Controlled surface doping for operating stability enhancement in organic field-effect transistors

The introduction of an inorganic/organic or organic/organic heterojunction in the pentacene-based organic field-effect transistors is demonstrated to be in favor of improving their operating stability. The heterojunction-induced p-type doping of pentacene is nondestructive, and it can be controlled by varying the adlayer thickness. The bias stress effects are compared at similar surface carrier density for the doped and undoped devices, and the current flow in the pentacene bulk is found to be more stable than that in the conducting channel close to the gate dielectric. In the initial stage of the bias stress characteristics, the carrier trapping associated with the gate dielectric is mainly responsible for the current instability. On the other hand, in the prolonged stage, the carrier trapping in the active layer may become dominant.     

Pentacene organic field-effect transistors with polymeric dielectric interfaces: Performance and stability

Low-voltage pentacene organic field-effect transistors (OFETs) with different gate dielectric interfaces are studied and their performance in terms of electrical properties and operational stability is compared. Overall high electrical performance is demonstrated at low voltage by using a 100 nm-thick high-κ gate dielectric layer of aluminum oxide (Al₂O₃) fabricated by atomic layer deposition (ALD) and modified with hydroxyl-free low-κ polymers like polystyrene (PS), divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) (Cyclotene™, Dow Chemicals), and as well as with the widely used octadecyl-trichlorosilane (OTS). Devices with PS and BCB dielectric surfaces exhibit almost similar electrical performance with high field-effect mobilities, low subthreshold voltages, and high on/off current ratios. The higher mobility in pentacene transistors with PS can be correlated to the better structural ordering of pentacene films, as demonstrated by atomic force microscopy (AFM) images and X-ray diffraction (XRD). The devices with PS show good electrical stability under bias stress conditions (V_GS = V_DS = −10 V for 1 h), resulting in a negligible drop (2%) in saturation current (I_DS) in comparison to that in devices with OTS (12%), and to a very high decay (30%) for the devices with BCB.     

Bias-stress stability of low-voltage p-channel and n-channel organic thin-film transistors on flexible plastic substrates

The bias-stress stability of low-voltage organic p-channel and n-channel thin-film transistors (TFTs) based on five promising organic semiconductors and fabricated on flexible polyethylene naphthalate (PEN) substrates has been investigated. In particular, it has been studied to which extent the bias-stress-induced decay of the on-state drain current of the TFTs is affected by the choice of the semiconductor and by the gate-source and drain-source voltages applied during bias stress. It has been found that for at least some of the organic p-channel TFTs investigated in this study, the bias-stress stability is comparable to that of a-Si:H and metal-oxide TFTs, despite the fact that the organic TFTs were fabricated at significantly lower process temperatures, which is important in view of the fabrication of these devices on plastic substrates.     

Low-power organic field-effect transistors and complementary inverter based on low-temperature processed Al2O3 dielectric

Organic-based complementary inverter could be a key component in future flexible and portable electronic products, which require low-power operation, high operating stability and flexible compatibility at the same time. A simple method for making excellent Al₂O₃ gate dielectric is developed toward the target, and it is a low-cost solution process with a low annealing temperature compatible with plastic substrates. Utilizing the Al₂O₃ dielectric, both p-type and n-type low-voltage organic field-effect transistors (OFETs) are realized. The device operating voltage is down to ±3 V, and the On/Off ratio is up to 10⁶. The hole and electron field-effect mobilities are 2.7 cm²/V and 0.2 cm²/V, respectively, and the subthreshold swing is as small as about 110 mV/decade. The high quality of the Al₂O₃ dielectric results in high operating stability of the devices. The p-type and n-type OFETs are integrated to achieve a low-power complementary inverter with a large gain, which can be successfully fabricated on a flexible substrate.     

Study of contact resistance of high-mobility organic transistors through comparisons

Realization of high-frequency low-cost organic electronics requires high-mobility organic field-effect transistors (OFETs) with short channels, where influence of contact resistance becomes more serious than either lower mobility or longer channel devices. To reduce the contact resistance, we systematically and quantitatively investigate the influence of the lowest unoccupied molecular orbital (LUMO) level of an electron acceptor layer, the active layer thickness, and the side chain of active layer itself on contact resistance of top-contact high-mobility OFETs through a series of comparative analysis. We find that the acceptor of 1,3,4,5,7,8-hexafluoro tetracyano naphtha quinodimethane (F₆TNAP) with a deeper LUMO level is efficient for carrier injection and that the bulk resistance plays an important role in such devices. By optimizing the parameters, we get the lowest contact resistance of only 110 Ω cm, and thus recorded effective mobility of 8.0 cm²/V s is attained for polycrystalline thin film transistors and still kept as high as 6 cm²/V s at shorter channel lengths.     

Enhanced performance of room temperature ammonia sensors using morphology-controlled organic field-effect transistors

Developing electronic sensors for ammonia (NH₃) is very useful for environmental monitoring and diagnostic purposes. In this work, a highly sensitive, organic field-effect transistor (OFET) based, room temperature sensor for NH₃ has been fabricated using dinaphtho [2,3-b:2′,3′-f]thieno [3,2-b]thiophene (DNTT), which showed a fast response to low concentration of the analyte down to 100 ppb. A thin film of solution-processed polymethyl methacrylate (PMMA) has been used as the gate dielectric material and its hydrophobic surface promoted structured growth of organic semiconductor, DNTT, by inducing mass transfer. By controlling the thickness and thereby exploiting the growth dynamics of the semiconductor film, the sensor performance was improved. The sensitivity of the device towards 1 ppm of NH₃ was almost doubled with a thinner and porous film of DNTT as compared to that with a thick film. Morphological studies of the sensing layers, using atomic force microscopy (AFM), have established this structure-property relation. The variations in different transistor parameters have been studied with respect to different analyte concentrations. The p-channel devices in the enhancement mode showed depletion upon exposure to NH₃. The devices exhibited a fast response and good recovery to the initial state within 2 min.     

High operational stability of solution-processed organic field-effect transistors with top-gate configuration

Electrical characteristics of top-gate field-effect transistors based on a wide range of solution-processed organic semiconductors are systematically investigated. The top-gate field-effect transistors based on different organic semiconductors—from an amorphous polymer semiconductor to a polycrystalline molecular semiconductor—exhibit higher operational stability compared with bottom-gate organic field-effect transistors reported in literature, in spite of significant difference in field-effect mobility. The correlation between charge transport and operational stability is discussed to gain insight into high operational stability of top-gate organic field-effect transistors.     

Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.     

Downscaling of n-channel organic field-effect transistors with inkjet-printed electrodes

In this contribution we demonstrate for the first time a downscaled n-channel organic field-effect transistors based on N,N′-dialkylsubstituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) with inkjet printed electrodes. First we demonstrate that the use of a high boiling point solvent is critical to achieve extended crystalline domains in spin-coated thin films and thus high electron mobility >0.1 cm² V⁻¹ s⁻¹ in top-gate devices. Then inkjet-printing is employed to realize sub-micrometer scale channels by dewetting of silver nanoparticles off a first patterned gold contact. By employing a 50 nm crosslinked fluoropolymer gate dielectric, ∼200 nm long channel transistors can achieve good current saturation when operated <5 V with good bias stress stability.     

Top-gate organic field-effect transistors fabricated on paper with high operational stability

We report on top-gate organic field-effect transistors (OFETs) fabricated on specialty paper, PowerCoat™ HD 230 from Arjowiggins Creative Papers coated with a buffer layer composed of a polyvinyl alcohol (PVA) and polyvinylpyrrolidine (PVP) blend. OFETs operate at low voltages and display average carrier mobility values of 1.7 ± 1.1 × 10⁻¹ cm²/Vs, average threshold voltage values of −1.4 ± 0.2 V, and average on/off current ratio of 10⁵. OFETs also display excellent operational stability demonstrated by stable 1000 scans of the transfer characteristics and by stable on-currents displaying less than 6% change during a DC bias stress test at V_DS = V_GS = −10 V for 1 h. Furthermore, OFETs on paper display a decrease of only 7% in their on-state current during a bending test. The performance of these OFETs on paper is comparable to that displayed by top-gate OFETs with the same geometry fabricated on glass substrates.     

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.     

Solution-processed organic crystals written directly with a rollerball pen for field-effect transistors

To deposit organic semiconducting crystals from solution, we propose the use of a rollerball pen as a simple and promising tool. These organic crystal grains of dioctylbenzothienobenzothiophene measured several hundred micrometers. The fabricated OFETs exhibited good device performance with a field-effect mobility (μ_FET) of 0.7 cm²/Vs and an on-off ratio of more than 10⁷. Simulation results reveal that the flow behavior of solution from the pen refill tube to the substrate intrinsically enhances the formation of large organic crystals.     

Flexible organic field-effect transistors with TIPS-Pentacene crystals exhibiting high electrical stability upon bending

Flexible organic field-effect transistors with high electrical stability upon bending are demonstrated on indium tin oxide coated polyethylene terephthalate substrates with TIPS-Pentacene semiconductor crystals formed by drop casting on a hybrid gate dielectric consisting hafnium dioxide grown by atomic layer deposition and spin coated poly(4-vinylphenol). Fabricated devices exhibited excellent p-channel characteristics with field-effect mobility up to 0.12 cm²/Vs with high current on/off ratio >10⁴ and low threshold voltage of −0.2 V. Device performance was slightly affected by mechanical strain applied by bending for 5 min with radius varying from 12.5 mm to as low as 5.0 mm; and a high stability in performance was demonstrated upon applying constant tensile strain for more than 48 h at bending radius of 5.0 mm. It was found that strain induced changes in the device performance primarily occur due to increase in dielectric surface roughness; and the semiconductor-dielectric interface uniformity is influenced more with magnitude of strain rather than its duration.     

Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells

Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,3′-hexyl-terthiophene (IDT-3Th-ID(I)) or 4,4′-hexyl-terthiophene (IDT-3Th-ID(II)) as π-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good film-forming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52 cm² V⁻¹ s⁻¹ for IDT-3Th-ID(I) and 0.61 cm² V⁻¹ s⁻¹ for IDT-3Th-ID(II) were achieved, with corresponding high I_ON/I_OFF of ca. 10⁷ and ∼10⁹ respectively. The OSCs based on IDT-3Th-ID(I)/PC₇₀BM (2:1, w/w) and IDT-3Th-ID(II)/PC₇₀BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100 mW/cm². The results demonstrate that the small molecules constructed with the highly π-conjugated IDT as donor unit, 3Th as π-bridges and ID as acceptor units, could be promising organic semiconductors for high-performance OFETs and OSCs applications.     

Charge-transfer complex modified bottom electrodes for high performance low voltage organic field-effect transistors and circuits

By using charge transfer complex silver and 2,3-dichloro-5,6-dicyano-p-benzoquinone (AgDDQ) modified silver as the bottom contact source/drain electrodes, high performance organic transistors and complementary inverter circuits using dinaphtho[2,3-b:2′,3’-f]thieno[3,2-b]thiophene (DNTT) as P-type organic semiconductors and N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) as N-type organic semiconductors were demonstrated. Devices with Ag-DDQ bottom contact electrodes exhibit good compatibility for both P and N-type organic semiconductors, the transistors and inverters exhibit excellent stability after storing in air ambient for more than 40 days. The fabrication process is compatible with photolithography technology, which is applicable for large area integrated circuits. All these results indicate the potential application of Ag-DDQ modified electrodes in all-organic, flexible, and low-power electronics.     

Below-one-volt organic thin-film transistors with large on/off current ratios

In many of the applications envisioned for organic thin-film transistors (TFTs), the electrical power will be supplied by small batteries or energy harvesters, which implies that it will be beneficial if the TFTs can be operated with voltages of 1 V or even below 1 V. At the same time, the TFTs should have large on/off current ratios, especially for applications in digital circuits and active matrices. Here we demonstrate p-channel and n-channel organic TFTs fabricated on a flexible plastic substrate that have a turn-on voltage of exactly 0 V, a subthreshold slope of 100 mV/decade, and an on/off current ratio of 2 × 10⁵ when operated with gate-source voltages between 0 and 0.7 V. Complementary inverters fabricated using these TFTs have a small-signal gain of 90 and a minimum noise margin of 79% at a supply voltage of 0.7 V. Complementary ring oscillators can be operated with supply voltages as small as 0.4 V.     

Dialkylated dibenzotetrathienoacene derivative as semiconductor for organic field effect transistors

A novel semiconductor material based on dialkylated thienoacene is designed and synthesized. The dihexyl-substituted dibenzotetrathienoacene derivative C6-DBTTA exhibits high stability which is evidenced by thermogravimetric analysis (TGA), UV–vis spectroscopy and electrochemistry. X-ray diffraction measurements of the vacuum-evaporated thin films show strong diffraction and indicate that the molecules are stacked nearly perpendicular to the substrate. AFM images reveal that the morphology of thin films depended on the deposition temperature. Thin film FETs devices based on C6-DBTTA were constructed and showed high mobility up to 0.48 cm² V⁻¹ s⁻¹ and an on/off ratio over 10⁷. These results suggest that this new dihexylated thienoacene is an important organic semiconductor for field effect transistors.     

Parylene based bilayer flexible gate dielectric layer for top-gated organic field-effect transistors

In this paper, we report on a bilayer insulating film based on parylene-c for gate dielectric layers in top-gate/bottom-contact inkjet-printed organic field-effect transistors (OFETs) with indacenodithiophene-co-benzothiadiazole (IDTBT) and poly([N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bitthiophene)) (P(NDI2OD-T2)) as with p- and n-channel semiconductors. The thin parylene-c film (t = 210 nm) show large gate leakage density (2.52 nA/cm² at 25 V) and low breakdown voltage (2.2 MV/cm). In addition, a degraded field-effect mobility (μ) was observed in printed IDTBT and P(NDI2OD-T2) OFETs with the parylene-c single-layered dielectric. X-ray photoelectron spectroscopy (XPS) analysis reveals that the degradation of μ is due to unwanted chemical interaction between parylene-c and the conjugated polymer surface during the parylene-c deposition process. By inserting 50-nm thick poly(methyl-methacrylate) (PMMA) and polystyrene (PS) layer in-between the parylene-c and conjugated polymer film, highly improved gate leakage density and breakdown voltage are achieved. The printed IDTBT and P(NDI2OD-T2) OFETs with a bilayer dielectric compose of parylene-c and PMMA and PS show significantly improved hole and electron μ of 0.47 cm²/Vs and 0.13 cm²/Vs, respectively, and better operation stability. In addition, we demonstrate inkjet-printed polymer complementary inverter with a high voltage gain of 25.7 by applying a PS/parylene-c bilayer dielectric.     

Performance enhancement in mechanically stable flexible organic-field effect transistors with TIPS-pentacene:polymer blend

Flexible organic field-effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS-pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability. Due to merit of high quality dielectric-semiconductor interface, pristine TIPS-pentacene: PS blend OFETs exhibited maximum mobility of 0.93 cm² V⁻¹ s⁻¹ with average of 0.44(±0.25) cm² V⁻¹ s⁻¹ compared to 0.14(±0.10) cm² V⁻¹ s⁻¹ for bi-layer OFETs with high current on-off ratios on the order 10⁵ for both. Both types of devices exhibited high electrical stability upon bending with increasing magnitude of strain or its duration up to 5 days. However, significant differences in electrical stability of devices were observed upon applying constant bias-stress for 40 min to 1 h. Pristine blend devices exhibited outstanding electrical stability with very low drain current decay of <5% compared to ∼30% for bi-layer devices. Even upon bias-stress after 5 days of bending, the drain current decay levels were only changed to <10% and ∼50% for blend and bi-layer devices respectively.     

Controlling organization of conjugated polymer films from binary solvent mixtures for high performance organic field-effect transistors

We investigate the effect of a binary solvent blend as a solvent for poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} P(NDI2OD-T2) on the characteristics of n-channel organic field-effect transistors (OFETs). To make the binary solvent blend, the low-boiling-point non-solvent propylene glycol methyl ether acetate (PGMEA, b.p ∼146 °C) is added to the high-boiling-point good solvent 1,2-dichlorobenzene (O-DCB, b.p ∼180 °C) at various mixing ratio from 0 to 40 v%. UV–vis spectra of P(NDI2OD-T2) solution dissolved in the binary solvent clearly show the formation of polymer aggregates through a gradual red shift of the intramolecular charge transfer band with the addition of high concentrations of non-solvent PGMEA. Higher edge-on oriented crystallinity is observed for P(NDI2OD-T2) films spin-coated from the binary solvent with 5–10 v% PGMEA by out-of-order x-ray diffraction. P(NDI2OD-T2) films are applied as the active layer in top-gate/bottom-contact OFETs. Improved n-type field-effect mobility of the P(NDI2OD-T2) semiconducting layer up to 0.59 cm²/Vs was achieved for on-center spin coated films compared to 1.03 cm²/Vs for off-center (parallel alignment) spin-coated films respectively employing the binary solvent with 10 v% PGMEA.