Halide perovskites have long been considered a low-cost alternative to silicon in solar cells, but instability has limited their use. Now, researchers report a way to improve durability while maintaining high performance. The findings show how chemical additives can stabilize perovskite films under heat and prolonged stress.

A team at Rice University developed a modified precursor solution that improves crystal formation and slows degradation. In lab tests, the films retained 98% of their efficiency after 1,200 hours at 194°F under accelerated aging conditions.

Guiding crystal growth

Perovskites can be processed as liquids or vapors, making them easier to manufacture than silicon. However, their crystal structure can shift into a less useful form that reflects light instead of absorbing it.

To address this, researchers added a two-dimensional perovskite and formamidinium chloride to the precursor solution. The first acts as a template for crystal growth, while the second regulates crystallization.

“This research began with a simple but persistent question: Can we truly make a solar cell that is extremely stable — one that never degrades,” said Rabindranath Garai. “That question stayed with us in the lab… It became clear that if we wanted real stability, we could not just study how the material forms but we also had to understand how it falls apart.”

The additives create compressive strain in the crystal lattice, stabilizing the black phase that efficiently absorbs sunlight. “When connected in this way, the crystal is great at absorbing light ⎯ so great at it, in fact, that it looks black, because all the light that hits it gets absorbed,” said Isaac Metcalf. “We call this the black phase of crystallization, and it is the only one that is useful as a solar cell.”

Formamidinium-based perovskites often suffer from structural mismatch, which leads to instability. Heating can temporarily fix this, but the structure tends to revert after cooling. The new approach avoids that cycle by guiding orderly crystal growth and enabling gradual crystallization.

“You can think of it as having a grid of holes on the ground and then throwing a handful of marbles down,” Metcalf said. “If there is no grid, the marbles will go everywhere. With the grid, they will all segregate into the different holes.”

Slowing degradation pathways

The researchers also found that chlorine changes how the material degrades. “Here we have shown that the chlorine actually goes into the lattice, and by doing so, it changes the way the material degrades,” said Aditya Mohite.

Instead of following the usual low-energy breakdown, the material degrades more slowly through a higher-energy pathway. “Unlike the conventional degradation pathway via the yellow phase, this co-additive approach completely bypasses it and introduces an alternative, energetically uphill route,” Garai said.

The films also show larger, better-aligned crystals, reducing weak points where degradation begins. Researchers tested up to 100 devices at once using a custom setup.

“Previously, we relied on a lamp and hot plate setup that allowed us to test one device at a time,” said Faiz Mandani. “With our new degradation unit… we can now test up to 100 devices simultaneously.”

The results could support wider use of perovskites in tandem solar cells, which can exceed 30% efficiency when paired with silicon.

The study is published in the journal Science.