ENHANCED PHOTOVOLTAIC PERFORMANCE IN DYE-SENSITIZED SOLAR
CELLS USING TiO2 BEAD/NANOFIBER COMPOSITE
Nirachawadee Srisamran1,2,3,*, Jutarat Sudchanham1,3, Thammasit Vongsetsakul3,
Pasit Pakawatpanurat1,2,3,#
1
Physical Chemistry Program and Center for Alternative Energy, Faculty of Science,
Mahidol University, Bangkok, Thailand
2
Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
Bangkok, Thailand
3
Department of Chemistry, Faculty of Science, Mahidol University, Bangkok, Thailand
*e-mail: tomoe-kh@hotmail.com, #e-mail: pasit.pk@gmail.com
Abstract
Enhancing light scattering and electron diffusion within a working electrode are one
of the common strategies to improving the performance of dye-sensitized solar cells
(DSSCs). In this work, the mesoporous TiO2 beads were mixed with electrospun TiO2
nanofibers to produce active material for DSSCs. The results showed that mesoporous TiO2
beads mixed with 5% wt electrospun-nanofibers gave DSSC with conversion efficiency of
7.30% and higher current density compared with that of Degussa P25 titania. The
mesoporous TiO2 beads synthesized via sol-gel and solvothermal method provided several
advantages over Degussa P25 titania, i.e., compact packing and well interconnection of TiO 2
particles inside each sphere and favorable light scattering effect without sacrificing specific
surface area. In addition, TiO2 nanofibers prepared through sol–gel technique followed by
electrospinning also helped facilitate the long-range electron transport within the active
material through their quasi-one-dimensional elongated structures.
Keywords: TiO2 nanofiber; TiO2 beads; electron transport; light scattering, dye adsorption
Introduction
Nowadays, owing to the decline of natural fuels, alternative energy sources such as
solar energy, wind, hydropower, and biological waste have gained deep interest from the
public. Solar energy is an unlimited source and environmentally friendly. More importantly,
it can be converted into electricity using specific device called solar cells. Dye-sensitized
solar cells (DSSCs) can convert sunlight into electricity with relatively high conversion
efficiency. Photoanode is one part of DSSC that can be modified to improve % conversion
efficiency, especially the oxide semiconductor. TiO2 is considered an effective semiconductor
material to transport the excited electron in the photoanode due to low cost, high availability
in nature, and long-term stability. To date, the most widely used TiO2 morphology in
conventional DSSCs is spherical nanoparticles which have size around 14-25 nm in diameter.
This morphology can provide high surface area for dye adsorption, but its drawbacks include
limited light scattering due to its small size compared with visible light wavelengths.
Moreover, the random electron transport through TiO2 nanoparticle film before being
collected at the transparent conductive oxide (TCO) glass resulting in an electron back
transfer or recombination reaction between photogenerated electron and the oxidized ions in
electrolyte [1-2]. On the other hand, one dimensional (1D) TiO2 nanostructures, such as
nanotubes, nanorods, nanofibers, and nanowires have been applied to the DSSCs to suppress
the back electron transfer, because they can provide direct-electron-pathways but less dyeloading on their surface [3-8]. This is the major factor for prevent the power conversion
efficiency (%) of the DSSC cells to be high. Consequently, there are some research had
reported about mixing two TiO2 morphologies utilized in DSSCs in order to enhance the
photocurrent density such as incorporated TiO2 nanotubes into TiO2 nanoparticle film to
improve electron transport in DSSCs and received higher power conversion efficiency [9].
For these reasons, TiO2 beads have been synthesized and utilized instead. This novel
morphology is gathering the good points from the others that mentioned before. It can
generate light scattering because of their large particle without sacrificing the surface area for
dye-uptake. It is different from large solid particles whether the spherical aggregate
constructed from the nanosized TiO2 building units inside. Moreover, the electron transport
can be enhanced due to the compact packing of nanosized TiO2 building units in spherical
aggregate. Furthermore, they facilitate electrolyte diffusion because of their open structure
[10-16]. However, 1D TiO2 nanostructures still have the best potential in electron transport to
the TCO glass due to their one direction pathways. Especially, electrospun TiO2 nanofibers
(NFs) have long length, highly porosity and large surface area for dye adsorption were
suitable for enhancing the performance of DSSCs compare to other 1D TiO2 nanostructures
and simply synthesis. For instance, Chuangchote et al. reported that TiO2 nanofibers
fabricated directly onto TiO2 nanoparticle electrodes as an over layer via electrospinning and
sol-gel techniques can increase the conversion efficiencies up to 8.14% [6]. Hasani et al.
demonstrated that electrospun- TiO2 nanofibers with diameter 300 nm enhanced power
conversion efficiency 35% over using the Degussa P25 reference photoanode [8]. Thus, this
research focused on utilizing the mixing of two different TiO2 morphologies e.g.
nanocrystalites beadss or beads and nanofibers in photoanodes for enhance power conversion
efficiency and investigated the effect of them regarding electron recombination on DSSCs
compare to conventional TiO2 nanoparticle based DSSCs.
Methodology
Synthesis of TiO2 beads: Mesoporous TiO2 beads were synthesized by using twostep method, the first is a surfactant aided sol-gel method and the second one obtain from
solvothermal reaction [16]. For prepare the amorphous precursor beads, Titanium (IV)
butoxide 10 mL was dissolved in ethanol solution 400 mL. 0.1 M KCl 2.0 mL was then
added into the solution to control the monodispersity of the precursor beads by adjusting the
ionic strength. Lastly, laurylamine 2.64 g was used as a structure-directing agent. This
process was carried out at room temperature under vigorous stirring for 2 h. A white
precipitate was washed by ethanol and dried in air. To prepare mesoporous titania beads, 1.2
g of TiO2 microspheres were transferred into Teflon-lined stainless steel autoclave with
ethanol 12 mL and H2O 8 mL followed by the addition of 25% ammonium hydroxy solution
0.5 mL. Annealed at 160ºC for 16 h. The products were filtered and washed with ethanol
after kept staic for 18 h, dried in air at 60ºC. The powders were calcined at 500 ºC for 2 h in
air to remove organic compounds [12].
Synthesis of TiO2 nanofiber: Sol–gel technique was utilized to prepare the spinning
solution from 1.5 g of the polyvinyl pyrrolidone, (PVP) ( Mw=1 300 000, Aldrich) was used
as a binder for electro-spinning, 3 ml Titanium (IV) isopropoxide (TTIP, [Ti(OCH(CH3)2)4,
Aldrich), 6 ml Acetic acid and Ethanol 21 ml were stirred together for 12 h. The yellow
solution was transferred to syringe which connected to a high voltage power supply. An
electrical potential was set at 15 kV. A collection distance of the fibers was located at 15 cm
from the needle attached to syringe. The result fibers were calcined in air at 500 ºC for 4 h
[8].
Preparation of TiO2 paste and fabrication of DSSCs: The TiO2 nanoparticles
pastes were prepared by adding P25 nanoparticles (purchased from Sigma-aldrich) into the
ethanol solution and triton-100 was also added in solution patse under vigorous stirred and
sonicated in ultrasonic bath. Aside from the standard cell using P25, the other photoanodes
are deposited by TiO2 beads and nanofibers and their composite as films instead. Four pastes
containing mixed TiO2 beads and nanofibers which are labeled as 1% NF, 5% NF, 10% NF
and 15% NF were prepared with TiO2 nanofibers amount of 1, 5, 10, 15 wt%, respectively.
The synthesized TiO2 powder was coated on TCO glass by doctor blade tecnique with film
thickness around 12 µm and active area 0.25 cm2. The FTO glasses with TiO2 films were
then sintered at 500°C for 30 min in air. Immersed photoanode cells into 0.5 mM ruthenium
dye (N719, Dyesol) solution in ethanol for 24 h in the dark. Washed dye-TiO2 photoanodes
by ethanol many times, dried and assembled them with Pt-counter electrodes using Surlyn as
a sealing materials. The I-/I3- redox couple electrolyte solution was then injected into space
through the pores on Pt-counter electrode. Seal the pores on Pt-counter electrode by
aluminium foil. Universal Photovoltaic Test System (UPTS) with Keithley model 2420
digital source meter is employed to determine the % conversion efficiency of cells.
Characterization: Scanning electron microscopy (SEM) was used to examined the
morphology of samples and film thickness. The X-ray powder diffractometer (XRD) were
also employed to confirm white powder’s crystallite structure. The photocurrent–voltage
characteristics were recorded with a Keithley model 2420 digital source meter under a
simulated AM 1.5G illumination with a light intensity of 100 mWcm-2 and carried out at 1
sun level.
Results
The X-ray diffraction patterns of the calcined TiO2 nanofibers and beads were
obtained with a Bruker Axs model D8 advance X-ray powder diffractometer to confirm their
crystallite structure as anatase or rutile phase. The TiO2 P25 nanoparticles (Sigma aldrich)
was used to be the reference pattern. Figure 1 (b) and (c) displayed the X-ray diffraction
patterns of the synthesized TiO2 beads and TiO2 nanofibers, respectively. These can be
indicated that synthesized TiO2 beads have anatase phase on their cyrstallinity similar to P25
nanoparticles commercial. On the other hand, TiO2 nanofibers showed the weak peak of
rutile phase together with anatase peak.
R
Figure 1. X-ray diffraction patterns of TiO2 with different morphologies (a) P25 nanoparticles,
(b) beads, (c) nanofibers.
(a)
(b)
Figure 2. SEM images of (a) mesoporous TiO2 beads after solvothermal process
(b) thickness of TiO2 film coated on TCO glass.
The SEM images, figure 2(a) showed the morphology of TiO2 beads
solvothermal process. This figure secured that TiO2 beads is differ from TiO2 large
sphere with having much of mesoporous on their surface and their sphere structurized
aggregation of small nanocrystallite, so their surface were not smooth as TiO2 large
after
solid
from
solid
sphere. The most diameter of beads were approximately 400-600 nm. The film thickness on
TCO glass substrate were around 12 m confirmed by figure 2(b). The TiO2 nanofibers (NFs)
have diameter around 250 nm.
The conversion efficiencies of fabricated cells were showed in table 1. The results
showed that TiO2 beads mixed with 5% wt of nanofibers (NFs) based DSSCs obtained the
highest conversion efficiencies (%) 7.30%. while the higher % wt mixing of them gave the
lower conversion efficiencies with 6.85% and 6.40% for 10% and 15% wt of NFs,
respectively. Which were lower than that of TiO2 beads based photoanode. For TiO2
nanofibers based photoanode have low conversion efficiencies around 4% almost the same as
P25 nanoparticle.
Table 1. The photovoltaic parameter of various TiO2 photoelectrode films.
Photoanode film
P25 nanoparticle
TiO2 Bead
5% NF in bead
10% NF in bead
15% NF in bead
TiO2 NF in bead
FF
0.63
0.67
0.69
0.66
0.63
0.66
Voc
0.80
0.79
0.77
0.78
0.80
0.79
Jsc
8.67
13.37
13.83
13.18
12.79
8.83
Efficiency
4.350.29
6.940.12
7.300.23
6.850.12
6.400.30
4.610.12
For the performances of DSSCs based on photoelectrodes made of TiO2 beads mixed
with 1% NF, 5% NF, 10% NF and 15% NF pastes employing N719 as a sensitizer, 5% NF
was offered the highest Jsc around 13.83 mA/cm2 as shows in figure 3.
Figure 3 I-V curves of DSSCs based on different films.
Discussion and Conclusion
From the SEM image (a), the diameter 400-600 nm of TiO2 beads has the important
role for light-scattering, which can improve the conversion efficiency as well as the highly
surface area for dye-loading. Unfortunately, there are many voids on film because of the large
size of them. This is the problem that caused the losing surface area for dye-uptake. TiO2 NFs
with diameter ca. 250 nm and their elongated structures were also scatterd the light
illuminated through the film and facilitated electron transportation in direct pathway.
In conclusion, the highest conversion efficiency was observed after mixing
electrospun-TiO2 nanofibers for 5%wt with TiO2 beads (NFs) based DSSCs. This effect
suggested that the 1D structure of TiO2 NFs based DSSCs can improve electron transport. In
addition, the high surface area and large-sized of TiO2 beads can enhance dye loading and
light-scattering property, respectively. However, the current dropped with further increase the
amount of TiO2 nanofibers more than 5%wt due to an insufficient surface area for dye
adsorption. For further study, an electron transport in photoanode will be investigated by
electrochemical impedance spectroscopy (EIS).
References
1. Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-Sensitized Solar Cells. Chem. Rev. 2010;
110: 6595–6663.
2. Zhang Q, Myers D, Lan J, Jenekhe S. A, Cao G. Applications of light scattering in dye-sensitized solar
cells. Phys. Chem. Chem. Phys. 2012; 14: 14982–14998.
3. Lee J, et.al. Metal Oxides and Their Composites for the Photoelectrode of Dye Sensitized Solar Cells,
Advances in Composite Materials for Medicine and Nanotechnology. www.intechopen.com
4. Q. Zhang, G. Cao, Nanostructured photoelectrodes for dye-sensitized solar cells. Nano Today. 2011; 6:
91-109.
5. Yan J, Zhou F. TiO2 nanotubes: Structure optimization for solar cells. J. Mater. Chem. 2011; 21, 94069418.
6. Chuangchote S, Sagawa T, and Yoshikawaa S. Efficient dye-sensitized solar cells using electrospun TiO2
nanofibers as a light harvesting layer. App. Phy. Lett. 2008; 93; 1-3.
7. Junga W H, Kwaka N S, Hwanga T S, Yi K B. Preparation of highly porous TiO2 nanofibers for dyesensitized solar cells (DSSCs) by electro-spinning. App. Surf. Sci. 2012; 261: 343– 352.
8. Bijarbooneh F, Zhao Y, Sun Z, Heo Y, Malgras V, Kim J. and Dou S. Structurally stabilized mesoporous
TiO2 nanofibres for efficient dyesensitized solar cells. Apl. Mate. 2013; 1: 1-7.
9. Zhang X, et.al. The different electron transport of two nanotubes incorporated in working electrode of
dye-sensitized solar cells. J. All. & Comp. 2013; 578: 309–313.
10. Zhang Q, Cao G. Hierarchically structured photoelectrodes for dye-sensitized solar cells. J. Mater. Chem.
2011; 21, 6769-6774.
11. Zhang Q, Park K, Xi J, Myers D, Cao G. Recent Progress in Dye-Sensitized Solar Cells Using
Nanocrystallite Aggregates. Adv. Energy Mater. 2011; 1: 988–1001.
12. Jia Q Y, Que W X, Qiu X K, et al. Preparation of hierarchical TiO2 microspheres for enhancing
photocurrent of dye sensitized solar cells. Sci China-Phys. Mech. Astron. 2012; 55: 1158-1162.
13. Xi J, Zhang Q, Park K, Sun Y, Cao G. Enhanced power conversion efficiency in dye-sensitized solar cells
with TiO2 aggregates/nanocrystallites mixed photoelectrodes. Electrochimica Acta. 2011; 56: 1960–1966.
14. F. Sauvage, et.al. Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 Beads
Achieve Power Conversion Efficiencies Over 10%. acsnano, 2010; 4: 4420–4425.
15. Kim Y J, et.al, Formation of Highly Efficient Dye-Sensitized Solar Cells by Hierarchical Pore Generation
with Nanoporous TiO2 Spheres. Adv. Mater. 2009; 21: 3668–3673.
16. Chen D, Cao L, Huang F, Imperia P, Cheng Y, Caruso R A. Synthesis of Monodisperse Mesoporous
Titania Beads with Controllable Diameter, High Surface Areas, and Variable Pore Diameters. J. Am.
Chem. Soc. 2010; 132: 4438–4444.
Acknowledgements: The authors acknowledge support from the Center of Excellence for
Innovation in Chemistry (PERCH-CIC), and the Chemistry Program, Faculty of Science,
Mahidol University.