Tokyo Institute of Technology scientists have discovered a new strategy to design incredibly efficient perovskite-based LEDs with record-setting brightness by leveraging the quantum confinement effect.

Several techniques for generation of light from electricity have been developed over the years. Devices that can emit light when an electric current is applied, are referred to as electroluminescent devices, which have become orders of magnitude more efficient than the traditional incandescent light bulb.

Light-emitting diodes comprise the most notable and ubiquitous category of these devices. A myriad of different types of LEDs exist nowadays, which has been made possible by advances in our understanding of quantum mechanics, solid-state physics, and the use of alternative materials.

(A) Photoluminescence and (B) electroluminsecence in low-dimensional and 3D perovskite-based devices. Photoluminescence (PL) refers to the emission of light caused by the absorption of incident photons, whereas electroluminescence (EL) is the emission of light owing to the energy supplied by an electric current. Although low-dimensional perovskite exhibits better PL properties than 3D perovskite, the latter has better EL properties, which can be exploited to design very bright and efficient PeLEDs. Image Credit: Tokyo Tech. Click image for the largest view.

Electroluminescent devices consist of several layers, the most important being the emission layer which emits light in response to an electric current. Metal halide perovskites, with the chemical formula CsPbX3 (X = I, Br, Cl), have been recently considered as promising materials for fabricating the emission layer. However, current perovskite-based LEDs (PeLEDs) perform poorly compared with organic LEDs, which are typically used to design displays of TVs and smartphones.

Several researchers have suggested fabricating PeLEDs using low-dimensional (i.e., emitting structural units are connected on a plane or linearly in the crystal structure) perovskites that offer improved light-emission performance based on the quantum confinement effect of excitons. An exciton is an electron-hole pair that emits photons efficiently. However, using low-dimensional perovskites has an intrinsic drawback in that the conducting properties, i.e. low mobility, of these materials are very poor, and this lack of low mobility leads to low power efficiency.

Of interest, as discovered by a team of researchers led by Prof. Hideo Hosono at the Tokyo Institute of Technology, it is possible to design highly efficient PeLEDs using three-dimensional (3D) perovskites, which have superior mobility of electrons and holes and hence would address the limitation of low-dimensional perovskites.

The team investigated if the quantum confinement effect that occurs in low-dimensional materials using a new electron transport layer adjacent to the perovskite and if results in attractive light-emission properties, could be achieved in 3D materials. In an electroluminescent (EML) device, the EML is sandwiched between two layers: the electron transport and hole transport layers. These two layers play a key role in ensuring good conducting properties of the device. The team found that the energy-level characteristics of these layers also play a crucial role in emission efficiency of the EML.

By tuning the characteristics of the electron and hole transport layers in PeLEDs, the team could prevent the above mentioned effect by ensuring that excitons remain confined in the emission layer. Prof Hosono explained, “The whole device structure can be regarded as a scaled-up low-dimensional material in a sense if the energy levels of the electron/hole transport layers are sufficient for exciton confinement.” The team reported 3D PeLEDs with record-setting performance in terms of high brightness and power efficiency and low operating voltage.

The research paper has been published in Applied Physics Reviews.

Beyond these tangible practical achievements, this research sheds light into how the exciton-related properties of a material can be influenced by the adjacent layers and provides a strategy that can be readily exploited in the development of optical devices. Hosono added, “We believe this study provides new insight into the realization of practical PeLEDs.” With such interesting advances in light-emitting materials, it seems that a (literally) brighter future awaits.

Of all the technology we’ve looked at over the past decade LEDs have covered the most ground and are omnipresent across the lighting market. Remember when this blog started, incandescent and fluorescent bulbs were mainstream and dominant. They were eclipsed by compact florescent and that has been eclipsed again by LED.

Perovskite may well be the next major market change. The research is worldwide and interest is high. What we don’t know is the interest of manufacturers and consumers. Its one thing to have a while new light bulb technology clearly marked on the box. It will be much harder to get market momentum if the same consumer view only shows a new material in an existing technology.

For perovskites it will need be very much a More Better Cheaper offering to get mass markets.


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