German and Dutch researchers have produced the first three-dimensional images
of nanoscale structures inside a polymer solar cell. Published in the journal
Nature Materials, the findings give new insights into the structure of solar
cells and may lead to their performance being enhanced.
Polymer solar cells are plastic solar cells with unique properties that have
the potential to outperform traditional silicon solar cells at lower costs.
Until now, polymer solar cells have not been very efficient at capturing light
and transforming it into energy.
Traditionally, solar cells have been based on a refined, highly purified silicon
crystal, similar to those used in the manufacture of integrated circuits and
computer chips. The high cost of these silicon solar cells and their complex
production process is a driving force behind the development of alternative
photovoltaic technologies.
Polymer solar cells, called 'third-generation solar cells', are lightweight,
low-cost, flexible, and can be modified on the molecular level. Until now, visualising
polymers at the nanoscale level has been problematic. Researchers at the Institute
of Stochastics in Ulm, Germany, have obtained highly detailed images of the
nanoscale structures in polymer solar cells using three-dimensional electron
tomography (3DET).
3DET is a popular tool for measuring structures of cellular components at the
nanometre resolution. Using a transmission electron microscope, the researchers
were able to visualise ways of enhancing the power-conversion efficiency of
the polymer solar cells by changing the compounds used in its manufacture.
Hybrid solar cells are made by mixing of two different materials, a polymer
and a metal oxide. When sunlight shines on the resulting solar cell, charges
are created where the materials meet. The precise relationship between the polymer
and the metal oxide determine the solar cell's efficiency. If the two mix closely,
then this creates a very dense cell structure, which means more obstruction
and lower efficiency. By making a cell with a more spaced structure, the researchers
are hoping to cut down on obstacles to the energy's flow to the collection point.
This would give the solar cell greater energy-generating efficiency.
The Dutch researchers have been able to create this larger structure by using
a precursor compound which mixes with the polymer. This is converted into the
metal oxide at a later stage.
The use of the three-dimensional visualisation showed the importance of the
mixing and solar cell morphology. This allowed the researchers to measure a
consistent and quantitative correlation between solar cell performance and the
underlying structure of solar cells with various cell structures.
The report concludes that the polymer solar cells produced during the experiment
were among the most efficient third-generation cells produced. However, they
still have a conversion efficiency of only 2% as compared to the 30% level frequently
found in first-generation cells.
Further study of polymers, metal oxide and their morphology in polymer solar
cells is needed in order to realise the further promise of the technology. Using
three-dimensional imaging of nanostructures may bring more rapid progress in
that area of research.
Source: Cordis
Posted September 15th, 2009
