20 R&DMagazine June 2014 www.rdmag.com
Fullerene-Free Organic Solar
Investigated heavily since the 1970s, solar cells have been the great unfulfilled promise for unlimited, almost free energy to power the world. The reasoning is solid: The Earth
absorbs almost as much energy per hour than
the entire human race uses in a single year.
But feasibility of solar energy has lagged well
behind its practicality. Decades of intensive R&D
has produced a proliferation of rooftop-mounted solar panels and major solar array installations. Yet solar energy still accounts for much less
than 1% of the generated electricity in the U.S.
today, and the overall influence on energy use
and generation has been relatively modest.
Factors and alternatives
Efficiency, cost and reliability concerns affect nearly every type of solar energy conversion technology. The challenge of converting photonic energy
to electricity has stimulated the development of a
wide variety of technical schemes, the most popular and successful of which is the crystalline silicon
solar cell. The first research-oriented solar cells,
based on selenium, recorded just 1% over solar
baseline efficiency. Twenty years later, researchers
in the U.S. achieved a modest 4. 5 to 6% efficiency
upon the adoption of silicon. Now, the best of
these crystalline cells can achieve 25%, and have
dominated the market with a combination of
stability, competitive cost and widespread production.
Still, silicon fabrication is relatively expensive
and these cells have seen volatile swings in price
over the years. Cost is one drawback. So is reli-
ability. The best commercial monocrystalline
silicon solar cells may last up to 50 years. But
efficiency can degrade a cell up to 0.5% per year.
The cells are bulky and fragile. Replacement is
inevitable, and comes at a cost.
The grid-scale solar industry has moved to
specialized mirror-based solar concentrator
installations. But in the laboratory, researchers
have increasingly looked toward cheaper, lighter
solar collectors that are easier to make. Dye-sen-sitized solar cells feature a photo-sensitized
anode and an electrolyte that can be sandwiched
together with a roll-to-roll printing process. This
yields an attractive price-to-performance ratio;
and the best ones achieve 15% efficiency. But
unlike silicon cells, stability is poor.
Another option is the organic solar cell,
sometimes known as polymer cell. These
devices are built from small organic structures,
including molecules and different types of
conductive organic plastics. The driver for the
technology is its very low cost, ease of manufacture and naturally high optical absorption coefficient of organic materials, which has helped
produce conversion efficiencies of 4 or 5% or
more using a small amount of material. Plus,
organic solar cells are inherently flexible, which
has prompted concepts for widespread applications on buildings. However, relatively low efficiency, low strength and poor stability have kept
this type of cell confined to the laboratory.
The main disadvantages associated with organic photovoltaic cells are low efficiency,
low stability and low strength compared to inorganic photovoltaic cells.
A recent breakthrough by researchers
at the semiconductor developer imec
(Leuven, Belgium), however, may
help change its fortunes.
New materials, new theory
The simplest form of an organic solar
cell is the single-layer organic electronic materials sandwiched between
two metallic conductors. One is an
active indium-tin-oxide layer and a
backing of aluminum or magnesium.
These early cells were poor
performers. When these organic photovoltaic
sandwiches, or stacks (OPVs), absorb a pho-
ton, it creates an excited state attached to a
molecule on a polymer chain. It’s basically an
electrostatic interaction that is managed by a
field created by the dissimilar metal layers. The
absorber material, in essence, releases the elec-
tron to the acceptor material.
The single-layer organic cell was a poor per-
former, so multi-layer OPVs are made from two
different materials in between conductive layers.
The materials are chosen to have much different
ionization energies, which further enhances the
charge collection efficiency. They also typically
use fullerenes as the acceptor material due to its
high electron mobility and inherent talent for
accepting stable electrons. However, the small
absorption overlap with the solar spectrum
limits the photocurrent generation in fullerene
acceptors, and their deep energy level for electron
conduction limits the open-circuit voltage.
Engineers at imec have developed a third
option, one that removes fullerenes from the
equation. The new structure came about in an
unexpected way for researchers.
“We had been working for some years now
with the Univ. of Madrid to develop new materials for use in OPVs,” says Tom Aernout, group
leader of Organic Photovoltaics at imec. The
research group had been investigating a variety
of organic molecules in the cyanine family and
was able to chemically modify the structure of
a specific acceptor already used in OPV prototypes, called subphthalocyanine (SubPC). The
process involved reducing the number of “out
rings” from four to three, promoting strong
absorption at low energy levels. The increase
in absorption was enough so that, when paired
with another organic acceptor material called
subnaphthalocyanine (SubNC), they achieved
enough electron mobility to substitute for
fullerenes. Even better, the bandgap of the new
acceptor was far more conducive to collection
of visible wavelengths and worked well with the
established donor material sexithiophene.
The three layers are arranged as discrete
heterojunctions. In addition to an established
exciton dissociation at the central donor-accep-tor interface, the excitons generated in the outer
acceptor layer of this new OPV are first relayed
by energy transfer to the central acceptor, and
subsequently dissociated at the donor interface.
Molecular-scale engineering set the stage for
a new organic solar photovoltaic conversion
record set earlier this year. Image: imec