Researchers at the University of Toronto have developed a three layer perovskite solar cell with a record efficiency of 24.3%, output of 3.21V and an extended lifetime.
“In addition to lower manufacturing cost, perovskites offer us the ability to stack multiple layers of light-absorbing materials on top of each other, and even on top of traditional silicon cells,” says Professor Ted Sargent who recently joined the Department of Chemistry and the Department of Electrical and Computer Engineering at Northwestern University.
Sargent’s group is among those developing new ways to unlock the potential of perovskite solar cells. Their previous work has included two-layered tandem cells, but their latest study, published in Nature, focuses on a three-layer design.
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“Multi-layered cells are typically designed so that the top layer with wide-bandgap perovskites absorbs the most energetic photons, meaning high-frequency light with short wavelengths, toward the violet end of the spectrum,” says postdoctoral fellow Dr. Zaiwei Wang, one of four co-lead authors on the new paper.
“The next layer will absorb medium wavelengths, and the bottom one will absorb longer wavelengths. But it’s in the top layer that we get the challenge of light-induced phase separation.”
The team used ABX3 perovskite, which is made from a mix of cesium, lead, tin, iodine, bromine and some small organic molecules. The top layer, in particular, is composed of mixed halide perovskites, which have a high proportion of bromine and iodine.
“What happens in light-induced phase separation of these mixed perovskites is that the bombardment of high-frequency photons causes the phases that are richer in bromine to get separated from those that are rich in iodine,” says Dr. Hao Chen, a postdoctoral fellow and co-lead author of the study.
“This leads to an increase in defects, and a decrease in overall performance.”
To overcome this problem, the team used detailed computer models to simulate the effect of altering the composition of the crystals. This work suggested two changes: removing the organic molecules for an all-inorganic perovskite structure and introducing the element rubidium.
“The introduction of rubidium suppresses the light-induced phase separation issue,” says Dr. Tong Zhu, another postdoctoral fellow and co-lead author.
“Our rubidium/cesium mixed inorganic perovskites show better light stability than reported perovskite materials, including cesium-based inorganic perovskites and widely used organic-inorganic hybrid perovskites with similar band gaps.”
The team measured its efficiency at 24.3% with an open-circuit voltage of 3.21 volts. They also sent it to be independently certified by the National Renewable Energy Laboratory, which measured a quasi-steady-state efficiency of 23.3%.
“In the past, triple-junction perovskite solar cells have demonstrated a maximum efficiency of around 20%, so this is a big improvement. To our knowledge, this is also the first reported certification efficiency of triple-junction perovskite solar cells,” says researcher Lewei Zeng, another co-lead author.
“Previous designs also tended to lose a lot of their performance in a matter of hours. By contrast, ours maintained 80% of its initial efficiency even after 420 hours of operation, so that’s a big step in terms of durability as well.”