Bottom-contact Organic Field-effect Transistors Research Articles

Patterned Graphene as Source/Drain Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

Patterned Graphene as Source/Drain Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

Influence of 2-mercapto-5-nitrobenzimidazole treatment on the electronic characteristics of bottom-contact organic field-effect transistors

The treatment of the Au electrodes of an organic bottom-contact thin film transistor with 2-mercapto-5-nitrobenzimidazole (MNB) led to improved current–voltage characteristics. X-ray diffraction, atomic force microscopy, soft X-ray emission spectroscopy, and ultraviolet photoelectron spectroscopy were used to investigate the structure-function relationship in the device components MNB and pentacene. By treating the Au contacts with MNB prior to the pentacene deposition, the morphological and structural properties of the pentacene come to resemble the ideal case of pentacene deposited onto SiO 2. The action of the MNB is therefore to remove the transition region at the Au/pentacene interface, leading to improved charge carrier injection.

Performance of poly(3-hexylthiophene) organic field-effect transistors on cross-linked poly(4-vinyl phenol) dielectric layer and solvent effects

Bottom-contact organic field-effect transistors (OFETs) were fabricated using a polymer gate insulator cross-linked poly(4-vinyl phenol) with regioregular poly(3-hexylthiophene) (RR-P3HT) as an active layer from different organic solvents. With this polymer dielectric, a field-effect mobility of 0.084±0.006cm2V−1s−1 was obtained. Solvents and interfacial properties have pronounced effects in determining the crystallinity and device performance of RR-P3HT on the polymer gate layer. Morphology correlation with the charge carrier mobility of RR-P3HT OFETs is investigated. Large nanoscale crystalline island densities of this polymer play an important role in the high charge carrier mobility of devices.

Effect of molybdenum electrode annealing on the interface properties between metal and pentacene

Contact resistance between molybdenum (Mo) electrode and pentacene was studied with transmission line method (TLM). The Mo electrodes were annealed at 200 °C, 400 °C, 600 °C and 800 °C for 1 h and pentacene layer of 300 Å thickness was vacuum deposited on patterned Mo to form Mo–pentacene contact. Current–voltage measurement for Mo–pentacene contact showed linear relationship and it was confirmed that ohmic contact was formed. XRD and AFM measurements showed that Mo could be crystallized at annealing temperatures above 600 °C. 800 °C annealed Mo showed larger grains and work function was increased from 4.60 eV to 4.80 eV due to the decrease in defect density. The contact resistance was reduced down to 11.2 MΩ cm from 37.8 MΩ cm of as-deposited Mo. Also the pentacene film deposited on annealed Mo was denser with better crystallinity. Bottom contact organic field-effect transistor made with 800 °C annealed Mo showed better performance than as deposited Mo.

Extended storage time of poly(3-hexylthiophene) field-effect transistors via immersion in common solvents

The influence of storage in three different commonly used solvents on the stability of bottom-contact organic field-effect transistors (OFETs) using poly(3-hexylthiophene)-2,5-diyl (P3HT) as the active layer was studied. It is demonstrated that the immersion in a nonpolar solvent, cyclohexane, caused the devices to degrade rapidly. In contrast, the immersion of the devices in two polar solvents, acetone and water, did not significantly affect the performance of these OFETs for periods as long as 20h. Due to the significantly extended device stability in comparison with that in ambient air, de-ionized water might be a convenient storage medium for P3HT OFETs during device fabrication if a temporary isolation from ambient atmosphere is required.

Solution-Processed n-Type Organic Field-Effect Transistors With High on /off Current Ratios Based on Fullerene Derivatives

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2-thienylethoxycarbonyl)-propyl)-{5}-1-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to <formula formulatype="inline"><tex>$\hbox{7.8}\times\hbox{10}^{-2}\ \hbox{cm}^{2}/\hbox{Vs}$</tex></formula> in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The <emphasis emphasistype="smcaps">on</emphasis>/<emphasis emphasistype="smcaps">off</emphasis> ratios reported in this letter, which are in the range of <formula formulatype="inline"><tex>$\hbox{10}^{5} -\hbox{10}^{6}$</tex> </formula>, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100–300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP. </para>

Influence of traps on top and bottom contact field-effect transistors based on modified poly(phenylene-vinylene)

We report on the investigations of the differences in the measured current characteristics of source/drain top contact (TOC) and bottom contact (BOC) organic field-effect transistors (OFETs). The active layer was made from poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and a highly doped silicon wafer with a thermal oxide was used for the gate and the insulator, respectively. Both transistors show a good subthreshold behavior characterized by a threshold voltage of Vth≈−5 V and an inverse subthreshold increase of S≈0.25 V∕dec. The estimated mobility is in the range of 10−3 cm2∕V s. The main difference between the TOC and the BOC transistors is a nonlinear increase of the drain current at small drain voltages in the output characteristics of the TOC OFETs. Numerical two-dimensional simulations show that a high concentration of donor-like traps with a Gaussian or exponential distribution are the reason for this peculiarity. The investigation of the influence of air confirms the presence of such traps.

New growth method of rubrene single crystal for organic field-effect transistor

A rubrene single crystal, for the active material of an organic field-effect transistor (OFET), was directly grown from a rubrene powder in the single growing zone of a furnace, which is different from a conventional organic vapor transport method for the crystal growth. The growth time of the needle-shaped rubrene single crystals was ∼30 min without any purification process. Structural properties and the geometrical shape of the rubrene single crystals were characterized through a single crystal X-ray diffractometer and a scanning electron microscope. The rubrene single crystal OFETs were fabricated through both the bottom and top contact methods. The Si 3N 4 and polymer were used for a gate insulator in the bottom and top contact OFETs, respectively. From the current–voltage characteristics of the OFETs, a typical p-type transistor nature was observed. The mobility of the top contact OFETs was measured to be ∼1.12 cm 2/V s, which was approximately seven times higher than that of the bottom contact OFETs, because of the direct contact of the rubrene single crystals with both Au electrode and polymer gate insulator.

High-Performance Low-Cost Organic Field-Effect Transistors with Chemically Modified Bottom Electrodes

The characteristics of organic field-effect transistors (OFETs) were dramatically improved by chemically modifying the surface of the bottom-contact Ag or Cu source-drain (D-S) electrodes with a simple solution method. The contact resistance and energetic mismatch typically observed with Ag D-S electrodes in pentacene bottom-contact OFETs can be properly eliminated when modified by the Ag-TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane). The pentacene transistors with low-cost Ag-TCNQ-modified Ag bottom-contact electrodes exhibit outstanding electrical properties, which are comparable with that of the Au top-contact devices. It thus provides a novel way toward high-performance low-cost bottom-contact OFETs.

Organic field-effect transistors with reduced contact resistance

In this letter, the authors report a reduction in the contact resistance between pentacene and Au source/drain electrodes of organic field-effect transistors (OFETs). By immersing the Au electrodes in a sulfuric acid and hydrogen peroxide mixture, the injection barrier between the Au electrodes and pentacene was lowered by approximately 0.2eV and the contact resistance significantly decreased. The fabricated bottom-contact OFETs revealed a field-effect mobility of more than 0.66cm2∕Vs at a channel length ranging from 3to30μm, which is comparable to that of top-contact OFETs with a 50μm channel length.

Influence of processing conditions on the stability of poly(3-hexylthiophene)-based field-effect transistors

Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene)-2,5-diyl were fabricated under different process conditions. The devices displayed drastic differences in their ambient-air stability. Whereas it took only about 10min in air for the off current to increase by one order of magnitude in OFETs prepared with chloroform and hexamethyldisilazane, a 120min exposure to air caused only a slight degradation of OFETs prepared using 1,2,4-trichlorobenzene, n-octadecyltrichlorosilane, and a heat treatment. The differences in the film surface morphology were analyzed and possible mechanisms for the enhanced stability are discussed.

High-mobility bottom-contact n-channel organic transistors and their use in complementary ring oscillators

The electrical characteristics of bottom-contact organic field-effect transistors fabricated with the air-stable n-type semiconductor N,N′-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) are described. The mobility, threshold voltage, subthreshold swing, and Ion∕Ioff ratio(VDS=40V, VG=0∼40V) are 0.14cm2∕Vs, 1.6V, 2.0V/decade, and 1.2×103, respectively. The effect of electrode/dielectric surface treatment on these devices is also examined, with a combination of 1-octadecanethiol and hexamethyldisilazane. Organic complementary five-stage ring oscillators were fabricated using pentacene and PDI-8CN2, and operated at an oscillation frequency of 34kHz and a propagation delay per stage of 3μs.

Improvement of Short Channel Mobility and Operational Stability of Pentacene Bottom-Contact Transistors with a Sulfuric Acid and Hydrogen Peroxide Mixture (SPM) Treatment of Au Electrodes

ABSTRACTWe report a reduction in the contact resistance between pentacene and Au source/drain electrodes of organic field effect transistors (OFETs) with bottom-contact structure. By immersing the Au electrodes in a sulfuric acid and hydrogen peroxide mixture (SPM), the injection barrier between the Au electrodes and pentacene was lowered by approximately 0.2 eV and the contact resistance significantly decreased. The fabricated bottom-contact OFETs revealed a field-effect mobility of more than 0.66 cm2/Vs at a channel length ranging from 3 to 30 μm, which is comparable to that of top-contact OFETs with a 50 μm channel length. The transfer characteristics of the OFET with the SPM treatment were stable even after 44days storage in air under room illumination without any passivation. Moreover, the drain current reduction due to threshold voltage (Vth) shift under continuous application of gate voltage quickly recovered toward the original value with unloading of gate voltage.

A comprehensive study of short channel effects in organic field-effect transistors

Abstract We present a comprehensive study of short channel effects in organic field-effect transistors by measuring the electrical characteristics of devices with fixed channel width and varying channel length. Our studies are conducted on a hole transport organic semiconductor, E,E-2,5-bis-{4′-bis-(4″-methoxy-phenyl)amino-styryl}-3,4-ethylenedioxy-thiophene, that is spin-coated from solution to form bottom contact organic field-effect transistors. Drain–source currents from transistors with a channel length of 50 μm show excellent agreement with the square law equations derived for crystalline Si MOSFETs in both the linear and saturation regimes. As the channel length is incrementally reduced to 1 μm, device characteristics such as saturation regime channel conductance, sub-threshold current and threshold voltage, behave in a manner similar to Si MOSFETs of decreasing channel length. Results of these studies indicate the presence of non-destructive current punch-through and in addition, behavior similar to channel-length modulation and threshold-voltage roll-off, neither of which have previously been reported in OFETs.

Organic field-effect transistors based on a crosslinkable polymer blend as the semiconducting layer

For fabrication of top-gate polymer-based organic field-effect transistors (OFETs), it is essential that the semiconducting layer remain intact during spin coating of the overlying dielectric layer. This requirement severely limits the applicable solvent and materials combinations. We show here that a crosslinkable polymer blend consisting of a p-type semiconducting polymer {e.g., TFB; poly[9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine]} and an electroactive crosslinkable silyl reagent {e.g., TPDSi2; 4,4′-bis[(p-trichloro-silylpropylphenyl)phenylamino]biphenyl} is effective as the semiconducting layer in a top-gate bottom-contact OFET device. The TFB+TPDSi2 semiconducting blend is prepared by spin-coating in ambient. The crosslinking process occurs during spin-coating in air and is completed by curing at 90 °C, which renders the resulting film insoluble in common organic solvents and allows subsequent deposition of dielectric layers from a wide range of organic solvents. We also show that the presence of TPDSi2 in the semiconductor layer significantly reduces typical TFB-source-drain threshold voltages in bottom-contact devices, likely due to favorable interfacial TPDSi2-gold electrode interactions.

Characterisation of Organic Field-Effect Transistor by Scanning Raman Spectroscopy

Organic molecular semiconductors, such as pentacene (C22H14), is of great interest for potential application in thin-film organic field-effect transistors (OFETs). Novel scanning Raman microscopy was applied to the analysis of OFETs. Effective channel sizes in top-contact and bottom-contact OFETs were determined from scanning Raman spectra. The bottom-contact OFET showed a sharp change in Raman intensity at the gold/pentacene interface over a short distance while the top-contact OFET showed a gradual change in Raman intensity over a wide region. In the case of a gold electrode deposited on pentacene, the real channel size could be as small as half of the intended size. This resulted in the decrease in effective channel size and thus in the overestimation of the calculated mobility. The small channel length could be attributed to the presence of gold (composite) inside the channel in the top-contact OFET.

Radiotracer Diffusion Measurements of Noble Metal Atoms in Semiconducting Organic Films.

AbstractThe application of organic field effect transistors (OFETs) for large scale low-cost electronic devices has lead to intense research. Diindenoperylene (DIP) thin films on SiO2 are a prominent system due to their high structural out-of-plane order. While bottom contact OFET structures can be realized easily, preparation of top contacts might cause diffusion of metal atoms (typically Ag or Au) deep into the organic film changing the injection properties at the interface. These properties are of great importance for device fabrication. Therefore, only by understanding the diffusion behaviour of metals into the organic layer, formation of well defined interfaces and control of their properties will become possible. For a better understanding of the diffusion of noble metal atoms into crystalline organic films, a radiotracer technique has been used to obtain diffusion profiles for Ag and Au diffusion in crystalline DIP films. For Ag diffusion in DIP, the decrease in Ag concentration of four orders of magnitude within the first few nanometers indicates that most of the metal atoms remain near the surface while small amounts can penetrate deep into the thin film and can even accumulate at the interface between organic film and the silicon substrate. A comparison with diffusion profiles obtained for polymers indicates that the interplay between diffusion and immobilization by aggregation also determine the diffusion behaviour of metals in organic crystalline materials. Latest experiments support this interpretation of the diffusion profiles. Single atoms are highly mobile in the organic crystalline material due to the weak interaction between the metal and the organic material. Therefore, most of the single atoms that penetrate into the material do so during the initial phase of the deposition. When more and more atoms arrive at the surface, cluster formation sets in. Due to the high cohesive energy of the metal the atoms can not leave the cluster and become immobilized. After deposition of a closed surface layer no further metal diffusion should be observed. With the knowledge about the diffusion processes gained by the radiotracer measurements control of process parameters and development of barrier layers in sub-monolayer range should be possible.

High ON/OFF ratio and stability of amorphous organic field-effect transistors based on spiro-linked compounds

We report high ON/OFF ratios for organic field-effect transistors (OFETs) based on amorphous thin films of spiro-linked compounds. Bottom-contact OFET structures are fabricated, using 2,2′,7,7′-tetra-( m-tolyl-phenylamino)-9,9′-spirobifluorene (Spiro-TPD) and 2,2′,7,7′-tetrakis-(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD) as active materials. The field-effect mobility of holes in Spiro-TPD and Spiro-TAD thin films is 7 × 10 −5 cm 2/Vs. We obtained ON/OFF ratios up to 3.6 × 10 6 with low OFF currents in the pA range. Time-dependent measurements show that the transistor characteristics do not change significantly over 9 months.

Influence of distributed trap states on the characteristics of top and bottom contact organic field-effect transistors

Numerical simulations of organic field-effect transistors (OFET) of bottom and top contact (BOC, TOC) design with different source/drain contacts were carried out considering an exponential distribution of trap states in the gap of the active layer (a-Si model). For ohmic contacts, the current-voltage characteristics are similar to the trap-free case and there is not much difference between the two designs. However, the currents are lower due to immobile trapped charges, the threshold voltage is shifted, and the inverse subthreshold slope increases due to trap recharging. An analytical approximation for the effective mobility deviates from the simulation up to 20%. For low source/drain work function, there occur particular dependencies of the current on the gate voltage for the two designs, which are explained with the internal concentration and field profiles. A series resistance between source and channel causes in the TOC structure an abrupt transition from the gate voltage independent active region into saturation. In the BOC case, the reverse-biased Schottky-type source contact dominates the current. Through simulation of measured characteristics of prepared OFETs based on a modified poly-(phenylene-vinylene), the observed hysteresis is analyzed.

Fabrication and analysis of polymer field-effect transistors

Parameters of organic field-effect transistors (OFET) achieved in recent years are promising enough for R & D activities towards a commercial low-cost polymer electronics. In spite of the fast progress, preparations dominated by trial and error are concentrated essentially on higher mobility polymers and shorter channel patterning, and the analysis of measured data is based on oversimplified models. Here ways to professionalize the research on polymer field-effect transistors are discussed exploiting experience accumulated in microelectronics. First of all, designing the devices before fabricating and subsequently analyzing them requires appropriate modelling. Almost independently from the nature of the transport process, the device physics is basically described by the drift-diffusion model, combined with non-degenerate carrier statistics. Therefore, with a modified interpretation of the so-called effective density of states, existing simulation tools can be applied, except for special cases which are discussed. Analytical estimates are helpful already in designing devices, and applied to experimental data they yield input parameters for the numerical simulations. Preparations of OFET's and capacitors with poly(3-ocylthiophene) (P3OT), poly(3-dodecylthiophene) P3HT, Arylamino-poly-(phenylene-vinylene) (PPV), poly(2-methoxy, 5 ethyl (2′ hexyloxy) paraphenylenevinylene) MEH-PPV, and pentacene from a soluble precursor are described, with silicon dioxide (SiO2) or poly(4-vinylphenol) (P4VP) as gate insulator, and with rather different channel length. We demonstrate the advantage of combining all steps from design/fabrication to analysis of the experimental data with analytical estimates and numerical simulation. Of special importance is the connection between mobility, transistor channel length, cut-off frequency and operation voltage, which was the starting point for the development of a low-cost fabrication of high-performance submicrometer OFET's by an underetching technique. Finally results of simulation studies are presented concerning the formation of inversion layers, the influence of a trap distribution (as in the a-Si model) and of different types of source/drain contacts on top and bottom contact OFET's, and short-channel effects in submicrometer devices. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)