3D organic-inorganic perovskites have the chemical formula (CH3NH3)PbX3 (where Pb is lead and X can be iodine, bromine or chlorine). They are one of the most promising thin-film solar-cell materials around today because they can absorb light over a broad range of solar-spectrum wavelengths. What is more, the power-conversion efficiency (PCE) of solar cells made from these materials has gone from just 3% to more than 22% in the last five years. This means that their PCE is now comparable to that of silicon-based solar cells.

Thanks to the fact that they can be easily processed in solution, their high-colour purity and tuneable bandgap, hybrid perovskites are also promising for LEDs. Indeed, researchers have recently made methylammonium lead bromide-based LEDs that have an external quantum efficiency (EQE) of as high as 8.5%. However, there is a problem in that the rapid crystallization speed of 3D perovskites means that grain sizes in the materials are generally hundreds of nanometres in size when they are produced using solution-based processing techniques. These grains also have very rough surfaces, which adversely affects their final electronic properties.

Adding long-chain ammonium halides to the mix

Now, a team led by Barry Rand has developed a solution process to form highly uniform, ultra-flat perovskite films with nanometre-sized grains that can be used to make highly-efficient LEDs. By adding long-chain ammonium halides (for example, n-butylammonium halides, or BAX) in the 3D perovskite precursor solution, the researchers were able to inhibit the growth of 3D perovskite grains and dramatically decrease film roughness to just 1 nm.

The grains produced this way boast stronger and blue-shifted photoluminescence and electroluminescence compared to the emission from the bulk 3D perovskite. Indeed, the EQE of iodide perovskite LEDs increased from 1% to more than 10% thanks to the incorporated BAI and the EQE of bromide perovskite devices from 0.03% to nearly 9.3% thanks to the incorporation of BABr. The power efficiency and current efficiencies reached 13 lm/W and 17.1 cd/A, respectively, for Br-perovskite LEDs.

Improved shelf life too

And that is not all: LEDs containing the long-chain halides also have a much improved shelf life – lasting for more than eight months compared to just days for devices without BAX added.

“The perovskite crystal structure that we are working with contains an organic cation,” explains Rand. “This organic cation, methylammonium, is very small. We introduced a bigger organic cation in addition, which served to make films with very small crystallites that were coated in the bigger organic molecule, stabilizing and protecting the surface. This made the films much brighter, and the LEDs more efficient and stable.”

Displays and lighting applications

According to the team, reporting its work in Nature Photonics doi:10.1038/nphoton.2016.269, the perovskites could find applications in displays or for lighting.

The researchers say they will now try to push the limits of efficiency in their materials and test out alternative bulky organic cations. “At the moment, our devices are no more than 50% internally efficient, which means that there is substantial room for improvement in terms of performance and stability,” Rand tells nanotechweb.org.