Recent progress in non platinum counter electrode materials for dye sensitized solar cells

Recent Progress in Non-Platinum Counter ElectrodeMaterials for Dye-Sensitized Solar CellsJayaraman Theerthagiri,[a] Arumugam Raja Senthil,[a] Jagannathan Madhavan,*[a] andThandavarayan Maiyalagan[b]

ChemElectroChem 2015, 2, 928 – 945 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim928

ReviewsDOI: 10.1002/celc.201402406

1. Introduction

The solar cell is a promising renewable energy device that con-verts sunlight into electricity; it has broad potential to contrib-

ute to the solution for the future energy problem that humani-ty faces.[1] Nowadays, solar energy technology is gaining tre-

mendous popularity due to its numerous advantages, which

include its ability to operate without noise, toxicity, or green-house gas emission. Among various solar cells, dye-sensitized

solar cells (DSSCs) demonstrate specific advantages over otherphotovoltaic devices, because of their high efficiency, low cost,

simple fabrication procedures, environmental friendliness,transparency, and good plasticity. Though DSSCs perform well

under laboratory conditions relative to other solar cells, param-

eters such as efficiency, lifetime, and cost determine their com-mercial applications. The major components of conventional

DSSCs include a nanocrystalline semiconductor oxide, a dyesensitizer, a redox electrolyte, and a counter electrode (CE).[2, 3]

Recently, extensive studies of the individual components ofDSSCs have been performed to reduce production costs and

to achieve high cell performance. The cell performance de-

pends on many factors such as surface morphology, particlesize, photoelectrode thickness of TiO2, and the nature of the

dye. An overall solar conversion efficiency of more than 12 %has been achieved by employing liquid electrolytes (I¢/I¢3redox couple) in DSSCs. However, the use of liquid electrolytescauses many problems in DSSCs such as short-term stability

due to organic solvent evaporation and leakage, difficulty insealing the device, electrode corrosion, and limited solubilityof inorganic salts such as KI, NaI, and LiI.[4, 5]

To overcome these disadvantages, many researchers havefocused on alternatives such as the gelation of solvents,[6, 7]

polymers incorporating I¢/I¢3 redox couple materials,[8, 9] or re-placing the liquid electrolytes with other solid materials.[10] Im-

portant criteria that must be considered in selecting a polymerinclude good conductivity and stability. The conductivity of an

electrolyte mainly depends on the concentration of the saltcontaining the mobile species as well as the extent up to

which the salt is dissociated, which thus makes all dissociatedions available for conduction.[11] However, the dissociation of

salts depends on the dissociation constant of the salt, the

donor number of the solvent, the dielectric constant of the sol-vent, and the nature of the salt. In polymer electrolytes, how-

ever, the conductivity arises from rapid segmental motion andthe interaction between the cations and the donor atom of

the main structure.The sensitizing dye should possess the following require-

ments for efficient energy conversion: The absorption spec-

trum of the dye should cover the whole visible region and thenear-infrared portion of the solar spectrum. The excited state

of the dye should be higher in energy than the conductionband edge of the n-type semiconductor, so that an efficient

electron-transfer process can take place between the exciteddye and the conduction band of the semiconductor.[12, 13] Themost common high-performing dyes reported so far are the

ruthenium-centered polypyridyl complexes, including N3,N719, and N749 (black dye), in addition to other dyes, such asporphyrins,[14, 15] phthalocyanines,[16] and metal-free organicdyes.[17, 18]

The counter electrode is an indispensable component inDSSCs, as it injects electrons into the electrolytes to catalyze

the reduction reaction (I¢3 to I¢) after charge injection from the

photooxidized dye. Conventional conductive glasses, such asindium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO)

without a catalyst present a low rate of reduction for the coun-ter electrode, and thus, the counter electrode must be coated

with a catalytic material to accelerate the reaction.[19, 20] In thisregard, platinum (Pt) is preferred due to its superior conductiv-

ity and high electrocatalytic activity. However, platinum is

a very expensive, scarce noble metal and it undergoes slowdissolution due to the corrosive I¢3 /I¢ redox electrolyte, which

deteriorates its long-term stability. These drawbacks restrictlarge-scale applications of Pt electrode in DSSCs.[4, 21] Hence, it

is important to develop alternative noble-metal-free materialsthat are capable of replacing platinum as electrocatalysts. The

Dye-sensitized solar cells (DSSCs) have gained increasing atten-tion with regard to photovoltaic devices, because of their low

cost and simple fabrication methods; they are mostly investi-gated in indoor light-harvesting and portable applications. The

focus has been on three main parameters of photovoltaic devi-ces, that is, lifetime, and cost effectiveness. A DSSC consists offour prominent components including a photoanode, a photo-

sensitizer, a redox electrolyte, and a counter electrode. Thecounter electrode is a crucial component, in which triiodide is

reduced to iodide by electrons flowing through the externalcircuit. An effective approach to improve the performance ofa counter electrode is to enhance the power conversion effi-ciency and to reduce the cost of the device. Platinum-coated

conducting glass electrodes give the best performance, buttheir high cost and the scarcity of platinum restricts large-scale

application in DSSCs. This has prompted researchers to devel-op low-costing platinum-free electrodes for DSSCs. In this

review, we focus mainly on counter electrode materials for theelectrocatalytic redox reaction for the I¢/I¢3 electrolyte, and

apart from this, other counter electrode materials for iodine-free redox electrolytes are discussed. Different counter elec-trode materials are highlighted in different categories such as

carbon materials, conducting polymers, oxide and sulfide ma-terials, transition-metal nitrides and carbides, and composite

materials. The stability of counter electrodes in DSSCs is alsopresented.

[a] J. Theerthagiri, A. R. Senthil, Dr. J. MadhavanSolar Energy Lab, Department of ChemistryThiruvalluvar University, Vellore-632 115 (India)E-mail : [email protected]

[b] Dr. T. MaiyalaganMaterials Science and Engineering ProgramThe University of Texas at Austin, Austin, TX 78712 (USA)

ChemElectroChem 2015, 2, 928 – 945 www.chemelectrochem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim929

Reviews

power conversion efficiency of DSSCs does not increase in pro-portion to an increase in the thickness of the platinum film,

and it is possible to obtain a high power conversion efficiencywith a very thin platinum film of 2 nm. Such results suggest

that production costs can be somewhat reduced by reducingthe amount of platinum used.

However, a more promising way to reduce the cost ofDSSCs is to explore other low-costing materials that possess

reasonably good catalytic activity.[22] The general requirementof a photoelectrode material is that it should have a maximal

optical function to absorb solar energy in the visible regionwith a high catalytic function. The electrocatalytic activity ofthe material can be improved by increasing the specific surface

area, so the material must be prepared with nanoscale dimen-sions. This is because nanosized materials show different physi-

cal and chemical properties than bulk materials.[23]

The cell performance of a DSSC can be evaluated by the fill

factor (FF) and the overall solar light to electrical energy con-version efficiency (h), which can be calculated according to

Equations (1) and (2):[13, 24]

FF ¼ Vmax Jmax

Voc Jsc

ð1Þ

h %½ ¤ ¼ Vmax Jmax

Pin 100 ¼ Voc Jsc FF

Pin 100 ð2Þ

in which Jsc is the short-circuit current density [mA cm¢2] , Voc is

the open-circuit voltage [V] , Pin is the incident light power, and

Jmax [mA cm¢2] and Vmax [V] are the current density and voltageat the point of maximum power output in the current versus

voltage (J–V) curves.

In this review, the recent progress in the replacement of the

platinum counter electrode with other cheaper materials for

DSSCs is presented. The platinum-free counter electrode mate-rials are categorized into carbon materials, conducting poly-

mers, inorganic metal oxides and metal sulfides, transition-metal nitrides and carbides, and composite materials, and the

advantages of these platinum-free catalysts for DSSCs are alsohighlighted.

2. Dye-Sensitized Solar Cells: GeneralMechanism

A schematic diagram of a conventional DSSC and the individu-

al components of the cell are shown in Figure 1. DSSCs aretypically composed of electrodes (anode and cathode), a dye

(D) as a sensitizer, and the I¢/I¢3 redox couple containing the

electrolytes.The excited-state level of a dye should be higher in energy

than the conduction band edge of the semiconductor, so thatefficient electron transfer from the excited dye to the conduc-

tion band of the semiconductor oxide will take place. Theenergy level of the oxidized state of the dye must be more

positive than the HOMO level of the redox potential of the

electrolyte for regeneration of the dye. Regeneration of theoxidized dye from the redox species of the electrolytes should

be faster than recombination of the oxidized dye with an elec-tron in the TiO2 photoanode. The semiconductor oxide must

be mesostructured to achieve a surface area that is largeenough to allow sufficient dye to harvest the incident light.

Mr. Jayaraman Theerthagiri received

his MSc degree in chemistry from the

Ramakrishna Mission Vivekananda Col-

lege, Mylapore, Chennai, in 2011. He is

currently a PhD student at the Thiru-

valluvar University, Vellore. His current

research is focused on the synthesis of

counter electrode materials for DSSCs,

polymer electrolytes, and photocataly-

sis for energy and environmental

applications.

Mr. Arumugam Raja Senthil received

his MSc degree in chemistry from the

Thiruvalluvar University, Vellore, in

2012. He is currently a PhD student at

the Thiruvalluvar University, Vellore.

His current research is focused on

polymer electrolytes for electrochemi-

cal device applications.

Dr. Jagannathan Madhavan is currently

working as an Assistant Professor in

the Department of Chemistry, Thiruval-

luvar University, Vellore, India. He re-

ceived his PhD degree from the De-

partment of Energy, University of

Madras, in 2007 and worked as a post-

doctoral fellow at the School of

Chemistry, University of Melbourne,

Australia from 2008 to 2010. His cur-

rent research interests include dye-

sensitized solar cells, photocatalysis,

polymer electrolytes, and nanomaterials for energy and environ-

mental remediation.

Dr. Thandavarayan Maiyalagan re-

ceived his PhD degree in physical

chemistry from the Indian Institute of

Technology, Madras, and completed

postdoctoral programs at the Newcas-

tle University (UK) and at the Universi-

ty of Texas, Austin (USA). His main re-

search interests concern new materials

and their electrochemical properties

for energy conversion and storage de-

vices, electrocatalysts, fuel cells, and

biosensors.


Reviews

Among semiconductor oxides, TiO2 is widely studied due to itshigh charge transport and its ability to suppress charge

recombination.[12, 13]

Upon illumination of the cell, an electron generated by thesensitizer is transferred to the conduction band of the oxide

semiconductor wherein the sensitizer is anchored. Then, thesensitizer is regenerated by the iodide ions from the electrolyte

(iodide/triiodide redox couple dissolved in an organic solvent),and I¢3 in the electrolyte is reduced by the electron reaching

the counter electrode through back contact. The cycle is com-

pleted by electron migration between the TiO2 semiconductorand the counter electrode.

Photoexcitation [Eq. (3)]:

Dþ hn! D* ð3Þ

Electron injection [Eq. (4)]:

D* ! Dþ þ e¢CB ð4Þ

Dye regeneration [Eq. (5)]:

2 Dþ þ 3 I¢ ! I¢3 þ 2 D ð5Þ

Regeneration of iodine [Eq. (6)]:

I¢3 þ 2 e¢ ! 3 I¢ ð6Þ

For efficient regeneration of the electrolyte, the charge-

transfer resistance of the catalyst in the counter electrode andthe diffusion constant of triiodide are very important.

The reactions involved in the reduction mechanism of I¢3 aredescribed by Zhang et al. [Eqs. (7)–(9)]:[25]

I¢3 ðsolÞ $ I2ðsolÞ þ I¢ðsolÞ ð7Þ

I2ðsolÞ þ 2* ! 2 I* ð8Þ

I* þ e¢ ! I¢ðsolÞ ð9Þ

in which * represents the free site on the electrode surfaceand sol indicates the solvent of the electrolyte solution phase.The equilibrium reaction (step 7) is usually fast. The subse-quent iodine reduction reaction (steps 8 and 9) occurs at the

liquid–solid interface, that is, I2 dissociates into two surface Iatoms (I*) upon adsorption on the electrode surface, and the

removal of I* through one-electron transfer to produce sol-

vated I¢(sol) determines the overall electrocatalytic activity.

In this review, we focused on Pt-free, low-costing counter

electrode materials for DSSC applications. Detailed review arti-cles on the individual components of DSSCs and an overview

of the energetics of the electron-transfer processes involved in

DSSCs have already been reported by various researchers.[26–35]

3. Platinum-free Counter Electrode Materials

3.1. Carbon-Based Materials

Carbonaceous materials are quite attractive to replace the plat-inum electrode in DSSCs due to their high electronic conduc-

tivity, large surface area, corrosion resistance towards iodine,high reactivity for triiodide reduction, and low cost. Several

carbonaceous materials such as graphene, carbon nanotubes,

activated carbon, graphite, and carbon black have been suc-cessfully employed as counter electrodes.[36–38]

In 1996, Kay and Gr�tzel first explored a graphitic–carbonblack mixture as a counter electrode material and achieved

a power conversion efficiency of 6.7 %.[39] Thereafter, intensiveresearch efforts have been focused on carbon materials. The

power conversion efficiency and the fill factor of DSSCs are

strongly dependent on the thickness of the carbon layer. It hasbeen reported that the fill factor increases as the carbon layer

thickness increases up to 10 mm.[40] Glassy carbon with lowcrystallinity shows high electrocatalytic activity for the reduc-tion of triiodide in the redox reaction. The enhanced electroca-talytic activity of glassy carbon can be attributed to increased

graphene stacks and active sites in glassy carbon.[41] Lee et al.fabricated DSSCs by using nanosized carbon as a counter elec-

trode material and reported a power conversion efficiency of7.56 %. After storing the devices in the dark at room tempera-ture for 60 days, the carbon counter electrode retained 84 % ofits initial efficiency.[42]

The optical and electrochemical properties of the carbon

counter electrode are dramatically affected by the compositionand concentration of the precursor used. The optimized

carbon counter electrode exhibits high transparency and suffi-

cient catalytic activity for I¢3 reduction.[43] Veerappan et al. haveemployed colloidal graphite as an efficient counter electrode

for triiodide reduction in DSSCs. Advantages of colloidal graph-ite are that it may act dually as a substrate as well as an elec-

trocatalyst so that it can be successfully replaced by both thetransparent conducting oxide (TCO) and platinum.[44]

Figure 1. Energy-level diagram of a DSSC. VAC = vacuum energy level,CB = conduction band, VB = valence band.


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Veerappan et al. have fabricated a DSSC with a spray-coatedcarbon counter electrode, and they investigated the impact of

spraying time of the carbon paste on the performance of theglass/plastic-coated carbon counter electrodes. The spray-

coated carbon counter electrode on glass and plastic can ach-ieve a power conversion efficiency of more than 6.0 %, and the

highest power conversion efficiency of 6.2 % is obtained witha spraying time of 420 s.[45] Chen et al. have prepared a carbonblack coated graphite counter electrode with a thickness of

0.2 mm by a spin-coating method; it shows a maximum powerconversion efficiency of 6.46 %, which is higher than the effi-ciency (6.37 %) of a thermally deposited Pt/FTO counter elec-trode.[46] Wu et al. have fabricated a DSSC by using different

types of carbon-based counter electrodes, such as mesoporouscarbon, activated carbon, carbon black, conductive carbon,

carbon dye, carbon fiber, carbon nanotube, discarded toner of

a printer, and fullerene. All of these carbon materials possessgood electrocatalytic activity for triiodide reduction in DSSCs,

and the corresponding SEM images of these materials areshown in Figure 2. However, mesoporous carbon shows cata-

lytic activity that is comparable to that of Pt. The mesoporouscarbon counter electrode shows a power conversion efficiency

of 7.5 % by using the I¢/I¢3 redox couple.[47]

Sebasti�n et al. synthesized carbon nanofibers (CNFs) at dif-ferent temperatures ranging from 550 and 750 8C, and these

CNFS have been used as a counter electrode for DSSCs. CNFssynthesized at 550 8C show higher conversion efficiency than

CNFs synthesized at other temperatures. These authors reportthat the surface area and crystallinity of the materials play key

roles in efficiency.[48] Wang et al. have prepared a counter elec-trode for DSSCs by using nitrogen-doped mesoporous carbon

(NMC). The NMC counter electrode can achieve a power con-version efficiency of 7.02 % with the liquid I¢/I¢3 redox couple;

this is comparable to the power conversion efficiency of a Ptcounter electrode (7.26 %) under the same test conditions. This

result clearly indicates that the NMC electrode is an alternativelow-costing material for the Pt counter electrode in DSSCs.[49]

Jian et al. have investigated a DSSC by using a nonionic surfac-

tant Triton X-100 modified mesoporous carbon (MC) counterelectrode. Such an electrode-based cell can reach a maximumpower conversion efficiency of 5.65 %.[50]

Among the carbon polymorphs, graphene-based materials

have captured more attention in Pt-free DSSCs. Graphene isa compound that has two dimension atomically thick sp2-

carbon atoms arranged in a hexagonal “honeycomb” lattice

structure. Graphene has been demonstrated to be a promisingcounter electrode material for DSSCs due to its excellent con-

ductivity, high electrocatalytic activity, and remarkable trans-parency over the entire solar spectrum.[51]

Roy-Mayhew et al. have reported that the charge-transfer re-sistance of functionalized graphene is 10 times greater than

that of platinum, as there is no applied bias, and the charge

transfer resistance approaches that of platinum at an appliedbias of 0.5 V. Also, they have studied the effect of tuning the

catalytic functional groups on functionalized graphene sheetsand have shown that increasing the number of oxygen-con-

taining functional groups results in an increase in the catalyticactivity of the material.[52] The low number of oxygen function-

alities in the graphene electrode

mainly affects the redox kineticsand the charge-transfer resist-

ance. The low current density ismainly attributed to a high

charge-transfer resistance for theI¢3 /I¢ redox reaction.[53] Yue et al.

have investigated a Pt/graphene

film as a counter electrode inDSSCs and compared its power

conversion efficiency with thatof a traditional platinum counter

electrode. The Pt/graphene-based DSSC can achieve

a higher power conversion effi-ciency (7.88 %) than the Pt coun-ter electrode (6.51 %). This is be-

cause of its higher conductivityand lower charge-transfer resist-

ance on the electrode/electrolyteinterface for the I¢3 /I¢ redox reac-

tion.[54] The lower cost of gra-

phene-based counter electrodematerials has attracted large-

scale applications in DSSCs.The combination of graphene

with a polymer produces an ef-fective counter electrode, for

Figure 2. SEM images of different carbon materials such as activated carbon (Ca), carbon black (Cb), conductivecarbon (Cc), carbon dye (Cd), carbon fiber (Cf), carbon nanotube (Cn), mesoporous carbon (Com), discarded tonerof a printer (Cp), and fullerene (C60).[47]


Reviews

which graphene is responsible for the high catalytic activityand the polymers act as a conducting support material.[55] The

combination of the high crystallinity of graphene with thehigh surface area of carbon black results in a material with low

interfacial transfer resistance and high catalytic activity ; thismaterial shows the highest power conversion efficiency of

5.99 %.[56] Kim et al. have used three different polymorphs ofcarbon for counter electrodes in DSSCs, including graphene,single-walled carbon nanotubes (SWCNTs), and a graphene–

SWCNTs composite (Figure 3). Of these three catalysts, the gra-

phene-deposited SWCNT electrode shows the highest power

conversion efficiency (5.87 %), and thus, graphene is the mostsuitable material for the counter electrode in DSSCs.[57] The

photovoltaic characteristics of DSSCs with different carbon-based counter electrodes are shown in Table 1.

Carbon materials are low-costing counter electrode catalystsin DSSCs; however, the main drawback of carbon-based coun-

ter electrodes is that these materials require large dosage toattain the targeted catalytic activity. Then, a poor connectionbetween the carbon film and the substrate limits the long-

term use of carbon-based counter electrodes.

3.2. Conducting Polymers

In recent years, research efforts have been directed towards

conducting polymers due to their potential uses as conductingsubstrates, hole conductors, and counter electrode materials

for DSSCs. Conducting polymers are promising candidates forcounter electrode materials in DSSCs due to their unique prop-

erties, such as high conductivity, low cost, good stability, andhigh catalytic activity for I¢3 reduction.[58] Polymers such as

polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenediox-ythiophene) (PEDOT) have been adopted as counter electrodes

for DSSCs.

Over the last decade, PANI has been one of the most inten-sively studied conducting polymers due to its easy synthesis,

high conductivity, good environmental stability, and interestingredox properties.[59, 60] Li et al. have reported that an increase in

the surface area of the PANI electrode improves its catalytic ac-tivity. It has a lower charge-transfer resistance and a higher

electrocatalytic activity for the I¢3 /I¢ redox reaction than the

platinum electrode. The overall energy conversion efficiency ofDSSCs with a PANI counter electrode is 7.15 %, which is higher

than that with the platinum electrode (6.90 %).[61] The thick-ness, surface morphology, and electrocatalytic activity of the

electrochemically synthesized PANI counter electrode havebeen investigated by Lin et al. After optimization of the

sweep-segment number in cyclic voltammetry, the 27-sweep

segment PANI counter electrode shows good electrocatalyticactivity for triiodide reduction. The energy conversion efficien-cy obtained by using this PANI counter electrode is 5.92 %.[62]

To improve the electrocatalytic and photovoltaic properties of

DSSCs, Ameen et al. have investigated sodium fluoroacetate(SFA)-doped PANI and compared the results with those ob-

tained by using undoped PANI and a platinum counter elec-trode.[20] The superior photovoltaic properties (e.g. h, Jsc, Voc,and incident photon-to-current efficiency) of the SFA-PANI-

based cell have been attributed to its high conductivity andelectrocatalytic activity. Another report on PANI-CSA (polyani-

line–camphorsulfonic acid) as a counter electrode for DSSCs isalso interesting. The performance of the porous PANI-CSA

nanostructured counter electrode with optimized surface

roughness is comparable to that of a conventional DSSC witha platinum counter electrode.[63] Xiao et al. have prepared poly-

aniline nanofibers to replace the Pt counter electrode inDSSCs; the PANI nanofibers show excellent electrocatalytic ac-

tivity for the I¢3 /I¢ redox reaction. A power conversion efficien-cy of 6.21 % can be achieved by using the PANI nanofiber

Figure 3. A multifunctional nanocarbon-based interlayer used in DSSCs andthe J–V curves with (c) and without (g) TiCl4 treatment, as well as withSWCNH interlayers of 15 (c) and 30 nm (c).[37]

Table 1. Photovoltaic characteristics of DSSCs with different carbon-based counter electrodes.

Sample Jsc [mA cm¢2] Voc [V] FF h [%] Ref.

glassy carbon 16.50 0.71 0.645 7.56 [41]carbon 10.52 0.72 0.60 6.07 [42]colloidal graphite 12.7 0.794 0.62 6.2 [44]carbon nanotube 13.25 0.80 0.65 7.0 [47]C60 (fullerene) 11.60 0.75 0.32 2.8 [47]carbon dye 13.38 0.80 0.69 7.5 [47]carbon nanofiber 7.4 0.71 0.42 2.17 [48]N-doped carbon 15.46 0.71 0.64 7.02 [49]Vor-ink P3 8.14 0.70 0.66 3.75 [51]Vor-ink P10 7.77 0.71 0.70 3.83 [51]graphene/Pt 14.56 0.75 0.68 7.43 [54]graphene/carbon 15.07 0.70 0.57 5.99 [56]graphene 13.1 0.70 0.63 5.87 [57]SWCNTs 13.0 0.71 0.52 4.97 [57]SWCNTs-GO[a] 12.7 0.70 0.56 5.17 [57]

[a] GO = graphene oxide.


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counter electrode; this is close to the efficiency achieved byusing the Pt counter electrode (6.39 %).[64] Li et al. have investi-

gated the effect of doping several anions such as SO42¢, ClO4

¢ ,BF4

¢ , Cl¢ , and TsO¢ (Ts = tosyl) into the PANI film. Among

them, the SO42¢-doped PANI film shows a higher reduction cur-

rent for the reduction of I¢3 and a lower charge-transfer resist-

ance than the Pt counter electrode. The PANI-SO4 counter elec-trode in DSSCs shows a conversion efficiency of 5.6 %.[65] Thesodium dodecyl sulfate (SDS)-doped PANI film exhibits the

highest conductivity, and this leads to a higher catalytic reduc-tion of I¢3 . DSSCs assembled with PANI-SDS as a counter elec-trode show a photocurrent conversion efficiency of 7.0 %,which is comparable to that of a conventional Pt counter elec-

trode (7.4 %).[66]

Wu et al. have synthesized polypyrrole (PPy) nanoparticles

coated with a conducting FTO glass to construct a PPy counter

electrode for DSSCs. The overall power conversion efficiency ofDSSCs with the PPy counter electrode is 7.66 %, which is

higher than that of the platinum counter electrode (6.90 %).[24]

Jeon et al. have synthesized PPy nanoparticles and have fabri-

cated a PPy-layered FTO glass as a counter electrode for DSSCsby the drop-casting method by using a PPy-dispersed colloidal

solution. The power conversion efficiency of these DSSCs can

reach a maximum of 7.73 %, which is the highest efficiency todate with the use of PPy as a counter electrode.[58] A polypyr-

role nanotube film has been employed as a novel counter elec-trode to substitute the expensive Pt counter electrode in

DSSCs. A PPy–nanotube membrane has also been used to re-place the Pt counter electrode and the FTO plate; a power

conversion efficiency of 5.27 % can be obtained for the DSSC

assembled with this PPy–nanotube membrane.[67] The Tafel po-larization curves and the J–V curves of a characteristic DSSC

fabricated by using Pt/FTO and a PPy counter electrode areshown in Figure 4. It is clear from this figure that the Tafel

zone of the Pt electrode is higher than that of the PPy counterelectrode, which indicates that the exchange-current density of

Pt is higher than that of the PPy electrode and that the catalyt-

ic activity of Pt is superior to that of the PPy electrode. Veer-ender et al. have used a free-standing PPy film as a substrate-

free and Pt-free counter electrode for DSSCs, and their resultshave revealed that the PPy film exhibits remarkable catalytic

activity for the I¢3 /I¢ redox reaction with a DSSC efficiency of3.5 %.[68] A fabric counter electrode prepared by coating PPy on

a cotton-fabric-coated Ni substrate shows comparable catalyticactivity towards the reduction of I¢3 .[69] The structure and prop-erties of PPy are greatly influenced by anion doping. Dodecyl-

benzenesulfate-doped PPy shows the highest electrocatalyticactivity for the I¢3 /I¢ redox reaction, and thus, PPy films couldreplace Pt counter electrodes in DSSCs.[70]

PEDOT is an another kind of conducting polymer that has at-

tracted much attention due to its ease of processing, remark-able stability, electrical conductivity, and catalytic capability.[71]

A low-costing and low-temperature method to fabricate the

high-performance PEDOT counter electrode for DSSCs hasbeen successfully developed by Yin et al. The overall power

conversion efficiency of the PEDOT counter electrode basedDSSC is 7.04 %, which is comparable to that of the platinum

counter electrode (7.35 %).[72] Sudhagar et al. have reportedthat the overall power conversion efficiency of DSSCs with PE-

DOT:PSS (PSS = polystyrene sulfonate) can be increased by the

addition of catalytic sulfide materials. A PEDOT:PSS DSSCshows efficiency of 3.8 %; the efficiency increases to 5.4 % in

CoS/PEDOT:PSS DSSC, which is comparable to the efficiency ofthe conventional platinum counter electrode (6.1 %).[73]

PEDOT:PSS/carbon counter electrode materials possessa large surface area, high conductivity, small sheet resistance,

and good redox properties. DSSCs fabricated with this counter

electrode reach an energy conversion efficiency of 4.11 %. Theirsimple nature and their low cost allow carbon conductive

paste PEDOT:PSS to be used as a credible alternative counterelectrode in DSSCs.[74] Yue et al. have reported that PEDOT:PSS/

PPy films have low surface resistance and good catalytic per-formance for the I¢3 /I¢ redox electrolytes. The energy conver-

sion efficiency of DSSCs based on PEDOT:PSS:PPy counter elec-

trodes is 7.60 %, and this efficien-cy is comparable to that ofDSSCs based on platinum elec-trodes. Figure 5 shows the pho-

tocurrent–voltage curves ofDSSCs with PPy, PEDOT:PSS, and

PEDOT:PSS/PPy counter elec-trodes.[75] Kwon and Park havesynthesized PEDOT with different

conductivities ranging from 100to 690 S cm¢1 by a simple poly-

merization method and opti-mized the conductivity

(340 S cm¢1) of the PEDOT coun-

ter electrode exhibiting highperformance for the I¢3 reduc-

tion.[76] Further, a PEDOT counterelectrode has also been synthe-

sized by the aqueous galvano-static polymerization technique

Figure 4. a) Tafel curves of symmetrical cells based on Pt/FTO and PPy electrodes and b) J–V characteristic curvesof DSSCs fabricated by using Pt/FTO and PPy counter electrodes.[67]


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with different polymerization times. The catalytic activity of thecounter electrode varies with the polymerization time. Upon

increasing the polymerization time the reaction rate also in-creases with a higher catalytic reduction of I¢3 . PEDOT synthe-

sized by this technique shows a higher power conversion effi-

ciency of 6.46 % than the Pt counter electrode (6.33 %) underthe same test conditions.[77] The photovoltaic characteristics of

DSSCs with different conducting polymers as counter electro-des are shown in Table 2. PEDOT exhibits high conductivity,

high catalytic activity, and good adhesion to FTO conductivesubstrates; even though PEDOT is more expensive than other

conducting polymers, it can be electrodeposited within a limit-ed area of the electrode. So far, upon analyzing various knownconducting polymers, PPy is of special interest due to its easeof synthesis, excellent stability in air, low cost, and high poly-merization yield.

3.3. Metal Sulfide Materials

The construction of an efficient cathode is important for all

types of electrochemical cells. Metal sulfides are consideredgood choices, as these electrocatalysts are deposited on

a plain FTO plate and are widely studied in DSSCs. Severalquantum dots such as CdS, CdSe, CdTe, CuSnI2, CuS, PbS, and

InAs have been used as sensitizers of nanocrystalline TiO2,which is used in the construction of quantum-dot-sensitized

solar cells.[78, 79] CoS deposited as a quasitransparent layer onFTO glass is more electrocatalytic than the bulk platinum elec-

trode for the reduction of triiodide. CoS and CuS have beenused as counter electrodes in DSSCs. A maximum solar conver-sation efficiency of 2.7 % is obtained with a CuS counter elec-

trode, whereas the platinum counter electrode shows an effi-ciency of only 0.51 % in combination with polysulfide electro-

lytes.[80] Wang et al. have demonstrated that CoS is highly effi-cient in the reduction of triiodide to iodide in DSSCs; also, it

possesses remarkable stability and thermal stress, which makeit an extremely interesting candidate as a replacement for plat-

inum in DSSCs.[81] Wu et al. have introduced MoS2 and WS2 into

DSSC systems as counter electrode catalysts, and these sulfidesshow excellent catalytic activities for the generation of the

conventional I¢/I¢3 redox couple. These counter electrodes ach-ieve high power conversations of 7.59 and 7.73 %, respectively,

which are close to the photovoltaic performance of DSSCswith a platinum counter electrode.[82] The corresponding J–V

curves are shown in Figure 6. The MoS2 counter electrode syn-

thesized by using a low-temperature wet chemical process ex-hibits a competitive power conversion efficiency of 7.01 %. The

MoS2 counter electrode holds promise as a replacement for ex-pensive Pt-based counter electrodes in DSSCs. A DSSC fabricat-

ed by using the Pt-counter electrode shows a power conver-sion efficiency of 7.31 %, which is close to the 7.01 % shown bythe MoS2 counter electrode.[83] Yang et al. have studied differ-ent phases of NiS as counter electrodes for DSSCs. a-NiS and

b-NiS can be synthesized by a solvothermal route, and their

performance has been compared in terms of power conversionefficiency in DSSCs. The performance of DSSCs with an a-NiS

counter electrode is much better than that of DSSCs with a b-NiS counter electrode.[84]

More recently, researchers have focused on binary and terna-ry metal sulfide materials as counter electrodes to further im-

Figure 5. The J–V curves of DSSCs assembled with PPy, PEDOT:PSS, and PE-DOT:PSS/PPy counter electrodes.[75]

Table 2. Photovoltaic characteristics of DSSCs with different conductingpolymers as counter electrodes.


PANI nanofibers 10.94 0.68 0.54 4.0 [20]SFA-PANI 13.62 0.74 0.53 5.5 [20]PPy 15.01 0.740 0.69 7.66 [24]PPy 11.0 0.785 0.64 7.773 [58]HCl-PPy 15.5 0.778 0.64 7.73 [58]PANI 14.60 0.714 0.69 7.15 [61]PANI 13.29 0.72 0.61 5.92 [62]PANI-SDS 14.0 0.72 0.59 7.0 [66]PPy-nanotube 13.10 0.71 0.56 5.27 [67]PEDOT 16.26 0.710 0.61 7.04 [72]PEDOT:PSS/C 7.5 0.83 0.66 4.11 [74]

Figure 6. The J–V curves of DSSCs with MoS2 and WS2 counter electrodes.[82]


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prove the activity of metal sulfides in DSSCs. Lin and Chouhave synthesized NiCo2S4 as a highly efficient counter elec-

trode in Pt-free DSSCs and an impressive efficiency of 6.14 %has been achieved; this value is comparable to the efficiency

of the Pt-based DSSC (6.29 %).[85] Zheng et al. have fabricatedDSSCs with different ternary metal sulfides such as CoMoS4

and NiMoS4 and porous chalcogels of CoMoS4 and NiMoS4.They have reported that these ternary compounds showpower conversion efficiency that is similar to that of DSSCs fab-

ricated with noble Pt counter electrodes.[86] The I¢3 /I¢ redox re-action, electrochemical impedance plot, Tafel polarization, andJ–V curves of DSSCs with ternary metal sulfides of CoMoS4,NiMoS4, CoMoS4-C, and NiMoS4-C in addition to those of Pt

counter electrodes are shown in Figure 7. This figure evidencesthat the cathodic and anodic peak current densities of

CoMoS4-C and NiMoS4-C are higher than those of CoMoS4 and

NiMoS4, which is due to the addition of graphite; this indicatesthat graphite can improve the catalytic activity of triiodide re-

duction. Furthermore, the mean time current density is lowerthan that for the Pt counter electrode. The Nyquist plot shows

that the series resistance (Rs) with CoMoS4 and NiMoS4 electro-des is 14.0 and 15.8 W, respectively. Both values are higher

than the Rs value of the Pt electrode (8.2 W), which may be

due to the lower conductivity of the electrode. The value of Rs

does not decrease even after the addition of graphite.

Mali et al. have synthesized Cu2ZnSnS4 nanofibers by electro-spinning by using polyvinylpyrrolidone (PVP) and cellulose ace-

tate (CA) as solvents. The fabricated DSSC shows an efficiency

of 3.10 and 3.90 % for the PVP-Cu2ZnSnS4- and CA-Cu2ZnSnS4-based counter electrodes, respectively.[87] CuSbS2 and CuInS2

nanocrystals have also been used as counter electrodes to re-place the expensive Pt counter electrode in DSSCs.[88, 89] CuInS2

nanocrystals can be synthesized from a simple one-spot route,and the DSSC fabricated by using this material can achieve

a conversion efficiency of 5.77 %. Hence, these low-costing ma-terials are suitable as replacements for the Pt counter electrode

in DSSCs. The photovoltaic characteristics of DSSCs with differ-

ent metal sulfide based counter electrodes are shown inTable 3.

3.4. Metal Oxide Materials

Metal oxides are also best-opted materials to replace the Ptcounter electrode in DSSCs due to their low cost, good ther-

mal properties, and high catalytic activity. Some semiconductoroxides have been shown to increase the catalytic activity for

the reduction of triiodide to iodide. Wang et al. have studiedthe catalytic activity of ZnO with a conductive polymer that ex-

hibits excellent photovoltaic performance with a maximumpower conversation efficiency of 8.17 %.[90] Tantalum oxide

(TaO), a non-platinum counter electrode, shows impressive

platinum-like electrocatalytic activity for triiodide reduction inDSSCs. The TaO counter electrode in DSSCs shows a high

power conversion efficiency of 6.48 %, which is better than thepower conversion efficiency of the platinum counter elec-

trode.[91] A DSSC with NiO as the counter electrode has been

Figure 7. a) The I¢3 /I¢ redox peaks of the CoMoS4, NiMoS4, CoMoS4-C, NiMoS4-C and Pt counter electrodes. b) Electrochemical impedance plots (Rct = charge-transfer resistance ZN = N element series circuit, and CPE = constant phase element). c) Tafel polarization curves of symmetrical cells. d) J–V curves of character-istic DSSCs fabricated with ternary sulfide and Pt counter electrodes.[86]


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fabricated by Feihl et al.[92] Of various weight ratios investigat-

ed, 10 % by weight of NiO nanoparticles in ethanol pasteshows better efficiency than other samples. The NiO electrode

also has good homogeneity, transparency, and consumes suit-able light harvest with good conductivity. NbO2 and WO2 have

also been used as counter electrodes in DSSCs.[93, 94] Chenget al. have reported the use of hydrogen-treated commercial

WO3 as a counter electrode to replace the Pt counter elec-

trode. The triiodide reduction reaction is fast if hydrogen-treat-ed commercial WO3 is used as the counter electrode. The

mechanism of the triiodide reduction reaction by using hydro-gen-treated commercial WO3 and commercial WO3 is shown in

Figure 8. The power conversion efficiency of hydrogen-treatedWO3 is 5.43 %, whereas commercial WO3 without hydrogen

treatment shows a power conversion efficiency of only

0.63 %.[95]

Two different phases of SnO2 have been used as counter

electrodes for DSSCs by Pan et al.[96] They have reported thatd-phase SnO2 shows a low charge-transfer resistance and

higher polarization current density. The power conversion effi-

ciency is 4.81 %, whereas the SnO2 phase shows power conver-sion efficiency of only 2.88 %. Zhou et al. have demonstrated

W18O49 nanofibers as a superior electrocatalyst due to theabundance of oxygen vacancies that offer sufficient active sites

for the reduction of I¢3 into I¢ . The W18O49 nanofiber counterelectrode shows an efficiency of 8.58 %; this is slightly lower

than the Pt counter electrode, which shows a power conver-sion efficiency of 8.78 %.[97] Hou et al. have used first-principles

quantum-chemical calculations for the electrocatalytic activityof potential semiconductor counter electrodes and have re-

ported that a-Fe2O3 can be used as a new counter electrodecatalyst for DSSCs.[98] a-Fe2O3 exhibits adsorption energy of

iodine that is almost identical to that of a Pt counter counterelectrode at the acetonitrile/electrode interface (shown inFigure 9). The photovoltaic characteristics of DSSCs fabricated

with different metal oxides as counter electrodes are presentedin Table 4.

3.5. Transition-Metal Nitrides and Carbides

The catalytic activity of carbides was first explored between1960 and 1970.[99] This led to similar findings for other early

transition-metal carbides and nitrides. However, the develop-ment of nitride research began after the 1980s due to its tech-

nological interest and applications. Nitrogen is located in the

periodic table of the elements between carbon and oxygen.Nitrides are close to oxides and carbides with more or less sig-

nificant ionic, covalent, and metallic character. The synthesis oftransition-metal nitrides provides interesting challenges be-

cause of the insertion of nitrogen into the interstitial sites ofthe metals. The potential applications of transition-metal ni-

Table 3. Photovoltaic characteristics of DSSCs with different metal sul-fides as counter electrodes.


CoS 11.2 0.52 0.32 1.9 [80]CuS 13.9 0.55 0.35 2.7 [80]MoS2 12.52 0.63 0.63 4.9 [82]WS2 12.94 0.64 0.64 5.2 [82]a-NiS 11.42 0.68 0.67 5.2 [84]b-NiS 9.80 0.68 0.63 4.2 [84]NiCo2S4 14.11 0.72 0.60 6.1 [85]CA-Cu2ZnSnS4 8.42 0.57 0.65 3.9 [87]PVPCu2ZnSnS4 6.83 0.57 0.64 3.1 [87]CuSbS2 5.58 0.74 0.54 2.6 [88]

Figure 8. Mechanism of the triiodide reduction by using commercial WO3

and hydrogen-treated WO3 electrodes.[95]

Figure 9. The calculated adsorption energy of the iodine atom of varioussemiconductor materials.[98]

Table 4. Photovoltaic characteristics of DSSCs fabricated with differentmetal oxide counter electrodes.


V2O3 10.19 0.78 0.63 5.40 [2]Nb2O5 9.98 0.80 0.60 4.84 [2]MoO2 7.84 0.76 0.40 2.40 [2]ZnO 10.4 0.39 0.18 0.77 [90]TaO 12.59 0.77 0.67 6.48 [91]Ta2O5 13.01 0.75 0.42 4.08 [91]H-WO3 15.46 0.74 0.47 5.43 [95]C-WO3 6.71 0.50 0.21 0.63 [95]SnO2 18.71 0.43 0.35 2.88 [96]SnO2-d 17.21 0.53 0.52 4.81 [96]W18O49 17.14 0.70 0.66 7.94 [97]


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trides and carbides have been widely explored in materialschemistry due to their unique physical and chemical proper-

ties, that is, their high chemical stability, high wear resistance,electronic conductivity, and magnetic properties. In general,

transition-metal nitrides are conventionally produced by usingmetals, metal halides, or metal oxides that are thermally con-

verted into nitrides by using high-temperature treatment withN2 or NH3(g) or into carbides by carbonation with CH4(g).

Transition-metal nitrides and carbides are considered to be

potential substitutes for platinum due to their platinum-likeelectrocatalytic activity. Nitrides and carbides have a widerange of applications; they can be used in dehydrosulfurizationand dehydrogenation, they can be used as supercapacitorsand in hydrotreating and hydrogenation, and they show goodphotocatalytic activity.[100–102] Vanadium nitride is used as a cata-

lyst for n-butane dehydrogenation, and its catalytic activity is

similar to that of platinum-based catalysts.[103] An oil refinerycatalytic process such as hydrotreating of heavy vacuum gas

oil (HVGO) uses Mo2N and W2N to remove impurities such asoxygen, sulfur, and unsaturated hydrocarbons.[104] Molybdenum

nitride is used for hydrodesulfurization in the petroleum indus-try.[105, 106] It is not surprising that certain metal nitrides and car-

bides may replace platinum as counter electrodes in DSSCs.

Wu et al. have studied several carbide and nitride materials inDSSCs as counter electrode catalysts.[2] A comparison of the

power conversion efficiencies of DSSCs with different carbideand nitride catalysts is shown in Figure 10.

A TiC counter electrode with an optimal film thickness of20 mm can achieve a high power conversion efficiency of

6.46 %, which matches the performance of DSSCs prepared

with a platinum counter electrode.[22] Jiang et al. have reportedTiN nanotube arrays as counter electrodes, the photovoltaic

performances of which are comparable to those of a conven-tional platinum counter electrode; they have also suggested

the replacement of electrocatalytically active materials asa better option than the noble metal platinum in fuel cells.[107]

Zhang et al. have developed several nitride-based catalysts for

the reduction of triiodide in DSSC systems and have reportedthat the catalytic activity of nitride materials can be improved

significantly by combining with N-doped graphene oxide.[21]

Transition-metal carbides, such as molybdenum and tungsten

carbides, are potential substitutes for platinum counterelectrodes in DSSCs because of their low cost and high electro-

catalytic performance in triiodide reduction. Furthermore, ithas been reported that the catalytic activity can be increased

by the addition of P25 into Mo2C and WC particles.[108] Highlyactive Mo2N and W2N have been synthesized and used ascounter electrodes in DSSC systems for the reduction of triio-

dide. Mo2N and W2N counter electrodes can achieve powerconversation efficiencies of 6.38 and 5.81 %, respectively, andthese efficiencies are about 91 and 83 %, respectively, of thephotovoltaic performance of DSSCs fabricated by using a plati-num counter electrode.[109] Wu et al. have prepared vanadiumnitride (VN) peas by a simple urea-precursor route and used

these peas as a counter electrode in a DSSC for the traditional

redox couple, that is, the reduction of I¢3 into I¢ . The VN peasshow a higher power conversion efficiency (5.57 %) than the Pt

counter electrode (3.69 %) in the new organic redox couple thi-olate/disulfide(T¢/T2).[110] Mesoporous WC shows an excellent

solar conversion efficiency if used as a counter electrode inDSSCs. Jang et al. have synthesized mesoporous WC through

a polymer-derived (WC-PD) and a microwave-assisted (WC-

MW) route. The power conversion efficiencies of WC-PD andWC-MW counter electrodes are 6.61 and 7.01 %,

respectively.[111]

He et al. have reported that a TiC counter electrode shows

a higher power conversion efficiency than the conventional Ptcounter electrode in DSSCs with a Co2 +/Co3 + polypyridyl

redox mediator. The TiC counter electrode can achieve an effi-

ciency of 7.74 %, whereas the conventional Pt counter elec-trode shows an efficiency of only 7.51 %.[112] Wu et al. have in-

vestigated TiC as a counter electrode material and have report-ed that the efficiency of TiC is 6.40 %, whereas the Pt-based

counter electrode shows only 7.16 % efficiency under the sametest conditions. Also, the efficiency of the DSSC can be in-

creased from 6.40 to 7.68 % upon introducing small amounts

of Pt into TiC, and this value is higher than the efficiency ofthe Pt counter electrode.[113] Zhong et al. have investigated

TiC–SiC–C as a counter electrode to replace the expensive Ptelectrode, and the DSSC fabricated by using the TiC–SiC–Ccounter electrode shows an efficiency of 5.7 %.[114] Figure 11shows the J–V curves of some transition-metal carbides and ni-

trides.[2] The photovoltaic characteristics of DSSCs assembledwith different transition-metal carbides and nitrides counterelectrodes are shown in Table 5.

Transition-metal nitrides and carbides have unique proper-ties in that they show material diversity, are low costing, and

have high catalytic activity ; furthermore, they can be preparedby simple synthesis and can be easily modified in comparison

to carbon materials, organic polymers, inorganic oxides, and

sulfides.

3.6. Composite Materials

Composites are made by combining two or more materialsthat can then be used as efficient catalysts for DSSCs. These

Figure 10. Distribution graph showing the power conversion efficiencies ofI¢3 /I¢ DSSCs.[2]


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new materials show better performance than the conventionalplatinum counter electrode. Recently, some platinum-basedcomposites such as Pt/carbon nanotubes, Pt/graphene, and Pt/

polymer have been introduced as counter electrode catalystsin DSSCs, and these composites show superior electrocatalyticactivitythan the traditional Pt counter electrode.[115–117] Thetransition-metal compounds TiC, WO2, and VN have been in-corporated with platinum and used as counter electrode cata-lysts for DSSCs. These materials show photovoltaic per-

formance that is almost 20 % higher than that of the unitarymaterials.[118] However, to reduce the cost of DSSCs utilizing Pt,we have to explore other low-costing materials with high cata-

lytic activity to replace platinum- and platinum-based compo-site catalysts. Joshi et al. have reported an efficient DSSC by

using a cheap carbon/TiO2 composite as an alternative to theplatinum counter electrode for triiodide reduction.[119] In the

carbon/TiO2 composite, carbon acts as the catalyst and TiO2

functions as a binder. A solar-to-energy conversion efficiencyof about 5.5 % has been reported by using this carbon/TiO2

composite, and moreover, this composite shows device per-formance that is comparable to that of platinum counter elec-

trodes in terms of Jsc, Voc, and h under similar fabricationprocedures.

Lee et al. have investigatedthe effect of inserting graphenebetween PEDOT in counter elec-trodes in DSSCs. A graphene/PEDOT film without TCO andplatinum were also used as

counter electrodes in DSSCs. Thepower conversion efficiency with

Pt/FTO can reach 6.68 %, where-as graphene/PEDOT and PEDOTshow efficiencies of 6.26 and

5.62 %, respectively.[120] Yue et al.have investigated a graphene/

PEDOT:PSS composite as a coun-ter electrode for the replace-

ment of the expensive Pt coun-

ter electrode in DSSCs. The gra-phene/PEDOT:PSS composite has low charge-transfer resist-

ance on the electrode/electrolyte interface and also possesseshigh electrocatalytic activity for the I¢3 /I¢ redox reaction. The

graphene/PEDOT:PSS counter electrode shows high efficiencyof 7.86 %, which is higher than the Pt counter electrode

(7.31 %).[121] Li et al. have prepared a TiS2/PEDOT:PSS composite

as the counter electrode for DSSCs. In the TiS2/PEDOT:PSS com-posite structure, TiS2 is distributed in the polymer matrix of PE-

DOT:PSS, which leads to high conductivity and high electroca-talytic activity for the redox reaction of the I¢3 /I¢ couple. An ef-

ficiency of 7.04 % can be obtained by using TiS2/PEDOT:PSS,and this is comparable to the efficiency of a Pt counter elec-

trode (7.65 %).[122] Song et al. have fabricated a DSSC by using

a SiO2/PEDOT-PSS composite as the counter electrode. TheSiO2/PEDOT-PSS counter electrode shows electrocatalytic activi-

ty that is comparable to that of a Pt counter electrode for thereduction of I¢3 into I¢ , and a power conversion efficiency of

5.66 % can be obtained with a DSSC fabricated with SiO2/PEDOT-PSS as the counter electrode.[123] Nitrogen-doped

carbon and an iron-carbide nanocomposite show remarkable

electrocatalytic activity for the reduction of I¢3 into I¢ . A totalconversion efficiency of 7.36 % has been observed for a DSSCfabricated with this composite material ; this value is higherthan that obtained with a conventional Pt counter electrode

(7.15 %).[124] A hydrothermally synthesized MoS2-C hybrid hasserved as a low-costing Pt-free counter electrode for DSSCs

due to the special structure and high surface area of the MoS2-C composite. It is able to achieve a high power conversion effi-ciency of 7.69 %, which is higher than the efficiency achieved

by the Pt counter electrode (6.74 %).[125] Yang et al. have report-ed a comparative study between 1D and 2D graphene bridges

and have shown that the 2D graphene bridge composite isbetter than the 1D materials. As a result, graphene introduced

as 2D bridges enhances photoinduced charge transport in

DSSCs and also lowers recombination.[126] The differences be-tween the 1D and 2D nanomaterial composite electrodes are

shown in Figure 12.The photovoltaic efficiency of DSSCs with the improved gra-

phene structure counter electrode is still lower than that ofDSSCs with a standard platinum counter electrode. To further

Figure 11. J–V curve of a) some transition-metal carbides and b) some transition-metal nitrides for I¢3 /I¢ inDSSCs.[2]

Table 5. Photovoltaic characteristics of DSSCs assembled with differentmetal carbide and metal nitride counter electrodes.


VC 10.94 0.803 0.56 4.92 [2]VN 11.74 0.788 0.64 5.92 [2]NbN 9.81 0.798 0.47 3.68 [2]TiC 13.30 0.788 0.62 6.50 [2]TiN 12.83 0.796 0.61 6.23 [2]MoN 14.11 0.774 0.58 6.437 [21]TiC 13.12 0.77 0.64 6.46 [22]TiN nanotube 15.78 0.760 0.64 7.73 [107]Mo2N 0.74 0.60 6.38 6.38 [109]W2N 12.96 0.78 0.57 5.81 [109]VN peas 15.06 0.73 0.66 7.29 [110]WC 14.17 0.76 0.65 7.01 [111]TiC-SiC-C 11.13 0.78 0.65 5.7 [114]


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improve the cell performance, a graphene structure incorporat-

ed with tungsten nanoparticles in ethanol has been used asa counter electrode. The graphene–tungsten composite struc-

ture counter electrode can achieve an overall photovoltaic effi-ciency of 5.88 %.[3] Nagarajan et al. have assembled a PEDOT-re-

inforced exfoliated graphite composite counter electrode in

DSSCs. The PEDOT/EFG (EFG = exfoliated graphite) electrodeshows high energy conversion efficiency of 5.7 %, whereas the

conventional platinum counter electrode shows energy con-version efficiency of 4.4 %.[127] A TiN–carbon nanocomposite

counter electrode exhibits outstanding energy conversion effi-ciency of 6.71 %, which is higher than that observed for DSSC

devices fabricated by using pure TiN nanoparticles, carbon,

and conventional platinum counter electrodes. The outstand-ing performance of TiN embedded in amorphous carbon is

a result of low charge-transfer resistance and good chemicalstability.[23] CoS-coated multiwalled carbon nanotubes (CoS/

MWCNTs) have also been employed as a counter electrode forDSSCs due to the superior catalytic activity of CoS and the spe-cific surface of MWCNTs. This electrode exhibits excellent cata-

lytic activity and a higher energy conversion efficiency (6.96 %)than pure platinum, MWCNT, and CoS counter electrodes.[128]

A MWCNTs/PPy composite has been used as a counter elec-trode to replace the expensive Pt counter electrode in DSSCs.

The MWCNTs/PPy composite shows an efficiency of 3.78 %, andthe efficiencies of MWCNTs and PPy are 0.72 and 2.68 %, re-

spectively. The performance of MWCNTs/PPy is high relative tothat of the MWCNTs and PPy counter electrodes, and the cor-responding cyclic voltammetry (CV) curves (Figure 13) reveal

that the MWCNTs/PPy composite shows superior catalytic ac-tivity for the I¢3 /I¢ redox reaction. This may be due to the

higher conductivity of the MWCNTs/PPy composite due to theaddition of PPy into the MWCNTs.[129] Niu et al. have prepared

an axle–sleeve-structured MWCNTs/PANI composite and used it

as a counter electrode to replace the Pt electrode. This compo-site shows an efficiency of 7.21 %, which is comparable to that

of the Pt counter electrode (7.59 %).[130] Wang et al. have inves-tigated a composite catalyst of rosin carbon/Fe3O4 with stun-

ning morphology and applied it as a counter electrode inDSSCs. The DSSC assembled by using this composite shows

a high power conversion effi-ciency of 8.11 %, which is com-parable to that of the traditionalsputtered Pt counter elec-trode.[131] The photovoltaic char-acteristics of DSSCs assembled

with different composite counterelectrodes are shown in Table 6.

The commercial mass produc-

tion of DSSCs will need a largeamount of platinum, which is

very expensive and not abun-dantly available. Therefore, com-posite materials can be viewedas promising alternatives to the

platinum counter electrode inDSSCs.

4. Counter Electrodes for Iodine-Free RedoxElectrolytes

The iodide/triiodide redox couple electrolytes are the mostubiquitous and the most efficient in DSSCs. Also, researchershave introduced some new redox couples, including Co3 +

/Co2 + , SCN¢/ SCNð Þ¢3 , Br¢/Br¢3 , Fe3 +/Fe2+ , and T2/T¢ , inDSSCs.[132–136] Low-costing Pt-free counter electrode materialshave been proven to be promising catalysts for the regenera-

tion of iodine-free redox electrolytes. Tian and Sun have ex-plained the drawbacks of I¢/I¢3 electrolytes and iodine-free

redox couple electrolytes in detail.[137] Mathew et al. have ach-ieved 13 % efficiency for a DSSC by using a Co3 +/Co2 + redox

shuttle.[138] Swami et al. have investigated a CoS counter elec-

trode for the regeneration of cobalt-based redox electrolytesand have achieved an efficiency as high as 6.72 %.[132] Dao et al.

have prepared a Au nanoparticle/graphene nanohybrid bya dry plasma reduction method and used it as a counter elec-

trode in DSSCs fabricated with the Co3 +/Co2 + redox electro-lyte.[139] Maiaugree et al. have developed a NiSO4/PEDOT:PSS

Figure 12. Model of 1D (a, b) and 2D (c, d) nanomaterial composite electrodes.[126]

Figure 13. CV curves for the I¢3 /I¢ redox reaction using Pt, PPy, MWCNTs, andMWCNTs/PPy composite counter electrodes.[129]


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composite film counter electrode for the regeneration of the

organic T2/T¢ electrolyte.[136] Another report by Al-Mamun et al.

describes the preparation of a silver nanowire–graphene coun-ter electrode for the Co3 +/Co2 + redox mediator electrolyte for

DSSCs, and a power conversion efficiency of 2.44 % has beennoted; this value is comparable to the power conversion effi-

ciency of the Pt-based counter electrode (2.55 %).[140] Mu andMa have reported that the TiC counter electrode shows higher

efficiency than the Pt counter electrode in DSSCs with a Co3 +

/Co2+ redox electrolyte. The power conversion efficiency usingTiC as the counter electrode is 4.13 % and that of the Pt-based

counter electrode is only 2.91 %.[141]

Pt-free counter electrode materials are cheap and suitable

for the regeneration of both theiodide/triiodide redox coupleand iodine-free redox couples.

Hence, the development of newcounter electrode materials hasattracted immense attention inrecent years.

5. Stability of CounterElectrodes

The commercialization of DSSCs

predominantly depends on thelong-term stability of the fabri-

cated cell, and this may be af-fected by factors such as degra-

dation of the dye, evaporation

or leakage of the solvent, andthe dissociation of the counter

electrode in the corrosiveiodide/triiodide electrolyte.[142]

The long-term stability of DSSCshas been investigated by Hariki-

sun et al. during analysis from light soaking at 55–60 8C andonly a minor decrease is observed in the open-circuit voltages(i.e. 50–80 mV) due to a shift in the conduction band towardsmore positive potentials.[143]

A decrease in the fill factor at high light levels is due to anincrease in the series resistance of the cell. There is no evi-

dence of dye degradation, no loss of iodine, and no loss of cat-alytic activity of the Pt-based counter electrode. However, stor-age at 80 8C results in a 10–20 % decrease in performance with

ionic-liquid- or solvent-based electrolyte systems. Syrrokostaset al. have synthesized Pt counter electrodes by two different

methods, namely, electrochemical deposition and thermal de-composition of hexachloroplatinic acid (H2PtCl6) solution. They

then stored the DSSCs fabricated with these electrodes in theair and electrolyte solution for up to 70 days and then checked

their stability by cyclic voltammetry tests. They observed deg-

radation from a decrease in the triiodide reduction peak cur-rent density and an increase in the activation energy. The cur-

rent density of the counter electrode decreases by about ¢15to ¢25 % of the initial value if it is stored in air, whereas it de-

creases by ¢40 % if it is stored in the electrolyte. Degradationof the stability can be attributed to the dissociation of Pt when

stored in the electrolyte.[144]

Yun et al. have studied the chemical stability of counter elec-trodes by using X-ray photoelectron spectroscopy. They have

reported that Cu and Au films scarcely dissolve as their respec-tive sulfides. The Al film gradually loses its metallic properties

in iodine-based liquid electrolytes. On the other hand, for theconducting polymer-based counter electrode, the PEDOT:PSS

film adsorbs the organic molecules in the electrolytes,[145] and

the degradation mechanism of the metal and the conductingpolymer counter electrodes in the iodine-based electrolyte is

shown in Figure 14. Murakami et al. have reported that a coun-ter electrode prepared with a heating process shows better re-

Table 6. Photovoltaic characteristics of DSSCs fabricated with differentcomposite counter electrodes.

Sample[a] Jsc [mA cm¢2] Voc [V] FF h [%] Ref.

GO-tungsten 12.21 0.73 0.66 5.88 [4]MoN nitrogen-doped GO 14.62 0.772 0.70 7.998 [21]N-doped GO 12.14 0.821 0.58 5.80 [21]TiN-carbon 14.36 0.697 0.67 6.71 [22]PEDOT:PSS 13.2 0.65 0.62 5.40 [73]CoS/PEDOT:PSS 12.53 0.73 0.69 6.31 [75]PEDOT:PSS:PPy 14.27 0.75 0.71 7.60 [75]Pt/WO2 12.91 0.78 0.68 6.94 [118]Pt/WO2/TiO2 12.54 0.83 0.70 7.23 [118]carbon/TiO2 12.53 0.70 0.57 5.50 [119]GO/PEDOT:PSS 15.7 0.77 0.65 7.86 [121]TiS2/PEDOT:PSS 13.06 0.68 0.60 5.46 [122]SiO2/PEDOT:PSS 13.5 0.72 0.58 5.66 [123]Fe3C@N-C 14.97 0.74 0.66 7.36 [124]PEDOT:EFG 10.2 0.77 0.72 5.78 [127]MWCNTs/PPy 14.83 0.77 0.65 7.42 [129]MWCNTs/PANI 16.08 0.78 0.57 7.21 [130]carbon/Fe3O4 16.01 0.75 0.68 8.11 [131]

Figure 14. Schematic diagram explaining the degradation mechanism of metal and conducting polymer counterelectrodes in the iodine-based liquid electrolyte.[145]


Reviews

sults than a counter electrode prepared without a heating pro-cess.[146] Storing the device in the dark at room temperature

enhances both the open-circuit voltage and the fill factor butreduces the short-circuit current density.[142]

Degradation of the electrode is expected to occur in DSSCsprepared with these kinds of counter electrodes, and this af-

fects their long-term stability. The stability of the counter elec-trode may be determined by cyclic voltammetry. The value ofthe peak current is largely determined by mass transport. The

potential difference between the oxidation and reduction peakcurrents is indicative of electrocatalytic activity. A reaction with

the I¢3 mechanism has a potential difference of 68 mV.[147] Yueet al. have investigated the stability of a Pt/graphene counter

electrode by using cyclic voltammetry. They performed 30 con-tinuous cycle scans of the Pt/graphene counter electrode at

a scan rate of 10 mV s¢1.[54] Figure 15 represents the 30 continu-

ous cycles of the Pt/graphene counter electrode at a scan rateof 10 mV s¢1 and the relationship between the number of

cycles and the maximum redox peak current densities, whichshows a good linear relationship after 30 cycles. It is clear from

Figure 14 that the unchanged curve shape and the constantredox peak current densities are indicative of excellent electro-

chemical stability of the Pt/graphene counter electrode.

The counter electrode used for the I¢3 reduction should pos-sess high catalytic activity, high conductivity, and stability. Also,

it is important to realize that apart from cell composition, thepurity of the starting materials and the processing conditions

play important roles in the stability of the fabricated DSSCs.

6. Summary

DSSCs are the focus of recent attention owing to their ability

to convert sunlight into electricity at a low cost. A DSSC iscomposed of an electrode (photoanode, generally a TiO2-

coated fluorine-doped tin oxideplate used as the anode and

photocathode), a dye as the sensitizer, and I¢3 /I¢ redox electro-lytes. At present, the major focus of DSSCs is to decrease the

cost and to increase the photo-to-current conversion efficiency.The electrolyte plays an important role in improving the effi-

ciency of the device by increasing the conductivity of theredox couple. From a cost point of view, the dye and counter

electrode are the focus. For the dye, Ru–bipyridyl complexesgenerally show high efficiency, but they are expensive. For the

counter electrode part, platinum gives the best performance,but it is also expensive and its rarity limits its practical applica-tion in DSSCs. It has been suggested that the expensive plati-num counter electrode in DSSCs could be replaced by alterna-tive materials.

Apart from the dye and the electrolyte, it is important to de-velop alternative materials capable of replacing platinum as

the electrocatalyst. In DSSCs, the counter electrode is a crucialcomponent, in which triiodide is reduced to iodide by elec-trons flowing through the external circuit. We have seen

a series of platinum-free counter electrode materials in DSSCssuch as carbon materials, conducting polymers, inorganic

oxides and sulfides, transition-metal nitrides and carbides, andcomposite materials. Among these materials, conducting poly-

mers show promise as alternatives to platinum-free counterelectrodes in DSSCs. Transition-metal nitrides and carbides

have a wide range of applications in many fields, and many ofthese materials demonstrate excellent catalytic activity for the

reduction of triiodide ions. Intense investigation should be

done to develop novel counter electrodes for DSSCs, anda new concept or design strategy is needed in this field. While

developing or designing counter electrode materials, the re-searcher should remember that electrodes should be cheap,

environmentally friendly for clean technology, and exhibita wide range of applications.

Acknowledgements

We gratefully acknowledge financial support from the Depart-

ment of Atomic Energy- Board of Research in Nuclear Sciences(DAE-BRNS) and the Department of Science and Technology–Sci-

Figure 15. a) 30 continuous cycles of the Pt/graphene counter electrode ata scan rate of 10 mV s¢1, and b) the relationship between the number ofcycles and maximum redox peak current densities.[54]


Reviews

ence and Engineering Research Board (DST-SERB). Also, we ac-knowledge Dr. Krishna Rao Ragavendran, Department of Physics,

University of Malaya, for his support.

Keywords: counter electrodes · dye-sensitized solar cells ·efficency · electron transfer · nanostructures

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Received: November 28, 2014

Revised: February 11, 2015

Published online on April 15, 2015


Reviews

Recent progress in non platinum counter electrode materials for dye sensitized solar cells

Engineering

Transcript of Recent progress in non platinum counter electrode materials for dye sensitized solar cells