All-perovskite tandem solar cell with 27.4% efficiency, high voltage

From pv magazine global

Scientists from Northwestern University, the University of Toronto, and the University of Toledo have fabricated an all-perovskite tandem solar cell with a high open-circuit voltage (VOC), due to limited recombination at the electron transport layer (ETL).

They initially coated the surface of the perovskite layer with a substance known as 1,3-propanediammonium (PDA). This increases the photoluminescence quantum yield (PLQY), defining the number of photons emitted as a fraction of the number of photons absorbed. They then spin-coated the perovskite film onto an indium tin oxide (ITO) substrate and the ETL and deposited an electron transport layer made of buckminsterfullerene (C60).

“PDA has a positive charge, and it is able to even out the surface potential,” said researcher Hao Chen. “When we added the coating, we got much better energetic alignment of the perovskite layer with the electron transport layer, and that led to a big improvement in our overall efficiency.”

Common monoammonium surface treatments do not energetically align the perovskite film and the ETL, but the diammonium molecules have the ability to modify the perovskite surface states and achieve a more uniform spatial distribution of surface potential.

“Using 1,3-propane diammonium (PDA), QFLS increases by 90 meV,” the scientists explained.

The cell achieved a power conversion efficiency of 26.3% and an open-circuit voltage of 2.13 eV, as certified by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL). The cell was also able to retain 86% of its initial efficiency after 500 hours of continuous operation.

The scientists said that a prototype of the cell even achieved an internally certified efficiency of 27.4%, a record-breaking open-circuit voltage of 2.19 eV, a short-circuit current of 15.1 mA/cm2, and a fill factor of 83.1%.

“The 2.19 V VOC represents a significant improvement over the previous highest reported VOC among the best all-perovskite tandems (2.05 V), due to the high VOC of the PDA-treated wide bangap subcell,” they said.

They described the cell technology in “Regulating surface potential maximizes voltage in all-perovskite tandems,” which was recently published in Nature.

“In our cell, the top perovskite layer absorbs well in the ultraviolet part of the spectrum, as well as some visible light,” said researcher Chongwen Li. “The bottom layer has a narrow band gap, which is tuned more toward the infrared part of the spectrum. Between the two, we cover more of the spectrum than would be possible with silicon.”