recent progress on reduced graphene oxide

DOI: 10.1002/ijch.201400213

Recent Progress on Reduced Graphene Oxide-BasedCounter Electrodes for Cost-Effective Dye-Sensitized SolarCellsSuresh Kannan Balasingam[a] and Yongseok Jun*[b]

1. Introduction

Since modern society relies entirely on an energy-basedeconomy, the existing fossil fuel reserves are expected torun out in a few decades, causing future generations toface a severe energy crisis. Moreover, the existing fossilfuel-based energy technologies emit greenhouse gases,which have a great impact on the environment, includingwhat is known as global warming.[1] To solve these twokey issues, alternative energy conversion systems thatcould solely depend on renewable energy resources (e.g.,solar, wind, tidal, or geothermal energy) need to be de-veloped. Of these, solar cells are one of the prominentenergy conversion devices, in which the direct conversionof solar to electrical energy could be feasible. In addition,the sun supplies an abundant amount of solar energythroughout the day, indicating that this energy source isunlimited. The solar cell technology currently availableon the market is based on first- and second-generationsolar cells (e.g., silicon, CdTe, and copper indium gallium[di]selenide solar cells) that have the merits of high effi-ciency and long-term stability. However, the cost of thesemodules is very high, which prevents this technologyfrom reaching widespread clients. To reduce the cost ofsolar cells, third-generation solar cells have been underprogressive research for the past few decades.[2] Dye-sen-sitized solar cells (DSSCs) are one type of third-genera-tion solar cells that has attracted many researchers be-cause of their numerous merits, including the following:low cost, ease of fabrication, variety of colors, and highpower conversion efficiency (PCE).[3] Recent literature

on DSSCs reported a maximum efficiency of around13%, which revealed that the technology is commerciallyviable if we could reduce the cost of the solar cell compo-nents.[4] DSSCs are composed of dye-coated mesoporousTiO2 on fluorine-doped tin oxide (FTO)-coated glass asa working electrode, an iodine-based electrolyte, anda thin Pt layer coated on FTO-coated glass as a CE. Thetwo most expensive components are the FTO-coatedglass substrate and Pt-coated CE. Substitution of theFTO-coated glass substrate with various metal substrateseither on the working or counter electrode part could cer-tainly reduce the cost of the DSSCs. This possibility wasbriefly discussed in our previous feature article.[5] Anoth-er approach is to replace the noble Pt metal-based CEswith carbonaceous materials and/or non-noble metaloxide/chalcogenide-based composite catalysts, which havebeen briefly described in previous review articles.[6] Of

Abstract : Dye-sensitized solar cells (DSSCs) are one type ofhighly efficient low-cost solar cells among third-generationphotovoltaic devices. Replacing the expensive componentsof DSSCs with alternative inexpensive and earth-abundantmaterials would further reduce their price in the solar cellmarket. Recently, graphene-based low-cost counter electro-des (CEs) have been developed, which could serve as a po-tential alternative to the expensive platinum-based CEs. In

this review article, we have summarized recent research onvarious reduced graphene oxide (rGO)-based composite CEmaterials, methods for their synthesis, their catalytic activity,and the effective utilization of such CEs in DSSCs. The pho-tovoltaic performance of DSSCs made of rGO-based compo-site CEs were compared with the reference Pt-based cells,and the photovoltaic parameters are summarized in tables.

Keywords: composites · counter electrodes · catalysis · reduced graphene oxide · dye-sensitized solar cells

[a] S. K. BalasingamDepartment of ChemistrySchool of Natural ScienceUlsan National Institute of Science and Technology (UNIST)Ulsan 689-798 (Republic of Korea)

[b] Y. JunDepartment of Materials Chemistry and EngineeringKonkuk UniversitySeoul 143-701 (Republic of Korea)Tel: (+82) 2-450-0440e-mail: [email protected]

Isr. J. Chem. 2015, 55, 955 – 965 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 955

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these, graphene, a new class of carbon allotrope, has beeninvestigated by many researchers because of its low cost,high surface area, high electrical conductivity, and me-chanical stability. Graphene materials have been inten-sively studied by many researchers for the past few years,and some review articles that generally focus on gra-phene-based materials for solar cell applications werepublished at least two years before.[7] According to theWeb of Science database, research on alternative CEs forDSSCs has dramatically increased in the past few years,particularly since 2010. A couple of years ago, Wang et al.published a mini review article, which focused particularlyon graphene-based CE materials for DSSCs.[8] In thepresent review article, we describe recent research trendsin the area of rGO-based alternative CE materials, themethod of reduction of GO into rGO, the catalytic activi-ty of rGO-based composite materials and their PCE in

real-time DSSCs, with comparison to reference Pt-basedCEs.

2. Reduced Graphene Oxide-Based Composites

2.1 Metal Nanoparticle/rGO Composites

Basic trials of rGO for DSSC CEs have been performed.Types of rGO used in recent representative trials includerGO ribbons,[9] jet-processed rGO,[10] and rGO producedby a green photothermal reduction process.[11] In additionto the standard method, rGO ribbons have also been pre-pared by wet-spinning, a process in which an aqueous gra-phene oxide (GO) solution is injected into a chitosan-based solution. The method provides various types of rib-bons such as oriented, randomly wrinkled, or spring-likeloops, by controlling drying conditions. Hydrochloric acidwas used to reduce the as-spun rGO ribbons, and theirconductivity reached approximately 100–150 Scm¢1. ADSSC device with the rGO ribbon CE exhibited 2.80 %efficiency with short circuit current density (Jsc) of8.77 mAcm¢2, open circuit voltage (Voc) of 0.61 V, andquite a low fill factor (FF). This low performance was im-proved to 3.87 % efficiency once Pt nanoparticles wereembedded in the materials. Another recently reportedwork, which applied a rapid atmospheric pressure plasmajet process method, showed improved results. To preparea CE of DSSCs according to this method, rGO was firstprepared as a paste by mixing it with an organic vehicle.The paste was then screen-printed on the FTO-coatedglass substrate, and then the electrode fabrication processwas completed by plasma jet flow. Because the tempera-ture of the substrate reaches over 400 8C, the organic ve-hicles are burned-out, and rGO forms. A DSSC devicewith this type of rGO exhibited efficiencies up to 5.19%,whereas a reference cell with a conventional Pt CE exhib-ited an efficiency of 5.65%.

In most cases, rGO itself was not a suitable candidatefor the CE of DSSCs. Therefore, many research groupsattempted to achieve improved performance by addingmetal nanoparticles to the rGO. The trials include silver,nickel, tungsten, as well as platinum, nanoparticles. Em-ploying metal nanoparticles provides a few advantages,such as reduction of the required loading of metal cata-lyst, improvement of the conductivity of the rGO, and anincrease in the transmittance of the CEs due to the re-duced amount of metal loading. Gong et al. reported thata Pt/graphene composite can reduce the amount of Ptneeded by approximately 1000-fold with comparable effi-ciency.[12] Guai et al. also utilized GO to prepare CEsusing an electrophoretic method. These researchers pre-pared a GO solution and then reduced the solution intographene nanosheets by the chemical reduction with hy-drazine.[13]

The dispersed graphene nanosheets were electrophoret-ically deposited onto Pt-sputtered indium tin oxide

Suresh Kannan Balasingam is currentlya Ph.D. candidate at the Ulsan Nation-al Institute of Science and Technology(UNIST), Republic of Korea. He re-ceived his B.Sc. degree in chemistryfrom the Madura College (affiliatedwith Madurai Kamaraj University),India, in 2006. Balasingam completedhis M.Sc. degree in industrial chemistry(specialization in electrochemistry)with a gold medal from Alagappa Uni-versity, India, in 2008. After that, he ob-tained two years of research experienceat the Central Electrochemical Research Institute (CECRI-CSIR, Gov-ernment of India) and one year of research experience at the Techni-cal University of Denmark, prior to joining UNIST. His current re-search interests mainly focus on supercapacitors and photoelectro-chemical energy conversion and storage, encompassing DSSCs,photoelectrochemical water splitting, and CO2 reduction.

Yongseok Jun is presently an associateprofessor at Konkuk University, Repub-lic of Korea. He received his B.Sc. andM.Sc. degrees at Korea University(Prof. Kang-Jin Kim’s group) and hisPh.D. degree at the University of Min-nesota at Minneapolis (Prof. XiaoyangZhu’s group). He obtained three yearsof research experience at the Electronicand Telecommunications Research In-stitute (2006–2009), followed by fiveyears of teaching and research experi-ence, first as an assistant professor(2009–2012) and then as an associate professor (2012–2013) atUNIST, before starting his position at Konkuk University. His mainresearch interests include DSSCs, perovskite solar cells, and photo-chemical reactions with TiO2 nanostructures. He has authoredmore than seventy peer-reviewed journal articles, twenty-seven pat-ents, and two book chapters, including those on flexible DSSCsbased on metal substrates.

Isr. J. Chem. 2015, 55, 955 – 965 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.ijc.wiley-vch.de 956

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(ITO)-coated glass, or Pt was sputtered on graphene-coated ITO. The highest temperature applied for the pro-cesses was only 95 8C for the chemical reduction using thehydrazine agent. When the researchers prepared the CEsusing this method, the amount of Pt loading varied from1.9–5.6 mgcm¢2. Irrespective of the amount of Pt loading,devices made of Pt/rGO CEs exhibited quite comparableperformance to devices made with only Pt/ITO. A refer-ence device with a conventional Pt CE exhibited a powerconversion efficiency (PCE) value of 7.03 %, whereas de-vices with graphene-assisted Pt exhibited PCE values of7.10–7.57%.

Tjoa et al. prepared a rGO/Pt composite using light-as-sisted spontaneous co-reduction of GO and chloroplatinicacid without any reducing chemicals.[14] The illuminationtransfers energy for the reduction, and the solvent (etha-nol) works as a hole scavenger to form very small Ptnanoparticles with simultaneous reduction of GO. Thismethod formed very uniform Pt nanoparticles that were3 nm in size, whereas hydrazine reduction often yields Ptparticles with 20–200 nm diameters because of the vigo-rous nature of the reduction. The light was from an incan-descent bulb (100 W), and the local intensity was50 Wm¢2. Briefly, the sheet resistance values of the nano-materials on the glass substrate using the two different re-duction methods (the light-assisted method and the con-ventional hydrazine method) are summarized in Table 1.

Dao et al. successfully applied a rGO/Pt nanohybrid asa robust CE for DSSCs.[15] To fabricate the CEs, they pre-pared a GO paste by simply mixing GO powder in ethylcellulose and terpineol. The paste was applied on theFTO-coated glass using doctor-blade method and driedout at 300 8C for 30 minutes. Finally, a small amount of Ptprecursor was dropped on the electrode, followed by Arplasma treatment for 15 minutes. Figure 1 shows the mor-phology of the Pt nanoparticles/rGO hybrid. The imagesshow the general morphology of the Pt/rGO hybrid films,including the uniform distribution of metal nanoparticleson the film and the reduced nucleation of Pt nanoparti-cles compared with the conventional Pt-only method. Ac-cording to the Nyquist plot from electrochemical impe-dance spectroscopy (EIS) analysis, the Pt/rGO CE exhib-ited a charge transfer resistance (Rct) value of 0.52 Wcm2,which is only half of the Rct value of the Pt-sputtered CE.The GO-only CE exhibited 20 times higher values for Rct

than that of the Pt/rGO CE. Finally, the PCE value ach-ieved for the Pt/rGO CE was approximately 8.56%,

whereas the Pt-sputtered CE exhibited a PCE value of8.18%.

Jang et al. reported a transparent rGO/Pt compositeCE for DSSCs, fabricated using a pulsed current electro-deposition method.[16] GO-dispersed deionized water/di-methylformamide was spin coated onto FTO-coated glass,followed by heating at 350 8C for 10 minutes in air. Forthe Pt addition, pulse current electrodeposition was per-formed in a proper electrolyte solution with a Pt precur-sor. Figure 2 presents the transmittance spectra of variousCEs.

Other types of metals, including Ni, Ag, and W, wereused to decorate the rGO surface.[17] Similarly to Pt, theother metal nanoparticles were evenly distributed on therGO surface. The distribution of Ni nanoparticles wasperformed using a pulsed laser ablation process. The stan-dard Pt CE exhibited Rct and PCE values of 7.73 Wcm2

and 2%, respectively, and the Ni/rGO CE exhibited Rct

and PCE values of 4.67 Wcm2 and 2.19%, respectively.

Table 1. Sheet resistance measurement of spray-coated Pt/rGO thinfilm on glass (the thickness is approximately 0.8–1.0 mm)

Method of reduction Light assisted Hydrazine reduction

Pt/GO GO

Sheet resistance 0.3–3.2 kW 11.39 kW 15.52 kW

Figure 1. SEM images of (a) a GO CE and (b) Pt-distributed rGO. (c)The TEM image shows that the Pt nanoparticles are evenly distrib-uted. (d) Energy-dispersive X-ray spectroscopy (EDS) analysis and(e) XRD pattern of the sample on a Si wafer substrate. Reprintedwith permission from reference [15].

Figure 2. Transmittance spectra of various CEs. Reprinted with per-mission from reference [16].


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Silver nanowires and nanoparticles were embedded onrGO either by drop casting or pulsed laser reduction. Thebest cell performance was obtained when silver nanopar-ticles were combined with rGO, which resulted in a PCEvalue of 7.72%. However, the standard Pt electrode ex-hibited a higher PCE value of 8.25%. Tungsten-decoratedrGO CE also exhibited comparable Rct and PCE values.The best PCE value of 5.88% was obtained for W/rGOCE, which is almost comparable to that of the Pt-basedcells (5.92 %). The photovoltaic characteristics of theabove-mentioned DSSCs made of metal nanoparticles/rGO composite CEs are summarized in Table 2.

2.2 Metal Compound/rGO Composites

Various types of metal compounds, such as transitionmetal oxides (TMOs) and transition metal dichalcoge-nides (TMDs), have been under consideration as alterna-tive CE materials for DSSCs. These compounds can beeasily synthesized, are low in cost, and are abundant innature. Therefore, many researchers have performed in-tense research on these materials, and their catalytic abili-ties have been investigated in depth. Because of the pos-sibility of increasing the conductivity and synergic catalyt-ic ability, the mixing of rGO with these metal compoundshas recently received intense attention. Dou et al. synthe-sized a Ni12P5/rGO composite using the simple thermalhydrolysis of elemental P, nickel chloride, and GO in anethylene glycol-water mixture.[18] When Ni12P5 was pre-pared without GO, the morphology of the Ni12P5 was sim-ilar to a honeycomb structure. When GO was introduced,Ni12P5 formed in between the inter-layers of rGO, al-though some nanoparticles were located on the outer sur-face. Evaluation using EIS indicated that the Rct values atthe CE/electrolyte interface of individual Pt (as a refer-ence), Ni12P5, Ni12P5/rGO, and rGO electrodes were11.05 W, 6.22 W, 4.93 W, and 17.41 W in their system, re-spectively.

The enhanced electrocatalytic performance of rGO/SiO2 nanoparticles was introduced by Gong et al.[19] A

schematic illustration of their preparation is presented inFigure 3. The fabrication method is primarily drop cast-ing, and the rGO/SiO2 composite was simply prepared bymixing the two components with a hydrazine hydrate re-ducing agent. SiO2 nanoparticles were used to increasethe volume for coating, as observed in Figure 3. The Bru-nauer-Emmett-Teller (BET) surface area and pore sizedistribution were investigated for the film. The theoreticalBET surface area of graphene is approximately2620 m2 g¢1. The BET surface area of the as-prepared gra-phene film in this experimental condition (without silicananoparticles) was only 8.6 m2 g¢1, with a pore size of0.002 cm3 g¢1. When silica nanoparticles were embeddedinto the rGO, the BET surface area was increased to229.0 m2 g¢1, with a pore volume of 0.66 cm3 g¢1. The BETsurface area for silica nanoparticles used was 151.8 m2 g¢1.The specific surface area of graphene in the CEs was cal-culated to be 383.4 cm3 g¢1, after subtracting the contribu-tion of the silica nanoparticles. A DSSC with the rGO/SiO2 CEs exhibited a PCE value of 6.82%.

NiO is a common TMO semiconductor material.Bajpai et al. demonstrated that NiO-decorated rGO ex-hibits better electrocatalytic ability than pure NiO, andthe material could be applied as a CE for DSSCs, whichshowed a PCE value of near 3.06%.[20] Another NiO/

Figure 3. Schematic illustration of preparation and structure ofrGO and rGO/SiO2 CEs. Reprinted with permission from reference[19].

Table 2. Photovoltaic parameters of DSSCs made of metal nanoparticle/rGO CEs and the reference DSSCs made of conventional Pt-basedCEs

Sl. No rGO-based CEs[a] Pt-based CEs[b] Ref.

Description Voc Jsc FF h Rct[c] Voc Jsc FF h Rct

[d]

(V) (mA cm¢2) (%) (Wcm2) (V) (mA cm¢2) (%) (Wcm2)

1 Pt/rGO 0.72 14.1 0.67 6.77 – 0.72 13.1 0.672 6.29 – [14]2 Pt/rGO 0.771 16.66 0.667 8.56 0.62 0.763 16.80 0.638 8.18 1.22 [15]3 Pt/rGO 0.71 14.98 0.663 7.05 ~5[e] 0.71 14.11 0.589 5.82 ~7[e] [16]6 Ag/rGO 0.732 14.67 0.72 7.72 1.02 0.733 16.24 0.69 8.25 0.76 [17a]4 Ni/rGO 0.7 5.21 0.6 2.19 4.67 0.68 5.05 0.59 2.00 7.73 [17b]5 W/rGO 0.72 10.02 0.63 4.55 – 0.69 13.62 0.63 5.92 – [17d]6 Ag NW/rGO 0.55 6.45 0.52 1.61 11.57 0.58 6.47 0.44 1.87 16.47 [17c]

[a] DSSCs made of rGO-based CEs. [b] DSSCs made of Pt-based conventional reference electrodes. [c] Rct of rGO-based CEs from EIS analy-sis. [d] Rct of Pt-based CE from EIS analysis. [e] No specific area, so the corresponding unit is W not Wcm¢2.


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rGO CE was attempted by Dao et al.[21] These researchersprepared Ni-decorated rGO by drop casting of a Ni pre-cursor, followed by Ar plasma reduction. The PCE valuefor the Ni/rGO CE was 7.42 %, whereas the Pt-sputteredreference cell exhibited a PCE value of 8.18%.

A few other metal oxides have also been combinedwith rGO. Hausmannite (Mn3O4) exhibits a distinctivestructure and unique physicochemical properties, such asmagnetic and catalytic effects. A Mn3O4/rGO suspensionwas prepared by mixing GO and metal precursors, fol-lowed by addition of hydrazine hydrate reducing agent.[22]

The morphology of the obtained product was aggregatednanoparticles. The Rct (PCE) values of the CEs withMn3O4/rGO and Pt reference electrode were 5.24 W

(5.90 %) and 1.43 W (6.84%), respectively. ZnO nanorodswere also used for the CE with rGO addition.[23] TheZnO nanorods were grown on FTO-coated glass, anda laser pulse-reduced GO suspension was spin coated ontop. The Rct value of ZnO is greater than 4000 Wcm2, andonce rGO was added, the Rct decreased to 4 Wcm2, whichremains higher than the value for Pt CE of 0.5 Wcm2. ThePCE value of the ZnO/rGO composite CE is 8.12%,which is quite comparable to the Pt-based CEs (8.82%).

MoS2 has also received intensive attention, and MoS2/rGO CE was introduced by Liu et al. for DSSC CEs.[24]

MoS2 has a hexagonal structure, where Mo metal atomsare located in between sulfur layers. The structure is verysimilar to the graphene structure. Therefore, it is consid-ered as an inorganic graphene analogue. A solution ofGO and ammonium tetrathiomolybdate was prepared,from which a MoS2/rGO composite was easily formed

under H2 flow at 650 8C. The higher conductivity of rGOhelps electrons shuttle to MoS2 sites. In addition, theMoS2 exhibited catalytic reduction of I3

¢ ions. The Rct

value for MoS2/rGO CE is 0.57 Wcm2, whereas the valuefor the Pt-sputtered reference CE is 1.93 Wcm2. However,the PCE value for a device with MoS2/rGO CE (6.04%)was lower than that for a device with the Pt-sputtered CE(6.38 %) by 0.34%. The main difference in performanceis due to the lower Jsc value. Similarly, Lin et al. reportedthe electrophoretic deposition of transparent MoS2/rGOcomposite films as the CE for DSSCs.[25] These research-ers synthesized the MoS2/rGO composite by thermal hy-drolysis/dissociation of thiourea in the presence of GO.The precursor of Mo was sodium molybdate. This CE ex-hibited a high transmittance of approximately 70 % nearthe visible light region (400–800 nm). The Rct values forMoS2, MoS2/rGO, and sputtered Pt CE are 3.65 W cm2,2.34 Wcm2, and 1.79 Wcm2, respectively.

NiS/rGO CE was introduced by Li et al.[26] The com-pound was also synthesized by the hydrothermal reactionof nickel ions and sulfur precursors with GO. These re-searchers compared the peak-to-peak separation (Epp),which is inversely correlated with the standard electro-chemical rate constant of a redox reaction. The peak cur-rent values are clearly important factors. The Epp valuesfor NiS2, NiS2/rGO, and Pt electrodes were observed tobe 0.65 V, 0.5 V, and 0.44 V, respectively. Note that Epp

for rGO was not obtained from the experimental windowof the cyclic voltammetry (CV) scanning. The PCE valuefor a device with the NiS2/rGO composite was 8.55%,whereas the devices made of Pt or NiS2 alone exhibited

Table 3. Photovoltaic parameters of DSSCs made of metal compounds/rGO CEs and the reference DSSCs made of conventional Pt-basedCEs



[d]


1 Ni12P5/rGO 0.727 12.86 0.61 5.70 4.93[e] 0.744 13.12 0.62 6.08 11.01[e] [18]2 SiO2/rGO 0.72 15.52 0.61 6.82 39.8[e] 0.72 15.79 0.64 7.28 2.6[e] [19]3 NiO/rGO 0.67 7.53 0.61 3.06 0.85 0.71 8.04 0.63 3.57 0.63 [20]4 NiO/rGO 0.763 15.57 0.624 7.42 3.06 0.763 16.8 0.638 8.18 1.22 [21]5 MoS2/rGO 0.73 12.51 0.66 6.04 0.57 0.72 13.42 0.66 6.38 1.93 [24]6 MoS2/rGO 0.773 12.79 0.59 5.81 2.34 0.763 13.12 0.62 6.24 1.79 [25]7 NiS2/rGO 0.749 16.55 0.69 8.55 2.9 0.739 15.75 0.70 8.15 0.5 [26]8 Ta3N5/rGO 0.837 13.53 0.693 7.85 1.39 0.828 13.38 0.685 7.59 1.80 [27]9 TaON/rGO 0.829 13.38 0.69 7.65 1.9 0.835 13.73 0.69 7.91 1.8 [28]

10 CZTS/rGO 0.71 16.77 0.656 7.81 13.33[e] 0.70 16.79 0.567 6.66 23.89[e] [29]11 CIS/rGO 0.728 16.61 0.576 6.96 0.98 0.751 14.77 0.624 6.92 1.73 [30]12 CoS2/rGO 0.73 15.12 0.60 6.55 1.3 0.73 14.69 0.58 6.20 1.9 [31]13 CoS/rGO 0.710 20.38 0.74 10.71 2.58 0.698 20.21 0.69 9.73 4.62 [32]14 Bi2S3/rGO 0.74 15.33 0.608 6.91 4.98 0.73 16.47 0.618 7.44 1.70 [33]15 Mn3O4/rGO 0.635 15.20 0.61 5.90 5.24 0.635 16.25 0.66 6.84 1.43 [22]16 ZnO/rGO 0.765 21.7 0.671 8.12 4 0.777 24.2 0.708 8.82 0.50 [23]17 CoS/rGO 0.718 14.95 0.66 7.08 1.01 0.711 15.53 0.65 7.18 0.85 [345]18 NiSe/rGO 0.73 15.82 0.61 6.94 0.15 0.71 16.00 0.60 6.82 1.43 [35]

[a] DSSCs made of rGO-based CEs. [b] DSSCs made of Pt-based conventional reference electrodes. [c] Rct of rGO-based counter electrodesfrom EIS analysis. [d] Rct of Pt-based CEs from EIS analysis. [e] No specific area, so the corresponding unit is W not Wcm¢2.


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values of 8.15% and 7.02 %, respectively. NiSe2/rGOcould be prepared by hydrothermal treatment with pre-cursors such as NiCl2, selenium, ascorbic acid, and GO.[35]

Microsphere- and octahedron-structured NiSe2 areformed in this reaction, the as-prepared materials exhibit-ed quite low Rct values of 0.15 Wcm2 and 0.25 W cm2, re-spectively, whereas Pt exhibited an Rct value of1.43 Wcm2.

Li et al. prepared Ta-containing metal compounds suchas Ta3N5 or TaON with rGO.[27,28] The Rct values for theCE of Pt, rGO, Ta3N5/rGO, and TaON/rGO are1.80 Wcm2, 7.0 Wcm2, 1.39 Wcm2, and 1.9 Wcm2, respec-tively. Ta3N5/rGO and TaON/rGO composite CEs exhibit-ed quite comparable PCE values (7.85% and 7.65 %, re-spectively) to the Pt-based reference cells (7.6%).

Copper indium zinc can be used to prepare sulfur com-pounds, which can also be applied as a CE material forDSSCs as well as for thin-film solar cells starting fromcopper indium gallium selenide (or sulfide). Bai et al. [29]

prepared a copper zinc tin sulfide (CZTS)/rGO compo-site, which was successfully applied as a CE for DSSCs.CZTS was successfully synthesized in an ethylene glycolsolvent, and it was then dispersed in ethanol and ethyleneglycol to make a paste. Similarly, Zhou et al. [30] preparedCuInS2/rGO. Although they did not mention the cell areafor the EIS experiment, a rough comparison might bepossible. For CZTS/rGO CE, the highest PCE value is7.81%, with an Rct value of 13.33 W. CIS/rGO CE exhibit-ed a PCE value of 6.96% with an Rct value of 0.99 W.

A few additional metal sulfide/rGO compounds havealso been investigated as CE materials. Hydrothermalsynthesis of a CoS/rGO composite at 180 8C was per-formed by two research groups, and the composite wassuccessfully applied as a CE for DSSCs.[31,34] These re-searchers achieved a PCE value of 7.08 % with an Rct

value of 1.01 Wcm2. Similarly, Bi et al. prepared CoS/rGO(N [nitrogen doped]) and applied it as a CE for DSSCs.[32]

The CoS/rGO (N) CE exhibited Rct and Epp values of2.58 W and 0.31 mV, respectively. The Epp value is quitesmall even when compared with the Pt CEs. The CE ofPt exhibited Rct and Epp values of 4.62 W and 0.4 mV, re-spectively.

Bi2S3 has also been combined with rGO and exhibitedcatalytic ability for the CE of DSSCs.[33,36] Bi2S3 is a semi-conductor with a direct bandgap of 1.7 eV. The compositewas synthesized by solvothermal treatment at 150 8C, andthe shape of the product was a microsphere composed ofnanorods. The mechanism of formation was suggested tobe the nucleation of a GO sheet, as illustrated inFigure 4. Mesoporous Bi2S3 prepared by Guo et al. exhib-ited a similar nanostructure. The highest PCE and Rct

values of the Bi2S3-treated CE were 6.91 % and 4.98 W,respectively, when a 9 wt% rGO composite was used.The reference Pt cell exhibited a PCE value of 7.44 %with an Rct value of 1.70 W. CdS particles were also treat-ed with rGO, and they exhibited good catalytic effects

on the electrochemical analyses.[37] The photovoltaiccharacteristics of the above-mentioned DSSCs made ofmetal compound/rGO composite CEs are summarized inTable 3.

2.3 Polymer/rGO Composites

Various conducting polymer/graphene composites weresynthesized by different methods. Of these, polyaniline/graphene nanocomposites were synthesized and appliedas a CE for DSSCs by Wang et al. in 2012.[38] Initially, theGO was reduced by a hydrothermal method in a Teflon-lined autoclave at 180 8C. Then, the reduced GO wasmixed with an aniline monomer, followed by an in situpolymerization reaction. The as-prepared polyaniline(PANI)/graphene composite was used as a CE materialfor DSSCs, and a corresponding PCE value of 6.09 % wasobtained, which is comparable to the reference Pt-basedcells. Liu and co-workers also synthesized graphene-modi-fied polyaniline-based CE materials for DSSCs.[39] Theyreduced GO into rGO using thermal exfoliation and re-duction method. The graphene obtained was mixed withan aniline monomer and subjected to an electropolymeri-zation reaction on an FTO substrate. The device made ofthe graphene-modified polyaniline CE exhibited a PCEvalue of 7.17%, which is nearly equal to that of the refer-ence Pt-based DSSCs (7.24 %) prepared under similarconditions.

Niu and colleagues followed the in situ reduction ofGO into rGO during the polymerization of aniline intopolyaniline.[40] These researchers prepared a polyaniline/rGO composite via a one-step chemical synthesis. The re-duction of GO into rGO was confirmed by Raman spec-troscopy. The DSSC made of 1-Fe2O3 as the photoanodematerial and PANI-rGO as the CE exhibited a PCEvalue of 1.24%, which is much lower than those of theother TiO2-based photoanode DSSCs.

Recently, Wang and co-workers directly depositeda PANI/rGO composite onto an FTO-coated glass sub-

Figure 4. The formation process of the Bi2S3 microspheres. Most ofthe nanostructure growing mechanisms on rGO are very similar.Reprinted with permission from reference [36].


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strate using electrostatic deposition.[41] Initially, the PANI/GO composite was deposited onto the FTO, which wasfurther subjected to chemical reduction with hydroiodicacid to form the PANI/rGO CE. DSSCs made of thePANI/rGO CE exhibited efficiency (7.84 %), which iscomparable to that of the Pt-based reference cells(8.19 %) during front illumination by sunlight. For therear-side illumination process, DSSCs made of both CEsexhibited similar PCE values (Pt—6.1%; PANI/rGO—6.08%). Therefore, the authors claimed that the PANI-rGO CE material could be a potential candidate for bifa-cial DSSCs. Wan et al. also synthesized a polyaniline/gra-phene nanocomposite via the facile in situ polymerizationof aniline in a graphene solution.[42] Initially, GO was re-duced to rGO by chemical reduction (hydrazine and am-monia solution). The device made of this nanocompositeCE exhibited a lower PCE value of 4.46%, comparedwith the Pt-based cells (5.71 %).

PANI-graphene/GO multilayer CEs were synthesizedby Wang and colleagues.[47] Initially, positively chargedPANI-graphene complexes were prepared by reflux. Thepositively charged PANI-graphene complex was multi-layered with negatively charged GO using repeated alter-nating immersion of an FTO-coated glass substrate in sol-utions of PANI-graphene and GO. The device made fromthe multi-layered CE exhibited the best PCE value of7.88% with a Voc of 0.727 V, Jsc of 16.79 mA cm¢2, and FFof 0.646. The authors did not compare their values withthose of reference Pt-based cells.

Polypyrrole (PPy)/rGO nanocomposite CEs were syn-thesized by Lim et al. , via in situ electrochemical polymer-ization.[43] The device made with the PPy/rGO nanocom-posite CE exhibited a similar PCE value (2.21 %) to thereference Pt-based cells (2.19 %). Liu and co-workers alsoprepared a PPy/rGO composite CE on rigid FTO andflexible plastic substrates via a low-temperature electro-chemical deposition method.[44] They followed a two-stepelectrochemical process. In the first step, the PPy/GOcomposite was electrochemically deposited onto the con-

ducting substrate. In the second step, the as-depositedPPy/GO was electrochemically reduced into PPy/rGO.DSSCs made of the PPy/rGO composite CE exhibiteda PCE of 6.45 %, which is comparable to that of the Pt-based cells. The authors also fabricated a PPy/rGO CEon a conducting plastic substrate (polyethylene naphtha-late-ITO). The efficiency of the DSSC made of the flexi-ble substrate was lower than that using the rigid glass-based substrate. However, the PCE value of PPy/rGO(4.25 %) was still comparable to that of the Pt-based ref-erence cells (4.83%). Gong and colleagues synthesizeda rGO/PPy composite CE via in situ reduction of GO/PPy to rGO/PPy.[45] The composite rGO/PPy exhibiteda superior PCE value compared with that of the individu-al components (PPy and rGO CEs). The Rct and PCEvalues of rGO/PPy are almost equal to those of the refer-ence Pt-based cells.

Xu et al. synthesized poly(diallyldimethylammoniumchloride) (PDDA)/rGO composite CEs for DSSCs.[46] Ini-tially, the PDDA/GO composite was formed by layer-by-layer assembly of negatively charged GO and positivelycharged PDDA. In the subsequent step, the as-depositedPDDA/GO was electrochemically reduced to PDDA/rGO. DSSCs made from PDDA/rGO CEs exhibitedhigher PCE values than the Pt-based reference cells. Thephotovoltaic characteristics of the above-mentionedDSSCs made with polymer/rGO composite CEs are sum-marized in Table 4.

2.4 Functionalized or Doped rGO Composites

Compared with other rGO-based materials, functional-ized or doped rGO CEs exhibit superior electrocatalyticactivity. In 2012, Xu et al. functionalized rGO with aniron-containing porphyrin, called hemin, using a micro-wave-assisted chemical reduction method (hydrazine hy-drate and ammonia).[48] The as-prepared hemin-function-alized rGO was coated on FTO by drop casting followedby drying. The Rct value (at the CE/electrolyte interface)

Table 4. Photovoltaic parameters of DSSCs made of polymer/rGO composite CEs and reference DSSCs made of conventional Pt-based CEs



[d]

(V) (mAcm¢2) (%) (Wcm2) (V) (mA cm¢2) (%) (Wcm2)

1 PANI/graphene nanocomposite 0.685 13.28 0.67 6.09 – 0.695 14.20 0.70 6.88 – [38b]2 Graphene modified PANI 0.67 16.28 0.67 7.17 0.64 0.69 15.01 0.70 7.24 0.22 [39]3 rGO/PANI composite 0.57 4.07 0.528 1.24 28[e] 0.55 2.94 0.580 0.94 30[e] [40]4 PANI/rGO 0.775 15.8 0.64 7.84 0.71 0.780 15.9 0.66 8.19 0.69 [41]5 PANI/graphene nanocomposite 0.69 12.19 0.53 4.46 – 0.75 11.36 0.67 5.71 – [42]6 PPy/rGO nanocomposite 0.70 7.49 0.42 2.21 14.8 0.71 5.12 0.60 2.19 14.3 [43]7 PPy/rGO composite 0.695 15.48 0.60 6.45 6.59[e] 0.70 15.45 0.66 7.14 4.19[e] [44]8 rGO/PPy 0.725 15.81 0.71 8.14 5.0 0.724 16.00 0.72 8.34 1.1 [45]9 PDDA/rGO composite 0.692 18.77 0.74 9.54 – 0.686 18.11 0.74 9.14 – [46]

[a] DSSCs made of rGO-based CEs. [b] DSSCs made of Pt-based conventional reference electrodes. [c] Rct of rGO-based CEs from EIS analy-sis. [d] Rct of Pt-based CEs from EIS analysis. [e] No specific area, so the corresponding unit is W not Wcm¢2.


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of the hemin-functionalized rGO was 9 W, which wasslightly higher than that of the reference Pt-based CEs(7 W). It is apparent that the DSSCs made of hemin-func-tionalized rGO CEs exhibited a lower PCE value(2.45 %) than that of the Pt-based reference cells. The de-tailed I-V curve parameters are listed in Table 5.

Nitrogen-doped reduced graphene oxide (NG) hasbeen investigated as an efficient CE material by many re-searchers. Yen and colleagues synthesized nitrogen-dopedgraphene sheets by combined chemical (ammonia and hy-drazine hydrate) and hydrothermal reduction of GO.[49]

The DSSCs made of NG film CEs exhibit a superior PCEvalue (4.75%) compared with that of the undoped one(1.92 %), yet their efficiency is slightly lower than the Pt-based reference cells (5.03 %). Wang et al. also preparednitrogen-doped graphene via the hydrothermal reductionof GO in the presence of ammonia.[50] CV studies re-vealed that the NG samples exhibited similar redox peakscompared with those of the reference Pt electrodes,whereas in the pristine graphene electrodes, no distinctredox peaks were observed. Therefore, the authors claim-ed that the higher electrocatalytic activity of the NG elec-trode is predominantly due to the structural defectscaused by the doping with nitrogen atoms, which havea lone pair of electrons and, in turn, bring negativecharge onto the delocalized p-electron system of gra-phene. The PCE value of the nitrogen-doped graphenewas 6.12%, with a Jsc of 15.19 mAcm¢2, Voc of 0.683 V,and FF of 0.59. The PCE value is comparable to that thePt-based reference cells (6.97 %).

Xue et al. fabricated three-dimensional nitrogen-dopedgraphene foams (3D-NGFs) by annealing freeze-driedGO foams in ammonia.[51] The three-dimensional NGFsexhibited a superior PCE value (7.07 %) compared withthat of the undoped rGO foams (rGOFs; 4.84 %). More-over, the PCE values of the NGFs and rGOFs were stillhigher than those of the corresponding film structures.Generally, nitrogen doping enhanced the electrical con-ductivity and resulted in good catalytic activity. In addi-tion, the 3D-foam structure with a large surface area andwell-defined pores enhanced the catalytic sites for I3

¢/I¢

redox.In 2013, Wang et al. also adopted the hydrothermal

route to prepare nitrogen-doped graphene sheets(NGSs).[52] The DSSC made of an NGS CE exhibited anefficiency of 7.01%, which is comparable to that of thePt-based reference cells (7.34 %). The NGS-based sym-metrical cells exhibited an Rct value of 0.9 Wcm2, which ismuch smaller than that of the pristine graphene-basedsymmetric cells (14.1 W cm2). Zhang and co-workers pre-pared NGSs by heating GO at high temperatures (600–900 8C for 2 h) in the presence of ammonia.[53] The NGSsample was mixed with polytetrafluoroethylene, and theresultant slurry was coated on a stainless steel substrate.The device made of the NGS electrode heated at 600 8Cexhibited a higher PCE value than those of Pt and sam-

ples treated at higher temperatures. Notably, the Voc

values of all the NGS-based DSSCs (0.858–0.868 V) arehigher than that of the reference Pt-based cells (0.796 V).The authors suggested that the higher Voc values may bedue to the shift of the flat-band potential, which, in turn,enhances the reactivity and electrocatalytic performanceof DSSCs.

Ju and colleagues synthesized N-doped graphene plate-lets (NGPs) via a novel two-step sequential method.[54] Inthe first step, edge-aminobenzoyl functionalized graphite(EFG) was attained using edge-selective functionaliza-tion. In the second step, the EFG powder was heated tohigh temperature (900 8C for 2 h) under a nitrogen atmos-phere to obtain the NGPs. The PCE of the NGP-basedDSSCs exhibited a superior value (9.05 %), which isslightly higher than the Pt-based reference cells (8.43%),a Jsc of 13.83%, Voc of 0.883 V, and FF of 0.74. Notably,the Rct value of the NGP electrodes (3.06 Wcm2) is muchsmaller than the Rct value of the Pt-based reference cells(8.44 Wcm2).

The same group also selectively doped nitrogen at theedges of graphene nanoplatelets (NGnPs) via a simpleball milling reaction in the presence of nitrogen gas.[60]

DSSCs made of an NGnP-based CE exhibited a lower Rct

value and a higher PCE value than those of the referencePt-based DSSCs. The authors also adopted a similar ballmilling process to selectively synthesize edge-carboxylat-ed graphene nanoplatelets (ECGnPs) in the presence ofcarbon dioxide.[61] The device made of oxygen-richECGnPs exhibited superior catalytic activity and a higherPCE value than that of the chemically reduced GO andPt-based CEs.

Recently, Song and co-workers adopted the previouslydescribed concept of Xue et al. , three-dimensional porousgraphene foams, but performed the nitrogen doping byhydrothermal treatment of a solution of GO and ammo-nia.[55] The N-doped porous graphene foams (NPGFs)that formed exhibited a PCE value of 4.5 %, which isslightly lower than their Pt-based reference cells (4.9%).Compared with the previous report by Xue et al. , the re-ported PCE value of the NPGFs is much smaller in bothiodine and sulfide-based electrolytes. Very recently, brick-like N-doped graphene/carbon nanotube (NGC) compo-site three-dimensional films were synthesized by Maet al.[56] Initially, the carbon nanotubes (CNTs) were pre-pared by chemical vapor deposition, and the GO solutionwas prepared by ultrasonic exfoliation. The GO/CNTscomposite film was made by mixing together solutions ofeach component, followed by vacuum filtration. The as-prepared film was first ground, then mixed with mela-mine, and finally subjected to high-temperature thermaltreatment under argon to dope with nitrogen. The Rct

value of the NGC CE is much smaller (1.78 Wcm2) thanthat of the Pt-based reference cells (8.97 W cm2). ThePCE value of the NGC-based DSSCs is similar to that ofthe Pt-based DSSCs.


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Luo and co-workers prepared nitrogen-doped rGO(NRGO), sulfur-doped rGO (SRGO), and nitrogen andsulfur dual-doped rGO (NSRGO) CEs via hydrothermalreactions of the various precursors.[57] DSSCs made of theSRGO-based CEs exhibited a higher PCE value (4.73%)than that of the device made of NRGO-based CEs(3.85 %). Of the above three types of CEs, the NSRGO-based CE exhibited a higher catalytic activity and, inturn, a higher PCE value for DSSCs made with a disul-fide/thiolate redox shuttle.

Boron-doped graphene (BG) CEs were synthesized byFang and colleagues.[58] B2O3 was used as a boron source,and GO and B2O3 were ground together and then sub-jected to high-temperature annealing. DSSCs made ofBG CEs exhibited a PCE value of 6.73 %, which is higherthan that of the reference Pt-based CEs (6.34 %). Veryrecently, Jung et al. also prepared BG CEs using BBr3

precursors, via a modified Wurtz reaction in a high-pres-sure reactor.[59] The device made of the BG CEs exhibiteda higher PCE value (9.21 %) than that of the referencePt-based cells (8.45%).

Wang et al. synthesized phosphorous-doped rGO(PRGO) CEs using a facile high-temperature annealingmethod.[62] For the first time, a PRGO-based CE wasused in DSSCs. Although the Rct value of the PRGO CEis lower than that of the reference Pt-based cells, theoverall PCE value of the Pt-based cells exhibited a highervalue (6.80 %) than that of the PRGO-based cells(6.25 %). The photovoltaic characteristics of the above-mentioned DSSCs, made of functionalized or doped/rGOcomposite CEs, are summarized in Table 5.

3. Summary and Future Outlook

We have briefly summarized the recent progress in thefield of rGO-based CEs for DSSC applications. GO con-taining several oxygen functional groups remains a goodcatalyst for the I3

¢/I¢ redox reaction. However, the elec-trical conductivity of GO is significantly lower than thatof rGO. Therefore, several researchers focused on variousreduction methods (e.g., hydrothermal, heat treatment, orchemical reduction) to obtain highly conducting rGO.Defect sites in rGO act as a pathway for the transfer ofelectrons at the CE/electrolyte interface, which enhancesthe catalytic activity. In recent years, heteroatoms such asnitrogen, sulfur, boron, and phosphorous were intention-ally used to dope the rGO matrix to enrich the defectsites on its surface. Adhesion of carbon-based materialson the FTO substrate is one of the key issues in CEs ofDSSCs. To overcome these issues, rGO-based compositematerials have been developed by researchers. Metalnanoparticles incorporated into rGO CEs have led tolower Rct, much higher catalytic activity, and increasedPCE values. TMOs and TMDs also display good catalyticactivity. However, their conductivity needs improvement.Composites made of rGO with TMOs and rGO withTMDs showed higher conductivity and promising electro-catalytic activity toward the I3

¢/I¢ redox reaction. Con-ducting polymer/rGO composites also showed high con-ductivity and remarkable electrocatalytic properties.Moreover, polymer acts like a binder, facilitating properadhesion of electroactive material on FTO substrates.Thus far, various materials have been synthesized and uti-lized in DSSC applications, but most of these compositesshowed lower PCE values than those of the Pt-based ref-erence cells. Only a few reports showed higher PCEvalues than those of the reference cells. To achieve higherPCE values, the electrocatalytic mechanism and kinetics

Table 5. Photovoltaic parameters of DSSCs made of functionalized or doped rGO composite CEs and of reference DSSCs made of conven-tional Pt-based CEs



[d]


1 Hemin-rGO 0.65 5.75 0.31 2.45 9[e] 0.73 6.56 0.67 3.18 7[e] [48]2 N-doped graphene sheets 0.82 10.55 0.55 4.75 – 0.77 9.37 0.70 5.03 – [49]3 N-doped graphene (NG) 0.683 15.19 0.59 6.12 – – – – 6.97 – [50]4 3D-N-doped graphene Foams (NGFs) 0.77 15.84 0.58 7.07 5.6[e] 0.79 14.27 0.66 7.44 8.8[e] [51]5 N-doped graphene sheets 0.695 15.76 0.64 7.01 0.9 0.691 16.11 0.66 7.34 0.75 [52]6 N-doped rGO 0.858 13.00 0.72 8.03 5.76 0.796 13.26 0.69 7.33 – [53]7 N-doped graphene nanoplatelets (NGPs) 0.883 13.83 0.74 9.05 3.06 0.885 13.48 0.70 8.43 8.44 [54]8 N-doped porous graphene foams (NPGFs) 0.708 13.14 0.48 4.5 15.2 0.756 14.64 0.45 4.9 9.8 [55]9 N-doped graphene/CNTs composite (NGC) 0.766 16.23 0.54 6.74 1.78 0.768 16.66 0.53 6.89 8.97 [56]10 N and S- dual doped rGO (NSRGO) 0.601 11.70 0.67 4.73 0.39 0.609 10.22 0.50 3.11 34.9 [57]11 B-doped graphene (BG) 0.73 13.93 0.66 6.73 1.37[e] 0.73 13.28 0.65 6.34 8.40[e] [58]12 B-doped graphene (BG) 0.887 13.73 0.756 9.21 1.41 0.885 13.44 0.711 8.45 2.84 [59]

[a] DSSCs made of rGO-based CEs. [b] DSSCs made of Pt-based conventional reference electrodes. [c] Rct of rGO-based CEs from EIS analy-sis. [d] Rct of Pt-based CEs from EIS analysis. [e] No specific area, so the corresponding unit is W not Wcm¢2.


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of these composite materials must be further investigatedin detail. Although most of the rGO-based composite ma-terials showed comparable efficiencies to those of the Pt-based cells, their long-term stability must also be verified.Future research should focus on measuring and, if neces-sary, improving the long-term stability of these materialsand the devices they comprise, hopefully promoting themtoward commercial applications.

Acknowledgments

This research was supported by the National ResearchFoundation of Korea (NRF), funded by the Korean gov-ernment, MSIP/KETEP (2014060255, 2014060440,20133030000140, and 20123010010070).

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Received: December 31, 2014Accepted: February 24, 2015

Published online: May 15, 2015


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