Korea University: A Method for Increasing the Efficiency of QLED Has Been Found

A research group led by Professor Cho Minhaeng of the Department of Chemistry in the College of Sciences, who is the head of the IBS Center for Molecular Spectroscopy and Dynamics, proposed a new method for improving the light-emitting efficiency of photoelectric devices using quantum dots, such as quantum dot light-emitting diodes (QLEDs).

The results of the study were published online in Advanced Optical Materials (IF 9.926), a globally acclaimed international journal in optics, on December 23.

Quantum dots, which are semiconductor particles having a diameter of several nanometers, have been applied to various photoelectric devices, such as QLEDs, due to their unique optical characteristics, including the emission of light of different frequencies depending on particle size.

Semiconductors such as quantum dots have two bands where electrons can exist. The lower band filled with electrons is referred to as the valence band, and the upper band without electrons is referred to as the conduction band; the energy gap between the two bands is called the band gap. When receiving external energy (light) greater than the band gap, the electrons in the valence band are excited into the conduction band.

The empty site without an electron is referred to as an electron hole, which is bound with the electron excited to the conduction band to form an exciton, a quasiparticle. As time passes, the exciton loses energy and the electron is recombined with the hole. The electron externally releases the energy supplied previously in the form of light, which is the light we observe from photoelectric devices such as QLEDs.

The problem is that not all excitons emit light in such an ideal manner. The exciton recombination happens through other processes depending on the characteristics of the quantum dot materials. A representative example is the biexciton Auger recombination (AR) caused by the interactions between two excitons. The AR refers to the combination of an electron and a hole that transfers the energy to another exciton without releasing light externally. This lack of external light emission is an obstacle to the improvement of the efficiency of quantum dot-based photoelectric devices, especially displays.

Many spectroscopic studies have been conducted to suppress the AR. Previous studies modified the structure of the quantum dots or synthesized quantum dots of new shapes and thus failed to actively control the AR.

Professor Cho’s group prepared quantum dots as a thin film having a thickness of less than 100 nm on a nanostructure of a metamaterial to investigate the AR. The researchers observed the Auger process that occurred for an extremely short time of several picoseconds (a picosecond [ps] is a trillionth of a second) and found that the AR was suppressed by the nanostructure.

The research group also investigated the mechanism of the nanostructure in order to prevent the AR. Generally, the AR is enhanced as the transition dipole moment is increased. When a metal is present near the dipole, an image dipole is created. The research group explained that the image dipole formed by the nanostructure interacts with the transition dipole moment of a quantum dot to reduce the size of the whole transition dipole moment, thereby suppressing the AR.

* •Transition dipole moment :An electric dipole moment related to the transition between two quantum mechanical states. As the value is increased, the transition between the two states is enhanced.

* Dipole :An arrangement of electric charges in which a positive charge and a negative charge of the same quantity are separated from each other at a certain distance.
* Image dipole :A dipole that is formed on the opposite side of another dipole on a metal layer by the metal. The change in the electric field caused by the metal is the same as the change in the electric field that is expected when it is assumed that an image dipole is present.

Professor Cho said, “We became the first research group that proved that the AR that happens frequently in quantum dots can be controlled by means of a nanostructure.” He added, “The introduction of an external structure allows us to prevent the AR, which is one of the non-radiative processes, and thereby increase the efficiency of photoelectric devices.”

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