Addressable growth of oriented organic semiconductor ultra-thin films on hydrophobic surface by direct dip-coating

Oriented organic field-effect transistor (OFET) stripe arrays on hydrophobic substrates were fabricated by fast dip-coating technique. The addressable growth was achieved by decreasing surface energy of the channel areas with respect to the electrodes via hydrophobic treatment. The higher surface energy of the electrodes allows solution to adhere and then organic semiconductors nucleate and bridge the channels after evaporation of the solvent. Area-selective behaviour can be controlled by adjusting surface property of transistor channel, geometry features of the gold electrodes, pulling speed and evaporation atmosphere. The mechanism behind is the competition between receding of the solution and evaporating of the solvent that generate the organic semiconductor films on the substrate. The patterned bottom-contact transistor arrays exhibit carrier mobility of 2.0 × 10⁻³ cm² V⁻¹ s⁻¹, while no field-effect characteristics can be detected for bottom-contact arrays without hydrophobic treatment. Such reliable, fast and solution-based patterned OFET arrays are highly desirable for large-scale and low-cost production.     

Effects of semiconductor/dielectric interfacial properties on the electrical performance of top-gate organic transistors

We investigated the effects of varying the properties of the interface between a semiconductor P3HT layer and a dielectric Cytop™ layer on the performances of the resulting transistor devices by comparing the mobilities of devices prepared with bottom gate/bottom contact or top gate/bottom contact architectures. The reduced channel roughness that arose from the thermal annealing step dramatically enhanced the field-effect mobility, yielding the highest mobility yet obtained for a top-gate transistor: 0.12 cm²/V s. High-performance OFETs may be fabricated by controlling the channel roughness and the properties of the interface between the semiconductor and the gate dielectric.     

Improved ambipolar charge injection in organic field-effect transistors with low cost metal electrode using polymer sorted semiconducting carbon nanotubes

Solution-processed thin film transistors can be implemented using simple and low cost fabrication, and are the best candidates for commercialization due to their application to a range of wearable electronics. We report an ambipolar charge injection interlayer that can improve both hole and electron injection in organic field-effect transistors (OFETs) with inexpensive source-drain electrodes. The solution processed ambipolar injection layer is fabricated by selective dispersion of semiconducting single walled carbon nanotubes using poly(9,9-dioctylfluorene). OFETs with molybdenum (Mo) contacts and interlayer (Mo/interlayer OFETs) exhibit superior performance, including higher hole and electron mobilities, device yield, lower threshold voltages, and lower trap densities than those of bare transistors. While OFETs with Mo contacts show unipolar p-type behaviour, Mo/interlayer OFETs display ambipolar transport due to significant enhancement of electron injection. In the p-type region, transistor performance is comparable to devices with gold (Au). Hole mobility is increased approximately ten-fold over devices with only Mo contacts. The electron mobility of Mo/interlayer OFETs is 0.05 cm²V⁻¹s⁻¹, which is higher than devices with Au electrodes. The p-type contact resistances of Mo/interlayer OFETs are half those of OFETs with Mo contacts. Trap density in Mo/interlayer OFETs is one order magnitude lower than that of pristine devices. We also demonstrate that this approach is extendible to other metals (nickel) and n-type semiconductors with different energy levels. Injection by tunnelling is suggested as the mechanism of ambipolar injection.     

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.     

High-speed organic single-crystal transistors gated with short-channel air gaps: Efficient hole and electron injection in organic semiconductor crystals

Short-channel, high-mobility organic filed-effect transistors (OFETs) are developed based on single crystals gated with short-channel air gaps. The high hole mobility of 10 cm²/Vs for rubrene, and high electron mobility of 4 cm²/Vs for PDIF-CN₂ crystals are demonstrated even with a short channel length of 6 μm. Such performance is due to low contact resistance in these devices estimated to be as low as ～0.5 kΩ cm at gate voltage of ?4 V for rubrene. With the benefit of the short channel length of 4.5 μm in a new device architecture with less parasitic capacitance, the cutoff frequency of the rubrene air–gap device was estimated to be as high as 25 MHz for drain voltage of ?15 V, which is the fastest reported for p-type OFETs, operating in ambient conditions.     

Self-assembled monolayers mediated charge injection for high performance bottom-contact poly(3,3′′′-didodecylquaterthiophene) thin-film transistors

Device performance of bottom-contact poly(3,3′′′-didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs) was significantly improved via surface-modification of Au source–drain (S–D) electrodes with 1-decanethiol and 1H,1H,2H,2H-perfluorodecanethiol self-assembled monolayers (SAMs). By improving the PQT-12 morphology and modulating the Schottky barrier at electrode/PQT-12 contacts, the thiol SAMs chemisorbed onto Au surfaces can improve the charge carrier injection at electrode/PQT-12 contacts and result in dramatic enhancements in device mobilities. Device mobilities up to 0.09 and 0.19 cm² V⁻¹ s⁻¹ were obtained in high performance bottom-contact PQT-12 TFTs with 1-decanethiol and 1H,1H,2H,2H-perfluorodecanethiol SAMs surface-modified Au S–D electrodes, compared with 0.015 cm² V⁻¹ s⁻¹ in PQT-12 TFTs with bare Au electrodes. This work may provide a simple path to the fabrication of high performance, low-cost, and solution-processable bottom-contact OTFTs using fine lithography technology.     

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.     

Phosphorene field-effect transistors using high-k gate dielectrics of epitaxial SrTiO3 layers grown on Nb-doped SrTiO3 substrates

Epitaxial SrTiO₃ (STO) thin film as a gate dielectric layer was grown on single crystalline (100) Nb-doped SrTiO₃ substrate. On the 100-nm-thick STO gate dielectric layer, a 5-nm-thick phosphorene sheet channel layer was exfoliated from a bulk crystal. A phosphorene field-effect transistor (P-STO-FET) was prepared by the formation of 90-nm-thick Au source/drain (S/D) contacts. The P-STO-FET exhibited the transport characteristics of a p-type transistor with a mobility of approximately 376 cm⁻²/Vs and an on/off ratio of approximately 10³. Furthermore, it was experimentally confirmed that the mobility of the P-STO-FET was significantly influenced by the flatness of the phosphorene sheet.     

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.     

The transition from bipolaron to triplet-polaron magnetoresistance in a single layer organic semiconductor device

We measure the current–voltage–luminescence (I–V–L) and Magneto-Conductance (MC) response of a poly(3-hexyl-thiophene) (P3HT) based device (Au/P3HT(300 nm)/Al) in forward and reverse bias. In reverse bias (<1 V), the negative MC is described by a single non-Lorentzian function, consistent with the bipolaron theory. In forward bias, the transition from negative saturation MC (low bias) to positive saturation MC (high bias) occurs when the current density exceeds ∼10⁻² A cm⁻² and coincides with electroluminescence. Under these conditions the triplet density (∼10¹⁵ cm⁻³) becomes comparable to the hole density (∼10¹⁶ cm⁻³), consistent with the triplet-polaron interaction theory. From the current density dependence of the MC we conclude that in forward bias both mechanisms must be occurring simultaneously, within a given device, and that the overall sign of the MC results from competition between the two mechanisms.     

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.     

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.     

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.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

Crystal structure and charge-transport properties of N-trimethyltriindole: Novel p-type organic semiconductor single crystals

We report on a new p-type organic semiconductor single crystal, 5,10,15-trimethyl-10,15-dihydro-5H-diindolo[3,2-a:3’,2’-c]carbazole (N-trimethyltriindole). This molecule crystallizes forming a highly ordered columnar structure in which stacked molecules are situated at two alternating distances (3.53 Å and 3.68 Å) along the column as determined by single crystal X-ray diffraction analysis. These short intermolecular distances between adjacent units, make this system an ideal candidate for charge-transport processes along the stacks.Relevant parameters for transport (i.e. internal reorganization energies, transfer integral) have been estimated by DFT calculations at a 6-311G(d,p)/B3LYP level of theory. As a double check for the transfer integral, the electronic band structure of a one-dimensional stack of molecules has been computed. The electronic properties of this material have been studied both theoretically and experimentally. Its HOMO value is found to coincide with Au work function (Φ_Au = 5.1 eV), thus low barriers are expected for hole injection from gold electrodes. The hole mobility of this material has been predicted theoretically considering a hopping-type mechanism for the charge-transport and determined experimentally at the space charge limited current (SCLC) regime of the current–voltage measurements. Both theoretical and experimental values are in good agreement. The high hole mobility (μ_min = 0.4 cm² V^?1 s^?1) of this material points towards its useful application in the organic electronics arena. N-Trimethyltriindole single crystals constitute an essential model to study transport properties of triindole-based materials and to design new derivatives with improved electronic performance.     

Solution-processable ambipolar organic field-effect transistor based on Co-planar bisphthalocyaninato copper

A soluble binuclear phthalocyaninato copper (II) complex, Cu₂[Pc(COOC₈H₁₇)₆]₂ (1), with planar molecular structure and extended conjugation system, has been designed and synthesized. By fusing two phthalocyanine rings side by side and introducing electron withdrawing groups at periphery positions, the energy levels of HOMO and LUMO have been tuned successfully into the range of an air-stable ambipolar organic semiconductor required as revealed by the electrochemical studies. With the help of a solution-based quasi-Langmuir–Shäfer (QLS) method, thin solid films of this compound were fabricated and organic field effect transistors (OFETs) based on these QLS thin solid films were constructed. Because of the promising electrochemical properties as well as the high ordered packing structure of the molecules in the thin solid films, the OFETs performed excellent ambipolar operating properties, with the electron and hole mobility in air as high as 1.7 × 10⁻¹ and 2.3 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. For comparison purpose, mononuclear compound Cu[Pc(COOC₈H₁₇)₈] (2) was comparatively studied. The QLS thin solid films of this compound possess similar ordered structure with that of Cu₂[Pc(COOC₈H₁₇)₆]₂ (1), but the OFETs based on the thin solid films of this compound can only show n-type properties under nitrogen atmosphere with an extremely small electron mobility of 1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹. This result suggests that extension on the conjugation system of an aromatic compound with multiple electron withdrawing groups can tune the molecule into an air stable ambipolar organic semiconductor.     

Effective mobility in amorphous organic transistors: Influence of the width of the density of states

The temperature dependence of poly(3-hexylthiophene-2,5-diyl) (P3HT)/polystyrene (PS) blend organic transistor current/voltage (I–V) characteristics has been experimentally and theoretically studied. The planar transistors, realized by drop casting the P3HT/PS ink, exhibit high mobilities (over 5 × 10⁻³ cm² V⁻¹ s⁻¹) and good overall characteristics. A transistor model accounting for transport mechanisms in disordered organic materials was used to fit the measured characteristics. Using a single set of parameters, the measured effective mobility versus gate bias, either increasing or decreasing with the gate bias depending on temperature, is well reproduced over a wide temperature range (130–343 K). A Gaussian density of states width of 0.045 eV was determined for this P3HT/PS blend. The transistor I–V characteristics are very well described considering disordered material properties within a self-consistent transistor model.     

Growth and characterization of indium oxide diodes prepared by reactive magnetron sputtering

The electrical properties of Cr/Pt/Au and Ni/Au ohmic contacts with unintentionally doped In₂O₃ (U-In₂O₃) film and zinc-doped In₂O₃ (In₂O₃:Zn) prepared by reactive magnetron sputtering deposition are described. The lowest specific contact resistance of Cr/Pt/Au and Ni/Au is 2.94 × 10⁻⁶ and 1.49 × 10⁻² Ω-cm², respectively, as determined by the transmission line model (TLM) after heat treatment at 300 °C by thermal annealing for 10 min in nitrogen ambient. The indium oxide diodes have an ideality factor of 1.1 and a soft breakdown voltage of 5 V. The reverse leakage current prior to breakdown is around 10⁻⁵ A.     

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.     

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.