Characteristics Of Organic Thin-film Transistors Research Articles

Study on 6,13-bis(Triisopropylsilylethynyl) (TIPS) Pentacene Crystalline Morphology for Organic Thin Film Transistor

We found that the crystallization of TIPS pentacene thin film plays an important role in determining the electrical property of organic thin film transistor (OTFT). OTFTs were fabricated using two different TIPS pentacenes, namely big grains and dendrite grains. Poly-4-vinylphenol (PVP) was used as a gate insulator, and Au electrode was evaporated using a shadow mask. The channel lengths of OTFTs with the bottom gate structure were 20-40 um. The mobility values of OTFTs fabricated with big grains reveal ten time higher values than those obtained from OTFTs fabricated with dendrite grains. The physical properties of the TIPS pentacene films were analyzed using optical microscope (OM), x-ray diffraction (XRD) and secondary electron microscopy (SEM). The electric characteristics of OTFT were obtained using Keithley-4200.

A high-vacuum deposition system for in situ and real-time electrical characterization of organic thin-film transistors

We present a home-built high-vacuum system for performing organic semiconductor thin-film growth and its electrical characterization during deposition (real-time) or after deposition (in situ). Since the environment conditions remain unchanged during the deposition and electrical characterization process, a direct correlation between growth mode and electrical properties of thin film can be obtained. Deposition rate and substrate temperature can be systematically set in the range 0.1-10 ML∕min and RT-150 °C, respectively. The sample-holder configuration allows the simultaneous electrical monitoring of up to five organic thin-film transistors (OTFTs). The OTFTs parameters such as charge carrier mobility μ, threshold voltage V(TH), and the on-off ratio I(on)∕I(off) are studied as a function of the semiconductor thickness, with a submonolayer accuracy. Design, operation, and performance of the setup are detailed. As an example, the in situ and real-time electrical characterization of pentacene TFTs is reported.

Organic Thin-Film Transistors with Tailored Liquid Sources of High-κ HfO2 Using Excimer Laser Irradiation

The characteristics of organic thin-film transistors (OTFTs) with tailored liquid sources of HfO2 as a high-κ insulator were investigated using excimer laser irradiation. We have demonstrated that the OTFT characteristics were improved more by short wavelength light source irradiation than UV irradiation to cleave chemical bonds of organic residues in the alkoxy-derived HfO2 films. The field-effect mobilities and on–off ratios obtained for one shot of excimer laser irradiation were 0.18 cm2 V-1 s-1 and 750, respectively.

Synthesis and properties of phenothiazylene vinylene and bithiophene-based copolymers for organic thin film transistors

A series of new phenothiazylene vinylene and thiophene copolymers (P1, P2, and P3) have been synthesized via Yamamoto and Stille coupling reactions. The number-averaged molecular weights (Mn) of P1, P2, and P3 were found to be 12,000, 10,000, and 8200, with polydispersity indices of 3.5, 1.4, and 1.6, respectively. The UV–visible absorption spectra of the polymers contain two strong absorption bands in the ranges 306–325nm and 430–480nm, which arise from the absorptions of the phenothiazine segments and the conjugated main chains respectively. Solution-processed field-effect transistors (FETs) fabricated with these polymers exhibit p-type organic thin film transistor characteristics. The field-effect mobilities for P1, P2, and P3 were measured to be 1.8×10−4, 5.7×10−4, and 2.5×10−7cm2V−1s−1, respectively, with the corresponding on/off ratios of 5×102, 1×104, and 5×102.

Light response characteristics of organic thin-film transistors with and without a P(MMA–GMA) modification layer

This paper describes the photo-response characteristics of pentacene-based organic thin-film transistors (OTFTs) with SiO2 as the gate dielectric. The operation current increases significantly and the threshold voltage is shifted toward positive bias at the incident illumination. The distinguishable light responsive characteristics can be observed at the OTFTs with and without poly(methyl methacrylate co glycidyl methacrylate) (P(MMA–GMA)) as the modification layer, respectively. Maximum light responsivities of 24 600 and 1173 A W−1 were obtained for OTFTs with and without P(MMA–GAM), respectively, under the lower illumination power of 0.64 µW cm−2. The corresponding photo-response mechanisms were analyzed, which demonstrated that the present organic phototransistor properties have a strong dependence on the trap density at the pentacene bulk and the interface of pentacene/dielectric.

Performance Enhancement of Organic Thin-Film Transistors Using Bathophenanthroline:Cs Electron Injection Layer

In this study, we fabricated an organic thin-film transistor (OTFT) with a bathophenanthroline (Bphen):Cs electron injection layer between an organic semiconductor (C60) and a metal electrode. We compared the electrical characteristics of OTFTs with and without Bphen:Cs insertion layer which depend on the insertion layer thickness. We found that the Bphen:Cs layer inserted between the active layer (C60) and the metal electrode played an important role in improving the electrical characteristics of the devices. When the OTFT with 5-Å-thick Bphen:Cs was compared with that without Bphen:Cs, the mobility and the output current were determined to increase from 0.029 cm2 V-1 s-1 and 4.32×10-7 A to 0.127 cm2 V-1 s-1 and 1.67×10-6 A, respectively. This improvement was attributed to the reduction in contact resistance between C60 and the Al electrode layer when a Bphen:Cs electron injection layer of optimum thickness was applied.

The influence of electron-deficient comonomer on chain alignment and OTFT characteristics of polythiophenes

The structure–property relations of polythiophene derivatives have been the subject of research for both materials and electronic device applications. We have designed and prepared two kinds of polythiophene derivatives with similar chemical structures but different electronic properties by Stille polymerization. One is composed of only electron-rich thiophenes (PQT2T), while the other is composed of both electron-rich thiophenes and electron-deficient thiazoles (PTZT2T). The number-average molecular weights of PQT2T and PTZT2T were 23,000 (PDI = 1.5) and 13,000 (PDI = 1.7), respectively. PTZT2T showed a large bathochromic shift of absorption maximum from solution to film by 57 nm. Further investigation by differential scanning calorimetry and X-ray diffraction revealed the formation of well-ordered interdigitated structure of PTZT2T in the solid state ( d 1 = 20.7 Å). From absorption onset, optical band-gap of PQT2T and PTZT2T was calculated to be 2.22 eV and 1.92 eV, respectively. A mobility of 1.1 × 10 −3 cm 2/Vs, an on/off ratio of 4.7 × 10 4 have been obtained from organic thin-film transistors (OTFTs) using PTZT2T as a channel. By comparison with PQT2T, the mobility was 18 times and the on/off ratio was 52 times higher for PTZT2T, which shows the usefulness of electron-deficient thiazoles as comonomer units of polythiophene derivatives for OTFT applications.

Ultrathin polymer gate buffer layer for air‐stable, low‐voltage, n‐channel organic thin‐film transistors

AbstractAn ultrathin poly(methyl methacrylate) (PMMA) buffer layer was developed to improve the performance of n‐channel organic thin‐film transistors (OTFTs). The 8 nm‐thick PMMA film, prepared by spin‐coating, provided a very smooth surface and a uniform coverage on SiO2 surface reproducibly, which was confirmed by X‐ray reflectivity (XR) measurement. Then, we fabricated N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide (PTCDI‐C13) thin‐film transistors with and without this 8 nm‐thick PMMA insulating layer on SiO2 gate insulators and achieved one‐order increase of field‐effect mobility (up to 0.11 cm2/(Vs) in a vacuum), one‐half decrease of threshold voltage, and reduction of current hysteresis with the PMMA layer. Only TFTs with the PMMA layer displayed n‐channel operation in air and showed field‐effect mobility of 0.10 cm2/(Vs). We consider that electrical characteristics of n‐channel OTFTs were considerably improved because the ultrathin PMMA film could effectively passivate the SiO2 insulator surface and decrease interfacial electron traps. This result suggests the importance of the ultrathin PMMA layer for controlling the interfacial state at the semiconductor/insulator interface and the device characteristics of OTFTs. Copyright © 2009 John Wiley & Sons, Ltd.

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.

Organic Thin-Film Transistors with Tailored Liquid Sources of HfO2 as a High-κ Insulator

Organic thin-film transistors (OTFTs) with tailored liquid sources of HfO2 as a high-κ insulator were investigated. To investigate the effect of UV irradiation, two types of light source were tested. The contact angles of water and polycrystalline condition of a pentacene were systematically changed and the resultant OTFT characteristics were also changed. The field-effect mobilities and on–off ratios obtained using a high-pressure mercury lamp and a deep-UV lamp as irradiation sources were 0.035 and 0.10 cm2 V-1 s-1, and 24 and 3 ×103, respectively.

Tuning surface properties in photosensitive polyimide. Material design for high performance organic thin-film transistors

A photosensitive polyimide (PSPI) is synthesized to serve as the gate dielectric in organic thin-film transistors (OTFTs). A method to control the charge density within the channel of the OTFT using the PSPI on a silicon dioxide gate dielectric is proposed. The surface energy, surface carriers, and capacitance of gate dielectrics can be tuned by varying irradiation doses of UV light on the PSPI surface to create ultrahigh performance pentacene-based OTFTs with an average field-effect mobility (μ) of above 6.0 cm2/Vs. The correlation between the surface energy of the PSPI films and the μ value of OTFT devices is investigated. Moreover, the impact of gate-dielectric properties on the structural properties of pentacene films is closely related to the electrical characteristics of OTFTs. The results of contact angle, X-ray diffraction, electrostatic force microscopy, and micro-Raman spectroscopy measurements indicate that the surface energy and doping carriers coming from the PSPI film play important roles in the carrier transport of pentacene film for OTFT applications.

Synthesis and properties of phenothiazylene vinylene-based polymers: New organic semiconductors for field-effect transistors and solar cells

A series of new phenothiazylene vinylene-based semiconducting polymers, poly[3,7-(4′-dodecyloxyphenyl)phenothiazylene vinylene] (P1), poly[3,7-(4′-dodecyloxyphenyl)phenothiazylene vinylene-alt-1,4-phenylene vinylene] (P2), and poly[3,7-(4′-dodecyloxyphenyl)phenothiazylene vinylene-alt-2,5-thienylene vinylene] (P3), have been synthesized via a Horner-Emmons reaction. FTIR and 1H NMR spectroscopies confirmed that the configurations of the vinylene groups in the polymers were all-trans (E). The weight-averaged molecular weights (Mw) of P1, P2, and P3 were found to be 27,000, 22,000, and 29,000, with polydispersity indices of 1.91, 2.05, and 2.25, respectively. The thermograms for P1, P2, and P3 each contained only a broad glass transition, at 129, 167, and 155 °C, respectively, without the observation of melting features. UV–visible absorption spectra of the polymers showed two strong absorption bands in the ranges 315–370 nm and 450–500 nm, which arose from absorptions of the phenothiazine segments and the conjugated main chains. Solution-processed field-effect transistors fabricated from these polymers showed p-type organic thin-film transistor characteristics. The field-effect mobilities of P1, P2, and P3 were measured to be 1.0 × 10−4, 3.6 × 10−5, and 1.0 × 10−3 cm2 V−1 s−1, respectively, and the on/off ratios were in the order of 102 for P1 and P2, and 103 for P3. Atomic force microscopy and X-ray diffraction analysis of thin films of the polymers show that they have amorphous structures. A photovoltaic device in which a P3/PC71BM (1/5) blend film was used as the active layer exhibited an open-circuit voltage (VOC) of 0.42 V, a short circuit current (JSC) of 5.17 mA cm−2, a fill factor of 0.35, and a power conversion efficiency of 0.76% under AM 1.5 G (100 mW cm−2) illumination. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 635–646, 2010

Organic Thin-Film Transistors: Part I—Compact DC Modeling

A generic analytical model for the current-voltage characteristics of organic thin-film transistors (OTFTs) is derived. Based on this generic model, a TFT compact dc model that meets the requirements for compact modeling, including for computer circuit simulators, is proposed. The models are fully symmetrical, and the TFT compact dc model covers all regimes of TFT operation-linear and saturation above threshold, subthreshold, and reverse biasing. The empirical fitting parameters are mostly eliminated from the characteristic equations. The developed models are also in close correspondence to several physical, parametric, and limiting models for current-voltage and mobility characteristics. An essential practical feature of the TFT compact dc model is that the model is both upgradable and reducible, allowing for easier implementation and modifications and also simultaneously allowing for separation of characterization techniques. This allows for systematic fitting of experimental data with large scattering in the values, but at the same time, preserving consistently the OTFT behavior in the model.

Ordering of pentacene in organic thin film transistors induced by irradiation of infrared light

The device performances of pentacene-based organic thin film transistors (OTFTs) were greatly improved by irradiation of infrared light. The field effect mobility and maximum drain current increase from 0.20±0.01 to 0.57±0.02 cm2/V s and 1.14×10−5 to 4.91×10−5 A, respectively. The (001) peak of the pentacene “thin film” phase increases in intensity by 4.5 times after infrared irradiation at 50 W for 2 h. Two types of crystal orientations, i.e., “crystal I” (2θ=5.91°) and “crystal II” (2θ=5.84°), coexist in the pentacene. The improvement of the characteristics of OTFTs is attributed to crystallization and crystal reorientation induced by infrared light.

Characteristics of Organic Thin-Film Transistors with Self-Assembled Monolayer Formed in Gas-Phase on PVP Gate Insulator

The electrical characteristics of organic thin film transistors (OTFTs) with self-assembled monolayer (SAM) on the poly(4-vinylphenol) gate insulator have been investigated. The SAM was formed in gas-phase using the atomic layer deposition method onto gate insulator. It is observed that SAM modifies the surface polarity of gate insulator and influence the growth of the subsequent pentacene layer. Improvements of the crystallinity and directionality of pentacene molecules are confirmed by X-ray diffraction measurement. Also, the roughness of the pentacene thin film and the device performance are improved.

Condensation and Annealing Effects of Polymer Gate Insulator on Organic Thin-Film Transistor Characteristics

Performances of organic thin-film transistors (TFTs) can be detrimentally affected by the state of gate dielectric layer. In this study, we report the condensation and annealing effects of a polymer gate dielectric layer. For the observations of the effect of condensation, the spin-coated polymer layers with various deposition conditions were fabricated and kept under low vacuum condition for several days. It is observed that the thickness of polymer layer and the electrical characteristics of organic TFTs vary with the condensation time, and the appropriate additional annealing process could stabilize the gate dielectric layer and even improve the device characteristics consequently.

Performance of Organic Thin-Film Transistors Controlled by Electrode and Device Structures

We have fabricated organic thin-film transistors (OTFTs) of top contact (TC) structures using silver electrode based on triethylsilylethynyl anthradithiophene (TES-ADT) with mobility above 0.41 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , current modulation higher than 5 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> and sub-threshold swing below 0.65 V/dec. The electrical characteristics of OTFTs are not only corresponding to the work function of source and drain electrodes materials but also to the surface tension and deposition energy of them. The effects of work function and surface tension dominate the electrical characteristics in bottom contact (BC) device. On the other hand, TC device is affected by deposition energy dominantly.

Investigation of the Thin Silicon Oxide Deposited with Different Main Gases and Gap Distances of Atmospheric-pressure Plasma Jet at Low Temperature for OTFTs Gate Insulator

Low operation voltage organic thin film transistors (OTFTs) were successfully fabricated with top contact structure at low fabrication temperature. The thin gate dielectric of OTFTs were deposited by atmospheric pressure plasma jet at substrate temperature about 150° and under atmospheric pressure. The environment of processes would significantly improve the abilities of large area application for display and decrease the cost of instruments. We found that the quality of silicon oxide deposited with atmospheric pressure plasma jet strongly depended on the main gas and the gap distance between the plasma nozzle and the surface of the device and even influenced the deposition rate. Due to the improve of gate insulator quality, excellent electrical characteristics of OTFTs were obtained, including carrier mobility as large as 0.66 cm2/ V . s, operation voltage as low as -2 V, and subthreshold swing as low as 0.7 V/dec.

Fabrication of the flexible pentacene thin-film transistors on 304 and 430 stainless steel (SS) substrate

Flexible organic thin-film transistors (OTFT) were fabricated on 304 and 430 stainless steel (SS) substrate with aluminum oxide as a gate insulator and pentacene as an organic semiconductor. Chemical mechanical polishing (CMP) process was used to study the effect of the SS roughens on the dielectric properties of the gate insulator and OTFT characteristics. The surface roughness was decreased from 33.8 nm for 304 SS and 19.5 nm for 430 SS down to ∼2.5 nm. The leakage current of the metal–insulator–metal (MIM) structure (Au/Al 2O 3/SS) was reduced with polishing. Mobility and on/off ratio of pentacene TFT with bare SS showed a wide range of values between 0.005 and 0.36 cm 2/Vs and between 10 3 and 10 5 depending on the location in the substrate. Pentacene TFTs on polished SS showed an improved performance with a mobility of 0.24–0.42 cm 2/Vs regardless of the location in the substrate and on/off ratio of ∼10 5. With self assembled monolayer formation of octadecyltrichlorosilane (OTS) on insulator surface, mobility and on/off ratio of pentacene TFT on polished SS was improved up to 0.85cm 2/Vs and ∼10 6. I– V characteristics of pentacene TFT with OTS treated Al 2O 3/304 SS was also obtained in the bent state with a bending diameter ( D) of 24, 45 or 70 mm and it was confirmed that the device performed well both in the linear regime and the saturation regime.

Effect of the work function of gate electrode on hysteresis characteristics of organic thin-film transistors with Ta 2O 5/polymer as gate insulator

Organic thin-film transistors (OTFTs) using high dielectric constant material tantalum pentoxide (Ta 2O 5) and benzocyclobutenone (BCBO) derivatives as double-layer insulator were fabricated. Three metals with different work function, including Al (4.3 eV), Cr (4.5 eV) and Au (5.1 eV), were employed as gate electrodes to study the correlation between work function of gate metals and hysteresis characteristics of OTFTs. The devices with low work function metal Al or Cr as gate electrode exhibited high hysteresis (about 2.5 V threshold voltage shift). However, low hysteresis (about 0.7 V threshold voltage shift) OTFTs were attained based on high work function metal Au as gate electrode. The hysteresis characteristics were studied by the repetitive gate voltage sweep of OTFTs, and capacitance–voltage ( C– V) and trap loss-voltage ( G p/ ω− V) measurements of metal–insulator–semiconductor (MIS) devices. It is proved that the hysteresis characteristics of OTFTs are relative to the electron injection from gate metal to Ta 2O 5 insulator. The electron barrier height between gate metal and Ta 2O 5 is enhanced by using Au as gate electrode, and then the electron injection from gate metal to Ta 2O 5 is reduced. Finally, low hysteresis OTFTs were fabricated using Au as gate electrode.