Perovskite materials gain stability by branching out

Perovskite materials have shown great promise for use in next-generation solar cells, light-emitting devices (LEDs), sensors and various other applications, but their instability remains a critical limitation.

Researchers at the University of California, Santa Cruz (UCSC) have attacked this problem by focusing on perovskite nanocrystals, in which the instability problems are magnified by the particles’ large surface area in relation to their volume. Atoms on the surface of these nanocrystals are vulnerable to reactions that can degrade the material, so molecules that bind to the surface – called surface ligands or capping ligands – are often used both to stabilize perovskite nanocrystals and to control their properties.

In a paper published in Angewandte Chemie, the UCSC researchers now report using novel branched ligands to produce perovskite nanocrystals with greatly improved stability and uniform particle size.

"This new strategy to stabilize organometal-halide perovskites is an important step in the right direction," said corresponding author Jin Zhang, professor of chemistry and biochemistry at UCSC. "Our hope is that this could be used not only for perovskite nanocrystals but also for bulk materials and thin films used in applications such as photovoltaics."

Zhang's team tested the effects of different types of capping ligands on the stability of perovskite nanocrystals. They initially found that perovskite nanocrystals capped with ligands consisting of long straight-chain amines showed poor stability in solvents such as water and alcohol, but then went on to identify unique branched molecules that proved much more effective.

According to Zhang, the branching structure of the ligands protects the surface of the nanocrystals by occupying more space than straight-chain molecules, creating a mechanical barrier through an effect known as steric hindrance. "The branching molecules are more cone-shaped, which increases steric hindrance and makes it harder for the solvent to access the surface of the nanocrystals," he said.

In addition, by adjusting the amount of branched capping ligands used during nanocrystal synthesis, the researchers were able to control the size of the nanocrystals. This allowed them to obtain uniform perovskite nanocrystals with high photoluminescence quantum yield, a measure of fluorescence that is critical to the performance of perovskites in a variety of applications, in sizes ranging from 2.5nm to 100 nm.

Zhang's team is now investigating the use of these perovskite nanocrystals in sensors to detect specific chemicals. He is also working with UCSC physicist Sue Carter on the use of perovskite thin films in photovoltaic cells for solar energy applications.

This story is adapted from material from University of California, Santa Cruz, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.