Low-voltage Organic Thin-film Transistors Research Articles

Electrical Characteristics of Organic Thin-Film Transistors with Polystyrene/Sol-Gel Derived Titania Composite Insulator

In this study, a sol-gel process is introduced to fabricate the composite film of polystyrene and titania, using titanium tetraisoproxide as a titania precursor. Pentacene-based organic thin-film transistor (OTFTs) with the composite film as a gate insulator operates at a low voltage below −4 V and exhibits high performances, such as threshold voltage of −0.9 V, subthreshold slope of 0.2 V/decade, and mobility of 4.7 cm2/Vs, which are mainly attributed to a high dielectric property of the fabricated composite insulator. These results demonstrate that sol-gel derived composite insulators are essentially promising for low-voltage OTFTs with high performances.

Dielectric Surface-Controlled Low-Voltage Organic Transistors via n-Alkyl Phosphonic Acid Self-Assembled Monolayers on High-k Metal Oxide

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO(2)) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO(2) with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO(2) hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 x 10(6), threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec(-1), and field-effect mobilities as high as 1.8 cm(2) V(-1) s(-1).

Temperature-dependent ultra-thin polymer layer for low voltage organic thin-film transistors

In this article, a low voltage organic thin-film transistor (OTFT) was accomplished by using a random copolymer, poly(styrene-co-methyl methacrylate) (PS-r-PMMA), as the gate dielectric. The thickness of PS-r-PMMA polymer can be controlled well by thermal process and shows an ultra-thin property after toluene rinse. The thickness of PS-r-PMMA can be controlled in the range of 2–14 nm. By the densely packed copolymer brush, a leakage current as low as 10 −9 A/cm 2 was obtained. By utilizing this polymer as gate dielectric, pentacene based organic thin-film transistor could be operated at 5 V with 0.1 cm 2/V s mobility and 0.27 V/dec subthreshold swing. In addition to good OTFT properties, the coating, annealing and removing sequential process of PS-r-PMMA ensure that this technique is compatible with the large area printing methods such as ink-jet printing and doctor blade coating. The random copolymer dielectric is therefore suitable for OTFT in large area printed flexible electronic applications.

Fluoroalkylphosphonic acid self-assembled monolayer gate dielectrics for threshold-voltage control in low-voltage organic thin-film transistors

An important prerequisite for the design of digital integrated circuits is the ability to control the threshold voltage of the individual transistors during manufacturing. To address the problem of controlling the threshold voltage of low-voltage organic transistors we have synthesized a fluoroalkylphosphonic acid that forms self-assembled monolayers on patterned, plasma-oxidized aluminum gate electrodes for use as high-capacitance, low-temperature gate dielectrics in p-channel and n-channel organic transistors. Compared with alkyl phosphonic acid-based monolayers, the strong electron-withdrawing character of the fluoroalkyl monolayers causes a change in the threshold voltage of the transistors by about 1 V, i.e. almost half of the supply voltage.

Low-voltage high-performance organic thin film transistors with a thermally annealed polystyrene/hafnium oxide dielectric

Low-voltage pentacene-based organic thin film transistors (OTFTs) are demonstrated with polystyrene (PS)/hafnium oxide (HfOx) hybrid dielectrics. Thermal annealing of PS films on HfOx at 120 °C (PS-120) induces a flatter orientation of the phenyl groups (tilt angle 65°) at the surface compared to PS films without annealing (PS-RT) (tilt angle 31°). The flatter phenyl group orientation leads to better matching of surface energy between pentacene and PS. Pentacene deposited on PS-120 display higher quality thin films with larger grain sizes and higher crystallinity. Pentacene OTFTs with PS-120/HfOx hybrid dielectrics can operate at low-voltage (<3 V) with high field-effect mobilities (1 cm2/V s), high on/off current ratios (106), and low subthreshold slopes (100 mV/dec).

Effects of the alkyl chain length in phosphonic acid self-assembled monolayer gate dielectrics on the performance and stability of low-voltage organic thin-film transistors

We have fabricated pentacene organic thin-film transistors (TFTs) using self-assembled monolayers (SAMs) based on alkyl-phosphonic acids with five different alkyl chain lengths as the gate dielectric and investigated the relationship between the SAM chain length and the electrical performance and stability of the transistors. A SAM chain length of 14 carbon atoms provides a maximum TFT mobility of 0.7 cm2/V s, along with an on/off current ratio greater than 105. We have also investigated the bias stress effect in these TFTs and found that the change in drain current is substantially less severe than in pentacene TFTs with polymer gate dielectrics.

Low-Voltage and Hysteresis-Free N-Type Organic Thin Film Transistor and Complementary Inverter with Bilayer Gate Insulator

In this paper, we report on a low-voltage-operation n-type organic thin film transistor (OTFT) and a complementary inverter circuit using a thiazolothiazole derivative as an organic semiconductor (OSC). To achieve the low voltage and stable operation of the n-type OTFT, a bilayer structure consisting of a thin low-k polymer layer and a high-k Ta2O5 layer was used and investigated as the gate insulator. Atomic force microscopy images of the OSC film on the hydrophobic polymer showed densely distributed small grains, and the corresponding OTFT exhibited hysteresis-free and excellent n-type performance, for example, an electron mobility of 0.23–0.35 cm2 V-1 s-1. In addition, the n-type OTFT with a thinner polymer layer exhibited a low operating voltage of less than 10 V while maintaining excellent characteristics. A complementary inverter based on the thiazolothiazole derivative and pentacene was also fabricated using the bilayer gate insulator. The switching operation of the inverter was achieved at a low voltage of 5–10 V.

Low-voltage organic thin-film transistors with hydrophobic hafnium oxynitride film as gate insulator

An excellent gate dielectric of amorphous HfON (a-HfON) can be fabricated by flashing a Hf metal layer prior to a-HfON deposition. Nitrogen incorporation in the HfON film can reduce leakage current density from 8 × 10 −7 to 10 −7 A/cm 2. And surface roughness of a-HfON is reduced by the flash Hf metal layer from 1.9 to 1.2 nm. With the flash Hf metal layer, field effect mobility (0.17 cm 2/V s), threshold voltage (−0.3 V), subthreshold swing (−0.209 V/decade), and on/off current ratio (3.2 × 10 3) can be achieved for organic thin-film transistors with a-HfON gate dielectric.

Low-voltage organic n-channel thin-film transistors based on a core-cyanated perylene tetracarboxylic diimide derivative

Using the small-molecule organic semiconductor bis(1 H,1 H-perfluorobutyl)-dicyano-perylene tetracarboxylic diimide, C 3F 7CH 2-PTCDI-(CN) 2, and a low-temperature-processed, high-capacitance gate dielectric based on a phosphonic acid self-assembled monolayer, we have manufactured n-channel thin-film transistors on glass substrates. The transistors operate with low voltages (2 V) and have an electron mobility of 0.04 cm 2/Vs and an on/off ratio of 10 5. By combining C 3F 7CH 2-PTCDI-(CN) 2 n-channel transistors with pentacene p-channel transistors, we have also manufactured low-voltage, low-power organic complementary inverters with good static and dynamic performance.

The impact of self-assembled monolayer thickness in hybrid gate dielectrics for organic thin-film transistors

We have investigated the electrical characteristics of hybrid dielectrics with a thickness of 6 nm or less that are composed of a plasma-grown aluminum oxide (AlO x ) layer and a self-assembled monolayer (SAM) of an aliphatic phosphonic acid. The impact of the quality of the AlO x layer on the insulating properties of the double-layer dielectrics was assessed by comparing two different oxidation procedures, and the influence of the thickness of the organic SAM was evaluated by employing molecules with five different chain lengths. In order to decouple the relative contributions of the oxide and the SAM to the performance of the double-layer dielectrics we have also performed cyclic voltammetry measurements on indium tin oxide (ITO)/SAM devices without AlO x layer. Finally, we have evaluated how the quality of the AlO x layer and the thickness of the SAM affect the performance of low-voltage organic thin-film transistors (TFTs) that employ the thin AlO x /SAM dielectrics as the gate dielectric. The results confirm the important role of the SAM in determining the breakdown voltage, in limiting the current density, and in compensating the somewhat lower quality of AlO x layers produced under mild plasma conditions.

The effects of hydroxyl-free polystyrene buffer layer on electrical performance of pentacene-based thin-film transistors with high-k oxide gate dielectric

We have investigated the effects of hydroxyl-free polystyrene (PS) as buffer layer on pentacene-based low-voltage organic thin-film transistors (OTFTs). The PS buffer layer is formed on the HfO 2 layer evaporated on Si substrate by spin-coating method prior to pentacene deposition, existence of which results in a dramatic increase of field effect mobility from 0.09 to 0.59 cm 2/Vs and negligible hysteresis. The improved mobility and hysteresis of the OTFTs can be attributed to the formation of smooth and nonpolar hydroxyl-free PS/HfO 2 gate dielectric surface. The PS insulator buffer layer can also effectively reduce gate leakage current by more than 70%. The results demonstrate that using appropriate polymer buffer layer is favorable to improve the performance of the OTFTs operating at low voltages with high mobility and good electrical stability.

Bias stress effect in low-voltage organic thin-film transistors

The bias stress effect in pentacene organic thin-film transistors has been investigated. The transistors utilize a thin gate dielectric based on an organic self-assembled monolayer and thus can be operated at low voltages. The bias stress-induced threshold voltage shift has been analyzed for different drain-source voltages. By fitting the time-dependent threshold voltage shift to a stretched exponential function, both the maximum (equilibrium) threshold voltage shift and the time constant of the threshold voltage shift were determined for each drain-source voltage. It was found that both the equilibrium threshold voltage shift and the time constant decrease significantly with increasing drain-source voltage. This suggests that when a drain-source voltage is applied to the transistor during gate bias stress, the tilting of the HOMO and LUMO bands along the channel creates a pathway for the fast release of trapped carriers.

Polaron and ion diffusion in a poly(3-hexylthiophene) thin-film transistor gated with polymer electrolyte dielectric

Electrolytes are finding applications as dielectric materials in low-voltage organic thin-film transistors (OTFT). The presence of mobile ions in these materials (polymer electrolytes or ion gels) gives rise to very high capacitance (>10 μF/cm2) and thus low transistor turn-on voltage. In order to establish fundamental limits in switching speeds of electrolyte gated OFETs, we carry out in situ optical spectroscopy measurement of a poly(3-hexylthiophene) (P3HT) OTFT gated with a LiClO4:poly(ethyleneoxide) (PEO) dielectric. Based on spectroscopic signatures of molecular vibrations and polaron transitions, we quantitatively determine charge carrier concentration and diffusion constants. We find two distinctively different regions: at VG≥−1.5 V, drift-diffusion (parallel to the semiconductor/dielectric interface) of hole-polarons in P3HT controls charging of the device; at VG<−1.5 V, electrochemical doping of the entire P3HT film occurs and charging is controlled by drift/diffusion (perpendicular to the interface) of ClO4− counter ions into the polymer semiconductor.

π-σ-Phosphonic acid organic monolayer–amorphous sol–gel hafnium oxide hybrid dielectric for low-voltage organic transistors on plastic

A vacuum-free solution processed hybrid dielectric composed of an anthryl-alkyl-phosphonic acid (π-σ-PA) self-assembled monolayer on an amorphous sol–gel processed hafnium oxide (HfOx) is demonstrated for low-voltage organic thin film transistors (OTFTs) on plastic substrates. The π-σ-PA/HfOx hybrid dielectric provides high capacitance (0.54 µF cm−2) and low leakage current (2 × 10−8 A cm−2), and has a chemically and electrically compatible dielectric interface for evaporated and solution processed acene semiconductors. The utility of this dielectric is demonstrated by fabricating pentacene and 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-PEN) based OTFTs with operating voltages under 2 V, subthreshold slopes as low as 100 mV dec−1, and average mobilities of 0.32 cm2 V−1 s−1 and 0.38 cm2 V−1 s−1, for pentacene and TIPS-PEN, respectively.

Influence of Molecular Structure and Film Properties on the Water-Stability and Sensor Characteristics of Organic Transistors

Low-voltage organic thin-film transistors were fabricated to investigate the stability of organic semiconductors in water and determine the critical parameters for aqueous phase chemical detection. We showed that the molecular structure, alkyl-chain length, and thin-film morphology strongly influence the electrical characteristics in water. Highly two-dimensional films of alkyl-substituted semiconductors greatly reduced the influence of water on the electrical characteristics. The sensitivity and response times of OTFT sensors were also impacted by the thin-film structure of the organic semiconductor. OTFT sensors were used to detect 10 parts per billion cysteine and 40 ppm TNT in aqueous solutions.

AFM, ellipsometry, XPS and TEM on ultra-thin oxide/polymer nanocomposite layers in organic thin film transistors

Here we report on the fabrication and characterization of ultra-thin nanocomposite layers used as gate dielectric in low-voltage and high-performance flexible organic thin film transistors (oTFTs). Reactive sputtered zirconia layers were deposited with low thermal exposure of the substrate and the resulting porous oxide films with high leakage currents were spin-coated with an additional layer of poly-alpha-methylstyrene (P alphaMS). After this treatment a strong improvement of the oTFT performance could be observed; leakage currents could be eliminated almost completely. In ellipsometric studies a higher refractive index of the ZrO(2)/P alphaMS layers compared to the "as sputtered" zirconia films could be detected without a significant enhancement of the film thickness. Atomic force microscopy (AFM) measurements of the surface topography clearly showed a surface smoothing after the P alphaMS coating. Further studies with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) also indicated that the polymer definitely did not form an extra layer. The polymer chains rather (self-)assemble in the nano-scaled interspaces of the porous oxide film giving an oxide-polymer "nanocomposite" with a high oxide filling grade resulting in high dielectric constants larger than 15. The dielectric strength of more than 1 MV cm(-1) is in good accordance with the polymer-filled interspaces.

Low-voltage organic thin-film transistors with large transconductance

We have developed an organic thin-film transistor (TFT) technology that aims at providing a good balance of static and dynamic performance parameters. An inverted staggered (bottom-gate, top-contact) device structure with patterned metal gates, a room-temperature-deposited gate dielectric providing a capacitance of 0.7μF∕cm2, and vacuum-deposited pentacene as the semiconductor were employed. The TFTs have a channel length of 10μm, a carrier mobility of 0.4cm2∕Vs, an on/off current ratio of 107, a subthreshold swing of 100mV/decade, and a transconductance per channel width of 40μS∕mm. Ring oscillators operate with supply voltages as low as 2V and with signal propagation delays as low as 200μs per stage.

Low-voltage organic thin film transistors with hydrophobic aluminum nitride film as gate insulator

This paper reports on the low-voltage (<5 V) pentacene-based organic thin film transistors (OTFTs) with a hydrophobic aluminum nitride (AlN) gate-dielectric. In this work, a thin (about 50 nm), smooth (roughness about 0.18 nm) and low-leakage AlN gate dielectric is obtained and characterized. The AlN film is hydrophobic and the surface free energy is similar to the organic or the polymer films. The demonstrated AlN–OTFTs were operated at a low-voltage (3–5 V). A low-threshold voltage (� 2 V) and an extremely low-subthreshold swing (� 170 mV/dec) were also obtained. Under low-voltage operating conditions, the on/off current ratio exceeded 10 6 , and the field effect mobility was mobility was 1.67 cm 2 /V s.

Liquid Crystal Display Cells Fabricated on Plastic Substrate Driven by Low-Voltage Organic Thin-Film Transistor with Improved Gate Insulator and Passivation Layer

Organic thin-film transistor (OTFT)-driven 5×5 polymer-dispersed liquid crystal (PDLC) display cells on a flexible plastic substrate have been developed. It is necessary to increase the maximum usable gate voltage of OTFT to obtain a high contrast ratio. We propose a stacked gate insulator that consists of polyvinylphenol and anodized Ta2O5 for decreasing the gate leakage current and increase the maximum voltage. The OTFT with the insulator showed a field-effect mobility of 0.4 cm2/(V s), a current on/off ratio of 105, a low threshold voltage of 1.1 V, and a subthreshold slope of 0.2 V/decade. Leakage current was successively decreased up to a gate voltage of 15 V, maintaining a low-voltage operation of OTFT. Double passivation layers using polyvinylalcohol and photosensitive acrylic material are also proposed to prevent the degradation of OTFT by liquid crystal. The bending characteristics of OTFT on plastic substrates were also measured for various radiuses of curvature. The OTFT can operate at a radius of curvature exceeding 20 mm. On the fabricated display cells, we confirmed a good display operation with a contrast ratio of 10:1 with a low driving voltage of 12–13 V.

Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta2O5 as gate dielectric material for organic thin-film transistors. Ta2O5 has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta2O5 thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (r=21), Ta2O5 facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta2O5 gate dielectric are discussed.