the effect of zno-coating on the performanceof a

Available online at www.sciencedirect.com

www.elsevier.com/locate/solener

Solar Energy xxx (2012) xxx–xxx

The effect of ZnO-coating on the performanceof a dye-sensitized solar cell

Chuen-Shii Chou a,b,⇑, Feng-Cheng Chou b, Yi-Geng Ding b, Ping Wu c

a Research Center of Solar Photo-Electricity Applications, National Pingtung University of Science and Technology, Pingtung 912, Taiwanb Powder Technology R&D Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology,

Pingtung 912, Taiwanc Engineering Product Development, Singapore University of Technology and Design, 20 Dover Drive, Singapore 138682, Singapore

Received 30 September 2011; received in revised form 3 January 2012; accepted 3 February 2012

Communicated by: Associate Editor Sam-Shajin sun

Abstract

This study investigates the effect of a ZnO-coated TiO2 working electrode on the power conversion efficiency of a dye-sensitized solarcell (DSSC). This electrode was designed and fabricated by dipping the TiO2 electrode with the TiCl4 treatment in a solution of zincacetate dehydrate [Zn(CH3COO)2�2H2O] and ethanol. The effects of the concentration of Zn(CH3COO)2�2H2O and the duration of dip-ping on the band gap of a working electrode and the power conversion efficiency of a DSSC were also examined. The band gap of theworking electrode increases to 3.75 eV [TiO2 electrode dipped in 0.05 M Zn(CH3COO)2�2H2O) for 3 min] from 3.22 eV (TiO2 electrode).Interestingly, the power conversion efficiency of the DSSC with a Zn-coated TiO2 electrode (6.7%) substantially exceeds that of the con-ventional DSSC with a TiO2 electrode (5.9%), and it may be originated from an increased energy barrier between ZnO and TiO2 thatreduces the electron recombination rate.� 2012 Elsevier Ltd. All rights reserved.

Keywords: Dye-sensitized solar cell; ZnO-coated TiO2 electrode; Band gap; ZnO energy barrier; Power conversion efficiency

1. Introduction

In order to relieve the energy crises, environment pollu-tion and global warming, the development of accessiblerenewable energy productions has received substantialattention. Solar power is the most noteworthy amongrenewable and sustainable energy resources because it is aclean and unlimited resource of energy and has the globalavailability. In recent years, several alternatives to Si-basedsolar cells or photovoltaic (PV) cells have becomeavailable, and considerable research is ongoing towards

0038-092X/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2012.02.003

⇑ Corresponding author at: Research Center of Solar Photo-ElectricityApplications, National Pingtung University of Science and Technology,Pingtung 912, Taiwan. Tel.: +886 8 7703202x7016; fax: +886 8 7740142.

E-mail address: [email protected] (C.-S. Chou).

Please cite this article in press as: Chou, C.-S. et al. The effect of ZnO-coatindoi:10.1016/j.solener.2012.02.003

substantially reducing the cost of electricity generation(Thavasi et al., 2009). The dye-sensitized solar cell (DSSC),as proposed by O’Regan and Gratzel (1991), is an attrac-tive alternative because of its properties, such as low pro-duction cost, the various choices of substrates that can beused, and low environmental impact during fabrication(Lewis, 2007; Kim et al., 2008; Caramori et al., 2010).However, a comparison with conventional solid-sate junc-tion devices made of crystalline silicon indicates that theDSSC has lower power conversion efficiency (Chou et al.,2008).

Decreasing the recombination of electrons in the dye orelectrolyte may be one of promising approaches to enhancethe performance of a DSSC. However, research on decreas-ing the recombination of electrons in the dye or electrolyte

g on the performance of a dye-sensitized solar cell. Sol. Energy (2012),

http://dx.doi.org/10.1016/j.solener.2012.02.003

mailto:[email protected]



Fig. 1. Schematic of the DSSC with a ZnO-coated TiO2 electrode.

Table 1Test conditions of preparing TiCl4 and Zn(CH3COO)2�2H2O solutions.

Material Mass (g) Solution mol/L

B1 TiCl4 0.95 100 mL DI water 0.05B2 Zn(CH3COO)2�2H2O 0.56 50 mL ethanol 0.05B3 0.056 0.005

2 C.-S. Chou et al. / Solar Energy xxx (2012) xxx–xxx

by creating an energy barrier in the working electrode hasbeen less studied. For example, Su et al. (2007) fabricatedAu nanoparticles layer-by-layer onto the working electrodeas a Schottky barrier in a water-based DSSC using chemi-cal reduction method. Chou et al. (2009) investigated theuse of a layer of TiO2/Au (or TiO2/Ag) composite particlesas a Schottky barrier, and Chou et al. (2011) presented alayer of TiO2/NiO composite particles as a n–p junctionelectrode. Further, the p-type oxide semiconductors, suchas NiO (Sumikura et al., 2008a), CuAlO2 (Bandara andYasomanee, 2007), and CuO (Sumikura et al., 2008b), weresuggested as a hole collector and a barrier for chargerecombination. The n–p junction electrodes, such asSnO2/NiO (Bandara et al., 2004) and TiO2/NiO (Bandaraet al., 2005), were fabricated using the chemical processto promote the performance of the DSSC, and (Bandaraet al., 2005) with researchers claimed that the efficiency ofthe TiO2/NiO solar cell increases 30% more than that ofthe bare TiO2. Accordingly, reducing the electron recombi-nation in the dye (or electrolyte) becomes an importantissue in increasing the power conversion efficiency of aDSSC.

Aside from this, Wu et al. (2008) fabricated DSSCs basedon ZnO-coated TiO2 electrodes by RF magnetron sputter-ing, and the power conversion efficiency was increased from4.76% to 6.55%. However, its highly efficient TiO2/ZnO bi-phase working electrode was obtained using an expensivevacuum technology that required a sophisticated processcontrol. In contrast, this study used a simple dip coatingmethod to fabricate a ZnO-coated TiO2 electrode byimmersing a FTO-glass (Fluorine doped tin oxide, SnO2:F)substrate with a TiO2 film in the solution of zinc acetatedehydrate [Zn(CH3COO)2�2H2O] and ethanol. Effects ofthe concentration of Zn(CH3COO)2�2H2O and the durationof dipping on the band gap of a working electrode and thepower conversion efficiency of a DSSC were noted. A com-parison of a DSSC with the proposed working electrode(Fig. 1) with the conventional DSSC was also made.

2. Experimental details

The TiO2 particles (P-25) of 30% rutile and 70% anatasewith a particle size range (<100 nm) and the zinc acetatedehydrate [Zn(CH3COO)2�2H2O] with a purity of 98%were used in this study. Test conditions in the preparationof a solution of TiCl4 and the solution of zinc acetate dehy-drate [Zn(CH3COO)2�2H2O] and ethanol are presented inTable 1.

2.1. Preparing the working electrode

The procedure for fabricating a ZnO-coated TiO2 work-ing electrode of a DSSC (Fig. 1) consists of two stages. Thefirst stage of preparing this working electrode included thefollowing steps: (1) a FTO-glass substrate was immersed inthe 0.05 M solution of TiCl4 (Table 1) at a temperature of70 �C for 30 min; (2) the colloid of TiO2 particles (P-25)

Please cite this article in press as: Chou, C.-S. et al. The effect of ZnO-coatidoi:10.1016/j.solener.2012.02.003

was prepared by mixing 2 g TiO2 particles with 8 mL etha-nol, 0.8 mL acetylacetone, and 0.1 mL Triton X-100, andthe colloid was then homogenized in an ultrasonic homog-enizer for a certain duration; (3) the colloid of TiO2 parti-cles was deposited on top of a FTO-glass substrate with theTiCl4 treatment using the spin coating method; and (4) thissubstrate was then sintered at a temperature of 450 �C for1 h in a high-temperature furnace (Thermolyne, 46100)(Table 2).

The second stage of preparing this working electrodeincluded the following steps: (1) the surface of TiO2 filmon the FTO-glass substrate was modified by immersing thissubstrate in the solution of TiCl4 and sintered at a temper-ature of 450 �C for 1 h (Table 3); (2) this substrate was thenimmersed in the solution of zinc acetate dehydrate[Zn(CH3COO)2�2H2O] and ethanol at a temperature of25 �C for a preset duration of soaking, and the concentra-tion of solution used in this step was either 0.05 or0.005 mol/L (Tables 1 and 3); and (3) finally, this substratewas sintered at a temperature of 450 �C for 0.5 h (Table 3).Lu et al. (2010) prepared an electrode of ZnO nanoparticlessynthesized via a direct precipitation method, and theyindicated that an optimal annealing condition for a porous

ng on the performance of a dye-sensitized solar cell. Sol. Energy (2012),


Table 2Test conditions of first-stage preparing the working electrode.

First layer Second layer

Soaking Colloid No. of layer daubed by spin coating Sintering

Solution Time (min) Temp. (�C) Time (h) Temp. (�C)

C1 B1 30 70 TiO2 7 1 450

Table 3Test conditions of second-stage preparing working electrodes.

Substrate First layer Second layer

Soaking Sintering Soaking Sintering

Solution Time (min) Temp. (�C) Time (h) Temp. (�C) Solution Time (min) Temp. (�C) Time (h) Temp. (�C)

W1 – – – – –W2 B2 0.5W3 1.0W4 C1 B1 30 70 1 450 3.0 25 0.5 450W5 B3 0.5W6 1.0W7 3.0

C.-S. Chou et al. / Solar Energy xxx (2012) xxx–xxx 3

ZnO electrode for DSSCs was at a temperature of 400 �Cfor 1 h.

The FTO-glass substrate with a ZnO-coated TiO2 filmwas immersed into the (3 � 10�4 M) solution of N-719dye [Ruthenium, RuL2(NCS)2] and ethyl alcohol(CH3CH2OH, 95%) at a temperature of 70 �C for 6 h.Finally, this substrate was then rinsed using ethanol. Asidefrom this, a conventional working electrode with a nano-crystalline TiO2 film on the FTO-glass substrate was fabri-cated to demonstrate the feasibility and advantages of theworking electrode with a ZnO-coated TiO2 film. The areaof TiO2 electrode (or ZnO-coated TiO2 electrode) of DSSCwas 0.25 cm2.

2.2. Measuring the working electrode

Before immersing the substrate in the dye solution, themicrographs of a ZnO-coated TiO2 film on the FTO-glasssubstrate were obtained using a field emission scanningelectron microscope (JSM-6330). An energy dispersivespectrometer (Horiba EX-200) was utilized to analyze theweight ratios of the elements in the cross-section of ZnO-coated TiO2 film on the FTO-glass substrate and to obtainthe dispersion of Zn in the cross-section of film through theback-scattered electrons (BSEs) images. The transmissionof a TiO2 (or ZnO-coated TiO2) film, which had beendeposited on a FTO-glass substrate, was obtained usingan UV–VIS–NIR spectrophotometer (Jasco V-600). Theabsorption coefficient of the TiO2 (or ZnO-coated TiO2)film was given by

a ¼ lnð1=T Þd

ð1Þ

In Eq. (1), d and T are the film thickness and the trans-mission, respectively. Eq. (1) was obtained by simplifyingEq. (1) of Muth et al. (1997).


The band gap and the absorption coefficient of a TiO2

(or ZnO-coated TiO2) film were related by

ðahmÞ2 ¼ Bðhm� EgÞ ð2Þ

In Eq. (2), h, B, and m are Planck’s constant (6.63� 10�34 J s),an arbitrary constant, the band gap energy, and the light fre-quency, respectively. Further, the band gap energy of a TiO2

(or ZnO-coated TiO2) film was obtained by the following: (1)the absorption coefficient (a) was determined using Eq. (1); (2)a graph of (ahm)2 vs. hm was plotted; (3) a tangent line wasobtained by extrapolating the curve in the graph of (ahm)2 vs.hm; and (4) this tangent line was extended to intersect with x-axis,and the band gap energy was obtained at (ahm)2 = 0 according toEq. (2).

After immersing the substrate in the dye solution, ana-step (Dekeak 6 M) surface profiler was utilized to obtainthe average thickness of the film on the FTO-glass sub-strate. Table 4 lists the average thicknesses of the film onthe FTO-glass substrate in tests W1–W7.

2.3. Assembling and testing the DSSC

The dye-covered electrode with a TiO2 (or ZnO-coatedTiO2) film and the counter electrode obtained by deposit-ing a platinum (Pt) film on a FTO-glass substrate via anE-beam evaporator were assembled, such that the spacebetween two electrodes was adjusted to approximately25 lm for embarking the liquid electrolyte. The liquid elec-trolyte used herein was obtained by mixing 0.3 M Potas-sium Iodide (KI), 0.2 M Lithium Iodide (LiI), 0.05 MIodine (I2), 0.5 M 4-tert-butylpridine (TBP), and 0.6 M1,2-dimethyl-3-propylimidazolium iodine (DMPII) with3-propylene carbonate (PC).

An AM 1.5 solar simulator with a 300 W xenon lamp(San-Ei Pioneer of Light Technology, XES-310S) was used



Table 4Voc, Jsc, FF, and g.

Working electrode Voc (V) Jsc (mA/cm2) FF (%) Efficiency g (%)

Substrate Thickness (lm) No. of layer daubedby spin coating

D1 W1 10.6 0.70 15.4 54.3 5.9D2 W2 10.7 0.77 7.6 65.6 3.8D3 W3 10.8 0.85 4.0 62.7 2.2D4 W4 10.9 7 0.86 2.9 64.9 1.7D5 W5 10.7 0.75 13.5 65.4 6.7D6 W6 10.7 0.75 12.3 62.9 5.7D7 W7 10.8 0.77 9.0 65.3 4.5


to illuminate the DSSC, and the incident light powerwas calibrated to 100 mW/cm2. A digital source-meter(Keithley 2400) was used to measure the open-circuitphotovoltage and the short-circuit photocurrent of theDSSC. The detailed calculations of the power conversionefficiency g of the DSSC was described in our previousworks (Chou et al., 2009, 2011).

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig. 2. FE-SEM micrographs (50k�) of ZnO-coated TiO2 films (a, c, e, and g)ZnO-coated TiO2 films (b, d, f, and h).


3. Results and discussion

3.1. Characteristics of the ZnO-coated TiO2 electrode

The purpose of immersing a FTO-glass substrate in theTiCl4 solution before depositing the TiO2 (P-25) colloid isto prevent the FTO-glass substrate from dye (or

, and SEM micrographs (3.5k�) and EDS analyses of the cross-section of




electrolyte) contamination, which might penetrate throughthe cavities of the TiO2 (P-25) film. Ito et al. (2008)observed that TiCl4 treatment induced improvements inthe adhesion and mechanical strength of a nanocrystallineTiO2 layer. Aside from this, the purpose of immersing aFTO-glass substrate with a TiO2 film in the solution of zincacetate dehydrate [Zn(CH3COO)2�2H2O] and ethanol is tocreate an energy barrier. Kang et al. (2007) presented ZnO-coated TiO2 nano-tubes to improve the open-circuit volt-age and the power conversion efficiency of a DSSC andto retard any back reaction. Kao et al. (2009) as well asLi et al. (2010) applied sol–gel techniques to study theZnO-coated TiO2 electrode for DSSC.

Fig. 2 shows FE-SEM micrographs (50k�) of ZnO-coated TiO2 films as well as SEM micrographs (3.5k�) andEDS analyses of the cross-section of ZnO-coated TiO2 filmsin tests W2 (0.05 mol/L, 0.5 min), W4 (0.05 mol/L, 3.0 min),W5 (0.005 mol/L, 0.5 min), and W7 (0.005 mol/L, 3.0 min).Through the BSE mapping, the black dots in the SEMmicrograph of Fig. 2 depict the dispersion of Zn in thecross-section of the film on the FTO-glass substrate. Thethickness of these ZnO-coated TiO2 film ranged from 10.7to 10.9 lm.

With a higher concentration of [Zn(CH3COO)2�2H2O]and a longer soaking time, the atomic percentage of zinc(Zn) dispersed in the ZnO-coated TiO2 film will be substan-tially increased. For example, at a fixed concentration of0.05 mol/L and a soaking time of 3.0 min, the atomic per-centage of Zn dispersed in the ZnO-coated TiO2 film is upto 2.90% (test W4), as shown in Fig. 2d. Aside from this,

Fig. 3. The band gaps of the working electrode in tests W1 (TiO2 electrode)electrode (0.005 M-3 min)], and W4 [ZnO-coated TiO2 electrode (0.05 M-3 mi


several larger particles of ZnO were precipitated on the sur-face of the ZnO-coated TiO2 film in test W4 (Fig. 2c), andthese larger particles of ZnO on the surface of the Zn-coated TiO2 electrode may decrease the short-circuit pho-tocurrent of a DSSC.

Fig. 3 shows the band gaps of the TiO2 electrode in testW1 and the ZnO-coated TiO2 electrodes in tests W4(0.05 mol/L, 3.0 min), W5 (0.005 mol/L, 0.5 min), andW7 (0.005 mol/L, 3.0 min). With a higher concentrationof [Zn(CH3COO)2�2H2O] and a longer soaking time, theband gap of the ZnO-coated TiO2 electrode will be sub-stantially increased. For example, the band gap of theworking electrode increases to 3.75 eV [ZnO-coated TiO2

electrode in test W4 (0.05 mol/L, 3.0 min)] from 3.22 eV(TiO2 electrode in test W1), as shown in Fig. 3.

This result can be attributed to the fact that with a higherconcentration of [Zn(CH3COO)2�2H2O] and a longer soak-ing time, more ZnO was deposited on the TiO2 film, andnew phases might be formed although small quantity. It ispossible that the ZnTiO3 phase might be formed betweenTiO2 and ZnO coatings as the TiO2 electrode was dippedin the solution of zinc acetate dehydrate and ethanol witha higher concentration of [Zn(CH3COO)2�2H2O] and alonger soaking time. Zhao et al. (2005) reported the forma-tion of the ZnTiO3 phase in a sol–gel experiment, and Yeet al. (2009) observed that the optical band gap of thecrystalline ZnTiO3 film is about 3.70 eV. Therefore, theZnO-coated TiO2 electrode with a larger band gap mightfacilitate the creation of the energy barrier in the workingelectrode of a DSSC.

, W5 [ZnO-coated TiO2 electrode (0.005 M-30 s)], W7 [ZnO-coated TiO2

n)].



Fig. 4. J–V characteristics of DSSC in tests D1–D7.


3.2. Voc, Jsc, g, and J–V curve of DSSC

Table 4 presents the open-circuit photovoltage (Voc), theshort-circuit photocurrent per unit area (Jsc), the fill factor(FF), and the power conversion efficiency (g) of the DSSCin tests D1–D7. Further, the variations in photocurrentdensity with photovoltage (i.e., J–V characteristics ofDSSC) in all tests are shown in Fig. 4. The Voc of the DSSCwith a ZnO-coated electrode (tests D2-D7) exceeds 0.7 V,but the Voc of the conventional DSSC (test D1) is 0.7 V.The highest power conversion efficiency of DSSC in thisstudy is up to 6.7%.

The amount of ZnO deposited on the porous nano-crystalline TiO2 (P-25) film substantially influences the per-formance of a DSSC. For a fixed concentration of[Zn(CH3COO)2�2H2O], as the soaking time increased, theshort-circuit photocurrent per unit area and the power con-version efficiency of DSSC decreased, but the open-circuitphoto-voltage increased. For example, at a fixed concentra-tion of 0.05 mol/L [Zn(CH3COO)2�2H2O], as the soakingtime increased from 0.5 min (test D2) to 3 min (test D4),the short-circuit photocurrent per unit area decreased from7.6 to 2.9 mA/cm2, the power conversion efficiencydecreased from 3.8% to 1.7%, but the open-circuit photo-voltage increased from 0.77 to 0.86 V (Table 4).

These can be probably attributed to the following facts:(1) the ZnTiO3 phase might be formed between TiO2 andZnO coatings; (2) because ZnTiO3 does not melt congru-ently but decomposes into Zn2TiO4 and TiO2 at a temper-ature (<900 �C), it is reasonable to speculate that the workfunction of ZnTiO3 is less than that of TiO2, and the con-duction band of ZnTiO3 is above that of TiO2; (3) excitedelectrons at the TiO2 conduction band have to climb theadditional ZnTiO3 conduction band and overcome ahigher energy barrier before they recombine with the holesin the dye. Therefore, the amount of electron–hole recom-bination is reduced, and the open-circuit photovoltage of


the DSSC increases (Fig. 4 and Table 4); (4) due to thehigher conduction band energy in ZnTiO3, the rate of theelectron injection from the LUMO of the dyes to the con-duction band of TiO2 becomes lower, and the short-circuitphotocurrent is reduced. A competition between these twomechanisms (recombination and electron injection) callsfor an optimization of this ZnO-coating process.

For a fixed soaking time, as the concentration of[Zn(CH3COO)2�2H2O] decreased, the short-circuit photo-current per unit area and the power conversion efficiencyof DSSC increased, but the open-circuit photo-voltagedecreased. For example, at a fixed soaking time of0.5 min, as the concentration of [Zn(CH3COO)2�2H2O]decreased from 0.05 mol/L (test D2) to 0.005 mol/L (testD5), the short-circuit photocurrent per unit area increasedfrom 7.6 to 13.5 mA/cm2, the power conversion efficiencyincreased from 3.8% to 6.7%, but the open-circuit photo-voltage decreased from 0.77 to 0.75 V (Table 4). Theseresults agree with our aforementioned speculation on theformation of ZnTiO3 layers.

Therefore, a suitable amount of ZnO depositionenhanced the power conversion efficiency of a DSSC. Forexample, the power conversion efficiency of the DSSC witha ZnO-coated electrode (6.7%) substantially exceeded thatof the conventional DSSC (5.9%). This result can be attrib-uted to the following: (1) an energy barrier was constructedby coating ZnO on the TiO2 (P-25) film probably due to theformation of ZnTiO3 phase; (2) this energy barrier mightsuppress the charge recombination and decrease the darkcurrent generated in a DSSC. Thavasi et al. (2009) indi-cated that raising the energy level of the metal oxide con-duction band should reduce the recombination losses andresult in high open circuit voltage; and (3) the open-circuitphoto-voltage in tests (D2–D7) was increased due to thenegative shift (toward vacuum level) of the Fermi-level ofthe Zn-coated TiO2 (Kang et al., 2007); and (4) the fill fac-tor (FF) of the DSSC with a ZnO-coated electrode in testD5 (65.4%) exceeds that of the conventional DSSC in testD1 (54.3%). Thavasi et al. (2009) indicated that the higherthe recombination of conduction band electrons with theelectrolyte, the lower will be the FF.

In contrast, a larger amount of ZnO deposition throughincreasing the concentration of [Zn(CH3COO)2�2H2O] (orextending the soaking time) might decrease the short-cir-cuit photocurrent per unit area due to the reduction ofthe electron injection rate. Furthermore, as the superabun-dant ZnO was deposited on the TiO2 (P-25) film, the aggre-gates, which were formed by the N719 dye and Zn2+ ions,could be more easily created. These aggregates mightobstruct the electron injection from the LUMO of N-719dye to the CB of ZnO, and the short-circuit photocurrentas well as the performance of the DSSC were thendecreased. Horiuchi et al. (2003) investigated the effect ofaggregates (N3–Zn2+) on the electron injection efficiencyfrom excited N3 dye into the nano-crystalline ZnO film.Aside from this, Wu et al. (2008) observed that after sput-tering ZnO for 3 min on TiO2 the power conversion effi-




ciency was significantly enhanced from 4.76% to 6.55%, butthe power conversion efficiency decreased with the increaseof sputtering time.

Furthermore, as the working electrode had an appropri-ate coating of TiO2 (P-25) colloid, it might have a higheradsorptive rate of the dye because more tiny cavities werecreated in the film on a FTO-glass substrate, so the largernumber of electrons were excited as the DSSC was exposedto the light. Huang et al. (2006) determined the theoreticalthickness of TiO2 film (10.0 lm) by calculating the diffusionlength of electrons (Ln) via Ln ¼

ffiffiffiffiffiffiffiffiffiffi

Dnsn

p, in which Dn and sn

represent the electron’s diffusion coefficient in the TiO2 par-ticles (5 � 10�5 cm2/s) and the lifetime of photo-injectedelectrons (2 � 10�2 s), respectively. In this study, the thick-ness of ZnO-coated TiO2 film ranged from 10.7 to 10.9 lm(Table 4), which were reasonably aligned with the theoret-ical thickness of TiO2 film (10.0 lm) (Huang et al., 2006).

4. Conclusion

A ZnO-coated TiO2 working electrode was preparedusing a dip coating method, and it was then applied inthe DSSC. Depositing an appropriate amount of ZnO onthe TiO2 electrode not only increased the band gap of theworking electrode, but also enhanced the power conversionefficiency of a DSSC. We explained the above experimentsby the formation of a thin ZnTiO3 layer between ZnO andTiO2, and this thin ZnTiO3 layer may modulate the compe-tition between the processes of electron recombination andinjection. Furthermore, the aggregates of N719 dye andZn2+ ions on the surface the TiO2 electrode may diminishthe performance of a DSSC. Accordingly, research on opti-mizing the fabrication of a ZnO-coated TiO2 electrodeusing a simple dip coating method and a statistical methodis worthy of continued study.

Acknowledgments

The authors would like to thank the National ScienceCouncil, R.O.C., Taiwan for financially supporting this re-search under Contract No. NSC 99-2221-E-020-028. Theenergy dispersive spectrometer (EDS) (Horiba EX-200)was performed in the Precision Instrument Center ofNational Pingtung University of Science and Technology.

References

Bandara, J., Divarathne, C.M., Nanayakkara, S.D., 2004. Fabrication ofn–p junction electrodes made of n-type SnO2 and p-type NiO forcontrol of charge recombination in dye sensitized solar cells. SolarEnergy Materials and Solar Cells 81, 429–437.

Bandara, J., Pradeep, U.W., Bandara, R.G.S.J., 2005. The role of n–pjunction electrodes in minimizing the charge recombination andenhancement of photocurrent and photovoltage in dye sensitized solarcells. Journal of Photochemistry and Photobiology A: Chemistry 170,273–278.

Bandara, J., Yasomanee, J.P., 2007. P-type oxide semiconductors as holecollectors in dye-sensitized solid-state solar cells. SemiconductorScience and Technology 22, 20–24.


Caramori, S., Cristino, V., Boaretto, R., Argazzi, R., Bignozzi, C.A.,Carlo, A.D., 2010. New components for dye-sensitized solar cells.International Journal of Photoenergy 2010, 458614.

Chou, C.S., Yang, R.Y., Weng, M.H., Yeh, C.H., 2008. Preparation ofTiO2/dye composite particles and their applications in dye-sensitizedsolar cell. Powder Technology 187, 181–189.

Chou, C.S., Yang, R.Y., Yeh, C.K., Lin, Y.J., 2009. Preparation of TiO2/nano-metal composite particles and their applications in dye-sensitizedsolar cells. Powder Technology 194, 95–105.

Chou, C.S., Lin, Y.J., Yang, R.Y., Liu, K.H., 2011. Preparation of TiO2/NiO composite particles and their applications in dye-sensitized solarcells. Advanced Powder Technology 22, 31–42.

Horiuchi, H., Katoh, R., Hara, K., Yanagida, M., Murata, S., Arakawa,H., Tachiya, M., 2003. Electron injection efficiency from excited N3into nanocrystalline ZnO films: effect of (N3�Zn2+) aggregate forma-tion. The Journal of Physical Chemistry B 107, 2570–2574.

Huang, C.Y., Hsu, Y.C., Chen, J.G., Suryanarayanan, V., Lee, K.M., Ho,K.C., 2006. The effects of hydrothermal temperature and thickness ofTiO2 film on the performance of a dye-sensitized solar cell. SolarEnergy Materials and Solar Cells 90, 2391–2397.

Ito, S., Murakami, T.N., Comte, P., Liska, P., Gratzel, C., Nazeeruddin,M.K., Gratzel, M., 2008. Fabrication of thin film dye sensitized solarcells with solar to electric power conversion efficiency over 10%. ThinSolid Films 516, 4613–4619.

Kang, S.H., Kim, J.Y., Kim, Y., Kim, H.S., Sung, Y.E., 2007. Surfacemodification of stretched TiO2 nanotubes for solid-state dye-sensitizedsolar cells. The Journal of Physical Chemistry C 111, 9614–9623.

Kao, M.C., Chen, H.Z., Young, S.L., 2009. Effects of ZnO coating on theperformance of TiO2 nanostructured thin films for dye-sensitized solarcells. Applied Physics A 97, 469–474.

Kim, S., Kim, D., Choi, H., Kang, M.S., Song, K., Kang, S.O., Ko, J.,2008. Enhanced photovoltaic performance and long-term stability ofquasi-solid-state dye-sensitized solar cells via molecular engineering.Chemical Communications 40, 4951–4953.

Lewis, N.S., 2007. Toward cost-effective solar energy use. Science 315,798–801.

Li, S.J., Lin, Y., Tan, W.W., Zhou, X.W., Chen, J.M., Chen, Z.C., 2010.Preparation and performance of dye-sensitized solar cells based onZnO-modified TiO2 electrodes. International Journal of Minerals,Metallurgy and Materials 17, 92–97.

Lu, L.L., Li, R.J., Fan, K., Peng, T.Y., 2010. Effects of annealingconditions on the photoelectrochemical properties of dye-sensitizedsolar cells made with ZnO nanoparticles. Solar Energy 84, 844–853.

Muth, J.F., Lee, J.H., Shmagin, I.K., Kolbas, R.M., Casey, H.C., Keller,B.P., Mishra, U.K., DenBaars, S.P., 1997. Absorption coefficient,energy gap, exciton binding energy and recombination lifetime of GaNobtained from transmission measurements. Applied Physics Letters 71,2572–2574.

Su, Y.H., Lai, W.H., Teoh, L.G., Hon, M.H., Huang, J.L., 2007. Layer-by-layer Au nanoparticles as a Schottky barrier in a water-based dye-sensitized solar cell. Applied Physics A: Materials Science andProcessing 88, 173–178.

Sumikura, S., Mori, S., Shimizu, S., Usami, H., Suzuki, E., 2008a.Syntheses of NiO nanoporous films using nonionic triblock co-polymer templates and their application to photo-cathodes of p-typedye-sensitized solar cells. Journal of Photochemistry and PhotobiologyA: Chemistry 199, 1–7.

Sumikura, S., Mori, S., Shimizu, S., Usami, H., Suzuki, E., 2008b.Photoelectrochemical characteristics of cells with dyed and undyednanoporous p-type semiconductor CuO electrodes. Journal of Photo-chemistry and Photobiology A: Chemistry 194, 143–147.

Thavasi, V., Renugopalakrishnan, V., Jose, R., Ramakrishna, S., 2009.Controlled electron injection and transport at materials interfaces indye sensitized solar cells. Materials Science and Engineering R:Reports 63, 81–99.

O’Regan, B., Gratzel, M., 1991. A low-cost, high-efficiency solar cell basedon dye-sensitized colloidal TiO2 films. Nature 353, 737–739.




Wu, S., Han, H., Tai, Q., Zhang, J., Chen, B.L., Xu, S., Zhou, C., Yang,Y., Hu, H., Zhao, X.Z., 2008. Improvement in dye-sensitized solarcells with a ZnO-coated TiO2 electrode by rf magnetron sputtering.Applied Physics Letters 92, 122106.

Ye, C., Wang, Y., Ye, Y., Zhang, J., Li, G.H., 2009. Preparation andphotoluminescence of undoped ZnTiO3 thin films. Journal of AppliedPhysics 106, 033520.


Zhao, L.L., Liu, F.Q., Wang, X.W., Zhang, Z.Y., Yan, J.F., 2005.Preparation and characterizations of ZnTiO3 powders by sol–gelprocess. Journal of Sol–Gel Science and Technology 33, 103–106.



the effect of zno-coating on the performanceof a

Documents