Microscope reveals key to big efficiency boost for solar cells
Crystalline silicon solar cells make up the majority of solar panels out in the world today, but scientists believe that other types have the potential to be more efficient and carry more benefits. One of those types is perovskite solar cells, called that because they are made from compounds that have the crystal structure of the mineral perovskite.
These solar cells are inexpensive and easy to fabricate, making them a great alternative to traditional solar cells. Scientists have been working with this technology for about seven years and in just that amount of time the efficiency of those cells has increased from just three percent in 2009 to 22 percent today -- similar to silicon solar cells. That's the fastest efficiency increase of any solar cell material so far.
Scientists at the Berkeley Lab's Molecular Foundry and the Joint Center for Artificial Photosynthesis have made a discovery that could push that efficiency up even higher -- up to 31 percent.
Using photoconductive atomic force microscopy to study the structures of the cells at the nanoscale level, the researchers were able to map photocurrent generation and open circuit voltage in the active layer of the solar cell -- two properties that affect the conversion efficiency. The maps revealed a surface composed of bumpy, gemstone-like grains measuring about 200 nanometers each.
Each grain had multiple facets that it turns out had varying conversion efficiencies. Some facets of the grains were highly efficient, reaching the 31 percent mark, while others were much lower.
The researchers believe that if they can study the high efficiency facets and understand what makes them better at converting sunlight to electricity, they can produce a much higher efficiency solar cell overall.
“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,” said Sibel Leblebici, a postdoctoral researcher at the Molecular Foundry.
The researchers found that each of the facets behaved like tiny solar cells all connected in parallel -- some performing really well and others not so much. The current flows towards the poorly-performing facets, which lowers the performance of the entire solar material. They believe that if the material could be constructed so that only the high efficiency facets connect with the electrode, then the efficiency of the solar cell would jump to as high as 31 percent, leading to a higher-performing and less expensive solar material than we use today.