# Low-Temperature Preparation of Amorphous-Shell/  Nanocrystalline-Core NanostructuredTiO2 Electrodes for  Flexible Dye-Sensitized Solar Cells

Authors: Dongshe Zhang; Hengyao Hu; Laifeng Li; Donglu Shi
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

An amorphous shell/nanocrystalline core nanostructuredTiO2 electrode was prepared at low temperature, in which the mixture of TiO2 powder and TiCl4 aqueous solution was used as the paste for coating a film and in this film amorphous TiO2 resulted from direct hydrolysis of TiCl4 at 100∘C sintering was produced to connect the particles forming a thick crack-free uniform nanostructured TiO2 film (12 μm), and on which a photoelectrochemical solar cell-based was fabricated, generating a short-circuit photocurrent density of 13.58 mA/cm2, an open-circuit voltage of 0.647 V, and an overall 4.48% light-to-electricity conversion efficiency under 1 sun illumination.


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## Body

## 1. Introduction

Dye-sensitized solar cells (DSSCs)
[1–3] have been extensively studied more than a decade because they presented
high-efficient, cost-effective, and environmentally friendly advantages. In the
cell dye-sensitized semiconductor, photoelectrode plays an essential role, and conventionally
nanocrystalline porous TiO2 electrode is prepared by coating a paste
containing organic additives on a rigid conductive glass substrate, following a
procedure of high-temperature sintering to remove the organic additives [1, 2],
which are necessary to form a thick crack-free uniform film and optimize the microstructure
of the electrode for photosensitization [1–4].Flexible DSSCs [5–14], based
on the substrates of indium tin oxide (ITO) coated polyethylene terephthalate (PET), or polyethylene naphthalate (PEN) substituting for
rigid glass substrates, are regarded as one possible breakthrough in
the field of DSSC regarding their commercialization, because flexible DSSCs
have presented great advantages of low cost of production and wide application.
Conductive plastic substrates, such as ITO/PET or ITO/PEN, can be processed by a
continuous process like roll-to-roll production for porous nanocrystalline film
coating, therefore, greatly decreasing the production cost of the solar cells.
Meanwhile, flexible DSSCs can become part of a variety of every-day products
and turn them into energy sources. The possibility to produce the flexible DSSCs in any shapes would open
almost endless opportunities to the designers of such products. In addition, it
is light weight, having portable character.Underlying the flexible DSSCs, the necessary low-temperature
preparation of porous nanocrystalline metal oxides semiconductor films has been
a well-highlighted and on-going challenge up to today, because
the conventional method of high-temperature preparation cannot be applied to
prepare films on flexible plastic substrates, which only endure temperature of
up to around 150°C. So far, there have been a number of efforts concerned with the preparation of
nanoporous films at low temperature. The methods
reported were low-temperature heating [5, 6], compression [7, 8], microwave
irradiation [9, 10], electron-beam annealing [11] and chemical-vapor deposition
with UV irradiation [12], and hydrothermal crystallization [13, 14]. However,
the conversion efficiencies of the flexible DSSCs achieved so far are lower
than those obtained by high-temperature sintering. One main reason is that low-temperature
films have low level of crystallization of interconnection between particles comparing
with high-temperature film [5–14]. It is showed that low-temperature film has
poor interconnection between nanocrystalline particles, because the
above-mentioned methods that have been developed so far cannot result in as
perfect interconnection as high-temperature sintering did [5–14]. In fact, the
part of low level of crystallization worked as the interconnection of
nanocrystalline particles in the low-temperature film existed in all flexible DSSCs.
So the part of low level of crystallization in the low-temperature film played
an important role in the chemical reaction at interface of the cell. To well
understand how it works and further improve the performance of low-temperature
film, therefore, in this study, we developed a simple
method and prepared an amorphous shell/nanocrystalline core nanostructured
film under 100°C sintering. The amorphous shell not only is
responsible for the interconnection between nanocrystalline particles, but also
plays an important role in the interface chemical reaction. The as-prepared
films were mechanically stable. It is showed that amorphous TiO2 can
work effectively in DSSCs. Its performance was compared with that of
nanocrystalline porous film prepared at both high and low temperature.


## 2. Experimental

Nanostructured TiO2 electrode with the structure of amorphous shell/nanocrystalline core was
prepared by the following method. 0.8 g P25 (Degussa, Germany, 30% rutile
and 70% anatase, BET surface area 55 m2/g, particle size 25 nm) and 0.5 M TiCl4 water solution were
ground in an agate mortar for about 2 hours to get viscous paste, then coated
on the fluorine doped SnO2-coated conductive glass (sheet resistance
ca. 10 Ω/□) by doctor-blade technique. Subsequently, the film was sintered at
100°C for 12 hours. The resulting film thickness was 12 μm but can be varied by changing the paste
concentration or the adhesive tape thickness. The electrode was directly
immersed in an ethanol solution of cis-bis(4,4′-dicarboxy-2,2′-bipyridine)bis(thiocyanato)ruthenium(II),
N3 dye (0.05 mM) overnight at room temperature. This dye-sensitized electrode
was employed as a working electrode and platinized conductive glass as a
counter electrode for assembling a sandwich-type dye cell. The electrolyte was
0.5 M KI and 0.03 M I2 in ethylene carbonate-acetonitrile (4 : 1 by
volume). No special efforts were made to optimize the composition of the
electrolyte. Photoelectrochemical measurements were performed on the TiO2 film electrodes under white light illumination by a 500 W Xe lamp equipped with
IR and <420 nm cutoff filters from the side of the conductive glass back
contact. Surface morphology of the electrode
was observed by a Topcon ABT-150FS scanning electron microscope (SEM). X-ray
diffraction patterns (XRD) of the electrodes were measured by a Rigaku RAD-2R
using Cu Kα radiation at 40 kV and 20 mA by scanning at 2° 2θ min-1.


## 3. Results and Discussion

The SEM photographs of the TiO2 electrodes before and after sintering at low
temperature of 100°C are presented in Figure 1. It revealed
morphological homogeneity of both electrodes with micropores and interconnected
particles, but before sintering at low temperature of 100°C the
average particle size was approximately like the one of P25, while after sintering
it was increased obviously and the connection between particles was also
improved. The XRD patterns of the TiO2 electrodes before and after sintering at low temperature of 100°C
are shown in Figure 2. No new peak was observed after
sintering at low temperature of 100°C and, even both the relative
intensity and line width of crystal peak were not changed before and after
sintering, showing neither new compound nor crystal TiO2 was formed
in the film during the sintering. According to the Scherrer equation [L=0.9 λ/B(2θ) cosθ, where L is the crystallite size and B(2θ) is the line width]
together with the results of XRD measurement, the crystal size should not be
changed before and after sintering at low temperature of 100°C. Therefore, it conflicted with the results of SEM
measurement. Figure 2 also shows XRD pattern of TiO2 resulted from
0.5 M TiCl4 water solution sintered at 100°C for 12 hours.
From Figure 2, one can see no any crystal
TiO2 peaks were observed, showing that amorphous TiO2 was
formed during the sintering. So we can think that in the film TiCl4 was condensed at the surface of the crystal TiO2 of P25 before
sintering and, during the sintering at low temperature of 100°C amorphous
TiO2 resulted from TiCl4 grew on the surface of crystal
TiO2 of P25 forming amorphous shell/nanocrystalline core
particles, resulting in increments of the sizes of particles as well as
improvement of the connection between particles in the film. So the formed
electrode was crack free, robust, and uniform.SEM photographs of the TiO2 electrodes (a) before and (b) after sintering.
(a)(b)Figure 2
XRD patterns of the TiO2 electrodes (a) before and (b) after sintering as
well as (c) the TiO2 resulted from 0.5 M TiCl4 water solution sintered at 100°C
for 12 hours (small peaks resulted from SnO2 conductive glass).The amount of adsorbed N3 dye
on the nanostructured TiO2 electrode was 1.1×10−7 mol/cm2,2 and from this data the
calculated surface roughness factor was about 1000,15 showing that
the electrode had large surface area and the dye of N3 can also strongly adsorb
on the amorphous TiO2 surface. The photocurrent-voltage
characteristic of the cell based on this nanostructured TiO2 electrode after sintering at low temperature of 100°C is presented
in Figure 3. Under 1 sun illumination, a short-circuit photocurrent density
(Isc) of 13.58 mA/cm2, an open-circuit voltage (Voc) of 0.647 V, and
a fill factor of 51% were obtained, yielding an overall 4.48%
light-to-electricity conversion efficiency. Figure 4 shows the dependence
of Voc on illumination intensity. Within the range of the
measurement the open-circuit voltage versus incident light intensity was a
liner relationship and its slope was 130 mV per decade, yielding a
rectification coefficient of 2.5. This value was higher than that of 1 to 2
[15–17] of dye-sensitized solar cell based on the nanocrystalline TiO2 electrode sintered at 450°C for 30 minutes, meaning the density of
surface state in this amorphous electrode was higher which may result in larger
recombination [15]. However, this value was lower than that of 3.2 [5] of the
cell based on TiO2 electrode sintered at 100°C for 24 hours,
showing that amorphous TiO2 improved the connection between
particles in the film and decreased some recombination therefore, larger Isc
and conversion efficiency was observed. All these
experiments showed that the dye of N3 can inject electrons into amorphous TiO2 effectively and the recombination rate was lower and amorphous TiO2 can also collect and transport electrons effectively. Therefore, the cell based
on the amorphous shell/nanocrystalline core nanostructured TiO2 electrode prepared at low temperature had high-conversion efficiency up to 4.48%.
However, when this amorphous electrode was further sintered at 450°C
the amorphous became into well crystal, which was confirmed by XRD measurement
of TiO2 from the decomposition of TiCl4 with 450°C
sintering. A DSSC based on it presented a short-circuit photocurrent density of
20.2 mA/cm2, an open-circuit voltage of 0.69 V, and a fill factor of
51% were obtained, yielding an overall 7.1% light-to-electricity conversion
efficiency under 1 sun illumination. Obviously, both photocurrent and
photovoltage were improved with the improvement of the level of nanocrystalline
interconnection, suggesting that amorphous interconnection has lower collection
efficiency of electrons and higher recombination rates of electrons. The
unchanged fill factor implies that amorphous interconnection has close
resistance in the real cell when it works. The lower photocurrent and
photovoltage should come from the higher surface states in the amorphous shell
which worked as interconnection.Figure 3
Photocurrent-voltage
characteristic of a cell based on the amorphous shell/nanocrystalline core nanostructured
TiO2 electrode after sintering. Light intensity was 100 mW/cm2. Electrode area was 0.28 cm2.Figure 4
Open circuit voltage as a function of incident light intensity for N3-sensitized
amorphous shell-nanocrystalline core nanostructured TiO2 electrode.In summery, amorphous TiO2 can effectively work as interconnection to form robust nanostructured films,
however, it is not effective for electron collection. It presented large
recombination rate of electrons comparing with nanocrystalline porous films
with nanocrystalline interconnection. It is suggested that low-temperature
preparation methods should improve the crystal lever of the interconnection,
which plays essential roles in the forming of film and chemical reaction in the
interface, while it is not easy to achieve at low temperature. The flexible
DSSCs would present as high conversion efficiency as that of sintered DSSCs
when it would be achieved.



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*Source: 271631-2008-11-23.xml*