Researchers from Rice University have created microscopic seeds for growing uniform 2D perovskite crystals. The latest discovery is both stable and highly efficient at harvesting solar power, said the scientists.

The name perovskite refers both to a specific mineral discovered in Russia in 1839 and to any compound with the crystal structure of that mineral, the researchers pointed out.

Halide perovskites are organic materials made from abundant, inexpensive ingredients, and Rice’s seeded growth method addresses both performance and production issues that have held back halide perovskite photovoltaic (PV) technology, the team emphasized.

The study has been published online on the research portal Advanced Materials.

The research was supported by the Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy, the Academic Institute of France, and the Office of Naval Research. The study made use of DOE facilities at Argonne National Laboratory and Brookhaven National Laboratory.

Talking about the latest innovation, chemical engineers from Rice’s Brown School of Engineering described how to make the seeds and use them to grow homogenous thin films – highly sought materials comprising uniformly thick layers.

In laboratory tests, PV devices made from the films proved both efficient and reliable, a previously problematic combination for devices made from either 3D or 2D perovskites.

“We’ve come up with a method where you can really tailor the properties of the macroscopic films by first tailoring what you put into solution,” said Aditya Mohite, study co-author of the paper and associate professor of chemical and biomolecular engineering and materials science and nanoengineering at Rice. “You can arrive at something that is very homogeneous in its size and properties, and that leads to higher efficiency. We got almost state-of-the-art device efficiency for the 2D case of 17%, and that was without optimization. We think we can improve on that in several ways,” Mohite elaborated.

Mohite further said that achieving homogenous films of 2D perovskites has been a huge challenge in the halide perovskite PV research community, which has grown tremendously over the past decade.

“Homogeneous films are expected to lead to optoelectronic devices with both high efficiency and technologically relevant stability,” he added.

Mohite credited study co-lead author Siraj Sidhik, a Ph.D. student in his lab, with the idea of pursuing seeded growth.

“The idea that a memory or history — a genetic sort of seed — can dictate material properties is a powerful concept in materials science,” Mohite said.

While seeded growth has often been demonstrated for inorganic crystals and other processes, Mohite said this is the first time it has been shown in organic 2D perovskites.

The researchers claimed that Rice’s seed-grown, high-efficiency PV films proved quite stable, preserving more than 97% of their peak efficiency after 800 hours under illumination without any thermal management.

The experts noted that in earlier research works, 3D halide perovskite PV devices have been highly efficient but prone to rapid degradation, while 2D devices have lacked efficiency but were highly stable.

The Rice study also details the seeded growth process — a method that is within the reach of many labs, said study co-author Amanda Marciel, assistant professor of chemical and biomolecular engineering at Rice.

The process for growing 2D perovskite films from seeds is identical in several respects to the classical process of growing such films, explained the research team. In the traditional method, precursor chemicals are measured out like the ingredients in a kitchen and dissolved in a liquid solvent. The resulting solution is spread onto a flat surface via spin-coating, a widely used technique that relies on centrifugal force to evenly spread liquids across a rapidly spun disk. As the solvent dissolves, the mixed ingredients crystalize in a thin film.

Mohite’s group has made 2D perovskite films in this manner for years, and though the films appear perfectly flat to the naked eye, they are uneven at the nanometer scale, the researchers pointed out. In some places, the film may be a single crystal in thickness, and in other places, several crystals thick.

In the seeded growth method, seeds are made by slow-growing a uniform 2D crystal and grinding it into a powder, which is dissolved into solvent instead of the individual precursors. The seeds contain the same ratio of ingredients as the classical recipe, and the resulting solution is spin-coated onto disks exactly how it would be in the original method. The evaporation and crystallization steps are also identical. But the seeded solution yields films with a homogeneous, uniform surface, much like that of the material from which the seeds were ground.

When Sidhik initially succeeded with the approach, Marciel and co-lead author Mohammad Samani provided the perfect tool for finding and studying any bits of undissolved seeds that might be templating the homogeneous films.

“We could track that nucleation and growth using light-scattering techniques in my group that we typically use to measure sizes of polymers in solution,” Marciel said.

The dynamic light scattering tool revealed that solutions reached an equilibrium state under certain conditions, allowing a portion of some seeds to remain undissolved in solution.

The research showed those bits of seed retained the “memory” of the perfectly uniform slow-grown crystal from which they were ground, and Samani and Marciel tracked the nucleation process that would eventually allow the seeds to produce homogeneous thin films.

Mohite said the collaboration produced a self-assembly method to make macroscopic materials that live up to the promise of the individual nanoparticles of which they are composed.

“At an individual, single-element level, you have wonderful properties that are orders of magnitude better than anything else, but when you try to put them together into something macroscopic and useful, like a film, those properties just go away because you cannot make something homogeneous, with just those properties that you want,” Mohite added.

“We haven’t yet done experiments on other systems, but the success with perovskites begs the question of whether this type of seeded approach might work in other systems as well,” he said.

In a step toward improving the long-term reliability of perovskite solar cells, prolific research in the field is being conducted.

In May this year, a research team at Brown University, Rhode Island, United States, devised a molecular glue that keeps a key interface inside cells from degrading. In a study published in the journal Science, the researchers explained that the treatment dramatically increases the cells’ stability and reliability over time while also improving the efficiency with which they convert sunlight into electricity.

Earlier this year, researchers at the Gwangju Institute of Science and Technology, South Korea, developed a new method to increase perovskite solar cells’ efficiency by utilizing ions. The researchers said perovskite solar cells are thin, light, and have a high conversion efficiency of over 25%.