Three Bond Technical News
Issued Jul. 1, 2005

65

Dye-sensitized solar cells
Introduction ________________________________________________In the recent energy problems, substitution energy for the fossil fuel has been developed
flourishingly. The photovoltaic power-generation, pronoun of clean energy, attracts the attention invarious countries in the world. Various specifications of the photovoltaic power-generation system has
been built as well as current widely-used silicone-based solar cells. The other hand, IT age become
established so that mobile-related devices are popular today. So ensuring power supply andhigh-performance of devices has continued a fight. In their movement, "Dye-sensitized solar cells"(referred to DSC) attract the attention as the next-generation power supply next to the fuel cell. Thesesolar cells have various advantages that are different from conventional silicone-based solar cells. But
power-generation mechanism is the type of chemical reaction so that high-reliability and safety are
required. Especially it is said that sealing technology can be the key to produce dye-sensitized solar
cells, and many ideas are proposed. This issue mainly provides the performance and characteristics ofsealing agents that produce dye-sensitized solar cells.

Contents
Introduction ...................................................... 1
1, Method of solar cells ................................. 2
2, What is Dye-sensitized solar cell? .......... 2
3, History of DSC ............................................. 3
4, Constitution of DSC...................................... 3

5, Prescribed properties of sealants for DSC ....56, Characteristic of sealing agent "31X-101" forDSC ...........................................................67, Conclusion ....................................................81

1, Method of solar cells
Solar cells are the solar-powered cells as its namesuggests, and it is one of our familiar cells. In thethinking of the future energy environment such as
limiting the volume of carbon dioxide discharge on
global warming, solar cells attracts the attention as
the hopeful energy media next to the wind generation.
Figure 1 shows the classified solar cells. Like thistable, solar cells are classified by semiconductormaterial:
silicone-based,compoundsemiconductor-based, organic semiconductor-based,
and metal-oxide semiconductor-based. Silicone-basedsolar cells are developed comparatively for a longtime, and each of 3 types of silicone-based solar cells
has own unique characteristic respectively: single
crystal, poly crystal, and amorphous crystal. Well,what kind of principle do solar cells operate or
generate based on? Figure 2 shows the principleroughly. As mentioned above, materials of solar cells
are semiconductor represented by silicone.
Semiconductor is the material used in LSI(Large-Scale Integration), transistor and so on. Solarcells generate the power by 2 types ofsemiconductors: P-type and N-type semiconductors.
In other words, solar cells are the cell that generates
by utilizing electronic passing from/to thesemiconductor layers, and if only sunshine conditionis supplied stably, it is utilized as infinite power
supply media. From the point of view like that, solar
cells attracts the attention as the next-generationenergy source. However, although the utility value isrecognized, solar cells to general household have low
diffusion rate.
2, What are Dye-sensitized solar cells?
Dye-sensitized solar cells are the cell that dye
adsorbed in nanostructure of titanium oxide generates
by the light. They are unfamiliar, but theirconstruction technology is familiar to us for a longtime. For instance, photo is a familiar techniqueexample that utilizes the sensitization ofdye-sensitizing solar cells. The principle of photo
printing is the chemical reaction occurred in the time
when both the emulsion (silver halide) coated on thefilm and the dye react to light. The difference fromdye-sensitizing solar cells is the difference between
titanium dioxide and emulsion, and the principle thatthe dye absorbs light and generates electron isconsidered equivalent in a broad sense. On the other
hand, plant photosynthesis is also dye-sensitization in
the natural world. This is also the same as
dye-sensitizing solar cells, and oxygen and plants
generate nourishment to grow instead ofpower-generation. In this way, dye-sensitization
technology is widely used in the natural world as wellas industrial world, and it is harmonized withnanotechnology that has been in the news so that it
can be also said that it is a technology that the newpotential is found.

Figure 1, Classification of solar cells

2The structure of these solar cells is very simple asshown in figure 3. Well, what is the difference
between this solar cell and the solar cells sold in thecurrent market? Compared with the existingsilicone-based solar cells, the remarkable point is tomake manufacturing facilities simple. Current solar
cells use high-purity silicone crystals so that theyrequire a large-scaled manufacturing equipment. Also
now in low-costed amorphous silicones, burden for
manufacturing facility is heavy and thus
manufacturers are limited that can produce and sellcells. Then relationship between price of solar cells
and power-generation efficiency is considered.
Compared with silicone-based solar cells,
power-generation efficiency of DSC (photocarrier
generation efficiency) is about 1/10 for small ones.
But cost to manufacture DSC is about 1/5 comparedwith the cost of manufacturing silicone-based so thatit takes a short time to depreciate. In other words,
compared with the existing solar cells, DSC has agreat advantage that it is low-costed, and it can beproduced by every manufacturer. In recently
announced model houses, their wall surfaces areconstructed with DSC. This utilizes the DSC's greatfeature that power-generation is hard to be affectedby incident light angular degree. In the same way,
external wall of the buildings may be processed with
DSC. It is normal that conventional solar cells areplaced on the roof, but DSC can be used in the wallas well as roof so that houses that all power are
generated by solar cells can be existed. DSC is also
sold as a module unit in the market. DSC is counted
because it can be also installed in the hard location to
install with silicone-based solar cells. It is expectedthat DSC has a unique market in its own category, nota substitution of existing solar cells. On the other
hand, it is said that the greatest problem for practicaluse is to ensure a long-term stability. In other words,
it is a problem that evaporation of liquid causes
performance deterioration because cell is filled withthe organic solvent-based electrolyte. It is also
required to eliminate the anxiety of the liquid leakage
from the view of safety too. Many methods to solvethese problems are developed actively.
3, History of DSC
DSC has a long history that belongs to the categoryof organic semiconductor. As mentioned above, plant

photosynthesis provides nourishment by using thisprinciple. When thinking of it, it can be said that DSC
is wisdom learned from the natural world by nature.
In the public domain announcement at the laboratorylevel, Tubomura and others at Osaka University
announced the DSC principle with zinc oxide on the
Nature magazine in 1976 as the oldest one (therelationship between DSC and Japanese were
surprising us). But the highest impact announcement
is that Graetzel and others announced on the Nature
magazine in 1991. This is a basic structure on thecurrent DSC by using titanium oxide and dye, and its
patent is acquired in 1988. This basic patent will
expire in 2008, and many manufacturers consider its
timing as a starting line to industrialization toresearch and develop currently. This technology has
attracted an attention to for a long time in Japan, andit has been researched and developed. Especiallyresearch facilities in the university have a cuttingedge rather than the enterprises. In foreign countries,
venture companies advance their research to scale up
dye-sensitized solar cells aiming at practical use, andlong-term stability is reported when using multiple
unit flotation cells of approximate 10cm x 10cm.
4, Construction of DSC
As shown in figure 3, DSC construction is very
compact. Dye-sensitized solar cells use glass
substrates with transparent conductive films as an
anode (a photoelectrode) that is coated on withnanosized titania particles in paste form and that issintered at approximate 450 degrees Celsius.
Thickness of the titania layer is approximate 10 to 15
micrometer, and titania layer has many nanosizedholes so that effective surface area reaches 1000
times or more from the apparent substrate area. Whenruthenium bipyridyl complex with carboxyl group is
supported to the inner surface of these holes,
carboxyl group enables pigments to be combinedwith the titania surface chemically. On the other hand,
as a cathode (counter electrode), platinum isevaporated in transparent conductive films on glass
substrates, and electrolyte is filled up between both eelectrodes. As electrolyte, solvent of nitrile-based isused, and redox-based of iodine and iodine ion are
dissolved as a solute to be elaborated. It is DSC's
great characteristic that electron is cycled in such aphotochemical reaction. At this point, difference from3

conventional solar cells is that dye absorbs the lights,
that is adsorbed in the semiconductor layer rather
than that the semiconductor layer absorbs the lights
directly. The amount of light absorption depends on
the amount of power-generation, but using dye meansthat colorful and high architectural solar cells can be
designed. It is largely different from conventional
solar cells that were black only. Conventional solar
cells were monotone and conservative, but

appearance of DSC enables solar cells to be colorfuland "harmony with environment". Figure 4 shows theconceptual diagram of manufacturing process. When
thinking of the process: bonding adherends, sealing,
implanting the electrolyte, and sealing, this is similar
to LCD panel manufacturing process one decade ago.
In other words, even when thinking of manufacturing
DSC, it is expected that it can be supported byapplying the existing facilities.

Figure 2, Principle of power-generation by solar cells

Figure 3, Overview of Dye-sensitized solar cells

45, Prescribed properties of sealants for DSC
As mentioned above of DSC construction, DSC
generates the power by chemical reaction between
dye and electrolyte. To generate electricity efficiently,
it is indispensable to prevent leakage of electrolyte.
Electrolyte takes the same role as a blood for DSC,
and this drying up means that power-generation is
stopped. In other words, to use DSC as an stable
power supply, sealing technology is required toprevent electrolyte from leaking. At this point, we
discuss a little about electrolyte. Electrolyte consistsof high-polarity organic solvents, iodides, andadditives. Organic solvent depends on required cell
performance, but organic solvent of general
nitrile-based is mainly used. Most organic
compounds are dissolved because this nitrile-basedorganic solvent has a high polarity. Therefore innormal high-polymer compounds sufficient resistancecannot be provided, and adhesive property withadherend is given as requirement of the sealing that it
is very likely that sealing agents themselves arecorroded by electrolyte before sealing, but highadhesiveness means to enhance the polarity of sealing
agents so that affinity with electrolyte becomes
higher and sealing capability causes to be declined. In
other words, relationship between sealing property
and electrolyte resistance property is a trade-off.
However, reliability of cells is not provided unlesssealing is performed. Sealing technology for DSC is
required to maintain the performance. In other words,
the highest priority as a performance required forsealing agents for DSC is "to control leakage ofelectrolyte" according to these information. To
prevent leakage of iodide itself to outside of the cell
as well as volatile, design is required that chemical
structure itself of the resin is hard to corrode to polar
solvent and iodine.

Figure 4, Conceptual diagram of DSC manufacturing process56, Characteristic of sealing agent "31X-101"
for DSC
From these backgrounds, we at Three Bond haveconsidered workability and productivity, andlight-curing sealing agent "31X-101" for DSC wasdeveloped. These characteristic values are
summarized as shown in table 1. The design pointon the resins is 31X-101's own high electrolyte
resistance property. Furthermore as of light-curing
property, we considered relationship between
shearing adhesive strength and curing intensity, andwe found that light at approximate 1000mJ/cm2 isenough for curing (figure 5). Figure 6 shows thecomparison of the electrolyte resistance property
between conventional reactive resins and 31X-101.
This figure shows that affinity with electrolyte islower than silicone-based resin, epoxy-based resin,
and acryl-based resin. This is originated by
31X-101's resin skeleton. To enhance the electrolyte
resistance property, it is required to lower thepolarity of resin skeleton. In other words, there is apoint that making the polarity of resin skeleton
lower than the polarity of electrolyte lowers affinity
with electrolyte. Creating "Water and oil don't mix"
environment between sealing agents and electrolyte
has controlled leakage of electrolyte via sealing
agents. Then sealing capability for 31X-101 is
considered by using dummy cells. Figure 7 shows
how to create dummy cells. To create dummy cells,
31X-101 is coated between two glass substrates,and electrolyte is implanted, the sealing state is
created. In this state several durability tests are
performed. Results of electrolyte leakage property
by UV-ray exposure test for dummy cells that weremade in this way, that are summarized as shown infigure 8 to figure 11. Figure 8 shows the results oneach environment test about seal width andelectrolyte leakage property. This figure shows that
if gap is 15 micron (height is 15 micron that sealing
agents are squeezed), when seal width is more thanor equal to 3mm, electrolyte leakage property isgreatly controlled. Furthermore as a result thatsealing agent's gap was also considered, as shownin figure 9, we found that narrowing the sealing
agent's gap can improve electrolyte leakage control
ability. Then we performed the similar tests bychanging the types of electrolyte. Figure 10 shows
the result. As this result, it is found that electrolyte
leakage control ability greatly depends on the typesof electrolyte. Especially in propylene carbonate(PC shown in the figure) and gamma-butyrolactone(gamma-BL in the figure) it was found that great
depression effect is shown, not depending on thewidth of sealing agents. As similar consideration,compared with the sealing agents (thermoplastic
resin) used in DSC historically, in a result of seal
width (3 mm), it was found that 31X-101 is a great
sealing performance in the every electrolyte (figure
11).

Table 1, Basic characteristics of sealing agent 31X-101 for DSC
31X-101
Test method
Appearance
Creamy white
3TS-201-01
Viscosity (Pa・s)
160
3TS-210-02
Specific gravity
0.98
3TS-213-02
Hardness
55
3TS-215-01*1
Chemical resitance (%)
2
3TS-620-01*2
Premeability (g/m2・24h)
40
JIS Z 0280
Peeling property (MPa)43TS-320-02
Elongation ratio (%)
70
3TS-301-02*1: Immersed in acetonitrile, *2: Thickness of 100 micron

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Remarks
Visually
BH-type, No.7, 60rpm
Specific gravity cup method
Durometer A
60℃x24h
60℃x95%RH
Peeling speed of 50mm/min
Peeling speed of 50mm/min

Figure 5, Relationship between irradiation time andshearing adhesive strength by UV-ray intensity
Figure 8, Sealing property on several environmental
tests

Figure 6, Comparison of electrolyte resistance property
on several sealing agents
Figure 9, Relationship between seal width and seal gap

Figure 7, How to create dummy cells

Figure 10, Sealing property on several electrolytes7Figure 11, Comparison with thermoplastic resin on
several electrolytes

7, Conclusion
DSC is in the news so that mass communication media take it up as well as fuel cells and organic ELs. It will
have a long way that DSC is in practical use, but its high design flexibility also enables the applications such as
notebook PCs, cellular phones, and PDAs to be greatly expected, and it attracts an attention as a ubiquitouselectric power-supply in the mobile age. Three Bond has studied and developed the sealing technology and theseal materials adequate to DSC for many years. We take advantage of them and we hope that they will contributeto the propagation of DSC in the future.
Katsuhiko Kishi
Hiroyuki Katsuhiko
Research Section
Development Department
Research Laboratory
Three Bond Co., Ltd.

1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, Japan
Tel: 81-426-61-1333

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