Solar Energy Materials & Solar Cells 95 (2011) 3152–3156

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

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

Letter

Fabrication of highly ordered and vertically oriented TiO2 nanotube arraysfor ordered heterojunction polymer/inorganic hybrid solar cell

Jiwon Lee, Jae Young Jho n

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

a r t i c l e i n f o

Article history:

Received 16 May 2011

Received in revised form

22 June 2011

Accepted 24 June 2011Available online 16 July 2011

Keywords:

TiO2 nanotube

Atomic layer deposition

Ordered heterojunction

Polymer/inorganic hybrid solar cell

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.06.046

esponding author. Tel.: þ82 2 880 8346; fax:

ail address: jyjho@snu.ac.kr (J.Y. Jho).

a b s t r a c t

Highly ordered and vertically oriented TiO2 nanotube arrays with a length of 250 nm and a diameter of

70 nm were prepared by atomic layer deposition (ALD) coupled with anodic aluminum oxide (AAO)

template. An ordered heterojunction (OHJ) polymer/inorganic hybrid solar cell was fabricated by

successful infiltration of P3HT into the nanotube arrays. Structural features of the nanotube arrays

enabling the interdigitated structure of the OHJ were discussed and the performance of the solar cell

was characterized to be the power conversion efficiency of 0.50%.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

For several decades, photovoltaic cells based on light harvest-ing polymers have been paid much attention due to theiradvantages of low-cost and large-area production [1,2]. The mostwidely researched structure of the polymer solar cells is a bulk-heterojunction (BHJ), in which a photoactive layer comprises aninterpenetrating network of electron donor and acceptor materi-als [3]. In this structure, a very large interfacial area between thedonor and the acceptor materials can be achieved, leading to theefficient dissociation of excitons into holes and electrons.Recently, it was reported that power conversion efficiency (PCE)as high as 5% has been achieved by the BHJ solar cells fabricatedwith poly(3-hexylthiophene) (P3HT) and fullerene derivatives [4].However, BHJ solar cells intrinsically suffer from the chargecarrier recombination during the charge transport through therandom network of the materials [5,6]. Electrons or holes inevi-tably encounter dead ends or islands of a certain phase ofmaterials before they reach the respective electrodes, eventuallylimiting the PCE. In this regard, ordered heterojunction (OHJ)solar cells were proposed as a promising structure in which theelectron donor and acceptor materials were interdigitated withinnanometer scale, offering direct pathways for the charge trans-port [7–10]. To construct this type of structure, organic/inorganichybrid solar cells using vertically oriented nanorods or nanotubesof inorganic semiconducting materials as an electron acceptor

ll rights reserved.

þ82 2 884 7355.

have been preferred due to their structural robustness andelectrical properties [9–15].

TiO2 is one of the best candidates for electron acceptormaterials in solar cells by virtue of high electron mobility andexcellent chemical and physical stability [16]. Vertically orientedTiO2 nanotubes for this purpose have been developed by variousmethods including the sol–gel method [13,17] and direct anodi-zation of titanium (Ti) [18,19]. However, these methods havelimitations in synthesizing an ordered structure of uniform TiO2

nanotubes with precise control of dimension and geometry,which influences facile dissociation of excitons, the transport ofcharge carriers, and consequently high PCE. In addition, theprevious studies have experienced difficulties in accomplishingthe interdigitated nanostructrues of OHJ solar cells [11,13,18],because the infiltration of the light harvesting polymers into thenanotube arrays is hindered by structural factors of the TiO2

nanotubes from such methods, such as inter-tube space andrough surface of sidewalls. Furthermore, incorporation of thepolymers into the inorganic nanoporous structures is influencedby complicated factors including pore shape, surface interactionbetween two materials, spin-coating condition, and so on. Eventhough a variety of methods has been attempted by manyresearchers to solve the problem, such as melt-infiltration, surfacetreatment, and modification of spin-coating conditions, it couldnot be a universal solution [11,20–22].

In this study, we suggest a template-assisted method coupledwith atomic layer deposition (ALD) for TiO2 nanotube arrays withcontrolled dimensions. Anodic aluminum oxide (AAO) template,one of the best studied nanoporous materials, exhibits highlyordered and uniform nanopores with the controllable dimensions

J. Lee, J.Y. Jho / Solar Energy Materials & Solar Cells 95 (2011) 3152–3156 3153

such as diameter, depth, and inter-pore distance [23]. Atomiclayer deposition is well known as a gas-phase deposition methodwith self-limiting growth mechanism, which makes possible todeposit thin films on the AAO templates with precise control ofthe thickness by varying the number of cycles [24]. We demon-strate the successful fabrication of highly ordered and verticallyoriented TiO2 nanotube arrays using ALD on AAO template andtheir application to the OHJ polymer/inorganic hybrid solar celldevices. To obtain the transparent electrode, indium tin oxide(ITO) was sputtered on the bottom of the TiO2 nanotube arraysand was attached to a glass substrate using transparent epoxyadhesives. Resulted TiO2 nanotube arrays were expected to allowP3HT polymer to infiltrate into the TiO2 nanotube arrays, result-ing an OHJ hybrid solar cell with favorable structural features.

2. Experimental

2.1. Synthesis of vertically oriented TiO2 nanotube arrays

The whole sequence of the fabrication method is depicted inFig. 1. AAO used was of hexagonally packed nanopores withdiameters of 70 nm. The AAO template was introduced to the ALDchamber for TiO2 deposition. TiO2 ALD was performed at 150 1Cusing TiCl4 and H2O as precursors. One cycle of ALD wascomposed of a 0.2 s exposure of TiCl4, a 3 s N2 purge, a 0.3 sexposure of H2O, and a 3 s N2 purge. The cycle was repeated for300 times to deposit TiO2 layer on the AAO template followed byannealing at 450 1C for 5 h in air. In order to form a transparentelectrode, ITO was sputtered onto the TiO2-deposited side of theAAO template with a thickness of 200 nm. After bonding Pt wireusing silver (Ag) containing epoxy adhesives (Elcoats A-200,CANS, Inc.) on the ITO surface, the ITO sputtered side of theTiO2 coated AAO template was attached to the glass substrateusing thermo-curable epoxy adhesives (Araldites, Huntsman,Inc.) with a heat treatment at 40 1C for 3 h. To achieve the TiO2

nanotubular structures, the AAO template was to be selectivelyremoved. Aluminum layer of the template was dissolved byimmersing in the saturated solution of HgCl2, and then theresidual alumina layer was removed by treatment with 0.1 MKOH solution for 1 h and subsequent rinsing with deionized waterfor several times.

2.2. Fabrication of OHJ hybrid solar cell

Regioregular P3HT (Rieke Metals) was used as a light harvest-ing polymer without any further purification. P3HT was dissolvedin chloroform to make a 20 mg/ml solution, followed by filteringwith a 0.22 mm PTFE filter. Resulted solution was spin-coated on

Fig. 1. Fabrication of the OHJ polym

the vertically oriented TiO2 nanotube arrays at 2000 rpm for1 min in an argon-filled glove box (o0.1 ppm of O2 and H2O).An Ag metal cathode was thermally deposited through a shadowmask defining an active area of 0.0314 cm2. Lastly, for a facileconnection to the measurement system, a Cu wire was bondedonto the aluminum electrode using Ag paste.

2.3. Characterization

The nanostructures were characterized using field-emission scan-ning electron microscope (FESEM) (SUPRA 55VP, Carl Zeiss) andtransmission electron microscope (TEM) (LIBRA 120, Carl Zeiss). Thecrystalline structures were analyzed by X-ray diffraction (XRD)with CuKa radiation (lambda¼1.54 A) using diffractometer (D5005,Bruker). Current density–voltage (J–V) characteristics of the solar cellswere measured using MP-160 I-V curve tracer (EKO) under 100 mW/cm2 and AM1.5G simulated light radiation (Xenon arc lamp withAM1.5 filters). Incident-photon-to-current conversion efficiency(IPCE) measurement was carried out using a 300 W Xenon lightsource and a monochromator (Polaronix K3100 IPCE measurementsystem, McScience).

3. Results and discussion

3.1. Highly ordered and vertically oriented TiO2 nanotube arrays

Fig.2(a) and (d) shows FESEM images of the AAO template,which have highly ordered nanopores with the pore diameter anddepth of 70 and 250 nm, respectively.

After removing the AAO template, it is clearly seen that theTiO2 nanotube arrays are successfully formed. As shown in thetop-view images (Fig. 2(b)), the nanotubes are highly ordered andcrack-free with a tube-diameter of 70 nm. Cross-sectional image(Fig. 2(e)) depicts the vertically oriented TiO2 nanotubes on theITO with a tube-length of 250 nm. The dimension of the resultedTiO2 nanotubes accurately corresponds to that of the template,indicating that the TiO2 thin film was formed on the entire surfaceof the nanopores of the AAO template. High order and verticalorientation of the nanotube arrays without cracks over the widearea were derived from the strong adhesion of the ITO surface tothe glass substrate and the complete removal of the AAO template(see Supplementary material, Fig. S1). This fabrication methodcan be readily applied to other transparent substrates including aplastic film for flexible solar cells.

The structure of the individual TiO2 nanotubes was investi-gated by TEM and XRD. The TEM specimen was prepared byplacing a drop of the ethanol suspension of the TiO2 nanotubesonto the carbon-coated TEM grid. Fig. 3(a) shows the structure

er/inorganic hybrid solar cell.

Fig. 3. (a) TEM image of the TiO2 nanotubes and (b) X-ray diffraction pattern of annealed and pristine TiO2.

Fig. 2. Top and cross-sectional FESEM images of the AAO template (a and d), the TiO2 nanotube arrays (b and e), and P3HT spin-coated TiO2 nanotube arrays (c and f).

J. Lee, J.Y. Jho / Solar Energy Materials & Solar Cells 95 (2011) 3152–31563154

with closed end and hollow body, suggesting that TiO2 nanotubein this work can be categorized as a closed nanotube or an openhollow nanorod. Wall thickness of the nanotube is observed to beapprox. 20 nm and fairly uniform along the length. It is visiblethat the bottom parts of the nanotubes are connected to eachother, which results in an ITO layer thoroughly covered by TiO2

layer as depicted in Fig. 1. In the OHJ solar cells, such structure isable to act as a hole blocking layer by blocking a direct contact ofthe polymer to the ITO electrode. The crystalline structure of theTiO2 nanotubes was characterized by XRD analysis. Since it ishard to prepare the XRD specimens purely composed of TiO2

nanotube arrays, the AAO templates with the deposited TiO2 wereground into a powder. In order to investigate the effect ofannealing, the TiO2 nanotubes before and after annealing at450 1C were prepared and measured. The X-ray patterns revealsthe diffraction peaks of 2y¼251, 371, 481, 541, and 551 indexed for(1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1) of the TiO2 anatasephase, which emerged after sintering whereas only amorphoushalo from the AAO template was seen before annealing (SeeFig. 3(b)). It verifies that TiO2 nanotubes became the anatasecrystalline phase by the heat treatment.

3.2. OHJ polymer/inorganic hybrid solar cell

Utilizing the prepared TiO2 nanotube arrays, an OHJ polymer/inorganic hybrid solar cell was fabricated by spin-coating a light

harvesting polymer, a regioregular P3HT, followed by thermaldeposition of an Ag metal cathode. To investigate the morphologyof the polymer within the TiO2 nanotube arrays, FESEM wasemployed after the spin-coating process. Top-view image(Fig. 2(c)) shows that the TiO2 nanotube arrays are entirelycovered by the P3HT without any voids, and cross-sectional image(Fig. 2(f)) reveals that the P3HT is fully infiltrated into thenanotube arrays. Infiltration of the polymer into the nanotubearrays has been one of the major concerns in realizing theinterdigitated structure for electron donor and electron acceptorof OHJ solar cells. In this work, to our best knowledge, we firstachieved the successful result in filling P3HT into TiO2 nanotubearrays with no additional treatment. Comparing with the TiO2

nanotube arrays reported to date, those of this work exhibitdistinguishing features. Firstly, in contrast to the TiO2 nanotubesfrom the anodization of Ti, those of this work are located apartfrom each other with a distance of approx. 30 nm, offeringadequate room for the polymer solution to penetrate. Incorpora-tion of P3HT between TiO2 nanotubes is required for the excitongeneration and subsequent dissociation at the interface betweenthose two materials in an OHJ solar cell. Secondly, the nanotubesare highly ordered and vertically oriented with a smooth anduniform surface of sidewalls, which allows a simple route for aswift flow of the polymer solution to the bottom of the nanotubearrays. In addition to these structural factors, a relatively higherpolarity of chloroform than that of general solvents for P3HT in

Fig. 4. J–V characteristics of the OHJ hybrid solar cell; performance parameters are

summarized at the inset table.

Fig. 5. IPCE of the OHJ hybrid solar cell as a function of the wavelength of

monochromatic irradiation.

J. Lee, J.Y. Jho / Solar Energy Materials & Solar Cells 95 (2011) 3152–3156 3155

solar cells, such as dichlorobenzene and chlorobenzene, presum-ably gave rise to better contact of P3HT solution to the hydro-philic TiO2 surface. Combination of these favorable factorspotentially induced the successful filling of P3HT into the TiO2

nanotube arrays and consequently accomplished the interdigi-tated structure of the OHJ polymer/inorganic hybrid solar cell. Asthe deposition of Ag electrode completed the solar cell device,performance of the solar cell was evaluated. Fig. 4 plots J–V

characteristics of the device under AM 1.5 conditions. The solarcell performance parameters are summarized at the inset table inFig. 4. IPCE of the device was also measured and shown in Fig. 5.The maximum value of 4.91% was reached at 520 nm. The resultindicates that the highly ordered and vertically oriented TiO2

nanotube arrays function as an electron acceptor in the OHJhybrid solar cell. Although the device performance with a PCEof 0.50% is only comparable to the values reported for polymer/inorganic hybrid solar cells [11,13–15,25,26], it is still noteworthysince the result was obtained without any additional chemicalmodification or heat treatment. As the performance is thought tobe improved by further optimization of the experimental condi-tion including the length of the nanotubes and the spin-coating

condition of the P3HT polymer, additional experiments are under-way in this laboratory.

4. Conclusion

We presented the development of highly ordered and verticallyoriented TiO2 nanotube arrays using ALD on the AAO template andan OHJ polymer/inorganic hybrid solar cell using the nanotubearrays was fabricated and characterized. The successful infiltrationof the P3HT polymer into the nanotube arrays was achieved by thefavorable structural features of the TiO2 nanotube arrays. Althoughthe PCE of our device (0.50%) is only comparable to those ofprevious reports, the present study has significant implications inthat the interdigitated nanostructure for OHJ polymer/inorganichybrid solar cells was realized without any additional treatment.Further study on the experimental condition is being on progressand will promisingly improve the performance of the solar cell.

Acknowledgements

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). The authors also would like to thank Yohan Cho and KyusoonShin in Seoul National University for providing the AAO template.

Appendix A. Supplementary material

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.solmat.2011.06.046.

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