Review Article Recent Progress in Dye-Sensitized Solar ...downloads.hindawi.com/journals/jnm/2015/247689.pdf · Dye-sensitized solar cells (DSSCs) are one potential alternative to

Review ArticleRecent Progress in Dye-Sensitized Solar Cells for ImprovingEfficiency TiO2 Nanotube Arrays in Active Layer

Won-Yeop Rho1 Hojin Jeon2 Ho-Sub Kim3 Woo-Jae Chung4

Jung Sang Suh3 and Bong-Hyun Jun1

1Department of Bioscience and Biotechnology Konkuk University Seoul 143-701 Republic of Korea2Department of Chemical and Biomolecular Engineering Rice University Houston TX 77005 USA3Department of Chemistry Seoul National University Seoul 151-747 Republic of Korea4Department of Genetic Engineering Sungkyunkwan University Suwon Gyeonggi-do 440-746 Republic of Korea

Correspondence should be addressed to Bong-Hyun Jun bjunkonkukackr

Received 31 December 2014 Revised 21 February 2015 Accepted 1 March 2015

Academic Editor Nageh K Allam

Copyright copy 2015 Won-Yeop Rho et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Dye-sensitized solar cells (DSSCs) have been widely studied due to several advantages such as low cost-to-performance ratio lowcost of fabrication functionality at wide angles and low intensities of incident light mechanical robustness and low weight Thispaper summarizes the recent progress in DSSC technology for improving efficiency focusing on the active layer in the photoanodewith a part of the DSSC consisting of dyes and a TiO

2film layer In particular this review highlights a huge pool of studies that

report improvements in the efficiency of DSSCs using TiO2nanotubes which exhibit better electron transport Finally this paper

suggests opportunities for future research

1 Introduction

For nearly two centuries mankind has employed fossil fuelsas the primary energy source and now we are facing seriousproblems as a result Excessive emission of carbon dioxideand other greenhouse gases leads to environmental risk [1]The price of fossil fuels continues to rise because of theincreasing worldwide energy consumption The need for anaffordable sustainable and carbon-free renewable energysource is greater than ever as the rapid growth rate ofrenewable energy consumption suggests [2 3]

Amongst all the renewable energy sources solar energyhas been regarded as very promising due to the abundance ofits resourcemdashsunlightmdashand the fact that it yields no harmfulbyproducts The amount of solar energy that radiates uponearth in one hour is equivalent to the annual energy need ofmankind [4] Such high energy output however has not beenachieved due to thermodynamic and technical constraintsas well as few other reasons Conventional commercialsolar cells which use crystalline and polycrystalline siliconachieve over 20 energy conversion efficiency for residential

end users [5] However there are some problems such ascomplicated and difficult fabrication processing and high cost[6]

Dye-sensitized solar cells (DSSCs) are one potentialalternative to silicon solar cells DSSCs separate the lightabsorption and charge transfer processes unlike Si-basedcells The organic dyes or sensitizer molecules adsorbed onthe surface of the metal oxide nanostructure take the role ofabsorbing the incoming light and the rest of the structuretransfers the generated charge An early version of a DSSCfabricated by OrsquoRegan and Gratzel used ruthenium-baseddye and 10-120583m-thick porous TiO

2nanoparticle films for the

photoelectrode [7] The cell yielded 7 power conversionefficiency (PCE) which was high enough to motivate furtherresearch on cells with similar structures DSSCs provideseveral advantages such as low price-to-performance ratiolow processing cost ability to work at wide angles andlow intensities of incident light mechanical robustnesslight weight and aesthetically appealing transparent designDSSCs have a number of remarkable properties that allowthem to be used for several niche applications they are low in

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 247689 17 pageshttpdxdoiorg1011552015247689

2 Journal of Nanomaterials

weight and able to display various colors and transparencyand they work effectively in a broad range of wavelengths[8 9] However there are still several weaknesses that requireimprovement such as relatively low efficiency high costof the ruthenium dyes conducting glass and platinumtemperature stability problems and low scalability Amongthe various potential solutions some of the most widelyinvestigated ones are the development of new sensitizersto cover a broader solar spectrum and the fabrication ofnew photoanode structures with enhanced light utilizationStudies have reported several different methods to improvethe efficiency of DSSCs such as the use of panchromaticsensitizers or 3-dimensional metal oxide nanostructures [10ndash14] This review highlights another huge pool of studies thatreport improvements in the efficiency of DSSCs especiallythose that employ the use of TiO

2nanotubes Moreover we

will introduce recent papers on high PCEs and the use ofTiO2nanotube arrays in DSSCs These papers describe the

use of the space in TiO2nanotube array channels and how to

overcome the barrier layer that exists at the bottom of thesearrays to achieve a high PCE in DSSCs

2 Principle of DSSCs

A DSSC consists of a photoelectrode counter electrode andelectrolyte The photoelectrode is made up of a transparentconductive oxide (TCO) glass substrate a metal oxide suchas mesoporous TiO

2nanoparticles and a sensitizer such as

ruthenium- (Ru-) complex dye The electrolyte consists ofa redox couple and diffusing reducingoxidizing agent ionsand the counter electrode is coated with Pt as shown inFigure 1(a) In general fluorine-doped tin oxide (FTO) glassis used on DSSCs instead of indium tin oxide (ITO) glassbecause of the better thermal stability at high temperatures[15] During the sintering at 450ndash500∘C which crystallizesthe mesoporous TiO

2nanoparticles the sheet resistance of

ITO increases and affects the energy conversion efficiencyof the DSSCs However the sheet resistance of FTO isindependent of the temperature up to 500∘C which makesit favorable for applications that utilize the sintering stepTiO2nanoparticles are primarily used as electron acceptors

on DSSCs that incorporate ruthenium dye The counterelectrode consists of FTO glass coated with platinum (Pt) orcarbon to resist the oxidation and to work as a catalyst duringthe redox reaction in the electrolyte

The principle of DSSCs is shown in Figure 1(b) Whensensitizers absorb the light energy and electrons are gen-erated by the sensitizers and are transferred to the metaloxide films During the hopping process in the metal oxidefilms the generated electrons are transferred to the TCO andcirculated After the circulation the sensitizers are reducedby an iodide redox process In general a Ru-complex dye isused as the sensitizer and TiO

2nanoparticles are used as the

electron acceptors on DSSCs due to their high efficiency [20ndash22]

The energy diagram shown in Figure 1(c) demonstratesthe electron transfer process of DSSCs When the Ru-complex dyes absorb the light the electrons change from

the ground state to the excited state within a time on the orderof nanoseconds (ns) as shown in the reaction (Figure 1(c)-(1))

S0 + ℎ] 997888rarr Slowast (1)

Although the detailed mechanism of the injection process isunder debate the electron injection from theRu-complex dyeinto the conduction band of mesoporous TiO

2nanoparticles

takes about a femtosecond (fs) following the reaction shownin Figure 1(c)-(2) The energy level of electrons in the Ru-complex dye is higher than that of the conduction band ofmesoporous TiO

2nanoparticles which enables the transfer

of electrons into the mesoporous TiO2nanoparticles

Slowast 997888rarr S+ + eminus

S+ + eminus 997888rarr S0(2)

The excited state of the Ru-complex dye lasts around 20ndash60 ns which is long enough for kinetic competition betweenelectron injection and excited state decay in the Ru-complexdye

The electrons injected into TiO2nanoparticles diffuse

through mesoporous TiO2nanoparticles in about a millisec-

ond (ms) in the reaction shown in Figure 1(c)-(3) Howeverwhen the injected electrons are trapped onmesoporous TiO

2

nanoparticles they return to the oxidized Ru-complex dye bythe reaction shown in Figure 1(c)-(4) or reduce the triiodideby the reaction shown in Figure 1(c)-(5) These processes arecalled as the recombination

I3

minus+ 2eminus 997888rarr 3Iminus (Reduction)

3Iminus 997888rarr I3

minus+ 2eminus (Oxidation)

(3)

The Ru-complex dye is reduced by an iodide ion to accept theelectron as shown in the reaction in Figure 1(c)-(6)

2S+ + 3Iminus 997888rarr 2S0 + I3

minus (4)

The injected electrons on TiO2nanoparticles drive the exter-

nal circuit and perform work as in the reaction shown inFigure 1(c)-(7)

When the triiodide ion reduces to iodide to accept theelectrons from the counter electrode by the reaction shown inFigure 1(c)-(8) the iodide ion causes the electron in the Ru-complex dye to return to the ground state with the reactionshown in Figure 1(c)-(6)

Figure 1(d) illustrates the performance of DSSCs and thesolar power (119875) can be calculated by the equation

119875 = 119868119881 (5)

The energy conversion efficiency of DSSCs is described bythe ratio of the maximum electrical power output 119875max tothe solar energy input 119875 The maximum electrical poweroutput 119875max is calculated by the maximum current 119869mp andthe maximum voltage 119881mp

119875max = 119869mp times 119881mp (6)

Journal of Nanomaterials 3

TCO glassPt

TiO2nanoparticles

TCO glass

Electrolyte

(a)

Pt FTO

FTOLight

Load

Sensitizer

eminuseminus

eminus

eminus

eminuseminus

eminus

Iminus

TiO2 nanoparticles

I3minus

(b)

(LUMO)

(HOMO)

(2)

(1)

(3)

(4)

(5)

(6)

E v

ersu

s NH

E (V

)

ElectrolyteDyeCounter electrodeFTO

(7) Load

(8)

eminus eminus

IminusI3minus

TiO2PtFT

O

FTO

S0S

+

Slowastminus05

0

+05

+10

(c)

Power density

Voltage (V)

Maximumpower point

Curr

ent d

ensit

y (m

Ac

m2 ) Jsc

Jmp

Vmp Voc

(d)

Figure 1 (a) Structure of DSSCs (b) principle of DSSCs (c) energy diagram of DSSCs and (d) performance of DSSCs

The solar power input is taken as the product of the irradianceof the incident light that is measured in Wm2

120578 =119875max119875

120578 =(119869mp times 119881mp)

119875

(7)

The fill factor 119891119891 is a measure of the quality of the solar celland it can be calculated by comparing the maximum short-circuit current 119869sc and maximum open-circuit voltage 119881oc tothe maximum current 119869mp and maximum voltage 119881mp

119891119891 =(119869mp times 119881mp)

(119869sc times 119881oc)

120578 =119875max119875

120578 =(119869mp times 119881mp)

119875

120578 =(119869sc times 119881oc times 119891119891)

119875

(8)

119875 (100mWcm2) is the solar cell testing standard underterrestrial conditions with air mass AM15

The ideal power density of DSSCs is the product of 119869scand 119881oc The maximum power density is the product of 119869mpand 119881mp and the difference between the two comes fromthe resistances of the cell electrolyte electrode and othercomponents The energy conversion efficiency of DSSCs isrelated to the ideal power density and maximum powerdensity as shown in Figure 2(a) When the fill factor is equalto 1 119869sc and 119881oc are the same as 119869mp and 119881mp The series resis-tance 119877

119904includes the cell electrolyte electrode and interface

resistance The increasing 119877119904means that electrons are lost

so the voltage decreases earlier as shown in Figure 2(b)The shunt resistance 119877sh results from the recombination ofelectrons that are transferred in another way or are trappedby something The decreasing 119877sh means that there are fewrecombination sites so the electrons are transferred to eachlayer as shown in Figure 2(c)

3 Dye

Thedyes commonly used for DSSC applications share certaincharacteristics [23] (i) excellent spectral absorption profile


Power density

Voltage (V)

Jsc

Jmp

Vmp Voc

Curr

ent d

ensit

y (m

Ac

m2 )

(a)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )

Rs (series resistance)increasing

(b)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )

Rsh (shunt resistance)decreasing

(c)

Figure 2 Characterization of DSSCs performance (a) Ideal power density of DSSCs (b) series resistance and (c) shunt resistance

like the panchromatic absorption (ii) a surface-anchoringgroup such as a carboxylate or phosphonate group (iii) aLUMO matching the edge of the conduction band of thephotoanode (iv) a HOMO comparable to the electrolytesrsquoFermi level and (v) the stability to endure 108 turnovers (20years of exposure)

Generally dyes fall into two categories metal-complex-based polypyridyl dyes and metal-free organic dyes Whilemetal-complexmdashmostly rutheniummdashdyes show high effi-ciencies due to high absorbance ranges extending to thenear-infrared range the metal-free organic dyes appeal tothe market with their low processing cost and limited Rurequirements

31 Metal-Complex-Based Dyes Metal-complex dye sensitiz-ers generally consist of a central metal ion and a subsidiaryligand typically a bipyridine or tetrapyridine that containsanchoring groups The central metal ion performs the lightabsorbance Subsidiary ligands can be structurally modifiedto fine-tune the photovoltaic properties

Ruthenium complexes as shown in Figure 3 havedemonstrated excellent photovoltaic properties such asbroad absorbance profiles and well-fitted energy levels [24]These factors combined with electrochemical stability makeRu complexes unrivaled for sensitizer uses [10 25 26] One

representative Ru-based dye is the N3 dye whose reportedconversion efficiency is over 10

Another one isN749 dye which is called the black dye dueto its enhanced absorption spectrum It shows an enhancedefficiency as high as 104 Recent research on this familyof dyes includes the work conducted by Robson et al inwhich the NCS ligands of the black dye were replaced withanionic tridentate chelating ligands to adjust the HOMO andLUMO of the dye sensitizers to increase the photocurrentwithout compromising other DSSC properties [27] When atriphenylamine group was substituted for the NCS group anda phenyl-bipyridine-OMe group was used as the chelatingligand an 802 PCE was measured

There have been attempts to use metals other than Ruas the dye sensitizer Such metals include Os [28] Re [29]Fe [30ndash32] Pt [33ndash36] and Cu [37 38] Os(II) complexeshave better absorption spectra compared to Ru complexesHowever Alebbi et al reported a lower energy conversionefficiency attributing it to the slower electron transfer fromthe iodine electrolyte to the osmium complex [39] Pt(II)diimine-dithiolate complexes showed a high IPCE of 47 at500 nm [33] but had a poor light absorption capacity at longerwavelengths The tenability of photophysical properties dueto possible structural modification of the substituents onthe diimine and dithiolate was notable Cu(I) complexeshave very similar photophysical properties to Ru complexes


N

NN

N

O

O

HO

HO

HO

HO

N

N

O

O

RuC S

C S

(a)

N

NN

N

O

O

HO

HO

N

N

O

O

Ru

N+

minusO

N+

minusO

C S

C S

(b)

N

OHO

HO

O

N

Ru

N

N

N

NCS

C

S

(c)

N

N

O

OH

O

O

N

N

N

N

Ru

N+

minusON+

minusO

C S

CS

CS

(d)

Figure 3 Ru-based dyes (a) N3 (b) N719 (c) Z907 and (d) N749

According to Constable et al the 119868-119881 characteristic of thesecomplexes was sensitive to the location of the conjugated pi-bond

Polyphyrins are also of interest due to their good absorp-tion and stability With zinc-polyphyrin dye and a Co(III)tris(bipyridyl)-based redox electrolyte Yella et al reporteda breakthrough efficiency of 123 and attributed this high

efficiency to the suppression of electron backtransfer by thedye structure [40]

32 Metal-Free Organic Dyes N3 and N719 dyes exhibit highperformances with very fast electron injections However themolar extinction coefficients in the visible and near-IR ranges


S H

O

OH

N

S

N

N

O

O

N

O

O

S

N

N

O

O

N

S H

O

OH

N

O

O

DPP-I

DPP-II

Figure 4 DPP-based dyesrsquo molecular structures Reprinted from [42]

are relatively lowMoreover there are concerns about the costeffectiveness of ruthenium and the environmental impacts

Organic dyes also enable easy synthesis and design ofthe dye molecules The predominant structure of metal-freeorganic dyes is the donor-120587-acceptor (D-120587-A) structure Itworks as a basic template to design new dye molecules byadjusting the functional group or the whole moiety of thedonor pi-bridge or acceptor of an existing dye

Zhang et al reported a high power conversion efficiencyof 115ndash128 with cyclopentadithiophene-benzothiadiazolederivatives [41] This research group cografted the dyes hav-ing poor light absorption abilities by which they successfullyremoved the penetration channels for Co(III) redox ions andthereby restricted the charge recombination at the titaniasurface The dyes they used contain triphenylamine and ben-zoic acid moieties on each end of the cyclopentadithiophene-benzothiadiazole group where different orders of the moi-eties generate two different dyes The high PCE of 128 iseven comparable to the Ru-based dyes which proves thatmetal-free organic DSSCs can be as efficient as conventionalDSSCs and sets a new standard for organic DSSCs for peerresearchers

Another approach taken by Qu et al studied the dike-topyrrolopyrrole (DPP) dyes and their photovoltaic charac-teristics as shown in Figure 4 [42] These dyes commonlyhad a triphenylamine donor carboxylic acid acceptor andDPP pi-bridge The introduction of DPP moieties improvedthe spectral absorption at longer wavelengths The PCE was344 relatively high and was attributed by the authors to

high dye aggregation and electron recombination Adding15mMof chenodeoxycholic acid (CDCA) partly resolved theaggregation issue and raised the PCE to 414 with 119869sc =978mAcmminus2 119881oc = 0605V and a fill factor of 07 underAM15 solar light and with an iodine electrolyte

Another approach to DSSC dyes involves extractingthe natural dyes and structurally adjusting them for solarcell applications [49] None of them so far has led to aPCE significantly higher than those of other organic dyesensitizers One reason for their low PCEs is thought to be thelow interaction between the dyes and TiO

2 High adsorption

of dyes onto the TiO2structure is vital for high 119881oc because

electron recombination can occur In fact most of the naturaldye-based DSSCs suffer from low119881oc Low stability of naturaldyes is another issue Therefore improvements either in 119881ocor in stability are essential in order to justify further studiesusing this approach

4 Metal Oxide

Metal oxide nanostructures have also been integrated intoDSSCs tomaximize the number of adsorbed dyes and therebymaximize the photocurrent OrsquoRegan and Gratzel used amesoporous layer of TiO

2to produce a 1000-fold increase

in the surface area [7] TiO2NPs are generally preferred for

DSSC applications due to their high stability The group alsostudied various alternatives such as ZnO SnO

2 or Nb

2O5

[50] whichwere chosen because their energy band structures


are similar to that of TiO2 However these alternatives rarely

yielded PCEs comparable to those of TiO2-based cells [51]

41 TiO2Nanoparticle Films Based DSSCs TiO

2has three

types of crystallinities anatase rutile and brookite A com-parative study between anatase and rutile TiO

2nanoparticle

films was conducted by Park et al [52] This study concludedthat the anatase TiO

2nanoparticles have better electron

transport than do rutile TiO2nanoparticles in DSSCs

While nanoparticle films have shown significantly higherPCEs compared to those of other nanostructures due totheir excessive number of adsorbed dyes they have a cleardisadvantage in charge transport In nanoparticle films thegenerated charge has to pass several nanoparticles beforeit reaches the substrate Since the particles slightly differin their energy states the combined energy band mightcontain several trap states and the charge may be trappedin such states losing the energy and delaying the chargetransfer process [53] Moreover the grain boundary betweennanoparticles is not effective for electron transport

In TiO2nanotubes the electron transport is better than

TiO2nanoparticles due to the absence of a grain boundary

However the small surface area for sensitizers is one of themain obstacles to achieving a high PCE in DSSCs To retainthe advantage in the surface area while improving the chargetransport numerous studies have been conducted to testDSSCs based on TiO

2nanotubes

42 TiO2Nanotube Arrays

421 Titanium Anodization Anodic titanium oxide TiO2

nanotube arrays have properties that make them moreeffective than many other forms of TiO

2for applications in

photocatalysis [54 55] gas sensors [44 56ndash59] photoelec-trolysis [60ndash62] and photovoltaics [63ndash65] Since Zwillingreported on the TiO

2nanotube arrays in chromic acid

and hydrofluoric acid in 1999 [66] TiO2nanotube arrays

have been studied in terms of fabrication characterizationapplication and formation mechanism [44 67]

Gong et al reported onTiO2nanotube arrays in a 05 wt

HF aqueous solution at room temperature using differentanodizing voltages [68] The film thickness could not beincreased above 400ndash500 nm when an HF-based electrolytewas used The fluoride solution dissolves TiO

2by forming

TiF6

2minus anions The acidity of HF electrolytes is too highand results in a rapid dissolution of TiO

2 To overcome this

problem Mor et al reported that the addition of acetic acidto a 05 wt HF electrolyte in a 1 7 ratio resulted in moremechanically robust TiO

2nanotube arrays without changing

their shape or size [57 69] The surface morphology of TiO2

nanotube arrays anodized in an electrolyte containing 25HNO

3and 1 HF at 20V for 4 hours exhibited a uniform

clean and regular TiO2nanotube array structure with a

length of about 400 nm as reported by RuanThe anodizationwith an electrolyte composed of 05M H

3BO3 25 HNO

3

and 1 HF at 20V for 4 h led to irregular pore of TiO2

nanotube arrays but a long length of about 560 nm [70]To increase the length of the TiO

2nanotube arrays Ti was

anodized with KF or NaF in the electrolyte [71] The acidity

of the electrolyte could be controlled by adding HF H2SO4

or Na2SO4to adjust the balance of the dissociation of TiO

2

at the electrolyteoxide interface and the oxidation of TiO2at

the oxidemetal interface [72 73]Grimes et al obtained TiO

2nanotube arrays up to

approximately 1000 120583musing a variety of organic electrolytessuch as dimethyl sulfoxide formamide ethylene glycol andN-methylformamide [74ndash76]

The key to constructing longer TiO2nanotube arrays is

to reduce the water content in the anodization bath to lessthan 5 In organic electrolytes a little water content reducesthe dissociation of the oxide in the fluorine-containingelectrolytes

The chemical reaction of titanium anodization shouldbe the same as that of aluminum anodization [44 74 7778] The key processes are the growth of an anodic TiO

2

when the surface of Ti is oxidized with oxide anion and thedissociation of oxide anions through dissociation and field-assisted dissolution in water as shown in Figure 5 [43] At theelectrolyteoxide interface the reaction is written as

H2O 997888rarr OHminus +H+ or

OHminus 997888rarr O2minus +H+(9)

The cations move to the cathode and undergo the reac-tion

2H+ + 2eminus 997888rarr H2

(10)

The oxide ions OHminus or O2minus migrate through the oxidelayer to form TiO

2

Ti + 119909OHminus 997888rarr Ti (OH)119909+ 119909eminus or

Ti + 2O2minus 997888rarr TiO2+ 4eminus

(11)

where 119909 is yet to be determined Titanium hydroxide decom-poses to formTiO

2at the oxidehydroxide interface Fminus in the

electrolyte moves to the anode and undergoes the reaction

TiO2+ 6Fminus + 4H+ 997888rarr [TiF

6]2minus

+ 2H2O (12)

The overall reaction of TiO2nanotube arrays is

TiO2+ 119899H2O + 6Fminus

997888rarr [TiF6]2minus

+ (119899 + 2 minus 119909)O2minus + 119909OHminus + (2119899 minus 119909)H+

(13)where 119899 was introduced to indicate the ratio of the dissocia-tion rate of water to the dissolution rate of TiO

2

Pores develop from pits on the Ti plate surface and aschematic diagram for the equifield strength model is shownin Figure 6

The initiation and growth of pores are associated withaccelerated dissolution of TiO

2due to the influence of an elec-

tric field [79] Many defects such as impurities dislocationgrain boundaries or nonmetallic inclusions could cause afaster dissolution rate and lead to pit growth [80 81] WhenTi4+ cations escape from the oxide surface because of anapplied field Ti4+cation vacancies can appear and accumulateto form high-density voids in the oxide layer which can aidin the propagation of pits [82 83]


eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)

2H++ 2eminus rarr H2

Ti + 2H2O rarr TiO2 + 4H++ 4eminus TiO2 + 6Fminus

rarr [TiF6]2minus

+ 2O2minus

Figure 5 Growth of regular TiO2nanotube arrays (a) cathodic reaction (b) anodic reaction (c) transition state of TiO

2layer (d) beginning

of nanotube formation and (e) TiO2nanotube arrays Reprinted with permission from [43] Copyright 2012 Elsevier

422 TiO2Nanotube Arrays Based DSSCs Grimes et al

reported backside-illuminated DSSCs based on TiO2nan-

otube array electrodes in 2006 They were based on 6 120583mlong highly ordered nanotube-array films sensitized by a self-assembled monolayer of N719 sensitizer which showed an879mA cmminus2 short-circuit current density an 841mV open-circuit potential and a 057 fill factor yielding a PCE of 424under AM15 sun This investigation was the first attemptto construct a DSSC of TiO

2nanotube arrays However

it used backside illumination Therefore incident sunlighthad to pass a Pt-coated counter electrode and electrolytethat absorbed light near 400 nm To increase the energyconversion efficiency using TiO

2nanotube arrays in DSSCs

the structure should be designed with front illuminationSeveral approaches using TiO

2nanotube arrays in DSSC

designs have incorporated a thin film consisting of TiO2nan-

otube arrays with the photoanode of the DSSCs improvingthe scattering-driven light harvesting and suppressing thecharge recombination Park et al reported a simple methodfor preparing TiO

2nanotube arrays on the FTO glass [16] In

this method a thin film of TiO2was anodized with NH

4F

to form an array of TiO2nanotubes The nanotube array

was then detached from the metallic Ti substrate by theuse of an HCl solution and was annealed onto a sheet ofFTO glass at 500∘C for 30min The photoanode fabricatedwith this method was equipped with a Pt counter electrodeRu(II)LL1015840(NCS)

2dye (L = 22-bipyridyl-44-dicarboxylic

acid L1015840 = 22-bipyridyl-44-ditetrabutylammonium carboxy-late) and an iodine electrolyte to yield 76 power conversionefficiency under 1 sun illumination

Chen and Xu produced a crystalline TiO2nanotube array

thin film [17] The crystalline TiO2nanotube arrays were

usually obtained by annealing the array at a high temperatureand the high-temperature annealing caused the TiO

2nan-

otube arrays to curl The researchers circumvented this prob-lem by introducing an additional step of anodization afterannealing which results in a double-layer structure of TiO

2

nanotube arrays in which the lower layer is later dissolved inanH2O2solutionThe resulting free-standing TiO

2nanotube

arrays did not need any support to maintain their crystalstructure enabling them to be attached to FTO glass and tofabricate a front-illuminated DSSCThe front illumination oftheDSSC enhanced the irradiation intensity by 11 comparedto the conventional back-illuminated method Additionallythe crystalline free-standing TiO

2nanotube arrays provided

a longer electron lifetime which caused the PCE to increasefrom28with a back-illuminatedDSSC equippedwith thickTiO2nanotube arrays on a Ti substrate to 55 with a front-

illuminated crystal-structured TiO2nanotube array-based

DSSCLin et al fabricated open-ended TiO

2nanotube arrays

which differed from the previously mentioned nanotubestructures in that there was no blocking layer of TiO

2at the

junction point between the FTO glass and the TiO2nanotube

arrays [18] The DSSC devices were fabricated with the open-ended TiO

2nanotube arrays interconnected to the FTO glass

by NP-TiO2paste N719 dyes Pt-FTO glass and an iodine-

based electrolyte for front-side illuminationThe open-endednanotube arrays were prepared by the simple introductionof an oxalic acid solution as an etching agent Comparedto the close-ended TiO

2nanotube arrays the open-ended

TiO2nanotube arrays dramatically increased the short-

circuit current resulting in a high PCE of 91 The increasein the short-circuit current was attributed by the authors to


Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)

Figure 6 Schematic diagram of the evolution of a nanotube array at constant anodization voltage (a) oxide layer formation (b) pit formationon the oxide layer (c) growth of the pits into scallop-shaped pores (d)metallic part between the pores undergoing oxidation and field-assisteddissolution and (e) fully developed nanotube array with a corresponding top view Reprinted with permission from [44] Copyright 2006Elsevier

the enhanced interaction between the electrolyte and NP-TiO2underlayer due to the absence of the bottom caps The

open-ended nanotube array showed a strong enhancement incapturing incident photons with longer wavelengths whichalso indicated that opening the tube bottom enabled theDSSC device to collect the excited electrons over the entirelength of the nanotubes

Li et al reported on the fabrication of crystalline TiO2

nanotube arrays using the secondary anodization step ina similar manner to Chen et al [17 19] With these free-standing crystalline TiO

2nanotube arrays they used a

dry-etching method with a plasma reactor under BCl3Cl2

to make the nanotube bottom open-ended and comparedthis array to a close-ended nanotube array fabricated withthe same methodology but without the etching step Theresulting PCEwas higher for the open-ended nanotube arrays

(624) than it was for the close-ended nanotube arrays(484)The authors explained this difference as the result ofthe interface between nanotube arrays and FTO being morecompactly adhered leading to improved electron diffusionand charge collectionThis conclusion is similar to that of Linet al

The open-ended TiO2nanotube arrays generally show

a better photoelectric performance than do the close-endednanotube arrays Rho et al studied the effect of the thicknessof the bottom cap (barrier layer) of the nanotube arraysas shown in Figure 7 [45] The authors fabricated the TiO

2

nanotube arrays with two anodization steps followed by ionmilling with argon ions The thickness of the barrier layerwas controlled by varying the time for the ion-milling stepThe resulting PCEs varied from 25 to 37 As the barrierlayer was removed the PCE increased When the DSSC


100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)

Figure 7 SEM images taken by Rho et al of the freestanding TiO2films after ionmilling of (a) 0 (b) 20 (c) 30 and (d) 90minutes Reprinted

with permission from [45] Copyright 2012 American Chemical Society

was designed with back-side illumination the efficiency waslower thanwhen it was designedwith front-side illuminationIn back-side illumination the existence of the barrier layerdoes not affect the incident light intensity as the light reachesthe barrier layer after it is absorbed by the dye moleculesRather in this case the more decisive factor influencing theconversion efficiency was the efficiency of electron diffusionas the barrier layer may significantly hinder the electrontransport

The energy conversion efficiency of DSSCs based onopen-end TiO

2nanotube arrays was much higher than the

efficiency based on closed-end ones This result means thatthe barrier layer corresponding to the under layer of thedetached TiO

2nanotube arrays has a marked effect on

the conversion efficiency since the barrier layer has beenremoved in open-end TiO

2nanotube arrays but left intact

in closed-end TiO2nanotube arrays The barrier layer could

affect the conversion efficiency by preventing the diffusionof materials like electrolytes and dye molecules by reducingthe transmittance of light and by hindering the electrontransport The electrons generated by excitation of the dyesadsorbed on the TiO

2nanotube arrays could transfer to the

electrode through the TiO2nanotube arrays In this case they

must pass the barrier layer to reach the electrode (DSSCswith free-standing TiO

2nanotube array were summarized in

Table 1)

423 Development of TiO2Nanotube Arrays for DSSC

The various approaches mentioned in the earlier sections

exploited TiO2nanotube arrays to achieve DSSC systems

with higher efficiencies Generally the techniques to increasethe efficiencies of TiO

2nanoparticle systems can be also

applied to TiO2nanotube systems For example combining

plasmonic enhancement andor TiCl4could benefit TiO

2

nanotube arrays which have large spaces in them

(1) Plasmonic TiO2 Nanotube Arrays for DSSCs Plasmonicsolar cells are photovoltaic devices that use noble metalsurface plasmons Most solar cells have weak absorbers Toabsorb larger amounts and longer wavelengths of incidentlight themorphology of the substrate is etchedwith pyramidswith sizes of 2ndash10120583m to obtain a wavelength-scale textureThis structure improves the light trapping for solar cells butincreases the surface recombination and reduces the materialquality [84 85]

There is another approach involving the use of noblemetal nanoparticles in solar cells The light can be absorbedand scattered from the noble metal nanoparticles that areexcited at the surface plasmon resonance

The mechanism of plasmonic solar cells can be used toexplain the photocurrent enhancement by metal nanoparti-cles incorporated into or on solar cells as shown in Figure 8[46] One is scattering from the metal nanoparticles whichcan be used as subwavelength scattering elements to coupleand trap plane waves propagating from the Sun into thesemiconductor thin film by folding the light into a thinabsorber layer as shown in Figure 8(a)

Light is preferentially scattered and trapped in thesemiconductor thin film by multiple high-angle scatterings


Table 1 Summary of DSSCs with free-standing TiO2 nanotube arrays

Year Types of free-standing TiO2 nanotubearrays after being detached from Ti plate 119881oc (V) 119869sc (mAcm2) 119891119891 PCE () Reference

2008 Closed-ended and without crystallinity 0733 168 062 76 [16]2009 Closed-ended and with crystallinity 0701 124 063 55 [17]2010 Open-ended and without crystallinity 0770 185 064 91 [18]2011 Open-ended and with crystallinity 075 1278 065 624 [19]

(a) (b) (c)

Figure 8 Plasmonic light-trapping geometries for thin-film solar cells (a) Light trapping by scattering frommetal nanoparticles at the surfaceof the solar cell (b) Light trapping by the excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor (c)Light trapping by the excitation of surface plasmon polaritons at the metalsemiconductor interface Reprinted with permission from [46]Copyright 2010 Nature Publishing Group

effectively increasing the optical path length in the cell Ininorganic plasmonic solar cells the photocurrent enhance-ment is increased by scattering from metal nanoparticlesAnother approach makes use of the near-field enhancementfrom metal nanoparticles that can be used as subwavelengthantennas in which the plasmonic near-field is coupled tothe semiconductor increasing its effective absorption cross-section as shown in Figure 8(b) In organic plasmonicsolar cells the photocurrent enhancement is increased bynear-field enhancement The direct generation of chargecarriers in the semiconductor substrate has also been stud-ied A corrugated metallic film on the back surface of athin photovoltaic absorber layer can couple sunlight intosurface plasmon polariton (SPP) modes supported at themetalsemiconductor interface as well as guided modes inthe semiconductor slab whereupon the light is converted tophotocarriers in the semiconductor as shown in Figure 8(c)[86ndash88]

Stuart andHall reported an 18-fold photocurrent enhance-ment for light-sensitive devices with a 165-nm-thick silicon-on-insulator photodetector at a wavelength of 800 nm byusing silver nanoparticles on the surface of the device [89]The gold nanoparticles used for scattering and absorption oflight on highly doped wafer-based solar cells were reportedon by Schaadt et al [90] Gold nanoparticles on thin-filmamorphous silicon solar cells gained an 8 overall increasein conversion efficiency as reported by Derkacs et al [91]Silver nanoparticles on 125-120583m-thick silicon-on-insulatorsolar cells and planar wafer-based cells obtained 33 and19 photocurrent increases respectively as reported byPillai et al [92] Enhancement in the photocurrent by upto a factor of 27 for ITO-copper phthalocyanine-indiumstructures was reported by Stenzel Enhancement for silverclusters incorporated in ITO and zinc phthalocyanine solarcells was reported by Westphalen et al [93] Enhanced

efficiencies for ultrathin film organic solar cells due to 5 nmdiameter silver nanoparticles were reported by Rand Anincrease in efficiency by a factor of 17 for organic bulkheterojunction solar cells was reported by Morfa et al [94]Increased photocurrents for CdSeSi heterostructures [95]and enhanced carrier generation in dye-sensitized solar cellshave also been reported [96]

Rho et al attached TiO2nanotube arrays which were

prepared by the electrochemical anodization method onto aTCO glass to fabricate a transparent photoanode and a front-illuminated DSSC as shown in Figure 9 [47] With this celldesign the N719 dyes were sensitized more directly by theincident light compared to the conventional back-illuminatedDSSCs in which light has to pass through the transparentcounter electrode in order to reach the dyes Ag nanoparticleswith diameters smaller than the light wavelength can improvethe light absorption dramatically due to the surface plasmonresonance effect The authors varied the time of exposureto irradiation to examine the Ag nanoparticle depositionkinetics and the existence of the optimal irradiation TheAg nanoparticles were deposited not only on the outsideof the TiO

2nanotube arrays but also on the inside The

photovoltaic properties measured for DSSCs without Agnanoparticles and DSSCs with nanoparticles and varyingirradiation times indicated the same tendency The highestenergy conversion efficiency achievedwas 614 around 32improvement of the energy conversion efficiency for DSSCswithout Ag nanoparticles 464 Comparisonwas alsomadebetween front-illuminated and back-illuminated DSSCsThehighest efficiency reported for the back-illuminated DSSCswas 3077 which was lower than the front-illuminatedDSSCsrsquo highest efficiency Notably the effect of incorporatingAg nanoparticles was also lower for back-illuminated DSSCsThe back-illuminatedDSSCs showed only a 946 increase in


(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)

Figure 9 (a) Schematic of plasmon DSSC based on method reported by Rho et al (A) Ti anodization in organic electrolyte (B) freestandingTiO2nanotube arrays (C) formation ofAg nanoparticles onTiO

2nanotube arrays byUV irradiation at 254 nm (b) Schematic of an assembled

cell Reprinted with permission from [47] Copyright 2014 Elsevier

the efficiency after the deposition of Ag nanoparticles whichwas lower than the 32 increase in the front-illuminatedDSSCs The authors attributed this difference to the reducedintensity of the light passing through the counter electrodeand electrolyte layer in the back-illuminated DSSCs whichcaused the surface plasmon radiation to occur only at the toppart of the TiO

2nanotube arrays

(2) TiCl4 with TiO2 Nanotube Arrays for DSSC Solar CellsTiO2nanoparticles have been deposited to fill the micro-

cracks formed in TiO2nanotube arrays using an elec-

trophoretic method [97] By depositing nanoparticles theconversion efficiency was improved from 4 to 52Nanoparticles were found in the cracks and on the surfaceof the nanotubes The total inner space of the nanotubesmight be much larger than the space of the microcracksHowever nanoparticles might not be deposited deep insidethe pores Since TiO

2nanotube arrays are semiconduc-

tors electrochemical depositions take place at or near theentrances of their pores There is no driving force to pushthe deposited nanoparticles into the deep parts of the poresIn vacuum filtering the solvent provides a driving forceand nanoparticles can be filled throughout the long channels

of the TiO2nanotube arrays The TiO

2nanoparticles that

fill the channels of TiO2nanotube arrays can make contact

with the tubes directly or through other nanoparticles Sincenanotubes are more efficient in electron transfer than arenanoparticles or composites of nanoparticles and nanotubeselectron transfer to the electrode could also be more efficient

DSSCs based on TiO2nanotube arrays filled with TiO

2

nanoparticles have also been fabricated as shown in Figure 10[48] The energy conversion efficiency was enhanced byabout 21 by filling the channels of the membrane withTiO2nanoparticles By treating with TiCl

4after filling with

nanoparticles the efficiency was enhanced significantly byabout 40 due to an increase in the total available surfacearea for dye adsorption and better electron transfer to theelectrode The method of filling the channels of the TiO

2

membrane with TiO2nanoparticles and then treating with

TiCl4was used to improve the energy conversion efficiency

of DSSCs based on TiO2nanotube arrays

(3) Additional Layer Technique Introducing a scattering layersuch as TiO

2 ZrO

2 or SiO

2can increase the total energy

conversion efficiency of DSSCs [98] TiO2is a good material

to use for a scattering layer due to its chemical stability and


(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO

Figure 10 Scheme for fabricating a DSSC based on a TiO2nanotube membrane filled with TiO

2nanoparticles (a) 1st anodization then

annealing (b) 2nd anodization and detachment of TiO2film (c) ion milling (d) filling TiO

2nanoparticles attaching on fluorine-doped thin

oxide (FTO) glass treating with TiCl4 and then annealing in air and (e) dye adsorption and fabrication of a DSSC Reprinted with permission

from [48] Copyright 2011 Elsevier

dye-adsorption capability hence many DSSCs are fabricatedusing TiO

2nanoparticle films with a TiO

2scattering layer on

top of the active layerComparison has been made between DSSCs using closed-

and open-ended TiO2nanotube arrays when a TiO

2scat-

tering layer is introduced The energy conversion efficiencywas enhanced by 515 due to the removal of the barrierlayer which was present in the closed-ended TiO

2nanotube

arrays causing an improvement in electron transport Byintroducing the TiO

2scattering layer on the open-ended

TiO2nanotube arrays the energy conversion efficiency was

enhanced by 1030 due to the improved light harvestingAdditionally the energy conversion efficiency of the open-endedTiO

2nanotube arrays treatedwith TiCl

4was enhanced

by 551 due to increased dye adsorption Each componentcould enhance the DSSC yields

5 Conclusions and Future Work

In the future photovoltaic cells will be used in many fieldssuch as mobile commerce building integrated photovoltaics(BIPVs) and vehicles Moreover photovoltaic cells are essen-tial in the Smart Grid which is utilized in our daily livesTo apply solar energy in the Smart Grid photovoltaic cellsare required to have transparency flexibility light weightlow cost and high energy conversion efficiency In terms of

low cost and light weight organics inorganics and hybridmaterials have brighter prospects than the semiconductorsIn hybrid materials photovoltaic cells have been preparedwith the advantages of organics or inorganics selectivelyHowever it is not easy to increase the energy conversionefficiency One potential solution is to use 3-dimensionalnanostructures such as nanotubes nanowires compositefilms with metal or other types of nanostructures As oftoday 0-dimensional nanoparticles in photovoltaic cells stillyield the higher energy conversion efficiency by supplyinglarger surface areas for the sensitizer adsorption Howeverthe electron transport and flexibility are sufficiently highin 3-dimensional nanostructures To improve the energyconversion efficiency of 3-dimensional nanostructure basedsolar cells and take both advantages of 0-dimensionalnanoparticles and 3-dimensional nanostructures the lightharvesting ability must be reinforced One way of achievingthe better light harvesting ability is to fill the 0-dimensionalnanoparticles in the cavities of 3-dimensional nanostructurewhich is another currently active field of study [99]

Conflict of Interests

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


Authorsrsquo Contribution

Won-Yeop Rho and Hojin Jeon contributed equally to thiswork

Acknowledgment

This research was supported by the Basic Science ResearchProgram through theNational Research Foundation of Korea(NRF) funded by the Ministry of Science ICT amp FuturePlanning (2014-A002-0065)

References

[1] EPA ldquoInventory of US greenhouse gas emissions and sinks1990ndash1998rdquo EPA 236-R-00-001 US Environmental ProtectionAgency 2000

[2] International Engergy Outlook 2013 U S Briefing 2013[3] M A Qaeed K Ibrahim K M Saron M A Ahmed and N K

Allam ldquoLow-temperature solution-processed flexible solar cellsbased on (InGa)N nanocubesrdquo ACS Applied Materials ampInterfaces vol 6 no 13 pp 9925ndash9931 2014

[4] L Tsakalakos ldquoNanostructures for photovoltaicsrdquo Materials Sci-ence and Engineering R Reports vol 62 no 6 pp 175ndash189 2008

[5] S OrsquoRourke P Kim and H Polavarapu Solar PhotovoltaicIndustry Looking through the Storm vol 56 Deutsche Bank2009

[6] TW Hamann R A Jensen A B F Martinson H van Ryswykand J T Hupp ldquoAdvancing beyond current generation dye-sensitized solar cellsrdquo Energy amp Environmental Science vol 1no 1 pp 66ndash78 2008

[7] 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[8] L M Goncalves V de Zea Bermudez H A Ribeiro and A M

Mendes ldquoDye-sensitized solar cells a safe bet for the futurerdquoEnergy amp Environmental Science vol 1 no 6 pp 655ndash667 2008

[9] M K Nazeeruddin E Baranoff and M Gratzel ldquoDye-sen-sitized solar cells a brief overviewrdquo Solar Energy vol 85 no 6pp 1172ndash1178 2011

[10] D Kuang C Klein S Ito et al ldquoHigh-Efficiency and stablemesoscopic dye-sensitized solar cells based on a high molarextinction coefficient ruthenium sensitizer and nonvolatileelectrolyterdquo Advanced Materials vol 19 no 8 pp 1133ndash11372007

[11] F Ribeiro J MacAira R Cruz J Gabriel L Andrade and AMendes ldquoLaser assisted glass frit sealing of dye-sensitized solarcellsrdquo Solar Energy Materials and Solar Cells vol 96 no 1 pp43ndash49 2012

[12] J Wu S Hao Z Lan et al ldquoAn all-solid-state dye-sensitizedsolar cell-based poly(N-alkyl-4-vinyl-pyridine iodide) elec-trolyte with efficiency of 564rdquo Journal of the AmericanChemical Society vol 130 no 35 pp 11568ndash11569 2008

[13] WWu J Li F Guo L Zhang Y Long and J Hua ldquoPhotovoltaicperformance and long-term stability of quasi-solid-state fluo-ranthene dyes-sensitized solar cellsrdquo Renewable Energy vol 35no 8 pp 1724ndash1728 2010

[14] J H Yum S J Moon C S Karthikeyan et al ldquoHeterolepticruthenium complex containing substituted triphenylaminehole-transport unit as sensitizer for stable dye-sensitized solarcellrdquo Nano Energy vol 1 no 1 pp 6ndash12 2012

[15] T Kawashima T Ezure K Okada H Matsui K Goto andN Tanabe ldquoFTOITO double-layered transparent conductiveoxide for dye-sensitized solar cellsrdquo Journal of Photochemistryand Photobiology A Chemistry vol 164 no 1ndash3 pp 199ndash2022004

[16] J H Park TW Lee andM G Kang ldquoGrowth detachment andtransfer of highly-ordered TiO

2nanotube arrays use in dye-

sensitized solar cellsrdquo Chemical Communications no 25 pp2867ndash2869 2008

[17] Q W Chen and D S Xu ldquoLarge-scale noncurling and free-standing crystallized TiO

2nanotube arrays for dye-sensitized

solar cellsrdquo Journal of Physical Chemistry C vol 113 no 15 pp6310ndash6314 2009

[18] C-J Lin W-Y Yu and S-H Chien ldquoTransparent electrodes ofordered opened-end TiO

2-nanotube arrays for highly efficient

dye-sensitized solar cellsrdquo Journal of Materials Chemistry vol20 no 6 pp 1073ndash1077 2010

[19] L-L Li Y-J Chen H-P Wu N S Wang and E W-GDiau ldquoDetachment and transfer of ordered TiO

2nanotube

arrays for front-illuminated dye-sensitized solar cellsrdquo Energyamp Environmental Science vol 4 no 9 pp 3420ndash3425 2011

[20] R Buscaino C Baiocchi C Barolo et al ldquoAmass spectrometricanalysis of sensitizer solution used for dye-sensitized solar cellrdquoInorganica Chimica Acta vol 361 no 3 pp 798ndash805 2008

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

[22] 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

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

[24] W Sharmoukh and N K Allam ldquoTiO2nanotube-based dye-

sensitized solar cell using new photosensitizer with enhancedopen-circuit voltage and fill factorrdquo ACS Applied Materials andInterfaces vol 4 no 8 pp 4413ndash4418 2012

[25] D Kuang S Ito B Wenger et al ldquoHigh molar extinctioncoefficient heteroleptic ruthenium complexes for thin film dye-sensitized solar cellsrdquo Journal of the American Chemical Societyvol 128 no 12 pp 4146ndash4154 2006

[26] S A Haque E Palomares B M Cho et al ldquoCharge separationversus recombination in dye-sensitized nanocrystalline solarcells the minimization of kinetic redundancyrdquo Journal of theAmericanChemical Society vol 127 no 10 pp 3456ndash3462 2005

[27] K C D Robson B D Koivisto A Yella et al ldquoDesign anddevelopment of functionalized cyclometalated rutheniumchro-mophores for light-harvesting applicationsrdquo Inorganic Chem-istry vol 50 no 12 pp 5494ndash5508 2011

[28] D Kuciauskas J E Monat R Villahermosa H B Gray N SLewis and J K McCusker ldquoTransient absorption spectroscopyof ruthenium and osmium polypyridyl complexes adsorbedonto nanocrystalline TiO

2photoelectrodesrdquo Journal of Physical

Chemistry B vol 106 no 36 pp 9347ndash9358 2002[29] G M Hasselmann and G J Meyer ldquoSensitization of nanocrys-

talline TiO2by Re(I) polypyridyl compoundsrdquo Zeitschrift fur

Physikalische Chemie vol 212 part 1 pp 39ndash44 1999


[30] S Ferrere ldquoNew photosensitizers based upon [Fe119868119868(L)2(CN)2]

and [Fe119868119868L3] where L is substituted 221015840-bipyridinerdquo Inorganica

Chimica Acta vol 329 no 1 pp 79ndash92 2002[31] S Ferrere and B A Gregg ldquoPhotosensitization of TiO

2

by [Fe(II)(221015840-bipyridine-441015840-dicarboxylic acid)2(CN)2] band

selective electron injection from ultra-short-lived excitedstatesrdquo Journal of the American Chemical Society vol 120 no4 pp 843ndash844 1998

[32] S Ferrere ldquoNew photosensitizers based upon [Fe(L)2(CN)2]

and [Fe(L)3] (L = substituted 221015840-bipyridine) yields for the

photosensitization of TiO2and effects on the band selectivityrdquo

Chemistry of Materials vol 12 no 4 pp 1083ndash1089 2000[33] A Islam H Sugihara K Hara et al ldquoDye sensitization

of nanocrystalline titanium dioxide with square planar plat-inum(II) diimine dithiolate complexesrdquo Inorganic Chemistryvol 40 no 21 pp 5371ndash5380 2001

[34] E AMGeary L J Yellowlees L A Jack et al ldquoSynthesis struc-ture and properties of [Pt(II)(diimine)(dithiolate)] dyes with331015840- 441015840- and 551015840-disubstituted bipyridyl applications in dye-sensitized solar cellsrdquo Inorganic Chemistry vol 44 no 2 pp242ndash250 2005

[35] E A M Geary K L McCall A Turner et al ldquoSpectroscopicelectrochemical and computational study of Pt-diimine-dith-iolene complexes rationalising the properties of solar cell dyesrdquoDalton Transactions no 28 pp 3701ndash3708 2008

[36] E A M Geary N Hirata J Clifford et al ldquoSynthesis structureand properties of [Pt(221015840-bipyridyl-551015840-dicarboxylic acid)(34-toluenedithiolate)] tuningmolecular properties for applicationin dye-sensitised solar cellsrdquo Dalton Transactions no 19 pp3757ndash3762 2003

[37] S Sakaki T Kuroki and T Hamada ldquoSynthesis of a new cop-per(I) complex[Cu(tmdcbpy)

2]+ (tmdcbpy = 441015840661015840-tetra-

methyl-221015840-bipyridine-551015840- dicarboxylic acid) and its appli-cation to solar cellsrdquo Journal of the Chemical Society DaltonTransactions no 6 pp 840ndash842 2002

[38] T Bessho E C Constable M Graetzel et al ldquoAn element ofsurprisemdashefficient copper-functionalized dye-sensitized solarcellsrdquo Chemical Communications no 32 pp 3717ndash3719 2008

[39] M Alebbi C A Bignozzi T A Heimer G M Hasselmannand G J Meyer ldquoThe limiting role of iodide oxidation in cis-Os(dcb)

2(CN)2TiO2photoelectrochemical cellsrdquo The Journal

of Physical Chemistry B vol 102 no 39 pp 7577ndash7581 1998[40] A Yella H-W Lee H N Tsao et al ldquoPorphyrin-sensitized

solar cells with cobalt (IIIII)-based redox electrolyte exceed 12percent efficiencyrdquo Science vol 334 no 6056 pp 629ndash634 2011

[41] M Zhang Y Wang M Xu W Ma R Li and P WangldquoDesign of high-efficiency organic dyes for titania solar cellsbased on the chromophoric core of cyclopentadithiophene-benzothiadiazolerdquo Energy amp Environmental Science vol 6 no10 pp 2944ndash2949 2013

[42] S QuWWu J Hua C Kong Y Long andH Tian ldquoNew dike-topyrrolopyrrole (DPP) dyes for efficient dye-sensitized solarcellsrdquo Journal of Physical Chemistry C vol 114 no 2 pp 1343ndash1349 2010

[43] S Minagar C C Berndt J Wang E Ivanova and C Wen ldquoAreview of the application of anodization for the fabrication ofnanotubes onmetal implant surfacesrdquoActaBiomaterialia vol 8no 8 pp 2875ndash2888 2012

[44] G K Mor O K Varghese M Paulose K Shankar and C AGrimes ldquoA review on highly ordered vertically oriented TiO

2

nanotube arrays fabrication material properties and solar

energy applicationsrdquo Solar EnergyMaterials and Solar Cells vol90 no 14 pp 2011ndash2075 2006

[45] C Rho J-H Min and J S Suh ldquoBarrier layer effect on theelectron transport of the dye-sensitized solar cells based onTiO2nanotube arraysrdquo Journal of Physical Chemistry C vol 116

no 12 pp 7213ndash7218 2012[46] H A Atwater andA Polman ldquoPlasmonics for improved photo-

voltaic devicesrdquoNatureMaterials vol 9 no 3 pp 205ndash213 2010[47] W-Y Rho H-S Kim S H Lee et al ldquoFront-illuminated

dye-sensitized solar cells with Ag nanoparticle-functionalizedfreestanding TiO

2nanotube arraysrdquo Chemical Physics Letters

vol 614 pp 78ndash81 2014[48] C Rho and J S Suh ldquoFilling TiO

2nanoparticles in the channels

of TiO2nanotube membranes to enhance the efficiency of dye-

sensitized solar cellsrdquo Chemical Physics Letters vol 513 no 1-3pp 108ndash111 2011

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

[50] R Nashed P Szymanski M A El-Sayed and N K AllamldquoSelf-Assembled nanostructured photoanodes with staggeredbandgap for efficient solar energy conversionrdquo ACS Nano vol8 no 5 pp 4915ndash4923 2014

[51] J Z Ou R A Rani M-H Ham et al ldquoElevated temperatureanodized Nb

2O5 a photoanode material with exceptionally

large photoconversion efficienciesrdquo ACS Nano vol 6 no 5 pp4045ndash4053 2012

[52] 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

[53] Q Zhang and G Cao ldquoNanostructured photoelectrodes fordye-sensitized solar cellsrdquo Nano Today vol 6 no 1 pp 91ndash1092011

[54] M Adachi Y Murata M Harada and S Yoshikawa ldquoForma-tion of titania nanotubes with high photo-catalytic activityrdquoChemistry Letters no 8 pp 942ndash943 2000

[55] S-Z Chu S Inoue K Wada D Li H Haneda and SAwatsu ldquoHighly porous (TiO

2-SiO2-TeO2)Al2O3TiO2com-

posite nanostructures on glass with enhanced photocatalysisfabricated by anodization and sol-gel processrdquo The Journal ofPhysical Chemistry B vol 107 no 27 pp 6586ndash6589 2003

[56] O K Varghese D Gong M Paulose K G Ong E C Dickeyand C A Grimes ldquoExtreme changes in the electrical resistanceof titania nanotubes with hydrogen exposurerdquo Advanced Mate-rials vol 15 no 7-8 pp 624ndash627 2003

[57] G K Mor M A Carvalho O K Varghese M V Pishko andC A Grimes ldquoA room-temperature TiO

2-nanotube hydrogen

sensor able to self-clean photoactively from environmentalcontaminationrdquo Journal of Materials Research vol 19 no 2 pp628ndash634 2004

[58] M Paulose O K Varghese G K Mor C A Grimes and K GOng ldquoUnprecedented ultra-high hydrogen gas sensitivity inundoped titania nanotubesrdquo Nanotechnology vol 17 no 2 pp398ndash402 2006

[59] O K Varghese G K Mor C A Grimes M Paulose and NMukherjee ldquoA titania nanotube-array room-temperature sen-sor for selective detection of hydrogen at low concentrationsrdquoJournal of Nanoscience and Nanotechnology vol 4 no 7 pp733ndash737 2004


[60] G K Mor K Shankar M Paulose O K Varghese and C AGrimes ldquoEnhanced photocleavage of water using titania nan-otube arraysrdquo Nano Letters vol 5 no 1 pp 191ndash195 2005

[61] G K Mor K Shankar O K Varghese and C A Grimes ldquoPho-toelectrochemical properties of titania nanotubesrdquo Journal ofMaterials Research vol 19 no 10 pp 2989ndash2996 2004

[62] O K Varghese M Paulose K Shankar G K Mor and C AGrimes ldquoWater-photolysis properties of micron-length highly-ordered titania nanotube-arraysrdquo Journal of Nanoscience andNanotechnology vol 5 no 7 pp 1158ndash1165 2005

[63] S Uchida R Chiba M Tomiha N Masaki and M ShiraildquoApplication of titania nanotubes to a dye-sensitized solar cellrdquoElectrochemistry vol 70 no 6 pp 418ndash420 2002

[64] M Adachi Y Murata I Okada and S Yoshikawa ldquoFormationof titania nanotubes and applications for dye-sensitized solarcellsrdquo Journal of the Electrochemical Society vol 150 no 8 ppG488ndashG493 2003

[65] M Paulose K Shankar O K Varghese G K Mor B Hardinand C A Grimes ldquoBackside illuminated dye-sensitized solarcells based on titania nanotube array electrodes (vol 17 pg 14462006)rdquo Nanotechnology vol 21 no 38 2010

[66] V Zwilling M Aucouturier and E Darque-Ceretti ldquoAnodicoxidation of titanium and TA

6V alloy in chromic media An

electrochemical approachrdquo Electrochimica Acta vol 45 no 6pp 921ndash929 1999

[67] N K Allam K Shankar andC A Grimes ldquoPhotoelectrochem-ical and water photoelectrolysis properties of ordered TiO2nanotubes fabricated by Ti anodization in fluoride-free HClelectrolytesrdquo Journal of Materials Chemistry vol 18 no 20 pp2341ndash2348 2008

[68] D Gong C A Grimes O K Varghese et al ldquoTitanium oxidenanotube arrays prepared by anodic oxidationrdquo Journal ofMaterials Research vol 16 no 12 pp 3331ndash3334 2001

[69] G K Mor O K Varghese M Paulose and C A GrimesldquoA self-cleaning room-temperature titania-nanotube hydrogengas sensorrdquo Sensor Letters vol 1 no 1 pp 42ndash46 2003

[70] C Ruan M Paulose O K Varghese and C A GrimesldquoEnhanced photoelectrochemical-response in highly orderedTiO2nanotube-arrays anodized in boric acid containing elec-

trolyterdquo Solar Energy Materials amp Solar Cells vol 90 no 9 pp1283ndash1295 2006

[71] Q Y Cai M Paulose O K Varghese and C A GrimesldquoThe effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxidationrdquoJournal of Materials Research vol 20 no 1 pp 230ndash236 2005

[72] R Beranek H Hildebrand and P Schmuki ldquoSelf-organizedporous titanium oxide prepared in H

2SO4HF electrolytesrdquo

Electrochemical and Solid-State Letters vol 6 no 3 pp B12ndashB142003

[73] J M MacAk K Sirotna and P Schmuki ldquoSelf-organizedporous titanium oxide prepared in Na

2SO4NaF electrolytesrdquo

Electrochimica Acta vol 50 no 18 pp 3679ndash3684 2005[74] M Paulose K Shankar S Yoriya et al ldquoAnodic growth of highly

ordered TiO2nanotube arrays to 134 120583m in lengthrdquo Journal of

Physical Chemistry B vol 110 no 33 pp 16179ndash16184 2006[75] K Shankar G K Mor H E Prakasam et al ldquoHighly-ordered

TiO2nanotube arrays up to 220 120583m in length use in water

photoelectrolysis and dye-sensitized solar cellsrdquo Nanotechnol-ogy vol 18 no 6 Article ID 065707 2007

[76] M Paulose H E Prakasam O K Varghese et al ldquoTiO2nan-

otube arrays of 1000 120583m length by anodization of titanium foil

phenol red diffusionrdquo Journal of Physical Chemistry C vol 111no 41 pp 14992ndash14997 2007

[77] L V Taveira J M Macak H Tsuchiya L F P Dick and PSchmuki ldquoInitiation and growth of self-organized TiO

2nan-

otubes anodically formed in NH4F(NH

4)2SO4electrolytesrdquo

Journal of the Electrochemical Society vol 152 no 10 pp B405ndashB410 2005

[78] Z X Su and W Z Zhou ldquoFormation mechanism of porousanodic aluminium and titanium oxidesrdquo Advanced Materialsvol 20 no 19 pp 3663ndash3667 2008

[79] T P Hoar and N F Mott ldquoA mechanism for the formation ofporous anodic oxide films on aluminiumrdquo Journal of Physics andChemistry of Solids vol 9 no 2 pp 97ndash99 1959

[80] K R Hebert H Wu T Gessmann and K Lynn ldquoPositronannihilation spectroscopy study of interfacial defects formedby dissolution of aluminum in aqueous sodium hydroxiderdquoJournal of the Electrochemical Society vol 148 no 2 pp B92ndashB100 2001

[81] R Huang K R Hebert T Gessmann and K G Lynn ldquoEffect ofimpurities on interfacial void formation in aluminumrdquo Journalof the Electrochemical Society vol 151 no 4 pp B227ndashB2322004

[82] C Y Chao L F Lin and D D Macdonald ldquoA point defectmodel for anodic passive films 1 Film growth kineticsrdquo Journalof the Electrochemical Society vol 128 no 6 pp 1187ndash1194 1981

[83] L F Lin C Y Chao and D D Macdonald ldquoA point-defectmodel for anodic passive films II Chemical breakdownandpit initiationrdquo Journal of the Electrochemical Society vol 128no 6 pp 1194ndash1198 1981

[84] J Meier S Dubail S Golay et al ldquoMicrocrystalline silicon andthe impact on micromorph tandem solar cellsrdquo Solar EnergyMaterials and Solar Cells vol 74 no 1-4 pp 457ndash467 2002

[85] J Muller B Rech J Springer and M Vanecek ldquoTCO and lighttrapping in silicon thin film solar cellsrdquo Solar Energy vol 77 no6 pp 917ndash930 2004

[86] H A Atwater and A Polman ldquoPlasmonics for improved pho-tovoltaic devices (vol 9 pg 205 2010)rdquo Nature Materials vol 9no 10 p 865 2010

[87] S Pillai andM A Green ldquoPlasmonics for photovoltaic applica-tionsrdquo Solar Energy Materials and Solar Cells vol 94 no 9 pp1481ndash1486 2010

[88] K R Catchpole and A Polman ldquoPlasmonic solar cellsrdquo OpticsExpress vol 16 no 26 pp 21793ndash21800 2008

[89] H R Stuart and D G Hall ldquoIsland size effects in nanoparticle-enhanced photodetectorsrdquo Applied Physics Letters vol 73 no26 pp 3815ndash3817 1998

[90] DM Schaadt B Feng and E T Yu ldquoEnhanced semiconductoroptical absorption via surface plasmon excitation in metalnanoparticlesrdquo Applied Physics Letters vol 86 no 6 Article ID063106 pp 1ndash3 2005

[91] D Derkacs S H Lim P Matheu W Mar and E T YuldquoImproved performance of amorphous silicon solar cells viascattering from surface plasmon polaritons in nearby metallicnanoparticlesrdquo Applied Physics Letters vol 89 no 9 Article ID093103 2006

[92] S Pillai K R Catchpole T Trupke and M A Green ldquoSurfaceplasmon enhanced silicon solar cellsrdquo Journal of Applied Physicsvol 101 no 9 Article ID 093105 2007

[93] M Westphalen U Kreibig J Rostalski H Luth and DMeissner ldquoMetal cluster enhanced organic solar cellsrdquo SolarEnergy Materials and Solar Cells vol 61 no 1 pp 97ndash105 2000


[94] A J Morfa K L Rowlen T H Reilly II M J Romero and Jvan de Lagemaat ldquoPlasmon-enhanced solar energy conversionin organic bulk heterojunction photovoltaicsrdquo Applied PhysicsLetters vol 92 no 1 Article ID 013504 2008

[95] R B Konda R Mundle H Mustafa et al ldquoSurface plasmonexcitation via Au nanoparticles in n-CdSep-Si heterojunctiondiodesrdquo Applied Physics Letters vol 91 no 19 2007

[96] C Hagglund M Zach and B Kasemo ldquoEnhanced charge car-rier generation in dye sensitized solar cells by nanoparticle plas-monsrdquo Applied Physics Letters vol 92 no 1 Article ID 0131132008

[97] K Shin Y JimGYHan and JH Park ldquoEffect of incorporationof TiO

2nanoparticles into oriented TiO

2nanotube based dye-

sensitized solar cellsrdquo Journal of Nanoscience and Nanotechnol-ogy vol 9 no 12 pp 7436ndash7439 2009

[98] SHore C Vetter R KernH Smit andAHinsch ldquoInfluence ofscattering layers on efficiency of dye-sensitized solar cellsrdquo SolarEnergy Materials and Solar Cells vol 90 no 9 pp 1176ndash11882006

[99] J Macaira L Andrade and A Mendes ldquoReview on nanostruc-tured photoelectrodes for next generation dye-sensitized solarcellsrdquo Renewable and Sustainable Energy Reviews vol 27 pp334ndash349 2013

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


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


weight and able to display various colors and transparencyand they work effectively in a broad range of wavelengths[8 9] However there are still several weaknesses that requireimprovement such as relatively low efficiency high costof the ruthenium dyes conducting glass and platinumtemperature stability problems and low scalability Amongthe various potential solutions some of the most widelyinvestigated ones are the development of new sensitizersto cover a broader solar spectrum and the fabrication ofnew photoanode structures with enhanced light utilizationStudies have reported several different methods to improvethe efficiency of DSSCs such as the use of panchromaticsensitizers or 3-dimensional metal oxide nanostructures [10ndash14] This review highlights another huge pool of studies thatreport improvements in the efficiency of DSSCs especiallythose that employ the use of TiO

2nanotubes Moreover we

will introduce recent papers on high PCEs and the use ofTiO2nanotube arrays in DSSCs These papers describe the

use of the space in TiO2nanotube array channels and how to

overcome the barrier layer that exists at the bottom of thesearrays to achieve a high PCE in DSSCs

2 Principle of DSSCs

A DSSC consists of a photoelectrode counter electrode andelectrolyte The photoelectrode is made up of a transparentconductive oxide (TCO) glass substrate a metal oxide suchas mesoporous TiO

2nanoparticles and a sensitizer such as

ruthenium- (Ru-) complex dye The electrolyte consists ofa redox couple and diffusing reducingoxidizing agent ionsand the counter electrode is coated with Pt as shown inFigure 1(a) In general fluorine-doped tin oxide (FTO) glassis used on DSSCs instead of indium tin oxide (ITO) glassbecause of the better thermal stability at high temperatures[15] During the sintering at 450ndash500∘C which crystallizesthe mesoporous TiO

2nanoparticles the sheet resistance of

ITO increases and affects the energy conversion efficiencyof the DSSCs However the sheet resistance of FTO isindependent of the temperature up to 500∘C which makesit favorable for applications that utilize the sintering stepTiO2nanoparticles are primarily used as electron acceptors

on DSSCs that incorporate ruthenium dye The counterelectrode consists of FTO glass coated with platinum (Pt) orcarbon to resist the oxidation and to work as a catalyst duringthe redox reaction in the electrolyte

The principle of DSSCs is shown in Figure 1(b) Whensensitizers absorb the light energy and electrons are gen-erated by the sensitizers and are transferred to the metaloxide films During the hopping process in the metal oxidefilms the generated electrons are transferred to the TCO andcirculated After the circulation the sensitizers are reducedby an iodide redox process In general a Ru-complex dye isused as the sensitizer and TiO

2nanoparticles are used as the

electron acceptors on DSSCs due to their high efficiency [20ndash22]

The energy diagram shown in Figure 1(c) demonstratesthe electron transfer process of DSSCs When the Ru-complex dyes absorb the light the electrons change from

the ground state to the excited state within a time on the orderof nanoseconds (ns) as shown in the reaction (Figure 1(c)-(1))

S0 + ℎ] 997888rarr Slowast (1)

Although the detailed mechanism of the injection process isunder debate the electron injection from theRu-complex dyeinto the conduction band of mesoporous TiO

2nanoparticles

takes about a femtosecond (fs) following the reaction shownin Figure 1(c)-(2) The energy level of electrons in the Ru-complex dye is higher than that of the conduction band ofmesoporous TiO

2nanoparticles which enables the transfer

of electrons into the mesoporous TiO2nanoparticles

Slowast 997888rarr S+ + eminus

S+ + eminus 997888rarr S0(2)

The excited state of the Ru-complex dye lasts around 20ndash60 ns which is long enough for kinetic competition betweenelectron injection and excited state decay in the Ru-complexdye

The electrons injected into TiO2nanoparticles diffuse

through mesoporous TiO2nanoparticles in about a millisec-

ond (ms) in the reaction shown in Figure 1(c)-(3) Howeverwhen the injected electrons are trapped onmesoporous TiO

2

nanoparticles they return to the oxidized Ru-complex dye bythe reaction shown in Figure 1(c)-(4) or reduce the triiodideby the reaction shown in Figure 1(c)-(5) These processes arecalled as the recombination

I3

minus+ 2eminus 997888rarr 3Iminus (Reduction)

3Iminus 997888rarr I3

minus+ 2eminus (Oxidation)

(3)

The Ru-complex dye is reduced by an iodide ion to accept theelectron as shown in the reaction in Figure 1(c)-(6)

2S+ + 3Iminus 997888rarr 2S0 + I3

minus (4)

The injected electrons on TiO2nanoparticles drive the exter-

nal circuit and perform work as in the reaction shown inFigure 1(c)-(7)

When the triiodide ion reduces to iodide to accept theelectrons from the counter electrode by the reaction shown inFigure 1(c)-(8) the iodide ion causes the electron in the Ru-complex dye to return to the ground state with the reactionshown in Figure 1(c)-(6)

Figure 1(d) illustrates the performance of DSSCs and thesolar power (119875) can be calculated by the equation

119875 = 119868119881 (5)

The energy conversion efficiency of DSSCs is described bythe ratio of the maximum electrical power output 119875max tothe solar energy input 119875 The maximum electrical poweroutput 119875max is calculated by the maximum current 119869mp andthe maximum voltage 119881mp

119875max = 119869mp times 119881mp (6)


TCO glassPt

TiO2nanoparticles

TCO glass

Electrolyte

(a)

Pt FTO

FTOLight

Load

Sensitizer

eminuseminus

eminus

eminus

eminuseminus

eminus

Iminus

TiO2 nanoparticles

I3minus

(b)

(LUMO)

(HOMO)

(2)

(1)

(3)

(4)

(5)

(6)

E v

ersu

s NH

E (V

)


(7) Load

(8)

eminus eminus

IminusI3minus

TiO2PtFT

O

FTO

S0S

+

Slowastminus05

0

+05

+10

(c)

Power density

Voltage (V)

Maximumpower point

Curr

ent d

ensit

y (m

Ac

m2 ) Jsc

Jmp

Vmp Voc

(d)



120578 =119875max119875

120578 =(119869mp times 119881mp)

119875

(7)


119891119891 =(119869mp times 119881mp)

(119869sc times 119881oc)

120578 =119875max119875

120578 =(119869mp times 119881mp)

119875

120578 =(119869sc times 119881oc times 119891119891)

119875

(8)






3 Dye



Power density

Voltage (V)

Jsc

Jmp

Vmp Voc

Curr

ent d

ensit

y (m

Ac

m2 )

(a)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )


(b)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )


(c)










N

NN

N

O

O

HO

HO

HO

HO

N

N

O

O

RuC S

C S

(a)

N

NN

N

O

O

HO

HO

N

N

O

O

Ru

N+

minusO

N+

minusO

C S

C S

(b)

N

OHO

HO

O

N

Ru

N

N

N

NCS

C

S

(c)

N

N

O

OH

O

O

N

N

N

N

Ru

N+

minusON+

minusO

C S

CS

CS

(d)







S H

O

OH

N

S

N

N

O

O

N

O

O

S

N

N

O

O

N

S H

O

OH

N

O

O

DPP-I

DPP-II








2 High adsorption



4 Metal Oxide





2 or Nb

2O5






2has three


2nanoparticle








2nanotubes











2by forming

TiF6


2 To overcome this








3BO3 25 HNO

3







2








2






(10)


2



(11)





6]2minus

+ 2H2O (12)






2






eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




TCO glassPt

TiO2nanoparticles

TCO glass

Electrolyte

(a)

Pt FTO

FTOLight

Load

Sensitizer

eminuseminus

eminus

eminus

eminuseminus

eminus

Iminus

TiO2 nanoparticles

I3minus

(b)

(LUMO)

(HOMO)

(2)

(1)

(3)

(4)

(5)

(6)

E v

ersu

s NH

E (V

)


(7) Load

(8)

eminus eminus

IminusI3minus

TiO2PtFT

O

FTO

S0S

+

Slowastminus05

0

+05

+10

(c)

Power density

Voltage (V)

Maximumpower point

Curr

ent d

ensit

y (m

Ac

m2 ) Jsc

Jmp

Vmp Voc

(d)



120578 =119875max119875

120578 =(119869mp times 119881mp)

119875

(7)


119891119891 =(119869mp times 119881mp)

(119869sc times 119881oc)

120578 =119875max119875

120578 =(119869mp times 119881mp)

119875

120578 =(119869sc times 119881oc times 119891119891)

119875

(8)






3 Dye



Power density

Voltage (V)

Jsc

Jmp

Vmp Voc

Curr

ent d

ensit

y (m

Ac

m2 )

(a)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )


(b)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )


(c)










N

NN

N

O

O

HO

HO

HO

HO

N

N

O

O

RuC S

C S

(a)

N

NN

N

O

O

HO

HO

N

N

O

O

Ru

N+

minusO

N+

minusO

C S

C S

(b)

N

OHO

HO

O

N

Ru

N

N

N

NCS

C

S

(c)

N

N

O

OH

O

O

N

N

N

N

Ru

N+

minusON+

minusO

C S

CS

CS

(d)







S H

O

OH

N

S

N

N

O

O

N

O

O

S

N

N

O

O

N

S H

O

OH

N

O

O

DPP-I

DPP-II








2 High adsorption



4 Metal Oxide





2 or Nb

2O5






2has three


2nanoparticle








2nanotubes











2by forming

TiF6


2 To overcome this








3BO3 25 HNO

3







2








2






(10)


2



(11)





6]2minus

+ 2H2O (12)






2






eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Power density

Voltage (V)

Jsc

Jmp

Vmp Voc

Curr

ent d

ensit

y (m

Ac

m2 )

(a)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )


(b)

Voltage (V)

Curr

ent d

ensit

y (m

Ac

m2 )


(c)










N

NN

N

O

O

HO

HO

HO

HO

N

N

O

O

RuC S

C S

(a)

N

NN

N

O

O

HO

HO

N

N

O

O

Ru

N+

minusO

N+

minusO

C S

C S

(b)

N

OHO

HO

O

N

Ru

N

N

N

NCS

C

S

(c)

N

N

O

OH

O

O

N

N

N

N

Ru

N+

minusON+

minusO

C S

CS

CS

(d)







S H

O

OH

N

S

N

N

O

O

N

O

O

S

N

N

O

O

N

S H

O

OH

N

O

O

DPP-I

DPP-II








2 High adsorption



4 Metal Oxide





2 or Nb

2O5






2has three


2nanoparticle








2nanotubes











2by forming

TiF6


2 To overcome this








3BO3 25 HNO

3







2








2






(10)


2



(11)





6]2minus

+ 2H2O (12)






2






eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




N

NN

N

O

O

HO

HO

HO

HO

N

N

O

O

RuC S

C S

(a)

N

NN

N

O

O

HO

HO

N

N

O

O

Ru

N+

minusO

N+

minusO

C S

C S

(b)

N

OHO

HO

O

N

Ru

N

N

N

NCS

C

S

(c)

N

N

O

OH

O

O

N

N

N

N

Ru

N+

minusON+

minusO

C S

CS

CS

(d)







S H

O

OH

N

S

N

N

O

O

N

O

O

S

N

N

O

O

N

S H

O

OH

N

O

O

DPP-I

DPP-II








2 High adsorption



4 Metal Oxide





2 or Nb

2O5






2has three


2nanoparticle








2nanotubes











2by forming

TiF6


2 To overcome this








3BO3 25 HNO

3







2








2






(10)


2



(11)





6]2minus

+ 2H2O (12)






2






eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




S H

O

OH

N

S

N

N

O

O

N

O

O

S

N

N

O

O

N

S H

O

OH

N

O

O

DPP-I

DPP-II








2 High adsorption



4 Metal Oxide





2 or Nb

2O5






2has three


2nanoparticle








2nanotubes











2by forming

TiF6


2 To overcome this








3BO3 25 HNO

3







2








2






(10)


2



(11)





6]2minus

+ 2H2O (12)






2






eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







2has three


2nanoparticle








2nanotubes











2by forming

TiF6


2 To overcome this








3BO3 25 HNO

3







2








2






(10)


2



(11)





6]2minus

+ 2H2O (12)






2






eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




eminuseminuseminus

eminuseminus

eminuseminuseminus

eminus

eminus

H2

H2

H2

H2

H+

H+

H+

H+

H+

H+

H+

+minus

H2O

H2O

H2O

H2O

H+

H+

H+

H+ H+

H+

H+

H+

O2minus

O2minus

O2minus

O2minus

O2minus

Ti2+

Ti2+

Ti2+

Ti2+

Ti2+

eminuseminus

eminuseminus

eminuseminus

eminuseminus

eminuseminus

FminusFminus

Fminus

Fminus

Fminus

Fminus

TiO2TiO2

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2[TiF6]

2minus

[TiF6]2minus

(c) (d)

(e)

(a) (b)



rarr [TiF6]2minus

+ 2O2minus
















4F








2nan-


2


2nanotube






2nanotube arrays


2at the










Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Oxide

Metal

(a)

Metal

(b)

Pores

Barrier layer

(c)

PoresVoids

Barrier layer

(d)

Tube

Void

Barrier layer

(e)












2



100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




100nm

(a)

100nm

(b)

100nm

(c)

100nm

(d)
















Table 1)








2










(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







(a) (b) (c)










(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(A)

Ti plate

(B)

(C)

Ag NPs

(a)

TCO glassPt

Electrolyte

TCO glass

(b)





2nanotube arrays







2nanoparticles that




2


4after filling with


2





2 ZrO

2 or SiO





(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(d)

(a)

(b)

Ti plate

(c)

FTO

(e)FTO

Pt

Load

Electrolyte

FTO












2scat-


2nanotube






4was enhanced










Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






Acknowledgment


References




























2nanotube




















2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







2












2]+ (tmdcbpy = 441015840661015840-tetra-











2


















2solar cellsrdquo









2-nanotube hydrogen


































2nan-





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials

































2nan-





































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










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

Review Article Recent Progress in Dye-Sensitized Solar ...downloads.hindawi.com/journals/jnm/2015/247689.pdf · Dye-sensitized solar cells (DSSCs) are one potential alternative to