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High-Performance Organic Field-Effect Transistorsfrom Organic Single-Crystal Microribbons Formed by aSolution Process

By Yan Zhou, Ting Lei, Lei Wang, Jian Pei,* Yong Cao, and Jian Wang*

Intensive research has been carried out on one-dimensional (1D)organic electronics in recent years because of their highly tunableenergy levels and good processability, their low-cost fabrication,flexibility, etc.[1] In the ever ongoing quest for enhancedperformance in the microelectronics industry, organic 1Dmaterials with high carrier mobilities for both holes and electronsare required. High-performance organic field-effect transistors(OFETs),[2] photo-transistors,[3] and complementary inverters[4]

have been successfully demonstrated based both on vacuum-processed and solution-processed organic 1D nano- or micro-structures. Hu and co-workers fabricated single-crystal nanor-ibbon OFETs and achieved a hole mobility of 0.6 cm2V�1 s�1

using copper phthalocyanine (CuPc) and an electron mobility of0.1 cm2V�1 s�1 using copper hexadecafluorophthalocyanine(F16CuPc).

[2a,2c] Bao reported a p-channel mobility as high as0.27 cm2V�1 s�1 based on hexathiapentacene (HTP) nanowiresdeveloped in the same group.[2f ] Kim et al. fabricated FETdevicesbased on triisopropylsilylethynyl pentacene (TIPS-PEN) micro-belts that showed a hole mobility as high as 1.4 cm2V�1 s�1.[2h]

Most recently, Bao has achieved an electron mobility of1.4 cm2V�1 s�1 using an n-channel organic nanowire.[2i]

Nevertheless, fabricating 1D organic structures with mobilitieshigher than 1 cm2V�1 s�1 remains a challenge, especially forsolution-processed, self-assembled, and dispersed nano- andmicrowires.[1]

Physical vapor and vacuum deposition are widely used for thefabrication of high-quality organic single crystals and theproduction of highly crystallized organic films. OFETs basedon vapor-phase fabricated single crystals and films generallyexhibit a higher performance than their counterparts based onsolution-processed small molecules or polymers.[5] The majorobstacles for high-performance OFETs lie in the interfacebetween the organic compound and the dielectric layer, and

[*] Prof. J. Pei, Y. Zhou, L. TingThe Key Laboratory of Bioorganic Chemistry andMolecular Engineering of Ministry of EducationCollege of Chemistry, Peking UniversityBeijing 100871 (P. R. China)E-mail: jianpei@pku.edu.cn

Prof. J. Wang, Prof. Y. Cao, Dr. L. WangInstitute of Polymer Optoelectronic Materials and DevicesSouth China University of TechnologyKey Laboratory of Specially Functional Materials of Ministry ofEducationGuangzhou 510640 (P.R. China)E-mail: jianwang@scut.edu.cn

DOI: 10.1002/adma.200904171

� 2010 WILEY-VCH Verlag Gmb

the crystal quality of the organic compound. It was suggested thatit is easier to address both issues by vacuum deposition than bysolution processing. However, in this contribution, we show thatwith the same solution-processed organic compound, a twoorders of magnitude improvement in FET performance can beachieved by enhancing the crystallinity via a slow crystallizationprocess, and by improving the contact between the dielectric andthe single crystals via mixed solvents. During the devicefabrication, we also improved the plastic-fiber-mask method toreduce the FET channel length by 25%. Moreover, through sometweaking of the evaporation process, we obtained asymmetricsource/drain electrodes which led to a significant reduction onthe FET threshold. Our state-of-the-art high-performance deviceattained holemobilities as high as 2.1 cm2V�1 s�1, an on/off ratioof 2� 105, and a threshold of �7V.

In our earlier work, we demonstrated an OFET with a holemobility of 0.005–0.01 cm2V�1 s�1 based on a single organicmicroribbon self-assembled via a solution process.[2g] Themicrometer-sized ribbons were fabricated by direct precipitationfrom a solution of compound 1 (Fig. 1A) in dichloromethane. Byusing different solvents, several 1D nano- or microstructureswere obtained with similar morphology. However, the crystal-lization speeds were quite different. In dichloromethane, thecrystallization was finished within two hours; while in dioxane, ittook less than 10min for the crystallization, and in tetrahy-drofuran (THF), it took a few days to complete the process.

To investigate the solvent effects, 2mg of compound 1 wasmixed with 1mL of THF in a sample vial. The vial was then sealedand the mixture was heated to 60 8C. After all the solids haddissolved, the solution was filtered by a syringe through a 0.4mmfilter into another clean sample vial. Then, the clean vial withfiltered solution inside was sealed, and stored without furthermovement. All the procedures were carried out in a clean room atroom temperature (25 8C). For the first two days, there was novisible precipitation in the vial. After two days, crystals started toshow, and the whole crystallization process took at least one week.During the process, the molecules of compound 1 self-assembledinto nano- or microribbons because of strong p–p stacking of thelarge aromatic planes and through van der Waals interactionsbetween the long alkyl chains. The self-assembled microribbonsfrom THF had a width varying from 200 nm to 1 micrometer, athickness from 50 nm to 1mm, and a length from 20mm to200mm. The chemical structure of compound 1, scanningelectron microscopy (SEM) images, X-ray diffraction (XRD)patterns, transmission electron microscopy (TEM) images, andselected-area electron diffraction (SAED) patterns of the self-assembled microribbons are displayed in Figure 1. The SAED

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Figure 2. SEM images of the microribbons dispersed from suspensions ofdifferent solvents. A) Microribbon from a suspension of THF and hexanewith a volume ratio of 1:3. B) Microribbon from a suspension of THF,hexane, and ethanol with a volume ratio of 1:3:0.1. The scale bar is 1mm.

Figure 1. A) Chemical structure of compound 1. B) SEM image of theself-assembled microribbons from THF. The scale bar is 4mm. C) The XRDpatterns of the microribbons self-assembled from THF. D) TEM image ofthe microribbon with a scale bar of 500 nm (inset), and SAED patterns.

pattern is indexed according to the XRD results, showing that themicroribbon grows along the (001) direction. The p–p stacking,indexed to (1-3-2), has a 358 rotation with respect to the (001)direction. The sharp peaks in the XRD patterns indicate that themicroribbons are highly crystalline, while the strong intensitydifference between the peaks can be attributed to the preferredorientation of the microribbons. Careful studies of the XRDpatterns reveal that the crystal structure of the microribbons istriclinic with lattice constants: a¼ 18.87 A, b¼ 17.35 A,c¼ 11.81 A, a¼ 101.838, b¼ 92.718, g ¼ 95.988 (SupportingInformation). The same SAED patterns were found in differentregions of one microribbon suggesting the single crystallinity ofthe 1D nano- or microstructures.[2a]

To fabricate the FETdevices, the THFmicroribbon suspensionwas mixed with hexane, and the suspension was dispersed on apoly(methyl methacrylate) (PMMA)-coated SiO2/Si substrate byspin-coating. The gold electrodes were fabricated by thermalevaporation in vacuum. All the devices exhibit good p-channelFET characteristics. The highest hole mobility reached around0.6 cm2V�1 s�1, which is a two orders of magnitude improve-ment over our earlier results.[2g] The substrates that had beenspin-coated with the suspension were examined under amicroscope and some amorphous stains were observed aroundsome of the ribbons (Fig. S1A and B in the SupportingInformation). Under UV excitation (350 nm–370 nm), the stainsemitted green light (Fig. S1E), suggesting that the stains camefrom the dissolved compound 1 molecules in the suspension.Such amorphous residues were considered to be the contamina-tion of the interface between the organic semiconductor and thedielectric layer, which was crucial for the FETperformance.[1a] Inorder to reduce the concentration of dissolved compound 1, 10 vol% of ethanol was introduced into the THF/hexane suspension.After spin-coating the suspension on the substrate, the stain wascompletely removed (Fig. S1C), and the substrate became cleaner.However, the devices made with the ethanol/THF/hexanesuspension failed to perform as a FET. To find out the reasonfor this, SEM images of the ribbons from different suspensions

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were studied. As shown in Figure 2, the microribbons from theTHF/hexane suspension have a clean and smooth surface, whilethe microribbons from the suspension containing ethanol havesome amorphous nodules on the surface. The results show thatthe addition of ethanol does reduce the concentration of dissolvedmolecules in the suspension, thereby leading to a cleanersubstrate surface, however, ethanol also seems to increase thepolarity of the suspension, which makes insoluble amorphousnodules adhere to the ribbons thereby causing bad contactsbetween the dielectric and the organic single crystals. Therefore,to further enhance the FET performance we tried to improve thecontacts by optimizing the ratio of THF to hexane. As the THF/hexane ratio increased, the area containing the amorphous stainsalso increased. When no hexane was present in the suspension,the PMMA layer on the substrate was dissolved by THF resultingin a poor FET performance. As the THF/hexane ratio decreased,the density of ribbons dispersed onto the substrate was reduced,and the ribbons started to form bundles (Fig. S1D). For a THF/hexane ratio equal to 1/3, the hole mobility of the p-channel FETwas as high as 1.2 cm2V�1 s�1, and an on/off ratio as high as5� 105 was obtained. The best device characteristics are shown inFigure 3A,B.

After optimization of the interface between the microribbonsand the dielectric we also modified the metal electrodes of theFET devices. The plastic-wire mask method was improved toreduce the channel length, and the evaporation process wastweaked to obtain asymmetric metal contacts for the source anddrain electrodes. A plastic fiber with a diameter of 20mm wasused as the shadow mask. The fiber was first mountedperpendicularly to the ribbon on the substrate. When thesubstrate was loaded into the vacuum chamber, not only was thesubstrate moved away from the center position, which wasdirectly above the evaporation source, but the substrate was alsoaligned in such a way that one side of the fiber was closer to theevaporator source than the other side, as illustrated in Figure 4A.The first layer of 50-nm-thick gold was thermally deposited with arate of 0.005 nms�1 under a pressure of 2� 10�4 Pa. Because ofthe shadow effect, the shadow region under the mask had littlegold (Fig. 4B). After the thickness reached 50 nm, the substratewas rotated by 1808, so that the shadow region was facing thesource directly (Fig. 4C). The subsequent gold-evaporation speedwas increased to 0.015 nm s�1, which was three times faster thanthe initial speed, and another 50-nm-thick layer of gold wasevaporated. By moving the substrate off the center, aligning

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Figure 3. FET characteristics of the devices. Transfer curves and output curves of the device withsymmetric gold electrodes (A) and (B); the device with asymmetric gold electrodes (C) and (D);and the best performance device (E) and (F). In (C), the solid lines and squares represent thetransfer curves for the high-evaporation-rate electrode and the low-evaporation-rate electrode setas the drain, respectively.

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the substrate to have asymmetric mask sides with respect to theevaporation source, rotating the substrate by 1808 during theevaporation, and using different deposition speeds, not only canthe gap between the electrodes be reduced from 20mm to 15mm,but also asymmetric metal electrodes for source and drain can beeasily fabricated during a one-time vacuum operation withoutexposing the devices to air or any other micromanipulation.[2a,2c]

The microscopic images of the devices with symmetric goldcontacts and asymmetric gold contacts are illustrated in Figure 5.

A total of 53 FETdevices were fabricated with an electrode gapof 15mm. Among those, 28 devices had symmetric goldelectrodes using an evaporation rate of 0.01 nms�1 for bothsource and drain electrodes, while the other 25 devices hadasymmetric gold electrodes. Whereas the average mobility andthe on/off ratio were identical for the devices in both groups, theaverage threshold of the devices with symmetric gold electrodeswas much higher than that of the devices with the asymmetricelectrodes (Fig. S2). The average thresholds were �18.6 V and�12.5 V for the devices with symmetric and asymmetricelectrodes, respectively. The substantial threshold reduction

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

may be caused by different gold diffusioninto the organic crystal and the work-functiondifference may be related to the differentdeposition speeds.[3c,6] Many groups havedemonstrated that the evaporation rate ofthe gold can significantly affect the FETperformance.[2] The asymmetric electrodedevices exhibit asymmetric transfer character-istics when reversing the source and thedrain. As illustrated in Figure 3C and 3D, themobility changes by a factor of 24, andthe threshold is changed from �2V to �6V.The mobility of the FET device where theelectrode with high deposition rate is set asthe drain was 0.73 cm2V�1 s�1. When theelectrode with low deposition rate was setas the drain, the mobility dropped to0.03 cm2V�1 s�1. We speculate that suchdramatic difference in mobility could beattributed to the different work function ofthe gold electrodes because of the differencein deposition rate. In the output character-istics, the nonlinear increase of the draincurrent in the linear region suggests that thereis an injection barrier. The barrier may berelated to a work-function mismatch or to thecontact resistance between the electrodes andthe organic single crystal. As shown in FigureS3, when the electrode with the high deposi-tion rate is set as the drain, the barrier is lowerthan in the case where the electrode with thelow deposition rate is set as the drain. Theresults show that the barrier is caused by acombination of the work-function mismatchand contact resistance. To the best of ourknowledge, this is the first time that asym-metric FET characteristics are observed in anorganic 1D nano-/microstructure-based FETdevice with the same electrode metal forsource and drain.

Comparing all 53 devices, the average mobility and the on/offratio were 0.40 cm2V�1 s�1, and 1� 105, respectively (Fig. S2).The best device with asymmetric gold electrodes had a mobility ashigh as 2.1 cm2V�1 s�1, and an on/off ratio of 2� 105 with achannel length of 17mm and a channel width of 210 nm. Thethreshold was �7V for the device. The characteristics of the bestdevice are shown in Figure 3E and 3F. All 53 devices were quitestable. After storing in air for one month at 25 8C with a relevanthumidity of 60%, the mobility, the on/off ratio, and the thresholdof 5 typical devices with mobilities higher than 1 cm2V�1 s�1, didnot change.

In conclusion, by enhancing the crystallinity through a slowcrystallization process in THF, optimizing the interface betweenthe dielectric layer and the crystal surface through using a mixedsolvent of THF/hexane with a volume ratio of 1/3, reducing thechannel length through an improved plastic fiber mask method,and realizing asymmetric metal electrodes for source and drainthrough some tweaks during evaporation, we achieved astate-of-the-art high-performance p-channel transistor based on

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Figure 4. Schematic illustration of the improved plastic-fiber maskmethod. A) The plastic fiber is mounted perpendicularly to the micro-ribbon, and the device substrate is moved to the left of the evaporationsource. The arrows illustrate the gold deposition direction. B) After theinitial metal deposition, the shadow region has little gold deposition. C)The substrate is rotated by 1808 in vacuum, and subsequent metaldeposition starts. D) After the evaporation is complete, the channel lengthhas shrunk, and different gold contacts for source and drain are obtained.

Figure 5. A,B) Microscopic images of the devices with symmetric goldcontacts (A), and asymmetric gold contacts (B). The scale bar is 10mm.

a single-crystal organic microsized ribbon. The highest mobilityattained was 2.1 cm2V�1 s�1 with an on/off ratio of 2� 105 and athreshold of �7V.

Experimental

General: All commercially available chemicals were used without furtherpurification unless otherwise noted. THF, 1,2-dichlorobenzene, and PMMAwere purchased from Acros. Hexane was purchased from Fisher. Themicroscopic images were taken using a DVC-1420 digital camera mountedon an Olympus BX51 microscope. The SEM images were obtained using acold field-emission scanning electron microscope (HITACHI S4800)operated at an accelerating voltage of 2 kV. X-ray diffraction was recordedon a D/Max-RA high-power rotating anode (12 kW) X-ray diffractometer(Cu Ka radiation l¼ 0.154 nm). The 2u angle was measured from 3 to 308with a speed of 2 degrees min�1.The TEM images and the SAED patternswere captured by a thermal-emission transmission electron microscope(JEM 2100) equipped with a digital camera operated at an acceleratingvoltage of 200 kV.

1D Organic Structure Self-Assembly and Dispersion: Compound 1 wassynthesized by the procedures described in our earlier work [2g] andpurified by recrystallization in a 1mgmL�1 dichloromethane solutiontwice.

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Substrate Cleaning and Preparation: 300-nm-thick SiO2 was thermallydeposited on the heavily n-doped Si substrate. The substrates were heatedat 100 8C for half an hour in a1:1 (volume ratio) sulfate acid and hydrogendioxide (30% solution in water) solution, followed by 15min of sonicationin pure water twice. After cleaning, the substrates were dried inside avacuum oven at 80 8C. Prior to PMMA coating, the substrates underwentoxygen-plasma treatment for 15min. A 1,2-dichlorobenzene PMMAsolution was spin-coated on the SiO2. Finally, the substrates wereannealed in a vacuum oven at 70 8C for 3 h.

FET Fabrication and Testing: First, the organic self-assembled micro-ribbon suspension was spin-coated onto the PMMA-coated substrates.Second, the substrates with microribbons were baked in the vacuumoven at 70 8C overnight. The FET characteristics were measured using aVector BX4000 probe station connected to a Keithley SCS 4200 in air.

Acknowledgements

This research was financially supported by the Major State Basic ResearchDevelopment Program (Nos. 2006CB921602), and the 973 Project (Nos.2009CB623601, 2009CB623604, and 2009CB930604) from the Ministry ofScience and Technology and by the National Natural Science Foundation ofChina. Thanks to Kuo Li for the TEM imaging and SAED patterns.Supporting information is available online fromWiley InterScience or fromthe author. This article is part of a Special Issue on the occasion of theCentennial Celebration of Chemical Research and Education at PekingUniversity.

Received: December 6, 2009

Published online: March 8, 2010

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