Perovskites have the chemical formula (CH3NH3)PbX3 (where Pb is lead and X can be iodine, bromine or chlorine). These direct-bandgap semiconductors could be real alternatives to other types of photovoltaic materials, such as those made from silicon, for applications like colour displays, lighting and optical communications. This is because they are cheap and easy to make and can be easily tuned to emit light of a variety of colours.

However, there is a problem in that devices like LEDs made from these materials do not perform very well because electrons and holes only weakly bind in perovskite thin films. This means that excitons (electron-hole pairs) spontaneously dissociate into free carriers in the bulk recombination layer, leading to low photoluminescence quantum efficiency (PLQE), high leakage current and low luminous efficiency.

Better confining electrons and holes

To improve matters, researchers need to be able to better confine electrons and holes so that they will more readily recombine in the bulk recombination layer. A team led by Qihua Xiong has now succeeded in doing this by developing a new and simple way to make a series of colloidal (CH3NH3)PbX3 nanoparticles with an amorphous structure that can be tuned to emit light in the ultraviolet to near-infrared range.

Xiong and colleagues studied the photoluminescence properties of the nanoparticles in detail and found that the PLQE of the perovskite NP film is much higher than that of the bulk film. They then made a highly efficient green LED based on amorphous (CH3NH3)PbBr3 nanoparticles that have a maximum luminous efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W and an external quantum efficiency of 3.8%.

High quantum yield

“We made our colloidal perovskite nanoparticles by first dissolving the perovskite precursors in a mixture of N,N-dimethylformamide and γ-butyrolactone, with octylamine as the organic ligand,” explains Xiong. When we then drop the precursor solution into toluene, we obtain a green-coloured colloidal solution, indicating that the perovskite material is precipitating out.”

One of the main good properties of perovskite nanoparticles is their high quantum yield, reaching 77%, he adds. “Previous research also reported on the high quantum yield of as-synthesized perovskites, but these could not be separated easily from the solution in which they were produced so they were not suitable for making LED devices. Our perovskite nanoparticles can be easily separated from their processing solution by centrifuging and dispersing them into a colloid.”

The nanoparticles could be not only be used to make LEDs, but also solar cells, nanolasers, optical waveguides and in quantum communications, Xiong tells

The team, which includes Hilmir Volkan Demir from Nanyang Technological University and Yawen Zhao from China Academy of Engineering Physics, says that it now plans to optimize the LEDs it has made to improve their external quantum efficiency. “We also want to construct LED structures with inorganic charge transport layers to improve the stability of the these devices,” says Xiong.

The work is detailed in ACS Nano DOI: 10.1021/acsnano.6b01540.

For more information on perovskites, visit the Nanotechnology focus collection at Focus on Perovskite Solar Cells, Topical Review and Lab Talk.