Researchers from the Energy Department's National Renewable Energy Laboratory (NREL) have found that surface recombination limits the performance of polycrystalline perovskite solar cells. In such cells, the sunlight creates mobile electrons whose movement generates power, but upon encountering defects can slip into a non-productive process. Known as a recombination, this process reduces the efficiency of a solar cell.

The NREL team examined the surface recombination in lead iodide perovskites, and determined that recombination in other parts of a methylammonium perovskite film is less important than processes that are happening on the surface, both the top and bottom. The team explained that multiple sources of recombination exist, and that surfaces are often overlooked when paying attention to recombination in favor of grain boundaries and bulk defects.

The research sees surface recombination as an obstacle to overcome. The ability to engineer surfaces will benefit perovskite-based optoelectronic applications - fast surface recombination can be used to design photodetectors, while lasers and light-emitting diodes require a slower speed.

A second study used high-resolution fluorescence-lifetime imaging to also show that surface recombination is the determining factor instead of grain boundary recombination. The researchers compared two types of samples: single crystals and polycrystalline films. Surprisingly surface recombination is worse for single crystalline samples compared to the polycrystalline samples found in solar cell devices. Chemically, excess methylammonium iodide was present on the surface of the polycrystalline film but absent on the single-crystal sample.

The research suggested a light coating of a protective material on the surface of the polycrystalline thin films could further improve the performance of perovskite solar cells.