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"ISIS Press Release 18/01/06
Organic Solar Power
Solar power could become the next big thing in homes, personal
accessories, the battlefield and other military applications

New affordable, durable and portable solar devices provide local energy
generation for maximum efficiency and minimum greenhouse gas emissions.
Dr. Mae-Wan Ho

A fully referenced and illustrated version of this article is posted on
ISIS members’ website. Details here

New materials for harvesting light
Organic solar cells emerged in the late 1970s, based on conjugated
polymers – polymers with alternating double and single carbon-carbon
bonds – when it was discovered that doping these materials - slightly
contaminating with appropriate chemical elements - increased
conductivity several orders of magnitude [1]. Since then electronic
conducting materials based on conjugated polymers have found many
applications including light emission diodes (LEDs) and solar cells.

Nowadays, organic semi-conductors include not only polymers (molecular
mass more than 10 000 atomic mass units), but also small molecules
(molecular mass less than a few thousand units), and dendrimers, with
molecular masses in between the polymers and small molecules. The
distinctions between the different kinds of molecular semiconductors are
important in determining the processes required in making films and
devices, but the way they work is identical.

New mechanisms
Organic solar cells work differently from conventional inorganic
semiconductor solar cells. Light absorbed by an inorganic semiconductor
produces free charge carriers – electrons and holes – that are
transported separately through the semiconductor material. In an organic
solar cell, however, light absorption produces excitons, electron-hole
pairs that are bound together and hence not free to move separately. To
generate free charge carriers, the excitons must be dissociated. This
can happen in the presence of high electric fields, at a defect site in
the material, or usually, at the interface between two materials that
have a sufficient mismatch in their energy levels.

Thus, an organic solar cell can be made with the following layered
structure: positive electrode/electron donor/electron acceptor/negative
electrode. An exciton created in either the electron donor or electron
acceptor layer can diffuse to the interface between the two, leading to
electron transfer from the donor material to the acceptor, or hole
transfer from the acceptor to the donor. The negatively charged electron
and the positively charged hole is then transported to the appropriate
electrode.

Endless new possibilities
Organic materials are diverse and versatile, offering endless
possibilities for improving a wide range of properties such charge
generation, separation, molecular mass, ‘wettability’ (between organic
molecules and inorganic material), bandgap (determining the ability to
harvest light efficiently in different parts of the solar spectrum,
especially the infrared), molecular energy levels, rigidity, and
molecule-to-molecule interactions. Different organic molecules can be
combined with one another, or with inorganic materials in many unique,
favourable formulations.

One major advantage of organic solar panels is the low cost involved in
manufacture. Organic molecules are cheap to make, they can have very
high light absorbing capacity so that films as thin as several hundred
nanometres would be sufficient for the purpose. Organic materials are
compatible with plastic and other flexible substrates; and devices can
therefore be fabricated with low-cost, high throughput printing
techniques that consume less energy and require less capital investment
than silicon-based devices and other thin-film technologies. One
estimate put the reduction in cost by a factor of 10 or 20 [3].
Consequently, organic solar cells do not need to have conversion
efficiencies as high as thin-film inorganic solar cells to become
competitive in the market.

Organic materials can be printed on in any pattern or colour, and
integrated into existing building structures, or even clothing or other
accessories. In a couple of years, we are told, it will be possible to
recharge one’s mobile phone from one’s jumper, or power up one’s laptop
by plugging into the beach tent.

Seriously these affordable new generations of solar devices will be a
boon for the energy needs of poor countries that do not have power grids
or other infrastructure support. Generating electricity for use on site
also avoids the huge losses incurred in generating electricity in power
stations and distributing through the grid, estimated to be as high as
69 percent [3]. This is why local ‘microgeneration’ of electricity is
also gaining favour in developed countries as a means of improving on
efficiency and minimising greenhouse gas emissions.

Plastic solar cells
Most organic solar cells are currently running at conversion
efficiencies less than 5 percent. These include flexible thin-film
modules made of light-harvesting organic plastic polymers.

Kornaka Technologies, a company based in Lowell, Massachusetts, USA,
announced the acquisition of Siemens’s organic photovoltaic research
activities in September 2004 in order to develop and commercialise new
plastic power cells [4] for “any electronic device or structure to carry
its own on-board source of renewable energy.” Siemens has its
headquarters in Berlin and Munich, and is one of the world’s largest
electrical engineering and electronics company. Konarka cofounder and
CEO Howard Burke said that the company was testing various product
applications for consumer electronics and military devices at its pilot
manufacturing facility in Lowell. However, he would not say whether the
tests have reached the company’s stated goal of 10 percent efficiency [5].

A year later, Konarka announced a joint research programme with Evident
Technologies, a company based in New York USA, to develop “ultra high
performance plastic solar cells” [6] that combine its novel polymers
with Evident’s quantum dot nanotechnology. The quantum dot power plastic
could be used for “demanding energy, communications and military
applications, such as battlefied or off-grid power generation.”

The Pentagon is hoping to use Konarka’s solar cells to create a tent
that can generate electricity from the sun, and tools that soldiers can
carry in the field to recharge the batteries in their cell phones, night
vision scopes, and global positioning systems.

A major problem with the plastic solar cells and organic solar cells in
general is stability and longevity. Apart from chemical decomposition of
the organic molecules, organic solar devices can degrade from
distortion, loss of adhesion of the layers, or the layers diffusing into
each other. So, careful design of the device and engineering more stable
molecules are needed to substantially improve the lifetimes of the
device. Rapid progress has been made on these fronts, especially with an
organic-inorganic hybrid solar cell.

Dye sensitised solar cells
Dye sensitised solar cells (DSSCs) are among the third generation
devices nearest to the market, or already in the market. These are not
purely organic solar cells, but are made of a hybrid of organic and
inorganic semi-conducting materials.

The basic scheme of a DSSC is shown in Fig. 1 [7]. A layer of
light-sensitive dye is attached to the surfaces of the 15-20 nm
nanoparticles of TiO2 in a 5-20 microns thick film. One main effect of
the nano-structured film is to greatly amplify the light-sensitive
surface. The actual surface area in a 10 micron thick film is 1 000
times greater than that projected; because of the small size of the
particles, these films are highly transparent. To help enhance light
harvesting in the red and near infra-red range, quantum dots of 100 nm
to 400 nm are incorporated into the TiO2 film (see “Quantum dots and
ultra-efficient solar cells”, this series). The TiO2 film is deposited
by screen-printing from a colloidal suspension and sintered (heated to a
high temperature to fix it).

The dye is a member of a class of red ruthenium complexes code named N3,
or N719. The dye-sensitized photocell shows a broad photon-to-current
efficiency – the number of electrons generated per photon striking the
cell - of more than 70 percent between 350 to 660nm. The excited
electrons are injected very rapidly (10-15-10-12 s with 100 percent
quantum efficiency into the conduction band of the TiO2. (The quantum
efficiency is the fraction absorbed by the dye that is converted into
conducting electrons.) Dye regeneration occurs in 10-12 s, an order of
magnitude slower than electron injection, while charge recombination
takes place even slower, on a millisecond timescale, which means they do
not interfere with efficient charge separation [8].

The dye has been optimized for its light absorption characteristics as
well as stability. It is stable enough to sustain about 108 turnover
cycles, corresponding to about 20 years of exposure to natural light,
which is longer lasting than amorphous silicon. The most recent record
in power conversion efficiency set by a cell of this type is 11 percent,
in the laboratory of the inventor Dr. Michael Gratzel in Lausanne
Switzerland [7]. This is considerable progress from an efficiency of 1
to 2 percent reported in 1988.

Figure 1. A dye-sensitized solar cell. The gray dots represent
nanoparticles covered with a single layer of dye (small red dots).
Electrons are represented by circled minus signs, an incident photon
absorbed by hv. ECB and EVB are the energy levels of the conduction band
and valence band respectively, which defines the band gap. Redrawn from [7]

Improving stability and efficiency
Some recent milestones in improving stability and efficiency include
turning the liquid electrolyte into a gel, thereby preventing leakage of
the electrolyte from the cell. The cell sustained heating for 1 000h at
80C, maintaining 94 percent of its initial performance. The device also
remained stable under light soaking at 55C for 1 000h in a solar
simulator equipped with an ultraviolet filter. This cell had a
conversion efficiency of more than 6 percent [9].

A further improvement of energy conversion efficiency to 8 percent or
more [10] was achieved with a cell that retained over 98 percent of its
initial performance after 1 000 h of accelerated tests including thermal
stress at 80C in the dark or 1 000h of visible light soaking at 60C.
This cell used a robust electrolyte of low volatility in conjunction
with an improved ruthenium dye code-named K-19, grafted together with
the co-absorbent, 1-decylphosphonic acid onto the TiO2 film.

Another new member of the same dye, code named Z-910 absorbed light more
completely and over a wider spectral range [11]. The light conversion
efficiency exceeded 80 percent between 470-620 nm, reaching a maximum of
87 percent at 520 nm. In full sunlight, a DSSC tested had an overall
power conversion efficiency greater than 10.2 percent.

Meanwhile, a research team at the National Institute of Advanced
Industrial Science and Technology in Ibaraki, Japan, found that simply
pretreating the TiO2 with hydrochloric acid was sufficient to
significantly improve the energy conversion efficiency of their DSSC to
10.5 percent [12]. This appeared to be due to increased efficiency in
light-harvesting, electron injection and/or charge collection. The
researchers used another ruthenium complex, the black dye, with
absorption extending into the near IR region up to 920nm, which gives a
theoretical efficiency of 19.6 percent for the DSSC. There is certainly
plenty of room for improvement, and this will happen rapidly.

In another significant development, Solaris Nanosciences, a company in
Providence, Rhode Island, made the cell rechargeable [13], thus claiming
the lowest manufacturing cost for a long-life photovoltaic system in the
world: less than $3 000 compared with current silicon technology outlays
of $12 000 or more.

The life-time of the cell is extended by a chemical process that allows
the degraded dye in already installed cells to be removed and replaced
with a new dye, restoring the performance of the original solar cell.

The recharging process was independently confirmed at the Swiss Federal
Institute of Technology Lausanne headed by the inventor Michael Grätzel.
“Our evaluation has shown without doubt that the cell performance after
three coloration cycles remained intact, and could even be pushed beyond
the initial cell output.” He said.

The recharging process has the advantage that the cell could be refilled
with new generations of dyes, thus effectively upgrading the solar cell
during its life-time, obviating the need for total replacement of the
expensive equipment, thereby saving on both the financial and
environmental costs involved.

Another advantage of these cells is that they are good for high
latitudes. They do not have the reflectivity of inorganic materials such
as silicon, which allows them to have greater conversion efficiency when
the sun is at high angles relative to the cell.

The Solaris cell is currently running at a conversion efficiency of
seven percent, but Dr. Nabil Lawandy, CEO of Solaris Nanosciences, is
confident that further improvements are in the pipelines. “We expect the
first prototypes to be through the testing cycle in about 12 months and
then we will be considering a manufacturing strategy with a target of
1000 panels (20 m2 module) annually for the first manufacturing plant.”
He said.

Commercial developments
In fact the first commercial products in DSSC have already appeared [7].
Companies like Konarka in the US, Aisin Seiki in Japan, RWE in Germany
and Solaronix in Switzerland, are developing new products based on it.
Particularly interesting are applications in construction, such as
electricity generating glass tiles. The Australian company Sustainable
Technologies International has produced such tiles on a large scale for
field-testing, and the first building has been equipped with a wall of
glass tiles.

Toxicities
Things are moving fast in dye-sensitised and other organic solar cells;
fast enough for people to overlook the serious toxicities of some of the
main components. There is justification for the opinion that ruthenium
dyes, and indeed all ruthenium compounds are “highly toxic” and
“carcinogenic” [14]. A study published in 2000 indicated that the N3
ruthenium dye used in the DSSCs is not mutagenic [15], but its other
potential toxicities have not been investigated.

Although conventional TiO2 may be relatively harmless, many ultrafine
nanoparticles (less than 1 micron), such as those used in DSSCs, are
pathogenic [16], and chronic exposure to the nanoparticles may result in
fibrosis and airflow obstruction in the respiratory tract [17].

It is important for proponents and developers of these very promising
solar cells and applications to ensure that researchers and workers as
well as the public are protected from the hazardous materials, that
appropriate containment and recycling of wastes take place to prevent
environmental pollution, and that research on safety and safe use goes
hand in hand with development and commercial exploitation. In addition,
effort should be devoted to finding safer alternatives for toxic materials."

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Can someone read that for me?

You could have just linked to the page you copied that from...

Can I have the Readers Digest version?

Even better, can I have the English Reader's Digest version of that?

quote:

Originally posted by OffroadX:
You could have just linked to the page you copied that from...

It was via (SANET )Email- I searched.

I guess you folks don't read the newspaper , either.