APPLIED PHYSICS LETTERS 91, 063514 共2007兲
`
`High mobility solution processed 6,13-bis„triisopropyl-silylethynyl…
`pentacene organic thin film transistors
`Sung Kyu Parka兲 and Thomas N. Jackson
`Center for Thin Film Devices and Materials Research, Department of Electrical Engineering, Penn State
`University, University Park, Pennsylvania 16802
`John E. Anthony
`Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506
`Devin A. Mourey
`Department of Material Science Engineering, Penn State University, University Park, Pennsylvania 16802
`共Received 22 January 2007; accepted 16 July 2007; published online 9 August 2007兲
`Using the small molecule organic semiconductor 6,13-bis共triisopropyl-silylethynyl兲 pentacene
`共TIPS-pentacene兲, the authors have fabricated the solution-processed organic thin film transistors
`共OTFTs兲 with carrier mobility ⬎1 cm2/V s, current on/off ratio greater than 107, and subthreshold
`slope ⬍0.3 V/decade. The high mobility TIPS-pentacene solution-processed films are deposited
`from high boiling point solvents and show strong molecular ordering including molecular terracing.
`Film ordering varies substantially for different solvents and film deposition techniques and OTFT
`mobility correlates well with film ordering. © 2007 American Institute of Physics.
`关DOI: 10.1063/1.2768934兴
`
`To date, most high mobility 共⬎1 cm2/V s兲 organic thin
`film transistors 共OTFTs兲 have used vapor-deposited organic
`semiconductors as the active material.1–3 Vapor-deposited or-
`ganic semiconductors can be deposited and processed at low
`temperature and allow fabrication of OTFTs on low-
`temperature substrates such as plastic4 or even paper.5 How-
`ever, the TFT fabrication process for vapor-deposited organic
`semiconductors is similar to that used for conventional inor-
`ganic TFTs and it is not clear if manufacturing cost can be
`significantly reduced using such materials. Solution-
`processed organic semiconductors and OTFTs are of interest
`because they may allow lower-cost manufacturing ap-
`proaches such as printing and easy roll-to-roll processing.
`Materials such as poly共3-hexylthiophene兲 共P3HT兲 are easily
`processed; however, such polymer materials typically have
`relatively low field-effect mobility, usually ⬍0.2 cm2/V s.6,7
`Soluble small molecule organic semiconductors such as a
`precursor route pentacene and a soluble rubrene have also
`been studied. Using relatively high-temperature processes
`共⬎200 °C兲, field-effect mobilities of precursor route penta-
`cene and soluble rubrene as large as 0.9 and 0.7 cm2/V s,
`respectively, have been demonstrated.8,9 However, this tem-
`perature is too high for low-cost polymeric substrates and the
`field-effect mobility falls to near 0.1–0.2 cm2/V s for low-
`temperature processing 共⬍100 °C兲.
`In this letter, we have used a functionalized small mol-
`ecule pentacene, 6,13-bis共triisopropyl-silylethynyl兲 penta-
`cene 共TIPS-pentacene兲, as the soluble organic semiconductor
`for OTFT fabrication. Precursor route small molecule semi-
`conductors typically use a temporary addition of functional
`groups to make the material soluble. Because the groups
`added limit or destroy the semiconducting properties of the
`material, the functional group must then be removed by ther-
`mal or light-induced processes to restore the material to its
`
`a兲
`Author to whom correspondence should be addressed; electronic mail:
`sxp938@psu.edu
`
`native state after film formation. The functionalized small
`molecules used in this work have permanently attached
`groups that modify the solubility and processing properties
`of the system, and are designed to minimize impact on the
`electronic properties of the material, or potentially to even
`improve those properties. Because there is no need to re-
`move the functional groups, no high-temperature steps are
`required.
`We have investigated the surface morphology and mo-
`lecular ordering of solution-processed films using a variety
`of solvents and deposition methods. Most studies of organic
`semiconductor film morphology or molecular ordering have
`been done on vacuum deposited organic thin films10 or
`solution-processed polymer semiconductors.6,7 Highly or-
`dered dendritic polycrystalline films with a herringbone mo-
`lecular crystal structure have been observed for high mobil-
`ity vacuum deposited pentacene thin films.11 Solution
`deposited regioregular P3HT films have shown a weakly or-
`dered two-dimensional lamella structure.6,7 In this work, we
`have observed the molecular ordering for solution deposited
`TIPS-pentacene thin films and a correlation between film
`ordering and OTFT mobility.
`1共a兲
`of
`structure
`Figure
`shows
`the molecular
`TIPS-pentacene.11–13 In this functionalized small molecule
`material, the bulky group substitution on the central aromatic
`ring disrupts aromatic edge-to-face interactions, preventing
`the adoption of the typical herringbone packing motif in
`crystals. The carbon-carbon triple bond serves to hold the
`substituent away from the aromatic surface, allowing adja-
`cent molecules to interact in a face-to-face 共␲-stacking兲 ori-
`entation; this interaction has been suggested to lead to im-
`proved ␲-orbital coupling and potentially increased carrier
`mobility. These substituents also significantly increase the
`oxidative stability of the pentacene chromophore, particu-
`larly in solution.14 The substituent at the end of the alkyne
`can be adjusted to subtly tune the degree of ␲ surface over-
`lap between the aromatic groups and to lend solubility to the
`system. TIPS-pentacene is easily prepared in near quantita-
`
`0003-6951/2007/91共6兲/063514/3/$23.00
`91, 063514-1
`© 2007 American Institute of Physics
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`Appl. Phys. Lett. 91, 063514 共2007兲
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`FIG. 1. 6,13-bis共triisopropyl-silylethynyl兲 pentacene 共TIPS-pentacene兲. 共b兲
`Simple bottom contact OTFT with TIPS-pentacene active layer.
`
`tive yield in a one-pot reaction from 6,13-pentacenequinone
`and is very soluble in a wide range of organic solvents. X-ray
`diffraction demonstrates that solution-grown molecular crys-
`tals have a two-dimensional ␲-stacked structure instead of
`the herringbone pattern of unmodified 共and vapor-deposited兲
`pentacene.15
`Simple bottom contact OTFTs 关Fig. 1共b兲兴 were fabri-
`cated using TIPS-pentacene as the active material. A 370 nm
`thick layer of silicon dioxide was thermally grown as the
`gate dielectric on heavily doped 共0.015 ⍀ cm兲 n-type silicon
`wafers. Au source and drain electrodes were deposited by
`thermal evaporation with no adhesion layer and patterned
`using lift-off. Prior to the active layer deposition, substrates
`were cleaned using UV ozone. After UV cleaning, to im-
`prove the metal/organic contact and device performance,
`self-assembled monolayers
`of
`pentafluorobenzenethiol
`共Aldrich兲16 and hexamethyldisilazane 共HMDS兲
`共PFBT兲
`共Aldrich兲17 were formed on the Au source/drain electrodes
`and gate dielectric, respectively. The PFBT monolayer was
`deposited by immersion in an ⬃10 mM ethanol or toluene
`solution for 2 min. Following this, neat HMDS was spun
`over the PFBT treated devices at 4000 rpm. The HMDS
`treated oxide surface had a water contact angle of about 60°.
`For
`the
`active
`layer,
`various
`concentrations
`共0.2–3 wt %兲 of TIPS-pentacene solutions in several differ-
`ent solvents were dispensed over the prepatterned source/
`drain electrodes and dried. All solution preparation and de-
`vice processing steps were done in either an air ambient or
`drophobic surface led to dewetting for all three techniques.
`solvent-rich ambient. Three different methods were used for
`Drop cast films show molecular terracing and large grains,
`the TIPS-pentacene deposition. In the first method, spin coat-
`while spin and dip cast films show less or no molecular
`ing was used with spin speeds of 1500–2500 rpm from
`terracing as well as smaller grain sizes.
`0.2 to 3 wt % solutions to obtain continuous and relatively
`Figure 2 shows the atomic force microscopy 共AFM兲 im-
`uniform films. In the second method, dip coating, the sample
`age and x-ray diffraction results for an ⬃200 nm thick TIPS-
`was pulled out from 0.5 to 3 wt % solutions at various rates
`pentacene thin film drop casted from a 0.5 wt % toluene so-
`and then dried to remove the solvent. This deposition tech-
`lution. The XRD result shows good molecular ordering for a
`nique also produced continuous, relatively uniform films. In
`solution deposited thin film and the same diffraction peaks as
`the third method, drop casting, a small volume of solution
`bulk crystal TIPS-pentacene.10 Single crystal TIPS-pentacene
`from 0.5 to 2 wt % solutions was dispensed over the prepat-
`has a triclinic structure with unit cell parameters a
`terned source-drain electrodes and the solvent was evapo-
`c=16.835 Å, ␣=89.15°, ␤
`=7.5650 Å,
`b=7.7500 Å,
`rated in a solvent ambient. This deposition technique pro-
`=78.42°, and ␥=83.63°.10 From these parameters, strong,
`duced continuous but less uniform films. Continuous films of
`sharp peaks observed at 5.4° indicate a well-organized mo-
`TIPS-pentacene were obtained using all
`three deposition
`lecular structure with vertical
`intermolecular spacing of
`techniques with large morphology variations observed as a
`16.8 Å, which is consistent with terrace step height measure-
`function of solvent, solution concentration, film formation
`ments from AFM images 共⬃16–17 Å兲. As shown in Fig.
`speed, and other factors. For all three deposition techniques,
`2共b兲, when comparing films of approximately the same thick-
`the film formation was strongly influenced by substrate sur-
`face energy. Treating the substrate to obtain a strongly hy-
`ness deposited by drop, dip, and spin casting, we found that
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`
`FIG. 2. AFM image of terraced structure for a drop cast thin film from a
`0.5 wt % toluene solution and 共b兲 x-ray diffraction results for drop, dip, and
`spin deposited films.
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`Appl. Phys. Lett. 91, 063514 共2007兲
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`voltage varied from about 0 to 10 V 共3.4 V average兲 and
`subthreshold slope from about 0.3 to 0.7 V/decade 共0.5 V
`average兲 for these devices. For films drop cast from a low
`boiling point solvent such as tetrahydrofuran 共bp 66 °C兲 mo-
`bility were typically less than 0.1 cm2/V s. The typical mo-
`bility range for films dip coated from 1 wt % chlorobenzene
`solutions was 0.1–0.6 cm2/V s, with threshold voltage of
`7–15 V and subthreshold slope of 0.5–1 V/decade. The
`typical mobility range for films spin cast from 0.2 wt %
`chlorobenzene solution 共2000 rpm兲 was 0.05–0.2 cm2/V s,
`with threshold voltage of −5–13 V and subthreshold slope
`of 0.9–1.5 V/decade. For each deposition techniques, sev-
`eral solvents with TIPS-pentacene concentrations ranging
`from 0.2 to ⬃3 wt % were evaluated; the results reported
`above are the approximate best obtained in this initial work.
`In summary, using soluble functionalized pentacene,
`TIPS-pentacene, we have fabricated solution-processed OT-
`FTs with mobility greater than 1 cm2/V s. This solution de-
`posited small molecule organic semiconductor requires no
`high-temperature processing and drop cast thin films show
`good molecular ordering. Molecular ordering varies widely
`for different solvents and film deposition techniques and
`OTFT performance correlates well with film ordering.
`
`This work was partially supported by the National Sci-
`ence Foundation through the Penn State Center for Nanos-
`cale Science, a NSF MRSEC, and a grant from the Office of
`Naval Research 共JEA兲.
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`
`共a兲 冑IDS-VGS and
`FIG. 3. Typical drop cast OTFT characteristics.
`log共IDS兲-共VGS兲 共VDS=−40 V兲, and共b兲 IDS-VDS characteristics for several val-
`ues of VGS.
`
`drop cast films typically have the best ordering, while spin
`cast films have the worst. We also found that molecular or-
`dering correlates well with device performance, that is, films
`with stronger molecular ordering typically also had higher
`mobility. For the results described here, it is likely that im-
`proved molecular ordering and device performance are re-
`lated to film formation speed. Slower solvent evaporation
`and film formation speed, encouraged by high boiling point
`solvents and drop casting, facilitate the growth of highly or-
`dered films.18
`For the three deposition techniques, devices where a
`continuous film covered the channel region were electrically
`characterized. All electrical measurements were performed in
`air ambient at room temperature and in ambient light condi-
`tion. Figure 3 shows 冑ID and log共ID兲 vs VGS characteristics
`for VDS=−40 V and ID vs VDS characteristics for several gate
`voltages for an OTFT with the organic semiconductor active
`layer deposited by drop casting from a 1 wt % toluene solu-
`tion. The device has an extracted field-effect mobility of
`1.2 cm2/V s, with on/off current ratio of ⬃108 and sub-
`threshold slope of less than 0.3 V/decade. This device was
`one of a group of 73 OTFTs fabricated using a drop cast film.
`For
`this group of devices,
`the mobility ranged from
`0.2 to 1.8 cm2/V s with
`an
`average mobility
`of
`0.65 cm2/V s and a standard deviation of 0.35 cm2/V s. The
`highest mobility devices showed I-V characteristics 共trans-
`conductance saturation at large gate voltage and nonlinear
`low ID-VDS characteristics for small VDS兲 likely related to
`contact limitations and were less well characterized by a
`square law fit to the square root of drain current than the
`device shown in Fig. 3. For this group of devices, threshold
`
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`Idemitsu Ex. 2003 (pg. 3)
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