Scientists from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a possible secret to dramatically boosting the efficiency of perovskite solar cells hidden in the nanoscale peaks and valleys of the crystalline material.
Solar cells made from compounds that have the crystal structure of the mineral perovskite have captured scientists’ imaginations. They’re inexpensive and easy to fabricate, like organic solar cells. Even more intriguing, the efficiency at which perovskite solar cells convert photons to electricity has increased more rapidly than any other material to date, starting at three percent in 2009—when researchers first began exploring the material’s photovoltaic capabilities—to 22 percent today. This is in the ballpark of the efficiency of silicon solar cells.
Now, as reported online July 4 in the journal Nature Energy, a team of scientists from the Molecular Foundry and the Joint Center for Artificial Photosynthesis, both at Berkeley Lab, found a surprising characteristic of a perovskite solar cell that could be exploited for even higher efficiencies, possibly up to 31 percent.
She didn’t have to go far to find the material. For the past two years, scientists at the nearby Joint Center for Artificial Photosynthesis have been making thin films of perovskite-based compounds, and studying their ability to convert sunlight and CO2 into useful chemicals such as fuel. Switching gears, they created pervoskite solar cells composed of methylammonium lead iodide. They also analyzed the cells’ performance at the macroscale.
The scientists 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. These films 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.
This was performed using a state-of-the-art atomic force microscopy technique, developed in collaboration with Park Systems, which utilizes a conductive tip to scan the material’s surface. The method also eliminates friction between the tip and the sample. This is important because the material is so rough and soft that friction can damage the tip and sample, and cause artifacts in the photocurrent.
Surprise discovery could lead to better solar cells
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.
“This was a big surprise. It shows, for the first time, that perovskite solar cells exhibit facet-dependent photovoltaic efficiency,” says Weber-Bargioni.
Adds Toma, “These results open the door to exploring new ways to control the development of the material’s facets to dramatically increase efficiency.”
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, lowering 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 percent,” says Sharp.
A theoretical model that describes the experimental results predicts these facets should also impact the emission of light when used as an LED. Linn Leppert, Sebastian Reyes-Lillo, and Jeff Neaton performed this particular work.
The Molecular Foundry is a DOE Office of Science User Facility located at Berkeley Lab. The Joint Center for Artificial Photosynthesis is a DOE Energy Innovation Hub led by the California Institute of Technology in partnership with Berkeley Lab.
The research was supported in part by the Department of Energy’s Office of Science.