Fabrication of glass-free photoelectrodes for dye-sensitized solar cells(DSSCs) by transfer method using ZnO nanorods sacrificial layer

Hui Song a,1, Hyun Ho Jeong b,1, Jeong Hoon Song b, Sung Woo Shin b, Jaeyi Chun a,Seong Ju Park a, Young Sun Won c,n, Gun Young Jung a,nn

a School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Koreab Korea Science Academy of KAIST, Busan 614-100, South Koreac Department of Chemical Engineering, Pukyong National University, Pusan 608-739, South Korea

a r t i c l e i n f o

Article history:Received 30 March 2014Accepted 7 June 2014Available online 16 June 2014

Keywords:Glass-freeTiO2 photoelectrodeTransfer methodDye-sensitized solar cellsZnO nanorods

a b s t r a c t

Glass-free TiO2 photoelectrodes for dye-sensitized solar cells (DSSCs) were prepared by a novel transfermethod using sacrificial ZnO nanorods. A TiO2 layer was formed on the hydrothermally grown ZnOnanorods by conventional doctor-blading and subsequent high-temperature calcination. Afterwards, anAg epoxy film on a PET substrate was adhered to the sintered TiO2 layer. A glass-free TiO2 photoelectrodewas finally generated by selective etching of the ZnO nanorods in an acidic solution. Glass-free DSSCswere fabricated with a platinium(Pt)-coated conductive PET counter electrode, having a powerconversion efficiency of 4.8% and a short-circuit current of 18.06 mA/cm2.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Most research on dye-sensitized solar cells (DSSCs) has beenfocused on enhancing the performance by introducing novelconstituents such as customized organic dyes or TiO2 photoelec-trodes with various nanostructures. Although the power efficiencyof DSSCs is competitive to other solar cells, there are still economicbarriers to their commercialization. The estimated price of a DSSCmodule ranges from 0.75$/W [1] to 3.22$/W [2], based on reason-able 5% efficiency and 98% yield of DSSC modules. Assuming thatthe price of crystalline silicon photovoltaic modules is simply 1.0$/W (becomes less in market nowadays), the averaged cost perDSSC module turns out to be two-fold less competitive in price.The only countermeasure then seems to be lowering the cost ofmaterials, which accounts for up to 80% of the total price [2].Because the glass substrates in DSSCs increase the materials costby 50%, replacing them with polymer substrates would be reason-able. The use of polymer substrates in DSSCs has been limitedbecause a high-temperature calcination step (at above 450 1C) isrequired for the formation of well-interconnected TiO2 photoelec-trodes. Although low-temperature (below 150 1C) calcinable TiO2

pastes have been synthesized commercially (e.g., Peccell Technol-ogies, Inc.) [3,4], the resulting interconnection between TiO2

particles is by far inferior [5].We thus reported a new fabrication process for glass-free TiO2

photoelectrodes that preserved a good particulate interconnectionvia high-temperature calcination. The key idea was to utilize cheapand highly etchable ZnO nanorods(NRs) as a sacrificial layer, asfollows: ZnO NRs were grown on a silicon substrate by hydro-thermal method; a TiO2 layer was coated on top of the ZnO NRslayer and then subjected to calcination; the TiO2 layer was then incontact with a metal epoxy paste on a polyethylene terephthalate(PET) substrate. By selectively etching the ZnO sacrificial NRs in anacidic solution [6], the TiO2 film was then transferred to thepolymer substrate, resulting in glass-free TiO2 photoelectrodes.The performance of the glass-free DSSCs, assembled with a Pt-coated PET counter-electrode, was also investigated.

2. Experimental details

Fig. 1 presents the fabrication of glass-free TiO2 photoelec-trodes. To grow the ZnO NRs by the hydrothermal method [7], asilicon (Si) substrate coated with a ZnO seed layer via a sol–gelmethod [8] was dipped into 500 mL of a nutrient solution contain-ing 25 mM of zinc nitrate hexahydrate (Zn(NO3)2 �6H2O) and25 mM hexamethylenetetramine (HMTA, C6H12N4) in deionized(DI) water. The growth was processed in a common laboratoryoven at 92 1C for 4 h (Fig. 1(b)). The substrate was then washed

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.06.0320167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ82 62 715 2324; fax: þ82 62 715 2304.nn Corresponding author. Tel.:þ82 51 629 6431; fax: þ82 51 629 6429.E-mail addresses: yswon@pknu.ac.kr (Y. Sun Won),

gyjung@gist.ac.kr (G. Young Jung).1 These authors contribute equally to this work.

Materials Letters 132 (2014) 27–30

under flowing DI water and dried with a nitrogen gas. A TiO2

particulate paste was coated on the grown ZnO NRs by doctor-blading and annealed at 450 1C for 1.5 h to remove the polymerbinders that could remain within the TiO2 particulate film. After-wards, a PET substrate coated with c.a. 30 μm thick silver (Ag)epoxy paste (90,000 cps., �10�4 Ω □�1

, Transene, Inc.) was placedin conformal contact with the TiO2 layer under a pressure of0.8 kg/cm2 at 90 1C for 2 h in a common laboratory oven for curingthe Ag epoxy paste to have an appropriate conductivity (Fig. 1(d)).Finally, a 0.5 M HCl solution was used to etch the ZnO NRssacrificial layer as shown in Fig. 1(e), resulting in the transfer ofa TiO2 particulate film to the Ag epoxy film on a PET substrate(hereafter, Ag epoxy film@PET, Fig. 1(f)).

The resulting glass-free TiO2 photoelectrode was dipped into a0.5 mM N719 (Ru[dcbpy(TBA)2]2(NCS)2) dye solution at roomtemperature for 24 h. Using a counter electrode of indium tinoxide(ITO)/PET substrate (45 Ω □�1, Aldrich) deposited with a20 nm thick platinum (Pt) and a Surlyn (Dupont) as a spacer,glass-free DSSCs were assembled with a gap of 60 μm. Theelectrolyte of 0.6 M 1-hexyl-2,3-dimethyl imidazolium iodide(C6DMI), 0.2 M LiI, 0.04 M I2 and 0.5 M tert-butyl pyridine (TBP)dissolved in 3-methoxypropionitrile (MPN)/acetonitrile (ACN) (1:1volume ratio) was injected into an active area (0.25 cm2) throughan entry port.

The ZnO NRs structure and glass-free TiO2 photoelectrode wereanalyzed by field emission scanning electron microscopy (FE-SEM,

Fig. 1. Scheme for the fabrication of a glass-free TiO2 photoelectrode. (a) Coating of the ZnO seed layer by sol–gel method, (b) hydrothermal growth of ZnO NRs, (c) coating ofthe TiO2 paste on the ZnO NRs by doctor-blading, (d) conformal contact of the Ag epoxy paste@PET to the TiO2 layer and curing of the Ag epoxy paste under pressure andheat, (e) selective-etching of the ZnO NRs in HCl solution and (f) the generated glass-free TiO2 photoelectrode.

Fig. 2. SEM images of (a) the ZnO NRs with an inset of a cross-sectional view and (b) the TiO2 particulate film on top of the ZnO NRs. (c) Magnified image of the dotted regionin (b). (d) XRD patterns of the ZnO seed layer, the ZnO NRs and the TiO2 film on ZnO NRs (●:ZnO, ■:TiO2).

H. Song et al. / Materials Letters 132 (2014) 27–3028

JEOL 2010F) and X-ray diffraction (XRD) using Cu Kα X-rayradiation (40 kV, 100 mA, Rigaku D/max-2400). The current versusvoltage (I–V) curves of the DSSCs were measured with a KeithleyModel 2400 source meter under back illumination of an AM1.5 simulated sunlight source (SANEI solar simulator, Class A) witha power density of 100 mW�2.

3. Results and discussion

Fig. 2(a) shows the hydrothermally grown ZnO NRs with aheight of 1 μm. A typical hexagonal wurtzite top facets areobserved. A 10 μm thick TiO2 particulate paste, composed of 15–20 nm TiO2 nanoparticles covered with polymer binders to pre-vent aggregation, was coated on the ZnO NRs layer by doctor-blading (Fig. 2(b)). Calcination at above 450 1C was then performedto remove the polymer binders and enhance the interconnectionamong the TiO2 nanoparticles. A magnified image (Fig. 2(c)) of thedotted region in Fig. 2(b) shows the TiO2 particulate film, whichdoes not penetrate down to the bottom of the ZnO NRs due to thestickiness of the TiO2 paste. The voids between the ZnO NRs cansupply the channels through which the ZnO etchant of HClsolution can flow and accelerate the etching of ZnO NRs uniformlyin the subsequent TiO2 film transfer step.

The crystallinity of TiO2 film on ZnO NRs was investigated viaXRD patterns, as shown in Fig. 2(d); the spectra show thatcrystalline ZnO NRs were grown on the ZnO seed layer. Sharpand intense peaks from the ZnO NRs with lattice constants ofa¼3.253 nm and c¼5.213 nm (JCPDS-65-3411) indicate the crystalplanes of (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) [9]. Likewise,the peaks from the TiO2 particulate film exactly match those fromthe crystal planes of (1 0 1), (0 0 4), (0 0 2), (2 1 1) and (1 0 5) of astandard TiO2 structure with lattice constants of a¼0.378 nm and

c¼0.951 nm (JCPSD-21-1272) [10]. No impurity peaks areobserved.

The need for high temperature calcination discourages the useof a polymer substrate because the glass transition temperature(Tg) of PET is generally as low as c.a. 100–120 1C. In our approach, atransfer method was thus introduced after TiO2 calcination withan Ag epoxy paste@PET substrate, where the Ag epoxy worked notonly as an adhesive but also as a conducting electrode. When theAg epoxy paste was thermally treated under a certain pressure, itwas turned into a solid film with a metallic sheet resistance of0.3 Ω □�1. The applied pressure is advantageous for conformalcontact at the interfaces, including the TiO2/Ag epoxy film and theAg epoxy film/PET substrate. Without applying the pressure, theAg epoxy film was likely to peel off the PET substrate locally.

Fig. 3(a) shows the transferred flat TiO2 film on the Ag epoxyfilm after selective wet-etching of the ZnO NRs. The XRD patternsof the TiO2 particulate film/Ag epoxy film@PET (Fig. 3(b)) show theTiO2 peaks along with the Ag peaks corresponding to (1 1 1),(2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystal planes, which is inaccordance with the standard XRD data (JCPSD-89-3722) [11]. NoZnO peaks are observed. These results illustrate a successfultransfer of TiO2 film to the Ag epoxy layer through the selectiveZnO wet-etching.

Glass-free DSSCs were fabricated by assembling the two poly-mer substrates of TiO2/Ag epoxy film@PET and Pt-coated ITO/PET

Fig. 3. (a) An SEM image of the glass-free photoelectrode tilted at 451. (b) XRD patterns of bare PET substrate (★), Ag epoxy film@PET and TiO2 particulate film on the Agepoxy film@PET (■:TiO2, ●:Ag).

Fig. 4. (a) Current (I)–voltage (V) characteristics of DSSCs with the Ag epoxy film@PET treated under only heat (■) and both pressure and heat ( ). (b) A cross-sectional SEMimage, showing the generated voids between the Ag epoxy film and PET substrate under only heat. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Table 1Photovoltaic performance of the glass-free DSSCs with different Ag epoxy filmtreatments.

Photovoltaic performance VOC (V) JSC (mA/cm2) Fill factor Efficiency (ŋ, %)

(a) Only heat 0.54 14.47 0.46 3.8(b) Both heat and pressure 0.55 18.06 0.48 4.8

H. Song et al. / Materials Letters 132 (2014) 27–30 29

as detailed in experimental part. The inset in Fig. 4(a) is thestructure of the glass-free DSSC. The photovoltaic properties aresummarized in Table 1. The photoanode subjected to pressureduring thermal curing of the Ag paste exhibited a higher photo-voltaic performance with a short-circuit current (JSC) of 18.06 mA/cm2 and a power conversion efficiency of 4.8%, compared to thecounterpart (only heat treatment). Without applying pressurewhile thermal curing, several voids were formed between the Agepoxy film and the PET substrate as shown in yellow boxes in Fig. 4(b). The electrolyte then filled within the voids unwantedly,causing the trapping or recombination of the generated electronsby the electrolyte interfacing the Ag film within the voids, andthus a lower JSC.

4. Conclusion

We demonstrated a transfer method to fabricate glass-free TiO2

photoelectrodes using a highly etchable ZnO NRs sacrificial layer,which was incorporated into glass-free DSSCs with a decent powerconversion efficiency of 4.8%. A significant decrease in the cost ofmaterials for DSSC modules, as well as robustness to externalshocks due to the elimination of brittle glasses, can be expected.

Acknowledgments

This work was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea

funded by the Ministry of Science and Technology (NRF-2013R1A1A2058739), the ICT & Future Planning (no. R15-2008-006-03002-0, CLEA, NCRC), the Research & Education Program bythe Korea Science Academy of KAIST (2013-Eng-12) and GSR(GISTSpecialized Research) Project through a grant provided by GISTin 2014.

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