Abstract: Novel Materials and new design for
DSSC technology
This thesis work was devoted to the study of possible application of new materi-
als for the development of Dye Sensitized Solar Cells (DSSC) with improved so-
lar conversion e ciency and stability. A DSSC is a photoelectrochemical device
allowing electric power generation from sunlight. The DSSC working principles
are illustrated in gure 1. Sunlight is absorbed by the dye which is absorbed on
a TiO
2
-based electrode. Absorbed photon excites an electron from the HOMO
to the LUMO of the dye. As a consequence, the excited electron is transferred
into the titania electrode up to the external circuit. The dye is regenerated
thanks to the presence of a redox couple (tipically I
=I
3
) contained into the
electrolyte (normally in a liquid phase). Up to now, the best e ciency of light
Figure 1: Scheme of Dye Sensitized Solar Cell working principles
conversion obtained by using DSSC technology is ca 12 % and is reached using
Ruthenium complexes as sensitisers, liquid electrolyte (based on acetonitrile or
methoxipropyonitrile solvents), TiO
2
nanoparticles for the electrode prepara-
tion and Platinium-based counter electrode.
In this thesis work, particular attention was given to the use of novel nanos-
tructured materials for the preparation of innovative and more stable non-liquid
electrolytes and novel organic dyes as light absorbers. The thesis work was de-
veloped in the frame of the European project Innovasol concerning Innovative
Materials for future generation excitonic solar cells that is devoted to the opti-
mization of DSSC technology; thanks to the collaboration of six academic insti-
tutions (Cambridge, EPFL, Unito, Unipmn, TUD, Universidade de Campinas)
and two industrial partners (Centro ricerche Fiat and Solaronix). One of the
major problems of DSSC is their low stability, due to the high volatility of the
liquid electrolyte together with the toxicity of some components (solvent used in
the electrolyte and dyes). In this work non-liquid electrolytes were prepared by
adding to methoxipropyonitrile-based electrolyte 5wt% of inorganic or hybrid
4
organic-inorganic nanoparticles prepared by researchers of University of East-
ern Piedmont and Campinas University. Synthetic clays with di erent chemical
composition (i.e. saponite and talcs) and TiO
2
aiming to prepare stable elec-
trolytes. Saponites are phyllosilicates whose TOT structure is represented in
Figure 2: (left)Schematic view of a saponite structure. (center) TEM image of
a saponite prepared by usingH
2
O=Si ratio equal to 20 (right) and equal to 150.
The nanoparticles size decreases from ca 200 nm up to 50nm
gure 2. These materials have a negative charge layer, due to interstitial sub-
stitutions of Si(IV) with Al(III) atoms. This charge is normally balanced with
cations located in the interlayer space. The nanoparticles size can be tuned by
varying the amount of H
2
O=Si ratio used for the preparation of the synthesis
gel. In this work, saponite materials prepared by usind H
2
O=Si ratio of 20,
50, 110, 150 were tested (characterized by particles with dimensions ranging
from 200 to 50 nm) were tested. The electrochemical measurements done on
cells prepared by using non-liquid saponite-based electrolyte revealed a slight
increase of e ciency for all cells. In addition, for all saponite-based devices
(except for saponite prepared with H2O/Si ratio of 150 ratio) an increase of
short circuit current (J
sc
) suggesting a light scattering phenomenon, due to
saponite nanoparticles, allowing to increase the overall cell e ciency. Beside
saponite, synthetic talcs (that are magnesium silicates with neutral layers) were
used as additive for non-liquid electrolyte preparation. In particular the surface
of tested talcs were functionalized, by one-pot synthesis, with organic pendent
groups (CH
2
CH
2
CH
2
NH
2
orCH
2
CH
2
CH
2
NHCH
2
CH
2
NH
2
) into the inter-
layer space. I-V curves of talc-based devices revealed that the addition of talc
has positive e ect on the overall cell e ciency especially thanks to an increase
in the open circuit voltage V
oc
. This e ect should be due to a possible positive
in uence of talc on avoiding recombination of electrons. The best performance
obtained was an improvement of the relative e ciency of ca +12 :5% for the
cells with talc with respect to those prepared by using liquid electrolyte. After
this preliminary work, quasi-solid electrolytes were prepared by adding 20%wt of
saponite (both inorganic or hybrid organic-inorganic saponite prepared by intro-
ducing COOH andNH
2
groups on the surface) to ionic-liquid based electrolytes.
Obtained results showed that the DSSC performances strongly depend on the
nanoparticle size. An increase on the relative e ciency of ca +15% was obtained
by using greater saponite particles. Beside layered solids, gel electrolytes were
also prepared by adding to ionic liquids TiO
2
nanoparticles. Obtained results
showed that the solar cells performance increase occurs only by using gel elec-
5
trolyte prepared by adding 15wt% or 20wt% obtaining an increase of relative
e ciency of ca +25 :6% with respect to the ionic liquid electrolyte. These results
suggested the occurring of a possible Grotthus-like mechanism which allows an
increase in the redox coupleI
=I
3
di usion. Novel sensitisers were in addition
Figure 3: (left) VG10CX dye (X=2,8; carbon chain length) structure. (right)
D5 dye structure
tested for DSSCs preparation. Two antisymmetric squaraine dyes (Fig. 3),
with di erent lenght of carbon chain prepared by researcher of University of
turin were tested. The obtained results indicated that the VG10C8 (squaraine
with longer chain) allowed to increase the performances of DSSC devices im-
proving both e ciency and stability. It was pointed out that an increase in the
carbon chain length allowed to obtain increased relative e ciency of 37% with
respect to the dye with the short chain. An emitting dye, based on dipheny-
laniline group, the so-called D5 dye ( g. 3), was tested in DSSC prepared
with a ionic-liquid-based gel electrolyte prepared by adding saponite nanopar-
ticles. The so-prepared DSSC showed good performances with respect to cells
prepared by using Ruthenium dyes and ionic liquid electrolyte overcoming the
RuL
2
(NCS)
2
:2TBA(L=2,2’-bipyridyl-4,4’-dicarboxylic acid;TBA = tetrabuty-
lammonium), N719 in literature, in combination with an IL electrolyte, and the
RuLL’(NCS)
2
2H
2
O(L=2,2’-bipyridyl-4,4’-dicarboxylic acid; L’ = 4,4’-dinonyl-
2,2’-bipyridine), Z907 in literature, in combination with the gel electrolyte. As
conclusion of this work, in order to solve the sealing problems encountered dur-
ing the preparation of QuasiSolid-DSSCs two new cell designs were tested. The
rst method, called electrolyte bath, consists in soaking the titania electrode
into the electrolyte solution before the sealing process. In this way, using gel
electrolyte it should be possible to obtain more stable junction and it should be
possible to solve the physical problem due to the sealing. Even if worse perfo-
mances were obtained, the V
oc
enhances of +11% with respect to the reference
acetonitrile-based electrolyte. The second method consists in the use of coloured
TiO
2
nanoparticles. The coloured TiO
2
nanoparticles were obtained by soak-
ing the untreated titania nanoparticles into a N719 dye solution for one night;
then the mixture was ltered, by using a Buchner funnel, obtaining a coloured
powder with the dye absorbed on the TiO
2
surface. The powder was injected
into a ionic liquid electrolyte. The method was tested adding 5wt% or 10wt%
of nanoparticles, coloured and untreated, demonstrating that the coloured par-
ticles into the electrolyte don’t a ect the performance of the cells.
6
Introduction: Solar Energy
market and new
technologies
"There is something fundamentally wrong
with treating the earth as if
it were a business in liquidation"
Herman Daly
Crisis, from ancient Greek "Krisis", decision, choice, or, in the younger and
newer meaning, any event that is an unstable and dangerous situation, is our
actual situation. In the last twenty years all over the world, people, politicians,
economist and governments have understood that our world is collapsing. The
last two centuries of industrialization and of exploitation of all useful resources
have brought us to a situation in which it is not possible to continue with
the same lifestyle, with the same principles which lead economy for years. In
particular it is not possible anymore to produce energy and electricity only
exploiting fossil fuel and non-renewable resources. The e ect of the Green House
Gases was recognized ten years ago with the Kyoto Protocol and nally countries
all over the world have started to ght the rampant pollution. O course the
control of GHG emissions is not enough if there are not e cient and ecologic
way to produce energy and electricity. Here the crisis, or the Krisis, from the
ancient greek meaning: decision. There are many possible routes to "save our
planet" and many possible technologies to produce clean and green energy: from
the wind or with the hydroelectric power plant, from the nuclear energy, if in
future will be possible to use the cold fusion or from the sun power. In any case
if, all us, we start to think with an ecologic-minded and we stop us to exploit
and waste each possible resource, our world and next generation will thanks us.
Solar Energy
In a global view, as said, it is not possible anymore to think to produce energy
only from fossil fuels. One of the best way to realize all EU limit about renew-
able energy and Green House Gas emissions, or in a more general idealist and
ethic way to preserve our planet, is obviously the solar energy. Solar energy
research was born in 1954 when Bell Labs announced the invention of the rst
modern silicon solar cell. The rst cells have about 6% e ciency. Obviously it
7
was too early and too much expensive for a huge spread of this technology, nev-
ertheless the future revolution in energy productions got its starting point. For
many years researches continued in di erent directions without seeing a satis ed
business market. From the primitive silicon solar cell built in the Bells labora-
tory, the solar devices have increased ( gure 4) their e ciency conversion, in a
incredible way, obtaining last conversion coe cient for aerospatial application
of 42:8% at the University of Delaware [1].
]
Figure 4: Research solar energy elds since 1975
I, II, III generation. Today’s solar energy technologies concern many dif-
ferent types. The di erent types of solar devices can be subdiveded into three
big categories evolving in time. I generation regards in major part today’s com-
mercial module production and it is based on a well-certi ed technology (for all
types a 20 years stability is the minimum guarantee); in other words it concerns
all silicon based modules: from heating solar panels to monocrystalline mod-
ules and from amorphous Si-PV to polycristalline Si-Panels. II generation is
the so-called thin lm technology, concerning CdTe, GaAs, InGaP, CIGS/CIS,
and other types which are conquering a big part of the solar energy world mar-
ket. In particular, their principal attractive should be less materials used, and
consequentely lower costs, respect to the I generation devices. A very inter-
esting innovation of the thin lm solar cells is the possibility to make exible
cells on polymer sublayers, instead of glass layers; this characteristic allows to
many commercial applications. III generation, instead, includes all new tech-
nologies which take advantage of di erent working principles. Particularly III
generation devices[2] should can overcome the Schkley-Queisser limit (around
30 40% of e ciency) imposed for p-n junction solar converter. Speci cally this
III generation concerns devices as: Dye Sensitized Solar Cells(DSSCs), Organic
8
(polymeric) Solar cells (OSCs), concentrated solar power and generic multilayer
devices. In particular the most utilized, even if costs and LCA prediction states
the contrary, is the silicon based PV. As shown in gure 5 the solar market in
the year 2006 was completely dominated by silicon technology, over the 70% of
the total production. In these years it is to consider that every prediction is
]
Figure 5: Solar energy production percentage in the world (year 2007) (on the
left) datas of 2006 (on the right) datas of 2007. All two graphs refers to world
market.
very di cult to do, due to the fact that solar market is in constant growth as
well as new solar technologies. In fact observing datas predicted in 2007 for the
whole world ( gure 5) and datas of 2009 only for EU countries ( gure 6), it is
clear that predictions strongly underestimate the solar market explosion. For
instance, it is interesting to know an italian law, the so called "conto energia",
which should be help all investments in renewable energy, particularly the solar
sector, that started to consider the thin lm technology and the third generation
only since the third "conto energia" of the 6/08/2010[3]. Symbolic, to under-
stand how much the politics is not able to evaluate this economic phenomenon,
is the white paper, where EU underestimated the solar market explosion6, which
it was made in 1997 when the European Commision set a target for the 2010 of
only 3000 MW[5]. If the EU, ten years ago, didn’t understand the phenomenon,
the italian politicians do not understand it yet very well. Quite funny, in fact,
is the italian environmental policy. With the third "conto energia", the italian
guvernment xed public investments for solar energy only for 8000 MW up to
the 2020. In the rst two months of the 2011 the limit was overbounded[4]!
Labour market. Apart from incentives, the development of photovoltaics
requires the transfer of knowledge from academic institutes and research centres.
In a world crisis period, especially for EU countries, the photovoltaic sector can
give some real answer to the huge problem of the unemployment all around
the Europe. In fact, a recent EU project, PVemployment, has tried to predict
the consequence of a green economy based on the explosion of the photovoltaic
sector in next years, up to the year 2030. The conclusion is incredibly satisfying:
in best simulations the new market can be overcome milions of new job places,
as direct and indirect jobs, with an average growth of more than 150000 new
worker each year.[7] Particularly interesting is the changing in the job market
9
]
Figure 6: Solar energy production percentage in the EU countries in the year
2009 [6]
mentality: from a vertical enterprise idea to a decentralised new way; in other
words the photovoltaic sector with its decentralised structure leads to jobs in
the less industrialised areas and decrease the disequality scissor of money and
power holder and moreover it can be brought di erent countries on the way of
the energy self-su ciency.
Solar energy and DSSC production costs. A very early study [8] esti-
mated the costs for a 20 x 30cm
2
sized 3 Wp DSSC to be 4-5 dollars according
to the following table ( gure 7). These predicted costs were equivalent to the
unthinkable price, in comparison with today’s price, of 28000 dollars for a 3KWp
DSSC module; nevertheless the huge total price was less than the rst monocrys-
talline silicon cells, which they were around 45000 US dollars for 3KWp.
This right prediction opened the doors to this young research eld up to now,
when today’s price are much more economical advantageous. For istance, the
levelized electricity cost (LEC) of OSCs, assuming 5% e ciency and a durability
of 5 year, should be around 0.85 dollars/KWh. Instead assuming 15% e ciency
and a durability of 20 years (next step for DSSC reasearch) the LEC decrease
untill the order of 0.10-0.05 dollars/Kwh, a very competitive price[9].
Interesting is to observe as the great di erence between the cost of a DSSC and
a OSC is due principally to the ITO glass price. In fact all the other costs are
almost equivalent or lower for DSSC ( gure 7). Finally, here below, it’s showed
the general comparison in cost among di erent solar technologies and among
OSC and all di erent common types of energy ( gure 8). The most incredible
innovation for organic PV is the industrial production process, based on screen
printing: it was demonstrated that it’s possible to produce 1000 100000 m
2
per
day(an equivalent production of Silica solar cell takes 1 year)[10]. In the end
the cost of the solar energy starts to be comparable and competitive, against
other energy production methods, with all organic types, while for the more
mature si-PV technologies it remains too much expensive for the world market.
As said just above, organic cells LEC varies between 0.07 0.13 dollars/KWh
(for = 15% and durability 20 years) and 0.49 0.85 dollars/Kwh (for = 5%
and durability 5 years) . Obviously higher prices are completely out of suitable
business investments.
10
Figure 7: (top) an early business study on DSSC (1996). (bottom) comparison
cost (2009) between DSSC and organic solar cell (OSC)
Figure 8: (top) Cost comparison between di erent solar technologies; (bot-
tom) Comparison between OSC energy production and common resources.
11
Environmental costs: a LCA study. There are many LCA studies on
DSSC but unfortunately, there no exist a precise prediction, as for the money
costs. This aspect is a direct consequence of the fact that a well-de ned indus-
trial production process doesn’t exist yet and of the fact that DSSCs are still
prototypes and su er of many "hand-build" processes. However a general, low
accurate studies can be take into account to observe which parts of the process
are more expensive in terms of energy and pollution.
A recent study of ECN Solar Energy in Netherlands [11] estimates very inter-
esting values of Energy Payback time (EPBT) and of GWP for the DSSC pro-
duction. In fact, in South Europe the EPBT was estimated around only 1 year,
while the life-cycle greenhouse gas emissions can be as low as 20gCO
2
eq=KWh,
depending on the lifetime of the DSC systems. For this study was used a per-
formance factor of 0.75, which takes into account all losses due to inverter,
not-optimal orientation, temperature uctuations and other factors. As said
Figure 9: LCA of di erent parts of a DSSC [12]
just above, many parts of the production process, unfortunately, are not taken
into account in this result because of di erent technique of the di erent research
centers. All results reported here refers on an hypothetical 1m
2
area module.
Figure 9 shows all percentage impact of di erent parts of the device. It can
be seen that large part of the environmental impact of DSSC derives from the
TCO glass. This situation can be improved by using thin glass or other types
of substrates, such as metal or polymer foil. The two nal histograms ( gure
10) the evaluation of the gCO
2
eq emissions. The GWP strongly depends on
the life time. It was estimated in a range of 20-120 gCO
2
eq=KWh. Comparing
this result with other renewable energy ( gure 10) it’s possible to see that if 120
is still too big for a promising technology, the result of 20 gCO
2
eq=KWh is a
completely satisfactory value to compete and substitute (in future) the classical
Si-PV modules. For the energy payback time the result is quite incredible. In
fact in the South Europe hypothesis ( AM 1.5, irradiation of 1700 kWh/m2/yr),
it is only of 0.8 years.
12