Materials For Organic Thin-film Transistors Research Articles

Low-temperature crosslinked soluble polyimide as a dielectric for organic thin-film transistors: enhanced electrical stability and performance

We have prepared a low-temperature cross-linked soluble polyimide (SPI) as a dielectric material for organic thin-film transistors (OTFTs) to improve their electrical stability. Two types of SPIs (DOCDA/6FHAB and 6FDA/6FHAB) were synthesized by a one-step polymerization process using 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and hexafluoroisopropylidene diphthalic anhydride (6FDA) as the dianhydrides and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FHAB) as a diamine. To further enhance the electrical performance, the SPI thin films were crosslinked with methylated/ethylated (hydroxymethyl)benzoguanamine (HMBG) through a low temperature process at 160 °C. Crosslinking considerably improved the insulating properties, resulting in a substantial reduction in leakage current from 10−7 A cm−2 to 10−9 A cm−2 at 2.0 MV cm−1. When crosslinked SPIs were used as gate dielectrics in OTFTs, device stability and reliability, as measured by the off-current, threshold voltage, and hysteresis, improved significantly. Our results demonstrate the potential of crosslinked SPIs as effective gate dielectric materials for advanced organic thin-film transistors.

Amorphous Fluorinated Acrylate Polymer Dielectrics for Flexible Transistors and Logic Gates with High Operational Stability.

Fluorinated amorphous polymeric gate-insulating materials for organic thin-film transistors (OTFTs) not only form hydrophobic surfaces but also significantly reduce traps at the interface between the organic semiconductor and gate insulator. Therefore, these polymeric materials can enhance the OTFT's operation stability. In this study, we synthesized a new polymeric insulating material series composed of acrylate and fluorinated functional groups (with different ratios) named MBHCa-F and used them as gate insulators for OTFTs and in other applications. The insulating features of the MBHCa-F polymers, including surface energy, surface atomic content properties, dielectric constant, and leakage current, were clearly analyzed with respect to the content of the fluorinated functional groups. At higher fluorine-based functional group content, the polymeric series exhibited higher fluorine-based contents at the surface and superior electrical properties, such as field-effect mobility and driving stability, at OTFTs. Therefore, we believe that this study provides a substantial method for synthesizing polymeric insulating materials to enhance the operational stability and electrical performance of OTFTs.

Self-Doping Naphthalene Diimide Conjugated Polymers for Flexible Unipolar n-Type OTFTs.

The development of high-performance organic thin-film transistor (OTFT) materials is vital for flexible electronics. Numerous OTFTs are so far reported but obtaining high-performance and reliable OTFTs simultaneously for flexible electronics is still challenging. Herein, it is reported that self-doping in conjugated polymer enables high unipolar n-type charge mobility in flexible OTFTs, as well as good operational/ambient stability and bending resistance. New naphthalene diimide (NDI)-conjugated polymers PNDI2T-NM17 and PNDI2T-NM50 with different contents of self-doping groups on their side chains are designed and synthesized. The effects of self-doping on the electronic properties of resulting flexible OTFTs are investigated. The results reveal that the flexible OTFTs based on self-doped PNDI2T-NM17 exhibit unipolar n-type charge-carrier properties and good operational/ambient stability thanks to the appropriate doping level and intermolecular interactions. The charge mobility and on/off ratio are fourfold and four orders of magnitude higher than those of undoped model polymer, respectively. Overall, the proposed self-doping strategy is useful for rationally designing OTFT materials with high semiconducting performance and reliability.

Hidden thin-film phase of dinaphthothienothiophene revealed by high-resolution X-ray diffraction

Dinaphthothienothiophene (DNTT) has attracted considerable attention as a next-generation material for organic thin-film transistors, replacing the conventional basic material of pentacene. Although the performance of DNTT devices is higher than that of pentacene, and has been reported numerously, a comprehensive understanding of thin-film growth is lacking. In fact, thin-film structures have long been believed to be identical to single-crystal structures. In the present study, the thickness-dependent structural evolution is revealed by means of high-resolution X-ray diffraction. This technique apparently discriminates the thin-film structure from the conventionally known bulk structure. Thus, we have revealed the thin-film phase of DNTT for the first time.

Synthesis and Characterization of Benzodithiophene (BDT) Quinoid Compounds as a Potential Compound for n-Type Organic Thin-Film Transistors (OTFT)

Two potential compounds as an n-Type organic thin-film transistor (OTFT) from benzodithiophene (BDT) derivatives have been synthesized and characterized. BDT was chosen as the core because it has π-conjugated bonds, rigid structures, and planar. Quinoid structure with end-cap (terminal group) in the form of dicyanomethylene is used because it can lower the LUMO value of the compound, and side chains are selected in the form of alkoxy so that two BDT derivatives are obtained namely BDTQ-6 (hexyloxy) and BDTQ-10 (decyloxy). Based on the results of TGA, BDTQ-6 and BDTQ-10 have decomposition points of 183°C and 203°C, which indicate the compound has excellent thermal stability. From the UV-Vis measurement, the λmax value of the two compounds is 599 nm with optical gap energy (Eg°pt) of 1.7 eV. From the DPV measurement, the LUMO value for the two compounds is -4.3 eV, with an energy gap (Eg) of 1.69 eV (BDTQ-6) and 1.70 eV (BDTQ-10). Based on observations of the crystal structure through x-ray diffraction, it was found that the BDTQ-10 crystal has a "brick type" layer arrangement with a distance between layers of 3.55 Å. With excellent thermal stability and suitable LUMO values and energy gaps, it is expected that BDTQ-6 and BDTQ-10 compounds have the potential to be n-Type OTFT materials.

A Simple Structure Conjugated Polymer for High Mobility Organic Thin Film Transistors Processed from Nonchlorinated Solvent.

Herein, a simple structure, nonchlorinated solvent processable high mobility donor–acceptor conjugated polymer, poly(2,5‐bis(4‐hexyldodecyl)‐2,5‐dihydro‐3,6‐di‐2‐thienyl‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione‐alt‐thiophene) (PDPPT3‐HDO), is reported. The enhanced solubility in nonchlorinated solvent is realized based on a denser alkyl side chains strategy by incorporating small size comonomer thiophene. An associated benefit of thiophene comonomer is the remarkable structural simplicity of the resulting polymer, which is advantageous for industrial scaling up. The alkyl side chain density and structure of PDPPT3‐HDO can efficiently control the self‐assembly properties in solution and film. By bar coating from o‐xylene solution, PDPPT3‐HDO forms aligned films and exhibits high hole mobility of up to 9.24 cm2 V−1 s−1 in organic thin film transistors (OTFTs). Notably, the bar‐coated OTFT based on PDPPT3‐HDO shows a close to ideal transistor model and a high mobility reliability factor of 87%. The multiple benefits of increased side chain density strategy may encourage the design of high mobility polymers that meet the requirements of mass production of OTFT materials and devices.

Revealing the mechanism of contrasting charge transport properties for phenyl and thienyl substituent organic semiconductors.

The aromatic substituents such as phenyl and thienyl have a direct impact on the structure-property relationship of conjugated semiconducting molecules and thus play a significant role in determining the performance of organic thin film transistors (OTFTs). To design a molecule with good charge transport properties, it is essential to understand the charge transfer process. Herein, through the study of two groups of classical OTFT materials with different substituents, we demonstrate the fundamental reasons for the contrasting carrier mobilities between the materials in each group by means of DFT calculations. The results from our theoretical analysis on the two groups of molecules provide very good estimates of their charge transfer properties and the numerical trend from our calculation is consistent with that from the corresponding experimental results. We reveal in detail the impacts of the subtle geometric differences caused by different substituents from two aspects including (i) the significant influence of steric hindrance on reorganization energy and (ii) the overlap of the adjacent molecular orbitals (MOs) on electron coupling. Overall, our systematic investigation into the charge transfer mechanism and geometry effects allows us to employ a reliable theoretical methodology for the prediction of the material performance, which is crucial for the design of high performance OTFT materials.

Phthalocyanine-Based Organic Thin-Film Transistors: A Review of Recent Advances

Metal phthalocyanines (MPcs) are versatile conjugated macrocycles that have attracted a great deal of interest as active components in modern organic electronic devices. In particular, the charge transport properties of MPcs, their chemical stability, and their synthetic versatility make them ideal candidate materials for use in organic thin-film transistors (OTFTs). This article reviews recent progress in both the material design and device engineering of MPc-based OTFTs, including the introduction of solubilizing groups on the MPcs and the surface modification of substrates to induce favorable MPc self-assembly. Finally, a discussion on emerging niche applications based on MPc OTFTs will be explored, in addition to a perspective and outlook on these promising materials in OTFTs. The scope of this review is focused primarily on the advances made in the field of MPc-based OTFTs since 2008.

A Multifunctional Material Based on Triphenylamine and a Naphthyl Unit for Organic Light-Emitting Diodes, Organic Solar Cells, and Organic Thin-Film Transistors

We have developed a new multifunctional material, 4,4',4"-tris(4-naphthalen-2-yl-phenyl)amine (2-TNPA), which can be used as a blue-emitting and hole-transporting material in organic light-emitting diodes (OLEDs), as well as a donor material in organic solar cells (OSCs) and an active material in organic thin-film transistors (OTFTs). The OLED device doped with 3% 2-TNPA shows a maximum current efficiency of 3.0 <TEX>$cdA^{-1}$</TEX> and an external quantum efficiency of 3.0%. 2-TNPA is a more efficient hole-transporting material than 4,4'-bis[N-(naphthyl-N-phenylamino)]biphenyl (NPD). Furthermore, 2-TNPA shows a power-conversion efficiency of 0.39% in OSC and a field-effect mobility of <TEX>$3.2{\times}10^{-4}cm^2V^{-1}s^{-1}$</TEX> in OTFTs.

Optically switchable transistor via energy-level phototuning in a bicomponent organic semiconductor

Organic semiconductors are suitable candidates for printable, flexible and large-area electronics. Alongside attaining an improved device performance, to confer a multifunctional nature to the employed materials is key for organic-based logic applications. Here we report on the engineering of an electronic structure in a semiconducting film by blending two molecular components, a photochromic diarylethene derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT's hole transport. As a proof-of-concept we exploited this blend as a semiconducting material in organic thin-film transistors. The device illumination at defined wavelengths enabled reversible tuning of the diarylethene's electronic states in the blend, which resulted in modulation of the output current. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits.

Pyrene end-capped oligothiophene derivatives for organic thin-film transistors and organic solar cells

We have demonstrated an efficient one-step synthesis of pyrene end-capped thiophene oligomers as active materials for organic thin-film transistors (OTFTs) and donor materials for organic solar cells (OSCs). In particular, the OTFT device based on 5,5′′-di(pyren-1-yl)-2,2′:5′,2′′-terthiophene (3s) exhibited a reasonable field effect mobility of 0.11 cm2 V−1 s−1, and a power conversion efficiency of 1.20% was achieved when 3s was used as active materials in OSCs. These pyrene end-capped thiophene oligomers are considered to be good organic semiconducting materials for OTFTs and OSCs.

Stabilization of organic field-effect transistors by tert-butyl groups in dibenzotetrathiafulvalene derivatives

For a material for organic thin-film transistors, not only high mobility but also low threshold voltage and long-term stability are important requirements. In order to realize these properties, materials with relatively large oxidation potentials, namely weak donors, have been designed as p-channel organic semiconductors. Here we propose a different strategy; transistor properties of dibenzotetrathiafulvalene (DBTTF) are significantly improved by the introduction of tert-butyl groups. Although this chemical modification does not much change the ionization potential, small threshold voltage and stability over several months are attained together with the improved mobility, probably due to some kind of passivation effect of the bulky tert-butyl groups. In contrast, the systematic fluorine substitution rapidly diminishes the transistor performance. There are two kinds of herringbone structures with much different dihedral angles of about 50° and 130°, and the tert-butyl compound falls into the former category.

Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors

We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline semiconductor (P1) for organic thin-film transistors (OTFTs). Compound P1 was synthesized by the Sonogashira coupling reaction between 2-bromo-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene. Top contact OTFTs were fabricated by spin casting with 2 wt% solution of P1 in chloroform and their best performance, which exhibited a hole mobility of 4.5 x 10(-5) cm2/Vs, was showed after annealing of the films at liquid crystalline temperature. Time-of-flight (TOF) mobility measured at liquid crystalline phase was observed to be 1.5 x 10(-6) cm2/Vs for both positive and negative carriers. These results indicate that the liquid crystallinity helps to improve the molecular packing and enhance charge mobility for P1. These advantages can be applicable to design and construct solution-processable OTFT materials for electronic applications.

4,4′,4″-Tris(4-naphthalen-1-yl-phenyl)amine as a multifunctional material for organic light-emitting diodes, organic solar cells, and organic thin-film transistors

We have developed the first multifunctional organic material, 4,4′,4″-tris(4-naphthalen-1-yl-phenyl)amine ( 1-TNPA), which can be used as a deep blue emitting and hole-transporting material in organic light-emitting diodes (OLEDs), as a donor material in organic solar cells (OSCs), and as an active material in organic thin-film transistors (OTFTs). In particular, in the case of 1-TNPA-based OLEDs, efficient emission characteristics, the emission of deep-blue light similar to the National Television Standard Committee (NTSC) standard blue, and more efficient hole transport than that in the case of NPD-based OLEDs were observed. 1-TNPA can be also used as a donor in OSCs and as an organic semiconductor in OTFTs.

HIGH MOBILITY CONJUGATED POLYMER SEMICONDUCTORS FOR ORGANIC THIN FILM TRANSISTORS

Organic thin film transistors (OTFTs) are promising candidates as alternatives to silicon TFTs for applications where light weight, large area and flexibility are required. OTFTs have shown potential for cost effective fabrication using solution deposition techniques under mild conditions. However, two major issues must be addressed prior to the commercialization of OTFT-based electronics: (i) low charge mobilities and (ii) insufficient air stability. This article reviews recent progress in the design and development of thiophene-based polymer semiconductors as channel materials for OTFTs. To date, both high performance p-type and n-type thiophene-based polymers with benchmark charge carrier mobility of &gt; 0.5 cm2 V-1 s-1 have been archived, which bring printed OTFTs one step closer to commercialization.

Gate Dielectrics for Organic Field‐Effect Transistors: New Opportunities for Organic Electronics

AbstractIn this contribution we review the motivations for, and recent advances in, new gate dielectric materials for incorporation into organic thin‐film transistors (OTFTs) for organic electronics. After a general introduction to OTFT materials, operating principles, and processing requirements for optimizing low‐cost organic electronics, this review focuses on three classes of OTFT‐compatible dielectrics: i) inorganic (high‐k) materials; ii) polymeric materials; and iii) self‐assembled mono‐ and/multilayer materials. The principal goals in this active research area are tunable and reduced OTFT operating voltages, leading to decreased device power consumption while providing excellent dielectric/insulator properties and efficient low‐cost solution‐phase processing characteristics.

Fabrication of RR-P3HT-based TFTs using low-temperature PECVD silicon nitride passivation

AbstractRegioregular poly(3-hexylthiophene) (RR-P3HT) is a commercially available semiconducting polymer. Its high processability makes it favorable for fabrication of organic thin film transistors (OTFTs). Depending on the processing technique and device configuration, the field effect mobility of this polymer ranges from 0.01 to 0.1 cm2/Vs. The mobility also shows a correlation with the choice of gate dielectric material. The most commonly reported dielectric materials for OTFTs are SiO2, Al2O3 and Ta2O5. In this work, we report a new fully encapsulated top-gate RR-P3HT-based TFT structure with a-SiNx implemented as the gate dielectric and passivation material. The fabrication process enables realization of discrete transistors and transistor circuits through four consecutive photolithographic steps. The process is compatible for various substrates including Corning glass, Si wafers, and any appropriate plastic substrates. This paper addresses a number of critical technological issues such as substrate surface treatment to improve film adhesion, optimal spin coating conditions for uniform polymer film formation, preparation of device quality a-SiNx films by plasma-enhanced chemical vapor deposition (PECVD) at 75°C substrate temperature, and a tailored etch process for patterning of the polymer film. Current-voltage characteristics of the fabricated transistors are analyzed to evaluate the quality of the polymer/a-SiNx interface.

High-performance semiconducting polythiophenes for organic thin-film transistors.

Conjugated polymers have been widely studied as potential semiconductor materials for organic thin-film transistors (TFTs). However, they have provided functionally poor transistor properties when the TFTs are fabricated in air. We have developed a class of liquid crystalline regioregular polythiophenes, PQTs, that possess sufficient air stability to enable achievement of excellent TFT properties under ambient conditions. These polythiophenes exhibit unique self-assembly ability and form highly structured thin films when deposited from solution under appropriate conditions. TFTs fabricated in air with PQT channel layers have provided high field-effect mobility to 0.14 cm2 V-1 s-1 and high current modulation to over 107, together with other desirable transistor properties. These high-performance polythiophenes will therefore help bring the long-standing concept of low-cost organic/polymer transistor circuits closer to commercial reality.

Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta2O5 as gate dielectric material for organic thin-film transistors. Ta2O5 has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta2O5 thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (r=21), Ta2O5 facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta2O5 gate dielectric are discussed.

Efficient organic photovoltaic diodes based on doped pentacene.

Recent work on solar cells based on interpenetrating polymer networks and solid-state dye-sensitized devices shows that efficient solar-energy conversion is possible using organic materials. Further, it has been demonstrated that the performance of photovoltaic devices based on small molecules can be effectively enhanced by doping the organic material with electron-accepting molecules. But as inorganic solar cells show much higher efficiencies, well above 15 per cent, the practical utility of organic-based cells will require their fabrication by lower-cost techniques, ideally on flexible substrates. Here we demonstrate efficiency enhancement by molecular doping in Schottky-type photovoltaic diodes based on pentacene--an organic semiconductor that has received much attention as a promising material for organic thin-film transistors, but relatively little attention for use in photovoltaic devices. The incorporation of the dopant improves the internal quantum efficiency by more than five orders of magnitude and yields an external energy conversion efficiency as high as 2.4 per cent for a standard solar spectrum. Thin-film devices based on doped pentacene therefore appear promising for the production of efficient 'plastic' solar cells.