Bottom-contact Organic Thin-film Transistors Research Articles

A study of bottom-contact organic thin-film transistors based on source/drain tunneling structure

During recent years, OLED display has become one of the important products for consuming electronics and industrial domains. For AMOLED display, thin film transistor (TFT) is necessary for driving OLED devices. As the polysilicon and amorphous silicon TFTs have become mainstreams, organic thin film transistors (OTFTs) have also been proposed and shown their advantages in low-temperature process compatible with flexible substrates. However, OTFTs applicable to drive OLED generally adopt bottom-contact configuration. The performance of bottom-contact OTFTs are often inferior relative to top-contact OTFTs, due primarily to the high contact resistance of bottom-contact OTFTs. To overcome this disadvantage, the source/drain tunneling structure is proposed in this article. This tunneling structure can not only decorate the electrode/organic semiconductor interface to obtain a good metal-semiconductor contact, but also can form a MOS configuration at source/drain contact, so as to increase the carrier concentration at the contact to further reduce contact resistance. In this paper, a typical P-type high-mobility small molecule material of Pentacene is employed as the active layer of the OTFT, while CuO film, Polystyrene (PS) film is separately selected as the tunneling layer at source/drain contact. The diverse parameters, especially field effect mobility, were analyzed by analyzing electrical characteristics of the top-contact and bottom-contact OTFTs. In addition, the contact resistances were confirmed by line transfer method. The bottom-contact OTFTs incorporating tunneling layer own a maximum hole mobility of 0.143cm2/V·s.This suggested that the source/drain tunneling structure can indeed improve the electrical characteristics of bottom-contact OTFT, and further improvement is necessitated to achieve the high performance comparable to top-contact device.

Advantages of adding a weak second gate in sub-micron bottom-contact organic thin film transistors

It is shown that in bottom gate, bottom-contact sub micrometer organic thin-film transistors (TFTs) with Dinaphtho[2,3-b:2′,3′-f]thieno-[3,2-b]thiophene active semiconductors, the addition of a top gate results in substantial improvements in a whole range of electrical performance characteristics. These improvements are not due to additional charge induced by the top gate but originate from the spatial redistribution of the charge induced by the bottom gate by the top gate. In the TFTs described in this work, the bottom gate insulator consists of a bilayer of SiO2 and solution-deposited high-k ZrO2, while the top gate insulator is a relatively thick CYTOP® layer deposited from a solution by spin coating. The capacitances per unit area of the bottom gate insulator and top gate insulator are 87 and 8 nF/cm2, respectively. On account of this large difference, the top gate is referred to as “weak,” as its ability to induce charges in the channel is very much limited. The use of a weak top gate is shown to result in large improvements in the on-state drain current, sub-threshold swing, threshold voltage, and contact resistance compared to single gate operation. Weak top gates are especially useful when processing constraints make it difficult to realize symmetric or close to symmetric dual gate designs. While this demonstration is for a specific materials system, the design principle is more general and is likely to benefit a range of short channel length TFTs including organic/polymer, amorphous metal oxide, and 2D semiconductor-based transistors.

Synthesis of thieno[3,4-c] pyrrole-4,6-dione-based small molecules for application in organic thin-film transistors

The synthesis of two alternating donor-acceptor (D-A) small molecules revolving around thieno[3,4-c]pyrrole-4,6-dione (TPD) as the main acceptor building block is reported herein. The materials are synthesized using a robust Stille coupling to attach the TPD core with thiophene donors on either of its side, and completed with an aldol condensation under mild conditions for the addition of terminal secondary acceptor building blocks, representing a sought out A2-D-A1-D-A2 structure. The two newly synthesized materials are set to be used as organic semiconductors in organic thin-film transistor (OTFT) devices. Comparative optical, computational, and electrochemical studies were carried out on these new materials to investigate their ability to act as potential n-type organic semiconductors. The materials were then incorporated into bottom-gate bottom-contact OTFT devices, and subsequent thin-film engineering studies were done to probe the effects that solvent and post-deposition annealing conditions had on device performance. From these conditions, the rhodanine-derived D-A material exhibited the greatest electron field-effect mobility (0.011 cm2 V−1 s−1), and a threshold voltage of 20.1 V under vacuum.

Photolithographic Fabrication of P3HT Based Organic Thin-Film Transistors with High Mobility

An original design and photolithographic fabrication process for Poly (3-hexylthiophene-2, 5-diyl) (P3HT) based Organic thin-film transistors (OTFTs) is presented. The structure of the transistors was based on the bottom gate bottom contact OTFT. The fabrication process was efficient, cost-effective, and relatively straightforward to implement. Most of the fabrication steps were performed at room temperature and atmospheric pressure, with the only exceptions being the high temperatures used for annealing the films and the low pressures used for depositing the metal contacts. More than 226 transistors were fabricated on a single wafer. The electrical characteristics and the geometry of the transistors were consistent across the wafer. Current–voltage (I–V) and atomic force microscopy (AFM) measurements were performed to characterize the primary electronic properties of the transistors and morphology of the P3HT, respectively. Two key performance parameters were extracted from these measurements, the threshold voltage and the field-effect mobility of the transistors. The measured mobility of these transistors was significantly higher than most results reported in the literature for other similar bottom gate bottom contact P3HT OTFTs. The higher mobility is explained primarily by the effectiveness of the fabrication process in keeping the interfacial layers free from contamination, as well as the annealing of the P3HT.

Fabrication, characterization, numerical simulation and compact modeling of P3HT based organic thin film transistors

This paper presents the fabrication, characterization and numerical simulation of poly-3-hexylthiophene (P3HT)-based bottom-gate bottom-contact (BGBC) organic thin film transistors (OTFTs). The simulation is based on a drift diffusion charge transport model and density of defect states (DOS) for the traps in the band gap of the P3HT based channel. It combines two mobility models, a hopping mobility model and the Poole–Frenkel mobility model. It also describes the defect density of states (DOS) for both tail and deep states. The model takes into account all the operating regions of the OTFT and includes sub-threshold and above threshold characteristics of OTFTs. The model has been verified by comparing the numerically simulated results with the experimental results. This model is also used to simulate different structure in four configurations of OTFT e.g. bottom-gate bottom-contact (BGBC), bottom-gate top-contact (BGTC), top-gate bottom-contact (TGBC) and top-gate top-contact (TGTC) configurations of the OTFTs. We also present the compact modeling and model parameter extraction of the P3HT-based OTFTs. The extracted compact model has been further applied in a p-channel OTFT-based inverter and three stage ring oscillator circuit simulation.

Device Performance Optimization of Organic Thin-Film Transistors at Short-Channel Lengths Using Vertical Channel Engineering Techniques

This paper presents a finite-element-based two-dimensional numerical simulation study of the vertical channel engineering approaches for controlling the short-channel effects (SCEs) in organic transistors based on thin-film transistor technology (OTFTs). The impact of gate-oxide thickness TOx scaling and usage of high-permittivity gate dielectric material has been analyzed for a bottom-contact organic thin-film transistors at channel length of 0.7 μm. The techniques have been used to investigate the impact on drain-induced barrier lowering (DIBL), sub-threshold slope, and IOn/IOff ratio. The results have shown a significant reduction in values of DIBL and sub-threshold slope in short-channel OTFTs when either of the channel engineering techniques are employed. A high IOn/IOff ratio of the order of ~107 has been achieved using a high-permittivity gate-oxide material. It has been observed that using a high-permittivity gate dielectric material, a peak value of IOn/IOff ratio can be achieved for an equivalent oxide thickness of 5 nm. The results suggest that the desirable transistor performance can be achieved through proper selection of gate-oxide material and thickness.

Contact resistance corrected-electrical characteristics with channel length effects in π-conjugated small-molecule benzanthracene organic thin film transistors

In this work, bottom-contact p-type organic thin-film transistors based on small-molecule benzanthracene with various channel lengths were fabricated, characterized, and theoretically investigated. The fabricated devices showed a pronounced shift for threshold voltage as the function of channel length. Electrical parameters characterizing fabricated devices have been systematically evaluated as a function of channel length with and without contact resistance in the calculation. Contact resistance (Rc) and channel resistance (Rch) are extracted individually using the transfer line method (TLM) for output characteristic curves that were done only at low drain voltage (linear regime). The effect of contact resistance on the values of various OTFTs parameters is more pronounced in short channel length devices. The obtained results revealed that the linear regime's corrected mobilities of short channel devices are about 15% smaller than uncorrected mobility. Comparatively, the corrected threshold voltage values were found to be nearly close to uncorrected values for all channel lengths. The analytical model was also applied to reproduce output characteristics in the linear regime of different channel lengths devices with or without contact resistance. It was found that the calculated results were in good agreement with the measured results (when considering the effect of contact resistance), which confirms that the used model can correctly describe the charge transport in these kinds of devices.

Real-Time Putrescine Detection by Triphenodioxazine-Based Organic Thin-Film Transistor Sensor

Air-stable p-type PFTPDOBT polymer with triphenodioxazine (TPDO) as the building block has been incorporated into a bottom-gate, bottom-contact organic thin-film transistor (OTFT) to detect putrescine (PUT) vapor in ambient conditions. Due to the doping effect, the PFTPDOBT devices showed increased mobility in air compared with in nitrogen. The highest mobility reached 0.03 cm2 V−1 s−1 after annealing at 250°C. The storage stability was evaluated based on the change in the mobility of PFTPDOBT sensors stored in air. Around 55% of the mobility remained after 1 month of air storage. Proper operational stability was obtained at an operating voltage of −100 V. Despite certain degradation, real-time detection of putrescine could be realized by IDS measurements through time under continuous bias. The detection limit of the PFTPDOBT sensor was found to be 9 ppm. Fast responses were shown with IDS and mobility decreases, which can be ascribed to a charge trapping effect induced by lone electron pairs in PUT molecules. Further improvements could be achieved to obtain better stability at low operational voltage by using dielectric materials with high dielectric constant, adding an encapsulation layer, or decreasing the capacitance per unit area. Such sensors with easy operation and low cost could be used for food safety inspection and marine environmental monitoring.

Phase segregation effect on TIPS pentacene crystallization and morphology for organic thin film transistors

In this study, we report that the vertical phase separation between a small-molecule organic semiconductor and a polymer additive can be utilized to drive semiconductor crystallization, enhance thin film morphology, and improve device performance of solution-processed organic thin film transistors (OTFTs). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was demonstrated as a benchmark semiconductor material to blend with a polyacrylate polymer additive poly(2-ethylhexyl acrylate) (P2EHA), it was found that a vertical phase segregation occurred between TIPS pentacene and P2EHA, providing a confinement of anisotropic semiconductor crystallization and resulting in an elevated TIPS pentacene concentration at the charge transport interface. Accordingly, distinctive TIPS pentacene thin film morphology in terms of areal coverage, grain width, and crystal orientation was obtained by varying the loading ratio of the P2EHA polymer additive. Bottom-gate and bottom-contact OTFTs were fabricated to test charge transport and a hole mobility of up to 0.27 cm2/Vs was demonstrated with 80% loading ratio of P2EHA. The effective experimental method and important results as we showcased in this work can be applied to drive crystallization and optimize film morphology of small-molecule organic semiconductors other than TIPS pentacene.

Electron Charge Transport in Non-Peripherally Substituted Copper Phthalocyanine

Bottom-gate, bottom-contact organic thin film transistors (OTFTs) were fabricated using solvent soluble copper-1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine as the active semiconductor layer. The compound was deposited as 70 nm thick spin-coated films onto gold source-drain electrodes supported on octadecyltrichlorosilane treated 250 nm thick SiO2 gate insulator. The analysis of experimental results showed the n-type field effect behaviour. Devices annealed at 100 oC under vacuum were found to exhibit the field-effect mobility of 0.0989 cm2 V−1 s−1, with an on/off current modulation ratio of ∼106, a reduced threshold voltage of 0.7 V and a sub-threshold swing of 2.12 V decade−1. The variations in surface morphology of the devices are found reflected considerably in the electrical measurements. The device contact resistance was found to be decreased as the gate bias increased and also with the annealing.

Low-Temperature Cross-Linking Benzocyclobutene Based Polymer Dielectric for Organic Thin Film Transistors on Plastic Substrates.

The synthesis of a new benzocyclobutene based polymer, PSBBB, designed as a dielectric material for use in organic thin film transistors was reported. Compared to conventional benzocyclobutene-based materials, the introduction of a butoxide substituent at the 7-position of the benzocyclobutene pendant unit on the polymer allowed PSBBB to be cross-linked at temperatures of 120 °C, thus rendering it compatible with the processing requirements of flexible plastic substrates. The cross-linking behavior of PSBBB was investigated by Fourier transform infrared spectroscopy and differential scanning calorimetry, demonstrating cross-linking of the polymer after curing at 120 °C. Bottom-gate bottom-contact organic thin film transistors were fabricated using PSBBB as dielectric, affording a performance comparable to that of other dielectric polymeric materials.

Hysteresis control by varying Ta2O5-nanoparticles concentration in PMMA-Ta2O5 bilayer gate dielectric of hybrid-organic thin film transistors

Abstract In this paper, we report the fabrication of a series of top-gate bottom-contact hybrid organic thin film transistors based on solution-processed semiconductors and dielectric layers on a glass substrate. Semiconductor active layers were based on a mixture of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) mixture and poly (3-hexylthiophene) (P3HT), whereas gate dielectric layers were based on PMMA mixed with various concentrations of high-k Ta2O5 inorganic particles. Experimental investigation indicated that the hysteresis phenomenon of the devices can be governed by changing of the Ta2O5 concentration in PMMA-Ta2O5 nanocomposites gate dielectric layer. In addition, significant enhancement of electron transport was also observed with the increase of Ta2O5 inorganic particles concentration in the gate layer, which arises from the change of the charge transport from unipolar to ambipolar type. The fabricated OTFTs, with significant hysteresis variation as a function of Ta2O5 nanoparticles content in PMMA-Ta2O5 gate dielectrics, prove particularly promising in consideration of possible use for low-cost nonvolatile memory elements and sensing array applications.

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.

Dimethylacetamide-promoted Direct Arylation Polycondensation of 6,6′-Dibromo-7,7′-diazaisoindigo and (E)-1,2-bis(3,4-difluorothien-2-yl)ethene toward High Molecular Weight n-Type Conjugated Polymers

A highly efficient and eco-friendly protocol for the synthesis of an alternating copolymer poly(7,7′-diazaisoindigo-alt-(E)-1,2-bis(3,4-difluorothien-2-yl)ethene) (PAIID-4FTVT) via direct arylation polycondensation (DArP) is presented. Through detailed study, we found that the inhibitory effect of 7,7′-diazaisoindigo on DArP stemmed from the coordination of N atom with catalyst can be overcome by using dimethylacetamide (DMAc) as the co-solvent. Thus, PAIID-4FTVT with number-average molecular weight (Mn) > 100 kDa was synthesized via DArP by optimizing the content of DMAc. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy revealed that PAIID-4FTVT was defect-free. Top gate and bottom contact (TG/BC) organic thin-film transistors (OTFTs) were fabricated to characterize the semiconducting properties of the polymers. PAIID-4FTVT displayed unipolar n-type characteristics with the electron mobility (µe) strongly dependent on Mn. The highest µe up to 0.25 cm2·V−1·s−1 was achieved with the high molecular weight sample.

Bottom-contact organic thin film transistors with transparent Ga-doped ZnO source-drain electrodes

In this study, transparent Ga-doped ZnO (GZO) films grown by magnetron sputtering were used as the source-drain (S-D) electrodes of bottom-contact organic field effect transistors (OTFTs). The GZO films exhibited good transmittance (beyond 80 %) in the visible region and the work functions of GZO films were found to be gradually increased with increasing the annealing temperature from 500 to 700 °C, leading to the reduction of contact barriers and improvement of device performances. OTFT fabricated by GZO S-D electrodes exhibited higher field-effect mobility of 0.0101 cm2/Vs and threshold voltage of −7.7 V when the GZO electrodes were annealed at 700 °C. To further increase the carrier injection, an ultra-thin CuPc film was used as the buffer layer prior to the deposition of Pentacene, and an increased mobility of 0.0257 cm2/vs was obtained. The results indicate that optimized transparent GZO films prepared by magnetron sputtering have great potential in the S-D electrodes of OTFTs.

An analytical modeling approach to the electrical behavior of the bottom-contact organic thin-film transistors in presence of the trap states

In this paper, a complete analytical model of the conduction behavior of organic thin-film transistors (OTFTs) within the framework of trap states has been presented. The aim of the work is to develop modeling equations which can be evolved into compact models generic to OTFTs rather than relying on the MOSFET-based models for the modeling and simulation of organic-based devices. The exponential density of trap states and the trap decay rate has been included in the modeling equations of the bottom-contact OTFTs with pentacene as the active layer. In this work, we propose for the first time a modeling approach to mobility. The mobility model takes into account the low-field mobility enhancement as well as high-field mobility degradation because of the presence of defect states. The current–voltage equations in the linear and saturation regimes have been modified to include the effect of trap states. The influence of trap states on the device capacitance has been modeled, and it has been found that the high density of trap states leads to deviation in the capacitance characteristics of OTFTs. Results obtained from the modeling equations of surface potential, total charge density, mobility and current–voltage relations show performance degradation with the increase in the density of trap states. The results predicted from the analytical modeling equations are in excellent agreement with the reported electrical behavior of the OTFTs, thus proving the feasibility of the proposed model. The proposed analytical model is valid for OTFTs in the linear and saturation regimes even for short-channel length transistors.

Polyacrylate polymer assisted crystallization: Improved charge transport and performance consistency for solution-processable small-molecule semiconductor based organic thin film transistors

Abstract In this study, we report on an effective approach to modulate crystallization, control charge transport and enhance performance consistency of small-molecule semiconductor based organic thin film transistors (OTFTs) with the addition of a polyacrylate polymer additive poly(2-ethylhexyl acrylate) (P2EHA). 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a benchmark semiconductor to blend with the P2EHA additive, leading to a vertical phase separation between these two components. The resultant TIPS pentacene film exhibited greatly reduced crystal misorientation, enlarged grain width and enhanced film coverage. Bottom-gate, bottom-contact OTFTs based on the TIPS pentacene/P2EHA blends were fabricated and showed an increased average hole mobility of 0.317 ± 0.047 cm2/V, as well as a performance consistency factor of 6.72, which is defined as the ratio of the average hole mobility to the standard deviation of mobility. Notably, it leads to a 10-fold and 7-fold enhancement of average mobility and performance consistency as compared to the pristine TIPS pentacene OTFTs. This great improvement of device performance can be attributed to the reduced crystal misorientation, less defects and trap centers at the grain boundaries as a result of the enlarged grain width, as well as increased film coverage, due to the addition of the P2EHA polyacrylate polymer additive.

Scaling of contact metal induced hysteresis in solution processed organic thin film transistors

Contact effect in organic thin film transistor (OTFT) with bottom contact configuration has been considered as a serious issue that renders inferior device performance. In this article, the contact between metal and semiconductor is unveiled to be responsible for hysteresis in bottom gate bottom contact OTFT, consisting of poly(3-hexylthiophene) as semiconductor and cross-linked poly(4-vinylphenol) as polymer dielectric. The correlation between the contact effect and hysteresis is demonstrated by comparing gold contact and silver contact devices with having low and high contact resistances respectively. Negligible hysteresis in gold contact devices in contrary to sizable hysteresis in silver contact devices ensures contact-assisted trapping of charge carriers as the origin of hysteresis. Variation of hysteresis on scaling down the channel length of silver contact devices validates the contact induced hysteresis. A simplistic analysis based on charge trapping in the vicinity of source terminal is proposed to explain the scaling behavior of hysteresis with channel length.

Donor–Acceptor Conjugated Polymers Based on Bisisoindigo: Energy Level Modulation toward Unipolar n-Type Semiconductors

Four bisisoindigo (bis-IID) derivatives containing different numbers of electron withdrawing fluorine (F) or nitrogen (N) atoms and eight alternating copolymers of them with 2,2′-bithophene (BT) or 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4FBT) were synthesized. Depending on the numbers of F and N atoms incorporated, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were feasibly tuned in the ranges of −5.51 to −6.06 eV and −3.64 to −4.01 eV, respectively. Top gate and bottom contact (TGBC) organic thin-film transistors (OTFTs) were fabricated to study the semiconducting properties of the polymers. Five polymers, i.e., P0F-BT, P2F-BT, P4F-BT, P2N2F-BT, and P0F-4FBT that contain 0–4 F and N atoms in the repeating unit, with HOMO and LUMO energy levels above −5.90 and −3.80 eV, respectively, all showed ambipolar OTFT characteristics. Three other polymers P2F-4FBT, P4F-4FBT, and P2N2F-4FBT that contain 6 or 8 F and N atoms with HOMO ...

Difluorobenzothiadiazole and Selenophene-Based Conjugated Polymer Demonstrating an Effective Hole Mobility Exceeding 5 cm2 V-1 s-1 with Solid-State Electrolyte Dielectric.

We report synthesis of a new poly(4-(4,4-bis(2-ethylhexyl)-4 H-silolo[3,2- b:4,5- b']dithiophene-2-yl)-7-(4,4-bis(2-ethylhexyl)-6-(selenophene-2-yl)-4 H-silolo[3,2- b:4,5- b']dithiophene-2-yl)-5,6-difluorobenzo[ c][1,2,5]thiadiazole (PDFDSe) polymer based on planar 4,7-bis(4,4-bis(2-ethylhexyl)-4 H-silolo[3,2- b:4,5- b']dithiophen-2-yl)-5,6-difluorobenzo[ c][1,2,5]thiadiazole (DFD) moieties and selenophene linkages. The planar backboned PDFDSe polymer exhibits highest occupied molecular orbital and lowest unoccupied molecular orbital levels of -5.13 and -3.56 eV, respectively, and generates well-packed highly crystalline states in films with exclusive edge-on orientations. PDFDSe thin film was incorporated as a channel material in top-gate bottom-contact organic thin-film transistor with a solid-state electrolyte gate insulator (SEGI) composed of poly(vinylidene difluoride-trifluoroethylene)/poly(vinylidene fluoride- co-hexafluroropropylene)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, which exhibited a remarkably high hole mobility up to μ = 20.3 cm2 V-1 s-1 corresponding to effective hole mobility exceeding 5 cm2 V-1 s-1 and a very low threshold voltage of -1 V. These device characteristics are associated with the high carrier density in the semiconducting channel region, induced by the high capacitance of the SEGI layer. The excellent carrier mobility from the PDFDSe/SEGI device demonstrates a great potential of semiconducting polymer thin-film transistors as electronic components in future electronic applications.