Solution-processed Organic Thin-film Transistors Research Articles

Performance enhancement of solution-processed organic thin-film transistors incorporating an improved Corbino structure

The solution-processed organic thin-film transistors (OTFTs) with a minimal device footprint and improved performance are desirable for flexible electronics and other circuit applications. It is demonstrated that the enhanced performance can be achieved for solution-processed OTFTs by adopting an improved Corbino structure. The bottom-gate bottom-contact OTFTs of W/L ratio of 500 are fabricated using the standard poly-3-hexylthiophene (P3HT) as a semiconductor with an improved interdigitated pseudo-Corbino (IPC) structure. The exhibited IPC structure is a combination of interdigitated structure in the enclosed-Corbino design to achieve infinite output resistance, suppressed parasitic leakage current, and high ON current by accommodating a high W/L ratio in a minimal device footprint. For the fabricated solution-processed OTFTs, infinite output resistance with an OFF-current of the order of 10−12 A and an ON/OFF ratio of drain current of the order of 107 is achieved. Incorporating an enhanced hexamethyldisilazane treatment of the SiO2 gate dielectric improves the ON/OFF ratio to a record value of 108 and the mobility of the order of 10−2 cm2/Vs for P3HT. Implementation of IPC-TFT structure for intentionally chosen moderate-mobility, solution-processed P3HT semiconductor results in a consistent low OFF-current, high ON/OFF ratio, and infinite output resistance with excellent device-to-device uniformity.

Experiment study and analytical modeling of fully solution processed organic thin film transistors with conductive polymer top-gate electrode: Performance optimization

In this paper, various fully solution-processed top-gate bottom contacts organic thin-film transistors based on printed silver nanoparticles electrodes were fabricated, electrically characterized, and analytically modeled. The performance of the designed devices thermally and laser sintered nanoparticles electrodes was investigated. The device based on electrode sintered thermally exhibits improved contacts with an on/off ratio of 1.6 × 103 and a saturation mobility of 1.23 × 10−2 cm2V−1s−1. The current–voltage characteristics of OTFTs in terms of the transfer and output characteristics were also modeled and reproduced using an analytical model. Furthermore, various effects were considered in the used model, including the channel length modulation and contact resistance effects as well as field-effect mobility dependence of the gate voltage. The proposed model results for the devices were found to be in a good agreement with the experimental data in terms of the transfer and output characteristics. It was found that the used analytical model correctly describes the electrical characteristics, and it is proved to be accurate enough to explain the charge transport in these types of OTFTs. The development of such model will certainly help in the design of high-performance nanomaterials based OTFTs and their future advancement as well.

Solution-processed ambipolar organic thin-film transistors and inverters in a single substrate through self-assembled monolayer-treated electrodes

Self-assembled monolayer-treated source–drain electrodes enabled the development of high-performance ambipolar organic thin-film transistors using a donor–acceptor-type polymer, PNDI-TVT, and a complementary-like inverter on a single substrate.

Optimized charge transport in N-substituted isatin-based acceptor–donor–acceptor small molecules by regulating the side chain length for solution-processable organic thin-film transistors

A series of acceptor–donor–acceptor small molecules based on fused-ring as donor and N-substituted isatin with different side-chains (from methyl to n-decyl) as acceptors were synthesized for solution-processable organic thin-film transistors.

Organic thin-film transistors with inkjet-printed electrodes on hydrophobic Teflon-AF gate dielectric with reversible surface properties

Printed organic thin-film transistors (OTFTs) are promising for flexible, low-cost electronics; however, a major challenge is that successive processing steps of multiple solution-processed layers can interfere with each other. In particular, hydrophobic fluoropolymers are a promising group of materials to fabricate gate dielectrics with low charge trap density and properties such as good thermal stability, chemical inertness, low dielectric constant, and water repellency. However, the main difficulty in incorporating hydrophobic fluoropolymers in printed electronic devices is their low surface energy. Plasma treatment is a common method for surface modification that renders these hydrophobic layers wettable to print subsequent layers. This plasma processing can also affect the interface with the organic semiconductor (OSC) and transistor performance, which is studied here for the first time. The morphological and surface chemical properties of Teflon amorphous fluoropolymer (Teflon-AF) films change after plasma treatment and gradually reverse after post-annealing of subsequent layers as demonstrated here. Here, we fabricate solution-processed OTFTs with Teflon-AF as the gate dielectric. We report OTFTs with inkjet-printed source and drain electrodes with a minimum channel length of 20 μm on Teflon-AF for the first time. We show that the annealing of the inkjet-printed electrodes changes the morphology and the surface chemistry of the Teflon-AF gate dielectric underneath, thus reversing the effect of the plasma. We show that this electrode post-annealing step can improve the transistor performance by improving the interface between the Teflon-AF gate dielectric and the OSC in terms of smoothness and hydrophobicity. The transistor parameters such as mobility, on/off current ratio, and threshold voltage all follow a trend explained here by the properties of the Teflon-AF films. Increasing the post-annealing temperature to just below complete surface reversibility decreases surface roughness and trap-sites created after plasma treatment leading to optimized device performance. We characterize the Teflon-AF surface with contact angle measurement, roughness measurement, Fourier transform infrared spectroscopy (FTIR), and x-ray photoelectron spectroscopy (XPS). This understanding of the reversibility of the Teflon-AF surface helps achieve optimal performance of devices and systems incorporating fluoropolymers. • The challenge of processing OTFT layers on hydrophobic dielectrics is addressed. • 20 μm channel OTFTs with inkjet-printed electrodes on hydrophobic dielectric are achieved. • Teflon-AF surface changes after plasma and reverses after post-annealing. • Electrode annealing changes the morphology and surface chemistry of the dielectric. • Electrode annealing can improve the gate dielectric/organic semiconductor interface.

Fused thiophene based materials for organic thin‐film transistors

AbstractThis review highlights our recent efforts in the development of organic semiconductors based on anthradithiophene (ADT), dithienothiophene (DTT), tetrathienoacene (TTA), benzothienodithiophene (BTDT), benzothienothiophene (BTT), chalcogen‐planarized BT, and some quinoidal oligothiophenes for the application of organic thin‐film transistors (OTFTs). We visualized various strategies that have been employed in molecular architecture to improve the stability as well as solubility, and the energy levels are tuned for efficient hole/electron injection, thus leading to high‐performance solution processable OTFTs. The obtained mobility was correlated with the molecular stacking pattern and film morphology/microstructure of the semiconductors. Overall, this review presents information that aids reliable OTFT data analysis and provide guidelines for the development of next‐generation organic semiconductors. At present, the p‐type and n‐type OTFTs developed by our group are able to reach the mobilities of over 4.01 and 2.5 cm2V−1 s−1, respectively. In addition, we also demonstrated the utilization of these conjugated moieties in development of high performance dye‐sensitized solar cells (DSSCs) and hole transporting materials (HTM) as well as non‐fullerene acceptor (NFA) for perovskite solar cells (PSCs).

Organic monolayers modified by vacuum ultraviolet irradiation for solution-processed organic thin-film transistors

A trimethylsilyl-monolayer modified by vacuum ultraviolet (VUV) light has been investigated for use in solution-processed organic thin-film transistors (OTFTs). The VUV irradiation changed a hydrophobic trimethylsilyl-monolayer formed from hexamethyldisilazane vapor into a hydrophilic surface suitable for solution processing. The treated surface was examined via water contact angle measurement and X-ray photoelectron spectroscopy. An appropriate irradiation of VUV light enabled the formation of a dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) film on a modified monolayer by spin-coating. Consequently, the C8-BTBT-based OTFT with a monolayer modified for an optimal VUV irradiation time exhibited a field-effect mobility up to 4.76 cm2 V−1 s−1. The partial monolayer modification with VUV can be adapted to a variety of solution-processes and organic semiconductors for prospective printed electronics.

Molecular subtractive surface-energy engineering of crosslinked Poly(4-vinylphenol) insulators for a solution-processed organic thin-film transistor

We propose a feasible method of manipulating the surface energy of crosslinked poly(4-vinylphenol) (c-PVP) thin films through the surface compositional modification assisted by an etchant. A physical picture of the surface-energy manipulation of c-PVP thin films based on the surface-selective molecular subtractive approach is clarified by investigating the chemical composition of the c-PVP film surfaces. We reveal that the molecular detachment by solvation on the surface leads to a reduction in the surface PVP density, thereby decreasing residual hydroxyl groups on the surface. In particular, it is found that the surface energy of a c-PVP thin film can be controlled by exploiting the thermal-treatment-time dependence of the soluble-PVP density. Organic thin-film transistors (TFTs) are fabricated via a solution process for demonstrating the applicability of our surface-energy-engineered c-PVP film as a gate insulator. The TFTs with the engineered c-PVP gate insulators exhibit improved electrical characteristics, compared to those with ordinary c-PVP gate insulators. • We propose a method for the surface-energy manipulation of a c-PVP thin film. • Surface-selective molecular detachment occurs with aid of an etchant. • The surface energy is manipulated exploiting the density-controllable soluble PVP. • The surface-energy engineering leads to the improvement of TFT characteristics.

Gold nanorods doping induced performance improvement of room temperature OTFT NO2 sensors

Solution-processed organic thin-film transistors (OTFTs) are regarded as the promising candidates for low-cost gas sensors due to their advantages of high throughput, large-area and sensitive to various gas analytes. Microstructure control of organic active layers in OTFTs is an effective route to improve the sensing performance. In this work, we report a simple method to modify the morphology of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) thin films via doping gold nanorods (Au NRs) for enhancing the performance of the corresponding OTFT sensors for nitrogen dioxide (NO2) detection. With the optimized doping ratio of Au nanorods, the TIPS-pentacene OTFT snesors not only exhibit a 3-fold increase in mobility, but also obtain a high sensitivity of 70% to 18 ppm NO2 with a detection limit of 270 ppb. The microstructures and morphologies of the modified TIPS-pentacene thin film characterized by atomic force microscopy and field scanning electron microscope. The experimental results indicate that the proper addition of Au NRs could effectively regulate the grain size of TIPS-pentacene, and therein control the density of grain boundaries during the crystallization, which is essential for the high-performance gas sensors.

The Chemistry and Applications of Heteroisoindigo Units as Enabling Links for Semiconducting Materials.

ConspectusBecause of their low-temperature processing properties and inherent mechanical flexibility, semiconducting materials are promising candidates for enabling flexible displays, renewable energy, biological sensors, and healthcare. Progress has been made in materials performance by developing judicious materials design strategies. For example, improvements in electron transport have required new electron-deficient aromatics. Among them, isoindigo (IID) is an important functional group utilized in conjugated aromatics, where the structure combines two sets of five-membered electron-withdrawing lactam rings, exhibiting enhanced solubility, excellent chemical and thermal stabilities, broad absorption, and intriguing electron affinity.In the past decade, researchers have mainly focused on IID-based materials. However, the effect of heteroatom modification of the IID core has rarely been systemically investigated. In conventional conjugated polymers, single bonds connect the monomers, leading to energetic disorder and torsion defects, while ladder-type polymers are often intractable because of their fused nature. In this regard, the molecular design of new π scaffolds based on IID is central to the development of high-performance semiconductor polymers. Especially, a complete refresh of molecular design strategies and novel conjugated polymers with unique structures are needed to circumvent the disadvantages of the conventional ladder-type polymers.In this Account, we systematically summarize our recent progress in the design, synthesis, and structure-property relationships of IID- and particularly hetero-IID-based functional materials. More specifically, starting with molecular engineering of hetero-IIDs with variable electronic effects, conjugation lengths, and numbers of heterorings, we discuss the effect of the heteroring on the absorption spectra and energy levels. Additionally, we investigate a series of electron-withdrawing substitution of IIDs and hetero-IIDs and their molecular self-assembly behavior and the device performance. Furthermore, we discuss a series of IID-bis(EDOT) copolymers with hydrophilic ethylene glycol side chains for accumulation-mode organic thin-film electrochemical transistors, in which the relationships among the molecular structure, operational stability, film morphology, and device performance were revealed. Compared with IID polymers, the HOMO levels and optical band gaps of the thiophene and thienothiophene IID copolymers markedly decrease, and these polymers exhibit ambipolar charge transport. When we further expanded the IID core to a thieno[3,2-b][1]benzothiophene isoindigo (TBTI) core, such as in TBTIT, bulk-heterojunction solar cells employing this polymer class as the electron donor achieved good efficiency for additive- and annealing-free device conditions. When we introduced electron-deficient pyridine on the IID core, both the LUMO and HOMO energy levels of the copolymers markedly decreased, which significantly improved the electron mobility. In addition, we compare the correlation between the polymer structures of IID and hetero-IID copolymers with thiophene and benzothiophene as comonomers and their absorption spectra and energy levels. In particular, we evaluate the planarity and the dihedral angle between the repeat units, with systematic analysis by theoretical calculations to support our design concepts. We discuss polymer designs arising from simple aldol condensation, where the rigid backbone conformation has been locked by the double bonds. Our polymers display broad absorption from the visible to the NIR-II region, and more importantly, the high electron affinities of these polymers provide a platform to realize ambient-stable electron transport in solution-processed organic thin-film transistors. These exciting results are expected to open doors to new horizons of semiconducting materials in terms of other charming applications and the design and synthesis of superior materials.

Control of polymorphism in solution-processed organic thin film transistors by self-assembled monolayers

Polymorphism of organic semiconductor films is of key importance for the performance of organic thin film transistors (OTFTs). Herein, we demonstrate that the polymorphism of solution-processed organic semiconductors in thin film transistors can be controlled by finely tuning the surface nanostructures of substrates with self-assembled monolayers (SAMs). It is found that the SAMs of 12-cyclohexyldodecylphosphonic acid (CDPA) and 12-phenyldodecylphosphonic acid (PhDPA) induce different polymorphs in the dip-coated films of 2-dodecyl[1]benzothieno[3,2-b][1]benzothiophene (BTBT-C12). The film of BTBT-C12 on CDPA exhibits field effect mobility as high as 28.1 cm2 V−1 s−1 for holes, which is higher than that of BTBT-C12 on PhDPA by three times. The high mobility of BTBT C12 on CDPA is attributable to the highly oriented films of BTBT C12 with a reduced in-plane lattice and high molecular alignment.

Rhodanine-Bridged Core-Expanded Naphthalene Diimide Derivatives for n-Type Semiconductors

The core expansion of naphthalene diimides (NDIs) is an effective strategy to modulate frontier molecular orbital energy levels and improve device performances. Herein two new rhodanine-bridged and core-extended NDIs T1 and T2 were designed and synthesized. The rhodanine moiety could act not only as a π-spacer to enlarge the molecular conjugated system, but also as an electron-donating unit to tune the molecular energy levels. As a result, both T1 and T2 showed slightly lower lying LUMO energy levels (< − 4.2 eV) by ca. 0.1 eV and narrower optical band gaps (ca. 1.5 eV) by 0.5 eV compared to those of n-type organic semiconductor (OSC) NDI2DT-DTYM2.The solution-processed organic thin-film transistors based on T1 and T2 exhibited electron motilities in the range of 10−4–10−3 cm2 V−1 s−1, and the inverted perovskite solar cells constructed using T2 as electron transport materials provided a power conversion efficiency value of 8.82%. The results demonstrated that embedding rhodanine units in a NDI2DT-DTYM2 backbone is an effective approach to tune the energy levels and optical properties of OSCs, providing a new way to construct novel n-type OSCs with multifunctional optoelectronic applications.

Damage-free Metal Electrode Transfer to Monolayer Organic Single Crystalline Thin Films

Solution-processed organic thin film transistors (OTFTs) are an essential building block for next-generation printed electronic devices. Organic semiconductors (OSCs) that can spontaneously form a molecular assembly play a vital role in the fabrication of OTFTs. OTFT fabrication processes consist of sequential deposition of functional layers, which inherently brings significant difficulties in realizing ideal properties because underlayers are likely to be damaged by application of subsequent layers. These difficulties are particularly prominent when forming metal contact electrodes directly on an OSC surface, due to thermal damage during vacuum evaporation and the effect of solvents during subsequent photolithography. In this work, we demonstrate a simple and facile technique to transfer contact electrodes to ultrathin OSC films and form an ideal metal/OSC interface. Photolithographically defined metal electrodes are transferred and laminated using a polymeric bilayer thin film. One layer is a thick sacrificial polymer film that makes the overall film easier to handle and is water-soluble for dissolution later. The other is a thin buffer film that helps the template adhere to a substrate electrostatically. The present technique does not induce any fatal damage in the substrate OSC layers, which leads to successful fabrication of OTFTs composed of monolayer OSC films with a mobility of higher than 10 cm2 V−1 s−1, a subthreshold swing of less than 100 mV decade−1, and a low contact resistance of 175 Ω⋅cm. The reproducibility of efficient contact fabrication was confirmed by the operation of a 10 × 10 array of monolayer OTFTs. The technique developed here constitutes a key step forward not only for practical OTFT fabrication but also potentially for all existing vertically stacked organic devices, such as light-emitting diodes and solar cells.

Solution-processed ultra-flexible C8-BTBT organic thin-film transistors with the corrected mobility over 18 cm2/(V s)

Solution-processed ultra-flexible C8-BTBT organic thin-film transistors with the corrected mobility over 18 cm2/(V s)

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.

Unconventional Redox-Active Gate Dielectrics To Fabricate High Performance Organic Thin-Film Transistors

The fabrication of solution-processed organic thin-film transistors (OTFTs) is the cleanest root for the development of green and eco-friendly disposable technologies as a crucial need of human soc...

Organic thin-film transistors with liquid crystalline polymer insulator integrated for solution-processed organic light-emitting devices

We develop a solution-processed organic thin-film transistor (S-OTFT) with high stability and uniformity using a liquid crystalline polymer (LCP) as a gate insulator. It is found that the liquid crystal order in the LCP is one of critical factors governing the electrical properties of the S-OTFT. The photo-polymerized LCP layer exhibits the chain ordering which significantly improves the dielectric characteristics of the insulator layer and the reliability of the S-OTFT. A prototype array of 3 × 3 solution-processed organic light-emitting diodes integrated with the S-OTFTs based on the LCP insulator shows the high uniformity of light emission during operation.

One-Volt, Solution-Processed Organic Transistors with Self-Assembled Monolayer-Ta2O5 Gate Dielectrics.

Low-voltage, solution-processed organic thin-film transistors (OTFTs) have tremendous potential to be key components in low-cost, flexible and large-area electronics. However, for these devices to operate at low voltage, robust and high capacitance gate dielectrics are urgently needed. Herein, the fabrication of OTFTs that operate at 1 V is reported. These devices comprise a solution-processed, self-assembled monolayer (SAM) modified tantalum pentoxide (Ta2O5) as the gate dielectric. The morphology and dielectric properties of the anodized Ta2O5 films with and without n-octadecyltrichlorosilane (OTS) SAM treatment have been studied. The thickness of the Ta2O5 film was optimized by varying the anodization voltage. The results show that organic TFTs gated with OTS-modified tantalum pentoxide anodized at 3 V (d ~7 nm) exhibit the best performance. The devices operate at 1 V with a saturation field-effect mobility larger than 0.2 cm2 V−1 s−1, threshold voltage −0.55 V, subthreshold swing 120 mV/dec, and current on/off ratio in excess of 5 × 103. As a result, the demonstrated OTFTs display a promising performance for applications in low-voltage, portable electronics.

Effects of thermal annealing conditions on the electrical characteristics of TES-ADT thin-film transistors

We investigated the annealing temperature and exposure period dependence of electrical characteristics of bottom-gate/top-contact solution-processed organic thin-film transistors (TFTs) fabricated on ITO-patterned substrates using 5,11-bis(triethylsilylethynyl)anthradithiophene(TES-ADT), PVP-co-PMMA, and silver as the semiconductor, gate dielectric, and drain and source electrodes, respectively. The semiconductor films were thermally annealed at 60, 80, and 90 °C for 60 min; then, the optimal annealing temperature, which proved to be 80 °C, was considered to investigate the impact of exposure time by exposing the film for 30, 60, and 90 min. Experimental results indicate that the medium temperature resulted in the better electrical performance, whereas the high temperature proved to be detrimental by causing ruptures through the film. According to our study, TES-ADT film annealed at 80 °C for 60 min results in the finest semiconductor microstructure, which improves the electrical characteristics, whereas lower or higher temperatures and shorter or longer annealing times lead to an unfavorable structure that hampers charge transport in the film.

Diketopyrrolopyrrole-based small molecules for solution-processed n-channel organic thin film transistors

n-Type small molecules exhibit a highly reliable mobility of up to 1.05 cm2 V−1 s−1 in solution-processed organic thin film transistors.