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Winding ultrathin, transparent, and electrically

conductive carbon nanotube sheets into high-
Cite this: DOI: 10.1039/c3ta12663c
performance fiber-shaped dye-sensitized solar cells†
Received 9th July 2013
Hao Sun, Xiao You, Zhibin Yang, Jue Deng and Huisheng Peng*
Accepted 29th August 2013

DOI: 10.1039/c3ta12663c

www.rsc.org/MaterialsA

Highly aligned carbon nanotube sheets, that are transparent, flex- DSC. The resulting photovoltaic ber achieves a maximal energy
ible, and electrically conductive, have been wrapped onto a modified conversion efficiency of 4.10%.
titanium wire to produce novel, fiber-shaped dye-sensitized solar Aligned TiO2 nanotubes are grown on a Ti wire by an elec-
cells. The designed coaxial structure offers a high energy conversion trochemical anodizing method (Fig. 2a and b). Aligned TiO2
efficiency and stability during the deformation. nanotubes with a uniform diameter of 150 nm and wall
thickness of 20 nm have been produced (Fig. 2c). The aligned
MWCNT sheet was pulled out of a spinnable MWCNT array that
Compared with silicon-based solar cells, dye-sensitized solar is described in the ESI.† The MWCNTs are highly aligned to
cells (DSCs) can be made into exible, thin devices with enable electrical conductivities on the level of 102 to 103 S cm 1
promising applications that remain difficult for rigid silicon along the MWCNT-alignment direction (Fig. 2d). The MWCNT
wafers.1–6 However, the planar structure cannot adequately meet sheet can be uniformly wrapped around the working electrode
the ongoing requirement for small and rapid electronic devices with a thickness of appropriately 20 nm (Fig. 2e). The aligned
in the future. To this end, if DSCs could be made in a exible, MWCNTs are closely attached at the tips of TiO2 nanotubes
coaxial ber format, they might be lightweight and efficient for (Fig. 2f).
many promising applications over the conventional planar TiO2 nanotubes with increasing lengths from 10, 19, 31, and
cells.7–9 For instance, they can be easily integrated into micro- 39 mm have been investigated for photovoltaic bers (Fig. 3a).
devices and woven into electronic textiles, which is required in The MWCNT sheets shared the same thickness of 20 nm if not
electronics. In this case, it is critical but remains challenging to specied. The open circuit voltage (VOC) and ll factor (FF) are
nd appropriate counter electrode materials. A good candidate maintained to be appropriately 0.72 V and 0.580, respectively,
needs to be exible, robust, transparent, conductive, stable, and while the short circuit current (JSC) is increased from 3.61 to
electrocatalytic to the redox couple. 9.93 mA cm 2 with the increasing length from 10 to 31 mm and
In this work, highly aligned multi-walled carbon nanotube then decreased to 8.53 mA cm 2 with the further increase to
(MWCNT) sheets are developed as a new family of materials for 39 mm (Table 1). Longer TiO2 nanotubes can absorb more dye
the counter electrodes to fabricate DSC bers (Fig. 1). The thin molecules to generate higher photoelectron currents under the
MWCNT sheet is transparent, exible, and robust. Due to the
highly aligned structure of MWCNTs, the sheet is also
mechanically strong, electrically conductive, and active in
catalyzing the redox couple of I /I3 . Aligned TiO2 nanotubes
that are perpendicularly grown on a Ti wire have been used as a
working electrode, and the MWCNT sheet is continuously
wrapped around the modied Ti wire to produce a ber-shaped

State Key Laboratory of Molecular Engineering of Polymers, Department of

Macromolecular Science, Laboratory of Advanced Materials, Fudan University,
Shanghai 200438, China. E-mail: penghs@fudan.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta12663c Fig. 1 Schematic illustration of the coaxial DSC fiber.

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Different thicknesses of MWCNT sheets, i.e., 20, 40, and 60

nm, have been also compared for the photovoltaic ber
(Fig. 3b). Here the length of TiO2 nanotubes was the same, 31
mm. Both VOC and FF are maintained at appropriately 0.72 V and
0.60, respectively, while the JSC is continuously decreased due to
the increased shield effect by the thicker MWCNT sheets
(Table 2). The thicker the MWCNT sheets are, the less the
incident light can reach to sensitize the dye with the lower
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photoelectron current (Fig. S1†).11

To demonstrate the high performance of the coaxial
structure, a DSC wire is fabricated by winding an aligned
MWCNT ber that has been twisted from the same MWCNT
sheet as the counter electrode onto the same TiO2 nanotube-
modied Ti wire (Fig. S2† and 4a). As expected, they exhibit
the same VOC of 0.72 V that is determined by the energy level
of the electrode, N719 dye, and electrolyte.12,13 The coaxial
DSC ber shows a lower JSC than the twisted DSC wire due to
the fact that a MWCNT ber shows higher conductivity than
a MWCNT sheet (typically about ve times), so more photo-
electrons can be transported to the external circuit. However,
the coaxial DSC shows a FF of 0.570 that is much higher
Fig. 2 Scanning electron microscopy (SEM) images of the coaxial photovoltaic than 0.375 of the twisted DSC wire as the coaxial structure
fiber. (a and b) A TiO2 nanotube-modified Ti wire: side and top views, respectively. produces a much larger contact area between the working
(c) TiO2 nanotubes at high magnification. (d) Highly aligned MWCNT sheet. and counter electrodes with a higher catalytic activity. As a
(e and f) The modified Ti wire being wrapped in a layer of MWCNT sheet at low
result, the DSC ber exhibits a maximal energy conversion
and high magnifications, respectively.
efficiency of 4.10%, compared with 3.20% of the twisted DSC
wire.
Besides the much improved energy conversion efficiency, the
coaxial DSC ber also shows a higher exibility than the twisted
DSC wire. The photovoltaic parameters had been compared
aer bending for 100 cycles (Fig. S3† and 4b). For the coaxial
DSC ber, the JSC is rst slightly increased in the rst 10 cycles
due to an increased contact between the electrolyte and
MWCNT sheet and then slightly decreased in the following 90
cycles. The energy conversion efficiency remains almost
unchanged during the studied bending cycles (with a variation
of less than 5%). In contrast, for the twisted DSC wire, the JSC is
quickly decreased by 19.5% in the rst 10 cycles and further
decreased by 26.9% in the following 90 cycles. As a result, the
energy conversion efficiency is decreased by 30%.
To better understand the different stabilities, the coaxial and
twisted DSCs are further studied by scanning electron micros-
copy. Fig. 5a and b have compared the coaxial DSC ber before
and aer bending for 10 cycles, and no obvious damage in the
structure is observed. Therefore, the energy conversion effi-
ciency remains almost unchanged during the bending. In
contrast, the two ber electrodes are separated from each other
in a twisted DSC wire during the bending (Fig. 5c and d). The
increased diffusion length for I ions slows the regeneration
Fig. 3 (a) J–V curves of coaxial DSC fibers with different lengths of TiO2 nano-
rate of the dye and makes the photoelectron recombination
tubes. (b) J–V curves of coaxial DSC fibers with different thicknesses of MWCNT
sheets. with I3 ions in the electrolyte more frequent. As a result, the JSC
of the twisted DSC wire is largely decreased aer bending. These
coaxial DSC bers also show many other unique advantages. For
same illumination. However, the recombination of charges instance, the energy conversion efficiency of the DSC ber is
becomes dominating due to the longer diffusion length of independent of the incident angle (Fig. 6). The photovoltaic
photons and electrons with the further increasing length of parameters remain unchanged with increasing incident angle
TiO2 nanotubes, which reduces the collection of electrons.10 from 0 to 180 .

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Table 1 Photovoltaic parameters of DSC fibers based on different lengths of

TiO2 nanotubes

Length of TiO2 JSC

nanotubes (mm) VOC (V) (mA cm 2) FF h (%)

10 0.717 3.613 0.615 1.59

19 0.712 6.208 0.560 2.47
31 0.722 9.980 0.570 4.10
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39 0.715 8.526 0.586 3.57

Table 2 Photovoltaic parameters of DSC fibers based on different thicknesses of

MWCNT sheets

Thickness of MWCNT JSC

sheet (nm) VOC (V) (mA cm 2) FF h (%)
Fig. 5 Morphology characterization of (a and b) a coaxial DSC fiber and (c and d)
20 0.722 9.378 0.596 4.03 a twisted DSC wire before and after bending for 10 cycles, respectively.
40 0.726 6.304 0.610 2.79
60 0.722 4.479 0.641 2.07

Fig. 6 Dependence of photovoltaic parameters on incident angle of a coaxial

DSC fiber.

Experimental section
Ti wires with a diameter of 127 mm were sequentially immersed in
acetone, isopropanol, and water to wash away impurities, and
aligned TiO2 nanotubes were grown on the surface of the Ti wire by
electrochemical anodization. A 0.3 wt% NH4F–8 wt% H2O–
ethylene glycol solution served as the electrolyte, and a voltage of
60 V was conducted between the Ti wire and Pt sheet as anode and
cathode, respectively. The lengths of TiO2 nanotubes can be
Fig. 4 (a) J–V curves of the coaxial DSC fiber and twisted DSC wire. (b) Depen- controlled by varying the oxidation time. Deionized water was used
dence of photovoltaic parameters on bent cycle number for the coaxial DSC fiber
to remove the residual electrolyte on the surface of the TiO2
and twisted DSC wire.
nanotube-modied Ti wire, followed by heating to 500  C for 1 h
and annealing in air. Aer the treatment in a 40 mM TiCl4 aqueous
solution at 70  C for 30 min, the wire was heated to 450  C for 30
In summary, a new coaxial DSC ber has been fabricated min and annealed in air. When the temperature was dropped to
with a highly aligned MWCNT sheet as the counter electrode, 120  C, the resulting wire was immersed in 0.3 mM N719 solu-
and a record energy conversion efficiency of 4.10% is achieved. tion in a mixture of dehydrated acetonitrile and tert-butanol
The coaxial DSC ber also shows a high exibility and stability. (volume ratio of 1/1) for 16 h prior to the device fabrication.
More efforts are under way to improve wire-shaped DSCs by Spinnable MWCNT arrays were synthesized by chemical
introducing the other metal oxide materials.14 Although a ber- vapor deposition.15 A transparent, conductive MWCNT sheet
shaped DSC has been demonstrated in this work, the aligned could be pulled out of a spinnable CNT array with a razor blade
MWCNT sheet can be also used as an electrode material to with a single edge, and it was then carefully wrapped around the
fabricate coaxial, ber-shaped polymer solar cells and various modied Ti wire. For the twisted structure, the MWCNT sheets
other optoelectronic and electronic devices. were spun into bers with a rotation rate of 2000 rpm to function

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as a counter electrode. The DSC ber and wire were sealed by 5 L. Heng, X. Wang, N. Yang, J. Zhai, M. Wan and L. Jiang, Adv.
transparent uorinated ethylene propylene tubes, followed by Funct. Mater., 2010, 20, 266–271.
injection of the electrolyte (0.05 M iodine, 0.1 M lithium iodide, 6 Z. Yang, T. Chen, R. He, G. Guan, H. Li, L. Qiu and H. Peng,
0.5 M 4-tert-butyl-pyridine, and 0.6 M 1,2-dimethyl-3-propylimi- Adv. Mater., 2011, 23, 5436–5439.
dazolium iodide in dehydrated acetonitrile). The active area of 7 S. Zhang, C. Ji, Z. Bian, R. Liu, X. Xia, D. Yun, L. Zhang,
the ber-shaped DSC was calculated by multiplying the length C. Huang and A. Cao, Nano Lett., 2011, 11, 3383–3387.
and diameter of the working electrode.7,10,16,17 8 C. Pan, W. Guo, L. Dong, G. Zhu and Z. L. Wang, Adv. Mater.,
2012, 24, 3356–3361.
Published on 29 August 2013. Downloaded by University of Missouri - St Louis on 16/09/2013 11:46:20.

9 T. Chen, L. Qiu, Z. Yang and H. Peng, Chem. Soc. Rev., 2013,

Acknowledgements
42, 5031–5041.
This work was supported by NSFC (91027025, 21225417), MOST 10 Z. Lv, Y. Fu, S. Hou, D. Wang, H. Wu, C. Zhang, Z. Chu and
(2011CB932503, 2011DFA51330), STCSM (11520701400, D. Zou, Phys. Chem. Chem. Phys., 2011, 13, 10076–
12nm0503200), Fok Ying Tong Education Foundation, and the 10083.
Program for Professor of Special Appointment at Shanghai 11 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Institutions of Higher Learning. Chem. Rev., 2010, 110, 6595–6663.
12 M. Grätzel, Acc. Chem. Res., 2009, 42, 1788–1798.
13 J. N. Clifford, E. Martı́nez-Ferrero, A. Viterisi and
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