Metal substrate based electrodes for flexible dye-sensitized solar cells: fabrication methods, progress and challenges.

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Suresh Kannan Balasingam,†a Man Gu kang†b and Yongseok Jun*c 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A step towards commercialization of dye-sensitized solar cells (DSSCs) requires more attention to engineering aspects, such as flexibility, roll to roll fabrication process, cost effective materials, etc. In this aspect, advantages of flexible DSSCs attracted many researchers to contemplate the transparent conducting oxide coated flexible plastic substrates and the thin metallic foils. In this feature article, the pros and cons of these two kinds of substrates are compared. The flexible dye-sensitized solar cells fabricated using metal substrates are briefly discussed. The working electrodes of DSSCs fabricated on various metal substrates, their fabrication methods, the effect of high temperature calcination and drawbacks of back illumination are reviewed in detail. A few reports on the flexible metal substrate based counter electrodes that could be combined with the plastic substrate based working electrodes are also covered at the end.

1. Introduction

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Photovoltaic devices are one of best choices among the group of energy conversion devices, wherein direct conversion of solar to electrical energy could be feasible with a low maintenance cost. To date many different kinds of photovoltaic devices have been developed, such as silicon cells, multijunction cells, thin film cells (CIGS, CdTe) and the third generation cells, including dyesensitized solar cells (DSSCs), organic solar cells, etc. However, most of these are expensive due to the high manufacturing cost that prevents the device from reaching wide spread consumers. Of the aforementioned, DSSCs are considered to be one of the low-cost solar cells, hence the research in this field has been blooming for the past two decades, since it’s invention in 1991.1 Despite the highest efficiency of over 12%, reported in the recent literature,2 the commercialization of DSSCs needs improvement in engineering aspects, such as flexibility, roll to roll ease of fabrication, and utilization of cost effective substrates, which must be an alternative to expensive transparent conductive oxide (TCO) based rigid glass materials. In this aim, flexible DSSCs fabricated using TCO- coated polymer substrates were introduced by many researchers. However, the working electrode of DSSCs consists of TiO2 based oxide material, which needs high temperature calcination at around 450°C. Generally, most of the transparent polymers such as poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), poly(ether sulfone) (PES) are unstable during high temperature calcination. Hence the usage of these polymer substrates limits the calcination of working electrode below 200°C. Even though 200°C is not an optimum sintering temperature for the TiO2 electrode fabrication process, various low temperature fabrication methods have been developed, such as microwave heating,3, 4 room temperature This journal is © The Royal Society of Chemistry [year]

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drying and low temperature sintering with low boiling point organic slurries,5-8 hydrothermal crystallization,9-11 electron beam annealing,12 mechanical compression,13-15 cold isostatic pressing (CIP),16, 17 lift off and transfer technique,18 pre- dye coated TiO2 particles followed by pressing method,19 chemical sintering,20 electrophoretic deposition,4, 6, 14, 21, 22 chemical vapour deposition with UV irradiation,22, 23 spray coating,3 pulsed laser deposition24 etc. Irrespective of fabrication methods, the efficiency value of flexible DSSCs using plastic substrate based working electrode was lower than the glass based DSSCs, due to the poor adhesion of TiO2 particles on their substrates and poor inter particle contact (necking) of TiO2 particles arising from a low temperature heat treatment process. Moreover, ITO/PET is difficult to produce with high transparency and good conductivity, and also, a TCO coated polymer substrate is prone to stripping and damaged electrode surface due to stress fatigue.25 Recently, Arakawa et al. fabricated a plastic substrate DSSCs with the highest efficiency of 8.1% using a TiO2 water paste via a mechanical pressing method. This is the highest efficiency so far reported for the complete plastic substrate based flexible DSSCs. The above mentioned low temperature processing techniques for flexible plastic substrates and their challenging aspects were briefly reviewed in a recent review article.26 In this feature article, we have mainly focussed on the study of metal substrate based working- and counter- electrodes for flexible DSSC applications. In contrast to the flexible plastic substrates, thin metal foil based substrates could withstand the high temperature sintering process, which favours better interparticle contact between TiO2 particles and a lower internal resistance of the cell. The effect of various substrates on the performance of DSSCs, different fabrication methods and a [journal], [year], [vol], 00–00 | 1

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Metal substrate based electrodes for flexible dye-sensitized solar cells: Fabrication methods, progress and challenges

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DOI: 10.1039/C3CC46224B

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Fig. 1 Conduction band energy levels (in Volts) of class I metal oxide semiconductors with respect to the normal hydrogen electrode in aqueous electrolyte at pH=1.0. Reprinted with permission from ref. 27

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diverse nature of electrode dimensions, such as 1D TiO2 nanotubes, nanowires, etc. fabricated on the metal foils and wires, are briefly reviewed. In addition, the metal substrate based counter electrodes, which could be coupled with the plastic substrate based working electrodes, are also reviewed at the end of this article.

2. Metal substrate based working electrodes

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Titanium metal substrate based working electrodes for dyesensitized solar cells were first fabricated by Gratzel et al. in 1988, without the intention of flexible devices.28 In this work, TiO2 film was deposited on the titanium metal sheets and metal wires; their light to electrical energy conversion was measured using the three electrode setup. After a decade, the first flexible solid state dye solar cells were fabricated by Meyer and coworkers.29 In this work, a TiO2 layer was formed on the flexible titanium foil and stainless steel (StSt) substrate using the rapid sintering method. This layer was stained with dye, followed by the impregnation of the hole conducting material (spiroOMeTAD) within the TiO2 active matrix. The circuit was completed with either transparent ohmic contact of the ITO layer or semi-transparent gold thin film. A flexible solid state device made of a Ti-foil based substrate showed an efficiency of around 0.8% under 1 sun illumination. In 2004, a European patent demonstrated the improvement of DSSCs using titanium and zinc- foil as current collecting substrates, and TiO2 and ZnO were used as respective photoelectrodes.30 However, the quantitative information, such as photovoltaic properties and efficiencies, are concealed in this patent. Potentiostatic electrosynthesis of TiO2 films on a stainless steel substrate was synthesized by Georgieva et al. using the aqueous precursors TiOSO4 and H2O2 at room temperature.31 The deposited film was annealed at 400°C in order to form crystalline anatase films, and their photoelectrochemical properties were studied using cyclic voltammetry under dark and ultraviolet light illumination conditions. However, the authors did not report the device performance of this flexible electrode. Based on the substrate nature of counter electrodes, DSSCs made of metal substrate based working electrodes are sub-divided in the section 2.1 and 2.2. Titanium nanotubes (TNTs) grown on Ti metal substrate based working electrode are explained under section 2.3 and Transparent conducting oxide (TCO) – free flexible DSSCs are reviewed under section 2.4. 2.1 Metal substrate based working electrodes coupled with glass-TCO based rigid counter electrodes 2 | Journal Name, [year], [vol], 00–00

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Fig. 2 (a) EIS spectra of DSSCs made using bare Ti substrate (solid diamond-black colour), bare stainless steel substrate (solid circle-red colour) and bare tungsten substrate (solid triangle-blue colour). (b) Schematic representation of conduction band edges (Ecb) of TiO2 working electrode (blue shade) and the Ecb of metal oxides formed on the corresponding metal substrates. Reprinted with permission from ref. 32

In 2005, our group fabricated flexible DSSCs using various metal substrates (StSt, W, Ti, Co, Ni, Pt Al and Zn).27 For the first time, the effect of different metal substrates on the solar to electrical conversion efficiency, and their mechanism were analysed in detail. From the observed results, the metals were grouped into two classes based on the formation of the corresponding oxide interface (during the high temperature calcination) between the metal surface and TiO2 film. Class I metals, such as Ti, StSt, W and Zn, lead to the formation of corresponding oxides (TiO2, Fe2O3, WO3 and ZnO) which are all basically n-type semiconductors. In contradiction to the class I metals, class II metals such as Al, Ni, Co and Pt lead to the formation of insulating oxides, which retard the transfer of electrons from the TiO2 film to metal substrate. Although class II metals lead to the formation of ohmic behavior oxide layers, modification of their interface with ITO and SiOx layers make them alternative flexible substrates for DSSC applications. DSSCs fabricated from class I metal substrates showed the decreasing Jsc and η values in the following order: Ti>W>StSt>Zn. The variation in Jsc and η values were ascribed to the conduction band energy level (Ecb) disposition of corresponding metal oxides and their mismatching with the TiO2 based working electrodes, as shown in Fig. 1. We have investigated the effect of an ITO and SiOx interlayer on the various metal substrates (Ti, W and StSt) using the electrochemical impedance spectroscopy (EIS).32 Three different schemes were designed in this study such as (1) metal substrate/TiO2, (2) metal substrate/ITO/TiO2 and (3) metal substrate/SiOx/ITO/TiO2. In the case of scheme 1, the titanium metal substrate showed the best efficiency of 4.0%, since the high temperature calcination of the metal substrate lead to the formation of a TiO2 anatase phase which has the same the This journal is © The Royal Society of Chemistry [year]



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Fig. 3 (a) Schematic representation of band energy diagram of the metal oxides including the TiO2 working electrode, ITO interlayer and the various metal oxides formed during the high temperature calcination process. (b) EIS spectra of DSSCs made using the ITO/SiOx interlayer between the TiO2 working electrode and metal substrates. Solid diamond-Ti substrate; solid circle-stainless steel substrate; solid triangle-tungsten substrate. Reprinted with permission from ref. 32

conduction band energy level of TiO2 working electrode. The titanium and tungsten substrate based devices showed no noticeable resistance in the first semicircle of EIS data (see Fig. 2a), but the StSt/TiO2 device showed a drastic increase in the internal resistance, which might be due to the high level Ecb mismatch between the TiO2 working electrode and Fe2O3 interface, as shown in Fig. 2b. In the scheme 2, when the thin ITO layer was introduced between the metal substrate and TiO2, the StSt/ITO/TiO2 device showed a better efficiency than that of the StSt/TiO2 device. Here, the thin film ITO layer bridging the conduction band energy level of TiO2 and Fe2O3 (formed during the high temperature calcination of StSt). The same ITO layer gave an adverse effect to the Ti/ITO/TiO2 system, because the interlayer acts like an electron well between the TiO2 working electrode and TiO2 thin film (formed during the high temperature calcination of Ti-foil) as shown in Fig. 3a, which also increases the internal resistance of the cell. Scheme 3 contains the presence of both underlayers, which increases the short circuit current density, as well as decreases the fill factor of all three devices. The lower fill factor value might be due to the increased internal resistance caused by the SiOx insulating layer, which was further confirmed by the additional semicircle (Z4) observed in the electrochemical impedance spectra as shown in Fig. 3b. Onodo and co-workers also compared the effect of different substrates (titanium, StSt and FTO glass) on the performance of DSSCs.33 The sheet resistance of the three substrates before and after calcination were compared. After high temperature calcination, the sheet resistance of FTO was much higher than the other two metal substrates. StSt substrate based cells showed a This journal is © The Royal Society of Chemistry [year]

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lower efficiency than the titanium based DSSCs due to the higher photocurrent leakage from StSt to the electrolyte. The best efficiency of titanium based cells was reported around 3.2 %. The effect of a roughened surface area of the stainless steel substrate on the efficiency enhancement of DSSCs was investigated by our group.34 The StSt-foil was roughened using sulphuric acid with sodium thiosuphate and propargyl alcohol. The chemical pickling of the StSt substrate resulted in the 23.6% increase in surface area over the non-treated StSt-foil, which was confirmed using the AFM analysis. This larger surface area lead to the increased Jsc value of the DSSC and showed the better electrical contact between the StSt substrate and TiO2 nanoporous film. The EIS spectra showed much lower electrical resistance (3.9 Ω) for the roughened StSt substrate compared to the nonroughened surface (17.9 Ω). Apart from the Jsc value, there was no noticeable change observed in the Voc and FF values. The DSSC made with the roughened StSt substrate showed the efficiency of around 5.7% which was 33% higher than the nonroughened StSt substrate (4.3%).We have also improved the efficiency of StSt based DSSCs up to 8.6% using the TiOx under layer.35 The introduction of TiOx underlayer in between StSt/SiOx/ITO and TiO2 working electrode suppressed the electron recombination loss and dark current density, which resulted to the improved conversion efficiency. In 2008, Liu and co-workers synthesized the single crystalline TiO2 nanowires (TNWs) on titanium foil using a novel alkali hydrothermal process without the addition of any titanium salt precursors.36 In the first step, single crystalline sodium titanate (Na2Ti2O5.H2O) nanowires were grown on the Ti-foil using 1 M aqueous NaOH via hydrothermal method. In the second step, the as-grown sodium titanate nanowires were converted to protonated bititanate (H2Ti2O5.H2O) nanowires by immersing the sodium titanate nanowires in a HCl solution, followed by high temperature calcination, a single crystalline TiO2 nanowires were formed. The DSSC made using 12 µm thick TNWs showed an efficiency of around 1.4%, still a lower value than the one shown by Grimes et al. using the single crystalline TNWs grown on a TCO substrate (5%).37 Kim et al. fabricated a non-thickness limited TiO2 mesosponge on titanium foil using an anodization method.38 In the first step, a compact (non-porous) titanium oxide layer was formed using the 10 wt% K2HPO4 in a glycerol electrolyte. The formed compact layer was etched with 30 wt% H2O2 or 0.2 M oxalic acid in a sonication bath, which leads to the formation of a mesosponge structure with uniform pore size. After calcination at high temperature the film shows the high crystallinity and has a good contact with the metal substrate. The mechanical scratching didn’t even cause the lift-off of the mesosponge layer, which is an advantage over the TNTs, where in the latter case, flaking is common due to high stress and mechanical scratches. The fabricated DSSC showed the best efficiency of around 4.2%. Tan and co-workers fabricated the electrophoretically deposited double layer TiO2 film (light scattering layer and transparent layer) on the titanium foil, followed by chemical treatment with tetra-n-butyl titanate (TBT) and then high temperature sintering.39 The post treatment with TBT precursor led to the formation of anatase nanoparticles within the double layer TiO2 matrix and the IMPS study confirmed that the Journal Name, [year], [vol], 00–00 | 3


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Fig. 4 (i) Mask patterns with hole widths of (a) 100 µm and (b) 200 µm. (ii) Optical images of net- like platinum counter electrodes (a) CE-B100 and (b) CE-B200. (iii) Schematic representation of different diffusion lengths of I3- to the neighbouring Pt particles. Reprinted with permission from ref. 40

formation of anatase nanoparticles facilitated faster electron transport and inferior electron recombination loss. Moreover, the presence of a light scattering layer also improved the cell performance with the efficiency value of 6.33% when combined with the platinized rigid TCO- counter electrode. Wu et al. fabricated a nanoporous TiO2 layer on the titanium foil using the micro-arc oxidation followed by an alkali etching process.41 The device fabricated with the Pt coated ITO-glass substrate and the nanoporous TiO2 layer showed a very low efficiency of around 2.2%. The vertically aligned zinc oxide nanosheets were deposited on the flexible titanium foil using the chemical bath deposition method.42 Compared with ZnO nanoparticles (NPs), ZnO film with a mosaic structure composed of ZnO nanosheets showed a higher surface area, more dye loading and longer electron life time of the cell. The device made of Ti/ZnO nanosheet working electrode and a platinum coated FTO- glass based counter electrode showed an efficiency of around 5.3%, which is higher than the cell made with Ti/ZnO nanoparticles working electrode (3.58%). Lin and colleagues studied the effect of the calcination temperature of binder free TiO2 paste on the flexible titanium metal foil.40 By increasing the calcination temperature, efficiency 4 | Journal Name, [year], [vol], 00–00

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of the DSSC increases until 350°C and reaches a saturated value beyond this temperature. They have compared the effect of a low temperature process on the metal (Ti-foil) and polymer (PEN/ITO) substrates. Calcination of binder free TiO2 paste at 120°C showed efficiencies of 3.40% and 2.08% for Ti-foil and PEN/ITO respectively. A higher efficiency of the metal based substrate was ascribed to a better conductivity and a higher scattering effect of titanium metal foil than that of the transparent plastic substrates. A novel concept of net-like Pt- counter electrodes was also introduced in the same article. A platinum thin film net with 100µm and 200µm square transparent windows was fabricated on the ITO glass using a photolithography method. Since the metal based DSSCs have the only choice of back illumination through the counter electrodes, net-like Pt- counter electrodes having transparent windows facilitates incident of more no. of photons onto the working electrode. A net-like Ptcounter electrode having 100 µm transparent windows showed the best efficiency of 4.77% with the optimized platinum deposition time of 10 seconds. The schematic representation of mask patterns and the optical images of two different counter electrodes are shown in Fig. 4. Fan et al. compared the performance of rigid glass-FTO/TiO2 and flexible Ti/TiO2 based solar cells.43 They have investigated the mechanism of electron transfer reaction using electrochemical impedance spectroscopy, open circuit voltage decay studies and Tafel plot analysis. Both the cells were illuminated through the back side with 1 sun irradiation. The cell made with flexible titanium foil showed 36% higher efficiency than that of the FTOglass based cell. From the electrochemical investigations, the Tisubstrate based flexible cells showed reduced sheet resistance, lower electron trap sites and a suppressed recombination process. The reduced sheet resistance is an inherent property of the metal substrate. The remaining factors arose due to the formation of a passivation layer at the Ti/TiO2 interface during the high temperature sintering process. In 2011, Chen and colleagues adopted the electrophoretic deposition of TiO2 nanoparticles on a Ti substrate followed by mechanical compression and high temperature sintering.44 The authors studied the effect of various pressures on the performance of DSSCs. The electrophoretically deposited photoanode compressed with 100 MPa pressure followed by high temperature sintering showed the best efficiency of 6.54%. Lee et al. studied the effect of an interfacial layer on the performance of Ti metal based flexible DSSCs.45 A sponge-like TiO2 underlayer was formed by the hydrogen peroxide treatment. TiO2 photoactive film was formed on the sponge-like underlayer using the screen printing method. Under optimized conditions, the device made with this underlayer structure showed the best efficiency of 6.75% which is higher than the bare, polished and TiCl4 treated titanium substrates. The higher efficiency value of H2O2 treated Ti substrate was ascribed to the increased electrical contact and better adhesion between TNPs and Ti substrate. Vijayakumar and co-workers utilized the StSt mesh- based flexible substrate and the TiO2 film as an active layer.46 In order to prevent the oxidation of StSt surface, a thin non-porous titania layer was coated on the StSt surface. The StSt-mesh with the protective titania layer showed a better efficiency of 1.68% than the bare StSt-mesh at 1.03%. But the obtained efficiency was

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Ti/ TNPsa W/ TNPsa StStb/ TNPsa Zn/ TNPsa Ti/ TNPsa Ti/ ITO/ TNPsa Ti/ SiOx/ ITO/ TNPsa W/ TNPsa W/ ITO/ TNPsa W/ SiOx/ ITO/ TNPsa StStb/ TNPs a StStb/ ITO/ TNPsa StStb/ SiOx/ ITO/ TNPsa Ti/ TNPsa StStb/ TNPsa Rough StStb/ TNPsa StStb/ SiOx/ ITO/ TiOx(under layer)/ TNPsa Ti/ TNWsc Ti/ Ti mesosponge Ti/ TNPsa (double layer) Ti/ TiO2 nanoporous layer Ti/ ZnO nanoparticles film Ti/ ZnO nanosheets Ti/ TNPsa

Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-ITO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-ITO/ Pt-mesh design Glass-FTO/ Pt Glass-ITO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-ITO/ Pt Glass-FTO/ Pt Glass-TCO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-TCO/ Pt Glass-TCO/ Pt Glass-FTO/ Pt Glass-ITO/ Pt

0.62 0.62 0.61 0.80 0.65 0.64 0.65 0.62 0.62 0.63 0.61 0.66 0.65 0.75 0.80 0.807 0.72 0.69 0.73 0.697 0.570 0.47 0.54 0.76

8.14 7.88 7.63 4.92 8.12 8.23 8.46 7.88 8.20 8.71 7.63 7.75 8.52 6.94 7.65 9.74 16.32 3.56 10.20 14.15 0.546 12.9 18.9 8.69

0.71 0.68 0.60 0.56 0.76 0.70 0.65 0.68 0.71 0.61 0.60 0.73 0.64 0.61 0.703 0.724 0.73 0.57 0.56 0.65 0.423 0.59 0.53 0.477

3.60 3.32 2.79 2.20 4.00 3.70 3.54 3.32 3.60 3.34 2.79 3.60 3.55 3.2 4.3 5.7 8.6 1.4 4.16 6.33 2.19 3.58 5.41 0.73

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15.13 11.36 13.03 4.72 5.32 14.9 3.93 4.22 11.94 12.72 10.73 7.6 9.9 4.01 14.38

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Ti/ TNPsa Ti/ TNPsa Ti/ TiO2 sponge/TNTsd StStb/ TNPsa StStb/ TiO2(under layer)/ TNPsa Ti/ TNPsa Ti/ SnO2 film Ti/ SnO2 film (post treated) Zn/ ZnO NRse + ZnO nanosheets Ti/ TNPsa/ SiO2 nanoparticles film Ti/ TNPsa/ SiO2 nanoparticles film (module) StStb/ TNPsa (1cm2) StStb/ TNPsa (compact layer)/ TNPsa (1cm2) StStb/ Ti (compact layer)/ TNPsa Ti/ TiO2 nanosheets/ TNPsa

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StSt: stainless steel. c TNWs: TiO2 nanowires. d TNTs: TiO2 nanotubes. e ZnO NRs: ZnO nanorods.

much lower than the value previously reported by our group.35 The effect of surface treatment of titanium substrates (with HNO3-HF acid) on the performance of DSSCs was investigated by our group.47 A surface treated Ti substrate showed a more enhanced efficiency of 9.20% which was remarkably higher than the untreated substrate (5.90%). Surface treatment developed sharp steps at the grain boundaries and also removed the disordered grain structure. High temperature calcination led to the formation of an oxide interface on the titanium substrate. The acid treated Ti surface produced a single crystalline rutile interface whereas untreated Ti substrate exhibited both anatase and rutile structures. Zhang et al. deposited nanocrystalline SnO2 films on a flexible titanium substrate using an electrochemical pulse-potential This journal is © The Royal Society of Chemistry [year]

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technique. DSSCs were fabricated using the SnO2 deposited Tifoil as a working electrode, and glass-ITO/Pt as a counter electrode. The optimized sintering temperature of SnO2 film showed a conversion efficiency of 0.47%.48 The successive steps of water vapour, HNO3, UV-O3 post-treatment of films led to the higher surface area and better crystallinity. The device made of post-treated SnO2 film showed an improved efficiency of 0.52%.49 Recently, Gao et al. fabricated ZnO nanorods-nanosheet hierarchical structures on zinc foil as a working electrode for DSSC applications.50 The device, made of a platinized FTO-glass counter electrode and ZnO hierarchical structure on zinc foil, showed an efficiency of 2.38%. Recently, a compact TiO2 layer was introduced as a blocking layer between the StSt substrate and TiO2 film, and the DSSCs [journal], [year], [vol], 00–00 | 5



Table 1 Metal substrate based working electrodes coupled with glass-TCO based rigid counter electrodes:

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Fig. 5 Schematic diagram of titanium foil based DSSCs without (a) and with SiO2 layer (b) on porous TiO2 working electrode. Reprinted with permission from ref. 51 5

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made of large area cells were evaluated by Lee et al.52 The presence of a compact blocking layer reduces the dark current and increases the onset potential to a more negative Fermi level. A cell made with compact blocking layers showed an efficiency of 4.51%, which is almost double the one made without a blocking layer (2.58%) for an active area of 1 cm2. Meng and co-workers studied the effect of a compact titanium layer between a StSt substrate and TiO2 film working electrode.53 The presence of a compact Ti-layer reduced the charge transfer resistance between the Ti/TiO2 interface, thus leading to the higher conversion efficiency. Under optimized deposition conditions, the best efficiency of around 2.26% was reported. The combined role of TiO2 nanosheets and TiO2 nanoparticles were investigated on the flexible DSSC device using titanium substrate by Tsai et al.54 TiO2 nanosheets were fabricated using H2O2 chemical treatment process. TiO2 NPs were screen printed on the nanosheets. The fabricated working electrode was coupled with platinized FTO-glass counter electrode which showed a highest efficiency of 7.1%. Lee et al. fabricated a SiO2 decorated TiO2 film on a flexible titanium foil substrate.51 Usually conventional DSSCs consist of a scattering layer of 500 nm TiO2 NPs film on the 20 nm TiO2 NPs base film, which enhances the performance of the DSSCs by enhanced light trapping behaviour during the front side illumination process. However, the presence of a scattering layer plays an adverse effect during the back illumination method. Therefore, the authors replaced the scattering layer with the SiO2 NPs layer which has a semi-transparent nature and better light scattering tendency leading to the high performance of DSSCs during the back illumination process. The schematic representation of DSSCs with and without SiO2 layer is shown in Fig. 5. A small area cell showed an efficiency of 6.76% when coupled with a platinized FTO-glass counter electrode, and a submodule cell having an area of 22.4 cm2 showed an efficiency of 5.54%. The photovoltaic characteristics of the above mentioned DSSCs made of metal substrate based working electrodes coupled with glass-TCO based rigid counter electrodes are summarized in Table. 1.

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2.2 Metal substrate based working electrodes coupled with polymer-TCO based flexible counter electrodes 45

In 2006, our group achieved the increased efficiency of 4.2% with the interlayer modification of StSt based flexible DSSCs using SiOx and ITO layers.55 We have also fabricated a prototype 6 | Journal Name, [year], [vol], 00–00

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Fig. 6 An image of DSSC prototype made with the stainless steel based flexible working electrode and polyethersulfone plastic substrate based counter electrodes. Reprinted with permission from ref. 55

of a large area flexible DSSC, as shown in Fig. 6. In this structure, an introduced ITO layer and SiOx acts as a blocking layer to prevent oxidation of stainless steel substrate. This modified scheme (StSt/SiOx/ITO/TiO2) prevents the direct contact of TiO2 with the StSt, which also avoids the Ecb mismatching of TiO2 with the Fe2O3, possibly formed during the high temperature calcination of a StSt substrate. The individual effect of the ITO layer and SiOx layer was also compared using the J-V characterization. The StSt based flexible working electrode (StSt/SiOx/ITO/TiO2) coupled with a Pt-coated plastic counter electrode, the efficiency of around 4.2% was reported during the back illumination process (illuminated through the Ptcounter electrode side, since StSt is opaque). But in the case of all FTO-glass based device, the efficiency of around 4.8% was reported under front illumination process (illuminated through the TiO2 working electrode side). The effect of reflection of light on the conventional glass-based DSSC was also studied in detail using the polished StSt substrate. The StSt substrate was kept either behind the counter electrode (for front illumination) or behind the working electrode side (for back illumination) and measurements without the StSt were also conducted for comparison purposes. In the absence of a stainless steel substrate, the conventional glass based DSSCs showed a higher efficiency through the front side illumination (4.8%) than the back illumination (4.0%). The lower efficiency of the back illumination was attributed to the higher recombination loss of photo-generated electrons at the front layer of the TiO2 film in the shorter wavelength region of sunlight. This phenomenon was also confirmed using the IPCE spectra. By keeping the StSt substrate behind the glass counter electrode, the efficiency of the device increased to 5.3% during the front illumination. And also, by keeping the StSt substrate behind the glass working electrode, the efficiency of the device increased to 4.8% during the back illumination. This increase in efficiency value inferred that the recombination loss during the back illumination was compensated by the enhanced reflection of the StSt substrate. This phenomenon also inferred that the uniformly sputtered SiOx and ITO layers, or the usage of highly reflective materials instead of SiOx, may enhance the reflection of light and in turn improve the efficiency of metal substrate based flexible DSSCs. A high efficiency flexible DSSC was made by Gratzel and colleagues using the titanium metal substrate with the optimized thickness of the TiO2 film.56 The schematic representation of the back illumination of the flexible device using a Ti/TiO2 working This journal is © The Royal Society of Chemistry [year]



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electrode and a PEN-ITO/Pt counter electrode is shown in Fig. 7. In order to investigate the effect of thickness of TiO2 film on the maximum efficiency value, the authors made glass-based DSSCs with various thicknesses from 3-21 µm of TiO2 film, and their front and back illumination efficiency were measured at 1 sun condition. Irrespective of the illumination direction, the glass substrate based DSSC made of 14 µm thick TiO2 film showed the maximum efficiency value. This optimized thickness was adapted to the flexible DSSC made with Ti-foil as a working substrate and a Pt coated ITO-PEN substrate as a counter electrode. Although the highest efficiency of around 7.2 % was obtained for flexible DSSC (with back illumination), still the reported conversion efficiency was lower than that of the glass based conventional DSSCs (with front illumination) with the same TiO2 film thickness. The similar Voc was observed for both the devices, but the Jsc and fill factor of flexible device was lower than the glass based DSSCs. The IPCE spectra revealed that the lower Jsc value of the flexible device was due to the absorption of light (from the 400 nm to 600 nm region) by the I3- electrolyte, and also due to the absorption of light (from the 540 nm to 680 nm) by the PEN-ITO/Pt counter electrode during the back illumination process. The observed lower fill factor value of the flexible device was due to the higher resistance of the electrodeposited Pt counter electrode (31 Ω) on the ITO-PEN substrate than the thermal deposited Pt/ITO-glass counter electrode (7.2 Ω). In 2007, we fabricated high efficiency, stainless steel substrate based individual DSSCs and modules (see Fig. 8) using TiO2 as a working electrode with the support of a ITO and SiOx interlayers.57 The platinum counter electrodes were made of flexible plastic substrates via the low temperature chemical reduction method. The individual laboratory standard cells (active area=0.25 cm2) and the modules containing each larger area cell (active area=5.18 cm2) showed the efficiency of 6.1% and 3% respectively. We have analysed the effect of different parameters on the performance of flexible StSt based DSSCs. Whenever the conducting plastic substrates (ITO-PET) are used, the internal resistance of the cell increases, due to the restriction of a low temperature sintering process which causes the lower fill factor value.56 In our case, however, we have observed the same fill factor value compared to the glass based DSSCs. Hence, we concluded that the internal resistance of the ITO-PET substrate was compensated by the higher conductivity of the stainless steel substrate. The transmittance of the conducting glass, conducting plastic and Pt/ITO-coated plastic film was investigated by using FT-IR spectroscopy. The highest transmittance was observed for the conducting plastic substrate. The Pt-ITO-coated plastic film showed lower transmittance than the bare conducting plastic This journal is © The Royal Society of Chemistry [year]

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Fig. 8 The flexible DSSC modules made of stainless steel based substrates. Reprinted with permission from ref. 57

substrate; still the transmittance value was higher than the glass based substrates, especially in the region shorter than 550 nm. This higher transmittance of the counter electrode compensated the electron recombination loss during the back illumination process. Moreover, the StSt substrate reflected the longer wavelength light (>580 nm, which could not be absorbed completely by the TiO2 and electrolyte), providing an additional energy for the dye excitation process. This phenomenon was confirmed using the IPCE spectra. In 2010, Chang et al. adopted the electrophoretic deposition followed by a high temperature calcination method for the coating of TiO2 film on the stainless steel and titanium substrate.58 Platinum coated ITO-PET counter electrodes were combined with the metal based working electrodes, in order to form the entire flexible device. The device made of a StSt based working electrode showed an efficiency of around 2.91% and the titanium foil based device showed an efficiency of around 2.4% under optimized conditions. The lower efficiency of the titanium metal based device than the StSt based device was explained by their internal resistance value. The resistance of the titanium substrate before (36 Ω) and after (64 Ω) calcination was higher than the resistance of the StSt based substrate before (27 Ω) and after (50 Ω) calcination. Lin and colleagues studied the effect of different parameters on the efficiency of flexible Ti/TiO2 based solar cells.59 They have varied the thickness of platinum film, sintering temperature of Ti/TiO2, thickness of the Ti-foil and the concentration of iodine in the electrolyte solution. The best optimized cell showed an efficiency of 5.95% using the Pt- coated PEN/ITO counter electrode. Zou et al. fabricated the dendritic ZnO nanowire arrays (NWs) on a StSt-mesh substrate for the large area flexible DSSCs.60 Initially, ZnO NWs are grown on the StSt-mesh and then etched with a thiacetamide based aqueous solution. This base structure was immersed into the fresh zinc precursor solution in order to form the hierarchical ZnO nanostructure. The etching time of ZnO NWs plays a crucial role on the performance of DSSCs. Different etching times led to the various dendritic configurations. The optimized etching time of 4 hours led to the best performance of 1.78% conversion efficiency. A bilayer structure consisting of ZnO nanorods (NRs) and ZnO nanoparticles (NPs) were deposited on the flexible titanium foil by Lin et al.61 A platinum coated ITO-PEN substrate was used as a counter electrode. The device consisting of a bilayer structure showed an efficiency of 2.19% which was higher than the individual ZnO NRs (0.90%) and ZnO NPs (1.80%) based Journal Name, [year], [vol], 00–00 | 7



Fig. 7 A sketch of flexible DSSC irradiated through the platinum counter electrode (back illumination). Reprinted with permission from ref. 56

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StSta/ SiOx/ ITO/ TNPsb Ti/ TNPsb StSta/ SiOx/ ITO/ TNPsb StSta/ SiOx/ ITO/ TNPsb Ti/ TNPsb StSta/ TNPsb Ti/ TNPsb StSta/ ZnO NWsc Ti/ ZnO nanoparticles film Ti/ ZnO NRsd Ti/ ZnO NRsd/ ZnO nanoparticles film Ti/ HNWse Ti/ TiO2 (nanoforest layer)/ TNPsb Ti/ TNPsb Ti/ ZnO NRsd + ZnO nanoparticles film Ti/ TNPsb (large area = 40 cm2) Ti/ TNPsb

PESf-ITO/ Pt PENg-ITO/ Pt PETh-ITO/ Pt PETh-ITO/ Pt PETh-ITO/ Pt PETh-ITO/ Pt PENg-ITO/ Pt Plastic-FTO/ Pt PENg-ITO/ Pt PENg-ITO/ Pt PENg-ITO/ Pt PETh-ITO/ PEDOTi PENg-ITO/ Pt PENg-ITO/ Pt-SWCNTs j PENg-ITO/ Pt PENg-ITO/ Pt- SWCNTs j PETh-ITO/ Pt

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11.2 13.6 13.1 10.6 6.71 7.7 11.69 6.25 6.50 4.14 7.00 7.91 16.05 11.20 7.06 11.525 11.53

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4.20 7.20 6.10 3.00 k 2.40 2.90 5.95 1.78 1.80 0.90 2.19 4.32 8.46 5.96 2.17 4.63 5.05

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StSt: stainless steel. b TNPs : TiO2 nanoparticles film. c ZnO NWs: ZnO nanowires. d ZnO NRs: ZnO nanorods. e HNWs: hierarchical nanowires. f PES: poly(ether sulfone). g PEN: poly(ethylene naphthalate). h PET: poly(ethylene terephthalate). i PEDOT: poly(3,4-ethylenedioxythiophene). j SWCNTs: single-wall carbon nanotubes. k module efficiency. 5

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Fig. 9 Schematic diagram of DSSC consists of Ti-grid on the counter electrode side. Reprinted with permission from ref. 66 10

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devices. The higher efficiency was explained based on their high surface area, better electron transport and least charge transfer resistance at the interface of photoanode and electrolyte. Recently, a composite film of ZnO nanorods (NRs) and ZnO nanoparticles (NPs) was deposited on a flexible titanium substrate by the same group.65 The DSSC made of optimized length ZnO nanorods and nanoparticles composite film as a working electrode, and a platinized PET-ITO counter electrode, which This journal is © The Royal Society of Chemistry [year]

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gave an efficiency value of 2.17%. This value was a 66.9% enhancement in efficiency over the DSSCs made of ZnO NPs alone (1.30%). Liao and co-workers fabricated hierarchical single crystalline TiO2 nanowire arrays (HNWs) on titanium foil using two steps hydrothermal processes.62 They adopted Liu et al’s alkali hydrothermal process36 in the first step to grow the TiO2 nanowires. In the second step, hierarchical nanorods on the base TiO2 nanowires were grown by a hydrothermal method using a K2TiO.C4O8 precursor. The hierarchical structure showed a higher surface area than the TiO2 nanowires (TNWs), which facilitates more dye adsorption and enhanced efficiency of the cell. The conversion efficiency of 4.51% is reported for the HNWs, which is higher than the bare TNWs (3.12%) based working electrodes. A completely flexible DSSC was also made with PET-ITO/PEDOT (poly (3, 4-ethylenedioxythiophene)) as a counter electrode and Ti/HNWs as a working electrode; the best efficiency of 4.32% was reported. Ma et al. fabricated a flexible TiO2 working electrode on titanium foil with the presence of a TiO2 nanoforest underlayer.63 The TiO2 nanoforest underlayer was formed on titanium foil using three-step chemical treatments. The successive steps of acid, H2O2, and TiCl4 led to the formation of a double-layer hierarchical structure made up of TiO2 nanoparticles in the bottom layer and a TiO2 nanoforest on the top layer. The commercial TiO2 paste was then screen printed on the nanoforest layer followed by high temperature calcination. A platinum coated ITO/PEN counter electrode was combined with the aboveprepared working electrode, and showed the cell efficiency of 8.46 %. [journal], [year], [vol], 00–00 | 8



Table 2 Metal substrate based working electrodes coupled with polymer-TCO based flexible counter electrodes:

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Fig. 10 (a) Top view of photoanode with micro hole arrays (MHAs). (b) Top view of titania nanotube arrays. (c) Side view of TNTs showing the 6µm length of nanotubes. (d) A typical MHA. (e) TNTs growing inside the MHAs. (f) 3D profile measurement of typical MHA with the diameter of 22 µm and the depth of ca. 0.5 µm. Reprinted with permission from ref. 68

Xiao and colleagues fabricated TiO2 film on titanium foil using the TiO2 colloidal paste via the doctor-blade method.64 For the counter electrode fabrication, single-wall carbon nanotubes (SWCNTs) were mixed with a platinum precursor and then sprayed on the PEN-ITO flexible transparent substrate. With the optimized Pt- and SWCNTs- concentrations, fabricated DSSC showed an efficiency of 5.96% under 1 sun conditions The same group also fabricated large area flexible DSSCs (40 cm2) with a Ti/TiO2 photoanode as a working electrode and PENITO/Pt-SWCNTs as a counter electrode.66 The optimized condition of the platinum and single-wall carbon nanotube concentrations were adopted from their previous studies.64 A titanium grid was introduced along the sides of the PEN-ITO counter electrode in order to reduce the internal resistance of the cell (see Fig. 9). The DSSC consisted of a titanium grid showed a much higher efficiency of around 4.63% than the cell made without any titanium grids (1.41%) under 1 sun conditions. Under natural light intensity (55 mW/cm2) conditions, the cell made with the titanium grid showed an efficiency of around 4.80%. This efficiency value was further improved to 5.29% when the UV-O3 post treated Ti/TiO2 electrode was used as a photoanode. Dao et al. fabricated a completely flexible DSSC consisting of a Ti/TiO2 working electrode and a platinized counter electrode deposited on a PET/ITO substrate using a dry plasma reduction method. 67 The efficiency of around 5% was reported from this device. The photovoltaic characteristics of the DSSCs are summarized in Table. 2. 2.3. TNTs on Ti-substrate working electrodes coupled with TCO- coated glass or polymer based counter electrodes

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Zhu and co-workers compared the charge collection efficiency of TNTs and TiO2 nanoparticles (TNPs) deposited on a titanium metal substrate for DSSC applications.69 For the same thickness of both electrodes, TNTs based cell showed higher Jsc and lower fill factor values than the TNPs-based DSSCs. The lower fill factor was due to the formation of an oxide interface which causes the higher sheet resistance at the Ti/TNTs interface. Both This journal is © The Royal Society of Chemistry [year]

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the electrodes showed almost similar electron transport times, whereas the electron recombination was 10 times slower in the case of TNTs, due to their better charge collection efficiency of a 1 D structure. Moreover, the TNTs based electrode has a stronger light harvesting effect. Based on the above merits, DSSCs fabricated with 5.7µm length TNTs showed a best efficiency of 3.0% using a glass-based platinized TCO counter electrode. Gratzel and co-workers synthesized a highly ordered TiO2 nanotube arrays (TNTs) on titanium foil.70 An ionic liquid electrolyte based DSSCs comprised of Ti/TNTs as a working electrode, with either a glass-ITO/Pt or PEN-ITO/Pt as a counter electrode, was fabricated. The efficiency of the Ti/TNTs+ glassITO/Pt device was around 3.3%, but in the case of the plastic counter electrode, Ti/TNTs+ PEN-ITO/Pt, the efficiency was observed around 3.6%. Lin et al. fabricated a DSSC with rough conical shaped TNTs as a working electrode and PET-ITO/Pt as a counter electrode.71 The efficiency of the device showed around 4.3%, which was higher than the device made of Ti/TNPs. The higher efficiency value of the device was attributed to the 1D structure having a rapid electron transport rate, prolonged electron life time, and reduced dark current. The combined effect of TNPs and TNT arrays on the performance of DSSCs was investigated by Nakayama et al.72 TNT arrays were electrochemically synthesized in an extremely dilute perchloric acid solution; the method was unique compared to the conventional anodization using fluoride-ion based electrolyte solutions. The device comprised of a bilayer structure showed a higher efficiency of 6.41% than the single layer TNTs (5.27%) and single layer TNPs (3.41%). In the bilayer structure, the nanoparticles (TNPs) layer deposited on the TNTs layer specifically reduced the back-scattered incident light out of the TNTs layer, which causes more light harvesting efficiency. In 2009, Schmuki et al. fabricated TiCl4 treated TNTs on titanium foil as a flexible working electrode for DSSC applications.73 After calcination at high temperature, the infiltrated TiCl4 precursor was converted into 3 nm TiO2 anatase

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Glass-TCO/ Pt Glass-ITO/ Pt PENd-ITO/ Pt PETe-ITO/ Pt PETe -ITO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt PETe -ITO/ Pt PETe -ITO/ Pt PETe -ITO/ Pt PET-ITO/ Pt Glass-FTO/ Pt PENd -ITO/ Pt Glass-FTO/ Pt Glass-FTO/ Pt Glass-ITO/ Pt Glass-ITO/ Pt Glass-ITO/ Pt Glass-ITO/ Pt Glass-ITO/ Pt Glass-FTO/ Pt Glass-TCO/ Pt Ti-mesh/ Pt Glass-FTO/ Pt PETe -ITO/ PEDOTf TCO/ Pt TCO/ Pt

0.61 0.743 0.709 0.741 0.751 0.764 0.806 0.775 0.64 0.68 0.52 0.52 0.703 0.705 0.731 0.72 0.67 0.722 0.635 0.707 0.690 0.698 0.71 0.71 0.79 0.717 0.72 0.754 0.761 0.71 0.63

9.0 6.11 8.99 7.03 11.12 7.1 10.1 12.9 6.11 11.2 4.95 4.66 8.08 10.13 11.25 10.45 9.3 8.33 14.37 11.1 13.1 13.9 10.69 12.94 13.30 14.67 17.48 13.45 11.96 7.12 16.05

0.55 0.725 0.561 0.63 0.51 0.631 0.645 0.64 0.48 0.49 0.56 0.50 0.61 0.64 0.65 0.46 0.43 0.71 0.69 0.57 0.58 0.59 0.73 0.73 0.69 0.51 0.421 0.71 0.69 0.63 0.62

3.0 3.29 3.58 3.3 4.3 3.41 5.27 6.41 1.89 3.80 1.47 1.23 3.46 4.56 5.39 3.45 2.67 4.28 6.28 4.44 5.14 5.75 5.55 6.68 7.23 5.36 5.30 7.20 6.26 3.36 6.23

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a TNTs: TiO2 nanotubes. b TNPs : TiO2 nanoparticles . c MHAs: microhole arrays. d PEN: poly(ethylene naphthalate). e PET: poly(ethylene terephthalate). f PEDOT: poly(3,4-ethylenedioxythiophene).

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particles. The nanoparticles-decorated TNTs showed increased surface area for the dye anchoring, which causes the two times higher efficiency value (3.8%) than the untreated TNTs (1.9%). Liu and colleagues utilized the flexible titanium mesh substrates instead of Ti-foil for the anodization reaction.74 Vertically oriented TNT arrays were grown on the Ti-mesh which was applied as a photoanode for DSSCs. The device with the glass- based Pt counter electrode showed an efficiency of 1.47% and the PET-ITO/Pt counter electrode showed an efficiency of 1.23%. The authors systematically studied the effect of nanotube length on the efficiency of the cell, and the TNTs with 40µm length showed the maximum efficiency value. They have also observed that the mesh number plays an important role in the efficiency value, i.e. as the mesh number increases, the area of the titanium surface, and in turn the formation of TNTs, increases, which has a direct influence on the efficiency of the cell.


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Chen and co-workers fabricated SrO coated TNTs filled with TiO2 nanoparticles (TNTs-SrO+TNPs) as a working electrode which was sandwiched with the platinum coated PET-ITO flexible substrate.75 The device made of the TNTs-SrO+TNPs electrode showed higher Jsc, Voc and η values than the bare TNTs, or TNTs filled with TNPs electrodes. The higher Jsc value was attributed to the light scattering effect of SrO on the TNTs, which was confirmed using IPCE spectra. The higher Voc value was due the presence of the SrO blocking layer, which lower recombination loss of electrons between the electrode/electrolyte interface. This phenomenon was further confirmed using the Mott-Schottky plots of the various photoanodes, whereas the TNTs-SrO+TNPs working electrode showed more negative flat band potential than that of the other electrodes. The authors assumed that the presence of a SrO blocking layer might form a wide band gap material such as SrTiO3, which might have acted [journal], [year], [vol], 00–00 | 10



Table 3 TNTs on Ti-substrate working electrodes coupled with TCO coated glass or polymer based counter electrodes:

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Fig. 11 Schematic representation of DSSC assembled using the titanium mesh based substrates. Reprinted with permission from ref. 81 5

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as a barrier layer to block the back electron transfer from the conduction band of TiO2 to the electrolyte (recombination loss). Liu et al. fabricated TNTs with the preformed micro hole arrays (MHAs) on the titanium substrate.68 MHAs were drilled on the titanium foil using the Nd:YAG laser system. TNTs were grown on the drilled titanium metal sheet using the electrochemical anodization method. TNTs grown on the Ti/MHAs have good mechanical properties and adhesion with the metal substrates. Moreover, TNTs were also grown inside the MHAs (See Fig. 10), which provided more surface area for the adsorption of many dye molecules. The DSSC made of Ti/MHAs/TNTs with the platinum coated glass-FTO counter electrode showed an efficiency of 3.45%, and the platinum coated PEN-ITO substrate showed an efficiency of 2.67%. Electrophoretically deposited TiO2 nanoparticles (TNPs) within the titanium nanotube structure was done by Wang et al.76 The device made of combined nanoparticles and nanotubes (Ti/TNTs/TNPs) showed a higher efficiency of 6.28% when compared to the bare TNTs (4.28%). The higher efficiency of the cell was claimed due to the higher surface area of working electrode and in turn more dye loading. Although hierarchical structure showed a higher efficiency, the Voc value of the device is lower than the bare TNTs. This lower Voc value was attributed to the faster recombination of electrons and the low flat band potential value of working electrode. Chen and colleagues deposited a composite of TiO2 nanoparticles (TNPs) and nanographite inside the TNTs.77 The device made of nanocomposite-infiltrated TNTs showed the best efficiency of 5.75% over the bare TNTs and that of the TNTs with TNPs. Lin and co-workers followed the Nakayama et al’s bilayer concept.78 In this structure, TiO2 nanoparticles are deposited on the TNTs. This bilayer structure showed an improved efficiency of 6.68% over the single layer made of TNPs. The laser-induced photo-potential and photo-current transient measurements confirmed that the higher efficiency was due to the improved electron transport and electron life time of the bilayer device than the single layer TNPs- based cells. In 2011, our group also followed Nakayama et al’s bilayer structure consisted of TNTs and TiO2 nanoparticles (TNPs).79 We have studied the effect of various thicknesses of the TNT layer and the TNPs layer. When TNPs are deposited on the titanium foil, the best efficiency of 5.9% was obtained for the 15 µm film. In the case of the bilayer structure, TNTs having a 4.36 µm length combined with a 15 µm TNPs layer showed the best efficiency of This journal is © The Royal Society of Chemistry [year]

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around 7.23%, which was higher than the previously reported value by Nakayama’s group.72 Pan and co-workers fabricated the TNPs-infiltrated TNTs as a working electrode on a flexible titanium substrate.80 TNPs are infiltrated using a TiCl4 precursor via hydrothermal method. The authors varied the nanotube lengths and number of cycles of nanoparticles loading. The DSSC made of 13µm TNTs film with the two times infiltration of TNPs showed the best efficiency of 5.36%. TCO-less DSSCs consisting of Ti-mesh substrate based a working and counter electrodes were fabricated by Hwang et al.81 Various lengths of TNTs were grown on the Ti-mesh. Either platinized Ti- mesh or bare Ti- mesh were used as a counter electrode. The schematic representation of TCO-free DSSC is shown in Fig. 11. An optimized length of 11.5 µm TNTs grown on Ti- mesh working electrode coupled with a platinized Ti-mesh counter electrode showed the best efficiency of 5.3%. Lei and colleagues deposited a hierarchical structure of TiO2 flowers (made of TNTs) on a flexible titanium metal substrate using a one-step direct hydrothermal process.82 The DSSC made of the Ti/TiO2 flower working electrode and a platinized FTO-glass based counter electrode showed the highest efficiency of 7.20%. The same working electrode (Ti/TiO2 flower) combined with a PEDOT/ITO-PET counter electrode showed a lower efficiency value of around 6.26%. Ultra-fine TiO2 nanotubes deposited on Ti-foil were introduced by Fan et al.83 Initially titanium substrate was treated under hydrothermal conditions using a NaOH solution. Ultra-fine TNTs were formed on the Ti substrate, which was then transferred to another Ti substrate with the help of an adhesion layer. The device made of Ti/ultra-fine TNTs working electrode coupled with the platinized TCO counter electrode showed the highest efficiency of 6.23%, which was two times higher than the TiO2 nanoparticles (Ti/TNPs) based device. The photovoltaic characteristics of the above mentioned TNTs based DSSCs are summarized in Table. 3. 2.4 Transparent conducting oxide (TCO) - free DSSCs

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2.4.1 TCO-free DSSCs having both metal substrate based working and counter electrodes Fan et al. fabricated conductive mesh-based flexible dyesensitized solar cells without using any transparent conducting oxide (TCO) substrates.25 In this scheme, stainless steel (StSt) mesh was coated with TiO2 porous layer and a 0.05 mm thick Ptfoil was used as a counter electrode, both the electrodes were sandwiched together with the electrolyte. The thickness of the porous TiO2 layer was optimized and the best efficiency of 1.49% was achieved with the 10 µm thick layer. TCO-less DSSC consisting of a StSt mesh-based floating working electrode and a platinized titanium as counter electrode was designed by Yoshida et al.84 The StSt mesh was first coated with titanium thin film. On that, TiOx was deposited using an arcplasma deposition method. The TiO2 dense layer was further deposited on the TiOx layer using the doctor-blade method. This novel scheme of StSt/Ti/TiOx/TiO2 reduces the thermal stress between StSt substrate and TiO2 film which avoids the cracking of the TiO2 dense layer. The schematic design of DSSC is shown in Fig. 12a. The cross-sectional scheme of the floating working electrode and its SEM image is given in Fig. 12b and c. Under optimized conditions, the best efficiency of around 5.56% was Journal Name, [year], [vol], 00–00 | 11


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Fig. 14 A sketch of flexible quasi-solid state DSSC. Reprinted with permission from ref. 86

Fig. 12 (a) Schematic diagram of DSSC consists of floating stainless steel mesh based TiO2 electrode. (b) Schematic representation of cross-sectional view of StSt mesh- based TiO2 working electrode. (c) SEM image showing the cross-sectional view of the floating electrode. Reprinted with permission from ref. 84 35

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Fig. 13 Schematic diagram of DSSCs based on all titanium substrates (a) front view; (b) Side view. Reprinted with permission from ref. 85

obtained for the floating electrode DSSC with a TiOx gradient layer. Wang et al. fabricated TCO-free DSSCs using Ti metal wires as a working electrode and the platinized Ti-foil as a counter electrode.85 The schematic design is shown in Fig. 13. The device consisting of two wire working electrodes connected in parallel showed the highest efficiency of 5.6% under optimized TiO2 thickness. Huang and colleagues made-up a stainless steel mesh-based quasi-solid state DSSCs using a gel electrolyte.86 A trace amount of MgO-deposited TiO2 colloid was spray coated on the StStmesh followed by the high temperature sintering. After that, a one dimensional nanofibrous titanium dioxide (1D-TiO2 NFs) web was deposited on the previous layer using the electrospinning method. Either StSt-foil coated with polypyrrole (StSt-PPy) or pure platinum foil was used as a counter electrode. The schematic assembly of the DSSC device is shown in Fig. 14. The working electrode of StSt-mesh/TiO2 NPs-MgO/TiO2 NFs combined with a platinum foil counter electrode showed the best efficiency of around 2.80%, whereas the same working electrode merged with

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Fig. 15 Schematic diagram of p-n junction type DSSC using copper mesh substrate. Reprinted with permission from ref. 87

PPy coated StSt-foil showed an efficiency of 2.30%. Due to the higher light reflection, the device made of a Pt-foil based counter electrode showed a higher efficiency than that of the StSt-PPy counter electrode. Moreover, the working electrode consisted of trace amounts of MgO nanoparticles could effectively passivate the surface sites of the TiO2 NPs wherein electron recombination reaction predominantly occurred. According to the BrunauerEmmett-Teller (BET) measurement, the 1D TiO2 nanofibrous web showed a 91% higher surface area than the TiO2 NPs, which could facilitate more dye anchoring and in turn improved the light harvesting efficiency. A novel, p-n junction-based flexible dye-sensitized solar cell was designed by Heng et al.87 The schematic diagram of the device is shown in Fig. 15. The p-type cuprous iodide (CuI, γphase) was deposited on the flexible copper mesh substrate. A micro/nano hierarchical structure of TiO2 was electrospun on the CuI layer. The TiO2 layer was sensitized with ruthenium-based N3 dye, and a platinum foil was used as the counter electrode. This journal is © The Royal Society of Chemistry [year]



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Fig. 16 Schematic diagram of TCO-free flexible DSSC. Reprinted with permission from ref. 88

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The fabricated DSSC was irradiated with light through the Cumesh/CuI layer. The direct contact of CuI layer with the TiO2 layer provided a p-n junction, which acted as a single directional conductive diode. This feature enabled the effective separation of electrons and holes, and the evaluation of charge transport in the cells, which in turn enhanced Jsc and the efficiency of the cell. For comparison purposes, a cell without a CuI layer was also fabricated by the authors. The presence of the CuI layer (Cumesh/CuI/TiO2/dye -cell) almost doubled the Jsc value and the efficiency of the cell compared to the Cu-mesh/TiO2/dye – cell. Xiao and co-workers fabricated a large area flexible TCO-free DSSCs using the Ti metal substrate based working electrode and platinized Ti-mesh as a counter electrode.88 The schematic design of the DSSC is shown in Fig. 16. In this work, homogenized platinum nanoparticles are electrodeposited onto a large area (80 cm2) titanium mesh. TiO2 nanoparticles film is coated using the doctor-scrapping technique. The fabricated cell showed an efficiency of around 6.13%. When they increased the area of the electrodes from 80 to 160 cm2, the efficiency value reduced to 5.23%. The surface treated Ti-meshes coated with platinum based counter electrodes were done by the same group.89 Ti-mesh was initially treated with an alkali hydrothermal process, followed by calcination at high temperature. In the second step, the calcined Ti-mesh was post-treated with hydrofluoric acid, leading to the formation of a high surface area. Platinum nanoparticles were deposited on the surface treated Ti-mesh using the chloroplatinic acid precursor via a heat treatment process. The conversion efficiency of around 6.17% was reported for the surface treated Ti-mesh counter electrodes, which was higher than the nontreated Ti-mesh counter electrodes (5.83%) under identical experimental conditions. The authors examined the effect of poly (3,4-ethylenedioxythiophene) PEDOT as an active counter electrode material for large area TCO-free DSSCs.90 PEDOT was electrodeposited on the Ti-mesh. Titania nanoparticles coated Tifoil was used as a working electrode. The fabricated large area DSSC (100 cm2) showed the solar to electrical energy conversion efficiency of 6.33%, which was higher than the DSSC fabricated using conventional platinized Ti-mesh-based counter electrodes (6.13%). By using the TiO2 nanowire interpenetrating network (TiO2-IPN) as a working electrode material, and Pt-single wall carbon nanotubes (Pt-SWCNTs) as a counter electrode material to the same DSSC scheme, the authors improved the efficiency to This journal is © The Royal Society of Chemistry [year]

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Fig. 17 Schematic diagram of TCO-less DSSC consists of two metal foils. 1st foil made of hole patterned stainless steel substrate which consists of dye coated TiO2 porous working electrode. 2nd foil platinum coated titanium substrate counter electrode. Reprinted with permission from ref. 91

6.43%.92 TiO2-IPN was formed on the Ti-foil using an alkali hydrothermal process. TiO2 NPs are deposited on the TiO2-IPN network using the doctor-scrapping method. The counter electrode was fabricated using the mixture of chloroplatinic acid solution and SWCNTs via spray coating technique. The same group further improved the efficiency of large area DSSC (100 cm2) to 6.69% by using the surface treated Ti-foil/TiO2 working electrode and Pt/PEDOT bi-layer coating on titanium mesh (Timesh/Pt/PEDOT) as the counter electrode. 93 The efficiency value for all the large area DSSCs are measured under the natural light irradiation intensity of 55 mW cm-2. Vertically aligned Zn2SnO4 nanowires (NWs) were grown on the flexible StSt-mesh substrate by Zou et al.94 A large area cell (10 cm x 5 cm) made with this anode and a flexible Pt- coated counter electrode showed an efficiency of 1.15%, which was higher than the Zn2SnO4 nanoparticles (NPs) deposited on the StSt-mesh. The higher efficiency of the nanowires-based device was attributed to the enhanced electron transport with the reduced transfer resistance. TCO-less DSSCs consisting of two metal foils were fabricated by our group in 2012.91 The schematic diagram of the DSSC is shown in Fig. 17. A perforated StSt substrate was made by photochemical machining. A TiO2 nanoporous layer was deposited on the perforated StSt substrate. Platinum coated Tifoil was used as a counter electrode. The light penetrated through the transparent glass or plastic substrate. The efficiency of the DSSC was around 2.25% for a 60 µm pitch. The lower performance was explained based on the perforation pitch, which limited the flow of electrolyte to the dye molecules and in turn enhanced the electron recombination reaction with the redox couple of electrolyte species. Bonilha and co-workers followed the same concept of TCO-free DSSCs using two metal foil substrates.95 However, the perforation was done using a laser micromachining method. The working electrode of a TiO2 coated perforated StSt substrate and the counter electrode consisted of a Pt coated StSt substrate. To avoid short-circuiting the cell, a Celgard membrane was placed in between two electrodes with the liquid electrolyte. For optimized conditions, the best efficiency of around 1.34 % was obtained for a 350µm pitch, but Journal Name, [year], [vol], 00–00 | 13



DOI: 10.1039/C3CC46224B

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StSta-mesh/ TNPsb StSta-mesh/ Ti/ TiOx/ TNPsb Ti -wires/ TNPsb StSta-mesh/ TNPsb-MgO/ TNFsc StSta-mesh/ TNPsb-MgO/ TNFsc Cu mesh/ TNPsb Cu mesh/ CuI/ TNPsb Ti/ TNPsb (large area - 80cm2) Ti/ TNPsb (large area – 80 cm2) Ti/ TNPsb (large area – 100 cm2) Ti/ TiO2 INPsd/ TNPsb (large area – 100 cm2) Ti (surface treated)/TNPsb (large area – 100 cm2) StSta/ Zn2SnO4 nanoparticle film StSta/ Zn2SnO4 nanowires StSta (perforated)/ TNPsb StSta (perforated)/ TNPsb StSta-mesh/ TNPsb-Glass paper

Pt-foil Ti/ Pt Ti/ Pt StSt/ PPye Pt-foil Pt-foil Pt-foil Ti-mesh/ Pt Ti-mesh (surface treated)/ Pt Ti-mesh/ PEDOTf Ti-mesh/ Pt- SWCNTsg Ti-mesh/Pt/PEDOTf Pt-foil Pt-foil Ti/ Pt StSt/ Pt Glass paper/ Pt –thin film

0.65 0.74 0.59 0.702 0.746 0.54 0.54 0.729 0.718 0.718 0.724 0.720 0.47 0.50 0.704 0.67 0.70

4.5 9.8 15.41 4.98 5.14 9.1 18.0 6.48 6.59 7.06 6.83 7.27 4.17 6.5 4.73 5.55 4.10

0.509 0.65 0.62 0.675 0.730 0.35 0.49 0.714 0.717 0.687 0.715 0.703 0.27 0.35 0.67 0.36 0.72

1.49 4.68 5.60 2.36 2.80 1.74 4.73 6.13h 6.17 h 6.33 h 6.43 h 6.69 h 0.55 1.15 2.25 1.34 2.05

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StSt: stainless steel. b TNPs : TiO2 nanoparticles film. c TNFs: TiO2 nanofibers. d TiO2 INPs: TiO2 interpenetrating network. e PPy: polypyrrole. PEDOT: poly(3,4-ethylenedioxythiophene). g SWCNTs: single-wall carbon nanotubes. h efficiency value measured under the natural light intensity of 55 mW/cm2. f

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Fig. 18 Schematic illustration of (a) conventional sandwichstructured DSSC (b) StSt mesh-based TiO2 working electrode supported on flexible glass paper design and its cross sectional view (c) Photograph of Korean traditional doors. Reprinted with permission from ref. 96

the corresponding fill factor value was much lower than the previously reported value by our group.91 Cha et al. fabricated a TCO-free, highly bendable DSSCs using a StSt mesh-based TiO2 working electrode supported on the flexible glass paper. 96 A thin platinum film was deposited on the This journal is © The Royal Society of Chemistry [year]

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rear side of the glass paper, which served as a counter electrode. Generally, conventional DSSC structures adopt a gasket type spacer, but this design is not suitable for the flexible DSSCs because more strain will accumulate on the gasket and bending capacity will be limited. The newly proposed design consisting of glass paper reduces the strain and makes the cell highly flexible. This structure was patterned for the traditional Korean doors, which consist of a wooden frame and bonded paper. The schematic representation of the conventional structure, the glass paper supported StSt-mesh structure, and the photograph of traditional Korean doors are represented in Fig. 18. The overall conversion efficiency of the cell was reported around 2%. By replacing the counter electrode with glass-FTO/Pt, the efficiency reached to 4%. Very recently, a monolithic design of back contact DSSC was fabricated by Fu et al.97 In this design, flexible titanium foil was used as a substrate and the Pt layer, ZrO2 porous layer, titanium thin film and TiO2 NPs layer was deposited subsequently on the Ti-foil. Electrolyte was injected into the porous matrix to facilitate the ionic conductivity. The electrical contact has been taken from the titanium substrate and the thin film of titanium layer. The schematic representation of this monolithic design is represented in Fig. 19. The efficiency of this monolithic back contact DSSC based on a titanium substrate was reported around 4.2%. The photovoltaic characteristics of all the TCO-free DSSCs [journal], [year], [vol], 00–00 | 14



Table 4 TCO-free DSSCs having both metal substrate based working and counter electrodes:

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Fig. 20 (a) Optical image of twisted wire shape flexible DSSC (WSFDSSC) (b) and (c) SEM images of top view of WSF-DSSC (d) Crosssectional view of WSF-DSSC. Reprinted with permission from ref. 98 55 10

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are summarized in Table. 4. 2.4.2 Metal wire based working and counter electrodes for fiber shape DSSCs TCO-free wire shaped flexible DSSCs were fabricated by Fan et al. in 2008.98 The working electrode consisted of TNPs coated StSt-wire and the counter electrode consisted of poly(vinylidene fluoride) (PVDF) polymer coated- Pt wire. The two electrodes were twisted together to form a wire shape DSSC, as shown in Fig. 20. Misra and co-workers fabricated a cylindrical shape DSSC by inserting the Ti-wire/TNTs working electrode, along with a platinum wire counter electrode into a capillary glass tube.99 The conversion efficiency of around 2.78% was reported for 55µm long TNTs. Liu et al. designed a three dimensional dyesensitized solar cell consisting of a spiral shaped Ti-wire/TNTs working electrode and platinized titanium metal wire counter electrodes, separated with a porous SiO2 layer.100 The schematic design of 3D-DSSC is shown in Fig. 21. The fabricated 3D-


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Fig. 21 Schematic diagram of 3-D DSSC. Titania nanotubes are formed on the Ti-wire served as the working electrode and the platinum film placed in between the spiral act as a counter electrode. Reprinted with permission from ref. 100

DSSC showed solar to electrical energy conversion efficiency of 4.1%. Zhang and co-workers made a single wire DSSCs consisted of carbon nanotubes film covered on the Ti-wire/TNTs based working electrode and the efficiency of the device was reported around 1.6%.101 A fiber shape solid state DSSC with the highest conversion efficiency of 1.38% was reported by Wang et al.102 An annealed Ti-wire substrate with the polymer-templated TiO2 nanocrystalline film was used as a photoanode, and a gold wire counter electrode was twisted around the photoanode surface. To replace the expensive Pt-wire counter electrodes in wire shape DSSCs, Hou and colleagues introduced carbon fiber-based counter electrodes.103 A Flexible TCO-free DSSC with Ti wire/TNPs as a working electrode and platinized carbon fiber based counter electrode showed the best efficiency of 5.8%. A large size flexible DSSC of around 9.5 cm was fabricated by Zou et al. with the high conversion efficiency of 5.41%.104 A conventional Ti-wire/TNPs working electrode and Pt-wire counter electrodes were used in this configuration. The same group made a highly efficient fiber shape DSSC with the conversion efficiency of 7.0%, using a Ti-wire/TNTs working electrode and a Pt-wire counter electrode.105 Recently, a fiber shape DSSC with the highest efficiency of around 7.2% was reported by Zou’s group using the platinized StSt–wire as a counter electrode and Ti-wire/TNPs as a working electrode.106 The authors introduced a one-step facile synthesis of TiO2 coating on Ti-wire and they have claimed that a platinized StStbased counter electrode would significantly reduce the cost of DSSCs.

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In 2004, Ma et al. investigated the various types of metal substrate based counter electrodes.107 The platinized nickel, stainless steel, copper and aluminium were used in this study. TiO2 nanoparticles (TNPs) deposited on the FTO-glass substrate were used as a working electrode. Though platinized copper and aluminium showed a lower sheet resistance, their stability in the iodine-based electrolyte was very poor; hence it was not suitable substrates for DSSC applications. StSt/Pt and Ni/Pt have relatively much lower sheet resistance than the PEN-ITO/Pt transparent counter electrodes, which might be due to their high Journal Name, [year], [vol], 00–00 | 15



Fig. 19 (a) Schematic cross-sectional view of (a) conventional DSSC design (b) monolithic back-contact DSSC. Blue arrows represent the light illumination directions. Reprinted with permission from ref. 97

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StSta/ Pt (plating) StSta/ Pt (thermal deposition) Ni/ Pt (sputtering) Ni/ Pt (electro-deposition) StSta/ Pt (sputtering) StSta/ Pt (sputtering) StSta/ Pt (sputtering) StSta/ Pt (thermal deposition) StSta/ Pt (thermal deposition) Carbon steel/ Pt (thermal deposition) Carbon steel/ Pt (thermal deposition) Cu/ graphite StSta(rough surface)/ Pt (sputtering) Ti/ PEDOT:PSSb + TiN nanoparticles Ti/ Mo2N (magnetron sputtering) Ti/ W2N (magnetron sputtering) StSta/ carbon aerogel Ti/ Pt nanoparticles + MWCNTsc Cu-Ni alloy mesh/ PEDOT:PSSb + carbon black Ti/ TiC Ti/ WO2 Ti/ VN Ti/MWCNTsc/CoS

Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd (10x10 mm) Glass-FTO/ TNPsd (15x15 mm) Glass-FTO/ TNPsd (20x20 mm) Glass-FTO/ TNPsd PETe-ITO/ TNPsd Glass-FTO/ TNPsd PETe-ITO/ TNPsd PETe-ITO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd Glass-FTO/ TNPsd

0.613 0.576 0.685 0.650 0.655 0.660 0.658 0.65 0.70 0.65 0.64 0.360 0.85 0.68 0.743 0.786 0.84 0.69 0.739

5.20 12.40 12.30 13.30 9.10 9.13 7.03 15.1 2.4 13.3 3.0 2.10 12.7 14.20 14.09 12.96 14.63 18.60 12.6

0.61 0.46 0.61 0.46 0.58 0.45 0.37 0.36 0.51 0.35 0.42 0.70 0.71 0.69 0.61 0.57 0.74 0.70 0.58

1.95 3.26 5.13 3.96 3.48 2.70 1.72 3.6 0.9 3.1 0.8 5.29 7.7 6.67 6.38 5.81 9.06 9.04 5.5

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StSt: stainless steel. PEDOT:PSS: poly(3,4-ethylene dioxythiophene) : poly(styrene sulfonate). MWCNTs: multi-wall carbon nanotubes. TNPs: TiO2 nanoparticles film. e PET: poly(ethylene terephthalate.

electrical conductivity and low inherent resistance of the metal substrates. Platinum deposition was done by three different methods in this study, such as sputtering, electroplating and thermal decomposition. Under optimized sputtering thickness of the Pt layer, the StSt and nickel substrates showed the best efficiency of around 5.24% and 5.13% respectively. The large area cells with the TiO2 electrode dimensions of 10x10, 15x15 and 20x20 mm were coupled with platinized StSt substrates and their cell efficiencies were reported as 3.48%, 2.70% and 1.72% respectively.108 All three dimensions of the DSSCs containing platinized TCO-glass substrate showed a lower efficiency than that of the platinized StSt substrates. Toivola and colleagues examined a few industrial sheet metal (carbon steel, zinc-coated carbon steel, StSt and copper) foils as counter electrodes for flexible DSSCs.109 Working electrodes were fabricated using TNPs on FTO-glass and PET-ITO substrates. Zinc-coated carbon steel and copper substrates have very low corrosion resistance in the iodine electrolyte medium. The platinized StSt counter electrode coupled with glassFTO/TNPs and PET-ITO/TNPs working electrodes gave the best efficiency of 3.6% and 0.9% respectively. Whereas the platinized carbon steel counter electrode paired with a glass-FTO/TNPs This journal is © The Royal Society of Chemistry [year]

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working electrode showed an efficiency of 3.1%, and the same counter electrode with a PET-ITO/TNPs working electrode gave an efficiency of 0.8% under optimized conditions. Bazargan and co-workers utilized the graphite spray coated on an industrial copper substrate as a counter electrode.110 The device made of this counter electrode showed a higher efficiency of around 5.29% than the graphite deposited ITO-PET substrates (3.38%). As per the previous reports,107 the stability of copper in iodine electrolyte is very poor, but the authors did not show any long term stability of DSSCs and corrosion stability tests. Kim et al. investigated the effect of surface morphology of StSt- based counter electrodes (StSt/Pt) on the performance of DSSCs.111 The researchers used a chemical mechanical polishing method and an acid etching method. The latter process produces a higher surface area than the mechanical polishing method. The platinum thin film sputtered onto the etched StSt substrate counter electrode showed the highest efficiency of 7.7% when coupled with glass-FTO/TNPs as a working electrode. Ho and colleagues deposited the composite film of PEDOT:PSS+TiN on a Ti substrate.112 The device fabricated using this counter electrode showed a conversion efficiency of 6.67%, which was higher than that of the platinum based Ti/Pt counter electrodes. [journal], [year], [vol], 00–00 | 16



Table 5 Metal substrate based counter electrodes:

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Wu et al. introduced novel flexible counter electrodes, such as molybdenum and tungsten nitrides deposited on Ti sheets.113 The efficiency of the Mo2N and W2N was reported around 6.38% and 5.81%. Zhao et al. applied various carbon based materials, such as carbon aerogel, carbon black and active carbon as active catalysts.114 The composite film made of carbon aerogel and poly(tetrafluoroethylene) deposited on StSt-mesh showed a high conversion efficiency of 9.06%, which was comparable to that of the conventional platinum based counter electrodes. Chang and co-workers fabricated Pt nanoparticles and a multiwall carbon nanotubes (Pt NPs+MWCNTs) based counter electrode film on a titanium substrate.115 The efficiency of around 9.04% was reported for this composite counter electrode, the value is much higher than the StSt/Pt- based conventional counter electrodes. The higher efficiency was explained based on the rough surface morphology of composite film. The PEDOT:PSS+carbon black composite catalyst deposited on a copper-nickel alloy mesh-based substrate was utilized as a counter electrode by Huang et al.116 The efficiency of the DSSC was around 5.5%, which was a little bit lower than the PEN/ITO substrate with the same redox catalyst (6.0%). Inexpensive counter electrode catalysts, such as titanium carbide (TiC), tungsten oxide (WO2) and vanadium nitride (VN) were introduced by Wang et al.117 The catalysts were deposited on the titanium foil substrate, TNPs coated on FTO-glass substrate was used as a working electrode. Coating of TiC or WO2 on the Ti substrate showed almost similar efficiency values. The efficiency of the Ti/VN based cell is a bit lower than the other two materials. Recently, Xiao and co-workers deposited cobalt sulphide and multi-wall carbon nanotubes hybrid film on flexible titanium substrate using electrophoresis and pulse Potentiostatic electrodeposition methods.118 The DSSC made of Ti/MWCNTs/CoS hybrid counter electrode achieved the solar to electrical conversion efficiency of around 8% which was higher than the conventional platinum (Ti/Pt) based counter electrodes (6.39%). The higher efficiency was attributed to the combination of good electrocatalytic activity of CoS and the higher electrical conductivity of MWCNTs which facilitates I3- reduction reaction. The photovoltaic characteristics of the above mentioned DSSCs made of metal based counter electrodes are summarized in Table. 5.

4. Conclusion and future outlook

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We have briefly summarized the metal substrate based flexible DSSCs so far reported in the literature. In general, the nature of the metal substrate (both working and counter electrode part) is one of the key factors to influence the efficiency of metal substrate based DSSCs. Most of the researchers used Ti-sheet as a working electrode substrate due to the Fermi level matching of TiO2 working electrode with the oxide layer formed (during high temperature calcination) on the Ti-substrate. But, usually the Timetal substrate has the finger-grained boundaries at the surface, which convert to the finger-grained TiO2 blocking layer. This kind of finger-grained blocking layer will not effectively retard the electron recombination reaction.47 Next to titanium, StSt is the frequently used substrate in DSSC applications due to their low cost. But the oxidation of StSt leads to the formation of Fe2O3 interlayer which has a large energy level mismatch with This journal is © The Royal Society of Chemistry [year]

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the TiO2 working electrode,32 and also the corrosion of StSt in iodine based liquid electrolyte is one of the key issues. Therefore, the effective blocking layer should be introduced between the metal substrates and TiO2 porous layer in order to avoid the electron recombination reaction and the Fermi level mismatch. The metal substrate based DSSCs have the only option of back illumination, therefore the transparency of the counter electrode (both substrate as well as active material) greatly influences the performance of DSSCs. We have classified the metal substrate DSSCs based on the nature of their counter electrode substrates such as rigid glass-TCO or flexible polymer-TCO substrates. Many laboratory scale DSSCs consists of glass-TCO based rigid counter electrodes as a pair for the flexible metal substrate based working electrodes.27, 32-54, 68-70, 72-74, 76-80, 82 Although, the DSSCs made of glass-TCO/Pt rigid counter electrodes has a lower transparency than the plastic-TCO/Pt flexible counter electrodes, the efficiency of the former one is higher than the later, due to the lower internal resistance value.56, 57 In order to realize an entire flexible DSSC, a better method of low temperature Pt deposition (with good adhesion and low resistance) on the plastic-TCO/Pt flexible counter electrode should be developed. One dimensional (1D) titanium nanotubes grown on Ti substrates were briefly investigated by several researchers due to their direct electronic path and lower electron recombination kinetics.119 TCO-free flexible DSSCs also have many advantages, such as high temperature sintering, low resistance, higher conductivity and cheaper price of metal substrates than the TCO, etc. Fiber shape flexible DSSCs made of metal wire based working and counter electrodes were also included in the TCOfree DSSC section. Fiber shape DSSCs have been applied to the textile and electronic industries, especially for the portable microelectronic devices due to their weavable wire shape. At the end, the metal substrate based counter electrodes were also summarized. Even though the flexible DSSCs have been investigated for a decade, not much literature exists in the field of metal substrate based flexible counter electrodes. A metal substrate based counter electrodes could be combined with the polymer based working electrodes; the former has higher electrical conductivity than the polymer-TCO substrates and also provides better mechanical strength with moderate flexibility. Very recently, solid state perovskite solar cells showed the highest efficiency of around 15%, where in, thin metallic silver film was used as a current collector and the FTO-glass played a role of substrate as well as current collector.120 Due to the high electrical conductivity and low cost, metal substrates could be implemented to the solid state perovskite solar cells which doesn’t even contain corrosive iodine electrolyte. The research on the metal substrate-based flexible solid state solar cells would fix another milestone to the scientists in the present solar cells era.

Acknowledgements This work was financially supported by (2013K000204, 20110030361 and 20110030211). 110

the

MEST

Notes and references a

Department of Chemistry and KIER-UNIST Advanced Center for Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea.



DOI: 10.1039/C3CC46224B

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Advanced Solar Technology Research Team, Electronics and Telecommunications Research Institute (ETRI), Gajeongno 218, Yuseong, Daejeon, Republic of Korea. c Department of Materials Chemistry and Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea. E-mail: [email protected]; Tel: +82-2- 450 -0440. † These authors contributed equally to this work

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