recent advances in dye sensitized solar cells

Review ArticleRecent Advances in Dye Sensitized Solar Cells

Umer Mehmood1 Saleem-ur Rahman1 Khalil Harrabi2

Ibnelwaleed A Hussein1 and B V S Reddy3

1 Department of Chemical Engineering King Fahd University of Petroleum ampMinerals (KFUPM) PO Box 5050Dhahran 31261 Saudi Arabia

2Department of Physics KFUPM PO Box 5050 Dhahran 31261 Saudi Arabia3 Indian Institute of Chemical Technology Hyderabad India

Correspondence should be addressed to Ibnelwaleed A Hussein ihusseinkfupmedusa

Received 28 December 2013 Revised 22 March 2014 Accepted 22 March 2014 Published 17 April 2014

Academic Editor Chi-Wai Chow

Copyright copy 2014 Umer Mehmood et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Solar energy is an abundant and accessible source of renewable energy available on earth and many types of photovoltaic (PV)devices like organic inorganic and hybrid cells have been developed to harness the energy PV cells directly convert solar radiationinto electricitywithout affecting the environmentAlthough silicon based solar cells (inorganic cells) arewidely used because of theirhigh efficiency they are rigid and manufacturing costs are high Researchers have focused on organic solar cells to overcome thesedisadvantages DSSCs comprise a sensitized semiconductor (photoelectrode) and a catalytic electrode (counter electrode) with anelectrolyte sandwiched between them and their efficiency depends on many factors The maximum electrical conversion efficiencyof DSSCs attained so far is 111 which is still low for commercial applicationsThis review examines the working principle factorsaffecting the efficiency and key challenges facing DSSCs

1 Introduction

The world energy demand is continuously increasing andthe world power consumption which is 13 terawatts (TW)currently is expected to reach about 23 TW in 2050 [1] Fossilfuels which are depleting rapidly meet 80 of the energyrequirement of the whole world [2] Moreover the burning offossil fuels raises the amount of carbon dioxide in the atmo-sphere Owing to growing energy demand exhaustion of oilresources and global warming issues there is a need for cleanand renewable energy technologies Photovoltaic technologyemploying solar energy is regarded as the most efficient tech-nology among all the sustainable energy technologies such astidal power solar thermal hydropower and biomass [3]

The solar radiation from the sun is approximately 3times1024 Jper year which is ten times the current energy demands [4]The first practical photovoltaic cell was designed in 1954 atthe Bell Laboratories [5] using diffused silicon p-n junctiontechnology with an efficiency of 6 [6] Although the light to

electricity conversion efficiency of silicon based solar cells hasreached 15 to 20 [7] the need for highly purified siliconuse of toxic chemicals in their manufacture and the highcost has restricted their worldwide use These constraintsencouraged the search for environmentally friendly and lowcost solar cells In 1991 OrsquoRegan andGratzel developed a newphotovoltaic cell working on the principle of plant photosyn-thesis The efficiency of this PV cell which became knownas dye sensitized solar cell (DSSC) was reported as 71 to79 [8] DSSCs comprise of a photoelectrode and a catalytic-electrode with an electrolyte between them The measuringparameters of DSSCs such as electrical conversion efficiency(120578) open circuit voltage (119881oc) close circuit current density(119869sc) fill factor (FF) interface charge resistance and anincident photon to current efficiency (IPCE) depend on themorphological properties of semiconductors spectroscopicproperties of dyes and the electrical properties of electrolytesDSSCs are an important type of thin film photovoltaictechnology because of their low cost of manufacturing ease

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2014 Article ID 974782 12 pageshttpdxdoiorg1011552014974782

2 Advances in Materials Science and Engineering

Mediator

Conducting glass

Dye Electrolyte Cathode

0

05

10

Red Ox

Max voltage

Injectioneminus

eminus eminus

h

minus05

Eve

rsus

NH

E (V

)TiO2

Diffusion

S∘S+

SlowastS+

Figure 1 Structure and working principle of DSSC with 119878∘119878+119878lowastrepresenting sensitizer in the ground state oxidized state andexcited state respectively RedOx = redox mediator [11]

of fabrication and light weight product [9] In the near futurethe venture of DSSCs as a competitive technology will thus berevealed The latest efficiency of DSSC is more than 11 [10]

Structure andMechanism ofDSSCThe structure andworkingprinciple of DSSC is shown in Figure 1 A charge separationprocess in a DSSC consists of the following steps [1]

(i) Photoanode absorbs incident solar energy Uponabsorption of photo energy the electrons in the dyebecome exited from ground state to the excited state

(ii) Due to the difference in energy levels of the electronicstates electrons from the exited state are injected tothe conduction band of the semiconductor As a resultthe dye becomes oxidized

(iii) The electrolyte which is in contact with the dye thendonates electrons to the dye restoring it to the initialstate

(iv) Electrolyte then diffuses towards the catalytic elec-trode where the reduction reaction takes place andelectrolyte restores its initial state by accepting elec-trons from the external circuit

(v) In addition to these forward charge transfer processesbackward charge transfer processes also occur inone complete cycle These backward electron transferprocesses drastically reduce the efficiency of DSSCsThese include the following

(a) transfer of electrons from the semiconductor tothe oxidized dye

(b) recombination of injected electrons with theelectrolyte (dark current)

(c) transfer of electrons from the dye in its excitedstate to the dye in the ground state

The following should be fulfilled to reduce the effects ofthe backward transfer processes

(i) Charge transfer to semiconductor must occur with aquantum yield [12]

(ii) The lowest unoccupied molecular orbital (LUMO)of the photosensitizer should be more negative thanthe conduction band of the semiconductor and thehighest occupied molecular orbital (HOMO) shouldbe more positive than the redox potential of theelectrolyte [13]

(iii) Electron injection rate to the semiconductor shouldbe higher than the rate of decay of electrons from theexcited state of the dye to its ground state [14]

2 Components of Dye-Sensitized Solar Cell

21 Transparent Conductive Substrate DSSCs are typicallyconstructed with two sheets of conductive transparent mate-rials which provide a substrate for the deposition of thesemiconductor and catalyst acting also as current collectorsSubstrates must be highly transparent (transparency gt 80)to allow the passage of maximum sunlight to the active areaof the cellThe electrical conductivity of the substrates shouldalso be high for efficient charge transfer and to minimizeenergy loss These two characteristics of substrate dictate theefficiency of DSSCs [3]

Typically FTO (fluorine tin oxide SnO2F) and ITO

(indium tin oxide In2O3Sn) are used as the conductive

substrate FTO and ITO substrates consist of soda lime glasscoated with fluorine tin oxide and indium tin oxide layersrespectively ITO films have a transmittance of over 80and sheet resistance 18Ωcm2 while FTO films exhibit atransmittance of about 75 in the visible region and sheetresistance of 85cm2 Sima et al conducted a study tocompare FTO and ITObasedDSSCs [15] Photoanodes basedon FTO and ITO glass substrates were sintered at 450∘Cfor 2 h in an oxygen atmosphere They found that the sheetresistance of FTO remained constant while that of ITOincreased from 18Ωcm2 to 52Ωcm2 upon sintering Theoverall 120578 of a DSSC based on FTO is 94 while for anidentical cell based on ITO it is 24 Thus FTO is highlyrecommended for DSSCs due to its low and temperature-stable sheet resistance

Polymers can also be used as an alternative to glasssubstrates because of their flexibility and low cost Murakamiet al used PET (polyethylene terephthalate) coated withITO and found an efficiency of 38 [16] Polyethylenenaphthalate (PEN) coated with ITO was used by Ito et al andreported an efficiency of 78 [17] However limitations inthe range of usability temperature restrict the use of polymersas substrates in DSSCs [18] Metals such as stainless steeltungsten and titanium that can form a conducting layer havealso been used as substrates Although Jun et al demonstratedan efficiency of 61 using a stainless steel as the substrate

Advances in Materials Science and Engineering 3

[19] the high cost and corrosion caused by the electrolyteprohibit the use of metals as substrates [20]

22 Mesoporous Semiconductor The semiconductor whichprovides a surface area for the adsorption of the dye acceptselectrons from the excited dye and conducts them to theexternal circuit to produce an electric current [21] The elec-tron transport rate which highly depends on the crystallinitymorphology and the surface area of semiconductors affectsthe efficiency of DSSCs Metal oxides like titanium oxide(TiO2) zinc oxide (ZnO) [22] and stannic oxide (SnO

2)

[23] have been used as the semiconductor material Howeverexperiments show that DSSCs based on ZnO and SnO

2

yield a lower efficiency in comparison to those based onnanocrystalline TiO

2[23ndash25]The latter has been considered

as an ideal semiconductor material for DSSCs since 1991due to this and its better morphological and photovoltaicproperties when compared to other semiconductors [8]

Of the two crystalline forms of TiO2 anatase and rutile

the former is preferred because of its high conduction bandedge energy (32 eV) when compared to rutile (sim3 eV) Highband gap energy makes anatase chemically more stable [26]The electron transport process in rutile is also slow whencompared to anatase due to the high packing density Shortcircuit photo current of an anatase-based DSSC is 30 morethan that of a rutile-basedDSSCwith the same film thicknessOwing to the smaller surface area per unit volume rutileabsorbs less dye and therefore rutile-based DSSCs are lessefficient [27]

The main loss path in DSSCs is the recombination ofinjected electrons with the electrolyteThis phenomenon theresulting current of which is known as the dark currentdiminishes the efficiency of DSSCs and it can be minimizedby employing structural changes use of insulating layersor surface treatment of TiO

2[28] Many morphologies of

anatase TiO2from nanoparticles nanofibers [29] nanowires

[30] hollow spheres [31] hollowhemispheres [32] nanotubes[33] and hierarchical spheres to ellipsoid spheres [34] havebeen fruitfully fabricated via solvothermal reactions of tita-nium 119899-butoxide and acetic acid A DSSC with a photoelec-trode based on the hierarchical anatase TiO

2spheres has an

overall 120578 of 935 with a 119869sc of 1794mA cmminus2 119881oc of 803mVand FF of 065 Its overall 120578 is much higher than that ofa DSSC based on nanoparticles (737) nanofibers (815)and ellipsoid TiO

2spheres (793) [35] The substantial

improvement of short-119869sc and 120578 for the hierarchical sphere-based DSSC when compared to other nanostructure-basedDSSCs is mainly due to the larger dye loading higher lightscattering ability faster charge transport and longer electronlifetime [36 37] Another approach to minimize the chargerecombination is the deposition of an insulating layer on thesemiconductor electrode Many metal oxides like ZnO [38]niobium pentoxide (Nb

2O5) [39] Al

2O3[40] and SiO

2[41]

have been used as an energy barrier for retarding the chargerecombination owing to their insulating properties Theseinsulating layers reduce the interaction between the electronsinjected to the semiconductor and the electrolyte solution[42] Similarly surface treatment of TiO

2with TiCl

4also

reduces the charge recombination process by increasing the

interfacial charge-transfer resistance of the TCOelectrolyteinterface [43 44] Recently Melhem et al synthesized anitrogen-doped TiO

2electrode (optically active electrode) by

laser pyrolysis [45] They found that the short-circuit currentdensity of DSSC based on an N-doped electrode increased bymore than 10 when compared to that of a DSSC based onpure anatase This progress is associated with electronic andoptical properties of the starting nanopowder

23 Dye (Photosensitizer) The function of dye is to absorblight and transfer electrons to the conduction band of thesemiconductor It is chemically bonded to the porous surfaceof the semiconductor An efficient photosensitizer should

(i) show intense absorption in the visible region (400 nmto 700 nm)

(ii) adsorb strongly on the surface of the semiconductor(iii) possess a high extinction coefficient(iv) be stable in its oxidized form allowing it to be

rereduced by an electrolyte(v) be stable enough to carry out sim108 turnovers which

typically correspond to 20 years of cell operation(vi) possess more negative LUMO than the CB of the

semiconductor and more positive HOMO than theredox potential of the electrolyte

In addition the performance of DSSCs highly dependson the molecular structure of the sensitizers Many chemicalcompounds such as the phthalocyanines [47ndash49] coumarin-343 [50ndash52] carboxylated derivatives of anthracene [53 54]and porphyrins [55ndash57] have been used for the sensitizationof semiconductors However photosensitizers based on tran-sitionmetals have been shown to be the best so far [46]Thereare three classes of photosensitizers metal complex sensitiz-ers metal-free organic sensitizers and natural sensitizers

231 Metal Complex Sensitizers Metal complex sensitizerscomprise of both anchoring ligands (ACLs) and ancillaryligands (ALLs) The adhesion of photosensitizers to thesemiconductor is strongly dependent on the properties ofACLs While ALLs can be used for the tuning of the overallproperties of sensitizers polypyridinic complexes of d6metalions possess intense metal to ligand charge transfer (MLCT)bands in the visible region which is shown by polypyridiniccomplexes of d6 metal ions Modification of ACLs as well aschanging the ALLs or its substituents can alter the energies ofthe MLCT states Many metal complex sensitizers have beenprepared by changing the ALLs However ruthenium (II)polypyridyl complexes show better light to electricity con-version efficiency [58] because of their good spectroscopicphotostability redox and excited state properties in the finaldevice [59 60] The general representation of carboxylic acidbased sensitizers is [Ru (dcbH

2)2LL1015840] while dcbH

2and L

andor L1015840 represent anchoring ligands and ancillary ligandsrespectively An example of a high performance carboxylicacid based sensitizer is cis-[Ru(dcbH

2)2(NCS)

2] which is also

known as the N3dye [61] Efficient performance has also


Table 1 Record efficiencies achieved for DSSCs of varying surface area [46]

No Dye Surface (cm2) 120578 () 119881oc (V) 119868sc (mAcm2) FF ()1 N-719 lt1 112 084 1773 742 N-749 0219 111 0736 209 723 N-749 1004 104 072 218 654 N-719 1310 101 082 170 725 N-3 2360 82 076 158 71

Wavelength (nm)

IPCE

()

80

60

40

20

0

400 600 800 1000

L = 4 4998400-COOH-2 2-bipyridine

L998400 = 4 4998400 4998400998400-COOH-2 2998400 6998400 2998400998400-terbipyridine

TiO2

RuL998400(NCS)3

RuL2(NCS)2

Figure 2 IPCE of efficient dyes and their chemical structures [75]

been predicted for other ruthenium based photosensitizerssuch as [Ru(tcterpy)(NCS)

3]minus which is also known as the

black dye Figure 2 shows that a cell based on black dyeis more efficient than that of cell based on red dye in thenear infrared region and attained an efficiency of 104 [4]Record efficiencies of DSSCs with varying surface area andthe structure of the photosensitizers are shown in Table 1 andFigure 3 respectively Although record efficiency and stabilityhave been achieved with ruthenium-based sensitizers thehigh cost and scarcity of ruthenium have necessitated theconsideration of other options

232 Metal-Free Photosensitizers Metal-free organic sen-sitizers have been used not only to replace the expensiveruthenium based sensitizers but also to improve the elec-tronic properties of devices However the efficiency of thesesensitizers is still low when compared to devices based onruthenium-based dyes But the efficiency and performancecan be improved by the proper selection or tuning of thedesigning components The general design mechanism ofmetal-free organic dye sensitized photoanode is shown inFigure 4 Substituents acting as the donor and the acceptorare separated by a 120587-conjugated spacer

The photoelectric properties of these dyes can be tunedby altering or matching different substituents within the D-120587-A structure The literature shows that the efficiency ofsuch dyes depends on the chemistry of the electrolytes Forexample efficiencies of metal-free organic dyes in liquidionic and solid state electrolytes are greater than 8 6 and

4 respectively [77] These results suggest that the donorgroups to form efficient sensitizers should be selected fromthe electron rich aryl amines family including phenylamineaminocoumarin indoline and (difluorenyl)triphenylamineThe 120587-conjugated connector must be selected from com-pounds containing thiophene units for example oligoth-iophenes thienylenevinylenes or dithienothiophene due totheir outstanding charge transfer characteristics Acrylic acidgroup is considered the best acceptor moiety However theefficiency of DSSCs based on metal-free organic sensitizersis still low for industrial applications Some important metalfree photosensitizers and their photoelectric properties aresummarized in Table 2

233 Natural Sensitizers Natural dyes have also been used inDSSCs because of their low cost easy extraction nontoxicityand the environmentally benign nature [78]

There are two classes of plant pigments namely caro-tenoids and flavonoids In addition there are three sub-classes of flavonoids anthocyanins proanthocyanidins andflavonols But only anthocyanins of the flavonoid group areresponsible for cyanic colors which range from salmon pinkthrough red and violet to dark blue ofmost flowers fruits andleaves Anthocyanins is the group most extensively investi-gated as natural sensitizers and their extracts showmaximumabsorption in the range of 510 to 548 nm depending on thefruit or solvent used [79] The basic chemical structures ofmost abundant anthocyanins are shown in Figure 5

The chain length of the substituent R also affects theperformance of anthocyanins The performance of a dyecontaining an R group with a long chain length will belower due to steric hindrance which restricts the transferof electrons from dye molecules to the conduction band ofthe semiconductor The efficiency of natural dyes is very lowbecause of the weak interaction between the semiconductor(TiO2) and dyes Dye aggregation on the nanocrystalline

film is another important cause of low efficiency Someimportant natural dyes and their photoelectric properties aresummarized in Table 3 Calogero et al demonstrated that redturnip yields the highest efficiency [67]

24 Electrolyte The function of the electrolyte is to regener-ate the dye after it injects electrons into the conduction bandof the semiconductor The electrolyte also acts as a chargetransport medium to transfer positive charges toward thecounter electrodesThe long-term stability of DSSCs dependson the properties of electrolyte Therefore the electrolytemust have the following characteristics [80 81]


Table 2 Metal free organic photosensitizers with different electrolytes

No Compound 120578 () 119881oc 119869sc FF () References Electrolytelowast

1

N

NS

O

S

CO2H

614941

06067087

1789777

577461

[62][63][64]

OEILSS

2

N

N

S

O

NO S

CO2H

C2H5

906442

065071055

20125141

6947254

[65][63][66]

OEIESS

Electrolytelowast OE organic electrolyte IE ionic liquid SS solid state

Table 3 Photoelectric parameters of DSSCs based on natural dyes

No Dye 120578 () 119881oc (V) 119869sc (mAcm2) FF () References1 Red turnip 17 043 950 037 [67]2 Rhoeo spathacea 149 05 109 027 [68]3 Shisonin 131 053 480 051 [69]4 Wild sicilian 119 038 82 038 [67]5 Mangosteen 117 067 269 063 [70]

(i) a high electrical conductivity and low viscosity forfaster diffusion of electrons

(ii) good interfacial contact with the nanocrystallinesemiconductor and the counter electrode

(iii) not causing desorption of the dye from the oxidizedsurface and the degradation of the dye

(iv) not absorbing light in the visible region

Electrolytes for DSSCs are classified into three typesliquid electrolytes solid state electrolytes and quasisolid stateelectrolytes

241 Liquid Electrolytes Liquid electrolytes are further clas-sified into two types organic solvent based electrolytes androom temperature ionic liquid electrolytes (RTIL) dependingon the solvent used

(1) Organic Electrolytes Each component of organic elec-trolytes such as the redox couple solvent and additives affectsthe performance of DSSCs The major component of organicelectrolyte is the redox couple Many types of redox couplessuch as BrminusBr

3[82] SCNminus(SCN)

2 SeCNminus(SeCN)

2[83

84] and substituted bipyridyl cobalt (IIIII) [85] have beeninvestigated But I

3

minusIminus is considered an ideal redox couple

[86] because of its good solubility rapid dye regeneration low

absorbance of light in the visible region suitable redox poten-tial and very slow recombination kinetics between injectedelectrons into the semiconductor and triiodide (I

3) [87]

Another basic component of the liquid electrolyte isthe organic solvent It is responsible for the diffusion andthe dissolution of the iodidetriiodide ions Many types ofsolvents such as acrylonitrile (AcN) ethylenecarbonate (EC)propylene carbonate (PC) 3-methoxypropionitrile (MePN)and N-methylpyrrolidone (NMP) which yield reliable per-formance have been investigated [88 89] The photoelectricperformance of DSSCs depends on the donor number (DN)of the solvents Using solvents with high DN increasesthe 119881oc and decreases 119869sc Values [90] due to the lowerconcentration of triiodide which in turn reduces the darkcurrent and therefore yields a high photovoltage (119881oc)The photovoltaic properties of DSSCs can be optimizedby employing electric additives The most efficient addi-tives are 4-tert-butylpyridine (TBP)N-methylbenzimidazole(NMBI) and guanidinium thiocyanate (GuNCS) [91] Theseadditives adsorb on the photoelectrodeelectrolyte interfaceand prevent the recombination of injected electrons with thetri-iodide ions Another important parameter is the concen-tration of I

3

minusIminus in the mixture when used as a redox couple

If the concentration is low then it will be difficult to maintainthe required conductivity But if the concentration is highthen it will absorb light in the visible region [3] Thereforethe concentration of I

3

minusIminus electrolyte must be optimized


Table 4 ILs and their efficiencies in DSSC applications

No Ionic liquids 120578 () References1 1-hexyl-3-methylimidazolium iodide 5 [71]2 1-methyl-3-(334455666-nonafluorohexyl) imidazolium 51 [72]3 1-butyl-3-methylimidazolium iodide 46 [72]4 S-propyltetrahydrothiophenium iodide 351 [73]5 Eutectic mixture of glycerol and choline iodide 388 [74]

NN

N

N

Ru

N

N

CS

CS

N C S

NN

N

N

Ru

N

NC

S

CS

N C S

N719

N

N

N

Ru

N

N

CS

CS

N C S

Black dye

CO2H

N3

CO2H

HO2C

HO2C

HO2C

HO2C

120578 = 1103 120578 = 1118

CO2minusTBA+

CO2minusTBA+

+TBAminusO2C

+TBAminusO2C

+TBAminusO2C

Figure 3 Structure of some efficient Ru-based photosensitizers [76]

Do Acc

SensitizerSemiconductor

120587-bridge TiO2

eminus

eminuseminush

Figure 4 Design mechanism of the use of an organic dye for TiO2

photoanodes in DSSCs [77]

(2) Ionic Electrolytes Although covalent electrolytes showrecord efficiency they have a high evaporation rate due tohigh volatility High evaporation rates and leakage limit thelong-term stability of DSSCs based on covalent electrolytesIn order tominimize these problems room temperature ionicliquids (RTIL) have been employed successfully They are agroup of organic salts containing cations such as pyridiniumimidazolium and anions from the halide or pseudohalidefamily [92] They act simultaneously as an iodine sourceand as a solvent The most commonly used ionic liquid(IL) for DSSC applications is N N1015840 bis-alkyl-substitutedimidazolium iodides [93] It has been found that the viscosityof these salts decrease with decreasing alkyl chain lengthdue to the decrease of van der Waals forces Therefore the

conductivity of IL electrolytes decreases because an increasein viscosity limits the diffusion of charges in the liquid [71]The preferred counter ions for alkyl imidazolium based ILsare Iminus N(CN)

2

minus B(CN)4

minus (CF3COO)

2Nminus BF

4

minus PF6

minus andNCSminus In addition to alkyl imidazolium based ILs alkylpyridinium and trialkylmethylsulfonium based ILs have alsobeen developed for DSSCs applications Some ILs and theirefficiencies in DSSC applications are presented in Table 4However low efficiency due to the high viscosity limits theirapplication as electrolytes in DSSCs

242 Solid-State Electrolytes Leakage is a key problem inliquid-electrolyte based DSSCs which drastically reduce thelong-term stability of solar cells In order to improve theperformance and stability solid state electrolytes have beendeveloped [94] They replace the liquid electrolyte with ap-type semiconductor or a hole transfer material (HTM)The band gap structure of the p-type semiconductor must becompatible with the HOMO level of the photosensitizer andthe conduction band of the n-type semiconductor (TiO

2) [3]

Copper based compounds such as CuI CuBr and CuSCNhave been employed as inorganic HTMs because of theirgood conductivity [86] Organic HTMs have advantages overinorganic HTMs because of their low cost and easy deposi-tion The pioneer compound of this class is 221015840771015840-tetrakis(NN-di-p-methoxyphenylamine) 991015840-spirobifluorene(OMeTAD) which has an efficiency of only 074 (underwhite-light illumination of 4mWcm2) [95] Its efficiencywas


OH

HO

OH

OHPelargonidinCyanidinPeonidinDelphinidinPeturidinMalvidin

H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)

Figure 5 (a) Chemical structures of anthocyanins (b) Structure in acidic and basic media (c) Chelation mechanism with TiO2[79]

increased to 32 by improving dye adsorption by the pres-ence of silver ions in the dye solution [96] Solid state elec-trolyte basedDSSCs have very low light-to-electricity conver-sion efficiency due to the poor intimate contact between thephotoelectrode-HTMs andhigh rate of charge recombinationbetween the semiconductor and the HTM However light-to-electricity conversion efficiency can be improved byintroducing a redox couple into the solid state electrolytewhich acts as a transport medium Some common HTMsand their efficiencies in DSSCs are presented in Table 5

243 Quasisolid State Electrolytes Though the leakage prob-lem can be solved by using solid state electrolytes contactbetween the mesoporous semiconductor and the HTM isweak As solid-state electrolytes do not penetrate into thepores of the semiconductor the above problem was solvedusing quasisolid state electrolytes A quasisolid state elec-trolyte is a composite of a polymer and a liquid electrolyte

Table 5 Performance of different HTMs in DSSCs

No HTMs 120578 [] References1 CuI 24 [97]2 CuI 38 [98]3 CuSCN 15 [99]4 Spiro-OMeTAD 32 [96]5 Polyaniline 115 [100]

[101 108] Because of the unique network structure ofpolymers quasisolid state electrolytes show better long-termstability high electrical conductivity and good interfacialcontact when compared to liquid electrolytes [101 108] Theconductivity of the quasisolid state electrolytes depends onthe molecular weight and the morphology of the polymerbecause of the higher mobility of charges in the amor-phous phase of polymers when compared to the crystalline


Table 6 Polymer electrolytes and their performance in DSSC applications

No Polymer Solvent 120578 () References1 1324-Di-O-benzylidene-D-sorbitol 3-methoxypropionitrile 61 [101]2 5 poly (acrylic acid)-poly(ethylene glycol) N-Methyl-2-pyrrolidone + 120574-butyrolactone 61 [102]3 Low molecular weight gelator 1-Hexyl-3-methylimidazoliumiodide iodine 5 [103]4 Poly(acrylonitrile-co-styrene) 4-tert-Butylpyridine + NaI + I2 275 [104]5 Poly(ethylene glycol) (PEG) Propyleneycarbonate + potassium Iodide Iodine 72 [105]6 Poly(ethylene oxide-co-propylene oxide)trimethacrylate EC + GBL 81 [106]7 Poly(vinylidene-fluoride-co-hexafluoropropylene) 12-Dimethyl-3-propyl imidazoliumiodide iodine gt6 [107]

phase The performance of polymer-electrolyte based DSSCsstrongly depends on the working temperature because anincrease of temperature causes a phase transformation froma gel state to a solution state [86] Some important polymerelectrolytes are summarized in Table 6

25 Counter Electrode The counter electrode is used for theregeneration of the electrolyte The oxidized electrolyte dif-fuses towards the counter electrode where it accepts electronsfrom the external circuit A catalyst is required to acceleratethe reduction reaction and platinum (Pt) is considered a pre-ferred catalyst because of its high exchange current densitygood catalytic activity and transparency The performanceof the CE depends on the method of Pt deposition onTCO substrate Among the deposition methods are ther-mal decomposition of hexachloroplatinic salt in isopropanol[109] electrodeposition [110] sputtering [111] vapor deposi-tion [112] and screen printing [112] It has been found that theactivity of the Pt catalyst decreases with time in the presenceof iodidetri-iodide redox couple [113] Experiments showthat two major factors are responsible for the deactivationof the Pt counter electrode alteration of its electrocatalyticproperties and the removal of Pt from the substrate [109]

Although the Pt catalyst possesses high catalytic activitythe high cost of Pt is a disadvantage

Therefore grapheme and conductive polymers have alsobeen used as alternative materials for counter electrode [1]But their electrical efficiencies were very low when comparedto the Pt catalyst

3 Key Challenges and Recommendations

Low efficiency and low stability are the major challenges forthe commercial deployment of DSSCs The following factorsare responsible for the low efficiency and stability of DSSCs

(i) nonoptimized dark current(ii) poor performance of dyes in the NIR region(iii) poor contact between the electrodes(iv) low volatility and high viscosity of electrolytes(v) degradation of electrolyte properties due to UV

absorption of lightThe following steps can be recommended in order to

enhance the efficiency and stability of DSSCs(i) improvement in the morphology of semiconductors

to reduce the dark current

(ii) improvement in the dye design to absorb radiation inthe NIR region

(iii) developing low volatile and less viscous electrolytes toimprove the charge transfer rate

(iv) improvement in the mechanical contact or adhesionbetween the two electrodes

(v) use of additives for dyes and electrolytes that enhancetheir properties

However the efficiency and stability of DSSCs do notdepend on a single factor There must be trade off amongdifferent factors to improve the performance of DSSCs

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors would like to acknowledge the support providedby King Abdulaziz City for Science and Technology (KACST)through the Science amp Technology Unit at King Fahd Uni-versity of Petroleum amp Minerals (KFUPM) for funding thiswork through project 11-ENE1635-04 as part of the NationalScience Technology and Innovation Plan KFUPM is alsoacknowledged for supporting this research The authorswould like to acknowledge the Center of Research Excellencefor Renewable Energy at KFUPM

References

[1] L AndradeHA Ribeiro andAMendes ldquoDye-sensitized solarcells an overviewrdquo inEncyclopedia of Inorganic andBioinorganicChemistry pp 1ndash20 2011

[2] B Li L Wang B Kang P Wang and Y Qiu ldquoReview of recentprogress in solid-state dye-sensitized solar cellsrdquo Solar EnergyMaterials amp Solar Cells vol 90 no 5 pp 549ndash573 2006

[3] J Gong J Liang and K Sumathy ldquoReview on dye-sensitizedsolar cells (DSSCs) fundamental concepts andnovel materialsrdquoRenewable and Sustainable Energy Reviews vol 16 no 8 pp5848ndash5860 2012

[4] K R Millington ldquoDye-sensitized cellsrdquo in Encyclopedia ofElectrochemical Power Sources pp 10ndash21 2009

[5] K A Tsokos Physics for the IB Diploma Cambridge UniversityPress Cambridge UK 5th edition 2008


[6] J Perlin The Silicon Solar Cell Turns 50 National RenewableEnergy Laboratory 2004

[7] C D Grant A M Schwartzberg G P Smestad J KowalikL M Tolbert and J Z Zhang ldquoCharacterization of nanocrys-talline and thin film TiO

2solar cells with poly(3-undecyl-221015840-

bithiophene) as a sensitizer and hole conductorrdquo Journal ofElectroanalytical Chemistry vol 522 no 1 pp 40ndash48 2002

[8] B OrsquoRegan and M Gratzel ldquoA low-cost high-efficiency solarcell based on dye-sensitized colloidal TiO

2filmsrdquo Nature vol

353 no 6346 pp 737ndash740 1991[9] F O Lenzmann and J M Kroon ldquoRecent advances in dye-

sensitized solar cellsrdquo Advances in OptoElectronics vol 2007Article ID 65073 10 pages 2007

[10] M K Nazeeruddin F de Angelis S Fantacci et al ldquoCom-bined experimental and DFT-TDDFT computational study ofphotoelectrochemical cell ruthenium sensitizersrdquo Journal of theAmerican Chemical Society vol 127 no 48 pp 16835ndash168472005

[11] J-H Yum E Baranoff S Wenger M K Nazeeruddin andM Gratzel ldquoPanchromatic engineering for dye-sensitized solarcellsrdquo Energy and Environmental Science vol 4 no 3 pp 842ndash857 2011

[12] M Gratzel ldquoConversion of sunlight to electric power bynanocrystalline dye-sensitized solar cellsrdquo Journal of Photo-chemistry and Photobiology A Chemistry vol 164 no 1ndash3 pp3ndash14 2004

[13] K Hara T Sato R Katoh et al ldquoMolecular design of coumarindyes for efficient dye-sensitized solar cellsrdquo Journal of PhysicalChemistry B vol 107 no 2 pp 597ndash606 2003

[14] N A Andersen and T Lian ldquoUltrafast electron transfer atthe molecule-semiconductor nanoparticle interfacerdquo AnnualReview of Physical Chemistry vol 56 pp 491ndash519 2005

[15] C Sima C Grigoriu and S Antohe ldquoComparison of thedye-sensitized solar cells performances based on transparentconductive ITO and FTOrdquo Thin Solid Films vol 519 no 2 pp595ndash597 2010

[16] T N Murakami Y Kijitori N Kawashima and T MiyasakaldquoLow temperature preparation of mesoporous TiO

2films

for efficient dye-sensitized photoelectrode by chemical vapordeposition combined with UV light irradiationrdquo Journal ofPhotochemistry and Photobiology A Chemistry vol 164 no 1ndash3pp 187ndash191 2004

[17] S Ito N-L C Ha G Rothenberger et al ldquoHigh-efficiency(72) flexible dye-sensitized solar cells with Ti-metal substratefor nanocrystalline-TiO

2photoanoderdquo Chemical Communica-

tions no 38 pp 4004ndash4006 2006[18] H C Weerasinghea F Huanga and Y-B Chenga ldquoFabrication

of flexible dye sensitized solar cells on plastic substratesrdquo NanoEnergy vol 2 no 2 pp 174ndash189 2013

[19] Y Jun J Kim and M G Kang ldquoA study of stainless steel-baseddye-sensitized solar cells and modulesrdquo Solar Energy Materialsamp Solar Cells vol 91 no 9 pp 779ndash784 2007

[20] K Kalyanasundaram Ed Dye-Sensitized Solar Cells 2010[21] M R Hoffmann S T Martin W Choi and D W Bahnemann

ldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995

[22] O Lupan V M Guerin I M Tiginyanu et al ldquoWell-alignedarrays of vertically orientedZnOnanowires electrodeposited onITO-coated glass and their integration in dye sensitized solarcellsrdquo Journal of Photochemistry and Photobiology A Chemistryvol 211 no 1 pp 65ndash73 2010

[23] D-W Han J-H Heo D-J Kwak C-H Han and Y-MSung ldquoTexture morphology and photovoltaic characteristics ofnanoporous FSnO

2filmsrdquo Journal of Electrical Engineering and

Technology vol 4 no 1 pp 93ndash97 2009[24] Y Fukai Y Kondo SMori and E Suzuki ldquoHighly efficient dye-

sensitized SnO2solar cells having sufficient electron diffusion

lengthrdquo Electrochemistry Communications vol 9 no 7 pp1439ndash1443 2007

[25] C Bauer G Boschloo E Mukhtar and A Hagfeldt ldquoUltra-fast studies of electron injection in Ru dye sensitized SnO

2

nanocrystalline thin filmrdquo International Journal of Photoenergyvol 4 no 1 pp 17ndash20 2002

[26] J A Rodriguez and M Fernandez-Garcıa ldquoMetal oxidenanoparticlesrdquo in Nanomaterials Inorganic and BioinorganicPerspectives 2007

[27] N-G Park J van de Lagemaat and A J Frank ldquoComparisonof dye-sensitized rutile- and anatase-based TiO

2solar cellsrdquo

Journal of Physical Chemistry B vol 104 no 38 pp 8989ndash89942000

[28] C Longo and M-A de Paoli ldquoDye-sensitized solar cells asuccessful combination of materialsrdquo Journal of the BrazilianChemical Society vol 14 no 6 pp 889ndash901 2003

[29] M Y Song D K Kim K J Ihn S M Jo and D Y KimldquoElectrospun TiO

2electrodes for dye-sensitized solar cellsrdquo

Nanotechnology vol 15 no 12 pp 1861ndash1865 2004[30] M Adachi Y Murata J Takao J Jiu M Sakamoto and F

Wang ldquoHighly efficient dye-sensitized solar cells with a titaniathin-film electrode composed of a network structure of single-crystal-like TiO

2nanowires made by the ldquooriented attachmentrdquo

mechanismrdquo Journal of the American Chemical Society vol 126no 45 pp 14943ndash14949 2004

[31] S-C Yang D-J Yang J Kim et al ldquoHollow TiO2hemi-

spheres obtained by colloidal templating for application in dye-sensitized solar cellsrdquo Advanced Materials vol 20 no 5 pp1059ndash1064 2008

[32] H-J Koo Y J Kim Y H Lee W I Lee K Kim and N-GPark ldquoNano-embossed hollow spherical TiO

2as bifunctional

material for high-efficiency dye-sensitized solar cellsrdquoAdvancedMaterials vol 20 no 1 pp 195ndash199 2008

[33] H Park W-R Kim H-T Jeong J-J Lee H-G Kim andW-YChoi ldquoFabrication of dye-sensitized solar cells by transplantinghighly ordered TiO

2nanotube arraysrdquo Solar EnergyMaterials amp

Solar Cells vol 95 no 1 pp 184ndash189 2011[34] Z-S Wang H Kawauchi T Kashima and H Arakawa

ldquoSignificant influence of TiO2photoelectrode morphology on

the energy conversion efficiency of N719 dye-sensitized solarcellrdquo Coordination Chemistry Reviews vol 248 no 13-14 pp1381ndash1389 2004

[35] J-Y Liao J-W He H Xu D-B Kuang and C-Y Su ldquoEffectof TiO

2morphology on photovoltaic performance of dye-

sensitized solar cells nanoparticles nanofibers hierarchicalspheres and ellipsoid spheresrdquo Journal of Materials Chemistryvol 22 no 16 pp 7910ndash7918 2012

[36] Y Saito S Kambe T Kitamura Y Wada and S YanagidaldquoMorphology control of mesoporous TiO

2nanocrystalline

films for performance of dye-sensitized solar cellsrdquo Solar EnergyMaterials amp Solar Cells vol 83 no 1 pp 1ndash13 2004

[37] L Feng J Jia Y Fang X Zhou and Y Lin ldquoTiO2flowers and

spheres for ionic liquid electrolytes based dye-sensitized solarcellsrdquo Electrochimica Acta vol 87 pp 629ndash636 2013

[38] C-S Chou F-C Chou and J-Y Kang ldquoPreparation of ZnO-coated TiO

2electrodes using dip coating and their applications


in dye-sensitized solar cellsrdquo Powder Technology vol 215ndash218pp 38ndash45 2012

[39] T-Y Cho K-W Ko and S-G Yoon ldquoEfficiency enhancementof flexible dye-sensitized solar cell with sol-gel formed Nb

2O5

blocking layerrdquo Current Applied Physics vol 13 no 7 pp 1391ndash1396 2013

[40] G R R A Kumara K Tennakone V P S Perera A KonnoS Kaneko and M Okuya ldquoSuppression of recombinations ina dye-sensitized photoelectrochemical cell made from a film oftin IV oxide crystallites coated with a thin layer of aluminiumoxiderdquo Journal of Physics D Applied Physics vol 34 no 6 pp868ndash873 2001

[41] T-V Nguyen H-C Lee M Alam Khan and O-B Yang ldquoElec-trodeposition of TiO

2SiO2nanocomposite for dye-sensitized

solar cellrdquo Solar Energy vol 81 no 4 pp 529ndash534 2007[42] E Palomares J N Clifford S A Haque T Lutz and J R

Durrant ldquoControl of charge recombination dynamics in dyesensitized solar cells by the use of conformally deposited metaloxide blocking layersrdquo Journal of theAmericanChemical Societyvol 125 no 2 pp 475ndash482 2003

[43] HChoi CNahm J Kim et al ldquoThe effect of TiCl4-treatedTiO

2

compact layer on the performance of dye-sensitized solar cellrdquoCurrent Applied Physics vol 12 no 3 pp 737ndash741 2012

[44] L Vesce R Riccitelli G Soscia TM Brown A di Carlo andAReale ldquoOptimization of nanostructured titania photoanodes fordye-sensitized solar cells study and experimentation of TiCl

4

treatmentrdquo Journal of Non-Crystalline Solids vol 356 no 37ndash40 pp 1958ndash1961 2010

[45] H Melhem P Simon J Wang et al ldquoDirect photocurrentgeneration from nitrogen doped TiO

2electrodes in solid-state

dye-sensitized solar cells towards optically-active metal oxidesfor photovoltaic applicationsrdquo Solar Energy Materials amp SolarCells vol 117 pp 624ndash631 2013

[46] K Kalyanasundaram and M Gratzel ldquoEfficient dye-sensitizedsolar cells for direct conversion of sunlight to electricityrdquoMaterial Matters pp 1ndash6 2009

[47] L Giribabu and V K Singh ldquoSterically demanded unsymmet-rical zinc phthalocyanines for dye-sensitized solar cellsrdquo Dyesand Pigments vol 98 no 3 pp 518ndash529 2013

[48] P Balraju M Kumar M S Roy and G D Sharma ldquoDye sen-sitized solar cells (DSSCs) based on modified iron phthalocya-nine nanostructured TiO

2electrode and PEDOTPSS counter

electroderdquo Synthetic Metals vol 159 no 13 pp 1325ndash1331 2009[49] H Huang Z Cao X Li et al ldquoSynthesis and photovoltaic

properties of two new unsymmetrical zinc-phthalocyaninedyesrdquo Synthetic Metals vol 162 no 24 pp 2316ndash2321 2012


[51] K D Seo H M Song M J Lee et al ldquoCoumarin dyes contain-ing low-band-gap chromophores for dye-sensitised solar cellsrdquoDyes and Pigments vol 90 no 3 pp 304ndash310 2011

[52] K Hara Y Tachibana Y Ohga et al ldquoDye-sensitized nanocrys-talline TiO

2solar cells based on novel coumarin dyesrdquo Solar

Energy Materials amp Solar Cells vol 77 no 1 pp 89ndash103 2003[53] K R J Thomas P Singh A Baheti Y-C Hsu K-C Ho and

J T Lin ldquoElectro-optical properties of new anthracene basedorganic dyes for dye-sensitized solar cellsrdquo Dyes and Pigmentsvol 91 no 1 pp 33ndash43 2011

[54] J AMikroyannidis D V Tsagkournos S S Sharma A KumarY K Vijay and G D Sharma ldquoEfficient bulk heterojunction

solar cells based on low band gap bisazo dyes containinganthracene andor pyrrole unitsrdquo Solar EnergyMaterialsamp SolarCells vol 94 no 12 pp 2318ndash2327 2010

[55] N Xiang W Zhou S Jiang et al ldquoSynthesis and charac-terization of trivalent metal porphyrin with NCS ligand forapplication in dye-sensitized solar cellsrdquo Solar Energy Materialsamp Solar Cells vol 95 no 4 pp 1174ndash1181 2011

[56] M K Panda K Ladomenou and A G Coutsolelos ldquoPor-phyrins in bio-inspired transformations light-harvesting tosolar cellrdquo Coordination Chemistry Reviews vol 256 no 21-22pp 2601ndash2627 2012

[57] W Zhou B Zhao P Shen et al ldquoMulti-alkylthienyl appendedporphyrins for efficient dye-sensitized solar cellsrdquo Dyes andPigments vol 91 no 3 pp 404ndash412 2011

[58] A S Polo M K Itokazu and N Y M Iha ldquoMetal complexsensitizers in dye-sensitized solar cellsrdquoCoordination ChemistryReviews vol 248 no 13-14 pp 1343ndash1361 2004

[59] Y Takahashi H Arakawa H Sugihara et al ldquoHighlyefficient polypyridyl-ruthenium(II) photosensitizerswith chelating oxygen donor ligands 120573-diketonato-bis(dicarboxybipyridine)rutheniumrdquo Inorganica ChimicaActa vol 310 no 2 pp 169ndash174 2000

[60] M Yanagida A Islam Y Tachibana et al ldquoDye-sensitizedsolar cells based on nanocrystalline TiO

2sensitized with a

novel pyridylquinoline ruthenium(II) complexrdquoNew Journal ofChemistry vol 26 no 8 pp 963ndash965 2002

[61] M K Nazeeruddin A Kay I Rodicio et al ldquoConver-sion of light to electricity by cis-X2bis(221015840-bipyridyl-441015840-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X =Cl- Br- I- CN- and SCN-) on nanocrystalline TiO

2elec-

trodesrdquo Journal of the American Chemical Society vol 115 no14 pp 6382ndash6390 1993

[62] T Horiuchi H Miura and S Uchida ldquoHighly efficient metal-free organic dyes for dye-sensitized solar cellsrdquo Journal ofPhotochemistry and Photobiology A Chemistry vol 164 no 1ndash3pp 29ndash32 2004

[63] D Kuang S Uchida R Humphry-Baker S M Zakeeruddinand M Gratzel ldquoOrganic dye-sensitized ionic liquid basedsolar cells remarkable enhancement in performance throughmolecular design of indoline sensitizersrdquo Angewandte ChemieInternational Edition vol 47 no 10 pp 1923ndash1927 2008

[64] W H Howie F Claeyssens HMiura and LM Peter ldquoCharac-terization of solid-state dye-sensitized solar cells utilizing highabsorption coefficient metal-free organic dyesrdquo Journal of theAmerican Chemical Society vol 130 no 4 pp 1367ndash1375 2008

[65] T Horiuchi H Miura K Sumioka and S Uchida ldquoHighefficiency of dye-sensitized solar cells based on metal-freeindoline dyesrdquo Journal of the American Chemical Society vol126 no 39 pp 12218ndash12219 2004

[66] A Konno G R A Kumara S Kaneko B Onwona-Agyemanand K Tennakone ldquoSolid-state solar cells sensitized withindoline dyerdquoChemistry Letters vol 36 no 6 pp 716ndash717 2007

[67] G Calogero G di Marco S Cazzanti et al ldquoEfficient dye-sensitized solar cells using red turnip and purple wild Sicilianprickly pear fruitsrdquo International Journal of Molecular Sciencesvol 11 no 1 pp 254ndash267 2010

[68] W H Lai Y H Su L G Teoh and M H Hon ldquoCommercialand natural dyes as photosensitizers for a water-based dye-sensitized solar cell loaded with gold nanoparticlesrdquo Journal ofPhotochemistry and Photobiology A Chemistry vol 195 no 2-3pp 307ndash313 2008


[69] G R A Kumara S Kaneko M Okuya B Onwona-AgyemanA Konno and K Tennakone ldquoShiso leaf pigments for dye-sensitized solid-state solar cellrdquo Solar Energy Materials amp SolarCells vol 90 no 9 pp 1220ndash1226 2006

[70] H Zhou L Wu Y Gao and T Ma ldquoDye-sensitized solar cellsusing 20 natural dyes as sensitizersrdquo Journal of Photochemistryand Photobiology A Chemistry vol 219 no 2-3 pp 188ndash1942011

[71] W Kubo T Kitamura K Hanabusa Y Wada and S YanagidaldquoQuasi-solid-state dye-sensitized solar cells using room temper-aturemolten salts and a lowmolecular weight gelatorrdquoChemicalCommunications no 4 pp 374ndash375 2002

[72] A Abate and A Petrozza ldquoA polyfluoroalkyl imidazolium ionicliquid as iodide ion source in dye sensitized solar cellsrdquoOrganicElectronics vol 13 no 11 pp 2474ndash2478

[73] L Guo X Pan C Zhang et al ldquoIonic liquid electrolyte based onS-propyltetrahydrothiophenium iodide for dye-sensitized solarcellsrdquo Solar Energy vol 84 no 3 pp 373ndash378 2010

[74] H-R Jhong D S-H Wong C-C Wan Y-Y Wang and T-C Wei ldquoA novel deep eutectic solvent-based ionic liquid usedas electrolyte for dye-sensitized solar cellsrdquo ElectrochemistryCommunications vol 11 no 1 pp 209ndash211 2009

[75] M Gratzel ldquoRecent advances in sensitized mesoscopic solarcellsrdquo Accounts of Chemical Research vol 42 no 11 pp 1788ndash1798 2009

[76] MMao J-BWang Z-F Xiao S-Y Dai andQ-H Song ldquoNew26-modified BODIPY sensitizers for dye-sensitized solar cellsrdquoDyes and Pigments vol 94 no 2 pp 224ndash232 2012

[77] AMishraM K R Fischer and P Buuerle ldquoMetal-Free organicdyes for dye-sensitized solar cells from structure propertyrelationships to design rulesrdquoAngewandte Chemie InternationalEdition vol 48 no 14 pp 2474ndash2499 2009

[78] Q Dai and J Rabani ldquoUnusually efficient photosensitizationof nanocrystalline TiO

2films by pomegranate pigments in

aqueous mediumrdquo New Journal of Chemistry vol 26 no 4 pp421ndash426 2002

[79] M R Narayan ldquoReview dye sensitized solar cells based onnatural photosensitizersrdquo Renewable and Sustainable EnergyReviews vol 16 no 1 pp 208ndash215 2012

[80] H Kusama and H Arakawa ldquoInfluence of pyrazole derivativesin IminusI

3

minus redox electrolyte solution on Ru(II)-dye-sensitizedTiO2solar cell performancerdquo Solar Energy Materials amp Solar

Cells vol 85 no 3 pp 333ndash344 2005[81] A F Nogueira C Longo and M-A de Paoli ldquoPolymers in dye

sensitized solar cells overview and perspectivesrdquo CoordinationChemistry Reviews vol 248 no 13-14 pp 1455ndash1468 2004

[82] Z-SWang K Sayama and H Sugihara ldquoEfficient eosin Y dye-sensitized solar cell containing BrminusBr

3

minus electrolyterdquo Journal ofPhysical Chemistry B vol 109 no 47 pp 22449ndash22455 2005

[83] B V Bergeron A Marton G Oskam and G J Meyer ldquoDye-sensitized SnO

2electrodes with iodide and pseudohalide redox

mediatorsrdquo Journal of Physical Chemistry B vol 109 no 2 pp937ndash943 2005

[84] G Oskam B V Bergeron G J Meyer and P C SearsonldquoPseudohalogens for dye-sensitized TiO

2photoelectrochemical

cellsrdquo Journal of Physical Chemistry B vol 105 no 29 pp 6867ndash6873 2001

[85] S A Sapp C M Elliott C Contado S Caramori and C ABignozzi ldquoSubstituted polypyridine complexes of cobalt(IIIII)as efficient electron-transfer mediators in dye-sensitized solarcellsrdquo Journal of the American Chemical Society vol 124 no 37pp 11215ndash11222 2002

[86] J Wu Z Lan S Hao et al ldquoProgress on the electrolytes for dye-sensitized solar cellsrdquo Pure and Applied Chemistry vol 80 no11 pp 2241ndash2258 2008

[87] B Gerrit andH Anders ldquoCharacteristics of the iodidetriiodideredoxmediator in dye-sensitized solar cellsrdquoAccounts of Chem-ical Research vol 42 no 11 pp 1819ndash1826 2009

[88] A Fukui R Komiya R Yamanaka A Islam and L Han ldquoEffectof a redox electrolyte in mixed solvents on the photovoltaic per-formance of a dye-sensitized solar cellrdquo Solar Energy Materialsamp Solar Cells vol 90 no 5 pp 649ndash658 2006

[89] Z Kebede and S-E Lindquist ldquoDonor-acceptor interactionbetween non-aqueous solvents and I

2to generate Iminus

3 and its

implication in dye sensitized solar cellsrdquo Solar Energy Materialsamp Solar Cells vol 57 no 3 pp 259ndash275 1999

[90] J Wu Z Lan J Lin M Huang and P Li ldquoEffect of solventsin liquid electrolyte on the photovoltaic performance of dye-sensitized solar cellsrdquo Journal of Power Sources vol 173 no 1pp 585ndash591 2007

[91] T Stergiopoulos E Rozi C-S Karagianni and P FalarasldquoInfluence of electrolyte co-additives on the performance ofdye-sensitized solar cellsrdquo Nanoscale Research Letters vol 6article 307 2011

[92] S M Zakeeruddin and M Gratzel ldquoSolvent-free ionic liquidelectrolytes formesoscopic dye-sensitized solar cellsrdquoAdvancedFunctional Materials vol 19 no 14 pp 2187ndash2202 2009

[93] P Wang S M Zakeeruddin J-E Moser and M Gratzel ldquoAnew ionic liquid electrolyte enhances the conversion efficiencyof dye-sensitized solar cellsrdquo Journal of Physical Chemistry Bvol 107 no 48 pp 13280ndash13285 2003


[95] U Bach D Lupo P Comte et al ldquoSolid-state dye-sensitizedmesoporous TiO

2solar cells with high photon-to-electron

conversion efficienciesrdquoNature vol 395 no 6702 pp 583ndash5851998

[96] A Fukui R Komiya R Yamanaka A Islam and L Han ldquoTheinfluence of acid treatment of TiO

2porous film electrode on

photoelectric performance of dye-sensitized solar cellrdquo SolarEnergyMaterials amp Solar Cells vol 90 no 5 pp 649ndash658 2006

[97] K Tennakone V P S Perera I R M Kottegoda and G R RA Kumara ldquoDye-sensitized solid state photovoltaic cell basedon composite zinc oxidetin (IV) oxide filmsrdquo Journal of PhysicsD Applied Physics vol 32 no 4 pp 374ndash379 1999

[98] Q-B Meng K Takahashi X-T Zhang et al ldquoFabrication of anefficient solid-state dye-sensitized solar cellrdquo Langmuir vol 19no 9 pp 3572ndash3574 2003

[99] B OrsquoRegan ldquoLarge enhancement in photocurrent efficiencycaused by UV illumination of the dye-sensitized heterojunc-tion TiO

2RuLLlsquoNCSCuSCN initiation and potential mecha-

nismsrdquoChemistry ofMaterials vol 10 no 6 pp 1501ndash1509 1998[100] S Tan J Zhai M Wan et al ldquoInfluence of small molecules in

conducting polyaniline on the photovoltaic properties of solid-state dye-sensitized solar cellsrdquo Journal of Physical Chemistry Bvol 108 no 48 pp 18693ndash18697 2004

[101] N Mohmeyer P Wang H-W Schmidt S M ZakeeruddinandMGratzel ldquoQuasi-solid-state dye sensitized solar cells with1324-di-O-benzylidene-D-sorbitol derivatives as low molecu-lar weight organic gelatorsrdquo Journal of Materials Chemistry vol14 no 12 pp 1905ndash1909 2004


[102] J Wu Z Lan J Lin et al ldquoA novel thermosetting gel electrolytefor stable quasi-solid-state dye-sensitized solar cellsrdquo AdvancedMaterials vol 19 no 22 pp 4006ndash4011 2007


[104] Z Lan J Wu D Wang S Hao J Lin and Y Huang ldquoQuasi-solid state dye-sensitized solar cells based on gel polymerelectrolyte with poly(acrylonitrile-co-styrene)NaI+I

2rdquo Solar

Energy vol 80 no 11 pp 1483ndash1488 2006[105] J Wu S Hao Z Lan et al ldquoA thermoplastic gel electrolyte

for stable quasi-solid-state dye-sensitized solar cellsrdquo AdvancedFunctional Materials vol 17 no 15 pp 2645ndash2652 2007

[106] R Komiya L Han R Yamanaka A Islam and T MitateldquoHighly efficient quasi-solid state dye-sensitized solar cell withion conducting polymer electrolyterdquo Journal of Photochemistryand Photobiology A Chemistry vol 164 no 1ndash3 pp 123ndash1272004

[107] P Wang S M Zakeeruddin J E Moser M K NazeeruddinT Sekiguchi and M Gratzel ldquoA stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizerand polymer gel electrolyterdquo Nature Materials vol 2 no 6 pp402ndash407 2003

[108] J Y Song Y Y Wang and C C Wan ldquoReview of gel-typepolymer electrolytes for lithium-ion batteriesrdquo Journal of PowerSources vol 77 no 2 pp 183ndash197 1999

[109] N Papageorgiou W F Maier and M Gratzel ldquoAn iodinetri-iodide reduction electrocatalyst for aqueous and organic med-iardquo Journal of the Electrochemical Society vol 144 no 3 pp 876ndash884 1997

[110] G Tsekouras A J Mozer and G G Wallace ldquoEnhancedperformance of dye sensitized solar cells utilizing platinumelec-trodeposit counter electrodesrdquo Journal of the ElectrochemicalSociety vol 155 no 7 pp K124ndashK128 2008

[111] X Fang T Ma G Guan M Akiyama and E Abe ldquoPerfor-mances characteristics of dye-sensitized solar cells based oncounter electrodes with Pt films of different thicknessrdquo Journalof Photochemistry and Photobiology A Chemistry vol 164 no1ndash3 pp 179ndash182 2004

[112] G Khelashvili S Behrens C Weidenthaler et al ldquoCatalyticplatinum layers for dye solar cells a comparative studyrdquo ThinSolid Films vol 511-512 pp 342ndash348 2006

[113] G Syrrokostas ldquoDegradation mechanisms of Pt counter elec-trodes for dye sensitized solar cellsrdquo Solar Energy Materials ampSolar Cells vol 103 pp 119ndash127 2012

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



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CompositesJournal of

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Biomaterials


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TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


Mediator

Conducting glass

Dye Electrolyte Cathode

0

05

10

Red Ox

Max voltage

Injectioneminus

eminus eminus

h

minus05

Eve

rsus

NH

E (V

)TiO2

Diffusion

S∘S+

SlowastS+

Figure 1 Structure and working principle of DSSC with 119878∘119878+119878lowastrepresenting sensitizer in the ground state oxidized state andexcited state respectively RedOx = redox mediator [11]

of fabrication and light weight product [9] In the near futurethe venture of DSSCs as a competitive technology will thus berevealed The latest efficiency of DSSC is more than 11 [10]

Structure andMechanism ofDSSCThe structure andworkingprinciple of DSSC is shown in Figure 1 A charge separationprocess in a DSSC consists of the following steps [1]

(i) Photoanode absorbs incident solar energy Uponabsorption of photo energy the electrons in the dyebecome exited from ground state to the excited state

(ii) Due to the difference in energy levels of the electronicstates electrons from the exited state are injected tothe conduction band of the semiconductor As a resultthe dye becomes oxidized

(iii) The electrolyte which is in contact with the dye thendonates electrons to the dye restoring it to the initialstate

(iv) Electrolyte then diffuses towards the catalytic elec-trode where the reduction reaction takes place andelectrolyte restores its initial state by accepting elec-trons from the external circuit

(v) In addition to these forward charge transfer processesbackward charge transfer processes also occur inone complete cycle These backward electron transferprocesses drastically reduce the efficiency of DSSCsThese include the following

(a) transfer of electrons from the semiconductor tothe oxidized dye

(b) recombination of injected electrons with theelectrolyte (dark current)

(c) transfer of electrons from the dye in its excitedstate to the dye in the ground state

The following should be fulfilled to reduce the effects ofthe backward transfer processes

(i) Charge transfer to semiconductor must occur with aquantum yield [12]

(ii) The lowest unoccupied molecular orbital (LUMO)of the photosensitizer should be more negative thanthe conduction band of the semiconductor and thehighest occupied molecular orbital (HOMO) shouldbe more positive than the redox potential of theelectrolyte [13]

(iii) Electron injection rate to the semiconductor shouldbe higher than the rate of decay of electrons from theexcited state of the dye to its ground state [14]

2 Components of Dye-Sensitized Solar Cell

21 Transparent Conductive Substrate DSSCs are typicallyconstructed with two sheets of conductive transparent mate-rials which provide a substrate for the deposition of thesemiconductor and catalyst acting also as current collectorsSubstrates must be highly transparent (transparency gt 80)to allow the passage of maximum sunlight to the active areaof the cellThe electrical conductivity of the substrates shouldalso be high for efficient charge transfer and to minimizeenergy loss These two characteristics of substrate dictate theefficiency of DSSCs [3]

Typically FTO (fluorine tin oxide SnO2F) and ITO

(indium tin oxide In2O3Sn) are used as the conductive

substrate FTO and ITO substrates consist of soda lime glasscoated with fluorine tin oxide and indium tin oxide layersrespectively ITO films have a transmittance of over 80and sheet resistance 18Ωcm2 while FTO films exhibit atransmittance of about 75 in the visible region and sheetresistance of 85cm2 Sima et al conducted a study tocompare FTO and ITObasedDSSCs [15] Photoanodes basedon FTO and ITO glass substrates were sintered at 450∘Cfor 2 h in an oxygen atmosphere They found that the sheetresistance of FTO remained constant while that of ITOincreased from 18Ωcm2 to 52Ωcm2 upon sintering Theoverall 120578 of a DSSC based on FTO is 94 while for anidentical cell based on ITO it is 24 Thus FTO is highlyrecommended for DSSCs due to its low and temperature-stable sheet resistance

Polymers can also be used as an alternative to glasssubstrates because of their flexibility and low cost Murakamiet al used PET (polyethylene terephthalate) coated withITO and found an efficiency of 38 [16] Polyethylenenaphthalate (PEN) coated with ITO was used by Ito et al andreported an efficiency of 78 [17] However limitations inthe range of usability temperature restrict the use of polymersas substrates in DSSCs [18] Metals such as stainless steeltungsten and titanium that can form a conducting layer havealso been used as substrates Although Jun et al demonstratedan efficiency of 61 using a stainless steel as the substrate




2)


2










2spheres has an




2O5) [39] Al

2O3[40] and SiO

2[41]


2with TiCl

4also














2and L


2)2(NCS)

2] which is also





Wavelength (nm)

IPCE

()

80

60

40

20

0

400 600 800 1000



TiO2

RuL998400(NCS)3

RuL2(NCS)2
















1

N

NS

O

S

CO2H

614941

06067087

1789777

577461

[62][63][64]

OEILSS

2

N

N

S

O

NO S

CO2H

C2H5

906442

065071055

20125141

6947254

[65][63][66]

OEIESS











3[82] SCNminus(SCN)

2 SeCNminus(SeCN)

2[83


3




3) [87]


3



3





NN

N

N

Ru

N

N

CS

CS

N C S

NN

N

N

Ru

N

NC

S

CS

N C S

N719

N

N

N

Ru

N

N

CS

CS

N C S

Black dye

CO2H

N3

CO2H

HO2C

HO2C

HO2C

HO2C

120578 = 1103 120578 = 1118

CO2minusTBA+

CO2minusTBA+

+TBAminusO2C

+TBAminusO2C

+TBAminusO2C


Do Acc


120587-bridge TiO2

eminus

eminuseminush





2

minus B(CN)4

minus (CF3COO)

2Nminus BF

4

minus PF6



2) [3]



OH

HO

OH


H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)



























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






2)


2










2spheres has an




2O5) [39] Al

2O3[40] and SiO

2[41]


2with TiCl

4also














2and L


2)2(NCS)

2] which is also





Wavelength (nm)

IPCE

()

80

60

40

20

0

400 600 800 1000



TiO2

RuL998400(NCS)3

RuL2(NCS)2
















1

N

NS

O

S

CO2H

614941

06067087

1789777

577461

[62][63][64]

OEILSS

2

N

N

S

O

NO S

CO2H

C2H5

906442

065071055

20125141

6947254

[65][63][66]

OEIESS











3[82] SCNminus(SCN)

2 SeCNminus(SeCN)

2[83


3




3) [87]


3



3





NN

N

N

Ru

N

N

CS

CS

N C S

NN

N

N

Ru

N

NC

S

CS

N C S

N719

N

N

N

Ru

N

N

CS

CS

N C S

Black dye

CO2H

N3

CO2H

HO2C

HO2C

HO2C

HO2C

120578 = 1103 120578 = 1118

CO2minusTBA+

CO2minusTBA+

+TBAminusO2C

+TBAminusO2C

+TBAminusO2C


Do Acc


120587-bridge TiO2

eminus

eminuseminush





2

minus B(CN)4

minus (CF3COO)

2Nminus BF

4

minus PF6



2) [3]



OH

HO

OH


H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)



























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






Wavelength (nm)

IPCE

()

80

60

40

20

0

400 600 800 1000



TiO2

RuL998400(NCS)3

RuL2(NCS)2
















1

N

NS

O

S

CO2H

614941

06067087

1789777

577461

[62][63][64]

OEILSS

2

N

N

S

O

NO S

CO2H

C2H5

906442

065071055

20125141

6947254

[65][63][66]

OEIESS











3[82] SCNminus(SCN)

2 SeCNminus(SeCN)

2[83


3




3) [87]


3



3





NN

N

N

Ru

N

N

CS

CS

N C S

NN

N

N

Ru

N

NC

S

CS

N C S

N719

N

N

N

Ru

N

N

CS

CS

N C S

Black dye

CO2H

N3

CO2H

HO2C

HO2C

HO2C

HO2C

120578 = 1103 120578 = 1118

CO2minusTBA+

CO2minusTBA+

+TBAminusO2C

+TBAminusO2C

+TBAminusO2C


Do Acc


120587-bridge TiO2

eminus

eminuseminush





2

minus B(CN)4

minus (CF3COO)

2Nminus BF

4

minus PF6



2) [3]



OH

HO

OH


H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)



























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






1

N

NS

O

S

CO2H

614941

06067087

1789777

577461

[62][63][64]

OEILSS

2

N

N

S

O

NO S

CO2H

C2H5

906442

065071055

20125141

6947254

[65][63][66]

OEIESS











3[82] SCNminus(SCN)

2 SeCNminus(SeCN)

2[83


3




3) [87]


3



3





NN

N

N

Ru

N

N

CS

CS

N C S

NN

N

N

Ru

N

NC

S

CS

N C S

N719

N

N

N

Ru

N

N

CS

CS

N C S

Black dye

CO2H

N3

CO2H

HO2C

HO2C

HO2C

HO2C

120578 = 1103 120578 = 1118

CO2minusTBA+

CO2minusTBA+

+TBAminusO2C

+TBAminusO2C

+TBAminusO2C


Do Acc


120587-bridge TiO2

eminus

eminuseminush





2

minus B(CN)4

minus (CF3COO)

2Nminus BF

4

minus PF6



2) [3]



OH

HO

OH


H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)



























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






NN

N

N

Ru

N

N

CS

CS

N C S

NN

N

N

Ru

N

NC

S

CS

N C S

N719

N

N

N

Ru

N

N

CS

CS

N C S

Black dye

CO2H

N3

CO2H

HO2C

HO2C

HO2C

HO2C

120578 = 1103 120578 = 1118

CO2minusTBA+

CO2minusTBA+

+TBAminusO2C

+TBAminusO2C

+TBAminusO2C


Do Acc


120587-bridge TiO2

eminus

eminuseminush





2

minus B(CN)4

minus (CF3COO)

2Nminus BF

4

minus PF6



2) [3]



OH

HO

OH


H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)



























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




OH

HO

OH


H HOH H

HOH OH

OH

R1

R1

R2R2

O+

OCH3

OCH3 OCH3

CH3

(a)

OH

HO

OH

OH

O

OH

HO

OH

OH

OH

Acidic Basic

R1R1

R2R2

O+

H+

OHminus

(b)

O

OH

HO

OH

O

O

R1

Ti+4

(c)



























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials























Acknowledgments


References






















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



















2films
















2




2solar cellsrdquo












2as bifunctional











2nanocrystalline









2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






2O5








2



4

























2sensitized with a



2elec-
























3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials


















3





3













2to generate Iminus

3 and its
























2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







2rdquo Solar


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



recent advances in dye sensitized solar cells

Documents