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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Performance comparison of pentacene organic field-effect transistors with SiO2 modified with octyltrichlorosilane or octadecyltrichlorosilane

Performance of pentacene organic field-effect transistors (OFETs) is significantly improved by treatment of SiO₂ with octyltrichlorosilane (OTS-8) compared to octadecyltrichlorosilane (OTS-18). The average hole mobility in these OFETs is increased from 0.4 to 0.8 cm²/Vs when treating the dielectric with OTS-8 versus OTS-18 treated devices. The atomic force microscope (AFM) images show that the OTS-8 treated surface produces much larger grains of pentacene (∼500 nm) compared to OTS-18 (∼100 nm). X-ray diffraction (XRD) results confirmed that the pentacene on OTS-8 is more crystalline compared to the pentacene on OTS-18, resulting in higher hole mobility.     

Potential of low-voltage organic transistors with high on-state drain current for temperature sensor development

Organic transistors with high on-state drain current at gate and drain voltages of −2 V fabricated on polyethylene naphthalate foils were investigated for sensor development. Two aspects were studied: (a) the ability of such transistors to raise the sensitivity of a temperature sensor and (b) the bias stress stability of the transistors subjected to square voltage pulses that turned them on and off repeatedly. To demonstrate the first aspect, the voltage-amplifying ability of the organic transistor was used to increase the response to the temperature, ordinarily achieved with a thermistor. To achieve voltage amplification, the transistor must have on-state drain current of at least 20 μA at gate and drain voltages of −2 V. Two transistors with on-state drain current of ~60 and ~120 μA were tested, leading to voltage gain of −2.8 and −4.9 V/V, respectively, thus increasing the sensitivity of the temperature sensor by a factor of up to 5. To study the second aspect, the same square voltage pulses were concurrently applied to the gate and drain electrodes, causing the transistor to turn on and off repeatedly. The turn-on and turn-off voltages were −2 and 0 V respectively and four different pulse periods were used: T of 5, 20, 40 and 60 s. For each T, 1000 pulses with turn-on time of 1 s and varying turn-off times were applied to the transistors, leading to the aggregate net stress time of 1000 s in all cases. The changes in the on-state drain current, threshold voltage, and field-effect mobility depended on T, in spite of the net stress time being the same. The reduction in the on-state drain current did not exceed 17%, stabilization was also observed after about 500 cycles in some cases, and the maximum drop occurred for medium T, thus making T = 60 s a favorable condition for sensor operation.     

A facile method for fabrication of highly integrated organic field-effect transistors on photoresist-unwettable insulators with remarkable stability

Environmental stability is one of the most important parameters for high-performance organic field-effect transistors (OFETs). Hydrophobic insulators usually possess much better air stability than some conventional inorganic and hydrophilic organic insulators. However, fabrication of devices with high integration by photolithography method cannot be directly performed on hydrophobic insulators due to their photoresist-unwettable properties. In this work, a simple yet efficient metal-assisted photolithography method is developed to achieve large-scale fabrication of highly integrated organic electronic devices on photoresist-unwettable insulators. By using copper (Cu) as sacrificial layer, photolithography can be performed on these insulators with nearly the same resolution, uniformity, and reproducibility as the conventional photolithography. This method shows excellent flexibility and is capable of fabricating high-integrated devices on a variety of hydrophobic insulators including hydrophobic amorphous fluoropolymer (CYTOP), poly(dimethylsiloxane) (PDMS), and octadecyltrichlorosilane (OTS)-modified SiO₂. OFETs based on 6,13-dichloropentacene (DCP) microwires (MWs) with CYTOP as the insulator layer were fabricated, which exhibited excellent device performance. Much improved device stability with very low mobility degradation (less than 9%) was observed after 24 days. While devices fabricated on hydrophilic insulators of poly-(vinyl phenol) (PVP) and bare SiO₂ experienced dramatic performances decay within 24 days. Furthermore, by using metal-assisted photolithography method, flexible OFETs arrays could be further fabricated on polyethylene naphthalate (PEN) substrate, which showed excellent mechanical bending flexibility and stability. This work unveils the great potential of metal-assisted photolithography method for long-term stable high-integration organic electronic devices.     

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.     

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.     

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.     

Realization of electrically stable organic field-effect transistors using simple polymer blended dielectrics

Organic field-effect transistors (OFETs) were fabricated using polymer blended gate dielectrics in an effort to enhance the electrical stability against a gate bias stress. A poly(melamine-co-formaldehyde) acrylated (PMFA) gate dielectric layer with great insulating properties was blended with polypentafluorostyrene (PFS), a type of hydrophobic fluorinated polymer. Although the overall electrical performance dropped slightly due to the rough and hydrophobic surfaces of the blend films, at the blend ratio (10%), the OFET’s threshold voltage shift under a sustained gate bias stress applied over 3 h decreased remarkably compared with an OFET based on a PMFA dielectric alone. This behavior was attributed to the presence of the hydrophobic and electrically stable PFS polymer, which provided a low interfacial trap density between the gate dielectric and the semiconductor. A stretched exponential function model suggested that the energetic barrier to create trap states was high, and the distribution of energetic barrier heights was narrow in devices prepared with PFS.     

Photoinduced negative magnetoresistance in 6,13-bis(triisopropylsilylethynyl)-pentacene field-effect transistors

We report on photoinduced negative organic magnetoresistance in low external magnetic-fields (<100 mT) in 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pentacene) field-effect transistors. An external magnetic field does not influence the dark current of our device. In contrast, there is a significant increase in photocurrent when magnetic field is applied to the irradiated device, which leads to negative magnetoresistance. The magnetoresistance and photoresponse values are strongly correlated and both are influenced by applied voltages and irradiation intensity. We attribute the observed photoinduced negative magnetoresistance in TIPS-Pentacene field-effect transistors to the presence of electron-hole pairs under irradiation. The overall dissociation probability of electron-hole pairs rises under the influence of an external magnetic field, which leads to a higher number of free charge carriers.     

Cardanol-based polymeric gate dielectric for solution-processed organic field-effect transistors on flexible substrates

A cardanol-based polymer, poly(2-hydroxy-3-cardanylpropyl methacrylate) (PHCPM), was utilized as the gate dielectric of an organic field-effect transistor (OFET). PHCPM has good surface properties, appropriate gate dielectric characteristics, and good compatibility with solution-processed semiconducting polymers. The electrical properties of an FET that was prepared with natural resource-based PHCPM as a gate dielectric layer and solution-processed poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) as a semiconducting layer were investigated on flexible substrates. The flexible PBTTT-OFET device with the PCHPM gate dielectric exhibited high mobility and reliable performance, even in the bending state, without significant hysteresis.     

聚酰亚胺为栅绝缘层的并五苯场效应晶体管

以真空蒸发的有机半导体材料并五苯为有源层,以旋涂的聚酰亚胺作为栅绝缘层,以真空蒸发的Al为栅、源和漏电极,成功制作了顶接触式并五苯有机场效应晶体管(OFET).测试表明,在源漏电压为70 V时,器件的载流子迁移率μ为0.079 cm2/V·s,器件的开关电流比为1.7×104.     

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.