Journal of Physics and Chemistry of Solids 73 (2012) 535–539

Contents lists available at SciVerse ScienceDirect

Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jpcs

Successive ionic layer adsorption and reaction of ZnSe shells for ZnOnanowire-based dye-sensitized solar cells

Jooyoung Chung, Jihyun Myoung, Jisook Oh, Sangwoo Lim n

Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120–749, Korea

a r t i c l e i n f o

Article history:

Received 6 April 2011

Received in revised form

1 August 2011

Accepted 1 December 2011Available online 17 December 2011

Keywords:

A. Nanostructures

B. Sol–gel growth

C. Electron microscopy

D. Optical properties

97/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jpcs.2011.12.001

esponding author. Tel.: þ82 2 2123 5754; fax

ail address: swlim@yonsei.ac.kr (S. Lim).

a b s t r a c t

Thin ZnSe layers were deposited on ZnO nanowires by a novel successive ionic layer adsorption and

reaction technique in order to solve recombination problems in ZnO nanowire-based dye-sensitized

solar cells (DSSCs). Cell efficiency increased from 0.1 to 1.3–1.4% with the deposition of a 9- to13-nm-

thick ZnSe shell on ZnO nanowires due to a large increase in JSC. The dramatic increase in JSC and cell

efficiency is due to the facilitation of electron transfer related to ambipolar diffusion by the formation of

a type II band alignment and the suppression of recombination in the presence of the ZnSe shell.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

ZnO nanowires are used as anode materials in dye-sensitizedsolar cells (DSSCs) because they are relatively easy to synthesizeand modify and exhibit outstanding electric 1-D II-VI semicon-ductor characteristics [1,2]. In addition, they provide a directelectric pathway along the direction of the c-axis to ensure therapid collection of electrons, and have internal electric fields thatcan assist electron collection by preventing recombination withelectrolytes [1]. Further, this type of nanostructure exhibits alarge electron diffusion coefficient of 1.8�10�3 cm2/s [2]. How-ever, many factors such as the kind of dye molecule, the amountof dye loading relative to the surface area of the ZnO nanowires,and the electrical and optical properties of the ZnO nanowiresaffect the efficiency of ZnO nanowire-based DSSCs [3–5]. Thesurface conditions of the ZnO nanowires are particularly impor-tant to DSSC performance [6,7]. For example, improvement inelectronic coupling between the surfaces of ZnO nanowires andthe dye increases light harvesting efficiency [6]. In addition, it hasbeen reported that an increased number of hydroxide functionalgroups on the surface of ZnO nanowires leads to facilitation of dyeloading, which contributes to the improvement of DSSC perfor-mance [7]. Efforts to increase the surface area of the ZnOnanostructure have also been made by preparing hierarchicalZnO NW growth through nanoforests [8] and nanoflowers [9] toimprove performance.

ll rights reserved.

: þ82 2 312 6401.

However, one of the major problems in operating ZnO nano-wire-based DSSCs is the recombination of injected electrons dueto defects in ZnO nanostructures [10]. Several methods have beendeveloped to resolve such recombination problems, such as theformation of core/shell structures [11–14] and Zn(OAc)2 treat-ment [15]. The formation of shells on ZnO nanowires is aparticularly good candidate approach for solving recombinationproblems because core/shell nanowires reduce radiative recom-bination losses [12], while dark exciton formation minimizes theexciton recombination rate and improves the carrier collection inthe solar cell [13]. In addition, core/shell structured nanowiresadjust the band gap alignment in solar cells [14]. For example,Greene et al. reported TiO2 core shell formation on a ZnOnanowire, though the cell efficiency rose only 0.29%. In thatreport, the TiO2 shell was deposited by an atomic layer depositionmethod followed by annealing at 220 1C, which made the processsomewhat complicated [16].

ZnSe is a prospective material for use in optoelectronic devicesdue to its direct band gap and large exciton binding energy(22 meV) [17,18]. In addition, ZnO/ZnSe structures form type IIband alignments that facilitate electron transfer [19,20]. Type IIband alignments of heterostructures, which are formed by coatingnanowires, induce charge separation to different regions [21].Charge separation is advantageous for photovoltaic devicesbecause injected electrons in such devices are confined withinthe core material [21]. In particular, when a ZnO/ZnSe core/shellnanostructure was prepared using Na2SeSO3 in an autoclave at160 1C on a Zn sheet, the structure exhibited superior photoelec-trochemical properties [22]. However, the actual effect of theZnSe shell on the solar cell was not elucidated. Wu et al. observed

J. Chung et al. / Journal of Physics and Chemistry of Solids 73 (2012) 535–539536

a strong photoresponse from well-aligned ZnO/ZnSe core/shellnanowire arrays with type-II energy alignment, which wereprepared by chemical vapor deposition at 620 1C [23]. However,in that study, the effect of the ZnSe shell on DSSC performancewas also not determined. Therefore, in this study, we preparedZnSe shells on ZnO nanowires using a relatively simple successiveionic layer adsorption and reaction (SILAR) method and investi-gated their effects on ZnO nanowire-based DSSCs. This strategymay offer opportunities for the generation of Zn-based type IIband alignment structures and for their application to solar cells.

2. Experiments

A ZnO seed layer was deposited on fluorine-doped tin oxide (FTO)glass by radio frequency (RF) sputtering and was annealed at 300 1Cin an O2 ambient atmosphere for 1 h. For the growth of ZnOnanowires, a precursor was prepared by dissolving 30 mM of zincnitrate hexahydrate (Zn(NO3)2 �6H2O, 98%, Aldrich) in deionizedwater. Then, ammonium hydroxide (NH4OH, 28% NH3 in water; J.T.Baker) was added to set the pH of the solution to 10.3. ZnOnanowires were grown on the seed layer by a hydrothermal methodat 60 1C for 24 h. The precursor solution was refreshed every 6 h. TheFTO glass on which the ZnO nanowires had grown was taken out ofthe precursor solution, rinsed in deionized water and dried.

The SILAR method was used to deposit ZnSe layers on the ZnOnanowires. A zinc nitrate precursor was prepared by dissolving2 mM of zinc nitrate hexahydrate in deionized water. A sodiumselenosulfate solution was prepared by stirring 0.02 M selenium(Se, 99.99%, Aldrich) and 0.05 M sodium sulfite (Na2SO3, 98–100%,Aldrich) at 70 1C for 24 h. Then, a 2 mM sodium selenosulfatesolution was prepared by dilution. ZnO nanowires grown on theFTO glass were sequentially immersed in 2 mM zinc nitratehexahydrate and 2 mM sodium selenosulfate solution at roomtemperature for 20 s each. These SILAR steps were repeated 30 or60 times, and the resulting samples were labeled as S-30 andS-60, respectively. The samples were rinsed in deionized waterbetween each step.

The ZnO nanowire-grown FTO glass with or without a ZnSeshell was immersed in 0.5 mM N719 dye (cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato)ruthenium(II)bistetra-buty-lammonium) in ethanol for 2 h. In order to form a cathode, a Ptlayer was deposited on another piece of glass using a DCsputtering process. Sputtering was performed using a Pt (99.99%purity) target in an Ar ambient atmosphere at 100 W. The ZnOnanowire-grown FTO glass was covered with the Pt-depositedglass separated by 50-mm-thick hot-melt spacers (Type 1702,Himilan of Mitsui-DuPont Polychemical). Then, the spacebetween the anode and the cathode was filled with a liquidelectrolyte (0.5 M LiI, 50 mM I2, 0.5 M 4-tertbutylpyridine in3-methoxypropionitrile).

The performances of the DSSCs were assessed by current density–voltage measurements using a potentiostat (CompactStat) under AM1.5 G of simulated sunlight (Sun 2000 Series Solar Simulators, 11000,

500 nm500 nm 500 nm

Fig. 1. FE-SEM images of ZnO nanowires with and without ZnSe layers: (a) top view of

as-grown ZnO nanowires; and (d) cross-sectional view of sample S-60.

Abet-Technologies). The morphologies of the nanowires wereobserved using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4200). The formation of ZnSe shells on the ZnOnanowires was examined by X-ray photoelectron spectroscopy (XPS,Thermo Electron Corporation, K-Alpha) and high-resolution trans-mission electron microscopy (HR-TEM, JEOL, JEM-3010). The crystal-linities of the ZnO and ZnO/ZnSe core/shell structures wereinvestigated by X-ray diffraction (XRD, Bruker D8 Discover). Photo-luminescence spectroscopy (PL, SPEX 1403) was used to investigatethe optical properties and surface defects of ZnO nanostructures.

3. Results and discussion

The morphologies of the ZnO nanowires and ZnO/ZnSe core/shell nanostructures observed by FE-SEM are shown in Fig. 1. Inorder to observe the effects of the addition of ZnSe layers, sampleS-60 (subjected to 60 cycles of ZnSe SILAR) was compared withas-grown ZnO nanowires in Fig. 1. The diameters and lengths ofthe ZnO nanowires were 50–100 nm and 5.3 mm, respectively,with or without the ZnSe SILAR process.

HR-TEM images of ZnO nanowires with and without ZnSelayers are shown in Fig. 2. While the as-grown ZnO nanowires hadatomically smooth surfaces without additional surface layers(Fig. 2(a)), thin ZnSe layers were observed on the ZnO nanowiresthat underwent the SILAR process (Fig. 2(b and c)). The averagethickness of the ZnSe shells was 9 nm and 13 nm after 30 and 60SILAR cycles, respectively, and a nearly linear increase in shellthickness with increasing SILAR cycles was observed. The thick-ness of the ZnSe shell was uniform over the ZnO nanowires (data

not shown). The change in crystallinity was investigated bymeasuring selected area electron diffraction (SAED) patterns ofthe ZnO nanowires. The SAED pattern of the ZnO nanowireswithout ZnSe layers shown in Fig. 2(d) indicates that ZnOnanowires were grown with a preferential [0002] orientation.Rings corresponding to the interplanar (111) orientation of theZnO/ZnSe core/shell nanostructure, as seen in Figs. 2(e and f),represent the zinc blend structure of ZnSe [24]. Consequently, theHR-TEM and SAED patterns of the ZnO/ZnSe core/shell nanos-tructures reveal that thin ZnSe layers were successfully synthe-sized on single crystalline ZnO nanowires.

We used a SILAR deposition mechanism to prepare a ZnSe shelllayer. In the first SILAR step, ZnO nanowires were immersed in thezinc nitrate solution, and Zn2þ ions were adsorbed onto the surfacesof the ZnO nanowires. In the next step, Se2� anions produced in thesodium selenosulfate solution [25] reacted with Zn2þ ions on thesurfaces of the ZnO nanowires to deposit ZnSe. Thus, the reactionsrequired for the formation of the ZnSe layer are as follows:

Zn(NO3)2-Zn2þþ2NO3

� , (1)

Na2SeSO3þOH�-Na2SO4þHSe� , (2)

HSe�þOH�-H2OþSe2� , (3)

Zn2þþSe2�-ZnSe. (4)

1 m 1 m

as-grown ZnO nanowires; (b) top view of sample S-60; (c) cross-sectional view of

5 nm 5 nm

0001

5 nm

ZnSe

ZnOZnSe

ZnO

ZnO

0110

111 111

Fig. 2. HR-TEM images of: (a) as-grown sample; (b) sample S-30; and (c) sample S-60. SAED patterns of: (d) as-grown sample, (e) sample S-30, and (f) sample S-60.

Voltage (V)0 0.1 0.2 0.3 0.4 0.5

0

20

40

60

80

Cur

rent

den

sity

(A /m

2 )

as-grownS-30S-60

Inte

nsity

(a. u

.)

2 (deg.)20 30 40 50 60 70 80

S-30

S-60

as-grown

as-grownS-30S-60

300 400 500 600 700 800

Inte

nsity

(a. u

.)

Wavelength (nm)

Binding energy (eV)

Inte

nsity

(a. u

.)

S-30

S-60

as-grown

53 54 55 56 57 58

Fig. 3. (a) XPS Se3d spectra of ZnO nanowires with and without ZnSe shells; (b) XRD patterns of ZnO nanowires with and without ZnSe shells; (c) PL spectra

of ZnO nanowires with and without ZnSe shells; (d) photocurrent density vs. voltage curves of DSSCs fabricated using ZnO nanowires with and without ZnSe

shells.

J. Chung et al. / Journal of Physics and Chemistry of Solids 73 (2012) 535–539 537

J. Chung et al. / Journal of Physics and Chemistry of Solids 73 (2012) 535–539538

In order to confirm the formation of the ZnSe layer on the ZnOnanowires, XPS Se3d peaks were observed for the samples, asshown in Fig. 3(a). Se3d peaks at 55.4–55.7 correspond to thebinding energy of Zn–Se [26], and therefore, the thin layersobserved on the ZnO nanowires were concluded to be ZnSe. Thecrystallinities of the nanowires were investigated by XRD, and theresults are shown in Fig. 3(b). All nanowires with and withoutZnSe layers had hexagonal wurtzite structures with preferential(002) c-axis orientation. Although the formation of ZnSe layerswas confirmed by XPS analysis, no additional XRD peaks repre-sentative of ZnSe were observed, possibly due to the fact that theZnSe layers were extremely thin [27].

In order to determine the effect of the ZnSe shell on the opticalproperties of the ZnO nanowire structure, room temperature PLspectra of ZnO nanowires with and without ZnSe layers weremeasured. As Fig. 3(c) shows the appearance of UV and visibleemission peaks in all PL spectra, additionally, the visible emis-sions of the ZnO nanowires at 560 nm decreased after ZnSe layerformation. The visible emissions can be attributed to defects inthe ZnO crystals, such as singly ionized oxygen vacancies [28] andinterstitial oxygen defects [29], while the UV emission representsnear band edge emission relative to free exciton recombination[30,31]. Therefore, the results in Fig. 3(c) suggest that theformation of ZnSe layers on ZnO nanowires minimizes ZnOsurface defects.

DSSCs were fabricated using ZnO nanowires in order toexamine the improved surface properties of ZnO nanowires afterZnSe shell formation. Surprisingly, as shown in Table 1, the cellefficiency of the DSSCs was dramatically increased after ZnSelayer formation from 0.11% for as grown ZnO nanowires to 1.37%and 1.29% for samples S-30 and S-60, respectively. The photo-current density vs. voltage curves in Fig. 3(d) also show that theJSC and fill factor of the DSSCs dramatically increased afterformation of the ZnSe layer. The S-30 and S-60 samples showedJSC measurements of 79.7 and 66.9 A/m2 and fill factors of 0.39and 0.40, respectively, while for the non-ZnSe-layered sample, themeasurements were 11.4 A/m2 and 0.26. The significant improve-ments in cell efficiency observed for the DSSCs with ZnO/ZnSe

Table 1Performances of the DSSCs prepared using ZnO nanowires and ZnO/ZnSe

core/shell nanostructures.

Sample JSC [A/m2] VOC [V] FF Efficiency [%]

as-grown 11.4 0.54 0.26 0.11

S-30 79.7 0.46 0.39 1.37

S-60 66.9 0.51 0.40 1.29

-8

-7

-6

-5

-4

-3

E v

s. v

accu

um(e

V)

-3.25 eV

ZnO ZnSe N719-7.6 eV

-5.05 eV

-4.4 eV

-6.28 eV

-3.89 eV

J SC

(A /m

2 )

Fig. 4. (a) Schematic illustration of energy band diagrams of ZnO, ZnSe, and N719 dye.

emission intensity.

core/shell nanostructures resulted from these large increases inthe JSC and fill factor. On the other hand, samples S-30 and S-60exhibited degraded VOC, which is thought to have resulted fromthe positive shift of the conduction band edge of the anode [32] orback electron transfer [33]. The positive shift in the conductionband edge may increase the injection rate of the dyes due to anincreased driving force for injection and a higher density ofconduction band states accessible to the dye excited state [34],which will result in an enhanced photocurrent. However, furtherstudy is needed to fully understand the current observation.

Increases in the JSC of the DSSCs can be attributed to theincreased amount of dye on the nanowires and/or suppression ofelectron recombination. However, because no significant changesin nanowire surface area, e.g., nanowire density and/or length,were observed after ZnSe layer deposition (Fig. 1), the amount ofdye on the nanowire surface was not the primary factor influen-cing JSC and cell efficiency. As illustrated in Fig. 4(a), ZnOnanowires with ZnSe shells and N719 dye form a type II bandalignment [18,35,36], which is known to facilitate electrontransfer [20]. The advantage of the ZnSe shell may be conferredonly when the LUMO-HOMO structures of the ZnSe shell and dyeform a type II band alignment, as shown in the energy banddiagram in Fig. 4(a). In addition, type-II alignment produceslonger recombination lifetimes [37], which is favorable in DSSCsbecause the increase in lifetime slows interfacial recombination[38]. In addition, the photocurrent is determined by ambipolardiffusion of electrons coupled to counterion charges [39]. Thus,small-radius Liþ in the liquid electrolyte may penetrate into thedye-coated anode surface and form an ambipolar Liþ–e� with theelectrons in the conduction band of the ZnO/ZnSe nanostructure. As aresult, electron transfer in the anode material accelerates, leading toan increase in the JSC of the DSSC [40,41]. Therefore, the behaviorillustrated in Fig. 3(d) can also be explained by a change in theambipolar diffusion constant for electrons due to the ZnSe coating. Ithas been reported that electrons need to diffuse to a near-substrateregion in the case of near-substrate recombination [42].

Conversely, without ZnSe shells the dye is adsorbed directlyonto the ZnO nanowire surfaces (Fig. 4(a)). In such cases, defectsnear the ZnO surface provide recombination sites and degrade JSC

in the DSSC [43]. The quantity of defects was decreased bydepositing ZnSe layers onto the ZnO nanowires (Fig. 3(a)). Therelationship between the JSC of the DSSCs and the PL visibleemission intensity of ZnO nanowires with and without ZnSelayers obtained from Fig. 3(c) is plotted in Fig. 4(b). Here, the PLvisible emission intensity was normalized to as-grown ZnOnanowires without ZnSe layers. In the figure, JSC increases withdecreasing visible emission intensity, which is proportional to theamount of defects on the ZnO. Therefore, deposition of ZnSe

Normalized PL visible intensity (%)9092949698100

0

20

40

60

80

100

(b) Changes in JSC of the ZnO nanowire-based DSSC with the normalized PL visible

J. Chung et al. / Journal of Physics and Chemistry of Solids 73 (2012) 535–539 539

layers on the ZnO nanowires likely diminished the recombinationof injected electrons. Finally, the suppression of the recombina-tion of injected electrons and the facilitation of electron transferdue to the formation of ZnSe layers on the ZnO nanowires werethe main factors leading to increases in JSC and efficiency in ZnOnanowire-based DSSCs.

4. Conclusions

In summary, thin ZnSe layers were synthesized on ZnOnanowires using a SILAR method. TEM, SAED, and XPS measure-ments indicated that uniform ZnSe layers were deposited on theZnO nanowires. ZnSe shell thickness increased almost linearlywith the number of SILAR cycles. Using the PL spectra, weconfirmed that the defect-related visible emission was decreasedby depositing ZnSe layers on the ZnO nanowires. DSSCs werefabricated using ZnO nanowires with and without ZnSe layers andcompared. DSSCs with ZnSe shells exhibited significant increasesin cell performance compared to unmodified ZnO nanowires dueto a large increase in JSC. Finally, we concluded that the increasedJSC of the DSSCs with ZnSe shells on ZnO nanowires may beattributed to the successful formation of type II band alignmentsof ZnO nanowires, ZnSe shells, and an N719 dye structure, whichreduced the number of defect sites on the ZnO nanowire surfacesand facilitated electron transfer.

Acknowledgments

This research was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technol-ogy (2010-0010573). This work was also supported by the PriorityResearch Centers Program through the National Research Founda-tion of Korea (NRF) funded by the Ministry of Education, Scienceand Technology (2009-0093823).

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