Pentacene OTFTs Research Articles

Low-Threshold pentacene OTFT by using NdTaON gate dielectric and ITO gate electrode

Ta-doped Nd oxynitrides (NdxTa(1-x)ON) are adopted as gate dielectrics in pentacene organic thin-film transistors on ITO glass for transparent applications. The device with Nd0.76Ta0.24ON can achieve the optimal performance with a high carrier mobility of 2.32 cm2/V·s and a small threshold voltage of −0.21 V. The capacitance–voltage results indicate that besides producing a high permittivity (high-k) for the gate dielectric, Ta incorporation in NdON can generate negative fixed charges in the oxide to assist the external gate voltage to produce a small threshold voltage. The threshold voltage is further reduced by the ITO gate electrode because ITO with higher work function can decrease the energy barrier for the charge-carrier injection at the source/channel interface. XPS and AFM reveal that the moisture resistance of pure Nd oxynitride can be enhanced by Ta doping to improve the smoothness of the dielectric surface, which results in weaker surface-roughness and grain-boundary scatterings, and consequently higher carrier mobility. The carrier mobility is further enhanced by the ITO gate electrode because similar to metal-gate/high-k stack, metallic ITO gate with higher electron concentration than Si gate can offer stronger screening effect on the carrier scattering induced by the severe atomic oscillation in the high-k NdTaON gate dielectric.

Hydrogen sensors based on TFT’s with catalytic source/drain electrodes: IGZO vs. pentacene

A novel H2 sensor based on an InGaZnO thin-film transistor (IGZO TFT) with Pd source/drain electrodes as sensing medium is fabricated. Without the need of heating, rapid and concentration-dependent response (in the form of drain-current increase) of the sensor is observed under various cycles of H2 exposure/evacuation with H2 concentration ranging from 200 to 15 000 ppm. The drain-current increase is due to a decrease in barrier height at the interface between the Pd source and IGZO channel induced by the formation of a hydrogen dipole layer at the interface. By comparing this sensor with its counterpart based on pentacene OTFT having an opposite response, it can be claimed that the same hydrogen dipole layer has opposite effect on the two TFTs: it creates an electric field that can assist the flow of electrons from the source electrode to the IGZO channel, but oppose the flow of holes from the source electrode to the pentacene channel. As a result, the IGZO TFT can work in an OFF-to- ON mode during sensing, and so the standby power of sensing circuit could be reduced. Although the OTFT sensor shows higher sensitivity and faster response due to the higher flexibility of the pentacene layer, the proposed sensor has a better stability at high temperature due to the larger bandgap of IGZO and so is suitable for hydrogen-sensing applications in a wider range of operating temperature.

Electrode configurations for pentacene-deposited organic thin film transistors

ABSTRACTWhen a pentacene OTFT is fabricated with a bottom gate structure, the top-contact configuration of gold electrodes is preferred because it results in better electrical performance than the bottom-contact configuration. Pentacene molecules are deposited only on the SiO2 dielectric layer in the top-contact configuration, and thus all pentacene molecules form a vertical orientation that induces efficient charge transport between molecules. However, in an OTFT device with the bottom-contact configuration, pentacene molecules adopt a random tilted orientation at the boundary of the gold electrode, resulting in poor charge transport between molecules.

(Invited) Strain Sensing Inverter and Photovoltaic Circuits Based on Novel Heptazole Organic Thin Film

Following solid pentacene films and other p-type small molecules, in the present work, we show a new small molecule, 8,16-dihydrobenzo[a]benzo[6,7]indolo[2,3-h]carbazole (heptazole; C26H16N2) which has almost the same HOMO level as that of pentacene but with a higher HOMO-LUMO gap of ~2.95 eV, so as to be photostable under visible light except deep blue-to-violet range. The field-effect mobility of our new OTFT turns out to be ~0.21 cm2/V s which is lower than that of the pentacene OTFT on the same dielectric layer, however our new OTFT on flexible substrate is found to work as a strain sensor because organic heptazole layer changes its electric resistance under tensile strain but such change appears elastically recoverable by removing the strain. We here display an inverter which is composed of two OTFTs but with 90 drgree different channel orientations each other: one would work as a strain sensor while the other works as a load under static and dynamic tensile bending strain conditions. Moreover, we also found that our heptzole organic film has very small exciton binding (~40 meV) properties under deep blue photons (wave length~410 nm). Taking advantages of such small exciton binding energy, we developed a Schottky diode circuit which can dynamically measure the photovoltaic effects without any assistacnce of electric power. Based on above-mentioned benefits of our heptazole layer in TFT and Schottky diode, we demonstrate in the meeting the dynamic strain sensing of the organic inverter on flexble substrate, and dynamic photovoltaic switching of the Schottky diode circuits as briefly shown below. Figure 1

High-mobility pentacene OTFT with TaLaO gate dielectric passivated by fluorine plasma

High-mobility pentacene OTFT with TaLaO gate dielectric passivated by fluorine plasma

UV-curable organic–inorganic hybrid gate dielectrics for organic thin film transistors

UV-curable hybrid thin films were prepared from ZrO2 hybrid sols containing the acrylic monomer, DPHA, on substrates. The prepared ZrO2 hybrid sols showed long-term storage stability. Hybrid dielectrics were prepared by sol–gel process and UV cross-linking below 160°C. Leakage currents of dielectric layers remained below 10−6A in 2MV/cm and dielectric constants were measured to be 3.85–4 at 1kHz. In addition, organic–inorganic hybrid thin films have smooth and hydrophobic surface. Pentacene OTFTs with thin hybrid dielectrics exhibit of mobility as large as 2.5cm2/Vs, subthreshold swing as low as 0.2V/decade, an on–off ratio of 105. These results demonstrated that UV-curable sol–gel hybrid systems are suitable for gate dielectrics in OTFTs.

Direct photo-patternable, low-temperature processable polyimide gate insulator for pentacene thin-film transistors

We have developed photo-sensitive, low-temperature processable, soluble polyimide (PSPI) gate insulator with excellent resistance to the photo-patterning process. The PSPI was synthesized through one-step condensation polymerization of monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and 3,5-diaminobenzyl cinnamate (DABC). PSPI thin film, fabricated at 160°C, has a dielectric constant of 3.3 at 10kHz, and leakage current density of <1.7×10−10A/cm2, while biased from 0 to 100V. PSPI could be easily patterned by selective UV-light exposure and dipping into γ-butyrolactone. To investigate the potential of the polyimide as the photo-patternable gate insulator, we fabricated pentacene OTFTs and confirmed the PSPI’s resistance to the photo-patterning process. The photo-patternable polyimide shows promise as gate dielectrics for OTFTs.

石墨烯电极有机薄膜晶体管研究

High-performance bottom-contact pentacene OTFTs with graphene source/drain electrodes by transferring and patterning CVD-grown graphene films in a room-temperature process has been developed.The AFM images show that the growth of pentacene grains was not affected by the electrode,because patterned graphene electrodes were significantly thinner compared to common metal electrodes.The output and transfer curve,on/off current ratio,threshold voltage and field-effect mobility of the device were obtained by electric mea-surements.The transfer curve showed an off-current level of ≈109 A and an on/off current ratio exceeding 103.The bottom-contact pentacene OTFTs based on graphene source/drain electrodes had a maximum field-effect mobility of up to 2×10-2 cm2 ·V-1 ·s-1.

Influence of structural properties on environmental stability of pentacene thin film transistors

We studied environmental stability of top contact pentacene TFTs with active layer evaporated at different growth rates. We measured the transfer characteristics in vacuum and in air and after storing the devices in oxygen for several days. Different pentacene growth rates result in different grain size of active layer. This morphology difference influences the hysteresis of transfer characteristics induced by water absorption. On the contrary, aging effects on the transfer characteristics of pentacene O-TFTs, induced by oxygen diffusion into the active layer, are not related to structural characteristics of pentacene film.

Improved Performance of Pentacene OTFT with HfLaO Gate Dielectric by Annealing in NH3

Pentacene organic thin-film transistors with HfLaO as gate dielectric were fabricated. The dielectric was prepared by sputtering method with a dielectric constant over 11, and hence the OTFTs could operate at a voltage less than 5 V. The dielectric was annealed in N2, NO or NH3 at 400 oC after sputtering to passivate the surface of the dielectric. The OTFT treated in NH3 displayed higher carrier mobility, smaller sub-threshold swing, and lower 1/f noise than the OTFTs annealed in N2 and NO respectively. SEM image indicated that pentacene inclined to form larger grains on the dielectric annealed in NH3 than the dielectric annealed in N2 or NO. The enhanced electrical performance of OTFTs annealed in NH3 can be attributed to the improved interfacial characteristics between the organic film and the gate dielectric.

Improved Performance of Pentacene OTFT with HfLaO Gate Dielectric by Annealing in NH3

not Available.

Pentacene thin film transistors using La2O3 as gate insulator

A study of Pentacene OTFTs using La2O3 as gate insulator is presented. The device characteristics were studied and analyzed. The OTFTs exhibit p-type conductivity with field effect mobility 6.5 × 10−8 m2/V.s, ON/OFF ratio 1.4 × 102, sub-threshold swing 2 mV/decade and hole concentration 4.5 × 1017 cm−3. The SEM and XRD analysis on the semiconductor film are also reported.

Fine patterning of glycerol-doped PEDOT:PSS on hydrophobic PVP dielectric with ink jet for source and drain electrode of OTFTs

This report describes the fabrication and the electrical characteristics of pentacene OTFTs that use the conducting PEDOT:PSS for the source and drain (S/D) electrodes. By adding a controlled amount (10–20 wt.%) of glycerol the conductivity of the PEDOT:PSS ink enhanced to 450 S/cm, and the viscosity of the ink was reduced to <20 cps in order for it to be suitable for ink jet printing. In order to create a patterned PEDOT:PSS layer for the S/D electrode on a hydrophobic polyvinylphenol (PVP) gate dielectric, oxygen plasma was applied to control the surface energy of S/D electrode area on the PVP surface. In addition, by adding a leveling agent, the pattern of PEDOT:PSS droplet was improved enough to be spread up to the corners of S/D electrode area, producing channel length of 10 μm with a fine shape. Pentacene TFTs with the PEDOT:PSS S/D electrodes exhibited a similar performance to those with Au S/D electrodes in terms of the electrode resistance, corresponding to sum of the electrode series resistance and the contact resistance, being ∼30 kΩ cm at V G = −25 V and the field effect mobility being 0.13 cm 2/V s and the on/off current ratio being 5.6 × 10 6. However, the electrode resistance of the PEDOT:PSS needs to be further reduced for the short channels <20 μm.

Effect of Nanocomposite Gate-Dielectric Properties on Pentacene Microstructure and Field-Effect Transistor Characteristics

In this study, the effect of surface energy and roughness of the nanocomposite gate dielectric on pentacene morphology and electrical properties of pentacene OTFT are reported. Nanoparticles TiO2 were added in the polyimide matrix to form a nanocomposite which has a significantly different surface characteristic from polyimide, leading to a discrepancy in the structural properties of pentacene growth. A growth mode of pentacene deposited on the nanocomposite is proposed to explain successfully the effect of surface properties of nanocomposite gate dielectric such as surface energy and roughness on the pentacene morphology and electrical properties of OTFT. To obtain the lower surface energy and smoother surface of nanocomposite gate dielectric that is responsible for the desired crystalline, microstructure of pentacene and electrical properties of device, a bottom contact OTFT-pentacene deposited on the double-layer nanocomposite gate dielectric consisting of top smoothing layer of the neat polyimide and bottom layer of (PI+ nano-TiO2 particles) nanocomposite has been successfully demonstrated to exhibit very promising performance including high current on to off ratio of about 6 x 10(5), threshold voltage of -10 V and moderately high filed mobility of 0.15 cm2V(-1)s(-1).

Properties of SiO2 Surface and Pentacene OTFT Subjected to Atomic Hydrogen Annealing

Properties of SiO2 Surface and Pentacene OTFT Subjected to Atomic Hydrogen Annealing

Application of Chemical-mechanical Polishing for Planarizing a High-K Nano-composite Polyimide Insulator used in Organic Thin Film Transistors

To solve the large leakage current of heavily blended nano-composite (PI and nano-TiO2 particles) with high capacitance gate dielectric film, chemical-mechanical polishing (CMP) was used to planarize the interface between the PI-TiO2 nano-composite film and pentacene. The hysteresis effect in OTFTs with the polished nano-composite dielectric surface was reduced because less charge was trapped in the interface. In pentacene OTFTs with high capacitance and a smooth surface, the threshold voltage improved from -22V to -5V, the sub-threshold voltage decreased from 3.44 to 0.50 (V/dec.), the current on/off ratio increased from 1.6x106 to 3.53x108, and the mobility increased from 0.416 to 0.624 (cm2V-1s-1).

N-doping of pentacene by decamethylcobaltocene

We demonstrate n-type doping of pentacene with the powerful reducing molecule decamethylcobaltocene (CoCp2*). Characterization of pentacene films deposited in a background pressure of CoCp2* by X-ray photoemission spectroscopy and Rutherford backscattering confirm that the concentration of incorporated donor molecules can be controlled to a level as high as 1%. Ultraviolet photoemission spectroscopy show Fermi level (EF) shifts toward unoccupied pentacene states, indicative of an increase in the electron concentration. A 1% donor incorporation level brings EF to 0.6 eV below the pentacene lowest unoccupied molecular orbital. The corresponding electron density of ∼1018 cm−3 is confirmed by capacitance–voltage measurements on a metal–pentacene–oxide–silicon structure. The demonstration of n-doping suggests applications of CoCp2* to pentacene contacts or channel regions of pentacene OTFTs.

Analysis of interface states of the pentacene organic thin-film phototransistor by conductance technique

A pentacene thin-film transistor with a channel width of 300 μm and a channel length of 30 μm has been successfully fabricated on n-Si substrate with thermally oxidized SiO 2 as a gate insulator. The photovoltaic and interface state density properties of the transistor have been investigated. A pentacene film of 200 nm thickness was deposited on the SiO 2 layer with a vacuum thermal evaporator. Atomic force microscopy images of the pentacene film on SiO 2 insulating layer show a homogeneous film surface with the rms roughness of 11 nm. The transistor shows p-channel characteristics, as a result of positive carriers generated in the pentacene film for the negative bias voltages applied to the gate. The photosensitivity ( I ph/ I dark) is measured as 1.45 at an illumination intensity of 3500 lux at the off state. This suggests that the pentacene thin-film transistor shows a phototransistor characteristic. The field-effect mobility of the pentacene OTFT was found to be 0.021 cm 2/(V s). The interface state density of the transistor was determined using conductance technique and was found to be about 1.191 × 10 10 eV −1 cm −2.

Evaluation of Pentacene Thin Films Fabricated on Sio2 Surface Modified by Phenyl-Radical

We fabricated pentacene OTFTs with chemically modified SiO2 surface by phenyltrichlorosilane (PTS), and studied the molecular ordering of the pentacene films and electrical characteristics of the OTFTs. The molecular ordering of the pentacene thin film was improved by PTS treatment. We also fabricated organic thin film transistors with phenyltrichlorosilane treatment. The carrier mobility of the transistor with PTS treatment was 3.7 × 10−2 cm2/Vs, it was larger than that of the transistor without PTS treatment (2.0 × 10−2 cm2/Vs). We believe that this improvement of the μ value was due to improvement of the molecular ordering by PTS treatment.

Comparison of Pentacene and Amorphous Silicon AMOLED Display Driver Circuits

Organic light-emitting diode (OLED) displays offer distinct advantages over liquid crystal displays for portable electronics applications, including light weight, high brightness, low power consumption, wide viewing angle, and low processing costs. They also are attractive candidates for highly flexible substrates. In active-matrix OLED (AMOLED) displays, a small transistor circuit is used to drive each OLED device. This paper compares the simulated performance of two state-of-the-art AMOLED drivers with a proposed 5 thin-film-transistor (TFT) voltage programmed driver circuit which combines the advantages of the first two configurations. A competitive evaluation is also done between amorphous silicon (alpha-Si) and organic TFTs (OTFTs,) using comparable empirical device models for alpha-Si) and pentacene OTFTs. The 5-TFT circuit is found to match the speed of the 2-TFT while achieving a stability closer to the 4-TFT circuits and demonstrating a better speed-stability tradeoff.