Researchers have found that constant illumination relaxes the lattice of a perovskite-like material, making it more efficient at collecting sunlight and converting it to energy. Image: Light to Energy Team/Los Alamos National Laboratory.
Researchers have found that constant illumination relaxes the lattice of a perovskite-like material, making it more efficient at collecting sunlight and converting it to energy. Image: Light to Energy Team/Los Alamos National Laboratory.

Some materials are like people: let them relax in the sun for a little while and they perform a lot better. A collaboration led by researchers at Rice University and Los Alamos National Laboratory have found this is true for a perovskite compound touted as an efficient material for collecting sunlight and converting it into energy.

The researchers included Aditya Mohite, a staff scientist at Los Alamos who will soon become a professor at Rice, Wanyi Nie, also a staff scientist at Los Alamos, and Hsinhan (Dave) Tsai, a graduate student at Rice. They discovered that constant illumination relaxes strain in perovskite's crystal lattice, allowing it to uniformly expand in all directions.

This expansion aligns the material's crystal planes and cures defects in the bulk. That, in turn, reduces energetic barriers at the contacts, making it easier for electrons to move through the system and deliver energy to devices. Not only does this improve the power conversion efficiency of a solar cell, but it does so without compromising its photostability. The researchers found negligible degradation over more than 1500 hours of operation under continuous one-sun illumination of 100 milliwatts per cubic centimeter.

This research, which is reported in a paper in Science, represents a significant step toward stable perovskite-based solar cells for next generation solar-to-electricity and solar-to-fuel technologies, according to the researchers.

"Hybrid perovskite crystal structures have a general formula of AMX3, where A is a cation, M is a divalent metal and X is a halide," Mohite explained. "It's a polar semiconductor with a direct band gap similar to that of gallium arsenide.

"This endows perovskites with an absorption coefficient that is nearly an order of magnitude larger than gallium arsenide (a common semiconductor in solar cells) across the entire solar spectrum. This implies that a 300nm-thick film of perovskites is sufficient to absorb all the incident sunlight. By contrast, silicon is an indirect band gap material that requires 1000 times more material to absorb the same amount of sunlight."

According to Mohite, researchers have long sought efficient hybrid perovskites that are stable in sunlight and under ambient environmental conditions. "Through this work, we demonstrated significant progress in achieving both of these objectives," he said. "Our triple-cation-based perovskite in a cubic lattice shows excellent temperature stability at more than 100°C (212°F)."

The researchers modeled and made more than 30 semiconducting, iodide-based thin films with perovskite-like structures: crystalline cubes with atoms arranged in regular rows and columns. They measured their ability to transmit current and found that when soaked with light, the energetic barrier between the perovskite and the electrodes largely vanished as the bonds between the atoms relaxed.

They were surprised to see that the barrier remained quenched for 30 minutes after the light was turned off. Because the films were kept at a constant temperature during the experiments, the researchers were also able to eliminate heat as a possible cause of the lattice expansion.

Measurements showed that the ‘champion’ hybrid perovskite device increased its power conversion efficiency from 18.5% to 20.5%. On average, all the cells had a raised efficiency above 19%. Mohite said that the perovskites used in the study were 7% away from the maximum possible efficiency for a single-junction solar cell.

He said the cells' efficiency was nearly double that of all other solution-processed photovoltaic technologies and 5% lower than that of commercial silicon-based photovoltaics. The cells also retained 85% of their peak efficiency after 800 hours of continuous operation at the maximum power point, and their current density showed no photo-induced degradation over the entire 1500 hours.

"This work will accelerate the scientific understanding required to achieve perovskite solar cells that are stable," Mohite said. "It also opens new directions for discovering phases and emergent behaviors that arise from the dynamical structural nature, or softness, of the perovskite lattice."

The lead researchers indicated that the study goes beyond photovoltaics, as it connects, for the first time, light-triggered structural dynamics with fundamental electronic transport processes. They anticipate it will lead to technologies that exploit light, force or other external triggers to tailor the properties of perovskite-based materials.

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