Pentacene-based Organic Field-effect Transistors Research Articles

Nature of Ohmic and Schottky contacts on pentacene-based organic field-effect transistor

Organic field-effect transistors (OFETs) have become promising components for a wide range of applications in the fast-growing field of organic electronics. Nevertheless, with the increasing need for high-performance OFETs, it is crucial to understand and challenge the contact resistance in these devices, because it represents a critical bottleneck that can significantly suppress the overall performance. Herein, we report the evaluation of the voltage dependence of the injection barrier in pentacene-based OFET devices with Au or Ag electrodes. The injection barrier of the device with Au electrodes showed an Ohmic nature with a barrier of approximately 170 ∼ 200 meV, whereas the barrier in the device with Ag electrodes behaved as a Schottky contact and the zero-field injection barrier reached 552 ± 2 meV.

Effect of Organic / Inorganic Gate Materials on the Organic Field-Effect Transistors Performance

The choice of gate dielectric materials is fundamental for organic field effect transistors (OFET), integrated circuits, and several electronic applications. The operation of the OFET depends on two essential parameters: the insulation between the semiconductor layer and the gate electrode and the capacitance of the insulator. In this work, the electrical behavior of a pentacene-based OFET with a top contact / bottom gate was studied. Organic polyvinyl alcohol (PVA) and inorganic hafnium oxide (HfO2) were chosen as gate dielectric materials to lower the operation voltage to achieve the next generation of electronic applications. In this study, the performance of the OFET was studied using monolayer and bilayer gate insulators. To model and analyze a device's electrical properties, MATLAB was used. Two main parameters were studied: switching ratio (Ion/Ioff) and subthreshold swing (SS), as well as the effect of dielectric capacitance on the gate dielectric materials. The PVA/HfO2 bilayer gate dielectric gave the best results in Ion/Ioff ratio, SS and transconductance of 9.05´10-7, -1.52, and -4.99 x10-5A/V respectively, which is because the dielectric capacitance has increased.

Effect of Variations in the Alkyl Chain Lengths of Self-Assembled Monolayers on the Crystalline-Phase-Mediated Electrical Performance of Organic Field-Effect Transistors.

Self-assembled monolayers (SAMs) of organic molecules are frequently employed to improve the electrical performance of organic field-effect transistors (OFETs). However, the relationship between SAM properties and OFET performance has not been fully explored, leading to an incomplete understanding of the system. This study investigates the effect of the SAM alkyl chain length on the crystalline phase of pentacene films and OFET performance. Two types of SAMs—with alkyl chain lengths of 10 (decyltrichlorosilane, DTS) and 22 (docosyltrichlorosilane, DCTS)—were examined, and variations in the performance of pentacene-based OFETs with the nature of the SAM treatment were observed. Despite the similar surface morphologies of the pentacene films, field-effect mobility in the DCTS-treated OFET was twice that in the DTS-treated OFET. To find the reason underlying the dependence of the OFET’s electrical performance on the SAM alkyl chain length, X-ray diffraction measurements were conducted, followed by a phase analysis of the pentacene films. Bulk and thin-film phases were observed to coexist in the pentacene film grown on DTS, indicating several structural defects in the film; this can help explain the dependence of the OFET electrical performance on the SAM alkyl chain length, mediated by the different crystalline phases of pentacene.

Study the performance of pentacene based organic field effect transistor by using monolayer, bilayer and trilayer with different gate insulators

In this paper, Pentacene based-organic field effect transistors (OFETs) by using different layers (monolayer, bilayer and trilayer) for three different gate insulators (ZrO2, PVA and CYEPL) were studied its current–voltage (I-V) characteristics by using the gradual-channel approximation model. The device exhibits a typical output curve of a field-effect transistor (FET). Source-drain voltage (Vds) was also investigated to study the effects of gate dielectric on electrical performance for OFET. The effect of capacitancesemiconductor in performance OFETs was considered. The values of current and transconductance which calculated using MATLAB simulation. It exhibited a value of current increase with increasing source-drain voltage.

A Compact Model and Parameter Extraction Method for a Staggered OFET With Power-Law Contact Resistance and Mobility

We present a drain current model and a consistent parameter extraction method for organic field-effect transistors (OFETs) considering the gate voltage-dependent contact resistance and mobility by power-law. The physical origin of the asymptotic power-law dependence of contact resistance is elaborated based on the current crowding model. The parameters of a new model can be completely extracted without ambiguity by the proposed extraction method based on the linear fitting. Pentacene-based OFETs with the Au source/drain contact in a staggered configuration experimentally validate the proposed model and the extraction method, which further reveal the bulk resistivity as the major contribution to contact resistance.

Photosensitivity and charge injection dynamics of pentacene based thin-film transistors: influence of substrate temperature

In this work, we have performed an in-depth analysis to investigate the effect of substrate temperature on the molecular packing arrangement and energy levels of pentacene films. We have also explored their influence on the charge injection mechanism and photosensing behaviour of pentacene-based organic field-effect transistors (OFETs). In this study, we find the contact resistance and photosensitivity of the devices to be severely influenced by the active layer processing condition owing to the aforementioned structural and energy level modifications. Contact resistance of the devices at metal-semiconductor interfaces was observed to be reduced significantly upon increase in the substrate temperature; however, above a certain temperature, formation of pentacene thin-films was severely affected and no transistor characteristics were obtained afterwards. Detailed experimental analysis and theoretical investigations revealed that the processing temperature could strongly influence the grain structure and unit cell volume of the pentacene molecules, which consequently enhanced the carrier injection across the interface through a control over carrier mobility and the distribution of electronic states in the proximity of Fermi energy. Furthermore, our study demonstrates the role of substrate temperature in effectively enhancing the photosensitivity of the transistors. This report thus represents a step forward towards understanding a correlation between the processing temperature and the dynamics of charge injection in pentacene based organic transistors. The results also illustrate the viability of using proper substrate temperature to achieve an efficient photosensitivity from OFET devices. • Effect of substrate temperature on the charge injection dynamics is investigated. • Substrate temperature induced structural and energy level modification of pentacene is explored. • Delocalization of electronic states strongly influenced the interfacial charge injection. • High substrate temperature processed devices exhibited excellent photo sensitivity.

Effects of trifluoromethyl substituents on interfacial and bulk polarization of polystyrene gate dielectrics

The ability to control the bulk and interfacial polarization of dielectric polymers is important to their application in organic electronics. We examine the effect of the trifluoromethyl substituent on poly(3-trifluoromethylstyrene) (P3TFMS) as compared to unsubstituted polystyrene (PS) on the I-V relationships of pentacene-based organic field-effect transistors (OFETs). Single- and double-layered films of these polymers were used, with lower layers crosslinked through vinylbenzocyclobutene comonomers before deposition of upper layers. Control experiments verified that the electronic effect of the crosslinking was negligible. We found that the TFM substituent markedly and independently affected both the initial threshold voltage Vth and the nonvolatile, shifted Vth observed after the application of static gate voltage, depending on its position adjacent or apart from the pentacene. The trifluoromethyl-bearing polymers exhibited significantly lower magnitude initial threshold voltages (Vth,i of ca. −17 V for P3TFMS compared to −35 V for PS), large threshold voltage shifts after charging by the application of static electric fields (ΔVth of ca. 32 V for P3TFMS and 17 V for PS), and greater stability of the ΔVth under repeated charge/discharge cycles. These results are consistent with P3TFMS having fewer interfacial trap states but more stable bulk trap states. The results are applicable to organo-electronic systems such as piezoelectrics for energy harvesting and nonvolatile OFETs such as memory, sensing, and logic elements.

Highly Contrasting Static Charging and Bias Stress Effects in Pentacene Transistors with Polystyrene Heterostructures Incorporating Oxidizable N,N'-Bis(4-methoxyphenyl)aniline Side Chains as Gate Dielectrics.

Charge storage and trapping properties of polymer dielectrics govern the charge densities of adjacent semiconductors and greatly influence the on-off switching voltage (threshold voltage, ) of organic field-effect transistors (OFETs) when the polymers are used as gate insulators. Intentional charging of polymer dielectrics in OFETs can change and affect the bias stress. We describe a chemical design and fabrication protocol to construct multilayer-stack dielectrics for pentacene-based OFETs using different polystyrene (PS)-based polymers in each layer, with oxidizable N,N-bis(4-methoxyphenyl)anilino (TPAOMe)-substituted styrene copolymers in arbitrary vertical positions in the stacks. Thermal, byproduct-free cross-linking of benzocyclobutene subunits provides integrity to the multilayer structure by preventing dissolution of the previous deposited layer. Neutron reflectivity data verified the multilayer morphology. We compared the shift before and after charging the stacks by application of ±100 V across 0.5-1 μm total film thicknesses. Bias stress was the dominant effect in bilayer devices with a TPAOMe layer in contact with the pentacene, indicated by the direction of shift associated with either polarity of external electric field. In structures with no TPAOMe subunit in contact with the pentacene, when charging with -100 V on top of the source and drain electrodes, electron injection from pentacene to dielectric was the major charging mechanism, again consistent with the bias stress direction. When charging with +100 V, bilayer devices without TPAOMe showed little change in , suggesting there was no bias stress effect or charge injection in these devices for this charging polarity. For the bilayer devices with the TPAOMe layer in the bottom, and the trilayer devices with TPOMe in the middle, when +100 V was applied, the shifts were opposite those expected from bias stress. Dipole formation or partial ionization of chargeable groups at the interface between the dielectric layers are likely polarization mechanisms in these cases. A simple analytical model levelports the plausibility of these mechanisms. This work provides examples of both stabilization and shifting of , and therefore controlling charge carrier density, in semiconductors overlying the dielectric multilayers.

Significance of the gate voltage-dependent mobility in the electrical characterization of organic field effect transistors

We studied the effect of the addition of free hole- and electron-rich organic molecules to organic semiconductors (OSCs) in organic field effect transistors (OFETs) on the gate voltage-dependent mobility. The drain current versus gate voltage characteristics were quantitatively analyzed using an OFET mobility model of power law behavior based on hopping transport in an OSC. This analysis distinguished the threshold voltage shifts, depending on the materials and structures of the OFET device, and properly estimated the hopping transport of the charge carriers induced by the gate bias within the OSC from the power law exponent parameter. The addition of pentacene or C60 molecules to a one-monolayer pentacene-based OFET shifted the threshold voltages negatively or positively, respectively, due to the structural changes that occurred in the OFET device. On the other hand, the power law parameters revealed that the addition of charge carriers of the same or opposite polarity enhanced or hindered hopping transport, respectively. This study revealed the need for a quantitative analysis of the gate voltage-dependent mobility while distinguishing this effect from the threshold voltage effect in order to understand OSC hopping transport in OFETs.

Effects of source/drain electrode contact length on the photoresponsive properties of organic field-effect transistors

With the continual increase of the carrier mobility of organic semiconductors, there is a great need for optimizing the contact between source/drain electrode and organics in order to further improve the performance of organic field-effect transistors. The effects of Au source/drain electrode contact length on the photosensitivity in pentacene-based organic field-effect transistors were systematically investigated. The results show that at a given gate voltage and drain voltage, the drain current increases with the contact length at first and then tends to saturate at a contact length of 0.7 mm. It is observed that the effective mobility under illumination, both in the linear region and the saturation region, as well as the photoresponsivity and the external quantum efficiency, increase with contact length. All of these can be attributed to the reduction of contact resistance with the increase of contact length. Moreover, analytical expressions were derived and successfully describe the measured dependence of the drain current on the contact length.

Use of bilayer gate insulator to increase the electrical performance of pentacene based transistor

In this study, bottom-gate top-contact pentacene-based organic field effect transistors (OFET) with various spin-coated ultrathin organic dielectrics on anodized aluminum oxide (Al2O3) bilayer gate dielectrics were fabricated. We have investigated the influence of the bilayer gate insulator having different combinations on the OFETs performance. Polystyrene (PS), poly-4-methylstyrene (P4MS), Poly-4-vinylphenol (PVP), poly-methylmethacrylate (PMMA) and Poly(4-vinylphenol-co-methyl methacrylate) (PVP_co_PMMA) were used as an organic dielectric. The results indicate that Al2O3 gate dielectric with Poly (4-vinylphenol) shows the optimum electrical performance with carrier mobility as large as 0.65cm2/Vs, on/off current ratio of 106, and threshold voltage as −3.8V.

Engineering the mobility increment in pentacene-based field-effect transistors by fast cooling of polymeric modification layer

In this work, we investigate the effect of the cooling rate of polymeric modification layers (PMLs) on the mobility improvement of pentacene-based organic field-effect transistors (OFETs). In contrast to slow cooling (SC), the OFETs fabricated through fast cooling (FC) with PMLs containing side chain-phenyl rings, such as polystyrene (PS) and poly (4-vinylphenol) (PVP), show an obvious mobility incensement compared with that of π-group free polymethylmethacrylate (PMMA). Atomic force microscopy (AFM) images and x-ray diffraction (XRD) characterizations have showed that fast-cooled PMLs could effectively enhance the crystallinity of pentacene, which might be related to the optimized homogeneity of surface energy on the surface of polymeric dielectrics. Our work has demonstrated that FC treatment could be a potential strategy for performance modulation of OFETs.

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm2/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm2/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.

Degradation Process in Pentacene-Based Organic Field-Effect Transistors Evaluated by Three-Terminal Capacitance-Voltage Measurements

ABSTRACTBy taking advantage of three-terminal capacitance-voltage (TT-CV) measurement, we investigated a formation of trapped charge in pentacene(Pn)-based organic field-effect transistors (OFETs) during the bias stress measurement. The shift of the turn-on voltage in transfer curve correlated well with the increase of trapped charge estimated from TT-CV curves. Moreover, TT-CV measurement revealed that the trapped charges were distributed inhomogeneously at the vicinity of the pentacene/insulator interface, indicating that the current does not obviously affect their formation. Thus we suggested that the trapped charges are formed by keeping Pn molecules as unstable cation (hole state) by the prolonged bias stress.

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.

Understanding the dependence of performance on the dielectric-semiconductor interface in pentacene-based organic field-effect transistors

The dependence of performance on the dielectric-semiconductor interface in pentacene-based organic field-effect transistors (OFETs) was investigated by modification of the substrate with octadecylsilane (OTS) compared with octadecyltrimethoxysilane (OTMS). The hole mobility of OFETs was enhanced from 0.43cm2/Vs to 0.72cm2/Vs when treating the substrate with OTMS versus OTS. However, the subsequent ammonia vapor did not lead to enhanced FET performance, which might be closely related to its negative influence on the pentacene growth. According to atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements, flat pentacene domains with larger grain sizes and less crystal boundaries were observed for the OTS treated surface, demonstrating higher crystallinity, which may play a positive role for the charge transport and thus accounts for the improved mobility.

Radical polymers improve the metal-semiconductor interface in organic field-effect transistors

Modifying the organic-metal interface in organic field-effect transistors (OFETs) is a critical means by which to improve device performance; however, to date, all of the interfacial modifying layers utilized in these systems have been closed-shell in nature. Here, we introduce open-shell oxidation-reduction-active (redox-active) macromolecules, namely radical polymers, in order to serve as interfacial modifiers in pentacene-based OFETs. Through careful selection of the chemistry of the specific radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), the charge transport energy level of the interfacial modifying layer was tuned to provide facile charge injection and extraction between the pentacene active layer and the gold source and drain electrodes of the OFET. The inclusion of this radical polymer interlayer, which was deposited in through straightforward inkjet printing, led to bottom-contact, bottom-gate OFETs with significantly increased mobility and ON/OFF current ratios relative to OFETs without the PTMA interlayer. The underlying mechanism for this improvement in device performance is explained in terms of the charge transport capability at the organic-metal interface and with respect to the pentacene grain growth on the radical polymer. Thus, this effort presents a new, open-shell-based class of materials for interfacial modifying materials, and describes the underlying physics behind the practical operation of these materials.

(Poly(3,4-ethylenedioxythiophene):Polystyrene Sulfonate):Polytetrafluoroethylene for Use in High-Performance and Stable Bottom-Contact Organic Field-Effect Transistors

We successfully fabricated highly stable pentacene-based bottom-contact organic field-effect transistors (OFETs) with good charge injection properties at the electrode/organic semiconductor interface, obtained by optimizing the composition of solution-processed (poly(3,4-ethylenedioxythiophene):polystyrene sulfonate):polytetrafluoroethylene ((PEDOT:PSS):PTFE)-treated Au source/drain (S/D) electrodes. The (PEDOT:PSS):PTFE layer was deposited on the Au layer by spin-coating a mixture solution. The work function of the electrode increased from 4.84 to 5.21 eV as the PTFE concentration increased, accompanied by an interface dipole at the electrode surface. The optimized (PEDOT:PSS):PTFE (0.95:0.05)-treated electrodes significantly reduced the charge injection barrier at the electrode/semiconductor interface to achieve efficient charge transfer in the OFETs. Bottom-contact OFETs prepared with the optimized (PEDOT:PSS):PTFE-treated S/D electrodes had a field-effect mobility of 0.16 cm2/(V·s), which exceeded tha...

Low-temperature sol–gel processed AlOx gate dielectric buffer layer for improved performance in pentacene-based OFETs

An approach to achieve improved performance in pentacene-based organic field effect transistors (OFETs) using high-k AlOx prepared by a low temperature sol–gel technique as a thin buffer layer on a SiO2 gate dielectric was demonstrated.

Pentacene organic ferroelectric transistors with [P(VDF-TrFE)] gate by Langmuir-Blodgett process

We report the fabrication and electrical properties of pentacene-based ferroelectric organic field-effect transistors (FeOFETs) with ultrathin poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] gate insulators. A ultrathin and uniform P(VDF-TrFE) film was successfully deposited by Langmuir-Blodgett (LB) deposition with well-defined ferroelectric microdomains at the interface between P(VDF-TrFE) films and pentacene active layers. The P(VDF-TrFE) films derived by LB deposition significantly enhance the crystallization of the upper pentacene channel films and thus the performance of our FeOFETs. Our FeOFET device achieves a threshold voltage shift of 8.56 V induced by ferroelectric polarization under different voltage sweeping directions and such enhancement indicates a great potential for future organic nonvolatile memory applications.