Nanocomposite field effect transistors based on zinc oxide/polymer blends

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  • Nanocomposite field effect transistors based on zinc oxide/polymer blendsZong-Xiang Xu, V. A. L. Roy, Peter Stallinga, Michele Muccini, Stefano Toffanin, Hei-Feng Xiang, and Chi-MingChe Citation: Applied Physics Letters 90, 223509 (2007); doi: 10.1063/1.2740478 View online: http://dx.doi.org/10.1063/1.2740478 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High efficiency hybrid solid state blended dyes sensitized solar cell based on zinc oxide nanostructures J. Renewable Sustainable Energy 5, 033134 (2013); 10.1063/1.4811818 Hole transport mechanism in organic/inorganic hybrid system based on in-situ grown cadmium telluridenanocrystals in poly(3-hexylthiophene) J. Appl. Phys. 109, 114509 (2011); 10.1063/1.3594647 Enhanced stability of zinc oxide-based hybrid polymer solar cells by manipulating ultraviolet light distribution inthe active layer Appl. Phys. Lett. 98, 203304 (2011); 10.1063/1.3593020 Organic field-effect transistors based on a crosslinkable polymer blend as the semiconducting layer Appl. Phys. Lett. 87, 183501 (2005); 10.1063/1.2119418 Double-gate organic field-effect transistor Appl. Phys. Lett. 87, 153511 (2005); 10.1063/1.2103403

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  • Nanocomposite field effect transistors based on zinc oxide/polymer blendsZong-Xiang Xu and V. A. L. Roya,b

    Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The Universityof Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, Hong Kong

    Peter StallingaUniversidade do Algarve, 8005139 Faro, Portugal

    Michele Muccini and Stefano ToffaninCNR-ISMN, Istituto per lo Studio dei Materiali Nanostrutturati, 40129 Bologna, Italy

    Hei-Feng Xiang and Chi-Ming Chea,c

    Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The Universityof Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, Hong Kong

    Received 15 November 2006; accepted 24 April 2007; published online 1 June 2007

    The authors have examined the field effect behavior of nanocomposite field effect transistorscontaining ZnO zinc oxide tetrapods or nanocrystals dispersed in a polymer matrix ofpoly2-methoxy,5-2-ethylhexyloxy-1,4-phenylenevinylene MEH-PPV. The electricalcharacteristics of ZnO tetrapods/MEH-PPV composite devices exhibit an increase in hole mobilityup to three orders of magnitude higher than the polymer MEH-PPV device. 2007 AmericanInstitute of Physics. DOI: 10.1063/1.2740478

    Nanocomposite materials are of growing interest particu-larly for their potential practical applications in various elec-tronic devices, especially light-emitting diodes andphotovoltaics.1 Despite these advances, using nanocompositematerials for organic field-effect transistor OFET applica-tions remain scarce. OFETs have been recently reported inthe fabrication of active-matrix displays and integrated cir-cuits for logic and memory chips.2 Even though numerousreports exist in this area, OFET devices based on nanocom-posite materials fabricated using solution processing are stillnot well known. In this context, dispersing nanomaterials ina polymer matrix to obtain solution processed devices with ahigh mobility is of interest for both basic research andapplication.

    In this work, we dispersed ZnO nanocrystals or tetrapodsin the poly2-methoxy,5-2-ethylhexyloxy-1,4-phenylenevinylene MEH-PPV polymer matrix for the constructionof inorganic/organic hybrid devices for OFET applications.We chose ZnO nanomaterials and MEH-PPV polymer be-cause these two materials are widely studied for their intrigu-ing optoelectronic properties.3 In literature, there have beenreports on inorganic tetrapods mixed with polymers to fabri-cate hybrid photovoltaic devices.4 Herein, the electrical prop-erties of solution processed MEH-PPV ZnO nanocompositedevices have been found to have enhanced p-type mobilityup to three orders of magnitude higher than the polymerMEH-PPV device.

    The ZnO nanocrystals were prepared via a modified Pa-cholskis method.5 An ethanol solution of sodium hydroxideNaOH 0.3 M, 20 ml was slowly added to a solution ofzinc II nitrate hexahydrate ZnNO32 6H2O in ethanol0.3 M, 10 ml at 0 C which was vigorously stirred for 2 h,and subsequently heated further for 2 h. A precipitate of

    nanocrystals was obtained, which was washed with ethanoland toluene.

    ZnO tetrapods were synthesized according to a proce-dure described in Ref. 6. In a typical synthesis, about 0.5 gof pure Zn 99.99% was kept in an alumina boat as a sourcematerial. Later the source was transferred into a long quartztube. A tube furnace with the temperature ramped up to1000 C at a rate of 50 C/min was used for chemicalvapor deposition and the tube furnace was kept at 1000 Cfor more than 20 min under a constant flow of nitrogen andoxygen mixture 1:1. The ZnO tetrapods were formed bymoving the quartz tube inside the tube furnace to place thealumina boat with the source at the heating center 1000 Cfor around 20 min. After the furnace was cooled to roomtemperature, the ZnO tetrapods were removed.

    A number of different nanocomposites have been madethat contained ZnO nanocrystals or tetrapods dispersed in thepolymer matrix, MEH-PPV. In this work, the nanocompos-ites which contained polymer with nanocrystals or terapodswere made in various conditions with the following weightproportions: 10 mg of polymer to X mg of ZnO tetrapods ornanocrystals where X=1, 3, 5, 7, 9, and 10. Therefore, theweight percentage of ZnO in MEH-PPV polymer in thesedevices are 9%, 23%, 33%, 41%, 47%, and 50% respec-tively. The ZnO tetrapods or nanocrystals were dispersed in atoluene solution of MEH-PPV molecular weight of 500 000from H.W. Sands Co. using an ultrasonic bath.

    The transistors were fabricated on bottom-gate transistorstructure with a spin coated ZnO/polymer nanocompositethin film as active layer. Gate oxide SiO2 layer 100 nm,relative permittivity=3.9 was thermally grown on heavilydoped n-type Si substrates the gate electrode. An imagereversal photolithography technique was used to form anopening on the photoresist layer for the source and drainpatterns on the gate oxide. Source and drain metal layers,consisting of Ti adhesion film 10 nm, lower and Au con-ductive film 50 nm, upper, were deposited by thermalevaporation. After metal film deposition, a standard lift-off

    aAuthors to whom correspondence should be addressed.bElectronic mail: valroy@hku.hkcElectronic mail: cmche@hku.hk

    APPLIED PHYSICS LETTERS 90, 223509 2007

    0003-6951/2007/9022/223509/3/$23.00 2007 American Institute of Physics90, 223509-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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    http://dx.doi.org/10.1063/1.2740478http://dx.doi.org/10.1063/1.2740478

  • process in acetone was used to remove the metal film on topof the photoresist pattern, leaving behind the Ti/Au source/drain contact patterns. The transistor output and transfercharacteristics were measured with a probe station under anitrogen atmosphere using a Keithley K4200 semiconductorparameter analyzer. The transistor channel length and widthswere 10 and 600 m, respectively.

    The fabricated devices that contained a layer of MEH-PPV only exhibited p-channel behavior with a hole mobilityup to 104 cm2/V s, similar to those previously reported.3

    Figures 1a and 1b show the transmission electron micro-scope TEM images of ZnO nanocrystals or tetrapods dis-persed in MEH-PPV solutions, respectively. The size of thenanocrystals is around 5 nm Fig. 1a and the legs of thetetrapods are around 100 nm in width Fig. 1b. Figure 2shows the electrical behavior of the devices fabricated fromMEH-PPV and nanocomposite with ZnO nanocrystals or tet-rapods. In Fig. 2, the I-V characteristics and the transfercurves of the devices based on MEH-PPV Figs. 2a and2b 9 mg of ZnO nanocrystals in 10 mg of MEH-PPV or47% of ZnO in weight Figs. 2c and 2d and 9 mg of ZnOtetrapods in 10 mg of MEH-PPV or 47% of ZnO in weightFigs. 2e and 2f, are depicted. A saturation of the holemobility is observed in the nanocomposite devices when theconcentration of ZnO tetrapods or nanocrystal exceeds 40%in weight, as shown in Fig. 3. From the I-V characteristics,incorporation of ZnO nanocrystals or tetrapods in the poly-mer enhances the drain current and the mobility. The calcu-lated hole mobility was up to 0.08 cm2/V s for the ZnO

    nanocrystals/MEH-PPV devices and up to 0.15 cm2/V s forthe ZnO tetrapods/MEH-PPV devices, at the saturation re-gime. Whereas in the linear regime, the hole mobility was upto 0.071 cm2/V s for the ZnO nanocrystals/MEH-PPV de-vices and up to 0.096 cm2/V s for the ZnO tetrapods/MEH-PPV devices. A decrease in the threshold voltage up to15 V was found for both nanocomposite devices ZnOnanocrystals or ZnO tetrapods/MEH-PPV. The subthresholdswing was found to be 2 V/decade for the ZnO/MEH-PPVnanocomposite devices and up to 10 V/decade for the MEH-PPV devices. The on/off ratio was calculated as 105 for thenanocomposite devices where it was only 103 for MEH-PPVdevices. Furthermore, a reduction in density of traps, givenby NT=VTCox/q, has been observed, as shown in the insetof Fig. 3, while the weight percentage of ZnO increases inthe polymer. However, the trap density seems to saturatewhen the concentration of ZnO tetrapods or nanocrystal inthe polymer exceeds 40% in weight.

    Incorporation of ZnO nanomaterials nanocrystals or tet-rapods into the MEH-PPV polymera p-type semicon-ductordid not change the nature of charge transport, as thenanocomposite devices were found to behave as p-channeltransistors. However, the hole mobility was enhanced in thenanocomposite devices. We also fabricated ZnO tetrapod de-vices, containing only ZnO tetrapods, by drop casting thesolution of ZnO tetrapods dispersed in ethanol on a bottomcontact device. The aggregated tetrapods exhibited ambipo-lar behavior Fig. 4, available at supplementary information7as evidenced from their electrical characteristics in whichcase the current increased at higher gate voltages in bothpositive and negative drain-source voltages. Since a clearsaturation was not observed in both the positive and negativegate biases, the charge mobility was calculated from the lin-ear regime and it was found to be 103 cm2/V s for holes.This value is lower than the ZnO tetrapods/MEH-PPV com-posite devices. Moreover, no electron current was observedin these composite devices. This indicates that charge trans-port takes place only in the MEH-PPV polymer. In addition,the energy diagrams of MEH-PPV and ZnO are well known.The highest occupied molecular orbital 5.3 eV and lowestunoccupied molecular orbital 3.0 eV levels of MEH-PPVand the valence 7.6 eV and conduction 4.4 eV bands ofZnO show clearly that a huge energy barrier exists for holesto be transferred from ZnO to MEH-PPV for transport.8 Con-sequently, holes are confined in MEH-PPV and we suggestthat the effect of ZnO is to reduce the density of traps in the

    FIG. 1. TEM images of a ZnO nanocrystals and b ZnO tetrapods dis-persed in MEH-PPV the inset picture shows a selected area electron dif-fraction SAED pattern of tetrapods.

    FIG. 2. I-V characteristics and the transfer curves for devices containingMEH-PPV a where VGS=0, 25 to 40 V in steps of 5 V and b whereVDS=40 V, 9 mg of nanocrystals in 10 mg of MEH-PPV c whereVGS=0 to 40 V in steps of 5 V and d where VDS=40 V, and 9 mg ofZnO tetrapods in 10 mg of MEH-PPV e where VGS=0 to 30 V in stepsof 5 V and f where VDS=30 V, respectively.

    FIG. 3. Correlation between the mobility and the concentration of ZnO inthe form of nanocrystals solid circles or tetrapods open circles in MEH-PPV. The lines are guides to the eye. The inset shows the density of traps asa function of ZnO concentration, calculated on basis of the found thresholdvoltage, NT=VT Cox/q. We propose that the increased mobility is mainlydue to the reduction of traps in MEH-PPV.

    223509-2 Xu et al. Appl. Phys. Lett. 90, 223509 2007

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  • polymer which probably is a reason for the enhanced mobil-ity and the reduced threshold voltage.

    Photoluminescence PL measurements were done forZnO tetrapods, MEH-PPV, and ZnO tetrapods/MEH-PPVcomposite films Fig. 5, available at supplementaryinformation7. The PL measurements explained that the sur-face state green emission from the ZnO tetrapods wasquenched in the composite film. This suggests that the poly-mer MEH-PPV covered the surface of ZnO tetrapods; thelatter were distributed randomly in the MEH-PPV polymermatrix. The nonaggregated and nonoriented random distri-butions of ZnO tetrapods in MEH-PPV matrix have alsobeen observed from the TEM image Fig. 1b. Hysteresismeasurements Figs. 6a and 6b, available at supplemen-tary information7, a close loop measurement by scanningVGS from 0 to 40 V and back to 0 V with constant drain-source voltage, for both MEH-PPV and ZnO tetrapods/MEH-PPV com...

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