Top-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.

Characterization and modeling of threshold voltage for organic and amorphous thin-film transistors

This work portrays the study and analysis of several extraction techniques that are typically used to ascertain the threshold voltage of organic thin-film transistors (OTFTs) and oxide-based transistors. This study evaluates the effects of physical and electrical parameters such as channel length, doping concentration, temperature, and trap on the threshold voltage (VTH) of both p-type and n-type bottom gate top contact (BGTC) OTFTs using graphical method. The methods employed in the present work to extract the threshold voltage are determined in the linear region of operation and are derived from the drain current (IDS) and the gate-source voltage (VGS) transfer characteristics. To compare and analyze the threshold voltage, various techniques such as the linear extrapolation method (ELR), constant current method (CC), transconductance linear extrapolation method (GMLE), second derivative method (SD), ratio method (RM), Inverse Ratio method (VX), and transition method (TM) are employed in the present work. In addition to DC characterization, the capacitance-voltage (C-V) method was also used to extract and compare the threshold voltage using all the conventional techniques listed above. 2-D numerical simulations were performed to assess the validity and usefulness of the various methodologies used for the extraction of the threshold voltage.

Impact of Scaling of Dielectric Thickness on Subthreshold Slope in Top-Contact Pentacene Organic Thin Film Transistors

In the present work, we have fabricated the bottom-gate top-contact configuration of the pentacene organic thin film transistors (OTFTs) using two different dielectric layers. The effect of scaling of poly(methyl methacrylate) (PMMA) and cross-linkable poly(4-vinyl phenol) (PVP) polymer dielectric thickness on the subthreshold slope (SS) in top-contact pentacene OTFTs was investigated. Subthreshold slope improved significantly with the scaling of the thickness of both dielectrics.

Polyferrocenylsilane Semicrystalline Polymer Additive for Solution-Processed p-Channel Organic Thin Film Transistors

In this study, we demonstrated for the first time that a metal-containing semicrystalline polymer was used as an additive to mediate the thin film morphology of solution-grown, small-molecule organic semiconductors. By mixing polyferrocenylsilane (PFS) with an extensively-studied organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene), PFS as a semicrystalline polymer independently forms nucleation and crystallization while simultaneously ameliorating diffusivity of the blend system and tuning the surface energies as a result of its partially amorphous property. We discovered that the resultant blend film exhibited a 6-fold reduction in crystal misorientation angle and a 3-fold enlargement in average grain width. Enhanced crystal orientation considerably reduces mobility variation, while minimized defects and trap centers located at grain boundaries lessen the adverse impact on the charge transport. Consequently, bottom-gate, top-contact organic thin film transistors (OTFTs) based on the TIPS pentacene/PFS mixture yielded a 40% increase in performance consistency (represented by the ratio of average mobility to the standard deviation of mobility). The PFS semicrystalline polymer-controlled crystallization can be used to regulate the thin film morphology of other high-performance organic semiconductors and shed light on applications in organic electronic devices.

Thermally stable organic thin film transistors based on 2-(anthracen-2-yl)tetracene

Abstract Organic semiconductors with high mobility and high thermal stability are of great importance for practical application of organic electronics. To explore new semiconductors by taking advantage of the intrinsic properties of tetracene molecule, herein, we report the design and synthesis of a novel p-type tetracene derivatives, 2-(anthracen-2-yl)tetracene (TetAnt). Top contact organic thin-film transistors (OTFTs) based on TetAnt show a hole mobility of up to 0.79 cm2 V−1 s−1. In addition, a high mobility of ~0.4 cm2 V−1 s−1 is maintained even after thermal stressed to a high temperature of 290 °C, indicating the excellent thermal stability of TetAnt.

Controlled Synthesis of Poly(pentafluorostyrene-ran-methyl methacrylate) Copolymers by Nitroxide Mediated Polymerization and Their Use as Dielectric Layers in Organic Thin-film Transistors.

A library of statistically random pentafluorostyrene (PFS) and methyl methacrylate (MMA) copolymers with narrow molecular weight distributions was produced, using nitroxide mediated polymerization (NMP) to study the effect of polymer composition on the performance of bottom-gate top-contact organic thin-film transistors, when utilized as the dielectric medium. Contact angle measurements confirmed the ability to tune the surface properties of copolymer thin films through variation of its PFS/MMA composition, while impedance spectroscopy determined the effect of this variation on dielectric properties. Bottom-gate, top-contact copper phthalocyanine (CuPc) based organic thin-film transistors were fabricated using the random copolymers as a dielectric layer. We found that increasing the PFS content led to increased field-effect mobility, until a point after which the CuPc no longer adhered to the polymer dielectric.

A facile and novel route to improve TIPS pentacene based organic thin film transistor performance with elastomer

In this study, we report a facile route to regulate the crystallization and enhance charge transport of small-molecule organic semiconductors by blending with a rubbery polymer additive polyisobutylene (PIB). Using a solution-processed semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) to mix with the PIB polymer, we show that the addition of PIB led to densely-arranged, lengthened TIPS pentacene needles with excellent long-range uniformity. Bottom-gate, top-contact organic thin-film transistors (OTFTs) based on TIPS pentacene/PIB hybrid film demonstrated a field-effect mobility of up to 0.15 cm2/Vs, and particularly, a 3-fold and 2-fold enhancement of average mobility and performance consistency (denoted by average mobility to standard deviation ratio). In addition, the effect of hexamethyldisilazane (HMDS) surfactant treatment on the semiconductor charge transport was studied. We ascribe such improved performance of TIPS pentacene/PIB blend OTFTs to the collaborative outcome of reduced crystal misorientation, diminished defects and charge trap centers, as well as a higher areal coverage. The effective method as demonstrated in this work can be generally implemented to attune the crystal growth and promote charge transport of newly-developed small-molecule organic semiconductors.

Ultra-low misorientation angle in small-molecule semiconductor/polyethylene oxide blends for organic thin film transistors

In this work, we report for the first time the use of a nonconjugated semicrystalline polymer as a film-forming agent to control the crystallization and tune the charge transport of solution-processed, small-molecule organic semiconductors. When 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene) was demonstrated as a representative material to blend with polyethylene oxide (PEO) polymer, it crystallized into uniformly-aligned needles with reduced random orientation, enhanced long-range order and elevated areal coverage. Specifically, an ultra-low misorientation angle of 7.9° ± 3.5° was obtained with 10% PEO additive, beneficial for charge transport in the TIPS pentacene/PEO hybrid film. Bottom-gate, top-contact organic thin film transistors (OTFTs) based on TIPS pentacene/PEO mixture were found to show a field-effect mobility up to 2.5 × 10−2 cm2/Vs. This work may be universally applied to other organic semiconductors to regulate their crystal formation, enhance film forming and improve device performance of OTFTs. It contributes to the utilization of flexible substrates for future-generation high-performance organic electronics.

Poly(α-methylstyrene) polymer and small-molecule semiconductor blend with reduced crystal misorientation for organic thin film transistors

The electrical performance of solution-processed, small-molecule organic semiconductors is largely restricted by their severe charge carrier mobility variations. In this work, we demonstrate an effective method to reduce such variations of the semiconductor mobilities and improve the performance consistency of organic thin film transistors (OTFTs) by adding poly(α-methylstyrene) (PαMS) as a polymer additive. By using 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) as an example, we found that while pristine TIPS pentacene film exhibited random crystal orientation and large film gaps, the addition of PαMS polymer promoted the growth of semiconductor crystals, which formed uniformly aligned needles with significantly improved orientation and coverage within the channel regions. An ultra-low misorientation angle of 2.2° ± 1° was obtained from TIPS pentacene/PαMS blend film, which is a 20-fold reduction as compared to pristine TIPS pentacene. Bottom-gate, top-contact OTFTs with TIPS pentacene crystals aligned perpendicularly from source to drain contact electrodes demonstrated a hole mobility of up to 0.26 cm2/V s, as well a 6-fold enhancement of average mobility as compared to the pristine TIPS pentacene based counterparts. Notably, the addition of PαMS led to a performance consistency factor of 3.35, as defined by the ratio of the average mobility to standard deviation, implying a great reduction of charge carrier mobility variations. The improvement of electrical performance of OTFTs can be attributed to the combined outcome of crystal rigid alignment, extended long-range order, and almost full coverage of charge transport channel.

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.

Augmenting n-Type Performance of Ambipolar Top-Contact Organic Thin-Film Transistors by Self-Generated Interlayers

We report n-type performance enhancement of an ambipolar donor–acceptor (D–A) polymer by suppressing its p-type behavior using a simple and versatile methodology of interlayer processing in top-contact organic field-effect transistors. In this approach, a judiciously selected interlayer material, polyethylene glycol (PEG), is codeposited as an additive with the semiconducting polymer DPP-T-TT active layer. During top-contact aluminum deposition, the PEG molecules migrate from within the bulk film to the organic/Al interface because of additive–metal interactions. The formation of the PEG interlayer was confirmed by X-ray photoelectron spectroscopy and its effect on the polymeric-interfacial microstructure was studied using angle-dependent grazing-incidence wide-angle X-ray scattering. We find that the PEG interlayer has two significant contributions to the n-type characteristics of the device: it suppresses hole injection while improving electron injection by tuning the effective work-function, and it enh...

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.

Nanoscale alignment of semiconductor crystals for high-fidelity organic electronics applications

In this work, we report for the first time the combination of silicon dioxide (SiO2) nanoparticles and poly (styrene-block-4-vinylpyridine) (PS-b-P4VP) block copolymer supplements to tune random crystal growth and improve charge transport of small-molecule organic semiconductors. We chose 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) as a model semiconductor material to combine with both supplements. This led to a rigid alignment of the crystals that extended in long-range order as well as an increased coverage of the charge transport channel. In particular, the addition of 5% PS-b-P4VP supplement contributed to a low irregularity angle of 8.8° ± 6.7°, which indicates an 80% reduction of crystal random orientation. Bottom-gate, top-contact organic thin film transistors (OTFTs) based on the TIPS-PEN/SiO2/PS-b-P4VP combination film showed an average hole mobility of up to 0.033 ± 0.02 cm2/Vs and a 50% improvement of charge transport fidelity. The improved charge transport of TIPS-PEN crystals can be ascribed to both increased channel coverage due to the uniformity property of PS-b-P4VP polymer, and an effective nanoscale confinement of the random crystal growth due to the aggregation of nanoparticles. The facile method we reported in this work can be applied to manipulate the crystal growth and charge transport of the general solution-processed, small-molecule semiconductors for high-fidelity organic electronics applications on flexible substrate.

Development of High-k Polymer Materials for Use as a Dielectric Layer in the Organic Thin-Film Transistors

In this article, we developed a series of novel high dielectric polymers based on 5-TPM-co-HEMA-co-GMA (TP) and 5-CTPM-co-HEMA-co-GMA (TP–CN) containing alkylated terphenyl and alkylated terphenyl with a cyano group, respectively. These organic insulating materials provide smooth surface topographies, lower leakage current densities (<6 × 10–7 A/cm2 at 2 MV/m), and small dielectric loss (<0.02, 103 to 105 Hz), which display excellent insulating properties. Simultaneously, two kinds of polymer films have higher dielectric constants of 6.8 and 8.6 than other dielectric constants reported in some literatures and most commercial organic dielectric materials. To investigate the potential of TP and TP–CN as gate dielectrics, we successfully prepared bottom-gate top-contact organic thin-film transistors, with para-hexaphenyl (p-6P) as the buffer layer and vanadyl phthalocyanine (VOPc) as the active layer, which yields a higher charge carrier mobility of 0.52 cm2 V–1 s–1 and on/off ratio > 104.

Biocompatible and degradable gelatin dielectric based low-operating voltage organic transistors for ultra-high sensitivity NH3 detection

We developed an ultra-high sensitive ammonia (NH3) gas sensor that incorporates a biocompatible/degradable gelatin dielectric layer in a top-contact organic thin-film transistor. The sensitivity study of the gas sensor involves detecting the saturation current change with analyte concentrations, exhibiting a saturation current changing from 17.6% to 45.1% for the concentration of NH3 from 500 ppb to 30 ppm. Importantly, this type of sensing platform shows an excellent detecting limit of 174.0 ppb with a low operating voltage of 4 V. To understand the mechanism of this phenomenon, we fabricated another group of devices that incorporates an additional thin layer of polystyrene covered on top of the gelatin layer. The response sensitivity of this type of sensor is only 5.1% for the NH3concentration of 30 ppm. Systematic studies including morphology analysis of the organic films and electrical properties of the sensors were performed. The results indicate that the functional groups on the gelatin dielectric surface groups have ability to adsorb NH3 analyte, thereby leading to a superior sensing performance. This work demonstrates that the biocompatible/degradable gelatin with unique functional groups is applicable for readily manufacturable low-operating voltage and high-performance thin-film transistor-based gas sensors.

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 ...

Hydrophobic modification of SiO2 surface with disilanobiphenyl and disilanobithiophene and the application to pentacene-based organic transistors

ABSTRACTSurface modification of inorganic oxide with organic modifiers is an important process in areas of surface and interface engineering. In this paper, we report the hydrophobic modification of SiO2 surface using tetramethyldisilanobiphenyl (DSBP) and tetramethyldisilanobithiophene (DSBT) as new modifiers. Irradiation of an ozone-cleaned glass plate in a toluene solution of DSBP with a low-pressure mercury lamp at room temperature in an argon atmosphere led to the hydrophobic modification of the glass surface. Treatment of a glass plate in the dark at room temperature provided similar hydrophobic surface, although the process proceeded less rapidly. Modification of glass surface was also possible by DSBT, but less effectively. This process with DSBP was applied to the fabrication of the top-contact p-type organic thin film transistor (OTFT) with a pentacene film as the active layer, prepared by vapor-deposition on SiO2/Si surface. Modification of SiO2/Si surface by DSBP led to enhanced mobility, by enlargement of grain size and enhancement of the crystallinity.

Flexible Organic Thin Film Transistors Incorporating a Biodegradable CO2-Based Polymer as the Substrate and Dielectric Material

Employing CO2-based polymer in electronic applications should boost the consumption of CO2 feedstocks and provide the potential for non-permanent CO2 storage. In this study, polypropylene carbonate (PPC) is utilized as a dielectric and substrate material for organic thin film transistors (OTFTs) and organic inverter. The PPC dielectric film exhibits a surface energy of 47 mN m−1, a dielectric constant of 3, a leakage current density of less than 10−6 A cm−2, and excellent compatibility with pentacene and PTCDI-C8 organic semiconductors. Bottom-gate top-contact OTFTs are fabricated using PPC as a dielectric; they exhibits good electrical performance at an operating voltage of 60 V, with electron and hole mobilities of 0.14 and 0.026 cm2 V−1 s−1, and on-to-off ratios of 105 and 103, respectively. The fabricated p- and n-type transistors were connected to form a complementary inverter that operated at supply voltages of 20 V with high and low noise margins of 85 and 69%, respectively. The suitability of PPC as a substrate is demonstrated through the preparation of PPC sheets by casting method. The fabricated PPC sheets has a transparency of 92% and acceptable mechanical properties, yet they biodegraded rapidly through enzymatic degradation when using the lipase from Rhizhopus oryzae.

Triacetate cellulose gate dielectric organic thin-film transistors

Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm2/Vs similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis.

Low Temperature Processed Organic Tfts with Solution-Processable Al2O3 Gate Insulator Using Hydrogen Peroxide

Organic semiconductors are considered as a replacement for amorphous silicon in thin-film transistor (TFT) applications, since they have shown comparable electrical properties and potential advantages for their compatibility with plastic substrates. However, the high operating voltages of these TFTs still need to be reduced for any practical application. Among dielectric materials, aluminum oxide (Al2O3) appears to be promising due to its high dielectric constant, its robustness and its possibility to be prepared on different type of substrates under easy process. However, high quality and high dielectric constant films generally require annealing temperature above 400 oC, which is not suitable for high-throughput and flexible electronic devices. In this study, we fabricated the bottom-gate and top-contact organic TFTs on glass substrate. Al gate electrode was deposited by thermal evaporation while Al2O3 thin films, used as gate insulators, were grown by spin coating. Employing hydrogen peroxide (H2O2) in the solution process helps reduce defects at the interface between channel and G/I through strong oxidation and minimize organic chemical-induced defects, such as pores, pin holes, and organic residues, through decomposing organic chemicals at low temperature. The growth of the Al2O3 thin films with H2O2 was studied with various annealing temperatures from 100 ~ 300 oC and solution concentration of 0.1 ~ 1 M. Pentacene was evaporated through shadow mask at a rate of 0.3 Å/s to form a 70 nm thick active layer. Gold source/drain (S/D) electrodes (100 nm) were patterned using shadow mask on the pentacene layer. The performance of the solution processed Al2O3 based TFT was validated using capacitance-voltage and current-voltage analysis.