Performance improvement mechanisms of pentacene-based organic thin-film transistors using TPD buffer layer

To improve the performances of pentacene-based organic thin-film transistors (OTFTs), a TPD buffer layer was inserted between the Au metal electrode and the pentacene channel layer. As shown by the ultraviolet photoelectron spectroscopy measurement, the Au work function was increased from 4.61 eV for Au in direct contact with pentacene to 4.74 eV and 4.78 eV for the sample inserted with 2-nm-thick and 3-nm-thick TPD buffer layers, respectively, between the Au metal electrode and the pentacene channel layer. Moreover, the contact resistance was reduced from 1 MΩ to 0.1 MΩ by inserting a 2-nm-thick TPD buffer layer. Compared with the transconductance of 2.67 × 10^?7 S, the field-effect mobility of 0.46 cm²/V s, and the substhreshold swing of 1.78 V/decade for the conventional pentacene-based OTFTs without TPD buffer layer, the transconductance, the field-effect mobility, and the subthreshold swing were improved to 9.77 × 10^?7 S, 1.68 cm²/V s, and 1.46 V/decade, respectively, for the pentacene-based OTFTs inserted with a 2-nm-thick TPD buffer layer. By considering the trade-off between the increase of Au work function and the tunneling effect, the optimal thickness of the TPD buffer layer in the pentacene-based OTFTs was 2 nm.     

Controllable carrier density of pentacene field-effect transistors using polyacrylates as gate dielectrics

We have studied the effect of the chemical structure of dielectrics by evaporating pentacene onto a series of polyacrylates: poly(methylmethacrylate), poly(4-methoxyphenylacrylate), poly(phenylacrylate), and poly(2,2,2-trifluoroethyl methacrylate) in organic thin-film transistors (OTFTs). In top-contact OTFTs, the polyacrylates had a significant effect on field-effect mobilities ranging 0.093 ∼ 0.195 cm² V⁻¹ s⁻¹. This variation neither correlated with the polymer surface morphology nor the observed pentacene crystallite size. This result implies that the PTFMA device generates the local electric field that accumulates holes and significantly shifts the threshold voltage and the turn-on voltage to −8.62 V and 3.5 V, respectively, in comparison with those of PMMA devices.     

Influence of passivation with non-charged fluorinated ethylene propylene on the properties of pentacene organic thin film transistor

High electronegativity of non-charged fluorinated ethylene propylene (FEP) affected the performance of pentacene organic thin film transistors (OTFTs). The threshold voltage of the OTFT shifted from −14.7 V to −12.5 V without deterioration of on–off ratio and charge mobility. The degree of shift was related to the coverage of the FEP film on an active layer. No further shift was observed upon reaching the full coverage of surface at a 3-nm thickness of FEP. Encapsulation of the FEP-covered pentacene OTFT in a multilayer of PMMA/PUA/PET protected the device from oxygen and moisture.     

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550 nm, a threshold voltage of −6.0 V, an on–off drain current ratio of 8.4 × 10⁵, and a field-effect mobility of 1.71 cm²/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.     

Oriented growth of rubrene thin films on aligned pentacene buffer layer and its anisotropic thin-film transistor characteristics

Oriented growth of polycrystalline rubrene thin film on oriented pentacene buffer layer was investigated. The oriented pentacene buffer layer was created by thermal evaporation of pentacene on a rubbed polyvinylalcohol (PVA) surface. The pentacene layer in turn induced the oriented growth of rubrene crystals upon thermal deposition. The structures of successive layers were characterized by using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy. Highly oriented rubrene crystallites with the a-axis aligning along the surface normal and the (0 0 2) plane preferentially oriented 45° away from the rubbing direction were found. In contrast, the rubrene thin film deposited on PVA or rubbed-PVA substrate without a pentacene buffer layer only gave amorphous phases. With the aligned pentacene/rubrene film as the active layer of organic field-effect transistor, anisotropic mobilities were observed. The highest field-effect mobility (0.105 cm²/V s) was observed along the direction 45° away from the rubbing direction and is ∼4 times higher than that for similar device prepared on unrubbed PVA. The direction was consistent with the GIXD observation that a large number of rubrene crystallites are having their [0 0 2] direction aligning in this direction. A favourable C–H⋯π interaction between an oriented pentacene layer and the rubrene layer on the control of molecular orientation in the conduction channel of the OFET is suggested.     

Pentacene organic thin-film transistors with solution-based gelatin dielectric

Gelatin is a natural protein in the field of food, pharmaceutical and tissue engineering, which works very well as the gate dielectric for pentacene organic thin-film transistors (OTFTs). An aqueous solution process has been applied to form a gelatin thin film on poly(ethylene terephthalate) (PET) or glass by spin-coating and subsequent casting. The device performance of pentacene OTFTs depend on the bloom number (molecular weight) of gelatin. The pentacene OTFT with 300 bloom gelatin as the gate dielectric in air ambient exhibits the best performance with an average field-effect mobility (μ_FE) value of ca. 16 cm² V^?1 s^?1 in the saturation regime and a low threshold voltage of ?1 V. The high performance of the pentacene OTFT in air ambient is attributed to the water resided in gelatin. The crystal quality of pentacene is not the key factor for the high performance.     

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.     

Teflon/SiO2 bilayer passivation for improving the electrical reliability of pentacene-based organic thin-film transistors

We demonstrate a bilayer passivation method using a Teflon and SiO₂ combination layer to improve the electrical reliability of pentacene-based organic thin-film transistors (OTFTs). The Teflon was deposited as a buffer layer using a thermal evaporator that exhibited good compatibility with the underlying pentacene channel layer, and can effectively protect the OTFTs from plasma damage during the SiO₂ deposition process, resulting in a negligible initial performance drop in OTFTs. Furthermore, because of the excellent moisture barrier ability of SiO₂, the OTFTs exhibited good time-dependent electrical performance, even after 168 h of aging in ambient air with 60–80% relative humidity.     

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.     

Fabrication of organic thin film transistors on Polyethylene Terephthalate (PET) fabric substrates

In this paper organic thin film transistors (OTFTs) are directly fabricated on fabric substrates consisting of Polyethylene Terephthalate (PET) fibers. A key process is coating the polymer layers on the fabric in order to reduce the large surface roughness of the fabric substrate. Two polymers, i.e. polyurethane (PU) and photo-acryl (PA), are used to reduce the large surface roughness and simultaneously improve the process compatibility of the layers with the subsequent OTFTs processes while also retaining the original flexibility of the fabric. The surface roughness of the PU/PA-coated fabric is significantly reduced to 0.3 μm. Furthermore, the original flexibility of the PET fabric remained after coating of the PU/PA polymer layers. The mobility of the OTFTs fabricated on the PU-PA coated fabric substrate is 0.05 ± 0.02 cm²/V s when three PA layers and 90 nm thick pentacene layer were used. The performance does not vary even after 30,000 bending test.     

Solution processable bilayered gate dielectric towards flexible organic thin film transistors

In this study, we have successfully explored the potential of a new bilayer gate dielectric material, composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) composite thin film and Polyacrylonitrile (PAN) through fabrication of metal insulator metal (MIM) capacitor devices and organic thin film transistors (OTFTs). The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material are optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques are applied to deposit PAN and PS-P123 as a bilayer dielectric layer on Polyimide (PI) substrates. Contact angle study is further performed to explore the surface property of this bilayer polymer gate dielectric material. This new bilayer dielectric having a k value of 3.7 intermediate to that of PS-P123 composite thin film dielectric (k ∼ 2.8) and PAN dielectric (k ∼ 5.5) has successfully acted as a buffer layer by preventing the direct contact between the organic semiconducting layer and high k PAN dielectric. The OTFT devices based on α,ω-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37 × 10⁻² and on/off current ratio of 10³ which is one of the good values as reported before. Several bending conditions are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which are exposed to open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90 °C.     

A cross-linked poly(methyl methacrylate) gate dielectric by ion-beam irradiation for organic thin-film transistors

The electrical characteristics of pentacene organic thin-film transistors (OTFTs) using cross-linked poly(methyl methacrylate) (PMMA) as the gate dielectric are reported. Ultra-thin films of cross-linked PMMA could be obtained by spin-coating and subsequent irradiation using a 1.515 MeV ⁴He⁺ ion beam. The resulting film, with a thickness of 33 nm, possessed a low leakage current density of about 10^?6 A cm^?2 for fields up to 2 MV cm^?1. OTFTs incorporating the cross-linked dielectric operated at relatively low voltages, <10 V, and exhibited a mobility of 1.1 cm² V^?1 s^?1, a threshold voltage of ?1 V, a sub-threshold slope of 220 mV per decade and an on/off current ratio of 1.0 × 10⁶.     

Direct photo-patternable,low-temperature processable polyimide gate insulator for pentacene thin-film transistors

We have developed photo-sensitive, low-temperature processable, soluble polyimide (PSPI) gate insulator with excellent resistance to the photo-patterning process. The PSPI was synthesized through one-step condensation polymerization of monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and 3,5-diaminobenzyl cinnamate (DABC). PSPI thin film, fabricated at 160 °C, has a dielectric constant of 3.3 at 10 kHz, and leakage current density of <1.7 × 10^?10 A/cm², while biased from 0 to 100 V. PSPI could be easily patterned by selective UV-light exposure and dipping into γ-butyrolactone. To investigate the potential of the polyimide as the photo-patternable gate insulator, we fabricated pentacene OTFTs and confirmed the PSPI’s resistance to the photo-patterning process. The photo-patternable polyimide shows promise as gate dielectrics for OTFTs.     

High performance of pentacene organic thin film transistors by doping of iodine on source/drain regions

Contact doping was conducted by iodine in a top contact configuration in a pentacene organic thin film transistor (OTFT), to investigate its effects on contact resistance and the resulting electrical performance. Iodine doping in the pentacene film caused the change of pentacene structure, thus leading to an increase in electrical anisotropy, i.e. ratio of lateral to vertical resistivity. The two resistive components of doped pentacene film underneath the Au contacts were major contributors to the contact resistance, and a model to explain the dependence of contact resistance on iodine doping was presented. Finally, OTFTs fabricated on iodine doped source/drain contacts exhibited high mobility of 1.078 cm²/V s, two times that of OTFTs with undoped contacts, due to the low contact resistance.     

Dual enhancing properties of LiF with varying positions inside organic light-emitting devices

A multilayer organic light-emitting device (OLED) has been fabricated with a thin (0.3 nm) lithium fluoride (LiF) layer inserted inside an electron transport layer (ETL), aluminum tris(8-hydroxyquinoline) (Alq₃). The LiF electron injection layer (EIL) has not been used at an Al/Alq₃ interface in the device on purpose to observe properties of LiF. The electron injection-limited OLED with the LiF layer inside 50 nm Alq₃ at a one forth, a half or a three forth position assures two different enhancing properties of LiF. When the LiF layer is positioned closer to the Al cathode, the injection-limited OLED shows enhanced injection by Al interdiffusion. The Al interdiffusion at least up to 12.5 nm inside Alq₃ rules out the possible insulating buffer model in a small molecule bottom-emission (BE) OLED with a thin, less than one nanometer, electron injection layer (EIL). If the position is further away from the Al cathode, the Al diffusion reaches the LiF layer no longer and the device shows the electroluminescence (EL) enhancement without an enhanced injection. The suggested mechanism of LiF EL efficiency enhancer is that the thin LiF layer induces carrier trap sites and the trapped charges alters the distribution of the field inside the OLED and, consequently, gives a better recombination of the device. By substituting the Alq₃ ETL region with copper phthalocyanine (CuPc), all of the electron injection from the cathode of Al/CuPc interface, the induced recombination at the Alq₃ emitting layer (EML) by the LiF EL efficiency enhancer, and the operating voltage reduction from high conductive CuPc can be achieved. The enhanced property reaches 100 mA/cm² of current density and 1000 cd/m² of luminance at 5 V with its turn-on slightly larger than 2 V. The enhanced device is as good as our previously reported non-injection limited LiF EIL device [Yeonjin Yi, Seong Jun Kang, Kwanghee Cho, Jong Mo Koo, Kyul Han, Kyongjin Park, Myungkeun Noh, Chung Nam Whang, Kwangho Jeong, Appl. Phys. Lett. 86 (2005) 213502].     

Ambipolar transport in transparent and flexible all-organic heterojunction field effect transistors at ambient conditions

We report on the realization of fully flexible and transparent n-type and ambipolar all-organic OFETs. A double layer, pentacene-C60 heterojunction, was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography MicroContact Printing (μCP). Interestingly, as demonstrated by atomic force microscopy and X-ray diffraction investigations, growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to obtain n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C60. As a result, it was possible to realize n-type and ambipolar all-organic OFETs by optimizing the thicknesses of the pentacene buffer layer. All devices, measured in air, worked in accumulation mode with mobilities up to 1 × 10⁻² cm²/V s and 3.5 × 10⁻⁴ cm²/V s for p-type and n-type regimes, respectively. This is particularly interesting because it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. These results confirm the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials.     

Improved pentacene device characteristics with sol–gel SiO2 dielectric films

The interfacial interactions between semiconductors and gate dielectrics have a profound influence on the device characteristics of field effect transistors (FETs). This paper reports on the concept of introducing a sol–gel SiO₂ as inorganic capping layer to significantly improve device characteristics of pentacene-based FETs. The smoother film surfaces of sol–gel SiO₂ (1.9 Å root-mean-square) induced larger pentacene grain sizes, and led to hole mobilities of 1.43 cm²/Vs, on–off ratio of 10⁷, and a subthreshold swing of 102 mV/decade when operating at −20 V.     

Improving the performance of organic thin-film transistor with a doped interlayer

Top contact organic thin-film transistors (TC OTFTs) based on pentacene are fabricated. For improving the contact characteristics between the organic semiconductor thin-film and gold electrodes, we doped the starburst molecular 4,4′,4″-tris{N,(3-methylpheny)-N-phenylamino}-triphenylamine) (m-MTDATA), which is an excellent hole injection material for the organic light-emitting devices (OLEDs), into the interlayer contact with the electrodes. Compared with conventional TC OTFT, the performances of the organic transistor with the doped interlayer are improved. The field-effect mobility increases from 0.16 to 0.51 cm²/V s, and threshold voltage downshifts from –11 to –2.8 V for the linear region. The on/off current ratio is more than 10⁴ when the gate voltage varies from 0 to –20 V. We ascribe the improvements to the doped interlayer for which the contact resistance is reduced and the hole injection is enhanced.     

Novel organic inverters with dual-gate pentacene thin-film transistor

We report on the fabrication and characterization of dual-gate pentacene organic thin-film transistors (OTFTs) with plasma-enhanced atomic-layer-deposited (PEALD) 150 nm thick Al₂O₃ as a bottom-gate dielectric and PEALD 200 nm thick Al₂O₃ as a top-gate dielectric. The V_th of dual-gate OTFT has changed systematically with the application of voltage bias to top-gate electrode. When voltage bias from −10 V to 10 V is applied to top gate, V_th changes from 1.95 V to −9.8 V. Two novel types of the zero drive load logic inverter with dual-gate structure have been proposed and fabricated using PEALD Al₂O₃ gate dielectrics. Because the variation of V_th due to chemical degradation and the spatial variation of V_th are inherent in OTFTs, the compensation technology by dual-gate structure can be essential to OTFT applications.     

Contact resistance between pentacene and indium–tin oxide (ITO) electrode with surface treatment

Contact resistance between indium–tin oxide (ITO) electrode and pentacene was studied by transmission line method (TLM). Organic solvent cleaned, inorganic alkali cleaned, and self-assembled monolayer (with OTS: octadecyltrichlorosilane) modified ITO electrode structures were compared. Pentacene layer of 300 Å thickness was vacuum deposited on patterned ITO layer at 70 °C with a deposition rate of 0.3 Å/s. Alkali cleaned and SAM modified ITO gave a lower contact resistance of about 6.34 × 10⁴ Ω cm² and 1.88 × 10³ Ω cm², respectively than organic solvent cleaned ITO of about 6.58 × 10⁵ Ω cm². Especially with the SAM treatment, the work function of ITO increased closer to the highest occupied molecular orbital (HOMO) level of pentacene, which lowers the injection barrier between ITO and pentacene. It was also believed that pentacene morphology was improved on SAM modified ITO surface due to the lowering of the surface energy. We could obtain the low contact resistance with SAM treatment which is comparable to the measured value of gold–pentacene contact, 1.86 × 10³ Ω cm². This specific contact resistance is still much higher than that of amorphous silicon thin film transistor (0.1–30 Ω cm²).