Polymer nanocomposite dielectric based on P(VDF-TrFE)/PMMA/BaTiO3 for TIPs-pentacene OFETs

TIPs-pentacene OFETs were fabricated on a plastic substrate using polymer nanocomposite dielectric. The blend polymer P(VDF-TrFE)/PMMA (30 wt%) was used as polymer matrix and BaTiO₃ nanoparticles modified by 3-glycidoxypropyltrimethoxysilane (GPTMS) were dispersed as ceramic fillers. The effects of different loadings of BaTiO₃ on the surface morphology and electrical properties of dielectric films were investigated. The formulation of screen-printable dielectric ink of P(VDF-TrFE)/PMMA/BaTiO₃/Silica (SII) was achieved by adding fumed silica as the viscosity modifier. TIPs-pentacene OFETs using SII as the gate dielectric features a mobility of 0.01 cm²/V s, and having a threshold voltage of −6 V. This screen-printable dielectric ink is promising for low operating-voltage fully-printed OFETs.     

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.     

Inkjet-printed copper electrodes using photonic sintering and their application to organic thin-film transistors

We report on copper (Cu) electrodes fabricated with inkjet-printed nanoparticle inks that are photonic sintered on a polymer dielectric layer and their application to source and drain electrodes in organic thin-film transistor (TFT). By using photonic sintering with a radiant energy density of 9 J/cm², printed Cu nanoparticle layers on a glass substrate showed very low electrical resistivity levels of 7 μΩ cm. By optimizing the sintering conditions on polymer dielectric, the pentacene-based TFT using these printed Cu electrodes showed good mobility levels of 0.13 cm²/Vs and high on/off current ratios of about 10⁶. In addition, we revealed that the crystal grain growth of pentacene near the printed Cu electrodes was inhibited by the thermal damage of polymer underlayer due to the high radiant energy density of the intense light.     

1 Volt organic transistors with mixed self-assembled monolayer/Al2O3 gate dielectrics

Control of the threshold voltage and the subthreshold swing is critical for low voltage transistor operation. In this contribution, organic field-effect transistors (OFETs) operating at 1 V using ultra-thin (∼4 nm), self-assembled monolayer (SAM) modified aluminium oxide layers as the gate dielectric are demonstrated. A solution-processed donor–acceptor semiconducting polymer poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thieno[3,2-b]thiophene) (PDPP2TTT) is used as the active layer. It is shown that the threshold voltage of the fabricated transistors can be simply tuned by carefully controlling the composition of the applied SAM. The optimised OFETs display threshold voltages around 0 V, low subthreshold slopes (150 ± 5 mV/dec), operate with negligible hysteresis and show average saturated field-effect mobilities in excess of 0.1 cm²/V s at 1 V.     

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.     

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.     

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.     

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.     

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.     

Forming semiconductor/dielectric double layers by one-step spin-coating for enhancing the performance of organic field-effect transistors

We report one-step formation of the gate dielectric and conduction channel for enhancing the performance of organic field effect transistors (OFETs). The resulting OFET with the semiconductor/dielectric bi-layers spun in ambient conditions exhibits μ_FET up to 1.6 cm²/V s and on–off ratio higher than 10⁶, no additional treatment needed. Contact angle measurements and absorption spectra reveals that a well-defined semiconductor-top and dielectric-bottom film form after spin-coating the mixture of the two components, which is due to the surface induced self-organized phase separation. Compared to the single layer semiconductor film, the staggered film exhibits over 5 times higher mobility and nearly 90% reduced hysteresis in OFET. The higher performance is attributed to the simultaneous optimization in the dielectric interface and semiconductor crystallization. The approach is significant for the fabrication of low cost, easy processed and high performance OFETs.     

Ultra-thin polymer gate dielectrics for top-gate polymer field-effect transistors

We have demonstrated top-gate polymer field-effect transistors (FETs) with ultra-thin (30–50 nm), room-temperature crosslinkable polymer gate dielectrics based on blending an insulating base polymer such as poly(methyl methacrylate) with an organosilane crosslinking agent, 1,6-bis(trichlorosilyl)hexane. The top-gate polymer transistors with thin gate dielectrics were operated at gate voltages less than ?8 V with a relatively high dielectric breakdown strength (>3 MV/cm) and a low leakage current (10–100 nA/mm² at 2 MV/cm). The yield of thin gate dielectrics in top-gate polymer FETs is correlated with the roughness of underlying semiconducting polymer film. High mobilities of 0.1–0.2 cm²/V s and on and off state current ratios of 10⁴ were achieved with the high performance semiconducting polymer, poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene.     

Development of a new diindenopyrazine–benzotriazole copolymer for multifunctional application in organic field-effect transistors,polymer solar cells and light-emitting diodes

A new donor–acceptor (D?A) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^?1 s^?1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light with a maximum brightness of 385 cd m^?2 at the turn-on voltage of 6 V.     

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.     

Low-operating-voltage polymeric transistor with solution-processed low-k polymer/high-k metal-oxide bilayer insulators

We have successfully demonstrated a polymeric semiconductor-based transistor with low-k polymer/high-k metal-oxide (TiO₂) bilayer as gate dielectric. The TiO₂ layers are readily processable from solution and cured at low temperature, instead of traditionally sputtering or high temperature sintering process, thus may suitable for a low-cost organic field effect transistors (FETs) manufacture. The low-k polymer capped on TiO₂ layer could further smooth the TiO₂ dielectric surface and suppress the leakage current from grain boundary of TiO₂ films. The resulting unpatented P3HT-OFETs could operate with supply voltage less than 10 V and the mobility and threshold voltage were 0.0140 cm²/V s and 1.14 V, respectively. The on/off ratio was 1.0 × 10³.     

Solution-processed organic crystals for field-effect transistor arrays with smooth semiconductor/dielectric interface on paper substrates

Large surface roughness is a major obstacle for electronic systems fabricated on paper substrates. Here, a mixture solution of organic semiconductor and polymer dielectric was spin-coated on paper substrate with a patterned wettability. This spin-coating process produced organic crystals and a very smooth semiconductor/dielectric interface with a low trap density in well-confined patterns. Despite the large roughness of the paper substrate, the fabricated transistor arrays exhibited high performance with a field-effect mobility reaching 1.3 cm²/V s and an on/off ratio of 10⁸. The presented results offer a simple fabrication method for the current rapidly developing technology of paper electronics.     

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.     

Free radical fast photo-cured gate dielectric for top-gate polymer field effect transistors

We demonstrated top-gate organic field effect transistors (OFETs) made with free radical photo-cured polymer gate dielectrics and poly(3-hexylthiophene). We introduced a new approach of cross linking dielectric polymers in OFETs by using acrylate monomers cured with UV irradiation directly on the semiconductor. Three different blends were formulated: one self-initiating acrylate oligomer and two epoxy acrylate monomers mixed with 4-phenylbenzophenone as photo initiator and N-methyldiethanolamine as amine synergist. Thin films of these blends were cured in air within one minute. The curing process was monitored with FT-IR spectroscopy and the effect of a wetting agent was studied by measuring the CV characteristics of metal–insulator-semiconductor (MIS) structures made with these formulations. OFETs made with the demonstrated formulations showed high on/off ratios (10⁵–10⁶) and low sub-threshold slopes (0.44–1.42 V/dec).     

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.     

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.     

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.