Upgraded perovskite design sets solar cells on path to stability

In solar science, a little structural harmony goes a long way. By finding the atomic equivalent of a perfect handshake between two types of perovskite—a class of crystalline materials prized for their ability to convert sunlight into electricity—researchers at Cornell have built solar cells that are not only high-performing, but exceptionally durable.

Three-dimensional (3D) perovskites consist of repeating atomic networks of metal halide “cages” that are connected at their corners and filled with small, positively charged molecules known as cations.

These materials have shown remarkable promise for enabling solar cells that are lightweight, low-cost and capable of efficiencies that surpass those of traditional silicon. But despite their potential, most 3D perovskites are vulnerable to heat, moisture and the very sunlight they’re designed to capture due to their salt-like ionic crystal structures.

New research details a first-of-its-kind, two-dimensional (2D) perovskite designed by Cornell researchers that can be layered on top of a 3D perovskite to act as a rugged, weather-resistant coating. The results are detailed in the paper “Phase-stabilized 2D/3D hetero-bilayers via lattice-matching for efficient and stable inverted solar cells,” published May 9 in the journal Joule.

Other researchers have attempted this protective 2D perovskite coating using methylammonium (MA) as a cage cation. However, MA is so unstable it starts to vaporize upon exposure to sunlight.

“With MA, you have good efficiency and charge transport, but the solar cell degrades rapidly in a few hundred hours of continuous operation,” said lead author Shripathi Ramakrishnan, a doctoral candidate in the lab of senior author Qiuming Yu, professor of chemical and biomolecular engineering at Cornell Engineering.

Attempts have been made to use formamidinium (FA)—a more stable cage cation—in the protective layer, but too much strain in the material’s crystal structure imposed by FA’s larger size destabilizes it and prevents the formation of stable 2D lattices.

The new breakthrough came from lattice matching—the idea that if the lattice of 2D perovskite is sized just right, it will “click” together with the 3D perovskite. By selecting special organic cations, called ligands, that naturally align with both the FA cage cation and the surrounding crystal structure, the researchers were able to form 2D perovskites with a layer thickness and configuration that balances conductivity and stability.

“The basic idea is that a ligand in a 2D perovskite tries to shrink the lattice, while the FA cage cation works to make it bigger and you have these two opposing forces at play,” Ramakrishnan said. “We selected a ligand that doesn’t try to compress the cage too much, allowing it to expand a little and make room for the larger FA cation to fit inside.”

The group successfully synthesized a new 2D perovskite using FA as the cage cation and applied it as a protective coating on top of a 3D perovskite. Characterization techniques—including synchrotron X-ray diffraction and confocal photoluminescence mapping—revealed that the new 2D FA-based layer possesses exceptional stability under combined light, temperature and humidity, outperforming its MA-based counterparts.

The 2D-on-3D combination not only resists degradation under sunlight and heat; it also improves electrical performance by enabling smoother flow of charges between the 3D and 2D layers. The resulting solar cells achieved a sunlight-to-electricity conversion efficiency of 25.3% and showed only 5% performance loss over nearly 50 days of intense testing under combined light and heat, making them exceptionally durable perovskite solar cells.

While perovskites have attracted great interest from scientists over the last decade, their instability has held back commercialization of their use in solar cells.

“Silicon had about 50 years to get to where we are with solar. Perovskite hasn’t had 50 years yet, but we can accelerate that progress by understanding it at the molecular level and applying what we learn,” Yu said.

Ramakrishnan said an internship, supported by the National Science Foundation’s INTERN program at the National Renewable Energy Laboratory in Colorado, gave him a window into the commercialization landscape, where lab-tested materials are exposed to real outdoor conditions and compared directly with industrial solar panels.

“This was really inspiring for me—not just the scientific aspect, but also the technological relevance,” Ramakrishnan said.