Monday, September 14, 2009
Researchers from the
Eindhoven University of Technology and the University of Ulm have made the first high-resolution
3D images of the inside of a polymer solar cell. This gives them important new
insights in the nanoscale structure of polymer solar cells and its effect on
the performance. The findings were published online in Nature Materials on
Sunday 13 September.
The investigations shed new light on the operational principles
of polymer solar cells.
Cost-effective, flexible and lightweight
These solar cells do not have the high efficiencies of their
silicon counterparts yet. Polymer cells, however, can be printed in
roll-to-roll processes, at very high speeds, which makes the
technology potentially very cost-effective. Added to that, polymer
cells are flexible and lightweight, and therefore suitable to be
used on vehicles or clothing or to be incorporated in the design of
objects.
This is a 3-D electron tomography image of a polymer-metal oxide solar cell. The 3-D nanoscopic morphology shows the interpenetrating metal oxide network in (yellow) below an aluminum contact (gray) inside a polymer matrix (black). Credit: Eindhoven University of Technology
Hybrid polymer solar cells
In these hybrid solar cells, a mixture of two different
materials, a polymer and a metal oxide are used to create charges
at their interface when the mixture is illuminated by the sun. The
degree of mixing of the two materials is essential for its
efficiency. Intimate mixing enhances the area of the interface
where charges are formed but at the same time obstructs charge
transport because it leads to long and winding roads for the
charges to travel. Larger domains do exactly the opposite. The
vastly different chemical nature of polymers and metal oxides
generally makes it very difficult to control the nanoscale
structure. The Eindhoven researchers have been able to largely
circumvent this problem by using a precursor compound that mixes
with the polymer and is only converted into the metal oxide after
it is incorporated in the photoactive layer. This allows better
mixing and enables extracting up to 50% of the absorbed photons as
charges in an external circuit.
Nanoscale mixing
The importance of the degree of mixing was clearly demonstrated
by visualization of the structure of these blends in three
dimensions. Traditionally such visualization has been extremely
challenging, but by using 3D electron tomography, the team has been
able to resolve the mixing with unprecedented detail on a
nanoscale. From these images the researchers at the Institute of
Stochastics in Ulm have been able to extract typical distances
between the two components, relating to the efficiency of charge
generation, and analyze the percolation pathways, that is, how much
of each component is connected to the electrode. These quantitative
analyses of the structure matched perfectly with the observed
performance of the solar cells in sunlight.
Future
Even though these hybrid polymer solar cells are among the most
efficient reported to date for this class, their power conversion
efficiency of 2% in sunlight must be enhanced to make them really
useful. This will be realized by improving the control over the
morphology of the photoactive blend, for example by creating
polymers that can interact with the metal oxide and by developing
polymers or molecules that absorb a larger part of the solar
spectrum. At such point, the intrinsic advantages of hybrid polymer
solar cells in terms of low cost and thermal stability of the
nanoscale structure could be fully exploited.
Publication
The publication "The effect of three-dimensional morphology on
the efficiency of hybrid polymer solar cells", by Stefan Oosterhout
et al. can be found at DOI 10.1038/NMAT2533.
The research was conducted at the Eindhoven University of
Technology and the University of Ulm. It was funded by the Joint
Solar Programme of FOM, NWO, and the Shell Research Foundation, the
Deutsche Forschungsgemeinschaft, SenterNovem, and the Dutch Polymer
Institute.
SOURCE