Journal of Industrial and Engineering Chemistry 18 (2012) 19–23

Short communication

Fabrication of dye-sensitized solar cells using ordered and verticallyoriented TiO2 nanotube arrays with open and closed ends

Jiwon Lee, Ki Sang Hong, Kyusoon Shin, Jae Young Jho *

School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea

A R T I C L E I N F O

Article history:

Received 3 June 2011

Accepted 7 July 2011

Available online 10 November 2011

Keywords:

TiO2 nanotube

Atomic layer deposition

AAO

Dye sensitized solar cell

A B S T R A C T

Highly ordered and vertically oriented TiO2 nanotube arrays were prepared and applied to dye sensitized

solar cell (DSSC) as working electrodes. The nanotube arrays were fabricated using atomic layer

deposition and AAO template. The two types of nanotube’s end, closed-end and open-end, were produced

by reactive ion etching (RIE) process. The structure of nanotube arrays was characterized by FE-SEM,

TEM, and XRD. DSSCs using the TiO2 nanotube arrays as working electrodes were fabricated and

characterized. The DSSCs prepared from the TiO2 nanotubes with open end exhibited higher power

conversion efficiency of 1.17% than that with closed end. This result is attributed to that the open-ended

TiO2 nanotubes provided larger surface area, leading to more amount of dye molecules to adsorb followed

by the higher light absorption.

� 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Since the low-cost dye-sensitized solar cell (DSSC) was reportedby O’Regan and Gratzel in 1991 [1], the DSSC has been expected toreplace the silicon-based solar cells. Among the DSSCs, the highestefficiency was achieved by the combination of liquid electrolyteand titanium oxide (TiO2) working electrode, which shows overalllight-to-electricity conversion efficiencies of over 11% [2]. Workingelectrodes of most DSSCs are fabricated with interconnected TiO2

nanoparticles which possess large surface area for adsorbing dyemolecules and good electron transport properties. When photonsare absorbed by dye molecules on the TiO2 nanoparticles, a currentis generated by electron injection into the conduction band of theTiO2. The resulted electrons transport through the TiO2 nanopar-ticles to reach the electrode. However, the random contactbetween the disordered TiO2 nanoparticles acts as a grainboundary leading to trapping and recombination of electrons,which limits the efficiency of the DSSCs. Recently, TiO2 nanotubeshave been proposed as a promising material for an orderedstructure of the working electrode. Electron transportationthrough the ordered TiO2 nanotubes should be more efficientthan that of the disordered TiO2 nanoparticles since there are lessgrain boundaries compared to the nanoparticles [3,4].

There have been various methods to develop TiO2 nanostruc-tures including sol–gel process [5,6], electrospinning [7] and

* Corresponding author. Tel.: +82 2 880 8346; fax: +82 2 884 7355.

E-mail address: jyjho@snu.ac.kr (J.Y. Jho).

1226-086X/$ – see front matter � 2011 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2011.11.116

anodization of Ti metal [8]. However, these methods havelimitations in synthesizing an ordered structure of uniform TiO2

nanotubes with precise control of dimension and geometry, whichstrongly influence the diffusion of excitons and electrons, andconsequently affect overall power conversion efficiency (PCE) ofthe DSSCs.

In this study, we report on a fabrication method of highlyordered, vertically oriented and uniform TiO2 nanotube arrays forapplication of DSSCs. The TiO2 nanotubes are synthesized bytemplate-assisted method using atomic layer deposition (ALD) andreactive ion etching (RIE). ALD is well known as a gas-phasedeposition method with self-limiting growth mechanism, whichmakes possible to deposit thin films on porous templates withprecise control of the thickness by varying the number of cycles. Byusing anodic aluminum oxide (AAO) as a template for ALD, fine andregular TiO2 nanotubular structure can be achieved. Dimensions ofthe TiO2 nanotubes, such as diameter and length, can be tuned byvarying the anodizing condition of AAO template. RIE techniquewas employed to modify the end type of the TiO2 nanotubes.Development of both close-ended and open-ended TiO2 nanotubearrays will be demonstrated, and characterization of the DSSCsfabricated with the nanotubes will be followed.

2. Experimental

2.1. Synthesis of TiO2 nanotube arrays

The whole sequence of the fabrication method for both ofclosed-ended and open-ended TiO2 nanotubes was illustrated in

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. Schematic of fabrication of DSSC working electrode using TiO2 nanotube arrays with closed end and open end.

J. Lee et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 19–2320

Fig. 1. AAO used in this work was of hexagonally packed nanoporeswith diameters of 70 nm. The AAOs were introduced into ALDchamber for TiO2 deposition. TiO2 layer was deposited onto thesurface of the AAO pore channel by ALD using titanium–tetraisopropoxide (Ti[OCH(CH3)2]4, TTIP) and water as precursors.The substrate temperature was kept at 130 8C and a base pressurewas 2 mTorr. One cycle of ALD was composed of a 2-s exposure ofTTIP, a 5-s Ar purge, a 2-s exposure of water, and a 5-s Ar purge. Thecycle was repeated for 300 times to deposit TiO2 layer by thicknessof 15 nm. TiO2 coated AAO template was annealed at 450 8C for 3 hin order to form anatase crystal structure of TiO2.

As depicted in Fig. 1, in order to improve the electrical contact ofTiO2 layer to the metal substrate, 200 nm-thick platinum (Pt) layerwas sputtered on the top surface of the TiO2-deposited AAOtemplate. The Pt layer side of the template was attached to atitanium foil (thickness of 0.25 mm, Aldrich) using silver contain-ing epoxy adhesives (Elcoat1 A-200, CANS, Inc.) followed by heattreatment at 150 8C for 30 min. In order to obtain the TiO2

nanotube arrays, the AAO template was removed by the followingprocedure. Aluminum layer was dissolved using a saturatedsolution of mercury chloride, and then both of alumina barrierlayer of the AAO template and the top of the TiO2 nanotubes wereetched by RIE (RIE 80 PLUS, Oxford, Co. Ltd.). The RIE wasperformed for 45 min using Ar ion with a gas flow rate of 10.0 sccm,a pressure of 15 mTorr, and a power of 150 W. Then the residualalumina layer was removed by treatment with a 0.2 M solution ofchromic acid (H2Cr2O7) at 70 8C for 4 h. For formation of close-ended TiO2 nanotubes, the RIE process was omitted.

2.2. Fabrication of DSSC using the TiO2 nanotube arrays

The both types of TiO2 nanotubes on the Ti substrates wereimmersed in an ethanol solution of 0.5 mM cis-dithiocyanate-N,N0-bis-(4-carboxylate-4-tetrabutylammoniumcarboxylate-2,20-bipyridine) ruthenium(II) (N719, Ruthenium 535 bis-TBA, Solar-onix) overnight for a dye adsorption. After rinsing the excess dyemolecules from the electrodes with ethanol several times, bothedges of the Ti substrates were covered with polyimide film tape(3 MTM) as a spacer. A conducting glass (Florine doped tin oxideglass) was put onto the spacer and then clamped. A drop ofelectrolyte solution was injected into the space between the

clamped electrodes to complete the DSSC. The electrolyte solutionwas a mixture of acetonitrile and valeronitrile (85:15 by vol.%)containing 0.6 M of butylmethylimidazolium iodide, 0.03 Miodine, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyr-idine.

2.3. Characterization

The morphology of TiO2 nanotubes was characterized by fieldemission scanning electron microscopy (FE-SEM, JSM-6700F, JEOL)and transmission electron microscopy (TEM, JEM-2000EXII, JEOL).The crystal structures were determined by X-ray diffraction (XRD)with Cu Ka radiation (l = 1.54 A) using diffractometer (D5005,Bruker). Current density–voltage (J–V) characteristics of DSSCswere measured using a 500 W Xenon lamp (XIL model 05A50KSsource unit laid on an AM 1.5 filter) with a light intensity of100 mW cm�2. The amount of dye adsorbed on the TiO2 nanotubeswas estimated by measuring UV–vis light absorption spectrum(8453 UV–Visible spectrophotometer, Agilent Technologies, Inc.)for a solution of the dye molecules which were desorbed from theworking electrode using 0.2 M NaOH solution.

3. Result and discussion

3.1. Formation of TiO2 nanotubes

Fig. 2 shows FESEM images of the AAO used as a template for ALD,which has highly ordered nanopores with the pore diameter 70 nmand depth of 8 mm. Fig. 3 shows FE-SEM images of the resulted TiO2

nanotube arrays; Fig. 3(a)–(c) for the closed-ended, and Fig. 3(d)–(f)for the open-ended TiO2 nanotube arrays. Diameter of both types ofnanotubes is 70 nm, which is consistent with the pore diameter ofthe AAO template. Length of the nanotubes also corresponds to thepore depth of the AAO template, 8 mm. These results verified thatTiO2 layer was successfully deposited onto the entire surface of thenanopores of the AAO template. In the overall top view images ofFig. 3(a) and (d), it is visible that bundles of TiO2 nanotubes formedwith size of several micrometers. Bundle formation of the nanotubescan be explained by that individual nanotubes gather into bundlesby lateral capillary force between themselves when dissolving theAAO template [9].

Fig. 2. Cross sectional FESEM image of the AAO template.

J. Lee et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 19–23 21

Cross sectional images of Fig. 3(c) and (f) indicate that thevertically oriented TiO2 nanotubes were successfully formed to beattached on the Pt layer. Vertically oriented TiO2 nanotube arrayswithout cracks or defects over the wide area is thought to be theresult of adhesion of the bottom of the nanotubes to the Pt layerand complete removal of the AAO template. Fig. 3(e) clearlyexhibits the open ends of the nanotubes, which indicates that RIE

Fig. 3. Top-view and cross-sectional FESEM images of the TiO2

process successfully has etched both the barrier layer of the AAOand the end of the TiO2 nanotubes.

TEM was employed to observe the individual TiO2 nanotube.The specimen for the TEM study was prepared by placing a drop ofethanol suspension of the TiO2 nanotubes onto the carbon-coatedTEM grid. Fig. 4 shows the TEM images of a part of the individualTiO2 nanotube. The wall thickness of the nanotube was measuredas approximately 15 nm and was found to be fairly uniform alongthe length.

To characterize the crystal structure of the TiO2 nanotubes, XRDanalysis was carried out. Since it is hard to prepare the XRDspecimens purely composed of TiO2 nanotube arrays, the AAOtemplates with the deposited TiO2 nanotubes were ground intopowder for the XRD study. In order to investigate the effect ofannealing, the TiO2 nanotubes before and after annealing at 450 8Cwere prepared and measured. The X-ray patterns reveal thediffraction peaks of 2u = 258, 378, 488, 548, and 558 indexed for(1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1) of the TiO2 anatase phase,which appeared after annealing whereas only amorphous halofrom the AAO template was seen before annealing (see Fig. 5). Itreveals that TiO2 nanotubes became crystalline with the anatasestructure by the heat treatment.

3.2. Photovoltaic performance of DSSC

Fig. 6 shows the J–V curve of DSSCs based on working electrodesusing closed-ended and open-ended TiO2 nanotubes which wedenote as CE-DSSC and OE-DSSC, respectively. As shown in Table 1,PCE of OE-DSSC (h = 1.17%) is higher than that of CE-DSSC(h = 0.63%). Both types of DSSCs showed nearly equal open circuitvoltages which are 0.71 V and 0.74 V for CE-DSSC and OE-DSSC,respectively. Short circuit current density (JSC) of OE-DSSCs was65% higher than that of CE-DSSC. Such result indicates that the PCEimprovement is consequence of the enhanced JSC. It is commonly

nanotube arrays with closed end (a–c) and open end (d–f).

Fig. 5. X-ray diffraction pattern of annealed and pristine TiO2.

Fig. 6. I–V characteristics of the dye-sensitized solar cells based on TiO2 nanotubes

with open end and closed end.

Table 1Performance parameters of the DSSCs.

Code End-type of nanotube VOC (V) JSC (mA/cm2) Fill factor PCE (%)

CE-DSSC Closed 0.71 2.32 0.38 0.63

OE-DSSC Open 0.75 3.81 0.43 1.17

Fig. 7. UV–vis light absorption spectra of the dye desorbed from TiO2 nanotubes

with open end and closed end.

Fig. 4. TEM image of the TiO2 nanotube.

J. Lee et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 19–2322

known that JSC value is dependent on the light absorption which isstrongly related to the amount of adsorbed dye molecules [10].

In order to compare the amount of dye molecules adsorbed onthe two types of electrodes, UV–vis light absorption analysis wascarried out. Fig. 7 depicts the UV–vis light absorption spectra of thedye molecules desorbed from the working electrodes based on CE-DSSC and OE-DSSC. It is visible that the light absorption intensityfrom OE-DSSC is stronger than that of CE-DSSC, suggesting thatlarger amount of the dye molecules are adsorbed on the open-ended TiO2 nanotubes. This is considered that the open-endednanotube structure allows the dye molecules to adsorb onto theinner area of the nanotubes in addition to the outer area. Largeramount of the dye molecules adsorbed on the electrode resulted inincreased light absorption leading to higher JSC and, consequently,higher PCE.

Although the best performance recorded in this study whichwas PCE of 1.17% is only comparable to the reported values ofDSSCs using TiO2 nanotube arrays fabricated by anodizationmethod [4,11], our fabrication method is significant for dimen-sional and geometrical modification of TiO2 nanotubes for betterperformance of DSSCs due to the facile and separate control of thedimensional and geometrical factors. As well as the geometricalmodification of the end-type of the nanotubes in this work, furtherstudy on dimensional control of the TiO2 nanotubes includinglength and wall thickness can be carried out by our fabricationmethod and is being on progress in this laboratory.

4. Conclusion

We presented the successful preparation of highly ordered andvertically oriented TiO2 nanotube arrays for working electrode ofDSSCs. The TiO2 nanotubes were synthesized by ALD using AAOtemplate, then the end type of the nanotubes were altered fromclosed end to open end by RIE process. DSSCs were fabricated andcharacterized with the both types of TiO2 nanotubes. OE-DSSCexhibited higher PCE than that of CE-DSSC, which is attributed tothe higher light absorption by utilization of the larger surface areaof the open-ended nanotubes for dye molecules to adsorb.

J. Lee et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 19–23 23

Acknowledgement

This research was supported by a grant from the FundamentalR&D Program for Core Technology of Materials funded by theMinistry of Knowledge Economy, Republic of Korea (M-2008-01-0026).

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