Polymer blended small molecule organic field effect transistors with improved device-to-device uniformity and operational stability

We report high performance organic thin-film transistors (OTFTs) with improved device-to-device uniformity and operational stability using polymer-blended small molecule organic semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). The diF-TESADT blended with poly(α-methylstyrene) was spin-cast to form bottom-contact OTFTs, and an average carrier mobility of more than 0.16 cm²/V s with more uniform surface morphology and device-to-device uniformity compared to neat diF-TESADT devices were achieved. Additionally, the polymer-blended OTFTs have shown improved operational stability under gate bias-stress possibly due to blocking of ambient oxygen and moisture by vertically separated insulating matrix polymer.     

High performance organic thin film transistors using chemically modified bottom contacts and dielectric surfaces

A compatible process of orthogonal self-assembled monolayers (SAMs) is applied to intentionally modify the bottom contacts and gate dielectric surfaces of organic thin film transistors (OTFTs). This efficient interface modification is first achieved by 4-fluorothiophenol (4-FTP) SAM to chemically treat the silver source–drain (S/D) contacts while the silicon oxide (SiO₂) dielectric interface is further primed by either hexamethyldisilazane (HMDS) or octyltrichlorosilane (OTS-C8). Results show that the field effect mobilities of the bottom-gate bottom-contact PTDPPTFT4 transistors were significantly improved to 0.91 cm² V⁻¹ s⁻¹.     

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.     

Bottom gate organic thin-film transistors fabricated by ultraviolet transfer embossing with improved device performance

Ultraviolet transfer embossing is optimized to fabricate bottom gate organic thin-film transistors (OTFTs) on flexible plastic substrates, achieving significant improved device performance (μ = 0.01–0.02cm²/Vs; on/off ratio = 10⁴) compared with the top gate OTFTs made previously by the same method (μ = 0.001–0.002 cm²/Vs; on/off ratio = 10²). The performance improvement can be ascribed to the reduced roughness of the dielectric-semiconductor interface (R_rms = 0.852 nm) and thermally cross-linked PVP dielectric which leads to reduced gate leakage current and transistor off current in the bottom-gated configuration. This technique brings an alternative great opportunity to the high-volume production of economic printable large-area OTFT-based flexible electronics and sensors.     

Photo-patternable polyimide gate insulator with fluorine groups for improving performance of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiopene (C10-BTBT) thin-film transistors

Surface properties of gate insulators strongly affect the device performance of organic thin-film transistors (OTFTs). To improve the performance of OTFTs, we have developed photo-sensitive polyimide gate insulator with fluorine groups. The polyimide gate insulator film could be easily patterned by selective UV exposure without any photoinitiator. The polyimide gate insulator film, fabricated at 130 °C, has a dielectric constant of 2.8 at 10 kHz, and leakage current density of <1.6 × 10^?10 A/cm² while biased from 0 to 90 V. To investigate the potential of the polyimide with fluorine groups as a gate insulator, we fabricated C₁₀-BTBT TFTs. The field-effect mobility and the on/off current ratio of the TFTs were measured to be 0.76 ± 0.09 cm²/V s and >10⁶, respectively.     

Photocurable propyl-cinnamate-functionalized polyhedral oligomeric silsesquioxane as a gate dielectric for organic thin film transistors

A polyhedral oligomeric silsesquioxane (POSS)-based insulating material with photocurable propyl-cinnamate groups (POSS-CYNNAM) was designed and synthesized through simple single step reaction for use as a gate dielectric in organic thin-film transistors (OTFT). POSS-CYNNAM was soluble in common organic solvents and formed a smooth thin film after spin-casting. A thin film of POSS-CYNNAM was cross-linked and completely solidified under UV irradiation without the use of additives such as photoacid generators or photoradical initiators. ITO/insulator/Au devices were fabricated and characterized to measure the dielectric properties of POSS-CYNNAM thin films, such as leakage current and capacitance. A pentacene-based OTFT using the synthesized insulator as the gate dielectric layer was fabricated on the transparent indium tin oxide (ITO) electrode, and its performance was compared to OTFTs using thermally cross-linked poly(vinyl phenol) (PVP) as the insulator. The fabricated POSS-CYNNAM OTFT showed a comparable performance to devices based on the PVP insulator with 0.1 cm²/Vs of the field effect mobility and 4.2 × 10⁵ of an on/off ratio.     

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.     

Fully aromatic polyimide gate insulators with low temperature processability for pentacene organic thin-film transistors

This paper demonstrates the effects of the imidization ratio of polyimide gate insulators on the performance of organic thin-film transistors (OTFTs). We report the synthetic results of polyimide films imidized at a temperature of 200 °C along with an easily removed organic base catalyst (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU), and their application in gate insulators of organic thin-film transistors. The degree of imidization increased to almost 100% after a thermal treatment at 200 °C for 40 min in the presence of DBU. The performance of the pentacene OTFT dramatically improved by using low temperature cured polyimide film as the gate insulator.     

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.     

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.     

Soluble pentacene thin-film transistor using a high solvent and heat resistive polymeric dielectric with low-temperature processability and its long-term stability

Solution processable organic thin-film transistors (OTFTs) were fabricated using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) and low-temperature processable polyimide gate dielectric. The TIPS-pentacene OTFT with the dielectric was found to have a field-effect mobility of 0.15 cm²/Vs, which is comparable to that of OTFT with an inorganic dielectric. The OTFTs with the polyimide dielectric did not show any significant performance degradation as time passed. A field-effect mobility of the OTFTs in 60 days was found to be almost identical to that of pristine OTFT. The combination of TIPS-pentacene and our polyimide gate dielectric can be one of the potential candidates for the fabrication of stable OTFTs for large-area flexible electronics.     

High performance top contact fused thiophene–diketopyrrolopyrrole copolymer transistors using a photolithographic metal lift-off process

High-performance fused thiophene–diketopyrrolopyrrole copolymer (PTDPPTFT4) top-contact transistors have been fabricated using a top contact metal lift-off process. The source–drain (S/D) top metal contacts were directly formed by i-line photolithographic patterning over the organic semiconductor channel through a lift-off method based on a negative tone photoresist. With a fluorinated imaging material (Orthogonal OScR 2312), devices exhibit field effect mobilities up to 2.56 cm² V⁻¹ s⁻¹ and on/off ratio >10⁷, with channel lengths precisely patterned down to 10 μm. Devices prepared from the lift-off process through a commercial negative tone photoresist such as AZ nLOF 2020 instead of the fluorinated photoresist exhibited considerable degradation. De-ionized (DI) water, aqueous tetramethyl ammonium hydroxide (TMAH) developer and organic stripper do not apparently degrade the performance of the PTDPPTFT4 transistors upon dip testing. Indeed the observed degradation originates from the incomplete development of the unexposed photoresist in the TMAH developer. This is detrimental to the PTDPPTFT4 channel due to formation of resist residue over the organic semiconductor surface. This residue eventually leads to a significant increase of the S/D contact resistance upon metal contact deposition.     

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.     

Hybridization of a low-temperature processable polyimide gate insulator for high performance pentacene thin-film transistors

We have synthesized a novel fully soluble and low-temperature processable polyimide gate insulator (KSPI) through the one-step condensation polymerization of the monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride and 4,4-diaminodiphenylmethane. Fully imidized KSPI was found to be completely soluble in organic solvents such as N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), γ-butyrolactone, dimethylsulfoxide (DMSO), and 2-butoxyethanol. Thin films of KSPI can be fabricated at only 150 °C and a pentacene OTFT with KSPI as a gate dielectric was found to exhibit a field effect mobility of 0.22 cm²/V s. To obtain a high performance organic thin-film transistor (OTFT), the KSPI surface was modified in our new technique by hybridization with a non-polar side chain containing a polyimide insulator (PI). The carrier mobility of a pentacene OTFT with a hybridized polyimide gate insulator (BPI-3) was found to be 0.92 cm²/V s. Our new low-temperature processable polyimides show promise as gate dielectrics for OTFTs.     

Improving the performance of the organic thin-film transistors with thin insulating lithium fluoride buffer layer

The pentacene-based organic thin-film transistors (OTFTs) with a thin insulating lithium fluoride (LiF) buffer layer between the pentacene and source/drain electrodes were fabricated. Compared with conventional OTFTs, the introduction of the buffer layer (1 nm) leads to field-effect mobility increases from 0.16 to 0.5 cm²/Vs, and threshold voltage downshifts from −19 to −8 V for the linear region. The on/off current ratio is improved to a level of 10⁵ for the off-state current decreasing. These improvements are attributed to (i) tunneling injection through the LiF layer and (ii) interface dipole energy barrier decreasing and contact resistance reduction between pentacene and Au. The results demonstrate that it is an effective method to improve the device characteristics by using a buffer layer.     

Organic thin film transistors with double insulator layers

We have investigated a double-layer structured gate dielectric for the organic thin films transistor (OTFT) with the purpose of improving the performance of the SiO₂ gate insulator. A 50 nm PMMA layer was coated on top of the SiO₂ gate insulator as organic insulator layer. The results demonstrated that using inorganic/organic compound insulator as the gate dielectric layers is an effective method to fabricate OTFTs with improved electric characteristics and decreased leakage current. Electrical parameters such as carrier mobility and on/off ratio by field effect measurement have been calculated. OTFT based on highly doped Si substrate with a field-effect mobility of 0.004 cm²/V s and on/off ratio of 10⁴ have been obtained.     

High field-effect mobility from poly(3-hexylthiophene) thin-film transistors by solvent–vapor-induced reflow

Room-temperature exposure of spin-coated poly(3-hexylthiophene) (P3HT) films to ortho-dichlorobenzene vapor increases the field-effect mobility of the P3HT organic thin-film transistors (OTFTs). The mobility increases moderately with unsaturated vapor exposure, owing to increased crystallinity of the P3HT films; on the other hand, the mobility increases abruptly with saturated vapor exposure, to 0.11 cm²/V s. The saturated vapor exposure causes the P3HT films to reflow, leaving in the active area approximately 2–3 P3HT monolayers whose molecular ordering is enhanced by the flow-generated shear against the gate dielectric. Although the reflowed OTFTs degrade in air much faster than do the non-reflowed OTFTs due to the susceptibility of the ultra-thin reflowed films, they become highly stable when encapsulated, obtaining a lifetime of more than 3000 h.     

AC characterization of organic thin-film transistors with asymmetric gate-to-source and gate-to-drain overlaps

This paper presents S-parameter characterization and a corresponding physics-based small-signal equivalent circuit for organic thin-film transistors (OTFTs). Furthermore, the impact of misalignment between the source/drain contacts and the patterned gate on the dynamic TFT performance is explored and a simple method to estimate the misalignment from the measured S-parameters is proposed. An excellent fit between theoretical and experimental S-parameters is demonstrated. For this study, OTFTs based on the air-stable organic semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) having a channel length of 1 μm and a gate-to-contact overlap of 5 or 20 μm and being operated at a supply voltage of 3 V are utilized. The intentional asymmetry between gate-to-source and gate-to-drain overlaps is precisely controlled by the use of high-resolution silicon stencil masks.     

Effect of electron-donating unit on crystallinity and charge transport in organic field-effect transistors with thienoisoindigo-based small molecules

We report the effect of an electron-donating unit on solid-state crystal orientation and charge transport in organic field-effect transistors (OFETs) with thienoisoindigo (TIIG)-based small molecules. End-capping of different electron-donor moieties [benzene (Bz), naphthalene (Np), and benzofuran (Bf)] onto TIIG (giving TIIG-Bz, TIIG-Np, and TIIG-Bf) is resulted in different electronic energy levels, solid-state morphologies and performance in OFETs. The 80 °C post-annealed TIIG-Np OFETs show the best device performance with a best hole mobility of 0.019 cm² V⁻¹ s⁻¹ and threshold voltage of −8.6 ± 0.9 V using top gate/bottom contact geometry and a CYTOP gate dielectric. We further investigated the morphological microstructure of the TIIG-based small molecules by using grazing incidence wide angle X-ray scattering, atomic force microscopy and a polarized optical microscope. The electronic transport levels of the TIIG-based small molecules in thin-film states were investigated using ultraviolet photoelectron spectroscopy to examine the charge injection properties of the gold electrode.     

Hybrid dual gate ferroelectric memory for multilevel information storage

Here, we report hybrid organic/inorganic ferroelectric memory with multilevel information storage using transparent p-type SnO semiconductor and ferroelectric P(VDF-TrFE) polymer. The dual gate devices include a top ferroelectric field-effect transistor (FeFET) and a bottom thin-film transistor (TFT). The devices are all fabricated at low temperatures (∼200 °C), and demonstrate excellent performance with high hole mobility of 2.7 cm² V⁻¹ s⁻¹, large memory window of ∼18 V, and a low sub-threshold swing ∼−4 V dec⁻¹. The channel conductance of the bottom-TFT and the top-FeFET can be controlled independently by the bottom and top gates, respectively. The results demonstrate multilevel nonvolatile information storage using ferroelectric memory devices with good retention characteristics.