexibility. The effects of incorporating different concentrations of barium zirconate (BZ) nanoparticles into poly-4-vinylphenol (PVP) gate dielectrics for flexible organic thin film transistors (OTFTs) were systematically studied. The effect of dielectric composition on the mobilities (μ), threshold voltages (VT), on/off ratios and subthreshold swings (S) has been discussed. The morphologies of dielectric and semiconductor films were investigated and shown to correlate closely with device performances. Two kinds of unipolar inverters based on these transistors have been fabricated and characterized. The depleted load inverter showed large signal gain and sharp switching. The electrical properties under various compressive and tensile strains evidenced the high mechanical stability of the dielectric films, hence the transistors and inverters. These findings will be of value for printable electronic devices on flexible substrates.

Vacuum thermally evaporated polymeric zinc acrylate as an organic interlayer of organic/inorganic multilayer passivation for flexible organic thin-film transistors

Polymeric zinc acrylate (pZA) was introduced as an organic interlayer for the inorganic/organic multilayer passivation of flexible organic thin film transistors (OTFTs). The pZA film was deposited by thermally evaporating a zinc diacrylate (ZDA) monomer under high vacuum (<10−5 Torr) and applying UV irradiation. The conversion of ZDA into a polymeric phase was confirmed by FTIR analysis, breakdown voltage measurements and the photopatternability of the film before and after UV irradiation. Vacuum-thermally evaporated pZA film showed good surface smoothness and a high permeation activation energy (53 kJ mol−1) compared to conventional polymeric films. As an interlayer for multilayer passivation, vacuum-thermally evaporated silicon monoxide (SiO) was introduced as the inorganic counterpart of the pZA interlayer. A multilayer film comprising 6.5 pairs of layers showed a water vapor transmission rate (WVTR) of 0.055 g m−2 per day, a 25-fold improvement over the WVTR of a single SiO film (1.207 g m−2 per day). OTFTs encapsulated with 6.5 pairs of polymeric zinc acrylate/silicon monoxide layers showed prolonged stability over 97 days. In addition, the passivation layer did not show crack formation, and sustained barrier characteristics, even after 500 bending tests.

Flexible low-voltage organic thin-film transistors and circuits based on C10-DNTT

Using the recently developed organic semiconductor, 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT), we have fabricated organic thin-film transistors and ring oscillators on flexible polymeric substrates. By utilizing a gate dielectric with a small thickness (5.3 nm) and a large capacitance (800 nF cm−2), the transistors can be operated with relatively low voltages of about 2 to 3 V. To improve the charge exchange between the organic semiconductor and the gold source and drain contacts, a thin layer of a non-alkylated organic semiconductor (DNTT) sandwiched between two thin layers of a strong organic dopant (NDP-9) was inserted between the C10-DNTT and the gold contacts. The optimized transistors have a field-effect mobility of 4.3 cm2 V−1 s−1, an on/off current ratio of 108, and a subthreshold swing of 68 mV per decade. The ring oscillators have a signal propagation delay of 5 μs per stage at a supply voltage of 3 V, making these the fastest organic circuits at supply voltages below 7 V reported to date.

Printed flexible OTFT backplane for electrophoretic displays

Printing technologies were applied to fabricate a flexible organic thin-film transistor (OTFT) backplane for electrophoretic displays (EPDs). Various printing processes were adopted to maximize the figures of each layer of OTFT: screen printing combined with reverse offset printing for the gate electrodes and scan bus lines with Ag ink, inkjet for the source/drain electrodes with glycerol-doped Poly (3,4-ethylenedioxythiophene): Poly (styrenesulfonate) (PEDOT:PSS), inkjet for the semiconductor layer with Triisopropylsilylethynyl (TIPS)-pentacene, and screen printing for the pixel electrodes with Ag paste. A mobility of 0.44 cm2/V s was obtained, with an average standard deviation of 20%, from the 36 OTFTs taken from different backplane locations, which indicates high uniformity. An EPD laminated on an OTFT backplane with 190×152 pixels on an 8-in panel was successfully operated by displaying some patterns.

Reliability of OTFTs on flexible substrate: mechanical stress effect

Flexibility will significantly expand the application scope of electronics, particularly large-area electronics. Over the last 10 years, printed organic electronic is believed to be one of the next major technological breakthroughs in the field of microelectronic and the use of printing technology to process organic field-effect transistors (OFETs) opens promising perspectives for low-cost, large-area circuits integrated on flexible, plastic substrates. With amorphous polymer-based thin film transistors acceptable electrical performances are now achieved with relatively good stability at ambient air. In the literature a lot of work has been devoted to study degradation of device characteristics under bias stress conditions but only few papers deal with the mechanical behavior. In this paper, we review our first reliability results obtained on flexible organic thin film transistors under mechanical stresses. The variations of electrical characteristics under bending tests, both in compression and tension, have been studied. Using specific equipment, we have also evaluated the reliability of transistors under cyclic bending tests. The stress dependency of the transfer characteristic deviates from the one observed for inorganic material like silicon.

Influence of mechanical strain on the electrical properties of flexible organic thin-film transistors

In this study, we systematically investigated the bending effect on the electrical properties of flexible organic thin-film transistors (OTFTs) fabricated on stainless steel substrates. We found that the compressive strain resulted in an increased mobility, while the tensile strain degraded the electrical performance. We further used a transfer line method to extract the channel and parasitic resistances under either compressive or tensile strain. The results indicated that the parasitic resistance increased apparently under the tensile bending condition, which probably could be attributed to the damage of the source/drain contacts. Additionally, we deduced that mechanical strains influence the energy barrier height between the grains of pentacene thin films, thereby resulting in the variation of channel resistances. Overall, the flexible OTFTs fabricated on the metal foils exhibited high mechanical flexibility and stability.

Transparent organic thin-film transistors and nonvolatile memory devices fabricated on flexible plastic substrates

Transparent and flexible organic thin-film transistors (OTFTs) and nonvolatile memory devices were developed. The memory device is based on OTFTs by incorporating self-assembled gold nanoparticles in the organic dielectric layers as the charge storage elements. The organic memory devices exhibited good programmable memory properties with respect to the program/erase operations, resulting in controllable and reliable threshold voltage shifts by hole trapping/detrapping in the gold nanoparticles. The organic memory devices could be electrically programmed and erased, leading to a large memory window of around 15 V. Additionally, the long term data retention (>105 s) and the stable cyclic bending tests (>2000 cycles) confirmed that the flexible memory devices exhibited good electrical reliability as well as mechanical stability. The organic memory devices were composed of a solution-processed organic dielectric layer/metallic nanoparticles and a low-temperature processed organic semiconductor layer. Additionally, transparent indium-tin oxide (ITO) deposited at room temperature was used for the gate and source–drain electrodes to fabricate transparent memory devices. Therefore, this approach could potentially be applied to advanced transparent and flexible electronic devices as well as integrated organic device circuits.

Flexible organic thin-film transistors using single-walled carbon nanotubes as an activated channel

We report on the fabrication of organic thin-film transistors (OTFTs) with a spun cross linked poly-4-vinylphenol (PVP) dielectric on a polyethersulphone (PES) flexible substrate. To improve the electrical performance of OTFTs, we employed a random single-walled carbon nanotubes (SWNTs) network as a carrier transfer underlay without sacrificing the flexibility of the TFTs. The random SWNTs showed that they can act as a semiconducting channel and conduction path to shorten the channel length in our TFTs. The flexible thin-film transistors (TFTs) with a random SWNTs/pentacene bilayer as an active channel exhibited an improved saturation field effect mobility ( µ sat ) of 2.6 × 10 − 1 cm 2/Vs compared to that of TFTs without the SWNTs underlay, while creating only a minor reduction of the current on/off ratio.

Double Self-Coated Benzimidazole–Copper Complex Structure as Gate Insulator for Flexible Organic Thin Film Transistors

A double-thin-film structure of self-coated benzimidazole-copper complex (SCBCC) is utilized as the gate insulator for pentacene organic thin film transistors (OTFTs). The SCBCC double thin film is composed of a 2-heptyl benzimidazole-copper complex and a 2-(naphthalene-2-ylmethyl)benzimidazole-copper complex. The fabrication features of the SCBCC double-thin-film are self-coating, water-based processing, selectivity of deposition, and short time. The SCBCC double-thin-film insulator OTFT exhibits better device performance than the SCBCC single-thin-film insulator OTFT made with the 2-heptyl benzimidazole-copper complex. The field-effect mobility, on/off current ratio, and subthreshold swing of the SCBCC double-thin-film insulator OTFT are determined to be ∼0.21 cm 2 V -1 s -1 , ∼ 1.2 × 10 4 , and ∼1.3 V/dec, respectively.