Delft Outlook
Further scientific news by TU Delft
Colophon
DO-Archive
Nano particles play with electrons
Fundamental research into electron transport inside dye-sensitised solar cells
door ROB VAN DEN BERG
We’re stuck with a chicken-and-egg-problem: solar
cells are expensive, so they don’t get sold, which keeps
the production volume low, so the price remains high.
However, within a decade the price of electricity from a
solar panel will be comparable to that of conventional
mains power,’ says Dr. Albert Goossens, associate
professor at the laboratory for Inorganic Chemistry at
tu delft.
He is currently investigating new methods and, in
particular, new materials, that will render the
production of electricity from solar energy more
efficient. The special focus is on the Graetzel cell, a
solar cell based on titanium dioxide (TiO2).
Measurements of the speed of electrons led Goossens to
the surprising conclusion that titanium dioxide nano
particles behave like quantum dots.
Plant leaves are tiny factories in which sunlight is used to
convert carbon dioxide gas and water into carbohydrates and
oxygen. They are not very efficient however, and the leaves
have to be replaced every year. Even so, the process has
worked for hundreds of millions of years, and forms the
primary energy source for all life on earth.
Excited
The basis of any solar cell is formed by semiconductors.
These are capable of generating an electric current if
sufficient energy is transferred to an atom to release an
electron. In physical terms, the electron is promoted to
the conduction band. The problem is that this cannot be
achieved using light of any wavelength, since a light
particle (photon) with a minimum level of energy is
required. Blue light photons represent more energy than
red light ones. On the other hand, if a photon contains
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (1 van 8) [13-02-2009 10:33:48]
Delft Outlook
more energy than required to excite a molecule or
semiconductor, this results in a loss. Both effects
combined ensure that approximately 70% of the
available solar energy remains unused. Many bacteria
and all green plants depend on the sun for their energy
needs, and they have developed an ingenious solution to
the problem, photosynthesis. This process uses sunlight
to convert water and carbon dioxide into carbohydrates
and oxygen. To prevent unnecessary loss of energy, the
absorption of light and the processing of that light into
free electrons are handled by two different systems. The
light is «harvested» by a complex of chlorophyll
molecules, which have an absorption spectrum that
exactly matches the spectrum of sunlight. In this way,
not a single photon is wasted. Once the energy in a leaf
has been absorbed by chlorophyll molecules, it is
transferred to a protein complex in which the actual
reactions take place that result in the forming of
carbohydrates and oxygen.
The sensitisation principle. A pigment molecule absorbs
light, causing it to reach a high-energy (excited) condition. In
this condition, the molecule is in a favourable position for
transferring an electron to the titanium dioxide. As a result of
this, the titanium dioxide globule is negatively charged, and
the molecule itself becomes positively charged.
Pigment
Ever since the nineteen seventies, attempts have been
made to create a better solar cell based on this principle.
Goossens: ‘The underlying idea is simple. You take a
layer of pigment molecules that will absorb most of the
light, and bring it into contact with a semiconductor. If
the pigment becomes excited, it will release an electron
which will disappear immediately into the
semiconductor’s conduction band. This means that you
no longer have to depend on the light absorption of the
semiconductor itself.’
It was none other than Melvin Calvin – 1961 winner of
the Nobel Prize for Chemistry, which he got for
explaining the photosynthesis mechanism – who was
one of the first to start research in this field. He covered
crystals of titanium dioxide, a semiconductor, with a
layer of chlorophyll. It wasn’t long before he and others
were facing what appeared to be an insurmountable
problem. Because the electrons were reluctant to move
through the layer of pigment, the layer had to very thin
if the electrons were to reach the semiconductor at all.
But, the thinner the layer becomes, the less light it can
absorb, and the fewer electrons it will release. As a
result of this effect, the efficiency of the first solar cells
sensitised in this way was no more than 0.01%.
Paste
It was the Swiss scientist Michael Graetzel who in 1991
found a way out of this deadlock, together with an
American student of his, Brian O’Regan. Instead of
using a single large titanium dioxide semiconductor
crystal, they opted for a large number of small spheres.
O’Regan is currently a temporary employee at the
Dutch Energy Research Centre (Energieonderzoek
Centrum Nederland, ecn), and visits Delft once a week.
As a student, he enjoyed playing with nanoballs, and he
knows a lot about their properties. It was the reason for
Graetzel to invite him to come to Switzerland, where
they hatched the idea of applying the balls to a sheet of
glass in the form of a sort of paste.
Goossens: ‘It’s like a white paste. In fact it looks just
like emulsion paint, which also gets its whiteness from
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (2 van 8) [13-02-2009 10:33:48]
Delft Outlook
titanium dioxide, and which is produced in huge
quantities, some 3 million tons worldwide per year.’ By
subjecting the layer of titanium globules to heat, a
spongy material was obtained. This was then immersed
in a pigment solution consisting of ruthenium
complexes dissolved in alcohol. The result was that the
globules were coated with an extremely thin layer of
pigment. Since the globules are very small – no more
than twenty nanometres across – this method increased
the effective surface area available for absorbing the
light by a factor of one thousand.
Goossens: ‘That was exactly the factor of one thousand
people that Calvin needed. The Graetzel cell also
contains two transparent conductors acting as
electrodes, and an electrolyte solution. These are needed
to replenish the electrons that the pigment molecules
lose to the TiO2.’
Luck
It was a resounding success. The required materials
were relatively cheap, and the manufacturing costs were
also low. However, the most important thing was that
the sunlight was very efficiently converted into an
electric current. The energy efficiency – the resulting
electric power as a fraction of the incident solar power –
was about 10%.
Goossens: ‘With hindsight, we can see that Graetzel
really had luck on his side. The pigment molecule
responsible for the high efficiency is now known to
insiders as lab code N3, not to be confused with a form
of nitrogen. In his laboratory, he now has five or six
people working to find new, even better pigments. They
have tested hundreds of different pigments, but none of
them has managed to surpass N3 so far.’
Manufacturing the cells so they are reproducible and
reliable still isn’t easy either. The greatest problem
plaguing the Graetzel cell was its lack of stability.
Goossens: ‘The liquid phase, the electrolyte, tends to be
its undoing. A solar cell like this must be capable or
working in full sunlight for at least ten years. Now the
operating temperature and pressure inside the cell can
reach very high values. To begin with, organic solvents
are not particularly resistant to light, and in this case
they contain loads of electrons flying all over the place
and starting all kinds of decomposition reactions. So,
like various other groups all over the world we started to
look for a solid state variant of the Graetzel cell.’
One variant being investigated in Delft uses conducting
polymers. These absorb the light and at the same time
act as hole conductors. The concept sounds very
promising in theory, but proves difficult to bring off.
Goossens: ‘The pores in the titanium dioxide are of the
same order of magnitude as the globules, about twenty
nanometres. The polymer on the other hand is about ten
times as long. Now how do you fit that length of cooked
spaghetti into a tiny hole like that?’ One method is to
construct the TiO2 sponge around the polymer, as it
were. To do this, you need to start with a so-called
titanium precursor, which you turn into titanium dioxide
by baking it in an oven at 300 – 400°C. At ECN, regular
partners of ours, they mixed a solution of the polymer
with a solution of a titanium precursor. By dropping the
The absorption of light results in a high-energy condition
called excitation. The excitation energy (exciton) is handed
from one electron to the next (like a relay baton). As the
process can only last for a few nanometres, very thin films
have to be used. The drawback of very thin films is that they
can collect only a small fraction of the incident sunlight. To
absorb sufficient quantities of sunlight in thin film, scientists
have to resort to all sorts of tricks.
The solar cell Prof. Michael Graetzel and his staff at
Lausanne came up with uses a spongy titanium dioxide
structure. The internal surface area of the sponge is almost
one thousand times that of the outside surface area.
Ruthenium pigment molecules have been bonded onto the
artificially expanded surface. These molecules catch the light
and inject an electron into the titanium dioxide, as a result of
which they become positively charged. By immersing the
TiO2 into a redox electrolyte with I– / I2 species, the positive
charge of the pigment molecules is neutralized (I– D 1/2 I2
electron). This closes the circuit.
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (3 van 8) [13-02-2009 10:33:48]
Delft Outlook
mixture onto a disc spinning at high speed you can
create a thin layer of liquid that will quickly evaporate,
leaving behind a thin film of the dissolved materials.
This spin coating technique creates very thin and flat
films, but when you heat them to create TiO2, you tend
to find that the mix was far from perfect. So, we’re now
trying to use a very fine mist of precursor droplets that
are fed through the oven and then precipitated onto the
spin coater, which carries a drop of the polymer
solution.’
Quantum dot
In addition to this type of synthesis work, Goossens and
his doctorate students are attempting to gain insight into
the underlying processes of the Graetzel cell, since
nobody expected it to work this well in the first place.
Goossens: ‘It is very unusual for the electrons to travel
through the titanium dioxide as easily as they do. Our
group has been measuring the electron transport
phenomenon. The electrons have to travel only a couple
of micrometres, but for an electron that is a long
distance. And not a single one of the electrons gets lost;
each incident photon results in an electron. The
experiment involves illuminating a solar cell with laser
pulses and measuring the current it produces. This
provides us with information about the time the
electrons need to reach the electrode. The travelling
time is shortened dramatically if the intensity of the
light is increased. To our surprise, measurements using
varying light intensities, different layer thicknesses, and
particle sizes, have shown that each TiO2 globule never
carries more than one electron. This appears to be a
universal law. Our expectation was that higher light
intensities would cause more electrons to gather on a
single nano particle. We think we can explain the
phenomenon. The speed at which the electrons are
transferred from the filled nano particles to empty ones
is so high that gatherings of several electrons on a single
nano particle simply never occur. We are pretty sure of
this, but we want so see if any other effects are also
playing a role. Together with John Warman of the
Interfaculty Reactor Institute we are also looking at the
mobility of charge carriers inside the cell in other ways.
For instance, we managed for the first time to actually
measure that the speed of the electrons in the porous
TiO2 is many times lower than inside a crystal.’
Future
Research in the Netherlands in the organic solar cell
field has reached a high level. There are no fewer than
six groups working on it, and according to Goossens,
the number in the U.K. and in Germany certainly isn’t
more than that. On top of that, Shell’s announcement
that it intends to participate in the research will result in
considerable extra funding. Over the next years, the
research effort will increase even more, bringing in the
Dutch Polymer Institute in Eindhoven, one of the
leading technological institutes in the Netherlands.
Goossens: ‘However, you mustn’t forget that twentyfive years of research went into the amorphous silicon
solar cell before it was ready for marketing.’
So, if he wants his prediction to come true that 2010
As the liquid electrolyte limits the service life of the solar
cell, researchers at TU Delft are looking for a method to
create a sensitised solar cell using a solid electrolyte. In the
experiment, the pores of the titanium dioxide sponge are
filled with a p type semiconductor. Two projects are
currently focusing on using copper sulphide and p type
conducting polymers in the pores. The difference in
refraction index between titanium dioxide on the one hand,
and copper sulphide or polymers on the other, results in light
scattering, causing the light to travel a longer path. The result
is that more light can be absorbed.
Infiltrating a titanium dioxide sponge can be done in several
ways. TU Delft group opted for a technique with gases (e.g.
an organic titanium compound). Atomic-Layer Chemical
Vapour Deposition (AL-CVD) is an advanced technique that
can be used to fill pores as small as 50 nanometres in a
highly controlled manner. The AL-CVD reactor was
developed and manufactured at the laboratory for Inorganic
Chemistry.
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (4 van 8) [13-02-2009 10:33:48]
Delft Outlook
will see the break-even point for traditional and solar
power, Goossens and all his Dutch colleagues have their
work cut out for them.
For more information, please contact
Dr. Albert Goossens, phone +31 15 278 4919, e-mail a.
[email protected], or
Prof. Dr. Joop Schoonman, phone +31 15 278 2647, email mailto:[email protected]
Infiltrating a titanium dioxide sponge can be done in several
ways. TU Delft group opted for a technique with gases (e.g.
an organic titanium compound). Atomic-Layer Chemical
Vapour Deposition (AL-CVD) is an advanced technique that
can be used to fill pores as small as 50 nanometres in a
highly controlled manner. The AL-CVD reactor was
developed and manufactured at the laboratory for Inorganic
Chemistry.
This is an example of a solar cell sensitised with ruthenium
pigment according to Graetzel’s principle. These cells were
manufactured as part of a test series at ECN, using an
automated production process. Together with their European
partners, ECN are looking for ways to improve the
production with a view to manufacturing cheap solar cells
with a long service life. The efficiency of this cell is
approximately 7 percent. On a fine summer’s day, one
square metre of this type of cell would yield about 70 watts.
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (5 van 8) [13-02-2009 10:33:48]
Delft Outlook
The heart of an AL-CVD reactor is formed by a reaction
chamber made entirely of quartz, in which process gases are
switched on and off by means of inert control gases (argon)
inside a 3-dimensional channel structure. The operating
temperature varies from 200 – 400ÞC at a pressure of 5
millibar.
Example showing what the AL-CVD process can do. The
grooves in a silicon wafer have been very uniformly coated
with a thin layer of aluminium oxide. No other technique
exists that can produce such fine results.
Test set-up for measuring the fundamental processes inside
solar cells using two coupled laser units. Part of the research
focuses on the transport of electrons inside the solar cell.
Electron counts have shown that there will never be more
than one electron attached to a titanium dioxide nano particle
at a time.
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (6 van 8) [13-02-2009 10:33:48]
Delft Outlook
This test set-up enables small changes in absorption of less
than one hundredth of a percent to be measured across the
entire visible and near infrared spectral range (400 - 4000
nm). It is based on the transient absorption technique, in
which a light beam is used to continuously measure the
absorption of the solar cell at a certain wavelength. The
absorption represents the degree of change of internal
processes such as electron transport.
The silicon solar cell contains two different semiconductors.
A quantity of atoms, e.g. phosphorus, which have one
electron more than silicon, is added to the silicon on what is
referred to as the n side. The reverse applies to the p side,
where the silicon has been doped with atoms like boron,
which has one electron less than silicon. These are usually
referred to as holes, since they are missing electrons.
Initially, the electrons will all move to the other side, where
there is a surplus of holes. The holes do the opposite. As the
number of moves increases, a voltage is built up that will
counteract any further transfer of electrons and holes across
the p/n barrier. As each electron on the p side is released by
the absorption of light, it is pulled to the n side by the
internal field. The electrons continue on their way through a
wire, and end up on the p side. The result is a solar current
that can be put to good use.
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (7 van 8) [13-02-2009 10:33:48]
Delft Outlook
A multi-crystalline solar cell like the ones produced by
Shell Solar. A p/n junction is created in a thin wafer of
silicon (0.3 mm thick). Light caught by the wafer will
generate electrons and holes. The electrons and holes
are separated on the boundary between the p type and n
type silicons. Metal electrodes attached to the other side
of the wafer collect and carry away the electrons and
holes.
file:///R|/BTUD/Teams/permanente%20teams/Team%20TU...tOutlook(retro)/2000/Delft%20Outlook2000-4nano.htm (8 van 8) [13-02-2009 10:33:48]