Performance Of Organic Thin-film Transistors Research Articles

Solution-Processable BODIPY-Based Small Molecules for Semiconducting Microfibers in Organic Thin-Film Transistors.

Electron-deficient π-conjugated small molecules can function as electron-transporting semiconductors in various optoelectronic applications. Despite their unique structural, optical, and electronic properties, the development of BODIPY-based organic semiconductors has lagged behind that of other π-deficient units. Here, we report the design and synthesis of two novel solution-proccessable BODIPY-based small molecules (BDY-3T-BDY and BDY-4T-BDY) for organic thin-film transistors (OTFTs). The new semiconductors were fully characterized by (1)H/(13)C NMR, mass spectrometry, cyclic voltammetry, UV-vis spectroscopy, photoluminescence, differential scanning calorimetry, and thermogravimetric analysis. The single-crystal X-ray diffraction (XRD) characterization of a key intermediate reveals crucial structural properties. Solution-sheared top-contact/bottom-gate OTFTs exhibited electron mobilities up to 0.01 cm(2)/V·s and current on/off ratios of >10(8). Film microstructural and morphological characterizations indicate the formation of relatively long (∼0.1 mm) and micrometer-sized (1-2 μm) crystalline fibers for BDY-4T-BDY-based films along the shearing direction. Fiber-alignment-induced charge-transport anisotropy (μ∥/μ⊥ ≈ 10) was observed, and higher mobilities were achieved when the microfibers were aligned along the conduction channel, which allows for efficient long-range charge-transport between source and drain electrodes. These OTFT performances are the highest reported to date for a BODIPY-based molecular semiconductor, and demonstrate that BODIPY is a promising building block for enabling solution-processed, electron-transporting semiconductor films.

Dendrons with urea/malonamide linkages for gate insulators of n-channel organic thin film transistors

A series of urea/malonamide dendritic molecules were prepared as gate insulators for organic thin film transistors (OTFTs). This series of molecules with different degrees of branching possess peripheral stearyl groups are dendrons generation 0.5 (G0.5), generation 1 (G1), generation 1.5 (G1.5), generation 2 (G2) and generation 2.5 (G2.5). In addition, two types of tetracarboxylic diimide derivatives, i.e. perylene diimide (PDI) and naphthalene diimide (NDI) with two different chain lengths of fluorinated alkyl end groups were prepared as semiconductors for OTFTs such as NDI-C4F7, NDI-C7F9, PDI-C4F7 and PDI-C7F9. The n-channel types of OTFTs were fabricated by spin-coating the gate insulators on Si/SiO2 substrates, and then depositing the semiconducting layers in vacuum without heating the substrate. Silver was used as contact electrodes for source and drain. The performance of OTFTs with dendrons as gate insulators were better than that of OTFTs modified by octadecyltrichlorosilane (ODTS). Moreover, the threshold voltages (Vths) of OTFTs shifted from positive voltage to negative voltage as the device was incorporated with higher generation of dendrons. This is because of different dielectric constants or surface energies between the interface of gate insulator and semiconducting layer. Among all samples in this study, the n-channel OTFT comprising PDI-C4F7 and G1.5 exhibited the best performance. In addition, an enhanced electron mobility and Ion/Ioff ratio measured under ambient condition were 4.71×10−4cm2V−1s−1 and 7.7×103, respectively. Apart from that, the influence of semiconducting molecular packing order on dendron gate insulator layers was investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). Furthermore, pentacene-based p-channel OTFTs with G1.5 gate insulator also exhibited the highest performance. These OTFTs achieved 0.1cm2V−1s−1 and 6.3×104 for mobility (μ) and Ion/Ioff ratio, respectively.

One-pot surface modification of poly(ethylene-alt-maleic anhydride) gate insulators for low-voltage DNTT thin-film transistors

This study investigates the one-pot surface modification of poly(ethylene-alt-maleic anhydride) (PEMA) gate insulators crosslinked with 1,5-naphthalenediamine (1,5-NDA) for enhancing the device performance of low-voltage dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) organic thin-film transistors (OTFTs). Surface properties of the PEMA gate insulator could be easily modified by adding poly(maleic anhydride-alt-1-octadecene) (PMAO) to the coating solution. The surface energy of the gate insulator is strongly correlated with the growth of organic semiconductors and the charge carrier transport at the interface between the semiconductor and gate insulator. The results indicate that the device performance of low-voltage DNTT OTFTs can be improved by one-pot surface modification of the PEMA gate insulator.

Regioregular and Random Difluorobenzothiadiazole Electron Donor–Acceptor Polymer Semiconductors for Thin-Film Transistors and Polymer Solar Cells

Random and regioregular electron donor–acceptor (D–A) polymers comprising of 3,6-bis(3-octyltridecyl)thieno[3,2-b]thiophene donor and 5,6-difluorobenzo[c][1,2,5]thiadiazole acceptor moieties were synthesized to study the effects of both structural regioregularity and molecular weight on their performance in organic thin-film transistors (OTFTs) and bulk-heterojunction (BHJ) polymer solar cells (PSCs). All the polymers displayed similar molecular orbital energy levels regardless of their structures and molecular weights (MWs). The regioregular D–A polymers exhibited higher molecular orders than their random counterparts, and the degrees in molecular ordering improved with increased MW, leading to higher charge transport capability. However, undesirably high MW led to limited polymer solubility and thus fabrication difficulties resulting in poorer device performance. A reasonably high power conversion efficiency of about 7.6% was provided by the new regioregular D–A polymer semiconductor of mid-MW propertie...

Vertical Phase Separation in Small Molecule:Polymer Blend Organic Thin Film Transistors Can Be Dynamically Controlled

Blending of small‐molecule organic semiconductors (OSCs) with amorphous polymers is known to yield high performance organic thin film transistors (OTFTs). Vertical stratification of the OSC and polymer binder into well‐defined layers is crucial in such systems and their vertical order determines whether the coating is compatible with a top and/or a bottom gate OTFT configuration. Here, we investigate the formation of blends prepared via spin‐coating in conditions which yield bilayer and trilayer stratifications. We use a combination of in situ experimental and computational tools to study the competing effects of formulation thermodynamics and process kinetics in mediating the final vertical stratification. It is shown that trilayer stratification (OSC/polymer/OSC) is the thermodynamically favored configuration and that formation of the buried OSC layer can be kinetically inhibited in certain conditions of spin‐coating, resulting in a bilayer stack instead. The analysis reveals here that preferential loss of the OSC, combined with early aggregation of the polymer phase due to rapid drying, inhibit the formation of the buried OSC layer. The fluid dynamics and drying kinetics are then moderated during spin‐coating to promote trilayer stratification with a high quality buried OSC layer which yields unusually high mobility >2 cm2 V−1 s−1 in the bottom‐gate top‐contact configuration.

Effect of molecular ordering in active layer on organic thin-film transistor performance

We investigate the effect of molecular ordering in the semiconductor of inverted staggered-type organic thin-film transistors (OTFTs) on device parameters. Molecular ordering is controlled by modifying the gate insulator with self-assembled monolayers and clarified by X-ray diffraction analysis. The reduction in the density of free carriers in a semiconductor with the alkyl-modified surface reduces off-current (IOFF) and increases contact resistance (RC). In contrast, IOFF increases and RC decreases for a phenyl-modified surface. Ultraviolet photoemission studies revealed that differences in the molecular ordering and the surface dipole moment caused by the insulator surfaces are crucial for device parameters for OTFTs.

Critical role of silk fibroin secondary structure on the dielectric performances of organic thin-film transistors

Correlation between silk fibroin (SF) secondary structure and dielectric performances of organic thin-film transistors (OTFTs) was investigated using various SF films.

PC70BM n-type Thin Film Transistors: Influence of HMDS Deposition Temperature on the Devices Properties

This study investigates the influence of the deposition temperature of hexamethyldisilazane (HMDS) on the performances of organic thin film transistors (OTFTs) using the [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as semiconductor. N-type OTFTs have been fabricated using this fullerene derivative, deposited from solution by drop casting technique on HMDS self-assembled monolayer (SAM) deposited at three different temperatures, 7°C, 25°C and 60°C, in order to evaluate the influence of these deposition conditions on the morphology of PC70BM films and on the electrical responses of fullerene derivative-based OTFTs.The effect of the treatments of the surfaces was observed through contact angle measurements. AFM imaging of the deposited material has been used to analyse its structure and morphology. The transistors performances have been evaluated through I vs. V static characterization and parameters extraction.Contact angle vs. HMDS deposition temperature shows the minimum value at 60°C, instead here field effect mobility presents a maximum. It has been observed that the lower hydrophobicity of the surface of the SAM induces the formation of more homogeneous surface of the PC70BM film, resulting in an increase of the OTFTs performances.

Bar-coated high-performance organic thin-film transistors based on ultrathin PDFDT polymer with molecular weight independence

We report high-performance organic thin-film transistors (OTFTs) with an ultrathin active layer of difluorobenzothiadiazole-dithienosilole copolymer (PDFDT) form by using the wire bar-coating process. The top-gate/bottom contact (TG/BC) OTFTs based on bar-coated PDFDT polymer as channel material and poly(methyl methacrylate) (PMMA) as gate dielectric show a hole mobility of up to 2.2 cm2 V−1s−1 with a current ON/OFF ratio (Ion/Ioff) of 104∼105, with the mobility being two times larger than that of the spin-coated PDFDT based OTFTs. The higher mobility of the bar-coated PDFDT polymer films can be attributed to the well-organized fibril structures of the polymer chains. Importantly, two different molecular weight polymers (Mn = 23 and 34 kDa) were employed to conduct these experiments and both batches showed about the same performance, which mitigates the typical batch-to-batch variation in OTFT performance. Furthermore, we explored the operational stability of the bar-coated OTFTs in ambient air and nitrogen environments. The bias-stress and cycling tests between the ON/OFF states of the bar-coated devices showed high stability in both nitrogen and air. Conclusively, here we demonstrate that (i) a simple bar-coating process is a better method to control and obtain good polymer morphology in comparison to spin-coating, and (ii) the PDFDT polymer has great potential to provide good reproducibility and stability in large-area OTFT devices.

Origin of mobility enhancement by chemical treatment of gate-dielectric surface in organic thin-film transistors: Quantitative analyses of various limiting factors in pentacene thin films

For the better performance of organic thin-film transistors (TFTs), gate-insulator surface treatments are often applied. However, the origin of mobility increase has not been well understood because mobility-limiting factors have not been compared quantitatively. In this work, we clarify the influence of gate-insulator surface treatments in pentacene thin-film transistors on the limiting factors of mobility, i.e., size of crystal-growth domain, crystallite size, HOMO-band-edge fluctuation, and carrier transport barrier at domain boundary. We quantitatively investigated these factors for pentacene TFTs with bare, hexamethyldisilazane-treated, and polyimide-coated SiO2 layers as gate dielectrics. By applying these surface treatments, size of crystal-growth domain increases but both crystallite size and HOMO-band-edge fluctuation remain unchanged. Analyzing the experimental results, we also show that the barrier height at the boundary between crystal-growth domains is not sensitive to the treatments. The results imply that the essential increase in mobility by these surface treatments is only due to the increase in size of crystal-growth domain or the decrease in the number of energy barriers at domain boundaries in the TFT channel.

An Investigation of the Ink-Transfer Mechanism During the Printing Phase of High-Resolution Roll-to-Roll Gravure Printing

Reducing the linewidth of electrodes is of high importance for increasing not only the efficiency of photovoltaic devices but also the performance of organic thin-film transistors. In particular, controlling the line pattern in printing processes has been difficult. In this paper, we report an analytical approach for obtaining high-resolution control over the ink-transfer mechanism in roll-to-roll (R2R) gravure printing. A dimensionless adhesion-force difference was defined for a simple ink-transfer model, and it was used to predict and evaluate the ink-transfer mechanism with respect to several parameters, such as the surface tension of the ink, surface energy of the substrate, and surface energy and aspect ratio (AR) of the cell. It was found that the low-surface-tension inks, high-surface-energy substrates, and low-surface-energy and high-AR cells are preferable to increasing the ink-transfer ratio during the printing phase. Finally, a matching-logic flowchart was developed for controlling the ink-transfer mechanism and fidelity of R2R gravure printing. The printed patterns obtained had an average width as small as 15.3 $\mu \text{m}$ (standard deviation $= \,\, 0.9~\mu \text{m}$ ).

Effects of different self-assembled monolayers on thin-film morphology: a combined DFT/MD simulation protocol.

Organic thin film transistors (OTFTs) are multilayer field-effect transistors that employ an organic conjugated material as semiconductor. Several experimental groups have recently demonstrated that the insertion of an organic self-assembled monolayer (SAM) between the dielectric and the semiconductive layer is responsible for a sensible improvement of the OTFT performances in terms of an increased charge carrier mobility caused by a higher degree of order in the organic semiconductor layer. Here, we describe a combined periodic density functional theory (DFT) and classical molecular dynamics (MD) protocol applied to four different SAMs and a pentacene monolayer deposited onto their surfaces. In particular, we investigate the morphology and the surface of the four SAMs and the translational, orientational, and nematic order of the monolayer through the calculation of several distribution functions and order parameters pointing out the differences among the systems and relating them to known experimental results. Our calculations also suggest that small differences in the SAM molecular design will produce remarkable differences in the SAM surface and monolayer order. In particular, our simulations explain how a SAM with a bulky terminal group results in an irregular and rough surface that determines the deposition of a disordered semiconductive monolayer. On the contrary, SAMs with a small terminal group generate smooth surfaces with uninterrupted periodicity, thus favoring the formation of an ordered pentacene monolayer that increases the mobility of charge carriers and improves the overall performances of the OTFT devices. Our results clearly point out that the in silico procedure presented here might be of help in tuning the design of SAMs in order to improve the quality of OTFT devices.

Synthesis, characterization and semiconducting properties of dioctyl substituted vanadyl phthalocyanines

Solution processible organic semiconductors have attracted great attention in recent years due to their applications in printable electronics, such as organic thin-film transistor (OTFT). This type of device requires semiconductors characterized by low reorganization energies and strong intermolecular electronic interaction. Phthalocyanines are 18 π -electron disc-like aromatic macrocycles with two-dimensional (2D) π -electron delocalization over the whole molecule, and are featured with low reorganization energies and close π - π stacking. Moreover, tight 2D cofacial π - π stacking, which is beneficial for charge transport in the channel of OTFTs, can be realized by incorporating axially substituted metals or large metals in the core of phthalocyanines. Introducing alkyl substituents can endow phthalocyanines good solubility in organic solvents. However, the number and positions of alkyl substituents have great influence on the packing structures thereby OTFT performance of phthalocyanines. In the current paper, four dioctyl substituted vanadyl phthalocyanines, i.e. 2,3-dioctyl vanadyl phthalocyanine (2,3-C8OVPc), 2,16(17)-dioctyl vanadyl phthalocyanine ( dp - C8OVPc), 1,15-dioctyl vanadyl phthalocyanine (1,15-C8OVPc) and 1,18-dioctyl vanadyl phthalocyanine ( dp -C8OVPc), were synthesized, and the effect of the positions of octyl groups on their photophysical properties, OTFT performance was studied in detail. All four compounds are soluble in organic solvents such as chloroform and exhibit excellent thermal stability with the decomposition temperature ( T d) beyond 400°C. In dilute solution, the positions of octyl groups have negligible influence on their absorption spectra and frontier molecular orbital energy levels. Their absorption maxima ( λ max) are all at ca. 700 nm and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels are - 5.2 and - 3.8 eV, respectively. From solution to film, their absorption spectra exhibited a remarkable bathochromic shift. In film state, four compounds all can form ordered thin films, in which they arrange in an edge-on way. Bottom gate and top contact OTFTs based on thermally annealed films of these four compounds were fabricated via solution spin-casting. All compounds showed p-type transport behavior with field-effect mobility above 0.1 cm2/(V s) 2,3-C8OVPc displayed the best OTFT device performance. A mobility of 0.19 cm2/(V s) has been demonstrated when the films were annealed at 150°C.

Separating the impact of oxygen and water on the long-term stability of n-channel perylene diimide thin-film transistors

A detailed understanding for the mechanisms that control degradation of the electrical performance of organic thin-film transistors (TFTs) during exposure to various environments, such as oxygen and humidity, is still developing. This is particularly true for n-channel organic TFTs. Here we present an investigation of the long-term stability of n-channel TFTs based on the small-molecule organic semiconductor N,N′-bis(2,2,3,3,4,4,4-heptafluorobutyl-1,7-dicyano-perylene-(3,4:9,10)-tetracarboxylic diimide (PDI-FCN2) during storage in dry nitrogen, dry air, wet nitrogen and ambient air. By monitoring the electrical characteristics of the TFTs over a period of six weeks, we are able to show that the degradation of the electrical parameters (charge-carrier mobility and the simultaneous shift of the threshold voltage) is caused by two distinct mechanisms with different time constants. Exposure to oxygen or nitrogen (in the absence of humidity) causes the carrier mobility to drop by a factor of two and the threshold voltage to shift towards more positive values within 20days, possibly due to a slight rearrangement of the conjugated molecules within the semiconductor layer. Storing the TFTs in saturated water vapor or in ambient air causes the threshold voltage and the carrier mobility to change much more rapidly, within just one day. The observed degradation in ambient air can be explained by an electrochemical instability of the radical anion of the organic semiconductor.

Patterning process of ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene layer using bank structures for organic thin film transistors

A layer of 6,13-bis(triisopropylsilylethynyl) pentacene(TIPS-pentacene) organic thin film transistors (OTFTs) deposited by an ink jet printer was patterned using bank structures. Normally, banks degrade the performance of OTFTs because ink adhering to the sidewalls of the bank disrupts the formation of crystal grains, lowering mobility. An appropriately designed bank structure, which had only two photo-resist sidewalls perpendicular to the channel direction of the OTFTs, produced a mobility of the 0.23±0.05cm2/V·s, comparable to 0.25±0.09cm2/V·s of OTFTs produced without banks. The minimum thickness of the TIPS-pentacene layer needed to achieve high mobility was determined to be about 20nm. A modified bank structure with a splitter bar in the middle of the open bank was also developed to simultaneously fabricate two OTFTs by jetting a single drop on the splitter. The mobilities of the two OTFTs were 0.24±0.04cm2/V·s and 0.28±0.09cm2/V·s, respectively.

P‐147: Organic Thin Film Transistors using Solution and Photolithography Process

The organic thin film transistors (OTFTs) were fabricated and investigated in this study. Solution and photography technology were used for organic thin film formation and patterning. Also, the process temperatures were kept below 120 degree C for low temperature process development. Furthermore, the performances of OTFTs were investigated in this study, including electrical characteristics and reliability.

PENTACENE-BASED ORGANIC THIN FILM TRANSISTOR WITH SiO2 GATE DIELECTRIC

An organic thin film transistor (OTFT) based on pentacene was fabricated with SiO 2 as the gate dielectric material. We have investigated the effects of the thickness of pentacene layer and the organic semiconductor (OSC) material on OTFT devices at two different thicknesses. Au metal was deposited for gate, source and drain contacts of the device by using thermal evaporation method. Pentacene thin film layer was also prepared with thermal evaporation. Our study has shown that the change in pentacene thickness makes a noteworthy difference on the field effect mobility (μFET), values, threshold voltages (VT) and on/off current ratios (Ion/Ioff). OTFTs exhibited saturation at the order of μFET of 3.92 cm2/Vs and 0.86 cm2/Vs at different thicknesses. Ion/Ioff and VT are also thickness dependent. Ion/Ioff is 1 × 103, 2 × 102 and VT is 15 V, 27 V of 40 nm and 60 nm, respectively. The optimized thickness of the pentacene layer was found as 40 nm. The effect of the OSC layer thickness on the OTFT performance was found to be conspicuous.

Synthesis and Evaluation of Self-Assembled Azido Monolayer as a Novel Dielectric Layer for Fabricating Pentacene-Based Organic Thin Film Transistors.

Self-assembled 3-azidopropyltriethoxysilane monolayer (SAM) is used as a dielectric layer to modify the interface between the silicon dioxide wafer and the pentacene semiconductor layer in an organic thin film transistor (OTFT), Au/pentacene/3-azidopropyltriethoxysilane/SiO2/Si. Compared to the commonly used alkyl siliane C18 dielectric, 3-azidopropyltriethoxysilane which possesses stable formal charges is far more effective in increasing the ON/OFF ratio of OTFT device with an improvement of nearly three orders of magnitude. Analysis and measurements reported in this paper have illustrated for the first time the improvement of OTFT performance by a SAM compound with stable formal charges.

Domain fracture and recovery process of metal phthalocyanine monolayers via NO2 and H2O

CuPc ultrathin films (5 monolayers) are employed to detect NO2 in chemFETs [organic thin film transistors (OTFTs)]; while the NO2 causes OTFT degradation, H2O restores OTFT performance. To develop an atomic understanding of this H2O induced performance recovery, NO2/CuPc/Au(111) was exposed to H2O, then observed using ultrahigh vacuum scanning tunneling microscopy. After dosing NO2 (10 ppm for 5 min) onto CuPc monolayers under ambient conditions, domain fracture is induced in CuPc monolayers, and CuPc aggregates are formed near new grain boundaries, consistent with dissociative O adsorption between CuPc molecules and Au(111). Conversely, after exposing H2O onto a fractured CuPc monolayer for 30 min, fractured domains merge, then large area domains are generated. As the duration of H2O exposure increases to 4 h, second layer growth of CuPc molecules is observed on the CuPc monolayers consistent with H2O breakdown of CuPc aggregates which have formed at the domain boundaries. The results are consistent with H2O driving the removal of atomic O between CuPc molecules and Au(111) consistent with previous sensing results.

Enhanced Performance of Organic Thin Film Transistor Devices Using Hydroxyethyl-Terminated P3HT as the Active Layer.

Hydroxyethyl-terminated poly(3-hexylthiophene) (P3HT-OH) have been synthesized via a catalyst-transfer polycondensation using Grignard metathesis mediated by a nickel-based catalyst. This hydrophilic P3HT-OH is compared against the hydrophobic P3HT when used as an active layer on silicon dioxide (SiO2) wafer for organic thin-film-transistor (OTFT) fabrication. Hydroxyl groups at a 7.5% weight content lead to more chain regularity when polymer is bonded to SiO2 wafer surface and thus enhance the performance of OTFT Device, such as an 114.2% increase in ON/OFF ratio, an 12.4% increase in mobility, a 23.3% decrease in threshold voltage and a 30.1% decrease in surface roughness. Analysis and measurements reported in this paper have illustrated for the first time the feasibility of imparting hydrophilicity to the active layer for improving the OTFT performance.