MIT research provides roadmap to perovskite passivation

With perovskite efficiency at converting sunlight to electricity edging up on that of silicon, the main challenges now are optimizing that efficiency and controlling degradation. A team of researchers at MIT in Cambridge, Mass. reveal a new way to engineer perovskites on the nanoscale.

The work is described in a paper published in Nature Energy by Dane deQuilettes, a recent MIT postdoc who is now co-founder and chief science officer of the MIT spinout Optigon, along with MIT professors Vladimir Bulovic and Moungi Bawendi, and 10 others at MIT and in Washington state, the U.K., and Korea.

The researchers describe their unique combination of nanoscale characterization techniques that enabled them to find a tunable passivation strategy in perovskite solar devices. They report that by using these techniques they were the first to reach the >25% PCE milestone.

The researchers explained that treatment with hexylammonium bromide forms an iodide-rich 2D layer along with a Br halide gradient that extends from defective surfaces and grain boundaries into the bulk three-dimensional layer. They report that the interface can be optimized to extend the charge carrier lifetime to record values >30 μs and to reduce interfacial recombination velocities to values as low as <7 cm s⁻¹.

“Ten years ago, if you had asked us what would be the ultimate solution to the rapid development of solar technologies, the answer would have been something that works as well as silicon but whose manufacturing is much simpler,” Bulovic says. “And before we knew it, the field of perovskite photovoltaics appeared. They were as efficient as silicon, and they were as easy to paint on as it is to paint on a piece of paper. The result was tremendous excitement in the field.”

Nonetheless, “there are some significant technical challenges of handling and managing this material in ways we’ve never done before,” he said. These challenges are what has driven researchers to study perovskites, and this work shows how to passivate the material’s surface so that the perovskite no longer degrades so rapidly or loses efficiency.

In previous work, research teams developed methods for passivation, but there wasn’t clear understanding of how the process works. The new MIT study provides a clearer roadmap. The team observed the interfaces between the perovskite layer and other materials, and how they develop, which resulted in “the clearest roadmap as of yet of what we can do to fine-tune the energy alignment at the interfaces of perovskites and neighboring materials,” and thus improve their overall performance, Bulovic says.

“The key is identifying the chemistry of the interfaces, the place where the perovskite meets other materials,” Bulovic says, referring to the places where different materials are stacked next to perovskite in order to facilitate the flow of current through the device.

The new study “addressed the ability to passivate those interfaces and elucidate the physics and science behind why this passivation works as well as it does,” Bulovic says.

“This paper is essentially revealing a guidebook for how to tune surfaces, where a lot of these defects are, to make sure that energy is not lost at surfaces,” deQuilettes says. “It’s a really big discovery for the field,” he says. “This is the first paper that demonstrates how to systematically control and engineer surface fields in perovskites.”

The new work builds on a common passivation method developed at MIT, which led to new world-record efficiencies. The recent efficiency records for a single perovskite layer range from about 24% to 26%, while the maximum theoretical efficiency that could be reached is about 30%, according to deQuilettes.

An increase of a few percent may not sound like much, but in the solar industry incremental advances are important.

“In the silicon photovoltaic industry, if you’re gaining half of a percent in efficiency, that’s worth hundreds of millions of dollars on the global market,” he said.