The role of water in the device performance of n-type PTCDI-C8 organic field-effect transistors with solution-based gelatin dielectric

Gelatin is a natural protein, which works well as the gate dielectric for N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C₈) organic field-effect transistors (OFETs). An aqueous solution process was applied to form the gelatin gate dielectric on poly(ethylene terephthalate) (PET) by spin-coating and subsequent casting. The field-effect mobility in the saturation regime (μ_FE,sat) and the threshold voltage (V_T) values of a typical 40 nm PTCDI-C₈ OFET are (0.22 cm² V⁻¹ s⁻¹, 55 V) in vacuum and (0.74 cm² V⁻¹ s⁻¹, 2.6 V) in air ambient. The maximum voltage shift in hysteresis is also reduced from 10 V to 2 V when the operation environment for PTCDI-C₈ OFETs is changed from vacuum to air ambient. Nevertheless, a slight reduction of electron mobility was found when the device was stressed in the air ambient. The change in the device performance has been attributed to the charged ions generation owing to water absorption in gelatin in air ambient.     

Dielectric surface-polarity tuning and enhanced operation stability of solution-processed organic field-effect transistors

The electrical performance of triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs) was significantly affected by dielectric surface polarity controlled by grafting hexamethyldisilazane and dimethyl chlorosilane-terminated polystyrene (PS-Si(CH₃)₂Cl) to 300-nm-thick SiO₂ dielectrics. On the untreated and treated SiO₂ dielectrics, solvent–vapor annealed TES-ADT films contained millimeter-sized crystals with low grain boundaries (GBs). The operation and bias stability of OFETs containing similar crystalline structures of TES-ADT could be significantly increased with a decrease in dielectric surface polarity. Among dielectrics with similar capacitances (10.5–11 nF cm⁻²) and surface roughnesses (0.40–0.44 nm), the TES-ADT/PS-grafted dielectric interface contained the fewest trap sites and therefore the OFET produced using it had low-voltage operation and a charge-carrier mobility ∼1.32 cm² V⁻¹ s⁻¹, on–off current ratio >10⁶, threshold voltage ∼0 V, and long-term operation stability under negative bias stress.     

Organic field-effect transistors based on low-temperature processable transparent polymer dielectrics with low leakage current

A solution-based transparent polymer was investigated as the gate dielectric for organic field-effect transistors (OFETs). Organic thin films (400 nm) are readily fabricated by spin-coating a polyhydrazide solution under ambient conditions on the ITO substrates, followed by annealing at a low temperature (120 °C). The smooth transparent dielectrics exhibited excellent insulating properties with very low leakage current densities of ～10^?8 A/cm². High performance OFETs with evaporated pentacene as organic semiconductor function at a low operate voltage (?15 V). The mobility could reach as high as 0.7 cm²/Vs and on/off current ratio up to 10⁴. Solution-processed TIPS-pentacene OFETs also work well with this polymer dielectric.     

Complementary-like inverters based on an ambipolar solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative

We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al₂O₃ bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1.2 and 0.01 cm² V^?1 s^?1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90 V/V (up to 122 V/V) at a supply voltage of 25 V.     

High-performance diketopyrrolopyrrole-based organic field-effect transistors for flexible gas sensors

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm²/V s on a glass substrate and 2.98 ± 0.19 cm²/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μ_FET, V_Th and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH₃ gas sensors. As the concentration of NH₃ increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼10⁷ to ∼10⁹ Ω at V_SD = −5 V and V_GS = −5 V.     

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.     

Natural polyelectrolyte: Major ampullate spider silk for electrolyte organic field-effect transistors

The major ampullate (MA) silk collected from giant wood spiders Nephila pilipes consists of 12% glutamic acid (Glu) and 4% tyrosine (Tyr) acidic amino residues. The MA silk may act as a natural polyelectrolyte for organic field-effect transistors (OFETs). Pentacene and F₁₆CuPc OFETs were fabricated with the MA silk thin film as the gate dielectric. The MA silk thin film with surface roughness of 4 nm and surface energy of 36.1 mJ/m² was formed on glass using a hexafluoroisopropanol (HFIP) organic process. The MA silk gate dielectric in pentacene OFETs may improve the field-effect mobility (μ_FE,sat) value in the saturation regime from 0.11 in vacuum to 4.3 cm² V⁻¹ s⁻¹ in air ambient at ca. 70% RH. The corresponding threshold voltage (V_TH) value reduced from −6 V in vacuum to −0.5 V in air ambient. Similar to other polyelectrolytes, the changes of μ_FE,sat and V_TH may be explained by the generation of electric double layers (EDLs) in the MA silk thin film in air ambient due to water absorption.     

High-performance organic field-effect transistors with binary ionic liquids

High-performance rubrene single-crystal field-effect transistors are developed with binary ionic liquid electrolytes used for gating. Inclusion of small amount of inorganic salts in the ionic liquids enhances the degree of dissociation for the organic ions and accelerates formation of the electric-double-layers in response to the gate voltage. High carrier mobility of 2.9 cm²/Vs is achieved in the rubrene single-crystal transistors with the mixture ionic liquid. In addition to the advantage of the low-voltage operation due to concentrated field in ultra-thin electric-double-layers, drastically increased capacitance at above 100 Hz makes the technique of the ionic liquid gating more attractive for fast-switching devices.     

Organic field-effect transistor and its photoresponse using a benzo[1,2-b:4,5-b′]difuran-based donor–acceptor conjugated polymer

Organic field-effect transistors (OFETs) were fabricated through a solution process with a donor–acceptor (D–A) conjugated polymer poly{4,8-bis(2′-ethylhexylthiophene)benzo [1,2-b;3,4-b′]difuran-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)} (PBDFTDTBT) as the active layer, which is a highly efficient D–A conjugated polymer as a donor in polymer solar cells with a power conversion efficiency (PCE) over 6.0%. The OFET devices showed a hole mobility of 0.05 cm²/Vs and an on/off ratio of 4.6 × 10⁵. Those are one of the best performance parameters for OFETs based on D–A conjugated polymers including benzo[1,2-b:4,5-b′]dithiophene (BDT) or benzo[1,2-b:4,5-b′]difuran (BDF) unit. The photoresponse of OFETs was investigated by modulating light with various intensities. The devices produced a photosensitivity (I_light/I_dark) of 1.2 × 10⁵ and a photoresponsivity of 360 mA W⁻¹ under white light illumination. The drain current in saturation region increases gradually with increasing illumination intensity. The threshold voltage exhibited a positive shift from −15.6 V in darkness to 27.8 V under illumination, which can be attributed to the well-known photovoltaic effect resulting from the transport of photogenerated holes and trapping of photogenerated electrons near the source electrode in organic phototransistors. Meanwhile, the devices showed good stability and with no obvious degeneration for 3 months in air. The study suggests that D–A conjugated polymers including BDF unit can be potentially applied in OFETs and organic phototransistors in addition to highly efficient polymer solar cells.     

Very facile fabrication of aligned organic nanowires based high-performance top-gate transistors on flexible,transparent substrate

Aligned single-crystalline organic nanowires (NWs) show promising applications in flexible and stretchable electronics, while the use of pre-existing aligned techniques and well-developed photolithography techniques are impeded by the large incompatibility with organic materials and flexible substrates. In this work, aligned copper phthalocyanine (CuPc) organic NWs were grown on flexible and transparent poly(dimethylsiloxane) (PDMS) substrate via a grating-assisted growth approach. Furthermore, a simple yet efficient etching-assisted transfer printing (ETP) method was used to achieve CuPc NW array-based flexible top-gate organic field-effect transistors (OFETs) with a high mobility up to 2.0 cm² V⁻¹ s⁻¹, a small operating voltage within ±10 V, a high on/off ratio >10⁴, and excellent bend stability with bending radius down to 3 mm. It is expected that the high-performance organic NW array-based top-gate OFETs with exceeding bend stability will have important applications in future flexible electronics.     

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).     

Megahertz operation of flexible low-voltage organic thin-film transistors

Bottom-gate, top-contact (inverted staggered) organic thin-film transistors with a channel length of 1 μm have been fabricated on flexible plastic substrates using the vacuum-deposited small-molecule semiconductor 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT). The transistors have an effective field-effect mobility of 1.2 cm²/V s, an on/off ratio of 10⁷, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 ns per stage at a supply voltage of 3 V. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V.     

Transparent organic thin-film transistors based on high quality polycrystalline rubrene film as active layers

Transparent organic thin-film transistors (OTFTs) with high performance are demonstrated by using high quality polycrystalline 5,6,11,12-Tetraphenylnaphthacene (rubrene) as an active layer, which is prepared by weak epitaxy growth (WEG) method. Benefiting from epitaxial relationship is formed between the inducing layer and the rubrene films, highly oriented and continuous organic polycrystalline thin films with large grains were obtained, which enhances the carrier transport in the film plane. The mobility of devices reaches 1.3 cm²/Vs, the threshold voltage is lower than ?0.9 V and the on–off current ratio (I_on/I_off) is higher than 10⁶ after the photolithography process. Moreover, the array consisting of the transparent thin-film transistors displays a high optical transparency more than 65% in visible light regions. The high-performance transparent OTFTs promote the practical applications for large-area and flexible active-matrix organic light-emitting diodes (AMOLEDs) display.     

All solution-processed organic single-crystal transistors with high mobility and low-voltage operation

High-mobility organic single-crystal field-effect transistors of 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]-dithiophene (C₁₀-DNBDT) operating at low driving voltage are fabricated by an all-solution process. A field-effect mobility as high as 6.9 cm²/V s is achieved at a driving voltage below 5 V, a voltage as low as in battery-operated devices, for example. A low density of trap states is realized at the surface of the solution-processed organic single-crystal films, so that the typical subthreshold swing is less than 0.4 V/decade even on a reasonably thick amorphous polymer gate dielectrics with reliable insulation. The high carrier mobility and low interface trap density at the surface of the C₁₀-DNBDT crystals are both responsible for the development of the high-performance all-solution processed transistors.     

Organic light-emitting diodes based on an ambipolar single crystal

Slice-like organic single crystals of 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB) are grown by the physical vapor transport (PVT) method, and exhibit a very high photoluminescence quantum efficiency (Φ_PL) of 75%. The ambipolar behavior of BCPEB single crystals are confirmed using the time of flight technique. The high efficiency and balanced (μ_h = 0.059 cm²/Vs and μ_e = 0.070 cm²/Vs) carriers’ mobility imply that the BCPEB single crystal is a promising light-emitting layer in the diodes structure. Intense green electroluminescence (EL) from a diode has been successfully demonstrated at an applied electric field of 2 × 10⁵ V/cm.     

Fabrication of high-mobility organic single-crystal field-effect transistors with amorphous fluoropolymer gate insulators

High-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable electrodes. Before constructing interfaces to crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize carrier mobility. Mobility values estimated in the saturation region reproducibly exceeded 15 cm²/V s for all the 10 devices, reaching 30 cm²/V s for the best two devices. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable for the excellent transistor performance.     

High-performance solution-processed organic transistors with electroless-plated electrodes

Electroless-plated gold and platinum films are used as source and drain electrodes in high-performance solution-processed organic field-effect transistors (OFETs), representing a promising large-area, near-room-temperature and vacuum-free technique to form low-resistance metal-to-semiconductor interfaces in ambient atmosphere. Developing non-displacement conditions using a Pt-colloidal catalyst for soft electroless plating, the electrodes are deposited on crystallized thin films of 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) without significant damage to the semiconductor material. The top-contact OFETs show remarkable performance, with a mobility of 6.0 cm² V^?1 s^?1. The method represents a practical fabrication technique to mass-produce circuitry arrays of nearly best-performing OFETs for the printed electronics industry.     

Improving solution-processed n-type organic field-effect transistors by transfer-printed metal/semiconductor and semiconductor/semiconductor heterojunctions

Solution-processed n-type organic field effect transistors (OFETs) are in need of proper metal contact for improving injection and mobility, as well as balanced hole mobility for building logic circuit units. We address the two distinct problems by a simple technique of transfer-printing. Transfer-printed Au contacts on a terrylene-based semiconductor (TDI) significantly reduced the inverse subthreshold slope by 5.6 V/dec and enhanced the linear mobility by over 5 times compared to evaporated Au contacts. Hence, devices with a high-work-function metal (Au) are comparable with those with low-work-function metals (Al and Ca), indicating a fundamental advantage of transfer-printed electrodes in electron injection. We also transfer-printed a poly(3-hexylthiophene) (P3HT) layer onto TDI to construct a double-channel ambipolar transistor by a solution process for the first time. The transistor exhibits balanced hole and electron mobility (3.0 × 10⁻³ and 2.8 × 10⁻³ cm² V⁻¹ s⁻¹) even in a coplanar structure with symmetric Au electrodes. The technique is especially useful for reaching intrinsic mobility of new materials, and enables significant enlargement of the material tanks for solution-processed functional heterojunction OFETs.     

High-mobility tetrathiafulvalene organic field-effect transistors from solution processing

We report on mobilities up to 3.6 cm²/V s in organic field-effect transistors (OFETs) with solution-processed dithiophene- and dibenzo-tetrathiafulvalene (DT- and DB-TTF) single crystals as active materials. In the devices, the channel length varies from 100 μm down to sub 100 nm, and the SiO₂ thickness is either 100 nm, 50 nm, or 20 nm. The devices exhibit excellent operation characteristics with an on/off-ratio exceeding 10⁶. Temperature dependent measurements between 50 and 400 K reveal a thermally activated transport with increased activation above 200 K. The mobility exhibits exponential activation with two distinct exponents.     

Low-voltage,high-performance n-channel organic thin-film transistors based on tantalum pentoxide insulator modified by polar polymers

Polar polymers (polyfluorene copolymers, PFN–PBT) with different polarities are utilized to modify the surface of tantalum pentoxide (Ta₂O₅) insulator in n-channel organic thin-film transistors (OTFTs). A high mobility of 0.55 cm²/Vs, high on/off current ratio of 1.7 × 10⁵, and low threshold voltage of 2.8 V are attained for the OTFT with the modification polymers, the performances of which are much better than those of OTFT with only Ta₂O₅ insulator. The performances of the OTFT with only Ta₂O₅ insulator are only 0.006 cm²/Vs in mobility, 5 × 10³ in on/off ratio, and 12.5 V in threshold voltage. Furthermore, it is found that the threshold voltage of the OTFTs with PFN–PBT modification layer is easily tuned by polarities of the polymers. Further studies show that self-assembly dipole moments in the polymers play an important role in the improvement of the OTFT performances.