Top-contact Thin-film Transistors Research Articles

Novel Thiophene–Phenylene–Thiophene Fused Bislactam-Based Donor–Acceptor Type Conjugate Polymers: Synthesis by Direct Arylation and Properties

Three new donor–acceptor copolymers based on thiophene–phenylene–thiophene fused bislactam and various donors (3,4-dodecylthiophene, 4,4′-didodecyl-2,2′-bithiophene, and ethylenedioxythiophene) were synthesized, characterized, and used in field-effect transistors. Polycondensation was performed using nonactivated thiophene derivatives by employing palladium-catalyzed direct arylation under phosphine-free conditions. This method is superior to traditional cross-coupling polymerization because it requires fewer synthetic operations and does not employ toxic organometallic intermediates. Regioselective polymers can also be generated by using β-substituted thiophene derivatives. The studied polymers were tested in a bottom gate top contact thin film transistor (OTFT) architecture. The best electronic performance was shown by polymer P3, with enhanced π-conjugation due to the appearance of intramolecular attractive interactions.

Trilayer ZnO Thin-Film Transistors With In Situ ${\rm Al}_{2}{\rm O}_{3}$ Passivation

In this letter, we report trilayer ZnO thin-film transistors (TFTs) with in situ Al2O3 passivation fabricated using plasma-enhanced atomic layer deposition. The bottom-gate, top-contact TFTs use an Al2O3-ZnO-Al2O3 trilayer deposited in one deposition run at 200°C that provides protection for the active layer back surface with no extra passivation step. Compared with Al2O3 passivated, nontrilayer ZnO TFTs, these trilayer devices have similar field effect mobility, but more positive turn-on voltage and improved bias stability. Seven-stage trilayer ZnO TFT ring oscillators operated at 3.5 MHz at a supply voltage of 17 V, corresponding to a propagation delay of ~ 27 ns/stage.

High-mobility organic thin-film transistors based on a small-molecule semiconductor deposited in vacuum and by solution shearing

The small-molecule organic semiconductor 2,9-di-decyl-dinaphtho-[2,3-b:2′,3′-f]-thieno-[3,2-b]-thiophene (C10-DNTT) was used to fabricate bottom-gate, top-contact thin-film transistors (TFTs) in which the semiconductor layer was prepared either by vacuum deposition or by solution shearing. The maximum effective charge-carrier mobility of TFTs with vacuum-deposited C10-DNTT is 8.5cm2/Vs for a nominal semiconductor thickness of 10nm and a substrate temperature during the semiconductor deposition of 80°C. Scanning electron microscopy analysis reveals the growth of small, isolated islands that begin to coalesce into a flat conducting layer when the nominal thickness exceeds 4nm. The morphology of the vacuum-deposited semiconductor layers is dominated by tall lamellae that are formed during the deposition, except at very high substrate temperatures. Atomic force microscopy and X-ray diffraction measurements indicate that the C10-DNTT molecules stand approximately upright with respect to the substrate surface, both in the flat conducting layer near the surface and within the lamellae. Using the transmission line method on TFTs with channel lengths ranging from 10 to 100μm, a relatively small contact resistance of 0.33kΩcm was determined. TFTs with the C10-DNTT layer prepared by solution shearing exhibit a pronounced anisotropy of the electrical performance: TFTs with the channel oriented parallel to the shearing direction have an average carrier mobility of (2.8±0.3)cm2/Vs, while TFTs with the channel oriented perpendicular to the shearing direction have a somewhat smaller average mobility of (1.3±0.1)cm2/Vs.

6,13-Dicyano pentacene-2,3:9,10-bis(dicarboximide) for solution-processed air-stable n-channel field effect transistors and complementary circuit

Aromatic diimides have attracted much attention due to their unique properties and potential applications for n-channel organic field effect transistors (OFETs). In this study, N,N′-dialkyl-6,13-dicyano pentacene-2,3:9,10-bis(dicarboximide) (1) was synthesized and used in n-channel OFETs. The photophysical, electrochemical and thermal properties of 1 were investigated. The low LUMO energy level (−4.15 eV) located within the air stability window ensures air-stable n-channel operation. Solution-processed top-contact thin film transistors based on such linear aromatic dicarboximide showed typical n-channel characteristics with a high electron mobility of up to 0.08 cm2 V−1 s−1 and a high Ion/Ioff ratio of 106 to 107. The devices exhibited good air stability when exposed to air for several months. Complementary inverters based on n-type 1 and p-type TIPS-pentacene were fabricated, which showed a maximum voltage gain (−dVOUT/dVIN) of 57.

Flexible Active-Matrix Organic Light-Emitting Diode Display Using Air-Stable Organic Semiconductor of $\hbox{Dinaphtho}[\hbox{2}, \hbox{3}\hbox{-b}: \hbox{2}^{ \prime}, \hbox{3}^{\prime} \hbox{-f}]\hbox{thieno}[\hbox{3}, \hbox{2} \hbox{-b}] \hbox{-} \hbox{thiophene}$

We developed a flexible active-matrix (AM) organic light-emitting diode (OLED) display driven by an organic thin-film transistor (TFT) (OTFT) using an air-stable organic semiconductor (OSC) of dinaphtho[2, 3 -b:2', 3' -f]thieno[ 3, 2 -b]-thiophene (DNTT) for the first time. We employed a low-temperature cross-linkable olefin-type polymer as the gate insulator (GI) and investigated the properties of the interface between DNTT and polymer GI. The top-contact TFT demonstrated a high mobility of up to 0.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s and a near-zero turn-on voltage. DNTT has a deeper highest occupied molecular orbital level than other OSC materials, and this leads to large contact resistance between the source/drain (S/D) contact and OSC. Surface modifications to the S/D contact were investigated to enable efficient carrier injection to fabricate high-performance bottom-contact TFTs. The short-channel TFT we fabricated exhibited a high hole mobility of 0.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s, a low subthreshold slope of 0.31, and excellent environmental and operational stability. The DNTT-based TFT also demonstrated good applications to processes with less deterioration. Finally, a 5-in flexible OLED display was successively fabricated by integrating the AM backplane with a phosphorescent OLED device. A high luminescence over 300 cd/ m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved by driving the DNTT-based OTFTs.

Chemical bath deposition of CdS channel layer for fabrication of low temperature-processed thin-film-transistors

∼66 nm thick CdS film with a hexagonal structure was uniformly generated via a low temperature-processed chemical bath deposition at 80 °C using a complexing agent of ethylenediaminetetraacetic acid and its crystal structure, surface morphology, optical transmittance, and Raman scattering property were measured. Grown CdS film was used as a channel layer for the fabrication of bottom-gate, top-contact thin-film-transistor (TFT). The TFT device with 60°C-dried channel layer exhibited a poor electrical performance of on-to-off drain current ratio (Ion/Ioff) of 5.1 × 103 and saturated channel mobility (μsat) of 0.10 cm2/Vs. However, upon annealing at 350 °C, substantially improved electrical characteristics resulted, showing Ion/Ioff of 5.9 × 107 and μsat of 5.07 cm2/Vs. Furthermore, CdS channel layer was chemically deposited in an identical way on a transparent substrate of SiNx/ITO/glass as part of transparent TFT fabrication, resulting in Ion/Ioff of 5.8 × 107 and μsat of 2.50 cm2/Vs.

Influence of substrate temperature on the performance of zinc oxide thin film transistor

Top-contact thin film transistors (TFTs) using radio frequency (RF) magnetron sputtering zinc oxide (ZnO) and silicon dioxide (SiO2) films as the active channel layer and gate insulator layer, respectively, were fabricated. The performances of ZnO TFTs with different ZnO film deposition temperatures (room temperature, 100 °C and 200 °C) were investigated. Compared with the transistor with room-temperature deposited ZnO films, the mobility of the device fabricated at 200 °C is improved by 94% and the threshold voltage shift is reduced from 18 to 3 V (after 1 h positive gate voltage stress). Experimental results indicate that substrate temperature plays an important role in enhancing the field effect mobility, sharping the subthreshold swing and improving the bias stability of the devices. Atomic force microscopy was used to investigate the ZnO film properties. The reasons for the device performance improvement are discussed.

Transparent dual-gate InGaZnO thin film transistors: OR gate operation

Transparent dual-gate (DG) InGaZnO4 thin film transistors for OR logic operation were fabricated on a glass substrate. A 100-nm-thick SiO2 layer used as both top and bottom gate dielectrics was deposited by plasma enhance chemical vapor deposition at 200°C. Compared to bottom gate, top gate thin film transistors (TFTs) exhibited better device performance with higher saturation mobility, drain current on-to-off ratio, lower threshold voltage, and subthreshold gate-voltage swing. This improved performance was mainly attributed to low process-induced damage or low parasitic capacitance between gate and source/drain and low parasitic resistance between channel and source/drain in top-contact TFT configuration (coplanar type). DG-mode TFTs showed saturation mobility of ∼16.9cm2V−1s−1, drain current on-to-off ratio of ∼1×106, subthreshold gate-voltage swing of ∼0.33Vdecade−1, and threshold voltage of ∼1.25V. The results demonstrate that DG InGaZnO4 TFTs are effective in improving the device performance because the channel layer is modulated independently by a top or, bottom gate signal and are well suited for OR gate operation.

Functionalized Asymmetric Linear Acenes for High‐Performance Organic Semiconductors

AbstractA series of compounds from the tetraceno[2,3‐b]thiophene and the anthra[2,3‐b]thiophene family of semiconducting molecules has been made. Specifically, synthetic routes to functionalize the parent molecules with bromo and then hexyl groups are shown. The bromo‐ and hexyl‐functionalized tetraceno[2,3‐b]thiophene and anthra[2,3‐b]thiophene were characterized in the top‐contact thin‐film transistor (TFT) geometry. They give high mobilities, ranging from 0.12 cm2 V−1 s−1 for α‐n‐hexylanthra[2,3‐ b]thiophene to as high as 0.85 cm2 V−1 s−1 for α‐bromotetraceno[2,3‐b]thiophene. Notably, grain size increases, going from the shorter anthra[2,3‐b]thiophene core to the longer tetraceno[2,3‐b]thiophene core, with a corresponding increase in mobility. The transition from undesirable 3D to desirable 2D thin‐film growth is explained by the increase in length of the molecule, in this case by one benzene ring, which results in an increase in intralayer interactions relative to interlayer interactions.

Enhanced performances of ZnO-TFT by improving surface properties of channel layer

Top-contact thin film transistors with ZnO as the channel layer and thermally grown SiO 2 as the gate dielectric were fabricated by using rf sputtering. The performances of ZnO-TFTs with different thicknesses of the active layer were investigated and the optimized condition was obtained. With the active layer thickness from 25 to 70 nm, the leakage current of devices increased from 10 −10 to 10 −8 A, and the on/off ratio decreased from 1.2×10 7 to 2×10 4. Atomic force microscope research indicated that with the thickness increased, the surface morphology of the active layer improved noticeably at first and then deteriorated. The 25-nm-thick ZnO TFT had the best surface morphology, and showed the best performance with a field effect mobility of 5.1 cm 2/V S, on/off ratio of 1.2×10 7 and threshold voltage of 20 V. This indicates that the surface properties of the channel layer have crucial affects on the performances of ZnO-TFTs.

Establishing Efficient Electrical Contact to the Weak Crystals of Triethylsilylethynyl Anthradithiophene

Triethylsilylethynyl anthradithiophene (TES ADT) forms weak van der Waals crystals in the solid state because its bulky TES side groups limit intermolecular interactions. Consequently, TES ADT melts easily and locally when it experiences heat conduction from the metal evaporation process to form electrical contacts. The performance of TES ADT thin-film transistors is thus highly dependent upon the manner in which electrical contacts are established to the organic semiconductor. Bottom-contact TES ADT thin-film transistors in which the electrodes are fabricated prior to the organic semiconductor deposition routinely exhibit a charge-carrier mobility of 0.11 ± 0.05 cm2/V·s. Top-contact thin-film transistors with electrodes patterned directly on top of TES ADT by metal evaporation through a shadow mask, on the other hand, exhibit highly variable device characteristics with a charge-carrier mobility 0.03 ± 0.03 cm2/V·s. To avoid thermal damage to TES ADT during electrode fabrication, we separately defined gold source and drain electrodes on elastomeric stamps and then laminated the electrodes against TES ADT to form top-contact devices. These laminated top-contact thin-film transistors exhibit device characteristics with minimal current−voltage hysteresis and an enhanced charge-carrier mobility of 0.19 ± 0.06 cm2/V·s.

Studies on poly(methyl methacrylate) dielectric layer for field effect transistor: Influence of polymer tacticity

Electrical properties of three kinds of poly(methyl methacrylate) (PMMA) with different tacticity, i.e. isotactic( i), syndiotactic( s), and atatic( a), were investigated for the application in field effect transistor. Metal-insulator-silicon structures were fabricated via spin coating PMMA on heavily doped p-type silicon (p +-Si) followed by evaporating gold electrode. The electrical characteristics were remarkably improved by heat-treatment at temperatures 40 K above glass transition temperatures of PMMAs. Among the three PMMA isomers, i-PMMA was observed to possess the highest dielectric strength (1.1 MV/cm) with the lowest leakage current density, but also the lowest dielectric constant ( k = 2.5). Top-contact thin film transistors fabricated with the configuration of NiO x /pentacene/ i-PMMA/p +-Si, where the NiO x being used as a source/drain electrode, displayed relatively a decent field effect mobility of 0.042 cm 2/V-s which is not that low with such a low dielectric capacitor as thick i-PMMA film.

Self-Assembly, Molecular Ordering, and Charge Mobility in Solution-Processed Ultrathin Oligothiophene Films

Symmetrical α,ω-substituted quarter-(T4), penta-(T5), sexi-(T6), and heptathiophene (T7) oligomers containing thermally removable aliphatic ester solubilizing groups were synthesized, and their UV−vis and thermal characteristics were compared. Spun-cast thin films of each oligomer were examined with atomic force microscopy and near-edge X-ray absorption fine structure spectroscopy to evaluate the ability of the material to self-assemble from a solution-based process while maintaining complete surface coverage. Films of the T5−T7 oligomers self-assemble into crystalline terraces after thermal annealing with higher temperatures required to affect this transformation as the size of the oligomers increases. A symmetrical α,ω-substituted sexithiophene (T6-acid) that reveals carboxylic acids after thermolysis was also prepared to evaluate the effect of the presence of hydrogen-bonding moieties. The charge transport properties for these materials evaluated in top-contact thin film transistor devices were found to correlate with the observed morphology of the films. Therefore, the T4 and the T6-acid performed poorly because of incomplete surface coverage after thermolysis, while T5−T7 exhibited much higher performance as a result of molecular ordering. Increases in charge mobility correlated to increasing conjugation length with measured mobilities ranging from 0.02 to 0.06 cm2/(V·s). The highest mobilities were measured when films of each oligomer had an average thickness between one and two monolayers, indicating that the molecules become exceptionally well-ordered during the thermolysis process. This unprecedented ordering of the solution-cast molecules results in efficient charge mobility rarely seen in such ultrathin films.

Novel thiophene-aryl co-oligomers for organic thin film transistors

A series of thiophene-aryl co-oligomers with phenyl-based central units, i.e. phenyl, biphenyl, fluorene and phenanthrene, were synthesized, and their optical properties and charge carrier transporting properties were characterized. The co-oligomers with biphenyl and phenanthrene units exhibited significantly improved thermal stability. Absorption measurements and electrochemical characterization revealed that all co-oligomers have broader band gap and lower HOMO levels than the corresponding thiophene oligomer, α,ω-dihexylsexithiophene (DHα6T). Introduction of biphenyl and phenanthrene units afforded co-oligomers with the broadest band gap and lowest HOMO levels in comparison with the counterparts of phenyl and fluorene central units. Depending on the structure of the central units, the field-effect mobility (μTFT) of the co-oligomers from top-contact thin film transistors is in the range of (1–6.7) × 10−2 cm2 V−1 s−1. The rigid and planar central unit is favorable for good device performance. Among the co-oligomers, 2,7-bis(5′-hexyl-2,2′-bithien-5-yl)phenanthrene (DH-TTPhTT) exhibits the highest μTFT of 6.7 × 10−2 cm2 V−1 s−1, comparable to that of DHα6T in the same device configuration and fabrication conditions.

Surface potential profiling and contact resistance measurements on operating pentacene thin-film transistors by Kelvin probe force microscopy

Surface potentials of operating pentacene thin-film transistors (TFTs) with two different contact geometries (bottom or top) were mapped by Kelvin probe force microscopy (KFM). The surface potential distribution was used to isolate the potential drops at the source and drain contacts. These potential drops were converted to resistances by dividing by the appropriate drain current values. The bottom contact TFTs were contact limited at large gate voltages, while the top contact TFTs were not contact limited. In both geometries, the contact and the channel resistances decreased strongly with increasing (negative) gate bias but did not depend strongly on the drain bias. This study demonstrates the utility of KFM for visualizing charge transport bottlenecks in operating pentacene devices and for correlating electrical behavior with device structure by comparison of surface potential and topographic maps.

1/f noise in pentacene organic thin film transistors

We report on the flicker (1/f ) noise in pentacene organic thin film transistors (TFTs) of different designs. Our studies show that the TFT design affects the noise level and the noise dependence on the gate- and drain-source biases. The measured noise level was the lowest for the TFTs with a top source and drain contacts design. For these devices, the noise dependence at low drain current values resembled that for n-type crystalline Si metal–oxide–semiconductor field-effect transistors. The extracted Hooge parameter α, which allows comparing the noise level in different devices and materials, was 0.045 for the top-contact TFTs. This parameter value is several orders of magnitude lower than that for conducting polymers and only several times higher than that for hydrogenated amorphous Si (α-Si:H) TFTs. The bottom source and drain contacts TFTs had a much higher noise level with a noise dependence on the terminal voltages that differed from the noise voltage dependence for the top-contact TFTs. The Hooge parameter values were in the range of 5–20 for the bottom-contact TFTs. We estimated that the contact noise could be comparable to the channel noise for both top-contact and bottom-contact TFTs.