A new means of maximizing the efficiency of perovskite solar cells, found in the nanoscale valleys and peaks of the material, has been discovered by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
This atomic force microscopy image of the grainy surface of a perovskite solar cell reveals a new path to much greater efficiency. 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. (Credit: Berkeley Lab)
The scientists were captivated by solar cells that are made of crystal compounds such as perovskite. These cells, much like organic solar cells, are easily produced and inexpensive. Moreover, the perovskite solar cells’ efficiency in converting photons to electricity has rapidly increased over the years. When the material was first tested, for its photovoltaic potential, in the year 2009 the conversion rate was 3%. After seven years, the current conversion rate of perovskite cells is 22%. The rate is within the limit of silicon solar cell efficiency.
A new study conducted by scientists from the Joint Center for Artificial Photosynthesis and the Molecular Foundry, both at Berkeley Lab, has discovered an unexpected feature of perovskite solar cells. This characteristic has the potential of increasing the efficiency of the cells to as much as 31%. The details of the study have been published in the July 4 issue of Nature Energy.
Two important characteristics of the solar cell’s active layer have been sketched by the scientists through photoconductive atomic force microscopy. The sketches show an uneven surface that is full of gemstone-like grains approximately 200 nanometers in length, and each grain has multi-angled facets similar to the faces of a gemstone.
The scientists were surprised to find that the energy conversion potential of the grains was different in different facets of the grains. Highly efficient facets were discovered beside facets with low efficiency. Moreover, some of the high efficiency facets were able to reach the conversion level of 31%.
Scientists believe that further research is necessary on the high-performance facets as they hold the key to improved solar cells with higher efficiency.
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 percent.
Sibel Leblebici, a postdoctoral researcher at the Molecular Foundry.
Ian Sharp, a Berkeley Lab scientist and Alexander Weber-Bargioni are the corresponding authors of the paper. In addition to Leblebici, who works in Weber-Bargioni’s lab, other researchers including Linn Leppert, Francesca Toma, and Jeff Neaton, the director of the Molecular Foundry were also part of the study.
A Team Effort
The study started when Leblebici was in search of a new project.
I thought perovskites are the most exciting thing in solar right now, and I really wanted to see how they work at the nanoscale, which has not been widely studied.
The material required was available at the Joint Center for Artificial Photosynthesis. Scientists at the Center have been producing perovskite-based compounds in thin films for a couple of years. The scientists were also analyzing the conversion-potential of the compounds, to see if they are able to convert CO
2 and sunlight into useable chemical fuels. They however decided to develop perovskite-based solar cells using methylammonium lead iodide and studied the large-scales operation of the cells.
The researchers also developed another batch of half cells without an electrode layer. Eight cells from this batch were wrapped in a square centimeter of thin film. The surface topography of the cells were sketched, and studied at the Molecular Foundry, at a ten nanometer resolution. The researchers also sketched two characteristics, open circuit voltage and photocurrent generation, that are connected to the photovoltatic potential of the cells.
The procedure was carried out using an advanced atomic force microscopy method, which has been created in association with Park Systems. The technique makes use of a conductive tip in order to scan the surface of the material while removing the friction between the sample and the tip. This is vital as the material is so soft and rough that friction can destroy the sample and tip, and further create artifacts in the photocurrent.
Surprise Discovery Could Lead to Better Solar Cells
The map sketched shows a variation pattern in the magnitude in photocurrent generation between different facets of the same grain. The sketch also shows a difference of 0.6volts in the open circuit voltage. Moreover, facets that produce high levels of photocurrent have high levels of open circuit voltage and those with low levels of photocurrent have low levels of open circuit voltage.
This was a big surprise. It shows, for the first time, that perovskite solar cells exhibit facet-dependent photovoltaic efficiency.
“These results open the door to exploring new ways to control the development of the material’s facets to dramatically increase efficiency,” added Toma.
The facets generally act like billions of small solar cells that are attached in parallel. The scientists found that while some cells have high efficiency levels others have very low levels. Under this circumstance, the current moves to poorly performing cells, thereby reducing the overall efficiency of the material. However, optimizing the materials so that only the high-performance facets receive the electrodes can eliminate the losses that will otherwise be incurred.
This means, at the macroscale, the material could possibly approach its theoretical energy conversion limit of 31 percent.
When tested in the theoretical model the findings of the experiment showed that the high-performance facets would influence light emission when used in LED. This experiment was conducted by Linn Leppert, Sebastian Reyes-Lillo, and Jeff Neaton.
Situated in Berkeley Lab, the Molecular Foundry is a DOE Office of Science User Facility. The Joint Center for Artificial Photosynthesis is a DOE Energy Innovation Hub headed by the California Institute of Technology in partnership with Berkeley Lab.
The study was partly supported by the Department of Energy’s Office of Science.