We report on the fabrication of crystalline nanoribbon network field effect transistors (FETs) using low molecular weight (M(W)) poly(3-hexylthiophene) (P3HT) with different surface treatments and compare with thin film FETs cast from the same M(W) regioregular P3HT. Nanoribbon FET shows improved performance with a maximum mobility of 0.012 cm(2)/V s and current on/off ratios of 6.5x10(4) due to unique crystalline structure and morphology. With various surface treatments, the nanoribbon FETs show less variation in device mobilities, while thin film FETs show more than ten times variation in device mobilities and up to 100 times change in current on/off ratios

Arif, M.

Khondaker, Saiful I.

Liu, Jianhua

Zhai, Lei

English

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2010 Poly(3-hexylthiophene) crystalline nanoribbon network for organic field effect transistors M. Arif University of Central Florida Jianhua Liu University of Central Florida Lei Zhai University of Central Florida Saiful I. Khondaker University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Arif, M.; Liu, Jianhua; Zhai, Lei; and Khondaker, Saiful I., "Poly(3-hexylthiophene) crystalline nanoribbon network for organic field effect transistors" (2010). Faculty Bibliography 2010s. 6956. https://stars.library.ucf.edu/facultybib2010/6956 Appl. Phys. Lett. 96, 243304 (2010); https://doi.org/10.1063/1.3455097 96, 243304© 2010 American Institute of Physics.Poly(3-hexylthiophene) crystallinenanoribbon network for organic field effecttransistorsCite as: Appl. Phys. Lett. 96, 243304 (2010); https://doi.org/10.1063/1.3455097Submitted: 13 January 2010 . Accepted: 24 May 2010 . Published Online: 16 June 2010M. Arif, Jianhua Liu, Lei Zhai, and Saiful I. KhondakerARTICLES YOU MAY BE INTERESTED INSolubility-driven thin film structures of regioregular poly(3-hexyl thiophene) using volatilesolventsApplied Physics Letters 90, 172116 (2007); https://doi.org/10.1063/1.2734387Soluble and processable regioregular poly(3-hexylthiophene) for thin film field-effecttransistor applications with high mobilityApplied Physics Letters 69, 4108 (1996); https://doi.org/10.1063/1.117834Thermal annealing-induced enhancement of the field-effect mobility of regioregularpoly(3-hexylthiophene) filmsJournal of Applied Physics 100, 114503 (2006); https://doi.org/10.1063/1.2400796Poly„3-hexylthiophene… crystalline nanoribbon network for organic fieldeffect transistorsM. Arif,1,2 Jianhua Liu,1,3 Lei Zhai,1,3 and Saiful I. Khondaker1,2,a1Nanoscience Technology Center, University of Central Florida, Orlando, Florida 32826, USA2Department of Physics, University of Central Florida, Orlando, Florida 32826, USA3Department of Chemistry, University of Central Florida, Orlando, Florida 32826, USAReceived 13 January 2010; accepted 24 May 2010; published online 16 June 2010We report on the fabrication of crystalline nanoribbon network field effect transistors FETs usinglow molecular weight MW poly3-hexylthiophene P3HT with different surface treatments andcompare with thin film FETs cast from the same MW regioregular P3HT. Nanoribbon FET showsimproved performance with a maximum mobility of 0.012 cm2 /V s and current on/off ratios of6.5104 due to unique crystalline structure and morphology. With various surface treatments, thenanoribbon FETs show less variation in device mobilities, while thin film FETs show more than tentimes variation in device mobilities and up to 100 times change in current on/off ratios. © 2010American Institute of Physics. doi:10.1063/1.3455097Regioregular poly3-hexylthiophene rr-P3HT is con-sidered to be one of the most promising organic semiconduc-tors for low cost electronic devices because of its easy solu-tion processing and high charge carrier mobility.1–4 In orderto fabricate devices with high mobility, one of the main chal-lenges is to control the film morphology of the P3HT and thedegree of crystallinity which depends on the material’s in-trinsic properties such as regioregularity,5 molecular weightMW,6,7and molecular orientation/ordering.4 The mobilityalso highly depends on the processing conditions such assurface treatments,8,9 thermal treatment,2,6 and castingsolvents,10 as well as deposition techniques.3,11 In particular,low MW P3HT films show larger dependence of processingconditions than high MW P3HT film.12 For example, the fieldeffect charge carrier mobility of low MW 3–10 kD P3HTcould change two to four orders of magnitude with differentsurface treatments and casting solvents due to the changes inoverall polymer crystallinity and morphology at the interfacebetween the polymer and dielectric.8,12In order to fabricate devices with enhanced morphologyfor improved charge carrier transport, attention has been re-cently devoted toward the development of one dimensionaland two dimensional 2D crystalline nanostructures basedon self-organized P3HT in the form of nanowires andnanofibers.13–16 Crystalline P3HT nanostuctures should showimproved electronic properties resulting from their increasedinterchain staking with – orbital overlap. Previous stud-ies have shown that low MW P3HT film can form well de-fined and highly ordered nanorods depending on the process-ing conditions with increased in-plane  staking along thenanorod axis. But the length of these nanorods is only about8 nm and the low orientational order between small aggre-gates of nanorods hinders charge transport.6,12 Moreover, themobility of the thin film of these nanorods is limited bynumerous grain boundaries. It is expected that if the aspectratio of the nanocrystalline structure could increase, it wouldprovide enhanced charge transport with improved mobilitydue to fewer grain boundaries and improved morphology.Here we report on the fabrication and charge transportproperties of high aspect ratio crystalline nanoribbon fieldeffect transistors FETs using low MW 10.2 kD P3HT. Thedevices show good transistor characteristics with saturationmobilities of up to 1.210−2 cm2 /V s and current on/offratios of 6.5104. We also show that when the FETs werefabricated with different surface treatments, the charge car-rier mobility is relatively insensitive due to the unique 2Dcrystalline nature of the nanoribbon. In contrast, our con-trolled experiment with the same MW P3HT thin film FETnot in the nanoribbon arrangement shows ten times varia-tion in carrier mobility and 100 times change in current on/off ratios with similar surface treatments.The nanoribbons have been fabricated from P3HT 10.2kD using a solution method following our recently devel-oped technique.17 In brief, P3HT was first dissolved into ani-sole at 90 °C with a concentration of 0.25 mg/mL in a glassvial. The solution was then incubated in a water bath at25 °C for 12 h to form the nanoribbon dispersion. Figure1a shows a transmission electron microscope TEM imageof as fabricated P3HT nanoribbons. From the TEM image,aAuthor to whom correspondence should be addressed. Electronic mail:saiful@mail.ucf.edu.FIG. 1. Color online a TEM image of the P3HT nanoribbon with width50–200 nm. b Selective area electron diffraction pattern of P3HT nanor-ibbon. c Schematic of device structure and transport measurement setupand d AFM image of P3HT nanoribbon network between source and drainelectrodes.APPLIED PHYSICS LETTERS 96, 243304 20100003-6951/2010/9624/243304/3/$30.00 © 2010 American Institute of Physics96, 243304-1the width of the nanoribbon was found to be 50–200 nm andlength varied from 3 to 5 m, giving a maximum aspectratio of 100. The thickness of the nanoribbon varied between15–17 nm as measured by tapping mode atomic force mi-croscopy AFM. Figure 1b shows the selective area elec-tron diffraction pattern of the nanoribbons which indicatestheir 2D crystalline nature.FETs were fabricated in the bottom-gate, bottom-contactconfiguration on heavily doped n-type Si substrates with a250 nm capped layer of SiO2. Source and drain electrodeswere defined using double layer photolithography LOR 3A/Shipley 1813 developing in CD26, followed by thermalevaporation of chromium Cr 3 nm and gold Au 45 nmand finally standard lift-off. The channel length L andchannel width W were 3 m and 200 m, respectively.The devices were cleaned in acetone and isopropanol, re-spectively, for 5 min each and then exposed to O2 plasma for10 min before an ethanol wash. Three types of FET deviceswere fabricated with different surface treatments. The firstdevice was only O2 plasma treated and is referred to as un-treated sample. In the other two devices after exposing to O2plasma, two types of silane molecules were deposited on thesubstrate, 1 octyltrichlorosilane OTS and 2 hexamethyl-disilazane HMDS, to form self-assembled monolayerSAM. After the surface treatment, all the devices were fab-ricated by drop casting 0.25 mg/mL P3HT nanoribbon dis-persion inside a N2 glove box. The fabricated devices werethen thermally annealed at 140 °C on a hotplate for 15 minto evaporate the solvent. Figure 1c schematically shows thedevice structure with transport measurement set up and Fig.1d shows the AFM image of one of our devices taken afterelectrical transport measurements. It can be seen from theAFM image that the electrodes are connected by many indi-vidual P3HT nanoribbons making a network FET. Thecurrent-voltage characteristics of the FET were measured us-ing Hewlett Packard HP 4145 B semiconductor parametricanalyzer in an enclosed glove box system with N2 flow.Figure 2a shows the source-drain current IDS versussource-drain voltage VDS characteristics of the HMDStreated P3HT nanoribbon FETs measured at different gatevoltages VG from 0 to 80 V 20V intervals. The deviceexhibits well defined p-type behavior in accumulation mode.Figure 2b shows the transfer characteristics IDS-VG atfixed VDS=−60 V, represented in the left vertical axis of thegraph. The right vertical axis of the graph shows IDS as afunction of VG. The field effect mobility  is determinedfrom the saturation regime using the relationship between IDSand VG from the equation IDS= WCi /2LVG−VT2,where VT is the threshold voltage; Ci is the capacitance/unitarea of the oxide layer 13.8 nF /cm2. From the plot, thesaturation mobility was calculated to be 0.012 cm2 /V s andcurrent on/off ratio was 6.5104.Figure 3a shows the output characteristics of our P3HTnanoribbon FETs at different VG for various surface treat-ments. All the devices show excellent gate modulation andcurrent saturation. Figure 3b represents transfer character-istics at VDS=−60 V and Fig. 3c shows IDS as a functionof VG for the same samples in the saturation regime. It canbe seen from these figures that the surface treatments have anegligible influence on the device properties. The maximumcharge carrier mobility was calculated from the saturationregime. The HMDS treated sample shows mobility of 1.210−2 cm2 /V s while the OTS treated and untreatedsamples show mobilities of 5.510−3 cm2 /V s and 7.810−3 cm2 /V s, respectively. Also the current on/off ratiosof these three samples are in the same order and varies from2.0104 to 6.5104. The maximum variation in mobilityand current on/off ratios with different surface treatments isabout 3 times. The lessened effect of surface treatments andimproved performance in the nanoribbon FETs are due to thepresence of unique crystalline structures with well-definedmorphology at the dielectric and polymer interface. There-FIG. 2. Color online a Output characteristics with HMDS treated P3HTnanoribbon FET for L=3 m and W=200 m and b transfer character-istics of the device measured at fixed VDS=−60 V left axis and IDS in thesaturation regime as a function of VG right axis.FIG. 3. Color online Output and transfer characteristics of FETs fabricatedfrom P3HT nanoribbon network with different surface treatments. a Out-put characteristics of the device at different VG, b transfer characteristicsmeasured at fixed VDS=−60 V and c IDS in the saturation regime as afunction of VG, and d transfer characteristics of controlled low MW P3HTpolymer FETs.243304-2 Arif et al. Appl. Phys. Lett. 96, 243304 2010fore, the surface treatments cannot further enhance the mor-phology. Nevertheless, small changes in mobility in differentdevices for various treatments could be attributed to a smallvariation in contact resistance. In order to find out whetherthermal annealing has effect on recrystallization of the P3HTnanoribbon FET devices, we have performed a controlledexperiment to measure the mobility before and after thermalannealing with same surface treatment condition. We havefound that there is negligible variation in the mobility of theP3HT nanoribbon FETs indicating that there is no furthercrystallization due to annealing.In order to further understand the quality of our nanor-ibbon FET devices and for a fair comparison of the mobility,we fabricated thin film FETs with similar surface treatmentsdrop cast from the solution of the same MW P3HT dissolvedin chloroform and having same concentration 0.25 mg/mLas nanoribbon. This is shown in Fig. 3d, where we plot IDSvalues at fixed VDS=−60 V as a function of VG. From thefigure it is evident that the devices show a significant changein IDS with surface treatments. The field effect mobility cal-culated from saturation regime of these devices varied be-tween 210−4 to 1.7510−3 cm2 /V s while current on/offratios changed from 10 to 103 with different surface treat-ments and is consistent with other reports on low MW P3HTfilms.6,7,11,12 With surface treatments, the mobility variationis upto ten times and current on/off ratio variation is up to100 times. In addition, the mobility values are at least tentimes lower while the current on/off ratios are 1000 timeslower compared to that of the nanoribbon P3HT devices re-ported here. It should be noted that the mobility values re-ported in our work is based on a network of nanoribbonstructure and our preliminary studies show that the mobilityof single nanoribbon FETs is as high as 0.07 cm2 /V s. Thedetails of this study will be published elsewhere.To understand the reason for the improved mobility inour nanoribbon network devices, we have studied the surfacemorphology of the nanoribbon network film and pristineP3HT thin film from AFM images. Figure 4a represents theAFM image of low MW P3HT film drop cast from chloro-form. The film morphology indicates the presence of smallnanorods with numerous grain boundaries. Although, filmscast from low MW P3HT shows crystalline order consistingof nanorods in amorphous matrix but due to its shorter chainlength it shows low charge carrier mobility. The twisted dis-order type crystalline structure can trap charge carrier andreduce the mobility.12 In contrast, Fig. 4b shows the AFMimage of P3HT nanoribbon network film consisting of larger2D crystalline structures with better ordering and less grainboundaries for improved charge transport.In summary, we have fabricated and studied the chargetransport properties of low MW crystalline P3HT nanoribbonnetwork FETs. Devices fabricated from a network of nanor-ibbons using a dropcast method in bottom contact geometrywith different surface treatments show p-type behavior witha maximum saturation mobility of 1.210−2 cm2 /V s andcurrent on/off ratios of 6.5104. The surface treatmentshave less influence on charge carrier mobility and currenton/off ratios, indicating the presence of a high level of struc-tural perfection of oriented crystal at the nanoribbon dielec-tric interface. Also, the performance of nanoribbon networkdevices is much better than that of the same MW P3HT poly-mer film devices with similar surface treatments. Our resultsfrom these studies will contribute in potential applications ofP3HT nanoribbon for organic nanoelectronics.This work is partially supported by U.S. National Sci-ence Foundation under Grant No. ECCS 0801924 to S.I.K.and under Grant No. DMR 0746499 to L.Z.1H. Sirringhaus, N. Tessler, and R. H. Friend, Science 280, 1741 1998.2S. Cho, K. Lee, J. Yuen, G. Wang, D. Moses, A. J. Heeger, M. Surin, andR. Lazzaroni, J. Appl. Phys. 100, 114503 2006.3G. Wang, J. Swensen, D. Moses, and A. J. Heeger, J. Appl. Phys. 93, 61372003.4D. H. Kim, Y. D. Park, Y. Jang, H. Yang, Y. H. Kim, J. I. Han, D. G.Moon, S. Park, T. Chang, C. Chang, M. Joo, C. Y. Ryu, and K. Cho, Adv.Funct. Mater. 15, 77 2005.5H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard,B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W.Meijer, P. 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Poly(3-hexylthiophene) crystalline nanoribbon network for organic field effect transistors

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Poly(3-hexylthiophene) crystalline nanoribbon network for organic field effect transistors

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