Pentacene Thin-film Transistors Research Articles

(Invited) Transferable Organic Semiconductor Nanosheets for Van Der Waals Heterojunction Devices

The combination of organic semiconductors (OSCs) and two-dimensional materials provides a new basis to harness the advantages of both material systems, for example to fabricate unipolar pentacene thin-film transistors using graphene contacts [1] and p-n junctions using pentacene and molybdenum disulfide [2]. However, the fabrication of such devices by vapor deposition of OSCs is a challenge because growth is limited by the surface-dependent diffusion of small molecules. In this talk, I will present a novel method for the preparation of transferable OSC nanosheets that overcomes this problem. Highly crystalline, wafer-scale (3") nanosheets as thin as 20 nm can be obtained by precisely delaminating the OSC layer from a water-soluble sacrificial film in an appropriate wetting geometry. These free-floating films are then deposited on arbitrary substrates while maintaining morphology and crystallinity. Consequently, we demonstrate functional OSC nanosheets on top of prefabricated electrodes and monolayer MoS2, resulting in devices such as unipolar, ambipolar, and anti-ambipolar field effect transistors [3].

Design of Pentacene Thin-Film Transistor Based Hydrogen Gas Sensor with High-K Dielectric Materials for High Sensitivity

Electrical properties of an organic field-effect transistor were modelled in top gate top contact (TGTC) geometry and H2 gas sensors were designed for increased sensitivity based on the structure. Safety concerns related to hydrogen usage must be addressed; these hazardous characteristics include a wide flammable range (4%–75%) that results in a rapid burning velocity, a low minimum ignition energy (0.017 mJ), a high heat of combustion (143 kJ g−1), and the high diffusivity of hydrogen gas (0.61 cm2 s−1 in the air). These characteristics make it impossible to control hydrogen combustion after a specific time. All simulations were performed in the Silvaco TCAD ATLAS tool. We analysed the driving principle of gas sensors and introduced gas sensing properties in OFET using platinum metal at the gate electrode for H2 gas detection. IOFF, ION, and VTH are sensitivity parameters that alter when the metalwork function of the gate changes with respect to the gas present on it. The designed sensor was analysed for different dielectric materials. Results demonstrate that the increase in sensitivity for OFET-based H2 sensors is 73.4%, 80.7%, 90.5%, and 95.6% when the work function changes by 50, 100, 150, and 200 meV for Pt gate electrodes with an increase in dielectric value of insulating layer from SiO2 (3.9) to La2O3 (27). Results were compared with the In1-xGaxAs CGNWFET-based H2 sensor as the work function varies at 200 meV,the sensitivity enhancement with OFET-based H2 sensors is 8.09%.

Interpretation of Hopping Transport Based on Pentacene Thin-Film Transistors

Interpretation of Hopping Transport Based on Pentacene Thin-Film Transistors

Polysilsesquioxane Gate Dielectric Layers Cured by UV Light Irradiation Using Thiol–Ene Reaction for Organic Thin‐Film Transistors

Polysilsesquioxane (PSQ) is used to make gate dielectric layers with pentacene to create thin‐film transistors (TFTs) by incorporating 3‐mercaptopropyl functional groups for UV light‐initiated thiol–ene polymerization with vinyl‐containing cross‐linkers. The dielectric constant (ε) of the polymerized PSQ films is controlled by the ratio of 3‐mercaptopropyl groups. When the 3‐mercaptopropyl group comprises 20%, it yields a polymerized film with a ε of 4.2 ± 0.2; upon decreasing this proportion to 10%, the ε for the subsequent film became 3.5 ± 0.2. The hole mobility of the fabricated pentacene TFTs is observed to increase as ε decreases, reaching a maximum of 0.96 cm2 V−1 s−1.

Low-temperature processable and photo-crosslinkable polyimide gate dielectric for flexible thin-film transistor

A fully soluble and photo-crosslinkable gate dielectric (6 F-SPI-Cinnamoyl: 6 F-SPI-CM) was synthesized. First, hydroxyl group substituted polyimide (6 F-SPI-Hydroxyl: 6 F-SPI-OH) polymer was prepared using monomers, 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and 4,4’-(perfluoropropane-2,2-diyl)dianiline (6FHAB) through polycondensation reaction. And, then 6 F-SPI-OH was further reacted with cinnamoyl chloride to prepare a photo-crosslinkable 6 F-SPI-CM. The thin film of photo-crosslinked 6 F-SPI-CM (C-6F-SPI-CM) showed the extremely low leakage current density of <1.0 x10−11 A/cm2. In addition, 6 F-SPI-CM was easily patterned by selective UV-light exposure. A flexible pentacene thin-film transistor (TFT) on polyethersulfone (PES) substrate with photo-crosslinked 6 F-SPI-CM as a gate dielectric showed a field effect mobility of 0.11 cm2/Vs with no hysteresis behavior.

Achieving high-mobility pentacene thin-film transistors by reducing the trapping density between insulators and organic semiconductors

We have demonstrated that it is important to control the carrier density in the channel region formed between an insulator and an organic semiconductor when preparing high-performance organic thin-film transistors (OTFTs). To clearly understand these effects, the surface roughness and surface energy of the insulator and the crystal structure were uniformly controlled. As the carrier density was controlled by adjusting the content of hydroxyl groups, the charge mobility was increased by 400%.

Contact resistance in sub-micron and nanoscale organic thin-film transistors

Two improved methods for evaluating contact resistance in organic thin-film transistors (TFTs) are presented. These methods can be applied to TFTs with any arbitrary geometry and are also suitable for retroactive analysis of current-voltage characteristics. The first method is based on the analysis of the differential conductance of TFTs and in separating the conductivity contributions from the contacts and the channel. This method provides accurate results when the contact resistance is approximately 0.1–10 times the channel resistance. The second method is especially applicable to nanoscale and short channel length TFTs. The minimum value of differential on-resistance as a function of drain bias is used to separate the contributions of the channel and contact resistances at various drain voltages. This method is accurate when the contact resistance is approximately 0.3–10 times the channel resistance, as verified by simulations. It works even when there is no distinct saturation in current-voltage characteristics with increasing drain voltage. Both methods have been applied to experimental data from organic TFTs with channel lengths in the range 10 nm to 5 μm. • Two novel methods of extracting contact resistance in OTFTs are introduced. • Contact resistance of various channel lengths OTFTs are extracted and analyzed. • Effects of contact resistance in short channel pentacene TFTs are discussed. • New design of ultra-short channel TFTs is proposed to improve carrier injection.

Role of Fluoride Doping in Low-Temperature Combustion-Synthesized ZrOx Dielectric Films.

Zirconium oxide (ZrOx) is an attractive metal oxide dielectric material for low-voltage, optically transparent, and mechanically flexible electronic applications due to the high dielectric constant (κ ∼ 14-30), negligible visible light absorption, and, as a thin film, good mechanical flexibility. In this contribution, we explore the effect of fluoride doping on structure-property-function relationships in low-temperature solution-processed amorphous ZrOx. Fluoride-doped zirconium oxide (F:ZrOx) films with a fluoride content between 1.7 and 3.2 in atomic (at) % were synthesized by a combustion synthesis procedure. Irrespective of the fluoride content, grazing incidence X-ray diffraction, atomic-force microscopy, and UV-vis spectroscopy data indicate that all F:ZrOx films are amorphous, atomically smooth, and transparent in visible light. Impedance spectroscopy measurements reveal that unlike solution-processed fluoride-doped aluminum oxide (F:AlOx), fluoride doping minimally affects the frequency-dependent capacitance instability of solution-processed F:ZrOx films. This result can be rationalized by the relatively weak Zr-F versus Zr-O bonds and the large ionic radius of Zr+4, as corroborated by EXAFS analysis and MD simulations. Nevertheless, the performance of pentacene thin-film transistors (TFTs) with F:ZrOx gate dielectrics indicates that fluoride incorporation reduces I-V hysteresis in the transfer curves and enhances bias stress stability versus TFTs fabricated with analogous, but undoped ZrOx films as gate dielectrics, due to reduced trap density.

Hybrid La2O3-cPVP Dielectric for Organic Thin Film Transistor Applications

In this work, we have studied the electrical performance of cross-linked polyvinyl phenol (cPVP) modified lanthanum oxide (La2O3) bilayer dielectric film in pentacene thin film transistors (TFT). A simple spin-coating and room temperature operated cross-linking reaction of the hydroxyl moieties of PVP and the nitrogen groups of PMF were carried out to form the cross-linked PVP. The deposition of a thin 30 nm cPVP layer over the La2O3 layer provided a low leakage current (<10−7 A cm−2), causing a reduction in the interface trap density. Besides, the modified surface properties of the La2O3 layer were favorable for the growth of pentacene organic semiconductors. As a result, the current on-off ratio and the sub-threshold slope was improved from 104 and 1.0 V/decade to 105 and 0.67 V/decade. The La2O3∕cPVP pentacene TFT operated at −10 V also exhibited improvement in the field-effect mobility to 0.71 cm2 Vs−1 from 0.48 cm2 Vs−1 for the single-layer La2O3 (130 nm) device. Thus, our work demonstrates that the rare earth oxide La2O3 with cPVP is an excellent dielectric system in the context of emerging transistors with hybrid polymer gate dielectrics.

Synthesis and characterization of a perfluorocyclobutane-containing cross-linked polyimide gate insulator for thin film transistor

We have synthesized a thermally curable polymeric gate insulator (TFVOB-SPI) through the modification of soluble polyimide (SPI) using a 4-(1,2,2-trifluorovinyloxy)benzoyl (TFVOB) chloride. The thermal cross-linking of TFVOB-SPI was done and prepared the cross-linked TFVOB-SPI thin film having the perfluorocyclobutane (PFCB) structure. The PFCB structure was formed by the radical mediated thermal cycloaddition of trifluorovinyl ether of TFVOB. The cross-linked TFVOB-SPI showed excellent thermal stability up to 495 °C with only 5% weight loss and excellent chemical resistance to common organic solvents. The pentacene thin-film transistor (TFT) with the cross-linked TFVOB-SPI as a gate dielectric exhibited a field effect mobility of 0.28 cm2/Vs with almost no hysteresis during the TFT operation.

Effect of the Active Layer Thickness of Pentacene Thin Film Transistor; Illumination Effect

Effect of the Active Layer Thickness of Pentacene Thin Film Transistor; Illumination Effect

Influence of Environmental Conditions on Electrical Stability of Pentacene Thin-film Transistors with Cross-linked Poly(4-vinylphenol-co-methyl methacrylate) Gate Dielectric Layer

Influence of Environmental Conditions on Electrical Stability of Pentacene Thin-film Transistors with Cross-linked Poly(4-vinylphenol-co-methyl methacrylate) Gate Dielectric Layer

A study on the effect of film crystallinity and morphology on charge carrier concentration-dependent hole mobility in pentacene thin-film transistors: advantages of high deposition rate

The combined effect of deposition rate and substrate temperature on the film crystallinity, morphology, and electronic properties of pentacene is studied. It is shown that the channel mobility in polycrystalline pentacene thin-film transistors is relatively immune to substrate temperature, and the films offer good hole mobility when grown at a high rate. This is advantageous when high throughput with low deviation in electrical parameters over devices are required. The surface morphology is characterized by atomic force microscopy measurements and the crystallinity is studied using x-ray diffraction. The effect of growth parameters on the crystalline phases of pentacene is correlated to the charge carrier transport. It is found that the field-effect mobility is primarily affected by the crystalline phases of the film rather than the grain size. The charge carrier dependence of the hole mobility is used to parameterize the dispersion (width) in the density of states (DOS) of the highest occupied molecular orbital of the films in the transistor channel region. It is found that the presence of multiple phases in the path of the charge carrier flow increases the dispersion of the DOS.

Energy distribution of interface states generated by oxygen plasma treatment for control of threshold voltage in pentacene thin-film transistors

Pentacene metal-oxide-semiconductor (MOS) capacitors with a SiO2 dielectric treated by oxygen plasma have been studied by capacitance-voltage (C-V) measurements to investigate the energy distribution of the interface states. Oxygen plasma treatment, which is used for control of the threshold voltage in pentacene thin-film transistors, shifted the C-V curves of pentacene MOS capacitors to a positive gate voltage as well as the transfer curves of pentacene thin-film transistors (TFTs). The shift is explained by electrons captured at interface states generated by oxygen plasma treatment. The interface states capturing the electrons are expected to locate at low energy levels. The energy distribution of the interface states locating at middle or high energy levels was extracted by a method equivalent to the Terman method. By use of the method in two steps, the interface state densities distributed at middle and high energy levels (D M and D H) were separately obtained. D M and D H were of the order of 1010–1012 cm−2 eV−1, and increased with an increase in plasma treatment time.

Poly(4-vinylphenol-co-methyl methacrylate)/Hafnium Oxide Nanocomposite Gate Insulators for Organic Thin-Film Transistors.

We fabricate 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) thin-film transistors (TFTs) with nanocomposite insulators. The insulator layers consist of both poly(4-vinylphenol-co-methyl methacrylate) and high-dielectric constant hafnium oxide (HfO₂) nanoparticles. The HfO₂ nanoparticles are ball-milled for sufficient dispersion in a nanocomposite solution to enable solution process methods to be used in preparing the insulator layers. The nanocomposite insulators demonstrate high capacitances and improve the performance of TIPS-Pn TFTs. Nonetheless, particle aggregates are produced in the nanocomposites solution with high HfO₂ concentrations, generating detrimental effects on the dielectric properties and the TFT performance. Our experimental result implies that the optimum concentration of HfO₂ nanoparticles in a mixed solution will find to be ~11.5 wt%.

Structural modification of poly(4-vinylphenol) insulators in pentacene transistors by using dimethyl ketone

We demonstrate a dimethyl ketone treatment for structural modification of cross-linked poly(4-vinylphenol) (c-PVP) gate insulator in pentacene thin-film transistors (TFTs). Here, the dimethyl ketone treatment was performed immediately after spin-coating the insulator solution, followed by thermal annealing for cross-linking of PVP molecules. It is found that the dimethyl ketone treatment lowered the surface energy of the PVP gate insulator and made the PVP film thinner. The results of X-ray diffraction and atomic force microscopy analyses showed that the dimethyl ketone-treated c-PVP insulator contributes to enhancing the crystallinity of pentacene semiconductor films and reducing the density of lamellar grains and bulk phase crystallites in pentacene films. Consequently, the performance improvement of the pentacene TFT is achieved by structurally modifying the c-PVP gate insulator through the dimethyl ketone treatment.

Zero drive load inverter with high gain and large noise margin based on organic weak epitaxy growth method

We constructed zero drive load inverters by employing two identical unipolar pentacene thin-film transistors, based on the fabrication technology of organic weak epitaxy growth, which exhibited the features of high gains and large noise margins. Pentacene films were grown on a p-6P inducing layer, which reduced the interface disorder and increased the scale of crystalline domains. The pentacene/p-6P transistors showed high-quality saturation characteristics and the negative shift of threshold voltages, which caused large noise margins and the operation range of full swing. Furthermore, the hole mobility was increased to 1.28 cm2 (Vs)−1. As a result, zero drive load inverters exhibited a large gain of up to 30, compared with the gains of only four based on single pentacene transistors. We also discuss the dependence of large gains and device performance. These results demonstrated the fabrication technology of organic weak epitaxy growth were promising and advantageous for achieving high performance organic inverters and logic circuits.

Hysteresis Reduction for Organic Thin Film Transistors with Multiple Stacked Functional Zirconia Polymeric Films

We show that transfer hysteresis for a pentacene thin film transistor (TFT) with a low-temperature solution-processed zirconia (ZrOx) gate insulator can be remarkably reduced by modifying the ZrOx surface with a thin layer of crosslinked poly(4-vinylphenol) (c-PVP). Pentacene TFTs with bare ZrOx and c-PVP stacked ZrOx gate insulators were fabricated, and their hysteresis behaviors compared. The different gate insulators exhibited no significant surface morphology or capacitance differences. The threshold voltage shift magnitude decreased by approximately 71% for the TFT with the c-PVP stacked ZrOx gate insulator compared with the bare ZrOx gate insulator, with 0.75 ± 0.05 and 0.22 ± 0.03 V threshold voltage shifts for the bare ZrOx and c-PVP stacked ZrOx gate insulators, respectively. The hysteresis reduction was attributed to effectively covering hysteresis-inducing charge trapping sites on ZrOx surfaces.

Electrical and surface properties of SiO2 films modified by ultraviolet irradiation and used as gate dielectrics for pentacene thin-film transistor applications

The effect of ultraviolet (UV) treatment on the electrical and surface properties of SiO2 is studied. UV treatment leads to the formation of the silicon peroxide in SiO2 near the surface, thus increasing the capacitance of SiO2. The effect of SiO2 gate dielectrics with UV treatment on carrier transport for a pentacene-based organic thin-film transistor (OTFT) is also studied. Experimental identification confirms that the shift of the threshold voltage towards positive gate-source voltages for OTFTs is due to an increase in the number of trap states in SiO2 near the pentacene/SiO2 interface. The existence of a peroxidation layer between pentacene and SiO2 serves to strengthen the pentacene-SiO2 interaction, reducing the field-effect carrier mobility and the leakage current in OTFTs.

Leakage conduction behavior for top- and bottom-contact pentacene thin film transistors

The leakage conduction mechanisms for top-contact and bottom-contact pentacene-based organic thin film transistors (OTFTs) are studied. OTFTs that use a bottom-contact design exhibit lower leakage conduction than those that use a top-contact design. For top-contact OTFTs, the dominant leakage conduction mechanism is via Schottky emission and the density of the leakage current increases significantly as the bias voltage increases. For bottom-contact OTFTs, the dominant leakage conduction mechanism is via displacement current. OTFTs that use a bottom-contact design exhibit lower leakage conduction than those that use a top-contact design.