Proceedings Volume 6658

Organic Field-Effect Transistors VI

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Proceedings Volume 6658

Organic Field-Effect Transistors VI

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Volume Details

Date Published: 13 September 2007
Contents: 10 Sessions, 28 Papers, 0 Presentations
Conference: Photonic Devices + Applications 2007
Volume Number: 6658

Table of Contents

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Table of Contents

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  • Front Matter: Volume 6658
  • Circuits and Displays
  • Gate Dielectrics and Contacts
  • Charge Transport
  • Organic Crystals
  • Molecular Design
  • OTFTs I
  • OTFTs II
  • Novel Processing of Soluble Organic Semiconductors
  • Poster Session
Front Matter: Volume 6658
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Front Matter: Volume 6658
This PDF file contains the front matter associated with SPIE Proceedings Volume 6658, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Circuits and Displays
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Capacitance-voltage characteristics of organic thin-film transistors
We have fabricated pentacene-based thin film field-effect transistors and analyzed quasi-static current and capacitance measurements as a function of gate bias. The latter provides an independent and accurate estimation of the threshold voltage, an important device parameter that cannot be extracted unambiguously from the I-V measurements. The C-V characteristics of the transistors were furthermore characterized using impedance spectroscopy as a function of frequency and gate bias for the zero drain bias case. We model the impedance data with a simple transmission line equivalent circuit and find that the frequency dependence of the capacitance and phase can be described adequately from the channel conductance, as determined from quasi static current-voltage measurements, and the geometrical values of the channel and source/drain to gate electrode overlap capacitances.
Organic thin-film transistor arrays for active-matrix organic light emitting diode
Sangyoon Lee, Hyunsik Moon, Do-Hwan Kim, et al.
We developed an active matrix organic light-emitting diodes (AMOLEDs) on a glass using two organic thin-film transistors (OTFTs) and a capacitor in a pixel. OTFTs switching-arrays with 64 scan lines and 64 (RGB) data lines were designed and fabricated to drive OLED arrays. In this study, OTFT devices have bottom contact structures with an ink-jet printed polymer semiconductor and an organic insulator as a gate dielectric. The width and length of the switching OTFT is 500μm and 10μm, respectively and the driving OTFT has 900μm channel width with the same channel length. The characteristics of the OTFTs were examined using test cells around display area. On/off ratio, mobility, on-current of switching OTFT and on-current of driving OTFT were 106, 0.1 cm2/V-sec, order of 8μA and over 70 μA respectively. These properties were enough to drive the AMOLEDs over 60 Hz frame rate. AMOLEDs composed of the OTFT switching arrays and OLEDs made by deposition of small molecule materials were fabricated and driven to make moving images, successfully.
13.56 MHz polymer rectifier by printing processes
Chang-Yu Lin, Chia-Hung Chou, Jack Hou, et al.
Printed electronics have attracted increasing attentions in recent years due to its flexible format, easy process and potentially low cost features. Achieving good performance organic Schottky diode as the key component of a rectifier for printed RFID applications became a critical task. The electrical performances of an organic Schottky diode electrical can be improved by inserting an intermediate layer between the semiconductor and electrode surfaces. This interface plays an important role in improving rectification ratio and lifetime of a polymer diode. In this work, a 13.56MHz rectifier based on a polymer Schottky diode will be demonstrated and a wireless energy transmission built on flexible substrates will be proposed.
Gate Dielectrics and Contacts
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High performance organic field-effect transistors with fluoropolymer gate dielectric
Wolfgang L. Kalb, Thomas Mathis, Simon Haas, et al.
Electrical stability is essential for a successful commercialization of organic semiconductor devices. We report on organic field-effect transistors with unprecedented electrical stability. The single crystal and thin-film transistors employ a fluorocarbon polymer as gate dielectric (CytopTM) and pentacene or rubrene as the organic semiconductor. CytopTM (Cyclic Transparent Optical Polymer) is easy to be used and can be deposited in air from solution. It is highly hydrophobic and has a very low permittivity of ∈i = 2.1 - 2.2. Moreover, the material is a good electrical insulator with a very high dielectric breakdown field. Its passive surface leads to extremely stable field-effect transistors with a high field-effect mobility, an outstanding subthreshold swing as low as 0.75 nFV/(decade cm2) and a near zero onset voltage. Of particular significance is the resistance of the devices against long-term gate bias stress. Oligomeric organic semiconductors can have a very high electrical stability when combined with a suitable gate dielectric. CytopTM is an ideal gate dielectric for organic electronics and it seems very likely that the material leads to outstanding transistors in combination with many small molecule organic semiconductors.
Charge Transport
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Determination of the density of trap states in organic thin film transistors
Polymer thin-film transistors based on poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) have been studied in the temperature range from 130 to 300K. In this temperature range both the field effect mobility and the drain current show thermally activated behaviour. The channel current at low gate voltage ( |Vgs|<|Vth| ) can be fitted with a power function of gate voltage. We deduce the presence of an exponential distribution of localized states above the dominant transport level from the gate bias and temperature dependence of the drain current.
Charge transport mechanisms in organic and microcrystalline silicon field-effect transistors
S. J. Konezny, M. N. Bussac, A. Geiser, et al.
Several organic and inorganic materials have emerged as promising candidates for the active layer of field-effect transistors (FETs) fabricated on flexible substrates. The charge transport models necessary for device optimization in these systems are at different stages of development. The understanding of charge transport in single-crystal and thin-film FETs based on organic materials such as pentacene, rubrene, and other related compounds has advanced considerably in recent years and a clear picture of the relevant transport mechanisms is forming. In contrast, the theoretical description of transport in hydrogenated microcrystalline silicon (μc-Si:H) is not as well known and the published results and theories are often contradictory. We review the paradigms we feel are useful in describing the current understanding of transport in organic and μc-Si:H field-effect transistors. In the case of organic materials these include the polarization and transfer integral fluctuation model [A. Troisi and G. Orlandi, Phys. Rev. Lett. 96, 086601 (2006), J.-D. Picon et al., Phys. Rev. B 75, 235106 (2007)], the Frölich polaron model [I.N. Hulea et al., Nat. Mater. 5, 982 (2006), H. Houilli et al., J. Appl. Phys. 100, 033702 (2006)], and several trapping models [M.E. Gershenson et al., Rev. Mod. Phys. 78, 973 (2006), V. Podzorov et al., Phys Rev. Lett. 95, 226601 (2005)]. Given the heterogeneous composition and structure of microcrystalline silicon thin films, a variety of theories to describe dark conductivity have been applied to μc-Si:H including those based on percolation theory [H. Overhof et al., J. Non-Cryst. Solids 227-230, 992 (1998)], hopping models [A. Dussan and R. H. Buitrago, J. Appl. Phys. 97, 043711 (2005)], thermionic emission, and tunneling. We give a brief overview of these models and present a fluctuation-induced tunneling model that we are developing to describe charge transport in microcrystalline silicon.
Current conduction in ambipolar organic field-effect transistors (OFETs)
Current conduction in organic field-effect transistors (OFETs) has attracted much attention. The most intriguing issue is that for most organic semiconductors only one carrier type is observed. To examine why this is so, effort has been spent to study ambipolar conduction in OFETs under strong gate bias. For the rubrene OFETs analyzed in this work, there is evidence that p-channel conduction, which occurs at low positive gate bias, is closely associated with negative charge states present at the insulator-semiconductor interface. Because of the somewhat small insulator capacitance only a fraction of the applied gate voltage drops across the OFET channel making it very difficult to achieve n-channel conduction at low positive gate voltage. Furthermore, electron conductivity is usually low due to the smaller density of states in the LUMO. As computed, the negative charge states appear to have peak energy at 0.8 eV above the HOMO and it is tempting to associate them with "polaron" states found in the insulator.
Organic Crystals
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Modification of charge transport in single crystal rubrene
Oleg Mitrofanov, D. V. Lang, T. Siegrist, et al.
To investigate the impact of impurities on properties of single crystals of rubrene (C48H24), we consider structural, charge transport and optical properties. We show that transport properties are strongly influenced by the presence of a bandgap acceptor state, commonly occurring in crystals of rubrene. The bandgap state is likely caused by incorporation of an oxygen-related impurity. We demonstrate that the impurity presence can be detected using optical spectroscopy. The impurity model explains commonly-observed variations of rubrene properties. The acceptor-like characteristics of the impurity suggests a possible approach for modification of the charge transport in molecular crystals.
Molecular Design
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Crystal design for organic semiconductors: the effects of substitution on crystal packing
John E. Anthony, Balaji Purushothaman
Electronic devices based on organic materials have been an area of intense research for the past several years. Pentacene, an oligoacene, is regarded as a benchmark due to its high charge carrier mobility. However it has poor solubilty, unoptimized π-overlap and is highly unstable. We have successfully demonstrated that peri-functionalization with trialkylsilyl ethynyl groups enhances the π-stacking, solubility and stability of pentacene and related compounds. TIPS pentacene exhibits both two dimensional (2D) π-stacking and good solubilty in common organic solvents. However changing the solubilizing groups or functionalizing the acene core results in a disruption of the desired 2D π-stacking. Molecules with one dimensional (1D) π-stacking have poorer mobility than those with 2D π-stacking and hence larger solubilizing groups are required to change the π-stacking to 2D. The emphasis of this paper is to show how small changes in substitution can disrupt π-stacking in these molecules and how one can achieve the desired π-stacking by careful selection of solubilzing groups.
High performance n-type FETs based on heterocyclic ring systems with trifluoromethylphenyl groups
Yoshiro Yamashita, Satoshi Shimono, Takahiro Kono, et al.
High performance n-type FETs have been accomplished by using novel heterocyclic systems with trifluoromethylphenyl groups. To enhance intermolecular interactions, selenophene rings were introduced. Some FET devices showed higher electron mobilities than 0.1 cm2V-1s-1. The mobilities of the selenophene-containing materials were higher than those of the corresponding thiophene analogues. The relationship between the structures and FET characteristics have been investigated. The threshold voltages were reduced by introducing heterocyclic units with higher electron affinity.
OTFTs I
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Microstructural mobility of the polymeric gate insulator affecting pentacene charge transport
Antonio Facchetti, Choongik Kim, Tobin J. Marks
Organic semiconductor-dielectric interfacial characteristics play a critical role in influencing organic thin-film transistor (OTFT) performance characteristics. In this study, we report new insights into how the bulk/surface physicochemical characteristics of the gate insulator modulate the thin-film growth mode, microstructure, and OTFT performance parameters of pentacene films deposited on bilayer polymer (top)-SiO2 (bottom) gate insulators. The results demonstrate that the pentacene growth mode varies substantially with the dielectric/substrate, and correlations are established between pentacene film growth mode, the thin-film to bulk microstructural phase transition, and OTFT device performance. Furthermore, we demonstrate here for the first time the key influence of the polymeric insulator layer microstructural mobility on pentacene film growth mode and OTFT response.
Organic phototransistor behavior and light-accelerated bias stress
Maarten Debucquoy, Stijn Verlaak, Soeren Stoedel, et al.
We studied pentacene thin film field-effect transistors to characterize their behavior as organic phototransistors. The shift in turn-on voltage (Von), responsible for the high sensitivity of these devices to illumination, is proved to be dependent on the illumination time and applied gate voltage during illumination, a relationship which was, until now, completely neglected in the description of these devices. Moreover, we show this behavior to be similar to the shift in Von during bias stress experiments in the dark and both processes can be described with the same Von vs time relationship, already previously reported for dark bias stress experiments on organic transistors. By comparing these characteristics in devices with a different treatment of the gate dielectric, trapping of electrons by OH-groups at the gate dielectric/organic semiconductor interface is indicated as a main origin for these shifts in Von. In this way we do not only reduce organic phototransistors behavior to light-accelerated bias stress in unstable thin film transistors, but also pin-point one major cause of organic transistor instability.
High mobility solution-processed n-channel organic thin film transistors
He Yan, Shaofeng Lu, Yan Zheng, et al.
N-channel organic thin-film transistors (OTFTs) based on N,N'-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10- bis(dicarboximide) (PDI8-CN2) were fabricated using different semiconductor film deposition methods, dielectric materials, and device structures. It was found that top -contact OTFTs fabricated on Si-SiO2 substrates with drop-cast or vapor deposited films afford comparable electron mobilities (0.01-0.1 cm2/Vs), much larger than those based on spin-coated PDI8-CN2 films (0.001 cm2/Vs). Furthermore, n-channel top-contact TFTs were fabricated using solution-processed PDI8-CN2 films and a UV-curable solution-processed polymeric dielectric. These devices exhibit typical gate leakage currents < 1nA for Vgate > 100V, which are negligible compared to the corresponding source/drain currents (> 0.1mA). OTFTs tested in ambient exhibit electron mobilities as high as 0.05-0.2 cm2/Vs and Ion:Ioff ~ 105. Furthermore, Isource-drain-Vgate hysterisis is negligible when the OTFTs were tested in both bias directions at different Vgate scan rates, demonstrating excellent insulator-semiconductor interfacial properties. Bottom-contact TFTs exhibit typical lower performance (~ ×0.1)compared to the top-contact structure. All of the devices stored in air for several months exhibit no degradation of the device characteristics.
High performance organic field-effect transistors using high-Κ dielectrics grown by atomic layer deposition (ALD)
Xiao-Hong Zhang, Benoit Domercq, Xudong Wang, et al.
We report on high performance field-effect transistors fabricated with pentacene as an active material and Al2O3 as a gate dielectric material grown by atomic layer deposition (ALD). These transistors were operated in enhancement mode with a zero turn-on voltage and exhibited a low threshold voltage (< -10 V) as well as a low subthreshold slope (< 1 V/decade) and an on/off current ratio larger than 106. Hole mobility values of 1.5 ± 0.2 cm2/Vs were obtained when using heavily n-doped silicon (n+-Si) as gate electrodes and substrates. Atomic force microscopy (AFM) images of pentacene films on Al2O3 treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray diffraction patterns showed characteristics of a "thin film" phase. Compared with thermally-grown SiO2, Al2O3 gate insulators have lower surface trap density and higher capacitance density, to which the high performance of pentacene field-effect transistors can be attributed.
OTFTs II
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Distinguishing between nonlinear channel transport and contact effects in organic FETs
B. H. Hamadani, J. L. LeBoeuf, R. J. Kline, et al.
We investigate charge injection and transport in organic field-effect transistors fabricated by using poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT-C14) or poly(3-hexylthiophene) (P3HT) as the active polymer layer. We show that in high mobility devices where the channel resistances are low compared to the contact resistances, the device performance can be dominated by the metal/organic semiconductor (OSC) contacts. However, in sets of devices where the channel resistance is dominant over the contacts (usually the lower mobility devices), we see pronounced field dependence in the saturation regime mobilities consistent with a Poole-Frenkel model of charge transport within the channel. The field-dependent mobility in short-channel devices produces nonlinear output current-voltage characteristics which can be modeled consistently in the Poole-Frenkel framework.
Dielectric interface modification by UV irradiation: a novel method to control OFET charge carrier transport properties
Niels Benson, Martin Schidleja, Christopher Siol, et al.
The charge carrier transport in organic field effect transistors (OFETs) is determined by the transport properties of the insulator / organic semiconductor interface. We demonstrate that an adequate treatment of this interface results in a polarity change of the OFET charge carrier transport properties, without further altering the device structure. Illuminating the utilized PMMA polymer dielectric, by using UV radiation, leads to the introduction of mainly electron traps at the dielectric interface. This results in the suppression of the electron transport for an otherwise n-type pentacene OFET. However, as a consequence of trapped electrons in the near surface layer of the PMMA dielectric, the hole transport of the device is enabled though a hole blocking source/drain metallization. This effect, as well as the impact of the UV irradiation on the PMMA dielectric will be discussed in detail. The UV treatment yields a PMMA interface rich on polar groups. The influence of these groups on the OFET characteristics is investigated by studying several polymer dielectrics with varying content of the emerging groups.
Transfer printing as a method for fabricating hybrid devices on flexible substrates
D. R. Hines, A. E. Southard, A. Tunnell, et al.
Printing methods are becoming important in the fabrication of flexible electronics. A transfer printing method has been developed for the fabrication of organic thin-film transistors (OTFT), capacitors, resistors and inductors onto plastic substrates. The method relies primarily on differential adhesion for the transfer of a printable layer from a transfer substrate to a device substrate. A range of materials applications is illustrated, including metals, organic semiconductors, organic dielectrics, nanotube and nanowire mats, a patterned inorganic semiconductor and graphene. Transfer printing can be used to create complex structures including many disparate materials sequentially printed onto the flexible substrate, with no mixed processing steps performed on the device substrate. Specifically, the fabrication and performance of model OTFT devices consisting of a polyethylene terephthalate (PET) substrate, gold (Au) gate and source/drain electrodes, a poly(methyl methacrylate) (PMMA) dielectric layer and either a pentacene (Pn) or a poly(3- hexylthiophene) (P3HT) organic semiconductor layer will be presented. These transfer printed OTFTs on plastic outperform non-printed devices on a Si substrate with a SiO2 dielectric layer (SiO2/Si). Transfer printed Pn OTFTs on a plastic substrate have exhibited mobilities of 0.237 cm2/Vs, compared to non-printed Pn OTFTs on a SiO2/Si substrate with mobilities of 0.1 cm2/Vs. Transfer printed P3HT TFTs on a plastic substrate have exhibited mobilites of 0.04 cm2/Vs, compared to non-printed P3HT TFTs on a SiO2/Si substrate with mobilities of 0.007 cm2/Vs.
Novel Processing of Soluble Organic Semiconductors
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Record high mobilities for regioregular poly(3-octylthiophene)
Geneviève Sauvé, Junying Liu, Rui Zhang, et al.
Regioregular poly(3-alkylthiophene)s are promising candidates for plastic electronics. In particular, poly(3- hexylthiophene) (P3HT) has attracted considerable interest due to its excellent field effect mobility. However, not much attention has been given to poly(3-alkylthiophene)s with longer alkyl side chains, mainly because a few studies reported that longer side chain length was detrimental to field effect mobility. However, these past studies used untreated SiO2 as the gate dielectric and commercially available polymers synthesized by the Rieke method. Here, all polymers tested were synthesized in house using our quasi-living GRIM method to yield well-defined clean polymers. For our first study, we synthesized P3HT and regioregular poly(3-octylthiohene) (P3OT), both with a high molecular weight (Mn of 40kDa and 35kDa, respectively). Regioregular P3HT required some sonication to dissolve in chloroform, whereas P3OT dissolved readily in chloroform at room temperature. Hole mobility was measured using bottom-contact geometry, with SiO2 as gate dielectric. The SiO2 surface was either untreated, or chemically treated with octyltrichlorosilane. Average mobility on untreated SiO2 was 0.09 cm2/Vs for P3HT and 0.03 cm2/Vs for P3OT. Average mobility on OTS-8 treated SiO2, on the other hand, was 0.13 cm2/Vs for P3HT and 0.19 cm2/Vs for P3OT. The OTS-8 treatment therefore dramatically improved the mobility of P3OT. Both polymers had a maximum mobility of about 0.2 cm2/Vs for channel lengths ≥10 μm, and a mobility of 0.22 cm2/Vs was obtained several times for P3OT. To our knowledge, this is by far the highest mobility reported for P3OT.
Ink-jet printing of self-aligned soluble-pentacene crystals for high-performance organic field-effect transistors
Jung Ah Lim, Wi Hyoung Lee, Yeong Don Park, et al.
We have reported the fabrication of the self-organized 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) crystals with highly ordered molecular structures by using evaporation-induced flows in a solution process. The one-dimensional microcrystal arrays of TIPS_PEN were fabricated by simple solution casting on a tilted substrate. By pinning a solution droplet on the tilted substrate, an array of ribbon-shaped crystals aligned in the tilted direction was formed on the substrate. In particular, self-aligned TIPS_PEN crystals with highly ordered crystalline structures via inkjet printing were successfully produced by controlling the evaporation-induced flow using solvent mixture, and arise when there is a recirculation flow in a inkjet printed droplet that is induced by a Marangoni flow (surface-tension-driven flow) in the direction opposite to the outward convective flow. The field-effect transistors fabricated with these self-aligned TIPS_PEN crystals via drop casting and inkjet printing exhibit significantly improved electrical performance. These results demonstrate that control of evaporation-induced flow in a solution process of organic semiconductor can be an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct-write fabrication of high-performance OFETs.
Solubility- and temperature-driven thin film structures of polymeric thiophene derivatives for high performance OFET applications
It has been shown that high charge mobility in solution-processible organic semiconductor-based field effect transistors is due in part to a highly parallel π-π stacking plane orientation of the semiconductors with respect to gate-dielectric. Fast solvent evaporation methods, generally, exacerbate kinetically random crystal orientations in the films deposited, specifically, from good solvents. We have investigated solubility-driven thin film structures of thiophene derivative polymers via spin- and drop-casting with volatile solvents of a low boiling point. Among volatile solvents examined, marginal solvents, which have temperature-dependent solubility for the semiconductors (e.g. methylene chloride for regioregular poly(3-alkylthiophene)s), can be used to direct the favorable crystal orientation regardless of solvent drying time, when the temperature of gate-dielectrics is held to relatively cooler than the warm solution. Grazing-incidence X-ray diffraction and atomic force microscopy strongly support that significant control of crystal orientation and mesoscale morphology using a "cold" substrate holds true for both drop and spin casting. The effects of physiochemical post-modificaiton on film crystal structures and morphologies of poly(9,9-dioctylfluorene-co-bithiophene) have also been investigated.
Performance improvement and evaluation of an all plastic organic field effect transistor
Shizuyasu Ochiai, Xin Wang, Narayana Perumal Rajesh, et al.
We have fabricated organic thin film transistors with a polyehylenenaphthalate (PEN) film as the substrate, a poly (3- hexylthiophene) (P3HT) thin film as the semiconductor layer and a cross-linked poly-4-vinylphenol (PVP) thin film as the gate dielectric layer. The performance of the P3HT-FET, fabricated using the spin-coated P3HT thin film, is as follows. The mobility is 4.0×10-4cm2/Vs and the threshold voltage is -13V. On the other hand, P3HT-FET fabricated using the drop-cast thin film is as follows. The mobility is 2.0×10-2cm2/Vs and the threshold voltage is -2V. Furthermore, the performances of the top and bottom contact P3HT-FETs were also evaluated as part of investigations into the charge injection from electrodes to the P3HT thin film and the interface traps between the P3HT thin film and electrodes. In terms of the performance of top contact P3HT-FET, the mobility is 7.0×10-2 cm2/Vs and the threshold voltage is 5V, which are values far superior to those of the bottom contact P3HT-FET. This indicates that in the top contact P3HT-FET, the contact resistance of the interface between the P3HT thin film and electrode is lower than that of the bottom contact P3HT-FET.
Poster Session
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Synthesis of bis-silylated oligothiophenes for solution-processable organic field effect transistors
Jung Hei Choi, Dae Won Cho, Sung-Ho Jin, et al.
A series of bis-dimethyl-n-octylsilyl end-capped oligothiophenes consisting of two to six thiophene units have been synthesized using palladium-catalyzed Stille coupling reactions. These oligothiophenes have been characterized by 1H-, 13C-NMR and high resolution mass spectrometry. The UV-vis spectral data indicate that these silyl end-capped oligothiophenes have longer conjugation lengths as evidenced by the higher λmax values than the corresponding unsubstituted thiophene oligomers. The thermal analyses indicate that the bis-silylated oligothiphenes show lower melting point (DSi-4T = 80°C; DSi-5T = 115°C; DSi-6T = 182°C) than the corresponding dialkylated thiophene oligomers by 100°C and hexamer DSi-6T exhibits a liquid crystalline mesophase at 143°C. The α,ω-bis(dimethyl-n-octylsilyl)oligothiophenes have a remarkably high solubility in chloroform which are comparable to the corresponding α,ω-dihexyloligothiophenes. The remarkably increased solubility by these silyl end groups leads the bis-silylated oligothiophenes to be applicable to solution processable devices for thin film transisitor (TFT) by utilizing a spin coating technique. We fabricated devices by spin-coating α,ω-bis(dimethyloctylsilyl)sexithiophene DSi-6T from chlorobenzene, toluene, chloroform onto device test structures. But the OFETs using DSi-6T solved in chloroform produced the transistor behavior. The hexamer DSi-6T, showed promising behavior as a solution-deposited semiconductor, with mobility up to 0.014 cm2V-1s-1.
A novel structure of directly patterned isolating layer for organic thin-film transistor-driven organic light emitting diodes
Yi-Kai Wang, Tsung-Hsien Lin, Jing-Yi Yan, et al.
Mono-chrome phosphorescence Organic light emitting diodes (OLEDs) operated by organic thin-film transistors (OTFTs) with a 32×32 array are fabricated with a novel method, and the results reveal a fabulous demonstration. The later isolation, which segregated source/drain electrodes and an OLED cathode, was designed in our OTFT-OLED pixel. In the OTFT-OLED process; we used the polymer isolating layer which was deposited by spin coating and patterned by traditional photo-lithography before the organic semiconductor and OLED deposition. However, the residue polymer affect of OTFT electric properties which have poor mobility (5×10-4 cm2/V-s), a lower on/off ratio (~103), and a positive threshold voltage (4.5 V), and devices, have poor uniformity. Using UV-Ozone treatment could enhance OTFT mobility (2×10-2 cm2/V-s) and permit higher devices uniformity, but the threshold voltage would still have a positive 5.1 V. This threshold voltage was not a good operation mode for display application because this operation voltage was not fit for our driving systems. In order to overcome this problem, a new structure of OTFT-OLED pixel was designed and combined with a new-material isolating layer process. This new process could fabricate an OTFT-OLED array successfully and have a nice uniformity. After the isolating layer process, OTFT devices have a higher mobility (0.1×10-2 cm2/V-s), a higher on-off ratio (~107) a lower threshold voltage (-9.7 V), and a higher devices uniformity.
Organic field-effect transistors with solution-processible thiophene/phenylene based-oligomer derivative films
Solution-processible thiophene/phenylene -based oligomer derivatives with different end substituents are presented as p-type semiconducting materials in OFETs. These films were deposited on OTS-treated SiO2/Si or polymeric bivinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB)/ITO/quartz substrates, via drop-casting and spin-casting. Synchrotron-based grazing-incidence X-ray diffraction and atomic force microscopy reveals that both drop- and spin-cast films have highly crystalline structures with edge-on molecules and parallel π-π stacking conjugated planes with respect to the substrate. In particular, temperature-dependent solubility of these materials can give a strategy for highly ordered crystalline structure in spin-cast films grown on cooler substrates, when compared to warmed solutions. Field-effect mobilities of these spin-cast films in a top-contacted electrode OFETs with BCB dielectrics are reached as high as ~0.01 cm2/Vs.
Understanding the mechanisms of photodecarbonylation of the photoprecursors of higher poly(acene)s
Rajib Mondal, Albert N. Okhrimenko, Bipin K. Shah, et al.
Polyacenes, linear poly(benzenoid) hydrocarbons, are significant compounds for various electronic applications. Smaller lower molecular weight members of the series (anthracene, tetracene, and pentacene) have been extensively investigated for their use as semiconductors, as active layers in organic field-effect transistors (OFETs) and in organic light-emitting diodes (OLEDs). Smaller HOMO-LUMO gaps might be achieved by synthesizing even higher poly(acene)s such as hexacene and heptacene. Nevertheless, an increased instability and insolubility associated with the increasing number of fused rings has become a synthetic challenge for chemists. We have recently developed a photochemical route that allows one to easily synthesize heptacene in rigid media from an α-diketone photoprecursor, thus opening a pathway to utilize these compounds in the construction of suitable devices. Photodecarbonylations of α-diketones are unusual reactions. In most instances, dione excited states become electron transfer acceptors. The decarbonylation is driven by the stability of the resulting aromatic hydrocarbon. We have studied the nature of the excited states involved in photodecarbonylation of the α-diketones and the structure of the carbon monoxide fragment(s). Results obtained on the mechanism of photodecarbonylation investigated using nanosecond flash photolysis and ultra-fast pump-probe techniques are discussed.
Highly sensitive thin film polymer phototransistors
Xuhua Wang, Kamol Wasapinyokul, Wei De Tan, et al.
We report solution processed highly photosensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-cobithiophene) (F8T2) as an active photoconducting material. Bottom gate contact coplanar device structure on Si wafer transistors was used. The photosensitivity of the drain photocurrent was investigated for different F8T2 annealing temperatures and illumination irradiances. Transistors annealed at 280oC show the highest drain current, approximately 8 times higher than the as-spincoated device at room temperature with a gate voltage of -40V. However, the field effect mobilities in the saturation regime for all devices at different annealing temperatures are in the same order of ~10-4 cm2/Vs. The field effect mobilities of the transistors were not affected by illumination, but the drain photocurrent of the transistor was significantly increased and the threshold voltage was shifted towards zero bias voltage when the polymer absorbs photons. The measured maximum responsivity was ~18.5 A/W for an LED light source with a peak wavelength of 465 nm and 19 nm bandwidth at 5 μW/cm2 light intensity. This is so far the highest reported for F8T2 phototransistors. The characteristics of transistors dominated by the photoconductive effect (turn-off) as well as the photovoltaic effect (turn-on) against a wide range of illumination intensities are reported.
Organic thin-film transistors using suspended source/drain electrode structure
Yong Uk Lee, Yong-Hoon Kim, Jeong-In Han, et al.
The electrical properties of triisopropylsilyl (TIPS) pentacene organic thin-film transistors (OTFTs) such as field-effect mobility, on/off ratio, threshold voltage and subthreshold slope were markedly improved by employing suspended source/drain (SSD) electrode structure. The SSD structure was fabricated by using Cr/Au double layer where Cr was used as a sacrificial layer. Using the SSD structure, the field-effect mobility in the linear region increases from 0.007 cm2/Vs to 0.29 cm2/Vs, on/off ratio from 104 to 107, threshold voltage decreases from +9 V to -3 V and subthreshold slope decreases from 4.5 V/decade to 0.6 V/decade.