U.S. patent application number 10/971236 was filed with the patent office on 2005-05-26 for dye-sensitized solar cell based on electrospun ultra-fine titanium dioxide fibers and fabrication method thereof.
Invention is credited to Jo, Seong-Mu, Kim, Do-Kyun, Kim, Dong-Young, Lee, Wha-Seop, Song, Mi-Yeon.
Application Number | 20050109385 10/971236 |
Document ID | / |
Family ID | 34420694 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050109385 |
Kind Code |
A1 |
Kim, Dong-Young ; et
al. |
May 26, 2005 |
Dye-sensitized solar cell based on electrospun ultra-fine titanium
dioxide fibers and fabrication method thereof
Abstract
A dye-sensitized solar cell comprising a semiconductor electrode
comprising electrospun ultra-fine titanium dioxide fibers and
fabrication method thereof are disclosed. The dye-sensitized solar
cell comprises a semiconductor electrode comprising an electrospun
ultra-fine fibrous titanium dioxide layer, a counter electrode and
electrolyte interposed therebetween. A non-liquid electrolyte such
as polymer gel electrolyte or the like having low fluidity, as well
as the liquid electrolyte, can be easily infiltrated thereinto. In
addition, electrons can be effectively transferred since titanium
dioxide crystals are one-dimensionally arranged.
Inventors: |
Kim, Dong-Young; (Seoul,
KR) ; Jo, Seong-Mu; (Seoul, KR) ; Lee,
Wha-Seop; (Seoul, KR) ; Song, Mi-Yeon; (Seoul,
KR) ; Kim, Do-Kyun; (Seoul, KR) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
34420694 |
Appl. No.: |
10/971236 |
Filed: |
October 22, 2004 |
Current U.S.
Class: |
136/252 ;
136/263 |
Current CPC
Class: |
B82Y 30/00 20130101;
H01L 51/4226 20130101; H01G 9/2009 20130101; C09C 1/3669 20130101;
Y02E 10/542 20130101; C01P 2002/72 20130101; Y02E 10/549 20130101;
C01G 23/053 20130101; C01P 2004/10 20130101; C01P 2004/03 20130101;
H01L 51/0086 20130101; H01G 9/2013 20130101; H01G 9/2031
20130101 |
Class at
Publication: |
136/252 ;
136/263 |
International
Class: |
H01L 031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2003 |
KR |
76964/2003 |
Claims
What is claimed is:
1. A dye-sensitized solar cell comprising: a semiconductor
electrode comprising an electrospun ultra-fine fibrous titanium
dioxide layer; a counter electrode; and an electrolyte interposed
between the semiconductor electrode and the counter electrode.
2. The cell according to claim 1, wherein the semiconductor
electrode comprises a glass substrate, a ITO or FTO transparent
conductive layer and the electrospun ultra-fine fibrous titanium
dioxide layer onto which dye molecules are adsorbed.
3. The cell according to claim 1, wherein the electrospun
ultra-fine fibrous titanium dioxide layer has a thickness of 5-20
.mu.m.
4. The cell according to claim 1, wherein the counter comprises a
glass substrate, an ITO or FTO transparent conductive layer and a
platinum layer.
5. The cell according to claim 1, wherein the electrolyte is a
liquid electrolyte containing iodine.
6. The cell according to claim 5, wherein the liquid electrolyte is
an electrolyte in which 0.1M lithium iodide, 0.05M iodine, 0.6M
1,2-dimethyl-3-propyl-imidazolium iodide and 0.5M tert-butyl
pyridine are dissolved in acetonitrile.
7. The cell according to claim 1, wherein the electrolyte is a
polymer gel electrolyte containing one or more polymers selected
from the group consisting of
poly(vinylidenefluoride)-co-poly(hexafluoropropylene),
poly(acrylonitrile), poly-(ethyleneoxide) and
poly(alkylacrylate).
8. The cell according to claim 7, wherein the polymer gel
electrolyte contains one or more polymers in an amount of 5-20% by
weight of a solvent mixture of propylene carbonate and ethylene
carbonate.
9. A method for fabricating a dye-sensitized solar cell, comprising
the steps of: adding titanium isopropoxide into a polymer solution,
followed by adding acetic acid as a catalyst thereinto, and then
stirring the resulting mixture at room temperature, to obtain a
solution for electrospinning; electrospinning the solution to form
a film made of ultra-fine titanium dioxide fibers onto an ITO or
FTO-coated transparent conductive glass substrate; pre-treating the
substrate having the film of the ultra-fine titanium dioxide fibers
formed thereon with acetone or dimethyl formamide; thermally
treating the pre-treated substrate to form a layer of the
ultra-fine fibrous titanium dioxide onto the substrate;
impregnating the thermally treated substrate in a solution of dye
molecules in ethyl alcohol to obtain a semiconductor electrode on
which the dye molecules are adsorbed into the electrospun
ultra-fine titanium dioxide fibers; coating a platinum layer onto
an ITO or FTO-coated transparent conductive glass substrate, to
obtain a counter electrode; performing a heating/pressing process
on the semiconductor electrode and the counter electrode with a
spacer having a thickness of 20 .mu.m therebetween, so as to attach
the semiconductor electrode and the counter electrode to each
other; and injecting electrolyte into empty space between the
semiconductor electrode and the counter electrode.
10. The method according to claim 9, wherein the polymer solution
is a solution in which a polymer selected from the group consisting
of polyvinylacetate, polyvinylpyrrolidone, polyvinylalcohol and
polyethyleneoxide is dissolved in dimethyl formamide, acetone,
tetrahydrofuran, toluene or a mixture thereof in an amount of 5-20
wt %.
11. The method according to claim 9, wherein the electrospinning is
performed so as to make the ultra-fine titanium dioxide fibers have
a thickness of 50-1000 nm.
12. The method according to claim 9, wherein the layer of the
ultra-fine fibrous titanium dioxide is formed at a thickness of
5-20 .mu.m after the thermal treatment.
13. The method according to claim 9, wherein the pre-treatment is
performed by a method i) the substrate is treated with vapor of
acetone or dimethyl formamide for 1-3 hours in a closed container,
ii) the substrate is immersed in acetone or dimethyl formamide
solvent for 1 hour or iii) that methods i) and ii) are
combined.
14. The method according to claim 9, wherein the electrolyte is a
liquid electrolyte or polymer gel electrolyte.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a dye-sensitized solar cell
to convert solar energy into electric energy by a
photo-electrochemical process and to a fabrication method thereof,
and particularly, to a dye-sensitized solar comprising a
semiconductor electrode consisting of electrospun ultra-fine
titanium dioxide fibers and to a fabrication method thereof.
[0003] 2. Description of the Background Art
[0004] Since Grtzel's research group in Swiss has reported a
dye-sensitized solar cell (B. O'Regan, M. Grtzel, Nature 353, 737
(1991)), researches thereinto have been actively conducted. The
dye-sensitized solar cell by Grtzel et al. is a
photo-electrochemical solar cell using an oxide semiconductor
electrode comprising photosensitive dye molecules that can absorb
light within visible region, thereby to generate an electron-hole
pair and nano crystalline titanium dioxide that can transfer the
generated electron. In this cell, electrons excited in dye
molecules upon receiving light within visible region are
transferred to titanium dioxide which is an n-type semiconductor,
and dye molecules are reproduced through an electrochemical
oxidation-reduction of I.sup.-/I.sub.3.sup.-contained in a liquid
electrolyte, by which current is generated.
[0005] The dye-sensitized solar cell is much expected as a solar
light conversion element due to its higher energy conversion
efficiency, while its fabrication cost is relatively low compared
to a conventional silicon solar cell.
[0006] However, because dye-sensitized solar cells provided until
now include a liquid electrolyte, stability problems have been
raised, and especially, because it is difficult for such solar
cells to be sealed, the liquid electrolyte can be leaked or an
electrochemical stability cannot be ensured in using the same for a
long time. Recently, in order to resolve such problems, researches
have been actively conducted for using an inorganic solid
electrolyte (Langmuir 19, 3572 (2003)), a polymer solid electrolyte
(Electrochemica Acta 47, 2801 (2002)), a gel electrolyte (J. Phys,
Chem. B 107, 4374 (2003)), an ionic liquid (J. Am. Chem. Soc. 125,
1166 (2003)), an organic hole carrier (Science 295, 2425 (2002)) or
the like, instead of using a liquid electrolyte. However, while a
liquid electrolyte can easily infiltrate throughout an entire
electrode plate having a thickness of 10 .mu.m or more which is
fabricated by sintering nanocrystalline titanium dioxide particles,
it is difficult for a non-liquid electrolyte to infiltrate into
such electrode plate, and therefore, energy conversion efficiency
is lowered for cells using a non-liquid electrolyte compared to
those using a liquid electrolyte (Chem. Lett. 30, 26 (2001); 31,
948 (2002)). Although a cell construction using a porous titanium
dioxide thin film based on a sol-gel method or particles in rod
shape has been proposed as a solution for such problems, it has
been known that its performance is much inferior compared to the
conventional nano-particle type. Therefore, there are problems yet
to be solved.
SUMMARY OF THE INVENTION
[0007] Therefore, one object of the present invention is to provide
a dye-sensitized solar cell comprising a semiconductor electrode
consisting of electrospun ultra-fine fibrous crystalline titanium
dioxide, which can solve electron mobility problems throughout
particles occurring in the conventional dye-sensitized solar cell
using an electrode made of sintered nanocrystalline titanium
dioxide, and which can also improve adhesiveness with solid
electrolyte and element characteristics.
[0008] Another object of the present invention is to provide a
method for fabricating the dye-sensitized solar cell comprising
electrospun ultra-fine fibrous titanium dioxide.
[0009] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention.
[0011] In the drawings:
[0012] FIG. 1 illustrates the construction of a dye-sensitized
solar cell in accordance with the present invention;
[0013] FIG. 2 is a schematic view of a general type of an
electrospinning apparatus;
[0014] FIGS. 3A to 3C are scanning electron microscopic photographs
of an electrospun ultra-fine fibrous titanium dioxide layer
fabricated in accordance with the present invention (FIG. 3A:
before pre-treatment; FIG. 3B: after pre-treatment but before
thermal treatment; and FIG. 3C: after thermal treatment);
[0015] FIG. 4 is a transmission electron microscopic photograph of
an electrospun ultra-fine fibrous titanium dioxide layer fabricated
in accordance with the present invention;
[0016] FIGS. 5A and 5B are photographs of a transparent conductive
substrate after thermal treatment, on which an electrospun
ultra-fine fibrous titanium dioxide layer was formed in accordance
with the present invention (FIG. 5A: without performing
pre-treatment; and FIG. 5B: with performing pre-treatment);
[0017] FIG. 6 is a graph showing X-ray diffraction peaks for an
electrospun ultra-fine fibrous titanium dioxide prepared in
accordance with the present invention after thermal treatment;
and
[0018] FIG. 7 is a graph showing current-voltage characteristics of
a dye-sensitized solar cell in accordance with the present
invention.
[0019] 10: Electrode
[0020] 11, 21: Glass
[0021] 12, 22: Transparent FTO conductive layer
[0022] 13: Electrospun titanium dioxide layer
[0023] 20: Counter electrode
[0024] 23: Pt layer
[0025] 30: Electrolyte
[0026] 40: Spacer
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The inventors of the present invention has made great
efforts to solve the problems in the conventional dye-sensitized
solar cell and finally developed an semiconductor electrode
consisting of an electrospun ultra-fine fibrous titanium dioxide,
so as to provide a new dye-sensitized solar cell having a titanium
dioxide layer in one dimensional structure that can facilitate
infiltration of a non-liquid electrolyte and can effectively
transfer electrons.
[0028] To achieve the above-mentioned objects and other advantages
in accordance with the present invention, as embodied and broadly
described herein, there is provided a dye-sensitized solar cell
comprising a semiconductor electrode containing an electrospun
ultra-fine fibrous titanium dioxide layer; a counter electrode; and
electrolyte infiltrated between the semiconductor electrode and the
counter electrode.
[0029] The semiconductor electrode consists of a glass substrate,
ITO or FTO transparent conductive layer, and an electrospun
ultra-fine fibrous titanium dioxide layer onto which dye molecules
are adsorbed. The electrospun ultra-fine fibrous titanium dioxide
layer has a thickness of 5-20 .mu.m.
[0030] The counter electrode can include a glass substrate, ITO or
FTO transparent conductive layer and a platinum layer.
[0031] The electrolyte is a liquid electrolyte containing iodine,
and preferably, an electrolyte of 0.1M lithium iodide (LiI), 0.05M
iodine (I.sub.2), 0.6M 1,2-dimethyl-3-propyl-imidazolium iodide and
0.5 M tert-butyl pyridine in acetonitrile, or a polymer gel
electrolyte containing at least one polymers selected from the
group consisting of
poly(vinylidenefluoride)-co-poly(hexafluoropropylene),
poly(acrylonitrile), poly(ethyleneoxide) and poly(alkylacrylate).
Preferably, the polymer gel electrolyte contains one or more
polymers as mentioned above in an amount of 5-20% by weight of a
mixture of propylene carbonate and ethylene carbonate.
[0032] To achieve another object of the invention, there is also
provided a method for fabricating a dye-sensitized solar cell
comprising the steps of:
[0033] adding titanium isopropoxide into a polymer solution,
followed by adding acetic acid as a catalyst therein, and then
stirring the resulting mixture at room temperature, to obtain a
solution for electrospinning;
[0034] electrospinning the solution to form a film made of
ultra-fine titanium dioxide fibers onto an ITO or FTO-coated
transparent conductive glass substrate;
[0035] pre-treating the substrate having the film comprising the
ultra-fine titanium dioxide fibers formed thereon with acetone or
dimethyl formamide;
[0036] thermally treating the pre-treated substrate to form a layer
of the ultra-fine fibrous titanium dioxide onto the substrate;
[0037] impregnating the thermally treated substrate in a solution
of dye molecules in ethyl alcohol, to obtain a semiconductor
electrode in which the dye molecules are adsorbed into the
electrospun ultra-fine titanium dioxide fibers;
[0038] coating a platinum layer onto an ITO or FTO-coated
transparent conductive glass substrate, to obtain a counter
electrode;
[0039] performing a heating/pressing process on the semiconductor
electrode and the counter electrode in which a spacer having a
thickness of 20 .mu.m is located therebetween, so as to attach the
above two electrodes to each other; and
[0040] injecting an electrolyte into the empty space between the
semiconductor electrode and the counter electrode.
[0041] The ultra-fine titanium dioxide fibers are electrospun so as
to have 50-1000 nm in thickness.
[0042] The pre-treatment can be performed such a manner that i) the
substrate is pre-processed with vapor of acetone or
dimethylformamide for 1-3 hours in a closed container; ii) the
substrate is immersed in acetone or dimethylformamide solvent for 1
hour, or iii) methods i) and ii) are combined. Preferably, the
thermal treatment is performed at 450.degree. C.-500.degree. C. for
30 minutes.
[0043] Below, the present invention will be described in more
detail by referring to the attached drawings.
[0044] FIG. 1 illustrates the construction of a dye-sensitized
solar cell comprising a semiconductor electrode consisting of
electrospun ultra-fine titanium dioxide fibers in accordance with
the present invention.
[0045] With reference to FIG. 1, the dye-sensitized solar cell
comprises a semiconductor electrode 10, a counter electrode 20 and
an electrolyte 30 interposed therebetween.
[0046] The semiconductor electrode 10 comprises a glass substrate
11, an indium tin oxide (ITO) or fluorine-doped tin oxide (FTO)
transparent conductive layer 12 and an electrospun ultra-fine
fibrous titanium dioxide layer 13. The ultra-fine fibrous titanium
dioxide layer 13 includes ultra-fine titanium dioxide fibers having
a diameter of about 50-1000 nm which are formed with a bunch of
fine fibrins having a thickness of about 10-30 nm one-dimensionally
arranged by electrospinning. In order to effectively generate
photocurrent, the titanium dioxide layer 13 preferably has a
thickness of about 5-20 .mu.m. Ruthenium-based dye molecules are
adsorbed onto the ultra-fine titanium dioxide fibers.
[0047] The counter electrode 20 comprises a glass substrate 21, an
ITO or FTO transparent conductive layer 22 and a platinum layer 23.
The platinum layer 23 of the counter electrode 20 is disposed to
face the fibrous titanium dioxide layer 13 of the semiconductor
electrode 10.
[0048] As the electrolyte 30 filled in the space between the
semiconductor electrode 10 and the counter electrode 20, an
electrolyte solution of I.sub.3.sup.-/I.sup.- in which 0.1M lithium
iodide (LiI), 0.05M iodine (I.sub.2), 0.6M
1,2-dimethyl-3-propyl-imidazolium iodide and 0.5M tert-butyl
pyridine are dissolved in acetonitrile. Alternatively, instead of
the liquid electrolyte, a polymer gel electrolyte in which one or
more polymers selected from the group consisting of
poly(vinylidenefluoride)-c- o-poly(hexafluoropropylene),
poly(acrylonitrile), poly-(ethyleneoxide) and poly(alkylacrylate)
are contained in an amount of 5-20% by weight of a mixture of
propylene carbonate and ethylene carbonate can be used.
[0049] A method for fabricating a dye-sensitized solar cell
comprising a semiconductor electrode consisting of electrospun
ultra-fine titanium dioxide fibers in accordance with the present
invention will be described below.
[0050] In order to fabricate the semiconductor 10, an anode, the
electrospun ultra-fine titanium dioxide fibers are fabricated.
[0051] To begin with, a solution for an electrospinning is prepared
by a sol-gel reaction of titanium isopropoxide. In detail,
polyvinylacetate having an excellent affinity to titanium dioxide
is dissolved in dimethylformamide, acetone, tetrahydrofuran,
toluene or a mixture thereof, to prepare a 5-20 wt % polymer
solution to be able to give a viscosity suitable for
electrospinning. Polyvinylacetate having a weight average molecular
weight of 100,000-1,000,000 g/mol is used. Instead of
polyvinylacetate, polyvinylpyrrolidone, polyvinylalcohol,
polyethyleneoxide and the like can be used to prepare a polymer
solution. Next, titanium isopropoxide is added into the
polyvinylacetate polymer solution in an amount of 5-25 wt % of the
polyvinylacetate polymer solution, to which acetic acid is added as
a catalyst in an amount of 20-60 wt % of the titanium isopropoxide.
The resulting mixture was then reacted for 1-5 hours at room
temperature so as to obtain a solution for electrospinning. This
solution is required to maintain a suitable viscosity required for
electrospinning. After being spun into fibers, the polymer is
completely decomposed by a thermal treatment at 450.degree. C. or
higher, and residual titanium dioxide is converted into an anatase
type crystal structure.
[0052] And then, electrospun ultra-fine titanium dioxide fibers are
obtained with an electrospinning apparatus. As shown in FIG. 2, a
generally used electrospinning apparatus includes a spinning nozzle
connected to a quantizing pump that can introduce a solution to be
spun quantitatively, a high voltage generator, an electrode for
forming a layer of fibers to be spun. An earthed transparent
conductive glass substrate, specifically a transparent conductive
glass substrate onto which ITO or FTO are coated and which have
conductivity of 5-30 .OMEGA. is used as an anode, and the spinning
nozzle equipped with a pump which can control discharging amount
per time is used as a cathode. 10-30 KV of voltage is applied and a
solution discharge rate is adjusted at 10-50 I/min, so as to obtain
ultra-fine titanium dioxide fibers having a thickness of 50-1,000
nm. Electrospinning is continued until a film consisting of
ultra-fine titanium dioxide fibers at a thickness of 5-20 .mu.m is
formed onto the transparent conductive substrate. By adjusting
discharging amount and voltage, the thickness and form of the fiber
can be controlled. In addition, in order to uniformly maintain the
thickness of the film entirely, it is preferred to use a robot
system which can repeatedly moves the position of the
electrospinning nozzle.
[0053] Thereafter, before thermal treatment, a pre-treatment is
performed such that the transparent conductive substrate, onto
which the film made of the electrospun ultra-fine titanium dioxide
fibers is formed, is treated with vapor of acetone or
dimethylformamide, which has been used as solvent for the polymer
solution, for 1-3 hours in a closed container or immersed the
substrate in acetone or dimethyl formamide for 1 hour. The
electrospun ultra-fine titanium dioxide fibers are in the form of a
film mixed with a polymer on the substrate (referred to as
`polymer-titanium dioxide composite film`, hereinafter). Thus, in
order to obtain the semiconductor electrode in accordance with the
present invention, it is necessary for the substrate on which
electrospun titanium dioxide film is formed to be subjected to a
thermal treatment at a temperature of 450.degree. C. or higher,
thereby to remove the polymer binder completely and to convert
residual ultra-fine titanium dioxide fibers into anatase type
crystal structure. However, if a polymer-titanium dioxide composite
film formed by a conventional electrospinning is thermally treated
at such a high temperature in the air, the transparent conductive
substrate and the titanium dioxide film are separated (refer to
FIG. 5A), and therefore, it can not be used as an electrode for a
preferable dye-sensitized solar cell.
[0054] Therefore, in the present invention, the polymer-titanium
dioxide composite film formed by electrospinning is subjected to a
pre-treatment so as to form a firm titanium dioxide film (refer to
FIG. 5B). The pre-treatment is to dissolve the polymer portion of
the polymer-titanium dioxide composite fibers by treat the
electrospun ultra-fine titanium dioxide fibers in which polymer and
titanium dioxide are mixed (referred to as `polymer-titanium
dioxide composite fiber`, hereinafter) with a solvent. During this
process, adhesiveness among fibers can be increased, while the
fibrous form of the titanium dioxide is maintained as it is, so
that its adhesiveness to the lower transparent conductive substrate
can be increased. As stated above, the pre-treatment includes the
method of treating the electrospun polymer-titanium dioxide
composite film with vapor of acetone or dimethylformamide used as a
solvent for the polymer solution for 1-3 hours, the method of
immersing the electrospun polymer-titanium dioxide composite film
in acetone or dimethyl formamide, and the method of employing both
methods. In the aspect of efficiency, it is preferred to use the
third method in a manner that the electrospun polymer-titanium
dioxide composite film is treated with vapor and then immersed in
the solvent.
[0055] The pre-processed transparent conductive substrate is
thermally treated at 450-500.degree. C. for 30 minutes in the air
to thermally decompose residual polymer so as to completely remove
the same, by which crystalline structure of the titanium dioxide is
converted into anatase type.
[0056] And then, the substrate, on which the film made of the
electrospun titanium fibers is formed, is impregnated in a solution
in which a Ruthenium-based dye molecules, for example, dye
molecules represented by a structural formula of
RuL.sub.2(NCS).sub.2, wherein L=2,2'-bipyridyl-4,4'-dicarboxyl
acid, is dissolved in ethyl alcohol in a concentration of
3.times.10.sup.-4M for 12 hours or more, to adsorb the dye
molecules therein. The substrate was then washed with ethyl alcohol
and then dried, to obtain the semiconductor electrode 10 comprising
dye-adsorbed electrospun ultra-fine titanium dioxide fibers.
[0057] Subsequently, in order to fabricate the counter electrode
20, which is a cathode, the platinum layer 23 is coated onto an ITO
or FTO-coated transparent conductive glass substrate.
[0058] And then, the counter electrode 20, the cathode, and the
semiconductor electrode 10, the anode, are assembled such that each
conductive surface of the cathode and anode comes inward so as to
make the platinum layer 23 and the fibrous titanium dioxide layer
13 faced with each other. At this time, the two electrodes are
attached with a spacer 40 having a thickness of about 20 .mu.m,
which is made of a thermoplastic Surlyn.TM. (available from Du Pont
Co.) and inserted the above two electrodes.
[0059] Thereafter, a liquid electrolyte or polymer gel electrolyte
is filled in spaces between the two electrodes. As mentioned above,
as the liquid electrolyte, the electrolyte solution of
I.sub.3.sup.-/I.sup.--obt- ained by dissolving 0.1M of lithium
iodine (LiI), 0.05M of iodine (I.sub.2), 0.6M of
1,2-dimethyl-3-propyl-imidazolium iodide and 0.5M of tert-butyl
pyridine in acetonitrile. As the polymer gel electrolyte, a mixture
obtained by dissolving one or more polymers selected from the group
consisting of
poly(vinylidenefluoride)-co-poly(hexafluoropropylene)- ,
poly(acrylonitrile), poly(ethyleneoxide) and poly(alkylacrylate) in
a mixture of propylene carbonate and ethylene carbonate in amount
of 5-20 wt % of the solvent mixture can be used.
[0060] The present invention will be explained in more detail in
the following examples. It is to be understood that these examples
are merely illustrative and it is not intended to limit the scope
of the present invention to these examples, and they can be
modified by the ordinary person skilled in the art within the
scope.
EXAMPLE
Example 1
Fabrication of an Electrospun Titanium Dioxide Fiber Layer
[0061] 6 g of titanium isopropoxide was slowly added in a polymer
solution in which 30 g of polyvinylacetate (Mw 500,000, a product
of Aldrich Co.) was dissolved in a 270 m of acetone and 30 ml of
dimethylformamide mixed solvent. As the reaction was initiated by
moisture contained in the solvent, the reaction mixture was changed
into a suspension. 2.4 g of acetic acid was then slowly added
dropwise as a reaction catalyst to the reaction mixture. As the
reaction was proceeding, the suspension was changed into a clear
solution. The resulting spinning solution should be spun to
ultra-fine titanium dioxide fibers within 24 hours once it was
prepared, because if it is left for a long time after the acetic
acid was added, the solution is changed into a dark brown color due
to hydrolysis of the polymer.
[0062] Electrospinning was performed with the electrospinning
apparatus shown in FIG. 2, wherein a FTO-coated transparent
conductive substrate (having the size of 10 cm.times.10 cm) was
used as a cathode and a metallic needle (No. 24) having a pump,
which can control discharge rate, attached thereto was used as an
anode, to which 15 kV of voltage was applied. While discharge rate
of the spinning solution was maintained at 30 .mu.l/min,
electrospinning was performed until the total discharge amount of
the spinning solution reaches 5,000 .mu.l, to form a layer of
ultra-fine titanium dioxide fibers onto the FTO-coated transparent
conductive substrate.
Example 2
Pre-Treatment and Thermal Treatment of the Substrate on Which a
Layer of Titanium Dioxide Fibers was Formed Fabricated in Example
1
[0063] The layer of titanium dioxide fibers fabricated in Example 1
includes polymer and titanium dioxide mixed therein. Thus, in order
to use the polymer-titanium dioxide composite film-formed substrate
as a semiconductor electrode of a dye-sensitized solar cell, the
substrate should be thermally treated at a high temperature to
remove polyvinylacetate, a polymer binder, and the spun ultra-fine
titanium dioxide fibers should be converted into a crystal form.
However, if the substrate fabricated in Example 1 is thermally
treated at a high temperature without pre-treatment, the titanium
dioxide film would not be attached on the FTO-coated substrate but
be separated therefrom, and thus, it cannot be used as a
semiconductor electrode for a dye-sensitized solar cell.
[0064] Thus, before the thermal treatment, the substrate, on which
the polymer-titanium dioxide composite film fabricated in Example 1
has been formed, was treated only with acetone vapor for 1 hour,
without being directly contacted with acetone in a closed
container, and then, thermally treated in an electric furnace of
450.degree. C. in the air for 30 minutes, thereby to stably form
titanium dioxide film on the FTO-coated transparent conductive
substrate.
[0065] FIGS. 3A to 3C are scanning electron microscopic photographs
of the electrospun titanium dioxide fiber layer in stages
fabricated in accordance with the present invention. Specifically,
FIG. 3A is a scanning electron microscopic photograph of the
titanium dioxide fiber layer after electrospinning but before
pre-treatment, FIG. 3B is a scanning electron microscopic
photograph of the titanium dioxide fiber layer after pre-treatment,
and FIG. 3C is a scanning electron microscopic photograph of the
titanium dioxide fiber layer after pre-treatment and thermal
treatment.
[0066] As shown in FIG. 3A, in the titanium dioxide fiber layer
that was thermally treated without pre-treatment, individual fibers
were fixed separately without being adhered to each other after
thermal treatment. When the titanium dioxide fiber layer was
subjected to the pre-treatment, fibers were well connected with
each other to form a dense film as shown in FIG. 3B because the
matrix polymer from fibers spun are partially dissolved.
[0067] The ultra-fine fibrous titanium dioxide layer obtained after
the pre-treatment and subsequent thermal treatment shows that the
layer of individual fibers were well adhered to each other, so as
to enable electron transfer to be more effective in a solar cell
device.
[0068] FIG. 4 is a transmission electron microscopic photograph of
the electrospun titanium dioxide fiber layer fabricated in
accordance with the present invention, and especially shows fine
structure of the ultra-fine titanium dioxide fibers remaining after
removing polymer by the pre-treatment and subsequent thermal
treatment at 450.degree. C. As shown in FIG. 4, the fibrous
titanium dioxide crystal arranged one-dimensionally increases
movement photocurrent in the solar cell, which are intended by the
present invention, thereby to remarkably improve performance of the
element.
[0069] FIGS. 5A and 5B are photographs of transparent conductive
substrate with the electrospun titanium dioxide fiber layer formed
thereon after the substrate was thermally treated in accordance
with the present invention. FIG. 5A is for the substrate without
performing the pre-treatment thereon, and FIG. 5B is for the
pre-treated substrate.
[0070] As shown in FIG. 5A, when the substrate was thermally
treated at 450.degree. C. without pre-treatment, the titanium
dioxide film was separated from the substrate. On the contrary,
when the substrates was pre-treated with vapor of acetone, followed
by thermally treated, the titanium dioxide film was firmly formed
on the substrate as shown in FIG. 5B.
[0071] FIG. 6 is a graph showing X-ray diffraction peaks after the
electrospun ultra-fine titanium dioxide fibers were thermally
treated in accordance with the present invention. It can be seen
from FIG. 6 that the crystalline structure of the titanium dioxide
was converted into the anatase type after the thermal
treatment.
Example 3
Fabrication of a Dye-Sensitized Solar Cell Using the Titanium
Dioxide Fiber Layer Fabricated in Example 2
[0072] Dye molecules were adsorbed onto the ultra-fine titanium
dioxide fibers in the substrate fabricated in Example 2.
Specifically, the transparent conductive glass substrate fabricated
in Example 2 was impregnated in 3.times.10.sup.-4M solution of
RuL.sub.2(NCS).sub.2 (L=2,2'-bipyridyl-4,4'-dicarboxyl acid)
(Ruthenium 535, available from Solaronix,), a ruthenium-based dye,
in ethanol for 12 hours, so as to adsorb the dye molecules therein.
The resulting substrate was washed with ethanol several times and
then dried, thereby to give a semiconductor electrode. Separately,
a platinum layer was coated onto a FTO-coated transparent
conductive glass substrate to obtain a counter electrode.
[0073] Next, a spacer having a thickness of about 20 .mu.m was
located between the semiconductor electrode and the counter
electrode fabricated as described above, and a certain pressure was
applied thereto at 120.degree. C. so as to attach the above two
electrodes. Iodine-based liquid electrolyte was then filled in the
space between the above two electrodes, and the resultant was
sealed to obtain the dye-sensitized solar cell in accordance with
the present invention. Liquid electrolyte used was prepared by
dissolving 0.25 g of iodine, 0.26 g of lithium iodide, 3.70 g of
1,2-dimethyl-3-propyl-imidazolium iodide and 1.34 g of tert-butyl
pyridine in 20 ml of acetonitrile.
[0074] Meanwhile, the dye-sensitized solar cell was also fabricated
in the same manner as described in Example 1 to Example 3, except
for using a polymer gel electrolyte instead of the liquid
electrolyte. The polymer get electrolyte used in this case was
prepared by dissolving 0.125 g of poly(vinylidene
fluoride)-co-poly(hexafluoropropylene) (Kynar 2801), 0.13 g of
1-hexyl-2,3-dimethyl imidazolum iodide (Im), and 0.008 g of iodine
in a mixture of 0.75 g of propylene carbonate and 0.5 g of ethylene
carbonate at 80.degree. C.
[0075] FIG. 7 is a graph showing current-voltage characteristics of
the dye-sensitized solar cell fabricated in the above Examples in
accordance with the present invention. As an electrolyte for the
dye-sensitized solar cell, the liquid electrolyte and the gel
electrolyte prepared as described in Example 3 were respectively
used. Table 1 below shows photoelectrochemical characteristics
which were calculated with current-voltage curves relative to the
types of the electrolytes, namely, photocurrent density (J.sub.SC),
voltage (V.sub.OC), fill factor (ff) and energy conversion
efficiency (.eta.).
1TABLE 1 Electrolytes J.sub.sc (mA/cm.sup.2) V.sub.oc (V) ff .eta.
(%) Liquid electrolyte 8.35 0.76 0.62 3.90 Get electrolyte 7.72
0.80 0.58 3.61
[0076] As can be seen from the above Table 1, the dye-sensitized
solar cell comprising a semiconductor electrode consisting of the
electrospun ultra-fine titanium dioxide fibers according to the
present invention exhibits energy conversion efficiency of above
90% even in using gel electrolyte relative to that of the cell
using liquid electrolyte.
[0077] As so far described, the dye-sensitized solar cell in
accordance with the present invention comprises a semiconductor
electrode comprising electrospun ultra-fine titanium dioxide
fibers, and therefore, the non-liquid electrolyte such as a polymer
gel electrolyte or the like having low fluidity, as well as a
liquid electrolyte, can even be easily infiltrated thereinto. In
addition, electrons can be effectively transferred because titanium
dioxide crystals are arranged one-dimensionally.
[0078] As the present invention may be embodied in several forms
without departing from the spirit or essential characteristics
thereof, it should also be understood that the above-described
embodiments are not limited by any of the details of the foregoing
description, unless otherwise specified, but rather should be
construed broadly within its spirit and scope as defined in the
appended claims, and therefore all changes and modifications that
fall within the metes and bounds of the claims, or equivalence of
such metes and bounds are therefore intended to be embraced by the
appended claims.
* * * * *