NREL scientists build all-perovskite tandem solar cell with 23.1% efficiency

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a tandem perovskite solar cell which it claims is the most efficient flexible thin-film solar cell of any non-III-V technology made to date.

The device is based on a new wide-bandgap perovskite recombination layer, dubbed Apex Flex, that is said to be able to withstand heat, light, and operational tests, and at the same time to provide a reliable and high voltage. Grown with atomic layer deposition (ALD), the new material is described as a “nucleation layer consisting of an ultra-thin polymer with nucleophilic hydroxyl and amine functional groups for nucleating a conformal, low-conductivity aluminum zinc oxide layer.”

The researchers explained that the two-terminal architecture of tandem cells is particularly challenging for making high-performance devices, due to the need to ‘tune’ different cell materials to each other to prevent them from limiting each other’s performance in certain light conditions. If this issue is addressed, they stated, tandem cells can be lighter and cheaper to manufacture due to the removal of two transparent contacts and potentially one substrate.

They addressed the two main challenges represented by shunting due to a thick recombination layer of indium tin oxide and large voltage losses in the wide-gap sub-cell due to iodide-bromide halide segregation. The first of the two problems was solved by building a dense and thin recombination layer, and the second by reducing the amount of bromine, which tuned the material’s bandgap to a level where losses were greatly reduced.

This composition tuning enables a 1.7 electron-volts bandgap perovskite with high, stable voltages, the NREL team said. “By combining these advances, we fabricate two-terminal all-perovskite tandem solar cells with 23.1% power conversion efficiency on rigid substrates and 21.3% on flexible plastic substrates,” it further explained.

“Pairing more than one metal halide perovskite absorber in a single solar cell enables a truly differentiated solar technology that is high-efficiency, low-cost, and lightweight,” said research co-author Axel Palmstrom. “This collaborative effort has brought all-perovskite tandems closer to commercial reality, and we are excited to see their real-world applications in the near future.”

The new perovskite material is presented in the study Enabling Flexible All-Perovskite Tandem Solar Cells, published in Joule.