Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Morphological, optical, and electrical investigations ofsolution-processed reduced graphene oxide and its application totransparent electrodes in organic solar cells

Jin-Mun Yun a, Chan-Hee Jung a, Yong-Jin Noh b, Ye-Jin Jeon c, Seok-Soon Kim d,Dong-Yu Kim c,*, Seok-In Na b,*a Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute (KAERI), 29 Geumgu-gil, Jeongeup-si,

Jeollabuk-do 580-185, Republic of Koreab Professional Graduate School of Flexible and Printable Electronics and Polymer Materials Fusion Research Center, Chonbuk National University, 664-14,

Deokjin-dong, Jeonju-si, Jeollabuk-do 561-756, Republic of Koreac Heeger Center for Advanced Materials, School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712,

Republic of Koread School of Materials Science and Chemical Engineering, Kunsan National University, Kunsan, Chonbuk 753-701, Republic of Korea

A R T I C L E I N F O

Article history:

Received 29 January 2014

Received in revised form 22 April 2014

Accepted 22 April 2014

Available online xxx

Keywords:

Transparent electrodes

Reduced graphene oxide

Organic photovoltaic cells

A B S T R A C T

Morphological, optical, and electrical properties of solution-processed reduced graphene oxide (r-GO)

thin-films are investigated by varying on the number of spin-coating cycles and annealing temperature.

Spin-coated r-GO thin-films all show full-covered morphologies with highly small rms roughness values

ranging from �1 to 2.5 nm. The sheet resistance of r-GO films can be controlled over two orders of

magnitude by controlling both the spin-coating cycles and annealing temperature while conserving the

transmittance values of 57–87%. The combination of coating cycles and heat-treatment temperature

make the r-GO films as a useful transparent hole-extraction electrode for the application to organic

photovoltaic cells.

� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Graphene, the emerging carbon material with one-atom-thickand two-dimensional (2D) honeycomb-lattices [1], has beenattracted increasing attention in electronic and optoelectronicfields such as organic light-emitting devices (OLEDs) and organicphotovoltaic cells (OPVs) due to its unique mechanical, optical, andelectrical properties [2,3]. Among the various applications,graphene-based thin-films have been studied immensely for useas transparent conducting electrodes due to its superior opticaland electrical properties [4–6]. In particular, these properties aswell as low-cost and abundant sources make them as a promisingtransparent conducting electrode to replace expensive, brittle, andacid-sensitive indium tin oxide (ITO) [7–9], which is widely used asa transparent electrode in optoelectronic applications. In practice,

* Corresponding authors. Tel.: +82 63 570 3055; fax: +82 63 570 3098.

E-mail addresses: kimdy@gist.ac.kr (D.-Y. Kim), nsi12@jbnu.ac.kr, seokinna@g-

mail.com (S.-I. Na).

Please cite this article in press as: J.-M. Yun, et al., J. Ind. Eng. Chem

http://dx.doi.org/10.1016/j.jiec.2014.04.026

1226-086X/� 2014 The Korean Society of Industrial and Engineering Chemistry. Publis

to replace the ITO with graphene-based electrodes, mostgraphene-based thin-films were made by chemical vapor deposi-tion (CVD) method followed by transferring on target substrates[10,11]. However, fabrication of graphene-based electrode throughthe CVD method can result in the cost-increase during the devicemanufacturing. In addition, a high temperature and complicatedtransfer process are not suitable for incorporating it into low-cost,fast, and solution-based production of organic solar cells that couldbe fabricated on large-area and various substrates [12,13]. In thisregard, an alternative strategy for producing graphene via achemical route has been enormously reported for the utilization ofthe graphene-based transparent electrodes prepared by thereduction of graphene oxide [14–21].

Typically, graphene oxide (GO) is insulator or semiconductordue to the disruption of the hexagonally sp2-bonded carbon latticeduring oxidation of graphite [22], thus reduction steps of GO viachemical or thermal methods are inevitably needed to restoregraphitic lattice structure that can enhance carrier transport andelectrical properties [14–21]. These methods show a great promiseto integrate r-GO into organic solar cells as a transparent electrode;

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hed by Elsevier B.V. All rights reserved.

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however, they are still incompatible with the low-cost andsolution-processed OPVs due to the following problems: (a) inthe case of the thermal reduction method, a high-temperatureranging from �400 to 1000 8C and an ultra-high vacuum conditionwere employed to make r-GO-based electrodes [14,15,17–19,25],which is not adequate for the production of OPVs. (b) In addition, r-GO prepared using reducing agents including hydrazine, sodiumborohydride, ascorbic acid, etc., showed a low dispersion-concentration that precludes a uniform and full-covered r-GOfilm through a spin-coating process [23]. For this reason, avacuum-filtration/transfer method is widely used to fabricatethickness-controllable and uniform r-GO thin-films [16–19]. Thesecomplicated, time-consuming, and high temperature fabricationprocesses over 1000 8C are not suitable for the low-temperature,solution-process, and fast production advantages of OPVs onvarious flexible substrates. Thus, it remains a big challenge toprepare a simple and solution-processable r-GO film withhomogeneous and thickness controllable properties under alow-temperature condition. Furthermore, considering that OPVparameters including current density (Jsc), shunt resistance (Rsh),and series resistance (Rs) are highly correlated with transmittance,film-morphology, and conductivity of the r-GO film, respectively[23], there have been limited publications for investigating optical,morphological, and electrical properties of solution-processed r-GO thin-films that can influence on the performance of OPVs.

In this study, we have investigated the morphological, optical,and electrical properties of the solution-processed r-GO thin-filmsand further demonstrated that the r-GO films have a potential astransparent conducting electrodes in OPVs. From the morphologi-cal and optical analysis, all r-GO thin-films fabricated through aspinning method showed a highly uniform morphology withthickness controllability. In addition, sheet resistance wasefficiently decreased from �103 to 10 kOhm/sq by the combinationof controlled coating cycles and annealing conditions. Finally, wefurther investigated the effects of r-GO-based electrodes on theOPV performances by controlling the number of coating cycles. Asa result, OPV performances were gradually increased withdecreasing the sheet resistance and the highest efficiency of0.33% was obtained using 200 8C-annealed and 7-times coated r-GO film.

2. Experiments

2.1. Sample preparation

To prepare graphene oxide (GO) via a modified Hummersmethod [24], graphite oxide (1 g) was added to concentratedH2SO4 (98%, 50 ml) with stirring, and then KMnO4 (3 g) as anoxidizing agent was slowly added to the mixture for 1 h. Theresulting mixture was then heated to 40 8C for 6 h andsubsequently quenched by pouring over crashed ice (500 g)containing hydrogen peroxide (30 wt%, 10 ml) to remove metalspecies and MnO2 compounds. The resultant suspension of GO wasfiltered and thoroughly washed with 1 M hydrogen chloridefollowed by washing with acetone and deionized water. Finally,gel-like GO was dialyzed in deionized water for one week withstirring.

The reduction of GO with a p-toluenesulfonyl hydrazidereducing agent was performed according to the previous ourreport [23].

3. Device fabrication

Glass substrates (Samsung Corning Co, Ltd.) were cleaned witha special detergent followed by ultrasonication in deionized water,acetone, and isopropyl alcohol each for 15 min, and then kept in an

Please cite this article in press as: J.-M. Yun, et al., J. Ind. Eng. Chem

100 8C oven for 30 min. Before the preparation of the r-GO film,cleaned glass substrates were treated with UV/O3 for 15 min toincrease wettability. After this, r-GO dispersed in N,N-dimethyl-formamide (DMF) at a concentration of �0.6 mg/ml was spin-casted onto the UV/O3-treated glass substrates varying on thenumber of coating cycles ranging from 1 to 7 cycles. To investigatethe effects of thermal treatment of r-GO films on the morphologi-cal, optical, electrical, and transparent electrode properties, as-deposited and annealed r-GO films each at 100, 150, and 200 8C for20 min in ambient atmosphere were fabricated. The resulting r-GO-based films were used as hole-collecting electrodes in organicsolar cells.

For the use of r-GO films as transparent electrodes in OPVs, Al(80 nm) was deposited using a thermal evaporator in vacuum witha pressure of 10�6 Torr. And then, poly (3,4-ethylenedioxythio-phene):poly 3-styrenesulfonate (PEDOT:PSS, Baytron P AI 4083)was spin-coated onto the ITO (Samsung Corning Co. Ltd, 18 Ohm/sq) and r-GO substrates at a rate of 5000 rpm for 40 s followed byannealing at 110 8C for 10 min in air. The active layer containing25 mg of P3HT (Rieke metals) and 25 mg of PCBM (Nano-C)dissolved in o-dichlorobenzene (1 ml) was spin-coated on top ofthe PEDOT:PSS-coated ITO and r-GO substrate at 700 rpm for 60 s.Then, the resulting active layer deposited onto substrates was keptin a glass jar at room temperature to slowly evaporate o-DCBsolvent for 2 h in an N2-filled glove box, followed by annealing at110 8C for 10 min. Finally, top electrodes consisted of Ca (20 nm)/Al(100 nm) with an area of 4.14 mm2 were deposited using a thermalevaporator in vacuum with a pressure of 10�6 Torr.

4. Characterization

Cell performance was measured using a Keithley 2400instrument operated under 100 mW/cm2 (1 sun condition) usinga xenon light source and AM 1.5 global filter. A reference Si solarcell certified by the International System of Units (SI) (SRC-1000-TC-KG5-N, VLSI Standards, Inc.) was used for calibration foraccurate measurement. The optical transmittance values of r-GOthin-films were measured by using UV–vis spectrophotometerwith a Varian, AU/DMS-100S. Surface morphologies of r-GO filmswere conducted by atomic force microscopy using a VeecoDimension 3100 instrument operated in tapping mode with asilicon cantilever. Electrical properties were obtained by 4-pointprobe measurement (FPP-RS8, Dasol Eng.). The work-function ofvarious r-GO films were taken by a Kelvin probe (KP 6500 DigitalKelvin probe, McAllister Technical Services. Co., Ltd.).

5. Results and discussion

To investigate the potential use of a solution-processed r-GOthin-film as transparent electrodes and characterize the effect of r-GO films varying the film thickness and heat-treatment on the cell-performances in OPVs, we prepared r-GO according to the previousour report [23]. As depicted in Scheme 1, the reduction of GO wasperformed using a p-toluenesulfonyl hydrazide (p-TosNHNH2)reducing agent. The prepared r-GO has a distinct hydrazone groupsattached to the edge and basal plane of graphene, which can renderr-GO a high-dispersion concentration and thin-film processabilityvia a facile spin-coating method due to a reduced van der Waalsinteraction, thus preventing re-aggregation between r-GO sheets.These features could overcome the problems of the previouslyreported solution-processed r-GO-based electrodes where theyfabricated by means of a time-consuming vacuum filtration/transfer method or a high-temperature reduction process of pre-coated GO [14–19,25], which is incompatible with simple, low-temperature, and fast production advantages of OSCs [12,13].

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Scheme 1. A schematic of reduced graphene oxide (r-GO) prepared by the reduction of GO using the p-toluenesulfonyl hydrazide reducing agent.

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Based on this advantage, we first evaluated morphologicalproperties of solution-processed r-GO thin-films varying thefabrication conditions such as spin-coating cycles and annealingtemperatures, as shown in Fig. 1. As a result, it can be seen thatindividually deposited and fully covered r-GO thin-films with alateral dimension of �1–3 mm were clearly observed in the AFMresult. In addition, as increasing the number of film coating cyclesfrom 1 to 7, the surface morphologies were gradually rougher,indicating that multi-layer deposition can be feasible by simplychanging the coating cycle. In particular, it is notable that as-deposited film morphology was preserved regardless of heat-treatment conditions, which is different from the previous reportswhere chemically prepared graphene films with wrinkles or folds

Fig. 1. AFM height images of as-deposited and annealed r-GO thin-films wit

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were easily observed depending on heat-treatment conditions[26]. This difference might be due to the fact that large amount ofoxygen functional groups such as hydroxyl, epoxy, and carboxylicacid were efficiently converted and removed to gaseous formsthrough thermal treatment, thus affecting r-GO film morphology[26].

To examine the precise surface roughness, we calculated theroot mean square (rms) roughness values based on the AFM resultsof Fig. 1. As shown in Fig. 2, the rms roughness was graduallyincreased with increasing the number of coating cycles due tooverlapping the individual graphene sheets. For pristine, 100, and150 8C-annealed samples with the same coating cycle, the rmsvalues were mostly decreased with an increase in annealing

h the different number of coating cycles onto corning glass substrates.

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Fig. 2. The rms roughness values of solution-processed r-GO thin-films varying the

number of coating cycles and heat-treatment conditions.

Fig. 3. The optical transmittance spectra of r-GO thin-films as-deposited (a), and annealed

of the dependence of transmittance values at 550 nm versus number of coating cycles

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temperature probably due to a removal of residual solvent. Thistrend in a decreased rms value was particularly distinct in at200 8C-annealed samples, which is attributed to the fact that theprepared r-GO exhibited large mass loss above at 150 8C aspreviously presented in thermogravimetric analysis (TGA) [23],thus resulting in more densely packed r-GO films.

More importantly, all samples showed very small rmsroughness values ranging from 1 to 2.5 nm, revealing that r-GOfilms deposited via a solution-process have a potential astransparent electrodes in OPVs, considering that homogeneouslycovered film morphologies with small surface roughness arerequired for reducing contact resistance and protecting devicebreakdown resulted from an inhomogeneous and rough filmmorphology of an electrode [23]. Thus, these results demonstratethat highly uniform and fully covered r-GO films with smallsurface roughness even in the case of multi-layer deposition can beeasily obtainable through a spin-coating process, which aredistinctive advantages compared to previous reports on r-GO-based electrodes.

Next, we investigated optical transmittance of the r-GO films asshown in Fig. 3. In Fig. 3(a)–(d), it is obvious that the transmittance

at 100 (b), 150 (c), and 200 8C (d) varying the number of coating cycles. Comparison

(e).

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Fig. 4. Sheet resistance of solution-processed r-GO thin-films as-deposited (a), and annealed at 100 (b), 150 (c), and 200 8C (d) varying the number of coating cycles.

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was shown to monotonously decrease in �300–1200 nm wave-length range without clear absorption peaks as increasing thenumber of film coating cycles. In addition, there is a distincttransmission difference between 200 8C-annealed r-GO films andothers as plotted in Fig. 3(e) where transmittance values weremore decreased in 200 8C-annealed r-GO films compared withother r-GO films, indicating that further reduction process wassignificantly occurred [23]. More surprisingly, the plot of thetransmittance values at 550 nm versus number of coating cycles ofr-GO films was shown in Fig. 3(e) and was found to a highly linearrelationship, which revealing the successful fabrication of thick-ness-controllable r-GO thin-films through a simple solution-process. To more precisely explain thickness-controllability ofthe r-GO thin-films, we measured the thicknesses of the r-GO filmsusing AFM. As a result, the corresponding thickness of pristine r-GO films fabricated by 1, 3, 5, and 7 times coating cycles wereapproximately 2, 5, 9, and 14 nm, respectively. From themorphological, optical, and thickness analysis, highly uniform

Fig. 5. J–V curves of cells based on ITO (a), and r-GO e

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and thickness-controllable r-GO thin-films can be successfullyfabricated by a facile spin-coating method and these findingssuggest that the r-GO films could become attractive for simple andsolution-processed transparent electrode applications.

To evaluate electrical performances of spin-coated r-GO thin-films with controlled thicknesses and degree of thermal reduction,we measured sheet resistance values, as shown in Fig. 4. For thefilms varying only spin-coating cycle, sheet resistance values weremoderately decreased by a factor of around 1. In addition, for thefilms processed with the same coating cycles but differentannealing temperature, a decrease in sheet resistance was also afactor of �1. Thus, the combination of the film thickness controland thermal treatment condition reduced the sheet resistance ofthe r-GO thin-films from �103 to �10 kOhm/sq, revealing that theresultant r-GO could be used as the anode electrode in proof ofconcept organic solar cells.

To investigate the effects of r-GO thin-films with different film-thicknesses and annealing temperatures on device performances

lectrodes fabricated with different coating cycles.

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Table 1Representative device parameters of cell with ITO and r-GO electrodes.

Electrode Voc [V] Jsc [mA/cm2] FF [%] PCE [%]

ITO 0.60 8.93 61.09 3.19

r-GO 1 0.56 0.71 25.05 0.10

r-GO 3 0.56 0.94 25.10 0.13

r-GO 5 0.56 1.33 25.24 0.19

r-GO 7 0.56 2.26 25.76 0.33

Table 2Work-function values of ITO and r-GO electrodes.

Electrode ITO r-GO (200 8C-annealed sample)

Coating cycle – 1 3 5 7

Work-function (eV) 4.62 4.65 4.65 4.66 4.64

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and thus to directly evaluate the feasibility and potential of themas transparent electrodes in OPVs, cells based on poly (3-hexylthiophene):phenyl-C61-butyric acid methyl ester(P3HT:PCBM) were fabricated. As observed in Fig. 4, the sheetresistance of r-GO films showed the lowest values when annealedat 200 8C compared with other samples. Thus, we tested thesesamples as transparent and conducting electrodes in OPVs. Inaddition, to enhance the hole transport property from the highestoccupied molecular orbital (HOMO) level of P3HT to the r-GOanode by blocking electron transport and reducing recombinationof holes and electrons at interfaces, a thin-layer of PEDOT:PSS wasintroduced between the anode and the photoactive layer. Thecurrent density–voltage (J–V) curves of cells based on ITO and r-GOelectrodes were shown in Fig. 5 and the corresponding deviceparameters were summarized in Table 1. For the cell with ITOelectrode, the open circuit voltage (Voc), the short circuit currentdensity (Jsc), the fill factor (FF), and the power conversion efficiency(PCE) were 0.60 V, 8.72 mA/cm2, 61.09%, and 3.19%, respectively.For the cells with r-GO electrodes, with an increase in the thicknessof r-GO films, the PCEs were obviously increased from 0.1 to 0.3%,however, these values are much more inferior to that of ITO-basedcell. To reveal the reason for poor performances of r-GO-basedcells, we first examined the Voc parameter. All devices with thedifferent thickness of r-GO films showed the same Voc of �0.56 V.In general, Voc was goverened by the energetic difference betweenHOMO level of the P3HT and the lowest unoccupied molecularorbital (LUMO) level of the PCBM in BHJ OPV devices [27].However, compared to the ITO-based cell, slightly reduced Voc

values were seen for devices on r-GO electrodes, which is probablydue to a higher series resistance (Rs) and a smaller shunt resistance(Rsh) as shown in the J–V curves of r-GO-based cells underillumination, considering that the work-function (WF) values of200 8C-annealed r-GO films regardless of the number of coatingcycles were approximately �4.64 to �4.66 eV that is similar tothat of ITO (�4.60 eV), as shown in Table 2. Moreover,PEDOT:PSS deposited onto the r-GO electrode formed ohmiccontact within the anode/active layer interface. For these reason,the reduced Voc might be attributed to a poor electrical property ofr-GO electrode.

Next, the effects of FF and Jsc values of devices with r-GOelectrodes on cell-performances were further investigated. Asincreasing the number of coating cycles of r-GO, the FF showed aslight increase from �10 to �26% due to a reduction of sheetresistance. Furthermore, with increasing the thickness of r-GO film,the Jsc value was noticeably increased from 1 to 3 mA/cm2, directlyresulting in a 3-fold enhancement of the PCE from 0.1 to 0.3, asshown in Table 1. These results indicate that the lower efficiency ofthe cell on r-GO electrode is mainly due to a poor Jsc compared withthe cell on ITO, which results from the high sheet resistance of thechemically prepared graphene electrode. Although the efficiencywas much lower than that of ITO, it is worth noticing that the PCEswere similar to the previous results of cell-performances based onchemically prepared r-GO electrodes where the r-GO electrodesfabricated by means of harsh heat-treatment up to �1000 8C ordoping treatments were used as transparent electrodes in OPVs[14,15]. More importantly, our r-GO electrodes can be fabricated

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via a spin-coating methods compatible with low-cost, simple, andlow-temperature solution-processing advantages of organic solarcells, compared with the previous reports of r-GO electrodesprocessed by vacuum filtration/transfer or reduction of a spin-casted GO at high temperature [14–20,25].

6. Conclusion

In conclusion, we have systematically investigated morpholog-ical, optical, and electrical properties of the solution-processed r-GO thin-films and further demonstrated the corresponding r-GOfilms as a transparent conducting electrode in OPVs. From themorphological and optical measurements, highly uniform, full-covered, and thickness controllable r-GO thin-films can besuccessfully fabricated via a simple spinning method, which isdifferent from the previously reported r-GO electrodes made byvacuum filtration/transfer or high-temperature reduction of GOfilm. By combination of annealing temperature and coating cycles,the sheet resistance of r-GO film was efficiently reduced by a factorof �3 while conserving the transmittance values between 57 and87%. As a result, the device with r-GO electrode fabricated by200 8C-annealed and 7 coating cycles showed the Voc of 0.56 V, theJsc of 3 mA/cm2, the FF of 30%, and the PCE of 0.33%, which wasinferior cell-performance in comparison with ITO-based cells. Inaddition, by comparing the OPV-performances of cells increasingthe coating cycle from 1 to 7, the Jsc values were obviouslyenhanced from 1 to 3 mA/cm2, reflecting that device performanceswere significantly influenced by sheet resistance of r-GO film.Thus, for improving an electrical property of solution-processed r-GO thin-film, further methods including composite of r-GO/carbonnanotube or r-GO/polyaniline are currently underway in order toimprove device characteristics of OPVs.

Acknowledgements

This work was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning(NRF-2013R1A1A1011880), the National Research FoundationGrant funded by the Korean Government (the Ministry ofEducation, Science and Technology) (2012M2A2A6035267).

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