Bottom-contact organic field-effect transistors based on single-crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene prepared by electrostatic spray deposition

Large crystalline domains (a few hundred micrometers in size) of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) were prepared by electrostatic spray deposition (ESD) and used as the active layers of bottom-contact organic field-effect transistors. The TIPS pentacene active layers were directly patterned via a shadow mask in the ESD process. The device, which had a 5-μm-long channel composed of a single-crystalline domain, exhibited a high field-effect mobility of more than 0.1 cm²/V s but resulted in a high threshold voltage of −17 V. The threshold voltage could be lowered to −6.4 V by reducing the thickness of the BC electrodes from 30 to 10 nm; this threshold voltage lowering was probably due to an improvement in the charge injection from the source electrode to the active layer.     

Understanding Lattice Strain‐Controlled Charge Transport in Organic Semiconductors: A Computational Study

The softness and anisotropy of organic semiconductors offer unique properties. Recently, solution‐sheared thin‐films of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐P) with nonequilibrium single‐crystal domains have shown much higher charge mobilities than unstrained ones (Nature 2011 , 480, 504). However, to achieve efficient and targeted modulation of charge transport in organic semiconductors, a detailed microscopic understanding of the structure–property relationship is needed. In this work, motivated by the experimental studies, the relationship between lattice strain, molecular packing, and charge carrier mobility of TIPS‐P crystals is elucidated. By employing a multiscale theoretical approach combining nonequilibrium molecular dynamics, first‐principles calculations, and kinetic Monte Carlo simulations using charge‐transfer rates based on the tunneling enabled hopping model, charge‐transport properties of TIPS‐P under various lattice strains are investigated. Shear‐strained TIPS‐P indeed exhibits one‐dimensional charge transport, which agrees with the experiments. Furthermore, either shear or tensile strain lead to mobility enhancement, but with strong charge‐transport anisotropy. In addition, a combination of shear and tensile strains could not only enhance mobility, but also decrease anisotropy. By combining the shear and tensile strains, almost isotropic charge transport could be realized in TIPS‐P crystal with the hole mobility improved by at least one order of magnitude. This approach enables a deep understanding of the effect of lattice strain on charge carrier transport properties in organic semiconductors.     

High charge-carrier mobilities in blends of poly(triarylamine) and TIPS-pentacene leading to better performing X-ray sensors

A new class of X-ray sensor – in which there is a blend of poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene in the active layer of a diode structure – has been developed. The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the sensor. The first time-of-flight charge-carrier mobility measurement of this blend is reported. The mobility of PTAA and TIPS-pentacene in a 1:25 molar ratio was found to be 2.2 × 10⁻⁵ cm² V⁻¹ s⁻¹ (averaged for field strengths between 3 × 10⁴ and 4 × 10⁵ V cm⁻¹), which is about 17 times higher than that obtained in PTAA over the same range of field strengths. This higher mobility is correlated with a fourfold increase in the X-ray detection sensitivity in the PTAA:TIPS-pentacene devices.     

High-performance thin-film transistor with 6,13-bis(triisopropylsilylethynyl) pentacene by inkjet printing

We have studied the performance improvement of organic thin-film transistor (OTFT) with a solution based TIPS pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) by inkjet printing. The TIPS pentacene with 1.0 wt.% solution in 1,2-dichlorobenzene was used for printing of an active layer of OTFT. The OTFT printed at room temperature shows a shoulder-like behavior but it disappears for the OTFT printed at the substrate temperature of 60 °C. The OTFT on plastic exhibited an on/off current ratio of ∼10⁷, a threshold voltage of −2.0 V, a gate voltage swing of 0.6 V/decade and a field-effect mobility of 0.24 cm²/Vs in the saturation region.     

Temperature gradient controlled crystal growth from TIPS pentacene-poly(α-methyl styrene) blends for improving performance of organic thin film transistors

6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) from simple drop casting typically forms crystals with random orientation and poor areal coverage, which leads to device-to-device performance variation of organic thin film transistors (OTFTs). Previously, a temperature gradient technique was developed to address these problems. However, this approach simultaneously introduced thermal cracks due to the thermally induced stress during crystallization. These thermal cracks accounted for a reduction of charge transport, thereby impacting the device performance of TIPS pentacene based OTFTs. In this work, an insulating polymer, poly(α-methyl styrene) (PαMS) was blended with TIPS pentacene to relieve the thermal stress and effectively prevent the generation of thermal cracks. The results demonstrate that the incorporation of PαMS polymer combined with the temperature gradient technique improves both the hole mobility and performance consistency of TIPS pentacene based OTFTs.     

Organic field-effect transistors based on single-crystalline active layer and top-gate insulator consistently fabricated by electrostatic spray deposition

An electrostatic spray deposition (ESD) method was applied to prepare both crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and insulating films of poly(methyl methacrylate) (PMMA) for fabricating top-gate single-crystal organic field-effect transistors (OFETs). The electrical characteristics of the top-gate device were compared to those of the bottom-gate one (SiO₂ bottom-gate insulator) with the same active layer, and the lower charge-trap density at the interface between the top-gate insulator and single-crystalline active layer was demonstrated. The drain current compression in the output characteristics of the top-gate device, however, occurred due to the large parasitic resistance between the source/drain electrodes and accumulation channel. Reducing the thickness of the single-crystalline active layer resulted in a high charge-carrier mobility of 0.29 cm²/V s (channel length of 5 μm).     

A 6,13-bis(Triisopropylsilylethynyl) Pentacene Thin-Film Transistor Using a Spun-On Inorganic Gate-Dielectric

We present the latest results of the use of soluble materials such as organic semiconductors (OSCs) or gate-dielectrics for simplified processing of organic thin-film transistors (OTFTs). In this paper, we described our fabrication of a solution-processed OTFT with 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) as the OSC and siloxane-based spin-on glass (SOG) as the inorganic gate-dielectric. Also, synthesized TIPS-pentacene and SOG were examined for use as the OSC and gate-dielectric in an OTFT. From electrical measurements, we obtained device performance characteristics such as charge carrier mobility, threshold voltage, current ON/OFF ratio, and subthreshold swing, which were 6.48 times 10^-3 cm²/V ldr s, -13 V, ~100, and 1.83 V/dec, respectively.     

Effect of active layer thickness on environmental stability of printed thin-film transistor

We have studied the effect of active layer thickness on the performance and environmental stability of the 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) thin-film transistor. The organic thin-film transistors (OTFTs) were fabricated by inkjet printing using a solution based TIPS pentacene. To get thick organic semiconductor, the surface of gate insulator was treated with n-octyltrichlorosilane (OTS-C₈) before jetting. The on-currents of the OTFT with ～1 μm active layer decreases a little in air, but the OTFT with 0.05 μm TIPS pentacene shows a significant degradation in drain currents.     

Improving performance of TIPS pentacene-based organic thin film transistors with small-molecule additives

This work demonstrates an effective approach to improve both charge transport and performance consistency in solution-processed organic thin-film transistors (OTFTs) by blending 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene, or TP) with a series of small-molecule additives: 4-butylbenzoic acid (BBA), 4-hexylbenzoic acid (HBA), and 4-octylbenzoic acid (OBA). These three small molecules share a benzoic acid moiety, but have different length of hydrophobic tails. The self-assembled interfacial layer of small molecules on the gate oxide surface leads to uniform deposition of TP crystal seeds and facilitates TP to grow along the tilted orientation of substrate, which results in a film of enhanced crystal orientation and areal coverage. OTFTs based on TP/small molecule blends demonstrate greatly improved average hole mobility and performance consistency, which correlates with the length of hydrophobic tail of the small-molecule additives.     

Quantitative Determination of Charge Transport Interface at Vertically Phase Separated Soluble Acene/Polymer Blends

Interfacial structure is critical for optimizing the electrical properties of organic field-effect transistors. In this study, the interfacial structures of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)/polymer blends are nondestructively determined by the complementary neutron and X-ray reflectivity. The TIPS-pentacene/deuterated poly(methylmethacrylate) (d-PMMA) blends exhibit a vertically phase-separated structure with a molecularly sharp interface (interfacial roughness ≈5 Å), whereas the TIPS-pentacene/d-polystyrene (d-PS) blend intermix near the interface. Ultrahigh molecular weight d-PMMA leads to the formation of surface-segregated hexagonal spherulites of TIPS-pentacene owing to the thermodynamic factors (e.g., surface/interface energy, polarity, and viscosity) of the blending materials. The well-developed hexagonal spherulites of TIPS-pentacene on molecularly sharp d-PMMA interface result in higher field-effect mobility as compared to the dendritic crystals from d-PS blends because of the higher perfectness, coverage, and interfacial roughness of the TIPS-pentacene crystals. The approach used in this study facilitates the understanding of the charge transport mechanism at the phase-separated interfaces in soluble acene/polymer blends.     

Directed self-assembly of organic semiconductors via confined evaporative capillary flows for use in organic field-effect transistors

We fabricated well-defined 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystal arrays for use in electronic applications via a simple but effective method, the confined evaporative capillary flow (CEC) method. This has been accomplished by systematically controlling the contact line pinning at the edge of glass stylus and the outward hydrodynamic flow within the drying droplet with various processing solvents and surface properties of the substrate during solidification. We found that after CEC coating of TIPS-PEN solution dissolved into toluene onto SiO₂ surface, ribbon-shaped TIPS-PEN crystals were well developed with a width of 20–100 μm and length of 300 μm – 2 mm, which is presumably owing to optimized capillary evaporation. Specifically, TIPS-PEN crystals present highly preferred crystal orientation along the (l 0 0) axis, which can lead to efficient charge transport in a lateral direction. Thus, TIPS-PEN field-effect transistors (FETs) exhibited a good hole mobility of 0.72 cm²/Vs.     

Enhanced Performance Consistency in Nanoparticle/TIPS Pentacene‐Based Organic Thin Film Transistors

In this study, inorganic silica nanoparticles are used to manipulate the morphology of 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS pentacene) thin films and the performance of solution‐processed organic thin‐film transistors (OTFTs). This approach is taken to control crystal anisotropy, which is the origin of poor consistency in TIPS pentacene based OTFT devices. Thin film active layers are produced by drop‐casting mixtures of SiO₂ nanoparticles and TIPS pentacene. The resultant drop‐cast films yield improved morphological uniformity at ～10% SiO₂ loading, which also leads to a 3‐fold increase in average mobility and nearly 4 times reduction in the ratio of measured mobility standard deviation (μ_Stdev) to average mobility (μ_Avg). Grazing‐incidence X‐ray diffraction, scanning and transmission electron microscopy as well as polarized optical microscopy are used to investigate the nanoparticle‐mediated TIPS pentacene crystallization. The experimental results suggest that the SiO₂ nanoparticles mostly aggregate at TIPS pentacene grain boundaries, and 10% nanoparticle concentration effectively reduces the undesirable crystal misorientation without considerably compromising TIPS pentacene crystallinity.     

All-Inkjet-Printed Bottom-Gate Thin-Film Transistors Using UV Curable Dielectric for Well-Defined Source-Drain Electrodes

Technological restrictions of the inkjet printing technology for printed electronics can hinder its application potential, mainly due to the limited resolution and layer homogeneity in comparison to conventional manufacturing techniques for electronics. The manufacturing of active devices such as thin-film transistors with appropriate performance using printing technologies is still one of the current challenges towards industrial applications. This work demonstrates the application of an ultraviolet (UV) curable ink as insulating material for the gate dielectric. The advantage of the UV curable ink is its fast curing and the smooth surface enabling high resolution patterns on top of it. In this way, all-inkjet-printed organic thin-film transistors (OTFTs) were fabricated with silver electrodes, UV curable gate dielectric, and 6,13-bis(triisopropylsilylethynyl)pentacene for the active semiconductor layer. By fine tuning of processing parameters and pattern geometries, a stable channel length of about 10 μm was obtained in the bottom-gate configuration without the need of additional steps, suggesting a way to build low-cost all-inkjet-printed OTFTs with well-defined source-drain electrodes and fast UV curable dielectric without any additional steps. The inkjet-printed device is characterized by an electron mobility of 0.012 cm² V^?1 s^?1 and on/off ratio of 10³.     

Solution-processed low leakage organic field-effect transistors with self-pattern registration based on patterned dielectric barrier

We demonstrated a new type of a solution-processed organic field-effect transistor (OFET) in a bottom-gate, top-contact geometry where low leakage current and self-pattern registration were achieved using a patterned dielectric barrier (PDB). The PDB of a hydrophobic fluorinated-polymer was produced on the top of a polymeric gate insulator of poly(4-vinylphenyl) by transfer-printing. The PDB enables to effectively screen out the vertical charge flow generated from the gate electrode, and thus the vertical leakage current between the gate and the drain was reduced by two orders of the magnitude compared to the leakage current in a conventional OFET without the PDB. Moreover, the PDB defines spontaneously an active channel pattern from a solution of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS PEN) by means of the selective wettability and the geometrical confinement.     

Electrical and optical analyses of trapping phenomenon with temperature dependence of organic device

Carrier mechanism in actual organic devices is not simple, owing to the dielectric nature of active organic semiconductor layers, the complexity of the organic device interface, carrier trapping effects by stress biasing, and others. By coupling the conventional electrical measurement, e.g., current–voltage, capacitance–voltage and capacitance-frequency measurements, with optical charge modulation spectroscopy (CMS) measurement, we studied the hysteresis behavior and the temperature dependence of indium tin oxide/polyimide/6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS-pentacene)/Au diodes to understand the effect of carrier trapping caused by injected carriers. The coupled electrical and optical measurements were very helpful to clarify carrier injection that was followed by carrier trapping in the diode, in terms of hysteresis behavior. CMS measurement was used to observe energetic states of carriers in TIPS-pentacene double-layer diode. Finally, the carrier mechanism in organic diodes was discussed by analyzing the diodes as a Maxwell-Wagner effect element.     

Air-flow navigated crystal growth for TIPS pentacene-based organic thin-film transistors

6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) is a promising active channel material of organic thin-film transistors (OTFTs) due to its solubility, stability, and high mobility. However, the growth of TIPS pentacene crystals is intrinsically anisotropic and thus leads to significant variation in the performance of OTFTs. In this paper, air flow is utilized to effectively improve the TIPS pentacene crystal orientation and enhance performance consistency in OTFTs, and the resulted films are examined with optical microscopy, X-ray diffraction, and thin-film transistor measurements. Under air-flow navigation (AFN), TIPS pentacene drop-cast from toluene solution has been observed to form thin films with improved crystal orientation and increased areal coverage on substrates, which subsequently lead to a fourfold increase of average hole mobility and one order of magnitude enhancement in performance consistency defined by the ratio of average mobility to the standard deviation of the field-effect mobilities.     

Cover Picture: Self‐Organization of Ink‐jet‐Printed Triisopropylsilylethynyl Pentacene via Evaporation‐Induced Flows in a Drying Droplet (Adv. Funct. Mater. 2/2008)

Charge carrier transport in organic electronic devices is influenced by the crystalline microstructure and morphology of the organic semiconductor film. Evaporation behavior during drying plays a vital role in controlling the film morphology and the distribution of solute in inkjet‐printed films. On p. 229, Kilwon Cho and co‐workers demonstrate the influence of the evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of inkjet‐printed 6,13‐bis((triisopropylsilylethynyl) pentacene. The results provide an excellent method for direct‐write fabrication of high‐performance organic electronics. We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm² V^–1 s^–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.     

Hybrid CMOS thin-film devices based on solution-processed CdS n-TFTs and TIPS-Pentacene p-TFTs

Fabrication and characterization of integrated hybrid complementary metal oxide semiconductor devices (CMOS) using 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PC) and cadmium sulfide (CdS) as the active layers deposited using solution based processes are demonstrated. The n- and p-type thin film transistors (TFTs), inverters, and NAND gate devices were fabricated using photolithography-based techniques. The hybrid CMOS technology demonstrated is compatible with large-area and mechanically flexible substrates given the low temperature processing (<100 °C) and scalable design. The integrated n- and p-type devices show saturation mobilities of 15 and 0.02 cm²/V s, respectively. The inverters exhibited a DC gain of ≈52 V/V with full rail-to-rail switching. The NAND logic gates switch rail-to-rail with a transition point of V_DD/2.     

Organic photovoltaic cells with high open circuit voltages based on pentacene derivatives

Heterojunction organic photovoltaic devices were fabricated using C₆₀ as the electron acceptor and several pentacene derivatives with triisopropylsilylethynyl functional groups as the electron donor. The open circuit voltage (V_oc) of functionalized pentacene-based cells is significantly higher (0.57–0.90 V) than for cells based on unsubstituted pentacene (0.24 V), due to the higher oxidation potentials of these pentacene derivatives. The performance of pentacene derivative cells is limited by lower current densities than the reference pentacene/C₆₀ cell. The absorption spectra of films and solutions of pentacene derivatives closely resemble one another, leading us to conclude that these films are amorphous in nature. Weak intermolecular coupling in the derivative films results in lower charge mobility and shorter exciton diffusion lengths relative to pentacene.