Solar power is on the rise. In 2008, the amount
of energy generated by solar cells rose by 80%.
The vast majority of solar cells are based on
silicon. According to consultancy Solarbuzz, they
account for more than 90% of the market, with
thin film solar cells – based on cheaper substrates
such as plastic – taking up only 7%. But the
European Photovoltaic Industry Association
says that thin film solar cells could account for
around 20% of the market by the end of 2010,
thanks to their lower price, rising efficiencies and
manufacturing flexibility.
Organic solar cells are much cheaper than
conventional solar cells, but have two main
disadvantages: they convert sunlight to electricity
much less efficiently – typically around 3% –
and they are relatively flimsy and short-lived.
And while a few organic solar cell technologies
are already commercial – Solterra’s Quantum
Dot and Konarka’s Power Plastic are two
examples – many research teams are working on
alternative technologies, or trying to boost system
performance further.
Better protection
IMEC, the Belgian electronics research centre – in
collaboration with materials specialist Cytec – is
looking to develop a commercially viable method
to make organic devices last ‘beyond five years’.
The team will attack the problem in two ways:
by stabilising the structure of the photoactive
substances on the cell surface; and by developing
a barrier encapsulation technology to protect the
surface from damage by oxygen and water.
It has already made progress in the first of
these tasks. The blend of conjugated polymers and
fullerene acceptor molecules in the active layer tends
to undergo phase segregation over time, which is
accelerated at higher temperature. ‘Fullerenes tend
to crystallise and aggregate into lumps,’ says Tom
Aernouts, team leader for organic photovoltaics at
IMEC. ‘This will affect device performance.’
Adding bulky side chains to the polymer
backbone helps to make the conjugated polymer
more soluble. The higher molecular weight also
makes the molecule heavier and less mobile and
more temperature stable, Aernouts says. The side
chains can be crosslinked with the fullerene, to
further stabilise the mixture. Standard cells have
lifetimes of ‘a few hundred hours’ before their
performance decreases by 20%. Aernouts says
that some of IMEC’s new compounds have seen a
tenfold increase in lifetime.
IMEC’s efforts to develop a barrier layer are
an attempt to replace silicon oxides, which work
well but are brittle and do not ‘flex’, with the
PET substrate.
‘The big challenge is to find barrier layers that
are transparent,’ Aernouts says.
The two-year project combines Cytec’s
expertise in interfacial engineering and coating
technology with IMEC’s experience in organic solar
cell processing and analysis.
In separate research, IMEC has demonstrated a
potentially cheap way of mass-producing organic
solar cells. It has used spray-coating – which is
commonly used in industrial coating – to deposit
an active layer and a metal top contact layer on a
plastic substrate. The active layer is a solution of
two substances: a conductive polymer known as
P3HT and a fullerene acceptor molecule, PCBM.
When deposited by spray-coating, it showed
power conversion efficiencies above 3%, which
is comparable to that of spin-coated devices.
For the metal top contact, IMEC spray-coated a
solution of silver nanoparticles, and sintered it
at 150°C – compatible with processing on
flexible substrates.
The spray method has several advantages: it
can cover larger areas, so could be used to make
larger cells; and it is easy to scale up, so is more
suitable for mass production.
‘R&D on organic solar cells has now reached
the stage where we can consider low-cost, highvolume
manufacturing,’ says Aernouts. ‘This
is essential for the uptake of this technology
by industry.’
These two strands of IMEC’s research – its spray
coating technique, and its attempts to extend solar
cell lifetimes – are complementary, Aernouts adds.
‘The end point will be to combine them. This could
happen within two years.’
As well as having a much lower price, organic
solar cells could be produced much more flexibly.
Instead of relying on a silicon foundry, plastic solar
cells could be produced as easily as a newspaper
or magazine.
US company Solterra has already begun to build
a commercial plant in Jeddah City in Saudi Arabia,
which will supply its Quantum Dot solar cells to
the booming construction market there. It expects
to begin mass production early in 2010. ‘Several
solar module manufacturers are waiting to transfer
our printed cells into large area devices,’ says ceo
Steve Squires. ‘We expect to start shipping at the
end of Q1.’
At the same time, Konarka has developed a line
of solar panels, for applications such as battery
chargers, handbags and restaurant umbrellas, that
will be available to product developers by the end
of this year. Its Power Plastic 20 Series includes the
Power Plastic 120 (1 Watt), 320 (3W) and 620 (7W)
products. Panels vary in size and are available with
or without integrated connectors – so are ready
to use or can be integrated into a manufacturer’s
device or product. ‘Manufacturers from new and
existing markets will integrate these solar panels
into their products,’ says Rick Hess, president and
ceo. ‘Initial applications will address portable and
remote power needs.’
In 2010, Konarka plans to launch products for
higher and lower voltage applications.
Cells by the metre
Solterra and Konarka may have stolen a lead in
their commercialisation plans, but other research
teams are developing their own methods to print
organic solar cells.
CSIRO, the Australian research organisation,
calls it ‘solar cells by the metre’. It is working
with a number of partners, including BP Solar,
roofing supplier Bluescope Steel and Merck, to
develop a ‘reel to reel’ printing process for organic
solar cells.
Securency International – whose products
include polymer banknotes for countries such as
Australia and Thailand – will carry out the printing.
Peter Batchelor, Victoria’s minister for energy and
resources, said at the launch of the project: ‘The
production of these film-like solar cells will literally
be as easy as printing money.’
The ability to print solar cells onto thin film
substrates will make energy generation far more
portable: solar cells could easily be incorporated
into building components, such as roofs or panels,
enabling efficient local energy generation, says
CSIRO.
By 2020, it expects to be producing 100,000km/
year of polymeric solar cells, which could generate
as much energy as a conventional coal-fired or nuclear power station.
CSIRO says its understanding of structure– property relationships in electronic polymer materials gives it a ‘unique competitive advantage’.
In similar fashion, at the University of Texas at Austin, US, chemical engineer Brian Korgel believes he can cut the costs of solar cells by 90%. He says that special conductive inks, which incorporate nanoparticles, could be printed onto substrates such as plastic, stainless steel or glass. Other than ‘printing’ reels of cells, it might allow roof structures or even windows to be turned into solar arrays. ‘You’d have to paint the light absorbing material and a few other layers,’ he says. ‘This is one step in the direction towards “paintable” solar cells.’
The light-absorbing ‘nano-inks’ were originally based on silicon, but are now made using copper indium gallium selenide technology, or CIGS, through spin-off company Innovalight, because it is cheaper and ‘benign in terms of environmental impact’.
Korgel and his team have published a proofof- concept (J. Am. Chem. Soc., doi: 10.1021/ ja905922j). Their solar cells currently have an efficiency of 1% which is yet to be optimised. ‘If we get to 10%, there’s a real potential for commercialisation,’ Korgel says. ‘If it works, you could see it being used in three to five years.’
Process improvement
Production processes also need to be understood and refined if solar cells are to reach their magic target of 10% efficiency. In the UK, the Printable Electronics Technology Centre (PETEC) recently received £20m government backing to enhance its ‘open access’ facilities, which help SMEs to develop prototypes in applications including organic solar cells. Facilities include a Class 1000 cleanroom and a reel to reel vacuum coater.
At Washington University in the US, David Ginger is leading a team looking at the minutest detail of solar cells in an attempt to find the most promising materials and process conditions.
Organic solar cells are manufactured by putting two electrically conductive substances on a matrix surface, then baking them. Microscopic – or even nanoscopic – bubbles and channels will form within the conductive layer, reducing conversion efficiency. By understanding exactly how much current these bubbles and channels carry, Ginger hopes to refine the production process in order to maximise efficiency (Nano Letters, doi:10.1021/ nl901358v). ‘The exact structure of bubbles and channels is critical to performance,’ Ginger reports. ‘But the relationship between baking time, bubble size, channel connectivity and efficiency has been difficult to understand.’
He studied a blend of polythiophene and fullerene, which are basic to solar cell research. While he says this particular blend is unlikely ever to attain a 10% efficiency, it is useful because the responses to heat can be extrapolated to other materials.
Various ‘recipes’ – baked at different temperatures for different lengths of time – were analysed using atomic force microscopy: a tiny platinum or gold tip, with a point less than 20nm wide, is traced back and forth across the material’s surface to record the current within a bubble or channel. The technique could be used to analyse other polymer mixtures, and quickly determine which are most likely to have higher efficiencies. As the efficiency of organic solar cells improves, Ginger believes they will be incorporated into purses or backpacks, to charge electronic devices such as mobile phones. But in future, their role will be in true power generation. ‘The solution to the energy problem is going to be a mix, but in the long term solar power is going to be the biggest part of that,’ he says.
Lou Reade is a freelance science writer based in Kent, UK.