A detailed view of how electrical charges behave inside perovskites could guide efforts to improve the perfor­mance of next-genera­tion solar cells based on these materials, KAUST research has shown. When light hits a perovskite, it excites negatively charged electrons and leaves behind posi­tively-charged holes within the material’s crystal­line structure. These electrons and holes can then move through the perovskite to generate an electrical current. But the charge carriers could also recombine instead, which wastes the energy they carry.

“The efficiency of perovskite solar cells has been greatly improved in the past decade, but funda­mental research on their photo­physics is relatively backward,” says Ming-Cong Wang at the KAUST Solar Center, part of the team behind the work. “One of the things that is not clear is how charge carriers behave before recombining.”

Some of the ions that make up the perovskite’s crystal lattice can help to localize electrons and holes in different regions, which prevents them from recom­bining and prolongs their lives. However, this locali­zation also tends to make the charge carriers less mobile, which may adversely affect the solar cell’s performance. Under­standing such effects could help researchers to fine-tune the compo­sition of perovskites and boost their ability to generate elec­tricity from sunlight.

The team studied two different aspects of charge-carrier behavior in thin films of a promising perovskite – CsFAMA (a cesium-con­taining triple-cation mixed halide perovskite). First, they used a series of brief laser pulses to excite the charge carriers and then examine them just a few pico­seconds later. They found that as the density of charge carriers increases, it linearly narrows the energy gap that electrons need to vault when they are excited by incoming light. This is different from the behavior of conven­tional semi­conductors, says Wang.

Then the team used a form of high-intensity terahertz radiation to study how the charge carriers moved around. This showed that as the density of charge carriers increases, they are more likely to stick in a parti­cular location. “Charge carriers are more localized at higher densities,” says Frédéric Laquai, who led the team. The researchers think that both of these obser­vations have the same root cause. When light hits the perovskite, it can cause small rapid distortions in the lattice of ions that affect the charge carriers’ behavior. Other researchers have very recently observed such lattice fluc­tuations in perovskites as well, lending support to the findings. (Source: KAUST)

Reference: M. Wang et al.: Photo-induced enhancement of lattice fluctuations in metal-halide perovskites, Nat. Commun. 13, 1019 (2022); DOI: 10.1038/s41467-022-28532-0

Link: KAUST Solar Center, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia