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Yan Gao , Hin-Lap Yip , Kung-Shih Chen , Kevin M. O’Malley , Orb Acton , Ying Sun , Guy Ting , Hongzheng Chen , * and Alex K.-Y. Jen *

Surface Doping of Conjugated Polymers by Graphene Oxide and Its Application for Organic Electronic Devices

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Conjugated polymers are a novel class of solution-processable semiconducting materials with intriguing optoelectronic prop-erties. [ 1 ] They have received great attention as active components in organic electronic devices such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and organic fi eld-effect transistors (OFETs) due to their light weight, facile tuning of electronic properties through molecular engineering, and ease of processing. The performance and lifetime of conju-gated polymer-based electronic devices are critically dependent on the bulk properties of the active materials and the interfacial properties of electrode/polymer contacts. [ 2–4 ] In these devices, the electrode(s) either inject charge into or extract charges from the organic semiconductor layer(s). Mismatch of the work func-tions between metal or metal oxide electrodes and molecular orbital energy levels of organic semiconductors can lead to high contact resistance, which decreases the charge injection and extraction effi ciency. Therefore, it is essential to mini-mize contact resistance at the electrode/organic semiconductor interface.

To improve charge injection/extraction across the electrode/organic semiconductor interface, several strategies have been developed. One is to tune the interfacial dipole across the electrode/semiconductor interface to reduce the injection/col-lection energy barrier. This can be achieved by modifying the electrode surface with self-assembled dipolar molecules to tune the energy level alignment at the semiconductor/electrode inter-face. [ 5–7 ] Alternatively, the introduction of a thin layer of polymer surfactant that contains polar side chains between the conju-gate polymer/electrode interface can also be used to improve the interfacial properties. The polar side chains can provide not

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 1903–1908

Y. Gao , Dr. H.-L. Yip , K.-S. Chen , O. Acton , Y. Sun , Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Box 352120, Seattle, WA 98195, USA E-mail: ajen@u.washington.edu Y. Gao , Prof. H. Chen State Key Laboratory of Silicon Materials MOE Key Laboratory of Macromolecule Synthesis and Functionalization Zhejiang-California International Nanosystems Institute Zhejiang University Hangzhou 310027, P. R. China E-mail: hzchen@zju.edu.cn K. M. O’ Malley , G. Ting , Prof. A. K.-Y. Jen Department of Chemistry University of Washington Box 351700, Seattle, WA 98195, USA

DOI: 10.1002/adma.201100065

only adequate processability in orthogonal polar solvents for the fabrication of multilayer devices, but also interact with metal electrode to realign the energy levels across the interface. [ 8 , 9 ]

Another widely employed strategy to improve the contact property in organic electronic devices is to introduce a highly conductive charge-transporting layer between the electrode and the active semiconductor layer to achieve Ohmic contact. [ 10 ] A conducting polymer, poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), is the most commonly used hole-transporting layer to improve the anode contact of OPVs and OLEDs. The high electrical conductivity of PEDOT:PSS is the result of protonic doping of PEDOT conjugated polymer by polystyrene sulfonic acid. However, PEDOT:PSS has several drawbacks as an interfacial layer including large electrical and microstructural inhomogeneities, [ 11 ] poor electron blocking property, [ 12 ] and the tendency of eroding the electrodes. [ 13 , 14 ] Doping of conjugated polymers using strong oxidizing agents (e.g., FeCl 3 , I 2 , and NOPF 4 ) [ 15 ] or strong electron acceptors (e.g., tetrafl uoro-tetracyanoquinodimethane (F 4 -TCNQ) [ 16 ] and poly-oxoymetalate [ 17 ] ) can also produce highly conductive polymers but their effect as charge-transporting layers in organic elec-tronic devices is less studied. [ 17 ]

In addition to using a doped bulk fi lm as an interfacial layer, surface doping through interfacial charge transfer between the dopants and organic semiconductors has also been explored as a new approach to improve the contact between semiconductor and electrode in organic electronic devices. [ 18 ] Reduction of contact resistance through the surface charge transfer doping in OFETs has been demonstrated by inserting a thin layer of F 4 -TCNQ acceptor at the pentacene/metal interface. [ 19 ] In addition, F 4 -TCNQ has also been utilized as a p-dopant for an oligothiophene-based self-assembled monolayer (SAM) modi-fi ed indium tin oxide (ITO) electrode to improve hole injection in OLEDs. [ 20 ] Recently, fl uorinated alkylsilane monolayers have been explored as p-type surface dopants for organic semicon-ductor single crystals and shown signifi cantly increased surface conductivities. [ 21 ] Although these surface doping approaches have shown some promising results for improving the electrical contact in organic electronic devices, the development of new surface dopants that are solution-processable, non-diffusive, and environmentally stable is needed to fully explore the poten-tial of surface doping to improve the performance of organic electronic devices.

Here, we demonstrate that graphene oxide (GO) can be used as a new type of solution-processed dopant to effi ciently p-dope the surface of conjugated polymers. GO is an oxidized deriva-tive of graphene, which is prepared by chemical oxidation of

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Figure 1 . a) FTIR spectrum of GO. The insert shows a structure model of GO. b) High-resolution C 1s XPS spectrum of GO deposited on Au (50 nm)/Si subtrate. c) The pH titration curve of GO versus the amount of added NaOH.

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naturally abundant graphite. [ 22 ] Its solution processibility and unique electrical and optical properties make it a promising nanomaterial for various applications. [ 23 ] We have found that when a thin layer of graphene oxide sheets is deposited on a poly(3-hexylthiophene) (P3HT) fi lm, the electrical conductivity of P3HT increased by six orders of magnitude. The dramatic increase in conductivity is attributed to the protonic doping of the surface of P3HT by the GO layer, which was confi rmed by the observation of new charge transfer absorption peaks in the near infrared region. In order to confi rm the surface doping mechanism of GO, various techniques such as X-ray photo-electron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and pH titration analysis were performed to inves-tigate the compositions of the surface chemical species. Finally, we have demonstrated that surface doping of GO can be used to improve the contact property of polymer solar cells to achieve much enhanced performance.

Although GO was fi rst prepared 150 years ago, its exact chemical structure is still ambiguous due to its amorphous and nonstoichiometric nature. It is widely accepted that GO consists of two regions: an aromatic region with sp 2 -hybridized carbon and an oxidized region that contains carbon with oxy-genated functional groups such as hydroxyl, epoxy, carboxylic, and carbonyl groups. [ 24–26 ] Moreover, Dékány et al. have also demonstrated the presence of phenolic groups in GO. [ 27 ] The fi nal structure of GO is dependent on the chemical oxidation process used during its synthesis. [ 28 ] In our study, GO was synthesized by the modifi ed Hummers’ method. [ 26 ] Figure 1 a shows the FTIR spectrum of GO. Various chemical species on GO can be readily identifi ed, namely, edge carboxyl groups (C = O stretching at 1733 cm − 1 and O–H vibration at 3425 cm − 1 ), phenolic groups (C–O stretching at 1225 cm − 1 with O–H vibra-tion at 3425 cm − 1 ), tertiary hydroxyl groups (O–H stretching at 3425 cm − 1 with a bending mode at 1375 cm − 1 ), epoxy groups (C–O stretching at 1285 cm − 1 and bending at 850 cm − 1 ), car-bonyls (C = O stretching at 1733 cm − 1 ), and sp 2 -hybridized C = C (asymmetric vibrational stretching at 1625 cm − 1 ).

In addition to using FTIR spectroscopy to study the sur-face species, XPS was also employed to study the quantitative elemental compositions and chemical states of GO. Figure 1 b shows the high resolution C 1s XPS spectrum of GO. Peak fi t-ting of this curve yields four components at 284.7 eV, 286.7 eV, 288.0 eV, and 290.9 eV. The component at 284.7 eV is assigned to be the C = C bonds in the non-oxidized aromatic network and also possibly to the C–C bonds at the defect sites. The defect C–C component is typically found to be at ≈ 285.0 eV. However, due to the convolution with the C = C peak, it is diffi cult to dis-cern them in the spectrum. The components at 286.7 eV and 288.0 eV can be attributed to the carbon in hydroxyl, phenol and epoxy groups (C–O bonds) and carbon in carboxyl and car-bonyl groups (C = O bonds), respectively. The smallest peak at 290.9 eV is assigned to π – π ∗ shake-up satellite in the aromatic network. [ 29 ] The atomic ratio of carbon to oxygen is calculated to be 2.58, which means for each six-membered carbon ring of GO, the empirical chemical formula is C 2 O 0.78 . See Supporting Information for the composition ratio of each component and detail calculation of the C:O ratio.

Due to the presence of chemical species such as carboxylic acid, phenolic and enolic hydroxyl groups that can lead to proton

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dissociation, GO is acidic in nature. [ 30 ] These proton sources can potentially be explored for surface doping at the GO/semiconducting polymer interface through a proton transfer process. In fact, improved conductivity of bulk polyaniline has been reported when doped with GO. [ 31 ] Protonic doping of con-jugated polymers has also been observed for other carbon-based

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Figure 2 . a) UV-vis absorption spectra of fi lms of P3HT, GO, and same fi lm of P3HT with GO on its surface. b) UV-vis absorption spectra of fi lms of P3HT, GO partially neutralized by alternatively spin coating of GO monolayer and tetrabutylammonium hydroxide, and same fi lm of P3HT with partially neutralized GO on its surface.

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nanomaterials such as sulfonated fullerenes [ 32 ] and sulfonated carbon nanotubes. [ 33 ]

To determine the amount of exchangeable protons on GO surface, GO was carefully titrated with 10 m M NaOH aqueous solution. As shown in Figure 1 c, the titration curve does not exhibit a distinctive infl ection point, suggesting that GO con-tains more than one kind of exchangeable protons. The protons may come from two equilibrium reactions: dissociation of i) COOH located at the edge of GO sheets and ii) phenolic and enolic groups, which are randomly distributed on the GO sur-face. [ 27 ] These ionizable groups are in different environments at molecular level, leading to a broad distribution of p K a , which is probably the reason for the relatively featureless titration curve. [ 34 ] It is known that carboxylic acid groups dissociate in the pH interval of 3–6 and that phenolic and enolic groups dis-sociate at higher pH, which is in agreement with the titration data. [ 35 ] The total amount of exchangeable protons was evalu-ated to be 3.15 mmol g − 1 of GO at the titration endpoint near pH 10. [ 36 ] The result from the semiquantitative analysis shows that the proton density of GO provides one exchangeable proton per 8.7 carbon rings (see detail calculation in the Supporting Information). Although the various acidic groups have different p K a values, the overall p K a is determined to be ≈ 3.71 (see Sup-porting Information for the detail calculation of p K a ). It should be noted that this p K a value is even lower than the frequently observed value for carboxylic acid (for example, the p K a value of acetic acid is 4.8). This can be explained by the delocalization of the 2p electrons of functional-oxygen into the π -conjugated system, resulting in a stabilized negative charge on the surface oxygen. Therefore, the mobility of associated protons increases, leading to lower p K a values. [ 35 ]

Doping of conjugated polymers by GO is evidenced by the measurements using UV-vis absorption spectroscopy. Figure 2 a shows the absorption spectra of a pristine P3HT thin fi lm, a GO thin fi lm ( ≈ 3 nm), and P3HT with GO deposited on top of it. The pristine P3HT fi lm exhibits a strong π – π ∗ absorp-tion peak at ≈ 560 nm. Upon the deposition of GO on top of the polymer surface, the intensity of the π – π ∗ absorption at ≈ 560 nm decreases accompanied by the appearance of a new, broad peak at ≈ 960 nm indicating the formation of polarons/bipolarons in P3HT. [ 16 ] Since the size of GO sheets are in the range of a few hundred nanometers to several micrometers measured by atomic force microscopy (AFM) (see Supporting Information for the AFM image of GO), it is unlikely that GO can migrate into the bulk of the conjugated polymer fi lm com-pared to smaller molecular dopants. As a result, doping is con-fi ned at the polymer surface. The depth of doping for the P3HT fi lm is determined to be ≈ 1 nm by measuring the loss of the absorption intensity of the π – π ∗ peak in doped fi lms. It is also found that the doping depth is independent of the thickness of the polymer fi lms. (see Supporting Information for the calcu-lation of doping depth). To further verify the protonic doping mechanism, neutralization of GO was carried out by soaking into 0.1 M tetrabutylammonium hydroxide in methanol after its deposition on P3HT fi lm (see Supporting Information for the detail of neutralization experiment). Tetrabutylammonium hydroxide is a strong organic base that can neutralize a variety of acids. Since tetrabutylammonium hydroxide molecules cannot throughtly penetrate into the interface of P3HT and GO

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 1903–1908

due to the large size of GO sheets, GO deposited on P3HT can only be partially neutralized. However, the absence of absorp-tion intensity decrease from the π – π ∗ peak of P3HT and the much weaker doping peak at the near-IR region confi rm the protonic doping mechanism of GO.

To investigate the effect of GO surface doping on the elec-trical properties of conjugated polymers, OFET devices based on the pristine P3HT fi lm and the P3HT fi lm doped with a thin layer of GO ( ≈ 3 nm) were tested. The pristine P3HT device exhibits typical p-type semiconductor characteristics with a hole mobility of 1 × 10 − 4 cm 2 V − 1 s − 1 and an on-off current ratio of 10 4 ( Figure 3 a,b). In this device, an applied gate voltage induces a hole accumulation layer at the interface of P3HT/SiO 2 , which constitutes the p-type conducting channel between the source and drain electrodes. Upon the deposition of GO, the device shows an increase of approximately six orders of magnitude in current with little depedence on the gate voltage, which means the device is always in the “on” (high-source–drain current) state (Figure 3 c). The output curve of the GO-doped device exhibits a linear I D – V D characteristic (Figure 3 d), where I D and V D are the drain current and drain voltage, respectively, suggesting a metallic-like conduction. As demonstrated, the surface of P3HT

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Figure 3 . Electrical characteristics of pristine P3HT, P3HT with the surface doped by GO (P3HT/GO), and P3HT with partially neutralized GO OFET devices. Transfer curves (– I D vs V G , (– I D ) 1/2 vs V G ) of a) pristine P3HT OFET, c) P3HT/GO OFET, e) GO OFET, and f) P3HT/partially neutralized GO (n-GO) with different soaking time in 0.1 M tetrabutylammonium hydroxide solution in methanol. The inserts shows the schematic cross sections of corresponding devices. Output characteristics of b) pristine P3HT and d) P3HT/GO OFET devices.

can be doped by GO sheets. We hypothesize that the high proton density in combination with the unique 2D structure of GO sheets creates a layer of stable, negatively charged GO that is immobilized at the GO/P3HT interface, with a heavily doped P3HT as the corresponding hole-transporting layer just below the interface, as shown in the insert of Figure 3 c. The sp 2 -regions of the GO sheets are in contact with electrodes and the bottom P3HT fi lm, however, they are isolated from each other by the oxidized non-conjugated regions, therefore, charges cannot be effi ciently transported laterally within the GO layer, as show in Figure 3 e, but can only tunnel through it. [ 37 ] The conductivity of the GO-doped P3HT thin layer is calculated to be ≈ 3.70 S m − 1 (Supporting Information), which is about six orders of magnitude higher than that of pristine

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P3HT. [ 24 ] Although an applied gate voltage can induce an accu-mulation layer in P3HT at the interface with SiO 2 , high con-ductivity makes the GO/P3HT interface the primary channel for injected charges. As a result, the applied gate voltage has little infl uence on the source–drain current, as shown in Figure 3 d. Compared with P3HT/GO OFET devices, P3HT/partially neutralized GO devices show one order of magnitude decrease in current (Figure 3 f), which gives another evidence for the pro-tonic doping mechanism of GO. Since the neutralization of GO deposited on P3HT fi lm depends on the diffusion of tetrabutyl-ammonium hydroxide molecules, longer soaking time leads to a lower current as shown in Figure 3 f.

The surface doping phenomenon can be exploited to improve the contact properties between electrode and polymer

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Figure 4 . The current density–voltage ( J – V ) characteristics of inverted polymer solar cells with a GO interfacial layer ( ≈ 3 nm, solid symbol) and without a GO interfacial layer (hollow symbol) using different top metal electrodes.

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for organic electronic devices such as OPV. Recently, Li et al. and Gao et al. have demonstrated that the incorporation of GO between the photoactive P3HT:[6,6]-phenyl C 61 butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) layer and the anode can result in dramatically increased power conver-sion effi ciency (PCE) of both conventional- and inverted-type OPVs. [ 37 , 38 ] We attribute the improved device performance to the formation of Ohmic contact between BHJ and anode due to surface doping.

The effect of surface doping on the performance of inverted P3HT:PCBM OPVs using metals (Al, Ag, Au) with different work functions as the top anode and ZnO-coated ITO as the bottom cathode was further investigated. [ 39 ] The current density–voltage curves under illumination are shown in Figure 4 . All devices without a GO layer between the BHJ and the metal electrodes show quite poor performance with low short circuit currents ( J sc ) and open circuit voltages ( V oc ). A clear trend of increased V oc with increasing anode work function is observed, which is in agreement with the metal-insulator-metal model in which the upper bound of V oc is governed by the difference between the work functions of the electrodes when the contacts are non-Ohmic in nature. [ 40 ]

However, when a GO thin layer is incorporated, all the devices show improved performance with a V oc of ≈ 0.62 V, inde-pendent of different metals used as the anode. This is because the heavily doped P3HT thin layer at the interface facilitates the formation of an Ohmic contact between the active layer and the top metal electrode. If Ohmic contact can be achieved at each electrode, the V oc is governed by the difference between the lowest unoccupied molecular orbital (LUMO) of PCBM and the highest occupied molecular orbital (HOMO) levels of P3HT. Compared to devices with a Ag anode, devices based on a Au anode exhibit a lower PCE because Au is less refl ective in the visible range, which may reduce the total optical density in the BHJ. [ 41 ] Devices based on a Al anode also shows lower performance, with a larger series resistance due to the poten-tial formation of thin insulating aluminum oxide layer at the buried interface between the metal electrode and BHJ.

In summary, we report that surface doping of conjugated polymers can be realized by the deposition of a thin layer of

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 1903–1908

GO on top of the polymers. The presence of carboxylic acid and phenolic and enolic groups in GO results in a high proton den-sity of one exchangeable proton per 8.7 carbon rings, leading to the protonic doping of conjugated polymer at the GO/polymer interface. The conductivity of doped P3HT thin fi lms was found to be as high as 3.70 S m − 1 with a metallic-like characteristic. The unique 2D structure of GO prohibits it from penetrating into the bulk of conjugated polymers, making it an ideal mate-rial for surface doping. This fi nding represents a new strategy for improving the charge transport across the metal/conjugated polymer interface, which is demonstrated in inverted polymer solar cells.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors are thankful for support from the National Science Foundation (DMR-0120967), the Department of Energy (DE-FC3608GO18024/A000), the Air Force Offi ce of Scientifi c Research (FA9550–09-1–0426), the Offi ce of Naval Research (N00014–08-1–1129), and the World Class University (WCU) program through the National Research Foundation of Korea under the Ministry of Education, Science and Technology (R31–21410035). A.K.-Y.J. thanks the Boeing Foundation for support. Y.G. thanks the State-Sponsored Scholarship for Graduate Students from China Scholarship Council. The authors also thank Prof. Fumio S. Ohuchi for his support of the XPS facility utilized in this study.

Received: January 7, 2011 Revised: February 9, 2011

Published online: March 15, 2011

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