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Page 1: Transparent and conducting electrodes for organic ...nanotubes.rutgers.edu/PDFs/transparent and conducting electrodes... · Transparent and conducting electrodes for organic electronics

Transparent and conducting electrodes for organic electronics fromreduced graphene oxide

Goki Eda,1,a� Yun-Yue Lin,2 Steve Miller,1 Chun-Wei Chen,2,a� Wei-Fang Su,2 andManish Chhowalla1,a�

1Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854,USA2Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

�Received 5 January 2008; accepted 9 May 2008; published online 13 June 2008�

The deposition and optoelectronic properties of reduced graphene oxide thin films are described.Thin films with thicknesses ranging from 1–10 nm have been deposited by the vacuum filtrationmethod. The conductivity of the thin films can be varied over six orders of magnitude by varyingthe filtration volume of the graphene oxide aqueous suspension while maintaining the transmittancebetween 60%–95%. In addition, enhancement in the conductance through Cl doping isdemonstrated. The combination of the reduction and Cl treatments make the reduced graphene oxidethin films sufficiently conducting to incorporate them as the hole collecting electrode in proof ofconcept organic photovoltaic devices. © 2008 American Institute of Physics.�DOI: 10.1063/1.2937846�

Interest in the unique properties of graphene has led tonumerous studies1–7 because it can be easily deposited by theScotch tape method2 and identified on 300 nm SiO2.3 Anideal monolayer of graphene has transparency of 98% �Ref.8� with sheet resistance of �6 k� /�, making it suitable fortransparent and conducting electrodes. However, for techno-logical feasibility, uniform deposition over large areas is nec-essary.

Chemical routes for obtaining single sheets of graphenefrom reduction of graphene oxide �GO� have beenreported.9,10 This is appealing because GO can be easily sus-pended in aqueous suspensions11 and deposited on a widerange of substrates.

Although the overall efficiency of the best OPVs is ap-proximately 5%,13,14 lower than inorganics,15 the economicscould allow their widespread use. Presently, state-of-the-arttransparent and conducting hole collecting electrode is in-dium tin oxide �ITO� �transparency �90% at �=550 nm andlow sheet resistance=10–30 � /��. However, ITO cracksupon flexing. Thus, to move toward truly flexible OPVs, al-ternatives to ITO and Al electrodes are needed.

The use of solution deposited single walled carbon nano-tube �SWNT� thin films as cathodes in OPVs has beendemonstrated.16,17 Transparent and conducting thin filmsfrom thermal reduction of GO �sheet resistance as low as�600 � /� with transparency of �60%� at 1100 °C inAr /H2 atmosphere have been reported18,19 and demonstratedto be suitable as electrodes for dye sensitized solar cells.18

These interesting properties make reduced GO thin films anemerging material for organic electronics.

The transparent and conducting reduced GO thin filmswere prepared from an aqueous suspension obtained by theexfoliation of graphite oxide using the modified Hummersmethod.20 The structure of GO has been widely studied andconsists of the attachment of hydroxyl, carboxyl, and ep-oxide groups21,22 which render it highly soluble in water,allowing exfoliation into individual sheets. We employed the

vacuum filtration technique to deposit uniform layers ofGO.12,22–25 The technique involves filtrating the suspensioncontaining the GO sheets through a 25 nm pore size mixedcellulose ester membrane. As the liquid is filtrated, the GOsheets block some of the pores where the permeation rate ofthe liquid is dramatically reduced. Therefore, in regionswhere the pores are not blocked, the permeation rate is en-hanced until covered by GO sheets. Once the GO films aredeposited on the ester membrane, it is necessary to transferthem onto substrates. The transfer process is performed byplacing the ester membrane with the film side down onto thesubstrate. The ester membrane is then etched away by se-quential acetone washes. The process is highly repeatableand the films are well adhered to glass and plastic substrates,allowing lithographic processing without delamination.12

Scanning electron microscopy �SEM� images of reducedGO thin films on SiO2 /Si substrate are shown in Figs.1�a�–1�d�. The lighter regions in the SEM images representeither single layer GO or voids between the GO sheets.12 Itcan be seen that the fraction of voids decreases with filtrationvolume until around 80 mL. The average thickness of thefilms can be varied from 1 to 5 nm by varying the filtrationvolume from 20 to 80 ml while keeping the concentration ofGO constant �0.33 mg / l�. A thickness of 1 nm correlates to asingle GO sheet due to functional groups and adsorbedwater.26 The thicknesses of our thin films have been mea-sured by Raman mapping of 2D peak,27 ellipsometry, andatomic force microscope �AFM� profilometry. Raman map-ping and ellipsometry results have been reportedelsewhere.12 The thicknesses versus effective filtration vol-umes measured by AFM are shown in Fig. 1�e�. We defineeffective filtration volume as the volume of 0.33 mg / l sus-pension required to deposit equivalent amount of GO whenthe concentration of the suspension is changed. Above filtra-tion volumes of 80 ml, the film thickness increases almostlinearly whereas no significant change is seen for 60 and70 ml. This is attributed to the fact that voids are present atlow filtration volumes and a continuous film is achieved atapproximately 80 ml.

a�Authors to whom correspondence should be addressed. Electronic ad-dresses: [email protected] and [email protected].

APPLIED PHYSICS LETTERS 92, 233305 �2008�

0003-6951/2008/92�23�/233305/3/$23.00 © 2008 American Institute of Physics92, 233305-1Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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GO thin films are electrically insulating and must bereduced in order to make them conducting. The reduction ofGO films can be performed by exposing to hydrazine vaporat room temperature28 or at slightly elevated temperatures�80 °C�.9,10 The reduction process is believed to occur bybreaking the sp3 carbon-oxygen bonds which leads to sp2

hybridized carbon, thereby recovering � electrons.21 Elimi-nation of the sp3 bonds leads to a higher concentration ofdelocalized � electrons which results in the decrease of re-sistivity. The reduction treatment has been monitored byx-ray photoelectron spectroscopy,12 which shows that theC–O bonds are removed with the reduction treatment.

We recently showed12 that a combination of hydrazinevapor and low temperature annealing �200 °C� in nitrogen orvacuum reduces the sheet resistance significantly. Figure 2�a�shows sheet resistance of GO films after the combined re-duction steps as a function of filtration volume. Figure 2�a� isdata from our previous study12 but is repeated here for thesake of completion. The sheet resistance decreases dramati-cally with the filtration volume after annealing but saturatesabove a critical volume because the reduction treatment isonly effective for the top few layers. The transmittance at550 nm versus the sheet resistance of the thin films is shownin Fig. 2�b� and was found to monotonously increase in300–900 nm wavelength range �not shown� with no clearabsorption peaks. It can be seen that the for the lowest sheet

resistance �70 k� /��, the transmittance of the film is ap-proximately 65%.

In order to further decrease the sheet resistance, we at-tempted to dope the films by dipping in thionyl chloride�SOCl2� for 1 h. We previously demonstrated in SWNT thinfilms that the attachment of Cl using this method leads top-type doping and enhanced sheet conductance.29 We foundthat for thin films of reduced GO, the sheet resistance can bereduced by a factor of nearly 5 while for thicker films theeffect is moderate. The increase in conductance with SOCl2treatment is demonstrated in the transfer characteristics ofdoped and undoped reduced GO thin film transistors �TFTs�shown in Fig. 3. The TFT characteristics are typical of re-duced GO devices measured in air.9,10 It can be seen thatexposure to SOCl2 leads to a fivefold increase in the source-drain current which can be translated to a similar decrease inthe sheet resistance, as reported in Ref. 12. The neutrality

FIG. 1. �Color online� SEM micrographs of reduced GO thin films preparedby filtrating 0.33 mg / l GO suspension in volumes of �a� 20 ml, �b� 30 ml,�c� 50 ml, and �d� 70 ml. Contrast is observed between the SiO2 /Si substrate�light� and reduced GO �dark�. The scale bar corresponds to 20 �m. �e�Reduced GO film thicknesses as a function of effective filtration volume.Films were prepared on SiO2 /Si substrates and the step height at the edgesof a scratch was measured with AFM at more than ten different locations foreach sample.

FIG. 2. �Color online� �a� Sheet resistance of reduced GO thin films as afunction of effective filtration volume. Measurements were made at morethan five different areas of the films. �b� Transmittance at �=550 nm as afunction of sheet resistance for reduced GO thin films.

FIG. 3. �Color online� �a� Transfer characteristics of a reduced GO thin filmbefore and after dipping in SOCl2 for 1 h. The film was prepared at afiltration volume of 30 ml with 0.33 mg / l GO suspension and transferredonto a Si substrate with 300 nm thermally grown oxide. Au was used assource and drain electrodes and the channel length was 21 �m. The gatevoltage was swept from positive to negative with 1 V step.

233305-2 Eda et al. Appl. Phys. Lett. 92, 233305 �2008�

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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point which is observed at the gate voltage of �60 V foruntreated film moves further toward the positive gate voltage��100 V, which is beyond the capability of our measure-ment apparatus� upon SOCl2 treatment indicating p-typedoping with Cl.

Proof of concept devices consisting of the reduced anddoped GO thin films as the cathodes in OPVs werefabricated.12 Here, we provide more details regarding devicefabrication and performance. In order to utilize the mostconducting material as the electrode, reduced thin filmsdeposited at a filtration volume of 480 ml and treated withSOCl2 by dipping in solution for 1 h were fabricated. OPVdevices were prepared by initially spin coating �4500 rpm� alayer of poly�3,4-ethylene dioxythiophene�:poly�styrene-sulfonate� �PEDOT:PSS� on top of the reduced and dopedGO thin films. Subsequently, regioregular �94%� poly�3-hexylthiophene� �P3HT� �molecular weight �58 000� andphenyl-C61-butyric acid methyl ester �PCBM� nanocompos-ite layer was deposited on top. The P3HT:PCBM ratio was1:1 and the concentration of solid to chlorobenzene solventwas 20 mg /ml. The spin coating speed for P3HT:PCBM was700 rpm to achieve a thickness of approximately 120 nm.Thermally evaporated Al was used as the top electrode forthe OPV devices.

The current density versus the applied voltage resultsfrom the reduced GO electrode devices in dark and underillumination are shown in Fig. 4. The efficiency of the de-vices was approximately 0.1%. We verified that the output inFig. 4 was not the result of PEDOT:PSS by fabricating de-vices where PEDOT:PSS was used as the hole collectingelectrode. In all of our devices fabricated with PEDOT:PSSalone, we were not able to make working devices. Thus, thereduced GO in our OPV devices indeed act as hole collectingelectrodes. The data under illumination are far from the idealdiode characteristics and the devices are limited by the largeresistance �on the order of 105 � /�� of the reduced GOelectrodes. However, the decrease in the sheet resistance ofreduced GO should lead to a significant improvement in theOPV device performance.

In conclusion, uniform deposition of transparent andconducting reduced GO thin films is described. The conduc-tance of the thin films was �105 � /� at a transmittance of

�65%. We also demonstrated that thionyl chloride �SOCl2�treatment reduces the sheet resistance by a factor of up to 5.The Cl doping allows the utilization of the transparent re-duced GO thin films as the hole collecting electrode in or-ganic photovoltaic devices. Thus, our work provides a pro-cess path for the incorporation of graphene in organicelectronics.

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FIG. 4. �Color online� Solar cell characteristics of an OPV with a reducedGO thin film as the transparent electrode. A Newport AM1.5 solar simulatorwas used to evaluate the photovoltaic characteristics. The film used for thefabrication of OPV had thickness of �14 nm, sheet resistance of�40 k� /�, and transmittance of 64%.

233305-3 Eda et al. Appl. Phys. Lett. 92, 233305 �2008�

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp