The researchers, led by Aditya Mohite of the Los Alamos Laboratory in New Mexico, studied 2D Ruddlesden–Popper perovskites. These materials are near single-crystalline, thin, layered perovskite films that have an out-of-plane orientation so that charge transport occurs predominantly, and uninhibited, through the inorganic components. They have recently emerged as promising alternatives to 3D perovskites thanks to the fact that their optoelectronic properties can be controlled.

When these materials absorb sunlight, electrons and holes are generated. These then combine to form excitons that must then be broken apart and transferred to different transport materials – one for the electrons and another one for the holes. The transport materials subsequently carry the charges to separate electrodes that generate both current and voltage to extract power from the cell.

The compound studied in this work has the chemical formula (BA)2(MA)n-1PbnI3n+1 (where n = 1 to 5) and it contains anionic layers derived from the parent 3D perovskite, methylammonium lead triiodide (MAPbI3). The anionic layers are isolated from one another by organic n-butyl ammonium (BA) spacer cations.

Lower energy states at the edges

Mohite and colleagues found that in thin perovskite layers just over two units thick, electrons move into a lower-energy state along the edges of the materials. “The presence of these edge states, which is the most important finding in our work, leads to intrinsic dissociation of strongly bound electron-hole pairs (excitons) to long-lived free carriers,” explains team member Jean-Christophe Blancon, who is lead author of the study. “What is more, once electrons and holes are trapped in these edge states, they remain protected and do not lose their energy via a non-radiative process. They can thus contribute to the photocurrent in a photovoltaic device or radiatively combine for light-emission applications.”

In our work, we have demonstrated that 2D perovskites are promising for solar cells with an efficiency of greater than 12% and they are stable for more than 2000 hours, he tells nanotechweb.org. “We also expect applications such as light-emitting diodes and lasers in which perovskites could provide a low-cost solution to making these devices. In the future, photo- and high-energy particle detectors are also an option, thanks to us being able to chemically design the properties and functionalities of 2D perovskites.”

These results also show that edges and surface states, which generally degrade optoelectronic properties, can now be chemically designed and engineered to allow for efficient flow of charge and energy, leading to high-efficiency electronic devices, adds Mohite.

The team, which includes researchers from Northwestern University in Illinois, the Brookhaven National Laboratory in New York and the University of Rennes 1 in France, says that it will now continue to explore the fundamental optical and electronic properties of these compounds. “We will be studying materials with different compositions to better understand them and to further improve their performance,” says Blancon. “We will also be trying to fabricate next-generation optoelectronic devices (like lasers, radiation detectors and transistors, for instance) based on low-cost, solution-processed 2D perovskites.”

The present work is detailed in Science DOI: 10.1126/science.aal4211

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