Organic field-effect transistors based on J-aggregate thin films of a bisazomethine dye

We report on the fabrication and the characterization of p-type organic field-effect transistors based on vapor-deposited J-aggregate bisazomethine dye thin films. The absorption spectra of this non-ionic organic semiconductor in the solid state show a strong influence of the film thickness on the J-aggregate formation. However, the electrical characteristics of the devices demonstrate that the hole transport properties do not vary significantly in films thicker than 100 nm. This is due to the fact that the J-aggregates are formed in this material at the surface of the crystalline grains and do not influence the semiconductor/gate dielectric interface and the charge transport properties of the devices. Hole field-effect mobilities as high as 2.4 × 10^?4 cm² V^?1 s^?1 were obtained and could be slightly improved by a solvent vapor treatment due to changes in the film crystallinity. Overall, this study demonstrates that J-aggregate bisazomethine dye thin films are good candidates for the realization of organic electronic devices.     

Vacuum-processed polyethylene as a dielectric for low operating voltage organic field effect transistors

We report on the fabrication and performance of vacuum-processed organic field effect transistors utilizing evaporated low-density polyethylene (LD-PE) as a dielectric layer. With C₆₀ as the organic semiconductor, we demonstrate low operating voltage transistors with field effect mobilities in excess of 4 cm²/Vs. Devices with pentacene showed a mobility of 0.16 cm²/Vs. Devices using tyrian Purple as semiconductor show low-voltage ambipolar operation with equal electron and hole mobilities of ～0.3 cm²/Vs. These devices demonstrate low hysteresis and operational stability over at least several months. Grazing-angle infrared spectroscopy of evaporated thin films shows that the structure of the polyethylene is similar to solution-cast films. We report also on the morphological and dielectric properties of these films. Our experiments demonstrate that polyethylene is a stable dielectric supporting both hole and electron channels.     

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.     

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.     

Flexible,plastic transistor-based chemical sensors

Flexible, plastic chemical sensors were fabricated using a thin polymer gate dielectric layer and polymer electrodes patterned via selective wetting directly on the surface of the organic semiconductor film. Low-voltage transistors based on DDFTTF with PEDOT:PSS electrodes had a mobility as high as 0.05 cm²/Vs with an on–off ratio of 1.2 × 10⁴ on ITO/PET substrates. These devices demonstrated stable operation in water with sensor characteristics similar to those reported on rigid silicon substrates, with sub-ppm detection for cysteine and 2,4,6-trinitrobenzene (TNB).     

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.     

MISFET structures with barium titanate as a dielectric layer for application in memory cells

This work shows investigations of La₂O₃ containing BaTiO₃ thin films deposited on Si substrates by Radio Frequency Plasma Sputtering (RF PS) of sintered BaTiO₃ + La₂O₃ (2 wt.%) target. Round, aluminum (Al) electrodes were evaporated on top of the deposited layers. Thus, metal–insulator–semiconductor (MIS) structures were created with barium titanate thin films playing the role of an insulator. The MIS structures enabled a subsequent electrical characterization of the studied film by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements. Several electronic parameters, i.e., ε_ri, ρ, V_FB, ΔV_H were extracted from the obtained characteristics. Moreover, the paper describes technology process of MISFETs fabrication and possibility of their application as memory cells. The influence of voltage stress on transfer and output I–V characteristics of the transistors are presented and discussed.     

Characteristics of tetracene-based field-effect transistors on pretreated surfaces

Tetracene-based organic thin-film transistors (OTFTs) were prepared using a neutral cluster beam deposition (NCBD) method. The effect of surface modification with an amphiphilic surfactant, octadecyltrichlorosilane (OTS), on the formation of thin films and the geometric influence of channel length and width on the transistor characteristics were systematically examined. The estimated trap density and temperature-dependence of the field-effect mobility in the range of 10–300 K demonstrated that surfactant pretreatment decreased the total trap density and activation energy for hole-transport by reducing structural disorder in the active layer. In particular, the room-temperature hole mobilities of 0.162 and 0.252 cm²/Vs for untreated and OTS-pretreated devices were among the best to date for polycrystalline tetracene-based transistors using SiO₂ gate dielectric layers without any thermal post-treatment.     

Enhancement in electrical performance of indium gallium zinc oxide-based thin film transistors by low temperature thermal annealing

We have studied the characteristics of transparent bottom-gate thin film transistors (TFTs) using In–Ga–Zn–O (IGZO) as an active channel material. IGZO films were deposited on SiO₂/Si substrates by DC sputtering techniques. Thereafter, the bottom-gate TFT devices were fabricated by depositing Ti/Au metal pads on IGZO films, where the channel length and width were defined to be 200 and 1000 μm, respectively. Post-metallization thermal annealing of the devices was carried out at 260, 280 and 300 °C in nitrogen ambient for 1 h. The devices annealed at 280 °C have shown better characteristics with enhanced field-effect mobility and high on–off current ratio. The compositional variation of IGZO films was also observed with different annealing temperatures.     

Low power flexible organic thin film transistors with amorphous Ba0.7Sr0.3TiO3 gate dielectric grown by pulsed laser deposition at low temperature

We deposited amorphous Ba_0.7Sr_0.3TiO₃ (BST) on silicon and plastic substrate under 110 °C by pulsed laser deposition (PLD) and use it as the dielectric of the organic transistor. Depends on the thickness of BST layer, the highest mobility of the devices can achieve 1.24 cm² V^?1 s^?1 and 1.01 cm² V^?1 s^?1 on the silicon and polyethylene naphthalate (PEN) substrate, respectively. We also studied the upward and downward bending tests on the transistors and the dielectric thin films. We found that the BST dielectric pentacene transistor can maintain the mobility at 0.5 cm² V^?1 s^?1 or higher while the bending radius is around 3 mm in both upward and downward bending. Our finding demonstrates the potential application of PLD growth high-k dielectric in the large area organic electronics devices.     

Memory effects in hybrid silicon-metallic nanoparticle-organic thin film structures

We report on the electrical behaviour of metal–insulator–semiconductor (MIS) structures fabricated on silicon substrates and using organic thin films as the dielectric layers. These insulating thin films were produced by different methods, including spin-coating (polymethylmethacrylate), thermal evaporation (pentacene) and Langmuir–Blodgett deposition (cadmium arachidate). Gold nanoparticles, deposited at room temperature by chemical self-assembly, were used as charge storage elements. In all cases, the MIS devices containing the nanoparticles exhibited hysteresis in their capacitance versus voltage characteristics, with a memory window depending on the range of the voltage sweep. This hysteresis was attributed to the charging and discharging of the nanoparticles from the gate electrode. A maximum memory window of 2.5 V was achieved by scanning the applied voltage of an Al/pentacene/Au nanoparticle/SiO₂/p-Si structure between 9 and −9 V.     

Spray deposition of Polyethylenimine thin films for the fabrication of fully-sprayed organic photodiodes

In the last years, a big effort has been put into the investigation of the scalability of deposition processes for organic optoelectronics to achieve large-scale and cost-effective fabrication of functional devices. One of the most prominent techniques that promises to obtain an easy-to-scale production is spray-deposition; however, so far, the feasibility of entirely spray-deposited optoelectronic devices has not yet been demonstrated. To fulfill this goal, in this work we investigate the spray-coating of Polyethylenimine (PEI) and the effect of the process parameters on the film characteristics, in terms of thickness, work-function and roughness. The achievement of thin layers of PEI (∼10 nm) with full coverage is the last step towards the realization of lithography-free and vacuum-free organic electronic devices. For the first time, we show the fabrication of fully-sprayed organic photodiodes (OPDs), initially on patterned Indium-Tin Oxide, and subsequently on bare glass. The resulting photodiodes yield peak EQE above 65% and dark currents lower than 10–4 mA/cm² at a reverse bias of −4 V. Moreover, both the cathode and anode electrode of the OPDs fabricated with the described process-flow are semi-transparent, granting the simultaneous collection of two different light signals from the top and the bottom side.     

Thermal stability of organic transistors with short channel length on ultrathin foils

We demonstrate an effective design for fabrication of short channel organic transistors (<3 μm channel length) on ultrathin 1 μm thick substrates that exhibit excellent thermal stability. For short channel transistors, we demonstrate durability up to 170 °C, with a theoretical cutoff frequency above 100 kHz, and stable performance in cyclic heating tests up to 120 °C. We fabricate inverter circuits to investigate their behavior upon heating and show that inverter gain can be improved by 150%. Device performance and topology changes were systematically analyzed after annealing steps to gain better understanding on the mechanism behind the performance change. This report on the thermal stability of short channel transistors on ultrathin films shows good durability at elevated temperatures and paves the way for high frequency imperceptible electronics.     

Flexible organic transistors on standard printing paper and memory properties induced by floated gate electrode

Integrating electronic devices with unconventional substrate has been a popular research direction. Among these substrates, cellulose fiber paper has advantages in low-cost, recyclable and bio-degradable. We demonstrated directing printing of all contact electrodes on standard untreated Fuji Xerox printer paper without using planarization layer. The screen-printed gate electrodes based on silver nanoparticles can smooth out the paper substrate surface by two orders of magnitude and allow us to use parylene and DNTT as the dielectric and active layer directly. The transistors show average mobility of 0.297 cm² V⁻¹ s⁻¹ and on/off ratio larger than 10⁵. The low leakage current allows us to demonstrate memory properties by employing the floated gate method. The devices show excellent memory retention time for more than 10,000 s. The unique flexibility and combustibility of the organic transistors on paper substrate manifest their applications as next generation of green electronics.     

Improving mobility and electrochemical stability of a water-gated polymer field-effect transistor

Water-gated organic transistors have attracted considerable attention in the field of biosensors, thanks to their capability of operating in the aqueous environment typical of biological systems at very low voltages (∼1 V). Some examples have been recently reported in the literature, employing different organic materials as the active semiconducting layer, ranging from small molecules to single crystals. Here we report on water-gated polymer-based organic-field effect devices using poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) as the active layer. Very promising electronic performances, in terms of mobility and operating voltages are obtained; notably, the charge carrier mobility is in the order of 0.08 cm²/V s, which is of the same order of magnitude of values reported for single-crystal based water-gated devices, and consistent with values reported for solid-state polymer dielectric transistors. Moreover, the pBTTT-based device shows improved electrochemical stability, as compared to previously reported polymer based water-gated devices. Importantly, good functioning of the device is demonstrated also when water is replaced by physiological-like solutions. Critical to the transistors operation, besides the good transport properties of the active material, is the key-role played by alkyl side chains and ordered morphology of the polymer at the interface with the liquid environment, which we highlight here for the first time. Our contribution overall provides a useful step towards the development of bio-organic sensors, with enhanced properties in terms of sensitivity and stability, and for a successful exploitation of organic based field effect transistors in biotic/abiotic interfaces.     

n-Type self-assembled monolayer field-effect transistors for flexible organic electronics

Within this work we present n-type self-assembled monolayer field-effect transistors (SAMFETs) based on a novel perylene bisimide. The molecule spontaneously forms a covalently fixed monolayer on top of an aluminium oxide dielectric via a phosphonic acid anchor group. Detailed studies revealed an amorphous, two-dimensional semiconducting sheet on top of the dielectric. Reliable transistors with electron mobilities on the order of 10^?3 cm²/V s with limited hysteresis were achieved on rigid as well on flexible substrates. Furthermore, a flexible NMOS-bias inverter based on SAMFETs is demonstrated for the first time.     

Nanoparticle-based,spray-coated silver top contacts for efficient polymer solar cells

We demonstrate a solution-processed top electrode for large area organic electronic devices. A Ag nanoparticle solution is spray-coated directly on top of an inverted bulk-heterojunction organic solar cell through a shadow mask. After sintering the Ag nanoparticle film at 150 °C, a temperature which is compatible with processes on flexible substrates, cells show performances comparable to those of reference devices with evaporated top-contacts.     

Self-aligned flexible organic thin-film transistors with gates patterned by nano-imprint lithography

Many applications that rely on organic electronic circuits still suffer from the limited switching speed of their basic elements – the organic thin film transistor (OTFT). For a given set of materials the OTFT speed scales inversely with the square of the channel length, the parasitic gate overlap capacitance, and the contact resistance. For maximising speed we pattern transistor channels with lengths from 10 μm down to the sub-micrometre regime by industrially scalable UV-nanoimprint lithography. The reduction of the overlap capacitance is achieved by minimising the source–drain to gate overlap lengths to values as low as 0.2 μm by self-aligned electrode definition using substrate reverse side exposure. Pentacene based organic thin film transistors with an exceptionally low line edge roughness <20 nm of the channels, a mobility of 0.1 cm²/Vs, and an on–off ratio of 10⁴, are fabricated on 4″ × 4″ flexible substrates in a carrier-free process scheme. The stability and spatial distribution of the transistor channel lengths are assessed in detail with standard deviations of L ranging from 185 to 28 nm. Such high-performing self-aligned organic thin film transistors enabled a ring-oscillator circuit with an average stage delay below 4 μs at an operation voltage of 7.5 V.     

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.     

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.