Revolutionary energy sandwich for solar energy and lighting

Researchers have achieved a new level of control over the atomic structure of a family of materials known as halide perovskites, creating a finely tuned “energy sandwich” that could transform the way solar cells, LEDs and lasers are made.

Because of their remarkable ability to absorb and emit light, and because they are cheaper and can be configured to convert more of the solar spectrum into energy than silicon, perovskites have long been touted as a potential replacement for silicon in solar cells, LEDs and quantum technologies.

However, due to their instability and durability, perovskite devices have been largely limited to laboratory use. Additionally, scientists have had difficulty precisely controlling the thickness of perovskite films and how different perovskite layers interact when stacked—an important step in building functional, multilayered structures.

Now a team of researchers led by the University of Cambridge has found a new way to grow ultra-thin layers of perovskite films so that their atoms are perfectly aligned, which could enable more powerful, longer-lasting and more efficient devices.

The researchers used a vapor-based technique to grow three-dimensional and two-dimensional perovskites layer by layer, allowing them to control the thickness of the films down to fractions of an atom. Their findings, reported in the journal Science, could open the door to viable perovskite devices that can be manufactured on a large scale using a process like that used to make commercial semiconductors.

Each layer in a semiconductor “sandwich” has a different role in moving electrons and their positively charged counterparts – called holes – and determining how the semiconductors absorb or emit light. Together, the layers act like one-way streets, directing the electrical charges in opposite directions, preventing them from bumping into each other again and wasting energy as heat.

For other widely used semiconductors such as silicon or newer materials such as gallium nitride, the properties of individual layers can be fine-tuned using various methods. However, despite their excellent performance, perovskites have so far proven difficult to control in layered devices, in part due to their “chaotic” atomic structure.

“Much of perovskite research uses solution processing, which is chaotic and difficult to control,” says Professor Sam Stranks from the Department of Chemical Engineering and Biotechnology, who co-led the research. “By switching to steam processing – the same method used for standard semiconductors – we can achieve the same level of atomic control, but with materials that are much more forgiving.”

The researchers used a combination of three-dimensional and two-dimensional perovskites to create and control their atomically tuned stacks, a phenomenon known as epitaxial growth. This fine control allowed the team to directly observe how the light emitted by the material changes depending on whether it is a single layer, a double layer or a thicker layer.

“The hope was that we could grow a perfect perovskite crystal by changing the chemical composition layer by layer, and we succeeded,” said co-first author Dr. Yang Lu from the Department of Chemical Engineering and Biotechnology and Cavendish Laboratory in Cambridge. “It’s like building a semiconductor from scratch, one layer of atoms at a time, but with materials that are much easier and cheaper to process.”

The researchers also found that they could design the connections between the layers to control whether electrons and holes remained together or separated – a key factor in how efficiently a material emits light.

“We have achieved a level of tunability that was not even on our radar when we started,” said Professor Sir Richard Friend of the Cavendish Laboratory, who co-led the research. “We can now decide what type of compound we want – one that holds charges together or one that pulls them apart – simply by slightly changing the growth conditions.”

The researchers found that they could adjust the energy difference between layers by more than half an electron volt and, in some cases, extend the lifetime of electrons and holes to over 10 microseconds: much longer than usual.

The team says this level of precision could pave the way for scalable, high-performance devices that use light in new ways, from lasers and detectors to next-generation quantum technologies.

“Changing the composition and performance of perovskites at will – and studying those changes – is a real achievement and reflects the time and investment we have made here in Cambridge,” Stranks said. “But more importantly, it shows how we can make working semiconductors from perovskites, which could one day revolutionize the manufacture of cheap electronics and solar cells.”

The research was supported in part by the Royal Society, the European Research Council, the Simons Foundation and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI). Richard Friend is a Fellow of St John's College, Cambridge. Sam Stranks is a Fellow of Clare College, Cambridge.