Solar cells using perovskite materials are inexpensive and easy to fabricate and the efficiency at which they convert photons to electricity has increased more rapidly than any other material to date, starting at 3% in 2009 and rising to 22% today. A team at the Molecular Foundry and the Joint Center for Artificial Photosynthesis, both at the Lawrence Berkeley National Laboratory (Berkeley, CA), used atomic force microscopy image of the surface of a perovskite solar cell to show a new path to much greater efficiency from manipulating the individual grain boundaries.
In the image (below) from the lab, the individual grains are outlined in black, low-performing facets are red, and high-performing facets are green. A big jump in efficiency could possibly be obtained if the material can be grown so that more high-performing facets develop.
Photoconductive atomic force microscopy allowed the researchers to map two properties on the active layer of the solar cell that relate to its photovoltaic efficiency. The maps revealed a bumpy surface composed of grains about 200 nanometers in length, with a huge difference in energy conversion efficiency between facets on individual grains. They found poorly performing facets adjacent to highly efficient facets, with some facets approaching the material’s theoretical energy conversion limit of 31%.
“If the material can be synthesized so that only very efficient facets develop, then we could see a big jump in the efficiency of perovskite solar cells, possibly approaching 31%,” says Sibel Leblebici, a postdoctoral researcher at the Molecular Foundry.
The team created pervoskite solar cells using methylammonium lead iodide, and also made a second set of half cells that didn’t have an electrode layer. They packed eight of these cells on a thin film measuring one square centimeter and these were analyzed at the Molecular Foundry, where researchers mapped the cells’ surface topography at a resolution of ten nanometers. They also mapped two properties that relate to the cells’ photovoltaic efficiency: photocurrent generation and open circuit voltage.
The resulting maps revealed an order of magnitude difference in photocurrent generation, and a 0.6-volt difference in open circuit voltage, between facets on the same grain. In addition, facets with high photocurrent generation had high open circuit voltage, and facets with low photocurrent generation had low open circuit voltage.
In practice, the facets behave like billions of tiny solar cells, all connected in parallel. As the scientists discovered, some cells operate extremely well and others very poorly. In this scenario, the current flows towards the bad cells, reducing the overall performance of the material. But if the material can be optimized so that only highly efficient facets interface with the electrode, the losses incurred by the poor facets would be eliminated.
“This means, at the macroscale, the material could possibly approach its theoretical energy conversion limit of 31%,” says Ian Sharp, a scientist at the Joint Center for Artificial Photosynthesis.
The discovery is not only about solar cells, as the theoretical model that describes the experimental results predicts these facets should also impact the emission of light when used as an LED.
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