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Electrochimica Acta 99 (2013) 230– 237

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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ickel substrate covered with a Sn-based protection bi-layer as ahotoanode substrate for dye-sensitized solar cells

u-Ting Huanga,b, Shien-Ping Fengb, Chih-Ming Chena,∗

Department of Chemical Engineering, National Chung Hsing University, Taichung 402, TaiwanDepartment of Mechanical Engineering, The University of Hong Kong, Hong Kong

a r t i c l e i n f o

rticle history:eceived 8 February 2013eceived in revised form 14 March 2013ccepted 16 March 2013vailable online xxx

eywords:hotoanodeye-sensitized solar cellickel

a b s t r a c t

Ni is a metallic substrate commonly used in microelectronic and opto-electronic devices due to its highstability, cost-efficiency and flexibility. Up to now, Ni has not yet been used as the substrate for thephotoanode in dye-sensitized solar cell (DSSC) because the formation of p-type Ni oxide hinders theelectron transfer from n-type TiO2 mesoporous film to Ni substrate. In this paper, a Sn-based protectionbi-layer coating on the Ni foil successfully turns Ni into a good photoanode substrate in DSSC. A Sn layeris initially electroplated onto the Ni surface to prevent its oxidation. The Sn layer is then transformed intoa bi-layer of SnO2 and Ni–Sn intermetallic compound after high-temperature TiO2 sintering. Unlike thep-type Ni oxide layer, the top n-type SnO2 layer enables the electron transfer from the TiO2 mesoporousfilm to the conductive Ni substrate because its energy level of conduction band well matches that of TiO2.

inlectroplating

In addition, the interfacial Ni–Sn intermetallic compound is a good electrical conductor which facilitatesthe electron transfer. By carefully controlling the plating current density and time, a 1.2-�m-thick Snlayer with a smooth surface morphology is formed and is recognized as the best protection layer over theNi substrate. The energy conversion efficiency of the resultant DSSC achieves 4.422% with a short-circuitcurrent density of 10 mA/cm2, an open-circuit voltage of 0.643 V, and fill factor of 0.689 under AM 1.5back-side illumination.

. Introduction

Dye-sensitized solar cells (DSSCs) have attracted considerablenterests in the past decades as an alternative energy device whichan convert solar energy into electricity [1]. DSSCs possess manyttractive properties such as low production cost, less environ-ental impact during fabrication, and high energy conversion

fficiency. A typical DSSC consists of three major components: aye-adsorbed TiO2 mesoporous film coated on a transparent con-uctive oxide (TCO) glass as the photoanode, an electrolyte systemontaining iodide/tri-iodide (I−/I3−) redox couple in a proper medi-tor, and a counter electrode capable to catalyze the tri-iodideeduction. The energy conversion efficiency of DSSC has reachedbove 12% [2] based on zinc-porphyrin dye which is a great progressor real commercialization. Fluorine-doped SnO2 (FTO) transpar-nt conduction glass has been widely used as the photoanodeubstrate due to its good conductivity and exceptional thermal

tability. However, the rigid FTO substrate precludes the DSSCsrom being used in flexible and lightweight applications. In ordero broaden the application of DSSCs, flexible metallic foils have

∗ Corresponding author. Tel.: +886 422859458; fax: +886 422854734.E-mail address: chencm@nchu.edu.tw (C.-M. Chen).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.03.126

© 2013 Elsevier Ltd. All rights reserved.

been extensively investigated as the electrode substrate of DSSCs[3–11]. The flexible characteristic of metallic foils can also makethe roll-to-roll production of DSSCs possible. In addition, metalsare superior electrical conductors which are beneficial for electrontransfer.

Although metals are potentially good electrode substrate forDSSCs, they need to meet some requirements. First, the chosenmetal needs to be highly resistive against the corrosive liquid elec-trolytes. Second, the oxide layer formed on the metal surface due tohigh-temperature TiO2 sintering must possess a conduction bandof an energy level lower than the TiO2 mesoporous film, so thatthe electrons can transfer smoothly. Third, metals are opaque, soa highly transparent counter electrode is necessary for back-sideillumination if a metal foil is used as the photoanode substrate.Kang et al. [7] investigated the feasibility of various metals as pho-toanode substrate for DSSC. Ti, W, stainless steel (StSt), and Znproduced n-type semiconductor oxides on the surface and wereworkable in the DSSC operation. The best energy conversion effi-ciency achieved 3.6% for the DSSC employed Ti as the photoanodesubstrate under 1 sun illumination. It was ascribed to the perfect

match between the oxide layer (TiO2) formed on the Ti surfaceand the TiO2 mesoporous film in the energy level which benefitedthe electron transfer from the TiO2 mesoporous film to the Ti sub-strate. However, Al, Co, and Ni were unsuitable for DSSCs because

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nsulating oxide layers were formed on the metal surfaces whichlocked the electron transfer. Kang et al. [12] further improved theell efficiency to 4.2% by sputtering an ITO/SiOx bi-layer on the StStubstrate to inhibit electron recombination. Park et al. [4] added

pre-treatment of spin-coating titanium tetraisopropoxide (TTIP)n the ITO/SiOx bi-layer prior to TiO2 film coating and increased theiO2 film to 20 �m thick, a high cell efficiency of 8% was achieved.

As mentioned above, Ti is a potential photoanode substrate forSSC because its surface oxide layer is chemically similar to theiO2 mesoporous film. Many studies have demonstrated that theSSCs employed Ti as the photoanode substrate exhibited supe-

ior photovoltaic performance [5–7]. Ito et al. [5] dipped a Ti sheetnto a 40 mM TiCl4 solution twice before and after the coating ofiO2 mesoporous film and used the treated Ti sheet as the pho-oanode of DSSC. A high energy conversion efficiency of 7.2% wasbtained for the DSSC under back-side illumination. Another effi-ient pre-treatment of Ti was to etch the Ti foil using H2O2 solutionfter which the surface area of Ti was significantly increased [6].igh-density porous TiO2 nanostructures were formed on the Ti

urface after H2O2 treatment which provided remarkably high sur-ace area for electrical contact between the TiO2 mesoporous filmnd the Ti substrate. The electron transfer at the TiO2/Ti interfaceas enhanced, and a high energy conversion efficiency of 7.1% was

chieved accordingly.Ni is a metallic substrate commonly used in microelectronic

nd opto-electronic devices. It has been indicated that Ni also hasigh stability against corrosive liquid electrolyte when using inhe DSSCs [10]. Ma et al. [10] used Ni coated with sputtered Pt ashe counter electrode for DSSC and obtained an energy conversionfficiency of over 5%. Pt could also be grown on the Ni foil usinghemical deposition by which the surface area of the catalytic Ptayer was remarkably increased due to its nanostructure [11]. Theharge transfer resistance at the Pt/electrolyte interface was sig-ificantly reduced, and the cell efficiency was enhanced to over% accordingly. Though Ni is a good electrical conductor and haseen shown to be good counter electrode substrate for DSSCs, it

s seldom used as a photoanode substrate because a p-type oxideayer is formed on the surface during TiO2 sintering [7,13–16]. Inhis present study, we coated various metals as a protection layerver the Ni substrate to prevent Ni from oxidation. The experi-ental results showed that the common Ni can also be used as

n efficient photoanode substrate for DSSC by covering a Sn-basedrotection bi-layer on its surface. A Sn layer was electroplated on

Ni foil to protect its surface from oxidation. The deposition ofn has two advantages. First, a thin intermetallic compound wasormed at the Sn/Ni interface during subsequent high-temperatureiO2 sintering and it enhanced the adhesion between Sn and Ni.econd, a SnO2 layer was formed on the Sn surface also during sub-equent high-temperature TiO2 sintering. The n-type SnO2 layern contact with the TiO2 mesoporous film is beneficial for electronransfer because its energy level of conduction band is lower thanhe TiO2 mesoporous film. It has been demonstrated that the SnO2anowires coated with TiO2 nanoparticles were workable as thehotoanode of DSSC and the cell efficiency could reach 4.1% [17].ffects of plating current density and time on the surface morphol-gy and thickness of the electroplated Sn layer were systematicallyxamined. The photovoltaic performance of DSSCs employing then-protected Ni foil as the photoanode substrate was investigatednd the correlation with the electroplated Sn layer was discussed.

. Experimental procedures

.1. Preparation of the photoanode and fabrication of DSSC

The preparation procedure of photoanode for DSSC wasescribed as follows. First, a 2 cm × 3 cm Ni foil (Alfa Aesar, 99+%

a Acta 99 (2013) 230– 237 231

purity, 127 �m thick) was polished using fine sandpapers toremove surface contamination and was micro-etched in an 5%H2SO4 solution to remove surface oxide layer. Subsequently, thebare Ni foil was coated with a protection layer to avoid further oxi-dation. The protection layer investigated here included Au, Pt, andSn. Au was grown on the Ni foil using immersion (SHA-5, ShengHung Chemical Engineering Co. Ltd, Taiwan) and its thickness wasabout hundreds of nanometers. Pt was grown using sputtering andits thickness was roughly 10 nm. Sn was grown using electroplating(SnSO4, Atotech Inc.). The plating current density was controlled at0.02, 0.1, and 0.5 A/cm2 and plating time varied from 5 to 600 s.After Sn electroplating, screen-printing technique was utilized tocoat a 0.16 cm2 TiO2 mesoporous film (Ti-Nanoxide T20/SP, Sola-ronix) on the Sn surface. The thickness of the TiO2 mesoporousfilm was about 12–14 �m. The TiO2-coated Ni foil was sintered at450 ◦C for 30 min to remove the organics in the original TiO2 pasteand to generate a TiO2 network structure as the photoanode. Thephotoanode was then immersed in a 0.4 mM N719 dye solution(Solaronix) at room temperature for 12 h to allow dye adsorptionon the TiO2 nanoparticles, followed by rinsing with ethanol anddrying in air.

A 1.5 cm × 2 cm tin-doped indium oxide (ITO, 7 �/�, 1.1 mmthick, Gem Tech.) glass sheet was cleaned in 4 wt.% detergent inan ultrasonic bath for 30 min, followed by rinsing with de-ionizedwater. Platinum was deposited on the ITO glass sheet using sput-tering at 20 mA for 20 s as the counter electrode. The thicknessof the sputtered Pt layer was only 2 nm, so the ITO glass sheetremained highly transparent. The average light transmittance inthe visible light region was about 70%, which is typically suffi-cient for DSSC operation under back-side illumination [6]. Thedye-adsorbed TiO2 photoanode and the Pt-coated counter elec-trode were stacked face-to-face and sealed with a 25-�m-thickthermal-plastic Surlyn spacer (SX1170-25, Solaronix). The effec-tive area of the TiO2 photoanode was 0.16 cm2. A proper amountof liquid electrolyte (0.2 M PMII, 0.03 M I2, 0.2 M LiI, 0.2 M TBAI,0.5 M TBP in AN/VN) was injected into the gap between the twoelectrodes.

2.2. Measurements and characterizations

The fabricated DSSC was evaluated under AM 1.5 (1 sun illumi-nation, 100 mW/cm2) back-side illumination with a solar simulator(YSS-E40, Yamashita Denso Corp., Japan). Photocurrent–voltage(J–V) curves were recorded using a computer-controlled digitalsource meter (Keithley, model 2400). The electron transport prop-erty, such as lifetime, in DSSCs was measured using intensitymodulated photocurrent spectroscopy (IMPS) and intensity mod-ulated photovoltage spectroscopy (IMVS) [18]. A light-emittingdiode (LED, 525 nm) with a light intensity up to 4.705 mW/cm2 onelectrodes was used as the light source.

The surface morphology of as-plated Sn layer on the Ni foilwas examined using a field-emission scanning electron microscope(FE-SEM, JEOL, Japan). To understand the effect of TiO2 sinteringat 450 ◦C on the surface morphology of plated Sn layer, the as-plated Sn layer was put into a tube furnace at 450 ◦C for 30 min andthen was observed using FE-SEM. Atomic force microscope (AFM,Seiko, SPA400) was also used to examine the surface morphologyof plated Sn layer and to obtain the root-mean-square roughness(Rrms). The composition analysis of the oxide layer formed on the Snsurface after heat treatment at 450 ◦C was carried out using an X-rayphotoelectron spectrometer (XPS). The Sn-plated Ni foil was also

cross-sectioned to expose the Sn/Ni interface for SEM observation.Compositional analysis of any new phase formed at the Sn/Ni inter-face was carried out using an energy dispersive X-ray spectrometer(EDX).

232 Y.-T. Huang et al. / Electrochimica Acta 99 (2013) 230– 237

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ig. 1. Photocurrent–voltage (J–V) curves of the DSSCs based on various protectionayers on the Ni substrate.

. Results and discussion

.1. Feasibility analysis of the protection layers on Ni

Fig. 1 shows the J–V curves of the DSSCs based on various pro-ection layers on the Ni substrate. Based on Fig. 1, the photovoltaicarameters can be deduced and are listed in Table 1. As seen,

ig. 3. Top-view SEM micrographs showing the surface morphologies of the as-deposited20 s, (b) 360 s, (c) 600 s; 0.1 A/cm2 for (d) 25 s, (e) 75 s, (f) 125 s; and 0.5 A/cm2 for (g) 5 s

Fig. 2. Photocurrent–voltage (J–V) curves of the DSSCs based on various protectionlayers on the Ni substrate under dark condition.

the cell employing bare Ni without any protection layer as thephotoanode substrate exhibited very low efficiency (� = 0.38%). Asmentioned above, a p-type NiO layer was formed on the Ni surfaceafter high-temperature TiO2 sintering. When the p-type NiO layerwas integrated with n-type TiO2 mesoporous film, a p–n junction

was formed. The p–n junction induced the charge separation so thatthe NiO layer acted as a barrier layer blocking the electron transferfrom the TiO2 mesoporous film to the Ni substrate [17]. As a result,the cell efficiency was worse. Au is an anti-oxidation protection

Sn layers on the Ni substrate. The electroplating conditions are 0.02 A/cm2 for (a), (h) 15 s, (i) 25 s.

Y.-T. Huang et al. / Electrochimic

Table 1Photovoltaic parameters of the DSSCs based on various protection layers on the Nisubstrate.

Protection layer VOC (V) JSC (mA/cm2) FF � (%)

Au 0.55 4.23 0.34 0.79Pt 0.20 6.01 0.37 0.44

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Sn 0.59 5.95 0.53 1.85Bare Ni 0.72 2.28 0.23 0.38

ayer commonly used in electronic applications, while it seems toe unsuitable in the DSSC application. As seen in Fig. 1 and Table 1,he DSSC based on Au-protected Ni photoanode also exhibited poorhotovoltaic performance and the cell efficiency was only 0.79%.

t was reported that the liquid iodide/iodine system was usuallysed to recycle Au from waste metals because Au was highly solu-le in the liquid iodide/iodine system and could form stable iodides19]. Since the DSSC employed liquid iodide-based electrolyte, thehin Au layer might dissolve in the electrolyte and affected theerformance of electrolyte. The DSSC based on Pt-protected Ni pho-oanode also performed poorly in the photovoltaic characteristics.ig. 2 shows the J–V curves of the DSSCs based on various protec-ion layers under dark condition (without illumination). The dark

urrent was the highest for the DSSC based on Pt-protected Ni pho-oanode, indicating that the back reaction (charge recombination)as very serious. Therefore, Pt is also not a suitable protection layer

or Ni on the photoanode side. In DSSC, Pt is commonly used as

ig. 4. AFM images showing the surface morphologies of the as-deposited Sn layers on thc) 600 s; 0.1 A/cm2 for (d) 25 s, (e) 75 s, (f) 125 s; and 0.5 A/cm2 for (g) 5 s, (h) 15 s, (i) 25 s

a Acta 99 (2013) 230– 237 233

the catalytic material on the counter electrode to assist the tri-iodide reduction. Though coating a Pt layer onto the Ni substratecan protect Ni from oxidation, it inevitably enhances the tri-iodidereduction reaction on the photoanode side, making back reactioneasy to occur. As a result, the dark current gets worse.

In comparison with Au and Pt, Sn is apparently more suitable asthe protection layer on the Ni substrate. As seen in Table 1, the effi-ciency of DSSC based on Sn-protected Ni photoanode could reach1.85%, which was a meaningful value for the DSSC efficiency. Asa preliminary test, the thickness of Sn was arbitrarily set at about10 �m. In the following section, the thickness of Sn was set as avariable by adjusting the plating conditions to see whether the cellefficiency could be further improved.

3.2. Characterization of the electroplated Sn layers on Ni

Table 2 lists all electroplating parameters used for the Sn deposi-tion on the Ni substrate including plating current density and time.The surface morphologies of the as-deposited Sn layers were exam-ined using SEM and AFM and the results were shown in Figs. 3 and 4,respectively. It was found that lower the current density, higher thesurface roughness and larger the grain size. At a lower current den-

sity (0.02 A/cm ), the nucleation rate was slower. So, the quantityof the Sn grains was fewer and the Sn grains were able to grow to abigger size as seen in Fig. 3(a)–(c). Upon increasing the plating timeto 600 s, the Sn grains gradually transformed into column-type with

e Ni substrate. The electroplating conditions are 0.02 A/cm2 for (a) 120 s, (b) 360 s,.

234 Y.-T. Huang et al. / Electrochimic

Table 2Electroplating parameters used for the Sn deposition on the Ni substrate.

No. Current density (A/cm2) Plating time (s) Sn thickness (�m)

A-1

0.02

120 1.0 ± 0.2A-2 240 1.7 ± 0.2A-3 360 2.2 ± 0.2A-4 480 4.0 ± 0.5A-5 600 4.0 ± 0.8

B-1

0.1

25 1.0 ± 0.2B-2 50 1.2 ± 0.2B-3 75 2.0 ± 0.2B-4 100 2.6 ± 0.2B-5 125 3.3 ± 0.2

C-1

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acets and stacked with each other as seen in Fig. 3(c). When the cur-ent density was increased to 0.1 A/cm2, the Sn grains stacked morelosely and the surface roughness was reduced significantly. After25 s of electroplating, as seen in Fig. 3(f), the surface became quitemooth and some wrinkles were observed locally. At a higher cur-ent density (0.5 A/cm2), the Sn grains became round but the grain

tacking was not so close as compared to that at 0.1 A/cm2 cur-ent density. The Sn layer structure looked a little loose. In general,igher current density leads to faster nucleation and deposition

ig. 5. Top-view SEM micrographs showing the surface morphologies of the Sn layers afa) 120 s, (b) 360 s, (c) 600 s; 0.1 A/cm2 for (d) 25 s, (e) 75 s, (f) 125 s; and 0.5 A/cm2 for (g)

a Acta 99 (2013) 230– 237

rates, while too fast deposition of the Sn atoms might cause randomatomic stacking and the layer structure was destroyed accordingly.Another reason was due to significant hydrogen reduction at highercurrent density and many small bubbles were generated to affectthe atomic stacking.

It needs to note that the DSSC assembly includes a high-temperature sintering step with respect to the TiO2 mesoporousfilm. Since the melting point of Sn is 232 ◦C, the Sn layer electro-plated on the Ni substrate would melt during the TiO2 sintering at450 ◦C. To understand the effect of high-temperature sintering onthe surface morphology of the Sn layers, the samples shown in Fig. 3were further annealed at 450 ◦C for 30 min and the results wereshown in Fig. 5. In comparison with Fig. 3, the surface roughness ofthe Sn layers after high-temperature annealing (Fig. 5) increased.The SEM observation could also be confirmed by comparing theAFM results shown in Figs. 4 and 6, where Rrms was higher for theSn layers after high-temperature annealing. Fig. 7 shows the cross-sectional SEM micrographs of the Sn/Ni interface of the B groupsamples before (as-deposited) and after high-temperature anneal-ing. It was found that the Sn layers had completely consumed by theinterfacial reaction with Ni and the intermetallic compounds wereformed at the interface. According to the EDX results, the inter-metallic compound was either the Ni3Sn2 or Ni3Sn4 phase. For athinner Sn layer, as seen in Fig. 7(a), the Sn supply was limited, so

Ni3Sn2 phase as seen in Fig. 7(f). For thicker Sn layers, as seen inFig. 7(b)–(e), the Sn-rich Ni3Sn4 phase was also formed in additionto the Ni3Sn2 phase as seen in Fig. 7(g)–(j). Though the original

ter annealing at 450 ◦C for 30 min. The electroplating conditions are 0.02 A/cm2 for 5 s, (h) 15 s, (i) 25 s.

Y.-T. Huang et al. / Electrochimica Acta 99 (2013) 230– 237 235

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C groups exhibited better efficiency as compared with those in theA group. Some cells in the B and C groups could exceed 4%, includ-ing B-1, B-2, C-1, and C-2 cells. In these four cells, the thickness ofthe electroplated Sn layer was only about 0.6–1.2 �m, indicating

Table 3Photovoltaic parameters and electron lifetimes of the DSSCs based on Sn-protectedNi photoanodes.

No. VOC (V) JSC (mA/cm2) FF � (%) �n(ms)

Bare Ni 0.69 4.43 0.25 0.78 –A-1 0.658 7.798 0.680 3.470 16.61A-2 0.643 8.502 0.615 3.355 –A-3 0.625 8.439 0.606 3.181 13.54A-4 0.610 8.153 0.622 3.079 –A-5 0.603 9.094 0.578 3.166 11.81B-1 0.653 8.984 0.725 4.246 21.34B-2 0.643 10 0.689 4.422 –B-3 0.615 8.805 0.660 3.558 18.51B-4 0.590 9.539 0.587 3.322 –B-5 0.590 9.430 0.583 3.245 13.47C-1 0.669 9.219 0.669 4.112 23.54

ig. 6. AFM images showing the surface morphologies of the Sn layers after anne60 s, (c) 600 s; 0.1 A/cm2 for (d) 25 s, (e) 75 s, (f) 125 s; and 0.5 A/cm2 for (g) 5 s, (h)

n layer was transformed into the Ni3Sn4 or Ni3Sn2 phase, bothhases still adhered to the Ni substrate closely. Fig. 8 shows thePS result of the Sn surface after high-temperature annealing. Oxy-en was detected on the surface and its penetration distance wasbout 250 nm, indicating that a Sn-based oxide layer was formed onhe surface during high-temperature annealing. The ratio of atomiconcentration of Sn to O on the top surface was close to 1:2, revea-ing that a very thin SnO2 layer was formed on the top surface. Basedn the analyses of Figs. 7 and 8, the electroplated Sn layer on the Niubstrate was transformed into a bi-layer structure of SnO2/Ni–Snntermetallic compound after high-temperature annealing. Such

phase transformation is beneficial to the electron transfer onhe photoanode in the DSSC operation. First, the top surface SnO2ayer allowed the electron injection from the TiO2 mesoporous filmecause the energy level of the conduction band of SnO2 was lowerhan that of TiO2. Second, the SnO2 layer adhered to the Ni–Snntermetallic compound closely and so was the Ni–Sn intermetallicompound to the Ni substrate as seen in Fig. 7. The Ni–Sn inter-etallic compounds are also good electrical conductors which are

dvantageous for the electron transfer.

.3. Photovoltaic characteristics of DSSCs based on Sn-protectedi photoanodes

The photovoltaic characteristics of the DSSCs based on Sn-rotected Ni photoanodes were examined and the resultanthotovoltaic parameters were listed in Table 3. The data listed in

t 450 ◦C for 30 min. The electroplating conditions are 0.02 A/cm2 for (a) 120 s, (b)(i) 25 s.

Table 3 were the average values obtained based on at least threecells. It was found that, by carefully controlling the plating currentdensity and time, the cell efficiency was improved to over 3% whichwas much better than those in Table 1. In general, the cells of B and

C-2 0.686 9.062 0.668 4.143 –C-3 0.634 8.663 0.627 3.441 19.05C-4 0.623 8.713 0.604 3.260 –C-5 0.604 9.034 0.587 3.206 17.4

236 Y.-T. Huang et al. / Electrochimica Acta 99 (2013) 230– 237

Fig. 7. Cross-sectional SEM micrographs of the Sn/Ni interface of the B group

Fig. 8. XPS result of the Sn surface after annealing at 450 ◦C for 30 min. (a) Theatomic concentrations of Sn and O. (b) The relationship between sputtering timeand thickness.

samples before (as-deposited) and after annealing at 450 ◦C for 30 min.

that such a thin Sn layer was capable of being a protection layerover the Ni substrate. In addition, the surface roughness of the pro-tection layer played an important role in the cell performance. Asseen in Fig. 3, the surface roughness of the as-electroplated Sn layerwas 72.97–153.7 nm, 25.57–69.54, and 15.74–51.78 nm for the A,B, and C groups, respectively. Because the cells of B and C groupsperformed better in the efficiency, it was helpful for the protec-tion layer to have a smooth surface morphology. In summary, athin Sn layer with a smooth surface morphology was recognizedas a good protection layer over the Ni photoanode in the DSSCapplication. The result can be explained as follows. In the DSSCassembly, the Sn layer must undergo a high-temperature TiO2sintering at 450 ◦C during which the Sn layer melted. Molten Snreacted with the Ni substrate to form the solid Ni–Sn intermetalliccompounds. For a thinner Sn layer, Sn would be completely trans-formed into the solid Ni–Sn intermetallic compounds at a fasterrate. The solid Ni–Sn intermetallic compounds were able to sup-port the TiO2 mesoporous film without significant morphologicalchanges. So, the adhesion between the protection layer and theTiO2 mesoporous film would not be affected. However, for a thicker

Sn layer, the phase transformation from Sn to Ni–Sn intermetalliccompounds took a longer time. The TiO2 mesoporous film woulddirectly pressed on molten Sn for a while. Uneven pressing mightdestroy the top surface oxide layer and simultaneously destroyed

Y.-T. Huang et al. / Electrochimic

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ig. 9. Photocurrent–voltage (J–V) characteristics of the DSSCs under dark condition.

he contact between the protection layer and the TiO2 mesoporouslm. As a result, the cell performance became worse for thickern layers. Fig. 9 shows the J–V characteristics of the DSSCs underark condition. It was found that the dark current increased with

ncreasing the Sn layer thickness. As mentioned above, the contactetween the protection layer and the TiO2 mesoporous film becameorse when the thickness of the Sn layer increased. It would hinder

he electron transfer and inevitably raised the probability of backeaction (charge recombination). A poor contact might also createefect sites which acted as electron traps. As a result, the dark cur-ent was increased. The electron lifetime (�n) was also measurednd the results were listed in Table 3. It was found that thicker then layer, shorter the electron lifetime, which was consistent withhe tendency of dark current.

. Conclusions

Ni is inherently unsuitable as the photoanode substrate in theSSCs because its p-type surface oxide layer did not favor the elec-

ron transfer when integrating with an n-type TiO2 mesoporouslm. To make Ni possible in the DSSC applications, three metallichin layers, Au, Pt, and Sn, were deposited on the Ni substrate torevent it from oxidation. After DSSC characterizations, Au and Ptere still not suitable as the protection layer on the Ni substrate.u suffers the dissolution problem in the liquid iodide-based elec-

rolyte, and Pt enhanced the back reaction (charge recombination)n the photoanode side. By contrast, Sn was recognized as a promis-ng protection layer on the Ni substrate. The Sn layer electroplatedn the Ni substrate can be transformed into a bi-layer structure ofnO2/Ni–Sn intermetallic compound after high-temperature TiO2intering. The top SnO2 layer facilitates the electron transfer fromhe TiO2 mesoporous film to the Ni substrate because its conductionand energy level well matches that of TiO2. The interfacial Ni–Sn

ntermetallic compound owning a good electrical conductivity alsoenefits the electron transfer. In summary, the DSSCs based on then-protected Ni photoanodes exhibited a good energy conversionith the average efficiency above 3%, and the peak efficiency of

[

a Acta 99 (2013) 230– 237 237

4.422% can be achieved by optimizing the thickness and surfacemorphology of the electroplated Sn layer. In addition, electroplat-ing is beneficial for cost-down fabrication, while for large-area cellfabrication, more experiments need to be done to tune the elec-troplating parameters in order to achieve the optimal Sn layerthickness and uniformity.

Acknowledgement

The authors thank the financial support of the National ScienceCouncil, Taiwan, ROC.

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