- Solar power's biggest hurdle is cost. A new prototype
self- assembling organic solar cell could clear that hurdle.
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- Combining two different kinds of carbon-based molecules,
the device converts up to 34% of incident blue-green light to electricity,
report developers Lukas Schmidt-Mende and colleagues at Cambridge University
and the Max Planck Institute for Polymer Research in Mainz, Germany1.
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- Better still, its molecular components organize themselves,
without help, into the layered structure necessary for efficient energy
conversion. Two-component organic solar cells are usually difficult and
expensive to engineer.
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- The photovoltaic device contains one type of molecule
based on the dye perylene, and another, denoted HBC-PhC12, that forms liquid
crystals. Under the right conditions, a mixture of these two separates
into a microscopically thin film containing predominantly perylene crystals
above a film of mostly HBC- PhC12.
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- Each substance conducts one of the two charged particles
responsible for the light-induced generation and flow of a current. When
the organic molecules absorb light, they spit out a negatively charged
electron, leaving behind a 'hole' which acts like a positively charged
particle.
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- Current is created when these particles pass through
the material to collector electrodes, electrons flowing in one direction,
holes in the other.
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- Perylene is an electron conductor; HBC-PhC12 conducts
holes. This latter's disk-shaped molecules stack like dinner plates, creating
hole-transporting channels that act like molecular wires. This stacked
structure - a discotic liquid crystal - transports holes efficiently.
-
- The segregation of the two organic materials into layers
facilitates the flow of electrons and holes in opposite directions to the
electrodes. If the materials stayed well mixed, the charged particles would
follow more convoluted paths and be collected less efficiently.
-
- Although the conversion efficiencies of these devices
are good for blue-green light, a useful solar cell has to absorb light
over the entire visible spectrum. This should be possible to engineer by
changing the chemical structure of the molecules. There are also concerns
about whether the organic molecules can be made stable enough for long-term
use.
-
- There are plenty of other obstacles to overcome before
these systems can compete with commercial solar cells, which are currently
made from slices of silicon. But the potential cheapness and ease of fabrication
of organic devices provides plenty of motivation.
-
-
- References Schmidt-Mende, L. et al. Self-organized discotic
liquid crystals for high-efficiency organic photovoltaics. Science, 293,
1119 - 1122, (2001).
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- © Nature News Service / Macmillan Magazines Ltd
2001 http://www.nature.com/nsu/010816/010816-3.html
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