High-performance flexible dye-sensitized solar cells

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Cite this: RSC Adv., 2015, 5, 88052

High-performance flexible dye-sensitized solar cells by using hierarchical anatase TiO2 nanowire arrays† Zhisheng Chai,a Jiuwang Gu,a Javid Khan,b Yufei Yuan,a Lianhuan Du,a Xiang Yu,c Mingmei Wub and Wenjie Mai*a Pioneering products in wearable technologies, such as Apple Watch, Google Glass or Nike smart sneakers, are leading a new trend in electronic devices. However, the fast developing field of wearable electronics urgently demands lightweight and flexible/bendable energy supplying devices. Here, we present thin, lightweight and flexible dye-sensitized solar cells (DSSCs) using three different kinds of TiO2 nanostructures grown on Ti meshes as photoanodes. The flexible DSSCs based on hierarchical anatase TiO2 nanowire arrays exhibit a higher power conversion efficiency (1.76%) than those based on TiO2 nanotube arrays (1.52%) or TiO2 nanowire arrays (0.85%). This enhancement can be attributed to (1) the large surface area and (2) the low recombination rate of electrons and the redox electrolyte. Moreover, commercial ink has been identified as a promising alternative to fabricate a flexible counter electrode and the corresponding DSSCs achieve a considerable conversion efficiency of 1.59%. The as-fabricated

Received 26th August 2015 Accepted 30th September 2015

mesh based DSSC retains 88% of its PCE after bending for 300 cycles, demonstrating its good flexibility and fatigue resistance. In addition, the as-fabricated sealed DSSCs have been integrated to light LEDs,

DOI: 10.1039/c5ra17294b

which demonstrates that our flexible DSSCs are promising candidates for wearable/flexible energy

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supplying devices in everyday applications.

Introduction Flexible and wearable electronic devices are becoming predominant in modern electronics.1–3 Given this, lightweight and exible energy supplying devices, such as exible lithiumion batteries, supercapacitors or solar cells, have attracted extensive scientic and social research interest in recent years.4–10 Compared with other kinds of solar cells, dyesensitized solar cells (DSSCs) have many advantages, including low manufacturing costs, great stability and high power conversion efficiency (PCE).11–16 Flexible DSSCs have been previously studied using conductive plastic lms,17–20 metal wire and foil,21–26 carbon bers27 or commercial thread28 as the exible substrate. Among them, indium-doped tin oxide/poly(ethylene terephthalate) (ITO/PET) is one of the most popular candidates for a photoanode substrate.20 However, the TiO2 photoanode lms have to be annealed at under only 150 C due

a

Department of Physics and Siyuan Laboratory, Jinan University, Guangzhou, Guangdong 510632, P. R. China. E-mail: [email protected]

b

School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, Guangdong 510275, P. R. China

c Analytical and Testing Center, Jinan University, Guangzhou, Guangdong 510632, P. R. China

† Electronic supplementary 10.1039/c5ra17294b

information

88052 | RSC Adv., 2015, 5, 88052–88058

(ESI)

available.

See

DOI:

to the heat resistance limitation of the ITO/PET substrate, which results in low crystallization and poorly interconnected TiO2 nanoparticles (NPs), leading to a low PCE. For the requirements of good electrical conductivity and heatresistance ability, metal foils have been developed as substitutes for exible DSSCs. As is well known, metal foils are opaque and a back-side illumination is needed, therefore this structure suffers energy loss while light passes through the counter electrode and redox electrolyte. Besides, the low exibility of metal foils also restricts the application of exible DSSCs. Therefore, a novel ber-shaped DSSC using metal wire as the substrate, which has advantages including being lightweight, easy encapsulation and good exibility, has been proposed.29–32 The ber-shaped DSSCs are generally assembled into a double wire structure: one wire coated with a nanostructured TiO2 lm as the working electrode, while another wire (e.g., Pt) used as the counter electrode. The ber-shaped DSSCs can be woven into exible solar cell textiles for practical applications. However, ber-shaped DSSCs are still facing many challenges, such as connecting devices in serial/parallel and large scale weaving. Recently, several attempts have been made to produce exible/bendable DSSC textiles based on woven metal mesh electrodes.33–35 Compared to other types of exible DSSCs, mesh based DSSCs exhibit competitive superiority since they possess excellent heat-durability, great transparency and exibility. Generally, micrometer-sized metal wires are woven together to

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form the mesh body. TiO2 nanotube (NT) arrays or NP lms are then coated on the metal wire surface, which is used as the working electrode and assembled with the mesh counter electrode to form the desired exible DSSC. Although some previous works have demonstrated that mesh based DSSCs have been fabricated and achieved considerable PCE, they mostly lack the complete seal of devices and are far from practical applications. Moreover, many challenges remain to fabricate mesh based exible DSSCs, such as producing low-cost, highperformance, bending-resistant, long-term-stable photoanodes and counter electrodes. Herein, thin and bendable mesh based DSSCs are developed by using hierarchical anatase TiO2 nanowire (NW) arrays as the photoanode. A two-step hydrothermal method is used to synthesize the TiO2 hierarchical nanowire (HNW) arrays, which achieve higher PCE within DSSCs than TiO2 NT arrays and TiO2 NW arrays. This improvement due to the large surface area and low recombination rate was investigated in detail. The fabricated exible DSSCs have excellent bending resistance and can be integrated to light up a group of LEDs, which demonstrates that the mesh based DSSC is a promising candidate for wearable energy supply devices.

Experimental Pre-treatment The Ti meshes (diameter: 100 mm, size: 100 meshes) were purchased from Sheng Zhuo Mesh Products Co., Ltd. The surface of the Ti wire was coated with graphite, which was used as a lubricant for weaving into the square Ti mesh, which was ultrasonically washed with concentrated hydrochloric acid to remove the graphite layer. Then, the Ti meshes were ultrasonically cleaned in acetone, ethanol and deionized water in sequence. Fabrication of highly aligned TiO2 NT arrays on the Ti mesh The Ti mesh was electrochemically anodized at a constant potential of 60 V in an ethylene glycol solution containing 0.3 wt% NH4F and 2 vol% deionized water for 6 h. The Ti mesh and Pt wire were used as the working electrode and counter electrode, respectively. The as-prepared samples were annealed in air at 500 C for 30 min and then soaked in a 0.1 M aqueous TiCl4 solution at 70 C for 30 min, followed by annealing at 450 C for 30 min. Fabrication of vertically TiO2 NW and HNW arrays on the Ti mesh A piece of Ti mesh was put into a 100 mL Teon-lined stainless steel autoclave lled with 30 mL 1 M NaOH aqueous solution. Aer that, the sealed autoclave was kept in an electric oven at 220 C for 24 h. The as-prepared sodium titanate NW arrays on the Ti mesh were immersed in a 1 M HCl solution for 30 min, and H2Ti2O5$H2O NW arrays were obtained. TiO2 NW arrays were obtained aer annealing the H2Ti2O5$H2O NW arrays in air at 500 C for 60 min. TiO2 HNW arrays were obtained through a two-step hydrothermal reaction. Specically, the TiO2


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NW arrays on the Ti mesh were immersed in a 20 mL solution containing 0.35 g potassium titanium oxide oxalate dihydrate (C4K2O9Ti$2H2O), 2.5 mL H2O and 17.5 mL diethylene glycol, and kept at 180 C for 6 h. The as-prepared TiO2 NW and HNW arrays were soaked in a 0.1 M aqueous TiCl4 solution at 70 C for 30 min, and then annealed in air at 450 C for 30 min. Preparation of exible counter electrode The dip-coating method was adopted to prepare ink coated Ti mesh, as follows: the cleaned Ti meshes were dipped into commercial carbon ink for 1 min, taken out, and heated in air at 300 C for 20 min. The platinum coated Ti mesh was fabricated by thermal deposition of H2PtCl6 onto Ti mesh in air at 420 C for 30 min. Device fabrication The as-synthesized photoanodes were immersed in alcohol containing 0.5 mM ruthenium dye N719 and sensitized for 24 h. A exible DSSC was fabricated by stacking the exible dyesensitized photoanode and counter electrode. A piece of PET with multiple tiny holes (20 mm in thickness) was used as a separator inserted between the photoanode and counter electrode. Then, the DSSC was sealed between two pieces of the PET lm (60 mm in thickness) by pressing with surlyn lm at 110 C. The electrolyte, consisting of 0.6 M 1-butyl-3methylimidazolium iodide (BMII), 0.03 M I2, 0.06 M LiI, 0.5 M 4-tert-butyl-pyridine (4-TBP) and 0.1 M guanidinium thiocyanate, was injected into the space between the two PET lms. Characterization A eld emission scanning electron microscope (FE-SEM, ZEISS ULTRA 55), an X-ray diffraction (XRD) analyzer (Rigaku, MiniFlex600, Cu Ka, l ¼ 0.15406 nm, 40 kV) and a transmission electron microscope (TEM, JEOL 2100F, 200 kV) were used to characterize the morphologies and nanostructure of the samples. The photocurrent density–voltage (J–V) curves of the DSSCs were obtained by using an AM 1.5 G simulator (Abet Model 11000, 100 mW cm2) with the incident light intensity calibrated using a standard silicon solar cell. The amount of dye loaded on the TiO2 photoanodes (desorbed in a 0.02 M NaOH solution) was measured with an UV-visible spectrophotometer (Shimazu UV-2550). The electrochemical impedance spectra (EIS) of the samples were recorded using a VersaSTAT 3-400 (Princeton Applied Research) electrochemical workstation, which was carried out in dark conditions with the frequency ranging from 10 mHz to 1 MHz.

Results and discussion The highly aligned one-dimensional TiO2 nanostructures (NT, NW and HNW) on the exible Ti mesh substrate were used as photoanodes in DSSCs. Fig. S1† shows a large-scale (75 90 mm2) exible Ti mesh, which has good exibility and high bending fatigue resistance. The diameter of the Ti wires in the mesh is 100 mm; the distance between two adjacent wires is 100–300 mm (Fig. S2†). Fig. 1a shows schematically the synthetic RSC Adv., 2015, 5, 88052–88058 | 88053

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process of the anatase TiO2 NT, NW and HNW arrays. Vertically aligned TiO2 NTs were grown on the Ti wire surface using electrochemical anodization (Fig. S3†). The TiO2 NWs were fabricated with an alkali hydrothermal treatment and a subsequent ion-exchange reaction. Aer a further hydrothermal reaction, lots of TiO2 nanorod branches had grown on the TiO2 NW trunks and the TiO2 HNWs were obtained. Fig. 1b shows photos of TiO2 NT arrays, TiO2 HNW arrays, and dye-sensitized TiO2 NT arrays and TiO2 HNW arrays. It can be noted that the NT/Ti sample is slightly more transparent than the HNW/Ti sample. A SEM image of the Ti mesh aer anodization at 60 V for 6 h is shown in Fig. 2a. It can be observed that the TiO2 NT arrays are grown around the Ti wires closely and uniformly. The diameter of the TiO2 NT/Ti wires is 110 mm. Fig. 2b and c are side-view SEM images of TiO2 NT arrays. The length and outside diameter of TiO2 NTs determined from the SEM images are 35 mm (Fig. 2b) and 100–150 nm (Fig. 2c), respectively. The anatase TiO2 NW arrays on the Ti mesh were prepared via an alkali hydrothermal treatment and subsequent ionexchange reaction. At rst, the sodium titanate (Na2Ti2O5$H2O) NW arrays on the Ti substrate surface were synthesized through an alkali hydrothermal reaction. Then, the samples were immersed in a HCl aqueous solution. The H2Ti2O5$H2O NW arrays were obtained when Na+ was replaced completely by H+, which could be converted to anatase TiO2 NW arrays aer annealing at 500 C for 30 min. The XRD patterns of Na2Ti2O5$H2O, H2Ti2O5$H2O and TiO2 NW arrays are shown in Fig. S4.† Fig. 2d is a typical SEM image of a Ti mesh covered with TiO2 NW arrays. It can be observed that the diameter of the TiO2 NW/Ti wires is 125 mm. The side-view SEM images (Fig. 2e and f) further show that the TiO2 NWs are 16 mm in length and 100 nm in diameter. The growth of the TiO2 NW arrays can be attributed to the reactions below:

(a) Schematic illustration of the synthetic process of anatase TiO2 NT, NW and HNW arrays on a Ti mesh substrate. (b) Photographs of the Ti mesh, TiO2 NT/Ti mesh, TiO2 HNW/Ti mesh, dye-sensitized TiO2 NT/Ti mesh and dye-sensitized TiO2 HNW/Ti mesh. Fig. 1

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Fig. 2 SEM images of the as-prepared TiO2 NT, NW and HNW arrays. (a) Ti mesh covered with TiO2 NT arrays. Cross-sectional SEM images of TiO2 NT arrays at (b) low and (c) high magnifications. (d) Ti mesh covered with TiO2 NW arrays. Cross-sectional SEM images of TiO2 NW and HNW arrays at (e and g) low and (f and h) high magnifications, respectively. (i) More detailed view of the TiO2 HNWs.

hydrothermal

2Ti þ 2NaOH þ 4H2 O ! Na2 Ti2 O5 $H2 O þ 4H2 [

(1)

ion-exchange Na2 Ti2 O5 $H2 O þ 2HCl ! H2 Ti2 O5 $H2 O þ 2NaCl

(2)

calcination

H2 Ti2 O5 $H2 O ! 2TiO2 þ 2H2 O

(3)

The as-fabricated TiO2 NW arrays/Ti mesh undergo a further hydrothermal reaction in the solution containing C4K2O9Ti, diethylene glycol and H2O, leading to TiO2 HNW arrays composed of numerous short nanorod branches on the surface of the NW trunks. Fig. 2g–i show the cross-sectional SEM images of the TiO2 HNW arrays at different magnications. The length of the TiO2 HNWs is 17 mm, which agrees with that of NWs (Fig. 2g). As shown in the enlarged views (Fig. 2h and i), a large number of short nanorods (50–100 nm in length) are growth on the TiO2 NW trunks. The morphologies and crystal structures of anatase TiO2 NW and HNW arrays are further characterized by XRD and TEM analysis. Fig. 3a displays the XRD patterns of the as-prepared H2Ti2O5$H2O NW arrays, TiO2 NW arrays, TiO2 HNW arrays and TiO2 NT arrays on the Ti mesh. The bottom curve is the diffraction pattern of the pristine Ti mesh. The diffraction peaks of the H2Ti2O5$H2O NW arrays prepared through the hydrothermal treatment and ion-exchange reaction are in agreement with the H2Ti2O5$H2O phase according to its PDF card (JCPDS 47-0124). Notably, the (101) and (200) diffraction peaks of the anatase TiO2 phase (JCPDS 21-1272) can be observed at 2q ¼ 25.3 and 48.1 for the TiO2 NW, HNW and NT arrays, respectively. As is well known, the anatase TiO2 nanostructures are more outstanding in their photovoltaic performance for DSSC applications. Fig. 3b is a typical TEM image of an anatase TiO2


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NW of 100 nm in diameter, which is in line with the SEM observations. Fig. 3b combined with the SEM image (Fig. 2f) conrm that the TiO2 NW is a cylindrically shaped nanostructure. Fig. 3c displays a high-resolution transmission electron microscopy (HRTEM) image of the edge area of the TiO2 NW (marked in a red color in Fig. 3b), which conrms that the NW is single crystalline. The inter-planar spaces of (200) and (101) are 0.19 nm and 0.36 nm, respectively, which are in excellent agreement with the anatase phase. Furthermore, Fig. 3c shows that the axis of TiO2 NW is along the [100] direction.24,36,37 TEM results (Fig. 3d and e) of the TiO2 HNWs clearly show that a number of short TiO2 nanorod branches (10 nm in diameter and 20–80 nm in length) are grown directly on the long NW trunks. From the HRTEM investigation (Fig. 3f), it is found that the short nanorods are single crystalline structures and grow along the [010] direction, which is perpendicular to the axial direction [100] of the NW. The interplanar spaces of (200) and (020) are both equal to 0.19 nm. This lattice structure greatly matches the boundary structure of the NW trunk (as shown in Fig. 3c and f). These results suggest that the nanorod branches are well perpendicular to the NW. The as-synthesized TiO2 NW, HNW and NT arrays were further explored as photoanode materials to fabricate mesh based exible DSSCs. As depicted in Fig. 4a, the dye-sensitized

TiO2 photoanode, separator and Pt/Ti mesh counter electrode were sandwiched together to assemble the cell. The exible device is lled with a redox electrolyte and sealed with two pieces of PET lm. For the mesh based TiO2 photoanode, the conductivity of the entire Ti mesh should be considered. The formation of TiO2 lms on the Ti wire surface may break the connection between the interlaced horizontal and longitudinal Ti wires. Therefore, all of the horizontal and longitudinal Ti wires coming out of the sealed device were attached to a copper foil electrode for electron collection (Fig. 4b). To avoid the short circuit between the photoanode and counter electrode while bending the device, a thin PET lm with multiple tiny holes was used as a separator, inserted between the photoanode and counter electrode. The as-fabricated mesh based DSSC is exible and can be bent at different angles without fracture (Fig. 4c). Fig. 5a shows the typical J–V curves of exible DSSCs based on the three different photoanodes (NT, NW and HNW). Similar to a planar DSSC device, the effective area for calculating the PCE and dye adsorption amounts is the product of the length and width of the whole Ti mesh (1 cm 1 cm), as shown in Fig. 5a. The resultant photovoltaic parameters of different DSSCs including the open-circuit voltage (Voc), short-circuit current density (Jsc), ll factor (FF) and PCE (ƞ) are summarized in Table 1. The PCE of the NT, NW and HNW cells are 1.52, 0.85 and 1.76%, respectively. The normalized PCE are calculated using the projected area of the TiO2/Ti mesh photoanodes. The projected area of the NT, NW and HNW cells are 0.68, 0.75 and 0.75 cm2, respectively; and their normalized PCEs are 2.21, 1.13 and 2.39%, respectively. The Jsc of these cells are 7.04, 3.36 and 7.24 mA cm2. Actually, the Jsc values of the DSSCs are closely related to the amounts of dye adsorbed on the photoanode, which is determined by the surface area of the TiO2 nanostructures. The amounts of dye adsorbed on different photoanodes are calculated using the Beer–Lambert Law according to the UV-vis absorption measurement (Fig. S5†). The dye adsorption amounts of NT, NW and HNW arrays are 72.3, 56.7 and 112.9 nmol cm2, respectively, as shown in Table 1. This result indicates that the surface areas of the different TiO2 nanoarrays are ascending in the sequence of NW < NT < HNW. Hence, the increased dye loading amounts of the larger surfaces denitely contributed to the enhanced photocurrents and thus

(a) Schematic diagram of the mesh based flexible DSSCs. (b) Photograph of a flexible DSSC device. (c) Photograph showing the flexibility of the device.

Fig. 5 (a) J–V curves of flexible DSSCs based on the TiO2 NT, NW and HNW arrays, which are measured by using an AM 1.5 G solar simulator (100 mW cm2). (b) EIS spectra of TiO2 NT, NW and HNW array based DSSCs.

(a) XRD patterns of Ti mesh, and H2Ti2O4(OH)2 NW arrays, TiO2 NW arrays, TiO2 HNW arrays and TiO2 NT arrays on Ti mesh. (b) TEM image and (c) HRTEM image of a TiO2 NW. (d and e) TEM images of TiO2 HNWs. (f) HRTEM image of the connecting region of the nanorod branch and NW trunk. Fig. 3

Fig. 4


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NT NW HNW

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Detailed photovoltaic performance parameters (Jsc, Voc, FF and ƞ) of DSSCs based on the different photoanodes Adsorbed dye (nmol cm2)

Rs (U)

R1 (U)

R2 (U)

Jsc (mA cm2)

Voc (V)

FF

ƞ (%)

Normalized ƞ (%)

72.3 56.7 112.9

13.5 12.9 17.4

6.6 6.1 4.2

192.0 152.1 294.7

7.04 3.36 7.24

0.68 0.61 0.72

0.32 0.47 0.34

1.52 0.85 1.76

2.21 1.13 2.39

higher efficiencies. Specically, despite the fact that the TiO2 HNW arrays are signicantly higher in dye loading amounts than the NT arrays (112.9 and 72.3 nmol cm2, respectively), the Jsc values of them are almost identical. This is owing to the fact that the NT arrays are more competent for light-harvesting through light scattering and trapping than the NW and HNW arrays. It also can be seen in Fig. 1b, the TiO2 NT arrays/Ti mesh sample is darker than TiO2 HNW arrays/Ti mesh sample. Table 1 shows that the Voc values of the NT, NW and HNW array based cells are 0.68, 0.61 and 0.72 V, respectively. It is well known that the Voc value is closely related to the recombination resistance and electron lifetime in the conduction band of TiO2.38,39 To obtain electrochemical behavior information within these cells, EIS measurements were performed. In Fig. 5b, the Nyquist plots of the DSSCs for the different types of TiO2 photoanode are presented. Generally, two semicircles are observed in a Nyquist diagram of a DSSC. The rst semicircle in the high frequency region represents the charge transfer resistance at the electrolyte/counter electrode interface (R1), while the second semicircle in the low frequency region represents the recombination resistance at the photoanode/electrolyte interface (R2). The impedance spectra can be analyzed using Z-view soware using an equivalent circuit and the corresponding parameters (internal ohmic resistance (Rs), R1 and R2) are obtained by tting the equivalent circuit. As shown in Table 1, these cells possess similar Rs and R1 values, which is due to using the same electrolyte and counter electrode in each case. However, the R2 value of the NW, NT and HNW cells are 152.1, 192 and 294.7 U, respectively. It is worth noting that the HNW cells exhibit greatly increased R2 values compared to the NW cells. This may be because a compact TiO2 layer was formed at the bottom of the HNW arrays while the short nanorod branches were grown on the surface of the TiO2 NW trunks, which is conducive to preventing the recombination of photo-generated electrons with I3 at the electrolyte/Ti substrate interface.40 The increase of the recombination resistance (NW < NT < HNW) accounts for the decline in the recombination reaction rate (increase in the electron lifetime), which will lead to increased Voc values. This result is in concurrence with the above photovoltaic data. Recently, mesh based exible DSSCs have received immense attention due to their great potential for portable and wearable electronic applications. Here, the anatase TiO2 HNW arrays/Ti mesh photoanode and the Pt/Ti mesh counter electrode are used to fabricate a complete device (1 cm 1 cm), which shows excellent exibility and bending stability. As shown in Fig. S6,† a DSSC device was bent 300 times. The Jsc, Voc, FF and PCE (ƞ) values as functions of bend cycle number are shown in Fig. 6. During the bending cycles, these photovoltaic parameters values

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varied slightly. The TiO2 HNW arrays/Ti mesh based DSSC has long-term and repeatable exibility, in which case the PCE decreased less than 12% over 300 bending cycles (1.33, 1.33, 1.22 and 1.17% for 0, 100, 200 and 300 cycles, respectively, as shown in Fig. S6b†). A detailed comparison to other works is summarized in Table S1.† Our TiO2 HNW/Ti mesh based DSSCs achieved a relatively high PCE compared to other works (1.49% for Fan et al.,22 1.23% for Liu et al.,35 3.67% for Pan et al.5 and 0.93% for Li et al.33). In Pan’s work, the TiO2 NT/Ti mesh was used as the photoanode, and the PCE of the DSSC device decreased 10% aer bending only 100 times. Our completely sealed exible DSSC device has good bendability and stability, which can maintain an unchanged PCE aer bending 100 times and maintain 88% of PCE aer bending 300 times. Our work demonstrated that the anatase HNW based DSSCs have better bending stability than NT based DSSCs, probably due to the larger spaces and higher compression tolerance of HNW (as shown in Fig. 2b and g, for comparison). A exible, inexpensive and effective counter electrode material is also a research hotspot in exible DSSCs.41,42 Here,

Fig. 6 Photovoltaic parameters (Jsc, Voc, FF and ƞ) of the flexible DSSCs as a function of bending cycle number up to 300 cycles.

Fig. 7 (a) Cyclic voltammograms of the Pt/Ti mesh and ink/Ti mesh

electrodes measured at a scan rate of 50 mV s1 (b) J–V curves of the DSSCs based on different counter electrodes (Pt and ink).


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Fig. 8 (a and b) Photographs of 32 green-LEDs lit up by four series-connected DSSCs (note: due to the strong background brightness from the solar simulator, the LEDs do not look as bright as they actually are). (c) J–V curve of the four mesh based flexible DSSCs in series.

commercial ink used as a counter electrode material to replace Pt has also been investigated. Cyclic voltammetry (CV) was performed to examine the catalytic activity of the commercial ink counter electrode in the DSSCs (Fig. 7a). The electrolyte is an acetonitrile solution containing 10 mM LiI, 1 mM I2 and 0.1 M LiClO4. Two pairs of oxidation and reduction peaks are observed for Pt and ink in the CV scan, which are attributed to the following reactions: left: I3 + 2e 4 3I

(4)

right: 3I2 + 2e 4 2I3

(5)

Generally, the counter electrode serves as the electrocatalyst to reduce I3 to I. The reduction peak current density (Jred) and peak-to-peak separation voltage (Vpp) are used to gauge the catalytic activities of counter electrodes. As the ink electrode material, with a high specic area (consisting of 20 nm carbon NPs, as shown in Fig. S7†), shows a high Jred, it is more reasonable to gauge their catalytic activities by comparing the Vpp values. The Pt/Ti mesh electrode exhibits a higher catalytic performance than the ink/Ti mesh due to a lower Vpp (0.7 and 0.82 V for Pt and ink, respectively). In Fig. 7b, a higher PCE of the DSSC device using the Pt/Ti mesh counter electrode is presented. This result indicates that the commercial ink is a great choice for costefficiency and widespread production consideration, but Pt still represents the optimal component within the DSSCs. As expected, the mesh based exible DSSCs could be integrated and applied to provide power for various electronic devices (Fig. 8a and b). Fig. 8c shows the J–V characteristics of a exible solar panel implemented by cascading four mesh based DSSCs. The Voc value has been increased from 0.72 to 2.4 V. As a result, these four series-connected solar cells can efficiently power a JNU logo consisting of 32 green-LEDs. These results suggest that the Ti mesh based exible DSSC device possesses promising potential for application as the lightweight and exible energy supply in wearable and portable electronics.

Conclusion In summary, thin, lightweight and exible mesh based DSSCs have been developed by using three different anatase TiO2


nanostructures on Ti meshes as efficient exible photoanodes. A two-step hydrothermal method is used to synthesize the TiO2 HNW arrays, which achieved a higher PCE of 1.76% than electrochemical anodized TiO2 NT arrays (1.52%). This improvement can be attributed to the large surface area and low recombination rate of electrons and redox electrolyte. Moreover, the commercial ink has been identied as a promising candidate to fabricate exible counter electrodes to replace Pt and achieve a considerable PCE of 1.59%. The as-fabricated mesh based DSSC retains 88% of PCE aer 300 bending cycles, demonstrating its good exibility and fatigue resistance. These sealed DSSCs have been integrated to light up a group of green-LEDs, which demonstrated that mesh based DSSCs are promising candidates for wearable/bendable energy supply devices in everyday applications.

Acknowledgements W. J. M. and M. M. Wu are thankful for the nancial support from the National Natural Science Foundation of China (Grants 21376104 and 21271190) and the Natural Science Foundation of Guangdong Province, China (Grants S2013010012876, 2014A030306010 and S2012020011113).

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