Tokyo Tech team discovers a way to improve perovskite-based light-emitting diodes

Researchers at the Tokyo Institute of Technology (Tokyo Tech) have designed a new strategy to make efficient perovskite-based LEDs with improved brightness by leveraging the quantum confinement effect.

Photoluminescence and electroluminsecence in low-dimensional and 3D perovskite-based devices image(A) Photoluminescence and (B) electroluminsecence in low-dimensional and 3D perovskite-based devices

Devices that 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 (LEDs) make for the most notable and prevalent category of these devices. Many additional types of LEDs also exist.

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

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 electron'hole pair that emits photons efficiently). However, using low-dimensional perovskites has an intrinsic drawback in that the conducting properties, and this leads to low power efficiency.

Interestingly, as discovered by the team of researchers led by Prof. Hideo Hosono from the Tokyo Institute of Technology, it is possible to design highly efficient PeLEDs using three-dimensional (3D) perovskites, which have superior mobility of electron and hole and so would address the limitation of low-dimensional perovskites.

The team investigated if the quantum confinement effect that occurs in low-dimensional materials using new electron transport layer adjacent to the perovskite and results in attractive light-emission properties, could be achieved in 3D materials. In an electroluminescent 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 EL.

By tuning the characteristics of the electron and hole transport layers in PeLEDs, the team could prevent the aforementioned effect by ensuring that excitons remain confined in the emission layer. "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," explains Hosono. The team reported 3D PeLEDs with record-setting performance in terms of high brightness and power efficiency and low operating voltage.

Aside from these tangible practical achievements, this research sheds light onto 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. 'We believe this study provides new insight into the realization of practical PeLEDs,' concludes Hosono.

Posted: Aug 11,2019 by Roni Peleg