Research Article A Facile Synthesis of Granular ZnO ...

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2013 Article ID 563170 6 pageshttpdxdoiorg1011552013563170

Research ArticleA Facile Synthesis of Granular ZnO Nanostructures forDye-Sensitized Solar Cells

Md Mahbubur Rahman1 Narayan Chandra Deb Nath1

Kwang-Mo Noh2 Jaecheon Kim3 and Jae-Joon Lee13

1 Department of Advanced Technology Fusion Konkuk University Seoul 143-701 Republic of Korea2Nanotechnology Research Center and Department of Nano Science and Mechanical Engineering Konkuk UniversityChungju 380-701 Republic of Korea

3 Nanotechnology Research Center and Department of Applied Chemistry Konkuk University Chungju 380-701 Republic of Korea

Correspondence should be addressed to Jae-Joon Lee jjleekkuackr

Received 15 February 2013 Accepted 5 April 2013

Academic Editor Theodoros Dimopoulos

Copyright copy 2013 Md Mahbubur Rahman et alThis is an open access article distributed under the Creative CommonsAttributionLicense which permits unrestricted use distribution and reproduction in anymedium provided the originalwork is properly cited

Granular ZnO nanostructures of single-crystalline wurtzite hexagonal phases were synthesized by a facile and low-cost chemicalmethod in aqueous condition The average size of ZnO nanograin increased with reflux time and it significantly affected the opencircuit potential (119881oc) while the short circuit current density (119869sc) was not changed much The overall energy conversion efficiencywas 182 with the smaller grain size of ca 250 nmwhen it was used as photoelectrode of DSSCsThe positive shifting of the Fermienergy (119864F) and low density of surface states (DOS) were consistent with the reduction of the recombination of excited electronwith electrolyte for smaller grains

1 Introduction

Zinc oxide (ZnO) is unique in exhibiting both semiconduct-ing and piezoelectric properties [1] Bulk ZnO is a direct anda wide band gap semiconductor (119864119892 sim 32 eV at 25∘C) withhigh excitation binding energy (sim006 eV) large saturationvelocity (32 times 107 cm sminus1) and high optical gain (300 cmminus1)[2] The high excitation binding energy leads to excitonictransitions (sim25∘C) high radiative recombination efficiencyfor spontaneous emission and a low threshold voltage forlaser emission The quantum confinement effect of photo-generated electron-hole pairs and the tunability of opticaland electronic properties of ZnO nanoparticles (sim10 nm)[3 4] allowed them to be used in various nanodevices suchas information storage sensing and surface acoustic wavedevices [5ndash7]

ZnO is potentially useful in dye-sensitized solar cells(DSSCs) because it has a similar band gap to TiO2 and showslower electron-hole (eminus-h+) recombination probability dueto its filled valence band (119864VB)(3119889

10) and s-p hybridization

properties of conduction band (119864CB) [2] The less populationof deep traps in ZnO nanostructures compared with TiO2nanoparticles induced the longer electron lifetime [2] Inspite of these superior physical properties of ZnO thephotoconversion efficiency of TiO2-based DSSC is generallymuch higher than ZnO This was mainly attributed to theacidity of the binding groups of dyes (eg N719 N3 andblack dyes etc) which reduced the chemical stability ofZnO by dissolving it to form a precipitation of dye-Zn2+complexes to lead electron injection and dye regenerationinefficiently [8] In general it is known that nanostructuressuch as nanorods nanofibers and nanowires induce lessrecombination to improve the charge collection efficiency[9ndash11] This work reports the synthesis of granular ZnOnanostructures by simple hydrothermal reactions of zincnitrate and KOH in the presence of tetra butyl ammoniumhexafluorophosphate (TBAPF6) The effects of the size ofZnO nanograin on the photovoltaic performances of DSSCswere studied in conjunction with their morphology as well asthe optical and the electronic properties

2 International Journal of Photoenergy

2 Experimental

Zinc nitrate [Zn(NO3)2sdot6H2O] (001mol) and KOH(002mol) were dissolved in 20mL of distilled water to forma white flocculate which was then diluted to 100mL Afterthe addition of 5 times 10minus3mol TBAPF6 while stirring it wasrefluxed for 30 60 and 120min to obtain white precipitatesThe samples being collected by centrifuge and dried for30 hours at room temperature after washing with waterand ethanol were sintered in an electric muffle furnaceat 500∘C for 30min in ambient condition before furthercharacterization They were denoted as Z1 Z2 and Z3respectively depending on the reflux time

Scanning electron microscopy (SEM Hitachi S-3000NJapan) was used to investigate the ZnO nanograinrsquos surfacemorphologies X-ray diffractometry (Philips Xrsquopert Nether-land)was conducted usingCuK120572 radiation (120582 = 015406 nm)over a 2120579 scan range of 20∘ndash80∘ Optical absorption andphotoluminescence emission spectra of the ZnO dispersedin ethanol were obtained using a UV-Vis spectrophotometer(Scinco S-3100 Republic of Korea) and spectrofluorometer(Horiba Flourolog-3 USA) The amount of dye adsorptionson photoelectrodes was measured from the absorption spec-tra of desorbed-dye solutions as reported in the literature [12]

ZnO paste was prepared by mixing ZnO powderwhich was pretreated by acetyl-acetone to minimize aggre-gation with ethanol terpineol and 10 ethyl celluloseas reported elsewhere [10 11] The ZnO films on FTO(Pilkington 8Ωsq) were prepared by doctor blading andwere sintered in an electric muffle furnace at 500∘Cunder ambient condition They were immersed in ethanolicsolution of 04mM cis-diisothiocyanato-bis (221015840-bipyridyl-441015840-dicarboxylato) ruthenium(II) bis-tetrabutylammonium(N719) for 12 h Counter electrodes were prepared byspin coating of 5mM chloroplatinic acid hexahydrate(H2PtCl6sdot6H2O) in ethanol on FTO and by sintering it inan electric muffle furnace at 380∘C for 20min The dye-loaded photoelectrodes (active area ca 02 cm2) and pla-tinized counter electrodes were sandwiched with 50120583m thicksurlyn film as a spacer and sealing agent at 110∘C for 10minThe electrolyte solution composed of 06M 12-dimethyl-3-propylimidazolium iodide (DMPII) 01M LiI 01M I2 and05M 4-tert-butylpyridine (tBP) in 3-methoxypropionitrile(MPN) was injected into the cell through the drilled holeson the counter electrodeThey were sealed with a transparentscotch tape for temporal sealing

An AM15 solar simulator with a 200W Xenon lamp(Polaronix K201 McScience Republic of Korea) was usedto illuminate the cells whose current density-voltage (J-V)properties were measured using a photovoltaic power meter(Polaronix K101 LAB20 McScience Republic of Korea) Theincident light intensity was adjusted to 100mWcmminus2 (1 sun)by a standard mono-Si solar cell (PVM 396 PV Measure-ment Inc USA) certified by the US National RenewableEnergy Laboratory Cyclic voltammetric (CV) experimentwas performed with a CHI430A electrochemical workstation(CH instruments Inc USA) ZnO nanograins on FTO aplatinumwire and a AgAg+ electrode were used as working

counter and reference electrodes respectively Electrochemi-cal impedance spectra (EIS)were obtained under open circuitand dark conditions at frequencies of 105ndash01Hz with a 5mVac amplitude (IM6ex Zahner-Elektrik GmbH amp Co KGGermany)Themeasured spectra were fitted to an equivalentcircuit appropriate for DSSCs using Zview software (version31 Scribner Associates Inc USA)

3 Results and Discussion

The SEM images showed that the average size of ZnOnanograins increased slowly with reflux time by reducingthe population of smaller size grains while the aspect ratio(ca 175) was almost unchanged The lengths of the grainsranged from ca 50 to 750 nm while the upper limit of thegrain length was ca 350 500 and 750 nm for Z1 Z2 andZ3 respectively (Figure 1(a)) The XRD patterns of all thesynthesized ZnO nanograins were consistent with the single-crystalline wurtzite (hexagonal-phase) with no impuritiessuch as Zn and Zn(OH)2 All the diffraction peaks were wellindexed to the hexagonal phase of ZnO reported in JCPDScard (no 36-1451) (Figure 1(b)(A))

All the absorption maxima (120582max) observed at ca 362376 and 378 nm for Z1 Z2 and Z3 respectively were blue-shifted relative to the bandgap energy (119864119892) of bulk ZnO[2] with the much broader absorption edges for Z1 (InsetFigure 1(b)(B)) The lowest value of the 120582max of Z1 indicatedthe larger 119864119892 and the positive shift of quasi-Fermi level (119864Fq)compared to those of Z2 and Z3 [13] Photoluminescenceemission spectra recorded at room temperature under exci-tation at 350 nm for Z1 and 360 nm for Z2 and Z3 clearlyshowed the strong UV emissions peaked at ca 368 nm (Z1)and ca 385 nm (bothZ2 andZ3) (Figure 1(b)(B))which couldbe attributed to the direct and instant radiative recombinationof excitons (RR0) (Scheme 1) The very broad and weakemission observed at ca 565 nm for Z1 was attributed to theother radiative recombination (RR1) through the defect levelspossibly caused by vacancies in the interstitial site of zinc andoxygen [14] as described in Scheme 1 Holes form the valenceband edge (119864VB) of ZnO recombinedwith electron by tunnel-ing through these defect levelsThe significantly low emissionintensities in the visible range indicated little density of defectlevels for Z2 and Z3 compared with Z1 [15ndash18] It indicatedthat the Z1 has the high probability of recombination ofexcitons underUV illuminationNevertheless the photoelec-trode constructed with Z1 showed the better performance inDSSCs as shown in the photocurrent density-voltage (J-V)characteristics (Figure 2(a) and Table 1) It was partly becausethere was no exciton generation in Z1 nanograins duringthe operation of DSSC and the source of electron was fromdyes which were mostly excited by visible light under AM15 conditionTherefore the existence of these intrinsic defectlevels of Z1 nanograins did not contributemuch to reduce thecell performance because the recombination of these electroswas more significantly involved with the highest occupiedmolecular orbital (HOMO) of dye which is energetically

International Journal of Photoenergy 3

Z1

Z2

Z3

(a)

20 30 40 50 60 70 80

Inte

nsity

(au

)

(100

)(0

02)

(101

)

(102

)

(110

)

(103

)(2

00)

(112

)(2

01)

(004

)

(202

)

Z1

Z2

Z3

2120579 (deg)

400 500 600 7000

005

01

015

02

Wavelength (nm)

Z1Z2

Z3

Z1Z2

Z3

250 300 350 400 450

1

15

2

25

Abso

rban

ce (a

u)

Wavelength (nm)

Bulk band gap wavelength

Inte

nsitytimes108

(CPS

)

(B)

(A)

(b)

Figure 1 (a) Scanning electron microscopic (SEM) images (on FTO) and (b) X-ray diffraction (XRD) pattern (A) and photoluminescencespectra in ethanol (inset the corresponding UV-visible absorption spectra) (B) of Z1 Z2 and Z3 grains

muchmore closer to the conduction band edge (119864CB) of ZnOthan these intrinsic defect levels of Z1 nanograins (Scheme 1)

The overall energy conversion efficiency with Z1 was ca182 and it was up to 11 and 152 higher than that ofZ2 and Z3 respectively The higher efficiency for Z1 wasmainly attributed to the higher open circuit potential (119881oc)compared to Z2 and Z3 Meanwhile the short circuit currentdensity (119869sc) was not affected significantly even though thedye loading for Z1 was higher up to ca 268 and 4142than those of Z2 and Z3 respectively (Table 1) It could beattributed to the scattering effect with bigger grains [19]The variation of 119881oc in DSSC was known to be dependentsignificantly on the distribution and density of surface statesin the mesoporous structure of photoelectrode which was

investigated by monitoring the nonfaradic capacitive currentflow at these electrodes The capacitive charging currentappeared in the forward scans in CVs (Figure 2(b)) couldbe interpreted as the filling of these surface trap states aspreviously studied in similar systems based onTiO2 nanopar-ticles [20ndash22] and the current essentially reached zero duringreverse scans to sufficiently positive potentials because of theregeneration of these surface states by dischargingThehighervalues of capacitive currents for Z2 and Z3 compared to thoseof the Z1 indicated the higher density of such trap states asdescribed in Scheme 1 It was found from the density of states(DOS) calculation [22] that they distributed exponentiallywith the onset potential being dependent on the size of ZnOgrain (Figure 2(c) Table 1) The downshift of the onset of the


00

1

2

3

4

5

Potential (V)minus02 minus04 minus06

Curr

ent d

ensit

y J (

mA

cm2)

Z1Z2

Z3

(a)

0 02

0

03

Z1Z2

Z3

minus12

minus09

minus06

minus03

Curr

ent d

ensit

y J (

mA

cm2)

minus02minus04minus06minus08119864 (V versus AgAg+)

(b)

0

02

0

DOS (au)1 times 1016 2 times 1016 3 times 1016 4 times 1016 5 times 1016

Z1Z2

Z3

minus02

minus04

minus06

minus08

119864(V

ver

sus A

gA

g+)

(c)

01 1 10 100 1000 10000 1000000

Frequency (Hz)

minus10

minus20

minus30

minus40

minus50

minus60 120596max

120579

Z1Z2

Z3

(d)

Figure 2 (a) Current density-voltage (J-V) characteristics of DSSCs with photoelectrodes of Z1 Z2 and Z3 grains (b) Cyclic voltammograms(CVs) of the photoelectrodes in 05M LiClO

4dissolved in acetonitrile at a scan rate of 50mVs (c)The exponential distribution of the density

of states (DOS) of different ZnO nanograins was deduced by capacitive charging of electrons into the surface states versus applied potentialand (d) the Bode plots from electrochemical impedance spectroscopy of DSSCs

Table 1 Photovoltaic and kinetic parameters of Z1 Z2 and Z3 DSSCs photoelectrodes along with their amounts of adsorbed dye and theonset potential for exponential distribution of the density of states (DOS)

Electrode 119869sc(mAcm2) 119881oc (V) FF () 120578 () Adsorbed dye

(molcm2) 119896119885119864

(sminus1) Onset potential(eV versus Vacuum)

Z1 46 073 5408 182 577 times 10minus7 558 445Z2 446 070 5217 164 455 times 10minus7 710 476Z3 444 066 5308 158 408 times 10minus7 1685 487

state distribution was consistent with the tendency of thenegative shift of the pseudo-Fermi level of the photoelectrode(Scheme 1) [23] which was responsible for the higher recom-bination rate to induce the decrease of 119881oc The electrontransfer kinetics measured by electrochemical impedance

spectroscopy (EIS) at the ZnO|electrolyte interface underdark condition (Figure 2(d)) was also consistent with thistrend by showing the lowest value of the back elec-tron transfer rate to I3

minus (119896119885119864) for Z1 as summarized inTable 1


Defect level

Surfa

ce st

ates

Surfa

ce st

ates

Surfa

ce st

ates

RR0

ZnO in solution

ZnO as photoanode in DSSCs

119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus6

119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus8

minus7

minus6

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864(IminusI3minus) 119864(IminusI3

minus) 119864(IminusI3minus)

119864CB

eminuseminus eminus eminus

Z1 Z2 Z3

Z1 Z2 Z3

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

eminus eminus

119864CB

119864VB 119864VB 119864VBh+ h+

ℎℎ

ℎ

ℎ

h+ h+h+ h+

RR0

119864CB 119864CB

RR0h+

RR1

119864119892

119864119892119864119892

Scheme 1 Schematic illustration of the possible relaxation pathways of photoexcited electron in pure Z1 Z2 and Z3 (upper panel) and Z1Z2 and Z3 photoelectrodes with energy levels of surface states or traps dye and electrolyte in DSSCs (lower panel) RR0 and RR1 and 119864FTOare the instant radiative recombination of excitons (RR0) radiative recombination of electron with holes by tunneling through these defectlevels (RR1) and the Fermi energy of FTO The shaded region in the lower panel shows the exponential distribution of the surface states

4 Conclusions

Crystalline ZnO nanograins being synthesized by afacile hydrothermal method in aqueous system wereused as photoanodes in DSSCs It was found that somephysicochemical properties of nanograins such as the densityand the distribution of intrinsic defects and surface stateswere major factors to control many optoelectronic propertiessuch as quasi-Fermi level and radiative recombinationprobabilities They were highly dependent on the size ofgrains and this also affected the performance of DSSCswhen used for photoelectrode Even though the maximumconversion efficiency obtained was 182 with the grainsize of ca 250 nm (Z1) it showed an improved performancecompared with similar previously reported system basedon ZnO nanostructures [24ndash27] It was expected that theuse of ZnO nanograin can offer a promising strategy toenhance the performance of DSSCs and further optimizationby controlling the size and aspect ratio to enhance the cellperformance is still under investigation

Conflict of Interests

The authors declare that they have no conflict of interests

Acknowledgments

This work was supported by the National Research Founda-tion of Korea (NRF) grant funded by the government of theRepublic of Korea (MEST) (NRF-20100026916) and by theConverging Research Centre Program through the Ministryof Education Science and Technology (2012K001287) and bySeoul RampBD Program (WR090671)

References

[1] B Wang S Nagase J Zhao and G Wang ldquoStructural growthsequences and electronic properties of zinc oxide clusters(ZnO)

119899(n = 2ndash18)rdquo Journal of Physical Chemistry C vol 111 no

13 pp 4956ndash4963 2007[2] J J Lee M M Rahman S Sarker N C Deb Nath A J S

Ahammad and J K Lee ldquoMetal oxides and their composites for


the photoelectrode of dye sensitized solar cellsrdquo in CompositeMaterials for Medicine and Nanotechnology B Attaf Ed pp181ndash210 InTech Rijeka Croatia 2011

[3] MMakkar andH S Bhatti ldquoInquisition of reaction parameterson the growth and optical properties of ZnO nanoparticlessynthesized via low temperature reaction routerdquo ChemicalPhysics Letters vol 507 no 1ndash3 pp 122ndash127 2011

[4] Y P Du Y W Zhang L D Sun and C H Yan ldquoEfficientenergy transfer in monodisperse Eu-doped ZnO nanocrystalssynthesized from metal acetylacetonates in high-boiling sol-ventsrdquo Journal of Physical Chemistry C vol 112 no 32 pp12234ndash12241 2008

[5] M M Rahman A J S Ahammad J H Jin S J Ahn and J JLee ldquoA comprehensive review of glucose biosensors based onnanostructured metal-oxidesrdquo Sensors vol 10 no 5 pp 4855ndash4886 2010

[6] J I Sohn S S Choi S M Morris et al ldquoNovel nonvolatilememory with multibit storage based on a ZnO nanowiretransistorrdquo Nano Letters vol 10 no 11 pp 4316ndash4320 2010

[7] V Chivukula D Ciplys M Shur and P Dutta ldquoZnO nanopar-ticle surface acoustic wave UV sensorrdquo Applied Physics Lettersvol 96 no 23 Article ID 233512 2010

[8] M Quintana T Edvinsson A Hagfeldt and G BoschlooldquoComparison of dye-sensitized ZnO and TiO

2solar cells

studies of charge transport and carrier lifetimerdquo Journal ofPhysical Chemistry C vol 111 no 2 pp 1035ndash1041 2007

[9] M H Lai A Tubtimtae M W Lee and G J Wang ldquoZnO-nanorod dye-sensitized solar cells new structure without atransparent conducting oxide layerrdquo International Journal ofPhotoenergy vol 2012 Article ID 497095 5 pages 2012

[10] M M Rahman H S Son S S Lim K H Chung and J JLee ldquoEffect of the TiO

2nanotubes in the photoelectrode on

efficiency of dye-sensitized solar cellrdquo Journal of ElectrochemicalScience and Technology vol 2 pp 110ndash115 2011

[11] K H Chung M M Rahman H S Son and J J LeeldquoDevelopment of well-aligned TiO

2nanotube arrays to improve

electron transport in dye-sensitized solar cellsrdquo InternationalJournal of Photoenergy vol 2012 Article ID 215802 6 pages2012

[12] H S Uam Y S Jung Y Jun and K J Kim ldquoRelation ofRu(II) dye desorption from TiO

2film during illumination with

photocurrent decrease of dye-sensitized solar cellsrdquo Journal ofPhotochemistry and Photobiology A vol 212 no 2-3 pp 122ndash128 2010

[13] N S Pesika K J Stebe andPC Searson ldquoRelationship betweenabsorbance spectra and particle size distributions for quantum-sized nanocrystalsrdquo Journal of Physical Chemistry B vol 107 no38 pp 10412ndash10415 2003

[14] Y S Kim and C H Park ldquoRich variety of defects in ZnO via anattractive interaction between O vacancies and Zn interstitialsorigin of n-type dopingrdquo Physical Review Letters vol 102 no 8Article ID 086403 2009

[15] T Tatsumi M Fujita N Kawamoto M Sasajima and YHorikoshi ldquoIntrinsic defects in ZnO films grown by molecularbeam epitaxyrdquo Japanese Journal of Applied Physics A vol 43 no5 pp 2602ndash2606 2004

[16] F Wen W Li J H Moon and J H Kim ldquoHydrothermalsynthesis of ZnO Zn with green emission at low temperaturewith reduction processrdquo Solid State Communications vol 135pp 34ndash37 2005

[17] K Vanheusden W L Warren C H Seager D R TallantJ A Voigt and B E Gnade ldquoMechanisms behind green

photoluminescence in ZnO phosphor powdersrdquo Journal ofApplied Physics vol 79 no 10 pp 7983ndash7990 1996

[18] B K Sharma N Khare and D Haranath ldquoPhotoluminescencelifetime of Al-doped ZnO films in visible regionrdquo Solid StateCommunications vol 150 pp 2341ndash2345 2010

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

2nanotube arraysrdquo Solar EnergyMaterials amp

Solar Cells vol 95 pp 184ndash189 2011[20] A Hagfeld and M Gratzel ldquoLight-induced redox reactions in

nanocrystalline systemsrdquo Chemical Reviews vol 95 pp 49ndash681995

[21] J Moser S Punchihewa P P Infelta and M Gratzel ldquoSurfacecomplexation of colloidal semiconductors strongly enhancesinterfacial electron-transfer ratesrdquo Langmuir vol 7 no 12 pp3012ndash3018 1991

[22] N C D Nath S Sarker A J S Ahammad and J J Lee ldquoSpatialarrangement of carbon nanotubes in TiO

2photoelectrodes to

enhance the efficiency of dye-sensitized solar cellsrdquo PhysicalChemistry Chemical Physics vol 14 pp 4333ndash4338 2012

[23] J Bisquert F Fabregat-Santiago I Mora-Sero G Garcia-Belmonte E M Barea and E Palomares ldquoA review of recentresults on electrochemical determination of the density of elec-tronic states of nanostructured metal-oxide semiconductorsand organic hole conductorsrdquo Inorganica Chimica Acta vol 361no 3 pp 684ndash698 2008

[24] J B Baxter and E S Aydil ldquoNanowire-based dye-sensitizedsolar cellsrdquo Applied Physics Letters vol 86 no 5 Article ID053114 2005

[25] A Umar ldquoGrowth of comb-like ZnO nanostructures for Dye-sensitized solar cells applicationsrdquo Nanoscale Research Lettersvol 4 no 9 pp 1004ndash1008 2009

[26] J J Wu G R Chen H H Yang C H Ku and J Y Lai ldquoEffectsof dye adsorption on the electron transport properties in ZnOnanowire dye-sensitized solar cellsrdquoApplied Physics Letters vol90 no 21 Article ID 213109 2007

[27] A Pandikumar K M Saranya and R Ramaraj ldquoSheaf-like-ZnO Ag nanocomposite materials modified photoanode forlow-cost metal-free organic dye-sensitized solid-state solarcellsrdquo Applied Physics Letters vol 101 no 9 Article ID 0931122012

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2 Experimental

Zinc nitrate [Zn(NO3)2sdot6H2O] (001mol) and KOH(002mol) were dissolved in 20mL of distilled water to forma white flocculate which was then diluted to 100mL Afterthe addition of 5 times 10minus3mol TBAPF6 while stirring it wasrefluxed for 30 60 and 120min to obtain white precipitatesThe samples being collected by centrifuge and dried for30 hours at room temperature after washing with waterand ethanol were sintered in an electric muffle furnaceat 500∘C for 30min in ambient condition before furthercharacterization They were denoted as Z1 Z2 and Z3respectively depending on the reflux time

Scanning electron microscopy (SEM Hitachi S-3000NJapan) was used to investigate the ZnO nanograinrsquos surfacemorphologies X-ray diffractometry (Philips Xrsquopert Nether-land)was conducted usingCuK120572 radiation (120582 = 015406 nm)over a 2120579 scan range of 20∘ndash80∘ Optical absorption andphotoluminescence emission spectra of the ZnO dispersedin ethanol were obtained using a UV-Vis spectrophotometer(Scinco S-3100 Republic of Korea) and spectrofluorometer(Horiba Flourolog-3 USA) The amount of dye adsorptionson photoelectrodes was measured from the absorption spec-tra of desorbed-dye solutions as reported in the literature [12]

ZnO paste was prepared by mixing ZnO powderwhich was pretreated by acetyl-acetone to minimize aggre-gation with ethanol terpineol and 10 ethyl celluloseas reported elsewhere [10 11] The ZnO films on FTO(Pilkington 8Ωsq) were prepared by doctor blading andwere sintered in an electric muffle furnace at 500∘Cunder ambient condition They were immersed in ethanolicsolution of 04mM cis-diisothiocyanato-bis (221015840-bipyridyl-441015840-dicarboxylato) ruthenium(II) bis-tetrabutylammonium(N719) for 12 h Counter electrodes were prepared byspin coating of 5mM chloroplatinic acid hexahydrate(H2PtCl6sdot6H2O) in ethanol on FTO and by sintering it inan electric muffle furnace at 380∘C for 20min The dye-loaded photoelectrodes (active area ca 02 cm2) and pla-tinized counter electrodes were sandwiched with 50120583m thicksurlyn film as a spacer and sealing agent at 110∘C for 10minThe electrolyte solution composed of 06M 12-dimethyl-3-propylimidazolium iodide (DMPII) 01M LiI 01M I2 and05M 4-tert-butylpyridine (tBP) in 3-methoxypropionitrile(MPN) was injected into the cell through the drilled holeson the counter electrodeThey were sealed with a transparentscotch tape for temporal sealing

An AM15 solar simulator with a 200W Xenon lamp(Polaronix K201 McScience Republic of Korea) was usedto illuminate the cells whose current density-voltage (J-V)properties were measured using a photovoltaic power meter(Polaronix K101 LAB20 McScience Republic of Korea) Theincident light intensity was adjusted to 100mWcmminus2 (1 sun)by a standard mono-Si solar cell (PVM 396 PV Measure-ment Inc USA) certified by the US National RenewableEnergy Laboratory Cyclic voltammetric (CV) experimentwas performed with a CHI430A electrochemical workstation(CH instruments Inc USA) ZnO nanograins on FTO aplatinumwire and a AgAg+ electrode were used as working

counter and reference electrodes respectively Electrochemi-cal impedance spectra (EIS)were obtained under open circuitand dark conditions at frequencies of 105ndash01Hz with a 5mVac amplitude (IM6ex Zahner-Elektrik GmbH amp Co KGGermany)Themeasured spectra were fitted to an equivalentcircuit appropriate for DSSCs using Zview software (version31 Scribner Associates Inc USA)

3 Results and Discussion

The SEM images showed that the average size of ZnOnanograins increased slowly with reflux time by reducingthe population of smaller size grains while the aspect ratio(ca 175) was almost unchanged The lengths of the grainsranged from ca 50 to 750 nm while the upper limit of thegrain length was ca 350 500 and 750 nm for Z1 Z2 andZ3 respectively (Figure 1(a)) The XRD patterns of all thesynthesized ZnO nanograins were consistent with the single-crystalline wurtzite (hexagonal-phase) with no impuritiessuch as Zn and Zn(OH)2 All the diffraction peaks were wellindexed to the hexagonal phase of ZnO reported in JCPDScard (no 36-1451) (Figure 1(b)(A))

All the absorption maxima (120582max) observed at ca 362376 and 378 nm for Z1 Z2 and Z3 respectively were blue-shifted relative to the bandgap energy (119864119892) of bulk ZnO[2] with the much broader absorption edges for Z1 (InsetFigure 1(b)(B)) The lowest value of the 120582max of Z1 indicatedthe larger 119864119892 and the positive shift of quasi-Fermi level (119864Fq)compared to those of Z2 and Z3 [13] Photoluminescenceemission spectra recorded at room temperature under exci-tation at 350 nm for Z1 and 360 nm for Z2 and Z3 clearlyshowed the strong UV emissions peaked at ca 368 nm (Z1)and ca 385 nm (bothZ2 andZ3) (Figure 1(b)(B))which couldbe attributed to the direct and instant radiative recombinationof excitons (RR0) (Scheme 1) The very broad and weakemission observed at ca 565 nm for Z1 was attributed to theother radiative recombination (RR1) through the defect levelspossibly caused by vacancies in the interstitial site of zinc andoxygen [14] as described in Scheme 1 Holes form the valenceband edge (119864VB) of ZnO recombinedwith electron by tunnel-ing through these defect levelsThe significantly low emissionintensities in the visible range indicated little density of defectlevels for Z2 and Z3 compared with Z1 [15ndash18] It indicatedthat the Z1 has the high probability of recombination ofexcitons underUV illuminationNevertheless the photoelec-trode constructed with Z1 showed the better performance inDSSCs as shown in the photocurrent density-voltage (J-V)characteristics (Figure 2(a) and Table 1) It was partly becausethere was no exciton generation in Z1 nanograins duringthe operation of DSSC and the source of electron was fromdyes which were mostly excited by visible light under AM15 conditionTherefore the existence of these intrinsic defectlevels of Z1 nanograins did not contributemuch to reduce thecell performance because the recombination of these electroswas more significantly involved with the highest occupiedmolecular orbital (HOMO) of dye which is energetically


Z1

Z2

Z3

(a)

20 30 40 50 60 70 80

Inte

nsity

(au

)

(100

)(0

02)

(101

)

(102

)

(110

)

(103

)(2

00)

(112

)(2

01)

(004

)

(202

)

Z1

Z2

Z3

2120579 (deg)

400 500 600 7000

005

01

015

02

Wavelength (nm)

Z1Z2

Z3

Z1Z2

Z3

250 300 350 400 450

1

15

2

25

Abso

rban

ce (a

u)

Wavelength (nm)


Inte

nsitytimes108

(CPS

)

(B)

(A)

(b)






00

1

2

3

4

5


Curr

ent d

ensit

y J (

mA

cm2)

Z1Z2

Z3

(a)

0 02

0

03

Z1Z2

Z3

minus12

minus09

minus06

minus03

Curr

ent d

ensit

y J (

mA

cm2)


(b)

0

02

0


Z1Z2

Z3

minus02

minus04

minus06

minus08

119864(V

ver

sus A

gA

g+)

(c)

01 1 10 100 1000 10000 1000000

Frequency (Hz)

minus10

minus20

minus30

minus40

minus50

minus60 120596max

120579

Z1Z2

Z3

(d)






(molcm2) 119896119885119864







Defect level

Surfa

ce st

ates

Surfa

ce st

ates

Surfa

ce st

ates

RR0

ZnO in solution


119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus6

119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus8

minus7

minus6

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast



119864CB


Z1 Z2 Z3

Z1 Z2 Z3

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

eminus eminus

119864CB

119864VB 119864VB 119864VBh+ h+

ℎℎ

ℎ

ℎ

h+ h+h+ h+

RR0

119864CB 119864CB

RR0h+

RR1

119864119892

119864119892119864119892


4 Conclusions




Acknowledgments


References













2solar cells

























2photoelectrodes to
















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Z1

Z2

Z3

(a)

20 30 40 50 60 70 80

Inte

nsity

(au

)

(100

)(0

02)

(101

)

(102

)

(110

)

(103

)(2

00)

(112

)(2

01)

(004

)

(202

)

Z1

Z2

Z3

2120579 (deg)

400 500 600 7000

005

01

015

02

Wavelength (nm)

Z1Z2

Z3

Z1Z2

Z3

250 300 350 400 450

1

15

2

25

Abso

rban

ce (a

u)

Wavelength (nm)


Inte

nsitytimes108

(CPS

)

(B)

(A)

(b)






00

1

2

3

4

5


Curr

ent d

ensit

y J (

mA

cm2)

Z1Z2

Z3

(a)

0 02

0

03

Z1Z2

Z3

minus12

minus09

minus06

minus03

Curr

ent d

ensit

y J (

mA

cm2)


(b)

0

02

0


Z1Z2

Z3

minus02

minus04

minus06

minus08

119864(V

ver

sus A

gA

g+)

(c)

01 1 10 100 1000 10000 1000000

Frequency (Hz)

minus10

minus20

minus30

minus40

minus50

minus60 120596max

120579

Z1Z2

Z3

(d)






(molcm2) 119896119885119864







Defect level

Surfa

ce st

ates

Surfa

ce st

ates

Surfa

ce st

ates

RR0

ZnO in solution


119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus6

119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus8

minus7

minus6

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast



119864CB


Z1 Z2 Z3

Z1 Z2 Z3

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

eminus eminus

119864CB

119864VB 119864VB 119864VBh+ h+

ℎℎ

ℎ

ℎ

h+ h+h+ h+

RR0

119864CB 119864CB

RR0h+

RR1

119864119892

119864119892119864119892


4 Conclusions




Acknowledgments


References













2solar cells

























2photoelectrodes to
















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


00

1

2

3

4

5


Curr

ent d

ensit

y J (

mA

cm2)

Z1Z2

Z3

(a)

0 02

0

03

Z1Z2

Z3

minus12

minus09

minus06

minus03

Curr

ent d

ensit

y J (

mA

cm2)


(b)

0

02

0


Z1Z2

Z3

minus02

minus04

minus06

minus08

119864(V

ver

sus A

gA

g+)

(c)

01 1 10 100 1000 10000 1000000

Frequency (Hz)

minus10

minus20

minus30

minus40

minus50

minus60 120596max

120579

Z1Z2

Z3

(d)






(molcm2) 119896119885119864







Defect level

Surfa

ce st

ates

Surfa

ce st

ates

Surfa

ce st

ates

RR0

ZnO in solution


119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus6

119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus8

minus7

minus6

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast



119864CB


Z1 Z2 Z3

Z1 Z2 Z3

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

eminus eminus

119864CB

119864VB 119864VB 119864VBh+ h+

ℎℎ

ℎ

ℎ

h+ h+h+ h+

RR0

119864CB 119864CB

RR0h+

RR1

119864119892

119864119892119864119892


4 Conclusions




Acknowledgments


References













2solar cells

























2photoelectrodes to
















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Defect level

Surfa

ce st

ates

Surfa

ce st

ates

Surfa

ce st

ates

RR0

ZnO in solution


119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus6

119864(e

V) (

Vacu

um)

minus3

minus4

minus5

minus8

minus7

minus6

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast



119864CB


Z1 Z2 Z3

Z1 Z2 Z3

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

FTO

119864FTO

119864119865119902

119881oc

1198630119863+

eminus

eminus

119863+119863lowast

119864CB

eminus eminus

119864CB

119864VB 119864VB 119864VBh+ h+

ℎℎ

ℎ

ℎ

h+ h+h+ h+

RR0

119864CB 119864CB

RR0h+

RR1

119864119892

119864119892119864119892


4 Conclusions




Acknowledgments


References













2solar cells

























2photoelectrodes to
















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of









2solar cells

























2photoelectrodes to
















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Research Article A Facile Synthesis of Granular ZnO ...

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