Light-activated contraction boosts ultrathin solar cells efficiency by 18%

Engineers at Rice raised the photovoltaic efficiency of 2D perovskites by up to 18%.
Engineers at Rice raised the photovoltaic efficiency of 2D perovskites by up to 18%. Credit: Jeff Fitlow

The research engineers at Rice University have achieved what they said is a new benchmark in the design of atomically thin solar cells made of semiconducting perovskites, boosting their efficiency while retaining their ability to stand up to the environment.

Researchers found that sunlight itself contracts the space between atomic layers in 2D perovskites enough to improve the material’s photovoltaic efficiency by up to 18%. In 10 years, the efficiencies of perovskites have skyrocketed from about 3% to over 25%, they said.

Perovskite solar cells have shown remarkable progress in recent years. These devices are compounds that have cubelike crystal lattices and are highly efficient at harvesting sunlight. However, their stability is quite limited compared with that of leading PV technologies. They don’t stand up well to moisture, oxygen, extended periods of light, or high heat.

“A solar cell technology is expected to work for 20 to 25 years,” said Mohite, an associate professor of chemical and biomolecular engineering and of materials science and nanoengineering. “We’ve been working for many years and continue to work with bulk perovskites that are very efficient but not as stable. In contrast, 2D perovskites have tremendous stability but are not efficient enough to put on a roof.”

The problem is to make the solar cells both efficient and stable. In their research, the Rice University team has discovered that in certain 2D perovskites, sunlight effectively shrinks the space between the atoms, improving their ability to carry a current. They found that placing a layer of organic cations between the iodide on top and lead on the bottom enhanced interactions between the layers. The enhanced charge transport boosts the photovoltaic efficiency of the 2D perovskite solar cells up to 18.3%.

The Rice engineers and their colleagues in France have confirmed the experiments by computer models. “This study offered a unique opportunity to combine state of the art ab initio simulation techniques, material investigations using large scale national synchrotron facilities and in-situ characterizations of solar cells under operation,” said Jacky Even, a professor of physics at INSA. “The paper depicts for the first time how a percolation phenomenon suddenly releases the charge current flow in a perovskite material.”

The results showed that after 10 minutes under a solar simulator at one-sun intensity, the 2D perovskites contracted by 0.4% along their length and about 1% top to bottom. It doesn’t sound like a lot, but this 1% contraction in the lattice spacing induces a large enhancement of electron flow. The lessening of space between atoms increases conductivity threefold and boosts efficiency.