Researchers from the US Department of Energy’s Lawrence Berkeley National Laboratory have made a discovery within the nanoscale peaks and valleys of perovskite that may allow solar cells utilising the material to reach their theoretical conversion limit of 31%.
Perovskite is a crystalline mineral that has been used to create solar cells that are inexpensive and easy to fabricate, much like organic solar cells (cells which use conductive organic polymers/small organic molecules).
The central point of interest for perovskite cells, however, is that 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. This puts them roughly alongside silicon solar cells (which work by placing a thin film of silicon on top of glass) in terms of efficiency.
However, a new discovery from a team of scientists from the Molecular Foundry and the Joint Center for Artificial Photosynthesis, both at Berkeley Lab, has found that the current composition of perovskite solar cells is not in fact taking advantage of the full possible efficiency.
Using photoconductive atomic force microscopy, which utilizes a conductive tip to scan the material’s surface, the scientists mapped 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 which are, in themselves, composed of multi-angled facets.
The importance of analysing this structure was revealed in that there is a huge difference in energy conversion efficiency between the facets. While some approach perovskite’s theoretical energy conversion limit of 31%, others immediately adjacent perform incredibly poorly.
“This was a big surprise. It shows, for the first time, that perovskite solar cells exhibit facet-dependent photovoltaic efficiency,” said Weber-Bargioni, corresponding author of the research paper.
Francesca Toma, another of the contributing scientists, added: “These results open the door to exploring new ways to control the development of the material’s facets to dramatically increase efficiency.”
The maps of the cell revealed several qualities of the facets and the differences between them, perhaps most notably that facets with high photocurrent generation had high open circuit voltage, and facets with low photocurrent generation had low open circuit voltage.
In action, the facets essentially behave like billions of microscopic solar cells, all connected in parallel. However, while some cells operate at a high efficiency, others work poorly. The difficulty comes from the current flowing towards the bad cells, thus reducing the overall performance.
The research team believes though that it is possible to optimise the material to eliminate the losses of the inefficient facets, by making it so that only the most efficient facets interface with the electrode.
“This means, at the macroscale, the material could possibly approach its theoretical energy conversion limit of 31%,” said Ian Sharp, another of the corresponding authors.