P-channel Organic Field-effect Transistors Research Articles

Complementary inverter and reward-modulated spike timing dependent plasticity circuit based on organic transistors

Organic materials are the key to the next generation of electric memory elements because of their low cost, high flexibility and high scalability. Solution-processed n-channel and p-channel organic field effect transistors (FETs) by spinning coating technique was demonstrated. We use based-poly (vinylcarbazole) (PVK) p-type transistor and based-6-phenyl-C61 butyric acid methyl ester (PCBM) n-type transistor as the channel of bottom gate structure FET, respectively, to fabricate the complementary inverter. The maximum voltage gain of the electrically isolated FET complementary inverter is up to 22, the power consumption is less than 3 μW, and the transition voltage is −4.2 V. In addition, our devices are able to implement “AND”, “NAND”, “OR”, and “NOT” gate logic well. But more than that, p-type transistor based on PVK and n-type transistor based on PCBM were combined with the memristor, reward-modulated spike timing dependent plasticity was implemented by introduced reward/punishment signals to guide synaptic weight updates. It shows great potential in the electronic application system with low cost and high energy efficiency.

High Mobility and ON/OFF Ratio of Solution-Processable p-Channel OFETs from Arylacetylene End-Capped Alkoxyphenanthrenes.

New arylacetylene end-capped alkoxyphenanthrenes were synthesized and demonstrated as the best active layer for solution-processable p-channel organic field-effect transistors. The alkoxy chain embedded compounds exhibited enhanced solubility and induced non-covalent interactions resulting in effective molecular packing. The 'Lewis soft' heteroatoms direct the most stable conformation with dihedral angles possible for molecular interactions, and energy levels. DFT studies supported the fine-tuning of FMOs, with high HOMO levels ∼-5.2 eV ensuring a low barrier for charge injection. OFET devices exhibited a maximum charge carrier mobility up to 1.30 cm2 /Vs with the highest ON/OFF ratio of 107 . The strong π-π interactions and the crystallinity of the films are well supported by GIXRD and SEM analysis.

Self-assembled dipoles of o-carborane on gate oxide tuning charge carriers in organic field effect transistors

A novel self-assembled monolayer of o-carborane on gate oxide not only realized p-channel organic field effect transistors with low threshold voltage and high field effect mobility, but also enabled ambipolar charge transport in a typical n-type organic semiconductor.

Functionalized D/A–A–D quinolines for application in solution-processable p-channel organic field-effect transistors

New functionalized quinolines with a D/A–A–D architecture and pi-end-groups at terminals were designed and synthesized. OFETs fabricated from compounds with a D–A–D architecture exhibited p-channel transistor characteristics with high hole mobilities.

Organic Field Effect Transistor Based on P3HT with Two Different Gate Dielectrics

The electrical performance of bottom-gate/top source-drain contact for p-channel organic field-effect transistors (OFETs) using poly(3-hexylthiophene) (P3HT) as an active semiconductor layer with two different gate dielectric materials, Polyvinylpyrrolidone (PVP) and Hafnium oxide (HfO2), is investigated in this work. The output and transfer characteristics were studied for HfO2, PVP and HfO2/PVP as organic gate insulator layer. Both characteristics show a high drain current at the gate dielectric HfO2/PVP equal to -0.0031A and -0.0015A for output and transfer characteristics respectively, this can be attributed to the increasing of the dielectric capacitance. Transcondactance characteristics also studied for the three organic materials and show the HfO2/PVP gate dielectric have higher value from the single layers which indicate the effect of dielectric capacitance, gm=-0.5517x10-4A/V, -0.9931x10-5A/V, and -0.6511 x10-4A/V respectively.

Surface-Treated Poly(dimethylsiloxane) as a Gate Dielectric in Solution-Processed Organic Field-Effect Transistors.

Poly(dimethylsiloxane) (PDMS) is a transparent and flexible elastomer which has a myriad of applications in various fields including organic electronics. However, the inherent hydrophobic nature and low surface energy of PDMS prevent its direct use in many applications. It is seldom utilized as a gate dielectric in solution-processed organic field effect transistors (OFETs). In this work, we demonstrate a simple method, extended ultraviolet–ozone (UVO) treatment, to modify the PDMS surface and effectively employ it in solution-processed OFETs as a gate dielectric material. The modified PDMS surface shows enhanced wettability and adherence to both polar and nonpolar liquids, which is contrary to the generally observed hydrophilic nature of UVO-treated PDMS surfaces because of the creation of polar functional groups. The morphological changes happening on the PDMS surface as a result of extended UVO treatment play a major role in making the surface suitable for all type of solvents discussed here. The contact angle measurements are used to give qualitative evidence for this observation. The modified PDMS is then used as a gate dielectric in solution-processed n- and p-channel OFETs using [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and regioregular poly(3-hexylthiophene) (rr-P3HT) semiconductors, respectively.

Asymmetric Pentacenes for Solution-Processed Organic Field-Effect Transistors

Background: Symmetrically substituted pentacenes have been traditionally used as semiconductors for solution-processed p-channel Organic Field Effect Transistors (OFETs). The aims of this paper are to introduce asymmetrically substituted pentacenes in the active layer and to examine the impact of the polyaromaticity of the pendant groups on the device characteristics. Methods: Research and online content related to asymmetrically substituted pentacenes is reviewed, the synthesis of the different pentacenes is described and the procedure used to introduce these semiconductors in devices is detailed. Comparison with a reference material is provided. Results: Extension of the polyaromaticity of the pendant group of pentacene greatly contributes to enhance the device performances. Impact of the pendant group on the morphology of the active layer is evidenced by the roughness decreasing with the increase of aromatic ring in the substituent. Conclusion: We demonstrate the extension of the π-conjugation of the aromatic end-group to drastically impact both ON/OFF ratio and charge carrier mobilities, mainly due to various degree of crystal formation characteristics in the film. Best results were obtained with the most extended one, namely the anthracene moiety which showed a mobility comparable to that of the benchmark TIPS-pentacene with OFETs that used hexamethyldisilazane as the gate dielectric passivation layer. Keywords: Mobility, OFET, organic materials, semiconductor, solution-process, TIPS-pentacene.

Ambient condition bias stress stability of vanadium (IV) oxide phthalocyanine based p-channel organic field-effect transistors

High bias-stress stability and low threshold voltage (Vth) shift under ambient conditions are highly desirable for practical applications of organic field-effect transistors (OFETs). We demonstrate here a 20-fold enhancement in the bias-stress stability for hexamethyledisilazane (HMDS) treated vanadium (IV) oxide phthalocyanine (VOPc) based OFETs as compared to the bare VOPc case under ambient conditions. VOPc based OFETs were fabricated on bare (non treated) SiO2 and a HMDS monolayer passivated SiO2 layer, with an operating voltage of 40 V. The devices with top contact gold (Au) electrodes exhibit excellent p-channel behavior with a moderate hole mobility for the HMDS-treated device. It is demonstrated that the time dependent ON-current decay and Vth shift can be effectively controlled by using self-assembled monolayers of HMDS on the VOPc layer. For the HMDS-treated case, the bias stress stability study shows the stretched exponential decay of drain current by only ~15% during the long-term operation with constant bias voltage under ambient conditions, while it shows a large decay of >70% for the nontreated devices operated for 1000 s. The corresponding characteric decay time constant (τ) is 104 s for the HMDS treated case, while that of the the non-treated SiO2 case is only ~480 s under ambient conditions. The inferior performance of the device with bare SiO2 is traced to the charge trapping at the voids in the inter-grain region of the films, while it is almost negligible for the HMDS-treated case, as confirmed from the AFM and XRD analyses. It is believed that HMDS treatment provides an excellent interface with a low density of traps and passivates the dangling bonds, which improve the charge transport characteristics. Also, the surface morphology of the VOPc film clearly influences the device performance. Thus, the HMDS treatment provides a very attractive approach for attaining long-term air stability and a low Vth shift for the VOPc based OFET devices.

Induced characteristics of n- and p-channel OFETs by the choice of solvent for the dielectric layer towards the fabrication of an organic complementary circuit

In this study, we proposed a facile and low-cost method to fabricate complementary circuits, by the careful selection of the solvent for the dielectric layer. We show that the dielectric solvent exerts a direct influence on performance of organic field-effect transistors (OFETs), but the difficulty rests in finding solvents whose orthogonality does not attack the semiconducting layer in a device of top-gate architecture. OFETs (p- and n-channel), in which such orthogonality was achieved, exhibited the best performance, most probably due to the lower roughness of the semiconductor/dielectric interface. The search for transistors that have similar characteristics (mobility, threshold voltages, Ion/Ioff, etc.) afforded the manufacture and characterization of an organic complementary circuit whose gain was higher than 7. This study therefore shows that the correct choice of solvents, especially that of the dielectric layer, is very important for the development of organic electronic circuits.

Thiophene quinoidal organic semiconductors for high performance n-channel organic field-effect transistors

Organic field-effect transistors have found wide applications in flexible displays, smart cards, sensors and so on because they can be fabricated on flexible substrates with large scale and low cost. Though great progress has been made on p-channel organic field effect transistors, the development of n-channel organic field-effect transistors is largely lagged behind their p-channel counterparts. The lack of high performance n-channel organic semiconductors has become one of the bottlenecks for organic field-effect transistors. Low LUMO energy level is one of the characteristics of n- type organic semiconductors. The low LUMO level of organic semiconductors can not only reduce the electron injection barrier, in favor of electrons transferring from the electrode into the semiconductor layer, but also prevent the electron charge carriers being trapped by air and water and obtain ambient stability. Thiophene quinoidal organic semiconductors have attracted great attentions recently because they have low-lying LUMO energy levels and strong intermolecular interactions, and usually display high electron mobility. This review summarizes the application of the thiophene quinoidal organic semiconductors in organic transistors in recent years. According to the characteristic of the molecular structures, in the review, the thiophene quinoidal organic semiconductors are divided into two classes: the oligo- thiophene quinoidal organic semiconductors and fused-ring thiophene quinoidal organic semiconductors. Compared to the extensive studied aromatic diimide n-type organic semiconductors, the research on thiophene quinoidal organic semiconductors is not sufficient and the molecular species of thiophene quinoidal compounds is not rich. However, the excellent electron transport properties of thiophene quinoidal organic semiconductors support their potential applications as high performance solution processable n-channel organic semiconductors. In addition to the application in organic transistors, thiophene quinoidal compounds usually have reversible redox property, high electron affinity and other characteristics, and will have potential applications in organic solar cells, charge transfer salts, organic memories and sensors. We believe that the research of thiophene quinoidal organic semiconductors will make more progress.

Spray-printed organic field-effect transistors and complementary inverters

We report the fabrication of high-performance organic field-effect transistors (OFETs) and complementary inverters using spray-printed films of n-type small-molecule semiconductors and p-type conjugated polymers. Highly crystalline organic semiconductor films could be obtained by controlling the droplet size, nozzle-to-substrate distance, and solvent drying speed during the printing process. After the optimisation of the spray-printing process, the performances of the spray-printed OFETs were comparable to those of spin-coated and inkjet-printed OFETs. In addition to excellent device-to-device uniformity, the spray-printed n- and p-channel OFETs also exhibited high field-effect mobilities, which were ∼0.3 (ActivInk™ N1450, Polyera), ∼0.01 (regioregular-poly(3-hexylthiophene) (rr-P3HT)), and ∼0.25 cm2 V−1 s−1 (ActivInk™ P2100, Polyera). Organic complementary inverters were fabricated by spray printing and shadow-mask patterning while using ActivInk™ N1450 and P2100 as the n- and p-type semiconductors, respectively. The complementary inverters exhibited a large voltage gain (∼17) and a low power consumption (∼0.02 mW) at VDD = 60 V.

Highly stable printed polymer field-effect transistors and inverters via polyselenophene conjugated polymers

We report the use of two polyselenophene-based conjugated polymers, poly(3,3′′-didodecyl-2,2′:5,2′′-terselenophene) (P3Se) and poly(3,3′′,3′′′,3′′′′-tetradodecyl-2,5′:2′,2′′:5′′,2′′′-pentaselenophene) (P5Se), as an active layer of printed p-channel organic field-effect transistors (OFETs). Top-gate/bottom-contact (TG/BC) P5Se OFETs showed a high-saturation hole mobility of up to ∼0.1 cm2 V−1 s−1 and a high on/off ratio of ∼105 with no hysteresis. In addition, polyselenophene-based OFETs exhibited a much better bias and ambient stability when compared with poly(3-hexylthiophene)-based OFETs. The excellent air stability of those polyselenophenes enables the realization of complementary metal-oxide semiconductor (CMOS) inverters via extended periods of ink-jetting under ambient conditions. CMOS inverters were demonstrated using p-[P5Se] and n-channel [poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-s,6-diyl]-alt-5,5′-(2,2′-dithiophene)}] ([P(NDI2OD-T2)], Polyera ActivInk N2200/OFETs) by inkjet printing of conjugated polymers. Printed CMOS inverters exhibited a stable voltage transfer characteristic with negligible hysteresis, a DC voltage gain of ∼10, and a power consumption of ∼0.025 mW at VDD = −60 V.

Influence of oxygen exposure on instabilities of perfluoropentacene transistor characteristics with different gate insulators

AbstractPentacene for p-channel organic field effect transistors (OTFTs) and perfluoropentacene for n-channel OTFTs have attracted strong attentions to be used as CMOS type organic semiconductor materials for flexible organic displays. n-channel OTFTs with different gate insulators of polyimide (PI) and SiO2 and perfluoropentacene as a semiconductor layer were fabricated, and their instability of the transistor characteristics measured in air, vacuum and oxygen was investigated. The results show that both of the transistors exhibit a significant increase of mobility when measured in vacuum. However, the mobility of the transistors is significantly reduced when measured in pure oxygen, and the threshold voltages continuously increase. These phenomena may be related to water diffusion into the perfluoropentacene/gate insulator interface in air and oxidation of perfluoropentacene in oxygen.

Water soluble poly(1-vinyl-1,2,4-triazole) as novel dielectric layer for organic field effect transistors

Water soluble poly(1-vinyl-1,2,4-triazole) (PVT) as a novel dielectric layer for organic field effect transistor is studied. Dielectric spectroscopy characterization reveals it has low leakage current and rather high breakdown voltage. Both n-channel and p-channel organic field effect transistors are fabricated using pentacene and fullerene as active layers. Both devices show device performances with lack of hysteresis, very low threshold voltages and high on/off ratios. Excellent film formation property is utilized to make AlOx and thin PVT bilayer in order to decrease the operating voltage of the devices. All solution processed ambipolar device is fabricated with simple spin coating steps using poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH–PPV) end capped with polyhedral oligomeric silsesquioxanes (POSS) as active layer. Our investigations show that PVT can be a very promising dielectric for organic field effect transistors.

Synthesis and characterization of soluble indolo[3,2- b]carbazole derivatives for organic field-effect transistors

The synthesis and characterization of new soluble materials based on the indolo[3,2- b]carbazole (IC) framework is presented. These new molecules are particularly suitable for p-channel organic field-effect transistors (OFETs) because of the high level of crystallinity usually found in IC derivatives. To obtain soluble materials, the synthesis of new 2- or 3-bromoindolo[3,2- b]carbazole backbone was performed which allowed the synthesis of molecule bearing the IC moieties on both ends. Either Suzuki cross-coupling or Horner–Emmons reaction were used as the final step. Thin films of these new materials, along with 3,9-diphenyl-5,11-dioctylindolo[3,2- b]carbazole (3POIND), a material previously reported, were obtained by using different processing techniques. Polarized optical microscopy (POM) and X-ray diffraction (XRD) experiments on these solution-processed films showed high crystallinity. Transistors fabricated with these thin films gave a hole mobility as high as 0.06 cm 2 V −1 s −1 with drop-cast films and 0.12 cm 2 V −1 s −1 when the films were processed with the shearing technique.

High mobility p-channel organic field effect transistors on flexible substrates using a polymer-small molecule blend

Organic field effect transistors have been fabricated on poly(ethylene terephthalate) film substrates with excellent operating characteristics and maximum mobilities of 1.1 cm 2/(V s). The semiconductor was a solution processible blend of 6,13-bis(triisopropyl silylethynyl) pentacene and a poly(triaryl amine) used to combine the favourable properties of small molecules and polymers. We have demonstrated that such systems are a good candidate for flexible organic electronics and that the surface energy of the substrate plays a key role in controlling the morphology of the semiconductor film.

High-performance C60 n-channel organic field-effect transistors through optimization of interfaces

High-performance C60 organic field-effect transistors (OFETs) have been obtained by engineering the essential electrode/semiconductor and dielectric/semiconductor interfaces. By using calcium (Ca) as the source and drain electrodes, the width-normalized contact resistance (RCW) at the electrode/semiconductor interface for devices with channel lengths ranging from 200 down to 25 μm could be reduced to a constant value of 2 kΩ cm at a gate-source voltage (VGS) of 2.6 V, leading to electrical properties that are dominated by gate-modulated resistance of the channel as in conventional metal-oxide-semiconductor field-effect transistors. Channel length scaling of the source-drain current and transconductance is observed. Average charge mobility values of 2.5 cm2/V s extracted at VGS<5 V are found independent of channel length within the studied range. Besides high mobility, overall high electrical performance and stability at low operating voltages are demonstrated by using a 100-nm-thick high-κ gate dielectric layer of aluminum oxide (Al2O3) fabricated by atomic layer deposition and modified with divinyltetramethyldisiloxane-bis (benzocyclobutene). The combined operating properties of these OFETs, obtained in a N2-filled glovebox, are comparable to the best p-channel OFETs and outperform those of amorphous silicon thin-film transistors.

Air Stability of p-Channel Organic Field-Effect Transistors Based on Oligo- p-phenylenevinylene Derivatives

We have investigated that the stability of organic field-effect transistors (OFETs) based on two oligo- p-phenylenevinylenes, 1,4-bis(4-methylstyryl)benzene (CH3-OPV) and 1,4-bis(4-dimethylaminostyryl)benzene [(CH3)2N-OPV], which is a new organic semiconductor for OFETs. The OFET fabricated using CH3-OPV with a highest occupied molecular orbital (HOMO) energy level of -5.55 eV shows good ambient stability even after storage in air for one year. On the other hand, the field-effect mobilities of organic semiconductors including (CH3)2N-OPV with the HOMO energy level ranging from -4.94 to -5.20 eV have decreased with time. The stability of CH3-OPV is suggested to be due to the relatively low HOMO energy level.

Low‐Voltage Polymer Field‐Effect Transistors Gated via a Proton Conductor

Low operating voltages for p-channel organic field-effect transistors (OFETs) can be achieved by using an electrolyte as the gate insulator. However, mobile anions in the electrolyte can lead to undesired electrochemistry in the channel. In order to avoid this, a polyanionic electrolyte is used as the gate insulator. The resulting OFET has operating voltages of less than 1 V (see figure) and shows fast switching (less than 0.3 ms) in ambient atmosphere.

Investigation of bottom-contact organic field effect transistors by two-dimensional device modeling

We present device simulations for p-channel organic field effect transistors. The current conservation equation and Poisson’s equation are solved self consistently in two dimensions in the drift-diffusion approximation. We focus on modeling transistor structures consisting of a p+ Si gate electrode, a silicon dioxide gate insulator, and a polymer layer as the active (channel) material. The source and drain contacts are taken to be deposited directly on the gate insulator (bottom contact structure). We focus particularly on calculations of the surface potential for which experimental data have recently been published [L. Burgi, H. Sirringhaus, and R. H. Friend, Appl. Phys. Lett. 80, 2913 (2002)]. We find that the surface potential on the polymer layer closely matches the channel potential as argued by Burgi et al. Furthermore, we show that the experimentally observed drops in the surface potential above the source and drain contacts can be explained as a consequence of polymer material defects near the contacts and, significantly, that these results cannot be explained solely by large contact Schottky energy barriers. We also present results for the transistor output characteristics and examine the effects associated with changes in charge carrier injection for different source and drain contact materials and different models for the injection process. These results elucidate the device physics of ideal organic field effect transistors and model the behavior of current experimental devices including the most likely parasitic effects.