Perovskite photovoltaic cells are much cheaper to produce, and have reached conversion efficiencies of 23 percent, matching that of silicon cells. However, the durability and lifetime of the cells has been a problem. The team at NREL has developed an unencapsulated cell that maintained 94 percent of its starting efficiency after 1,000 hours of continuous use under ambient conditions, according to research published in Nature Energy. This new cell has a starting conversion efficiency of just over 16%.
“During testing, we intentionally stress the cells somewhat harder than real-world applications in an effort to speed up the aging,” said NREL researcher Joseph Luther. “A solar cell in the field only operates when the sun is out, typically. In this case, even after 1,000 straight hours of testing the cell was able to generate power the whole time.”
While more testing is needed to prove the cells could survive for 20 years or more in the field for commercial use, and the actual efficiency figures for the new structure are slightly lower than existing designs, this is a key step forward in making commercially viable solar cells.
A typical organic perovskite solar cell sandwiches the perovskite material between a hole transport material, a thin film of an organic molecule called spiro-OMeTAD doped with lithium ions and an electron transport layer made of titanium dioxide (TiO2).
“What we are trying to do is eliminate the weakest links in the solar cell,” said Luther. The team replaced the spiro-OMeTAD with a different organic material called EH44, developed by Alan Sellinger at the Colorado School of Mines, that doesn’t contain lithium and repels the water that causes the charge to leak.
“Those two benefits led us to believe this material would be a better replacement,” said Luther.
Using EH44 as the top layer resolved the later more gradual degradation but did not solve the initial fast decreases that were seen in the cell’s efficiency. Swapping the