Halide perovskites are gaining ground on silicon as a critical material for solar cell technologies: A new study published in the journal Science reports a method to make perovskite-based photovoltaics more durable, allowing the films to attain the desirable black phase of crystal configuration quicker and at lower temperatures while also making it harder to degrade into the inactive yellow phase.

Perovskites are solution-processable materials and can be readily processed as a solution or deposited as vapor. By mixing two key ingredients in the precursor solution, Rice University chemical engineer Aditya Mohite and collaborators have developed perovskite crystalline films that retain 98% of their initial efficiency even after 1,200 hours of exposure under open circuit voltage conditions to accelerated aging at 90 degrees Celsius (194 degrees Fahrenheit).

The two additives used were a two-dimensional perovskite, which served as a template to guide crystal growth, and formamidinium chloride, a salt molecule that regulates crystallization and has the optimal size to sustain the atomic bonds in the crystal in the right configuration. The two additives create compressive strain in the lattice, driving the formation of the black perovskite phase and stabilizing it, while also steering degradation toward a harder-to-form phase, significantly improving durability.

“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, a former Fulbright-Nehru Postdoctoral Fellow and current research specialist at Rice, who is a first author on the study. “That question stayed with us in the lab, especially on days when our black films slowly faded into the unwanted yellow phase after a certain time. 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.”

Formamidinum lead iodide crystals consist of a scaffold of lead-iodide octahedra ⎯ clusters made up of a central lead atom surrounded by six iodine atoms ⎯ separated by large voids known as “A-sites.” For a solar cell to work well, neighboring octahedra in a three-dimensional lattice must connect at their corners rather than along their edges or faces. This geometry keeps the atoms aligned, so electrons can move freely through the material.

“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, a Rice doctoral alum and postdoctoral researcher in the Mohite research group who is a co-author on the study. “We call this the black phase of crystallization, and it is the only one that is useful as a solar cell.”

To keep the crystal structure stable and prevent it from collapsing into more compact versions, the voids between the octahedra have to be filled. Formamidinium cations, positively charged ions derived from formamidine, are well-suited to this task, yet they are slightly too large to fit easily into the A-sites.

Because of this mismatch, the crystal often rearranges itself into a compact configuration in which octahedra share faces rather than corners. That arrangement bends the atomic bonds away from the ideal alignment needed for electronic coupling. As a result, instead of absorbing the full solar spectrum, the material reflects much of it — turning from the desired black phase to a pale yellow one that does not function well as a solar absorber.

“At room temperature, the perovskite crystal does not accommodate the formamidinium cations and instead forms a more compact configuration which is awful at absorbing light,” Metcalf said.

The typical way to get around this is to heat a film in the yellow phase up to around 150 C (300 F), making the crystal lattice expand enough to allow the formamidinium cations to slide into the A-sites. However, once cooled back to room temperature, the structure tends to revert to the yellow phase. To prevent that from happening, the researchers added small amounts of chemical impurities during film formation.

One of the key ingredients used was a 2D perovskite, which forms sheets of corner-sharing octahedra with slightly more flexible internal voids or A-sites that can more easily accommodate formamidinium cations. When mixed into the precursor solution, these sheets act as structural templates that guide crystal growth.

“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.”

 Faiz Mandani, a Rice doctoral alum and co-author on the study, helped develop a degradation unit to simulate exposure to heat and irradiance in order to test how devices built from the new perovskite films would perform. (Photos by Jorge Vidal/Rice University)

Formamidinium chloride was the other key ingredient: Because chlorine forms stronger bonds with lead than iodine does, it was better at enabling the corner-sharing geometry needed for efficient charge transport. This offers a stepwise growth mechanism, which facilitates an energetically favorable phase transition.

“You can think of it as taking one step at a time on a staircase with control and ease rather than expending strenuous effort by jumping multiple steps in one go,” Garai said. “The two additives’ collective effect results in superior crystallization through a uniform, gradual transition pathway that induced a compressive strain and provided exceptional stability.”

One of the study’s surprising findings is that chlorine does more than guide crystallization.

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

When perovskite films break down, they typically follow the lowest-energy chemical pathway. Incorporating chlorine forces degradation to proceed through a much higher-energy route, effectively slowing the process.

“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.

Together, the additives not only chemically improve the stability of the photovoltaic films, but they also structurally improve the size and orientation of the crystals in those films, giving them better defenses against moisture, light and heat: The larger the crystals, the fewer surface areas sites there are for them to degrade at.

Nilanjana Nandi, a research specialist at Rice and study co-author, said that this thorough “understanding of the fundamental formation and degradation mechanisms opens up a new design strategy for developing materials with practical durability.” Nandi specifically highlighted the critical role of the accelerated degradation experiments for “uncovering the true underlying mechanisms.”

“We think that this is going to have a huge impact in terms of the stability of these materials,” said Mohite, Rice’s William M. Rice Trustee Professor, professor of chemical and biomolecular engineering and faculty director of the Rice Engineering Initiative for Energy Transition and Sustainability.

Mohite pointed out that silicon solar cells in use today operate at about 22-23% module efficiency, while “so-called tandem configurations where silicon- and perovskite-based photovoltaics are used together achieve efficiencies as high as 30-35%.”

Aside from harnessing solar power for electricity, photovoltaics can also be used to power chemical reactions, including for producing alternative fuels like hydrogen.

Faiz Mandani, a Rice doctoral alum and co-author on the study, helped develop a degradation unit to simulate exposure to heat and irradiance in order to test how devices built from the new perovskite films would perform.

“Previously, we relied on a lamp and hot plate setup that allowed us to test one device at a time,” Mandani said. “With our new degradation unit, which provides a large uniform heating surface and light source, we can now test up to 100 devices simultaneously. This enables us to generate statistically meaningful data and better understand how a representative population of solar cells degrades over time.”

 Aditya Mohite leads a research group at Rice University that is one of the world-leading hubs of expertise on perovskites for photovoltaics. (Photo by Jeff Fitlow/Rice University)

The Mohite group at Rice is one of the world-leading hubs of expertise on perovskites for photovoltaics. Mohite and his team have forged collaborations across the U.S. and abroad to advance scientific understanding of this semiconductor class.

Jacky Even, a longtime collaborator based at the University of Rennes in France, said that working on a relatively “new family of semiconductors with great potential for real applications” makes the research exciting.

“Scientific research is international by nature,” Even said. “We must distribute the knowledge ⎯ that is part of our scientific mission, and we learn a lot by interacting with other research groups. We have been fortunate to collaborate with the Mohite group at Rice for the past decade.”

Mohite credits collaborators at Lawrence Berkeley National Laboratory, Rennes, University of Lille, University of Cambridge and Northwestern University for creating a research ecosystem “where we can get really meaningful, breakthrough results and move the field by leaps and bounds.”

The research was supported by the U.S. Department of Energy (DE-EE0010738), the U.S.-India Educational Foundation (USIEF), the Hertz Foundation, the U.S. National Science Foundation (20587), the Swiss National Science Foundation (P500PN_206693), the University of Lille and the Centre national de la recherche scientifique. The content in this press release is solely the responsibility of the authors and does not necessarily represent the official views of funding entities.