Hidden Confinement Induced by Charged Excitons: External Electric Field Adjustment to Achieve Highly Efficient Fluorescence in PLEDs

Abstract

Experiments have shown that intensive charge injections are able to greatly enhance the efficiency of polymer light emitting diodes (PLEDs). Yet, under strong external electric filed/high voltages, there is a certain degree of efficiency roll-off. For this paradox, this article reveals the following underlying mechanism: Under a low electric field, a charged polaron is driven to the potential well created by an exciton, where they fuse together to form a "charged exciton". The carrier fusion induces the triplet state to fluoresce, which greatly enhances the performance of the device (ideally, the internal quantum efficiency can exceed 95%). As long as the external field continues to increase and surpasses 4.5 × 10 4 V/cm, the above confinement is broken, and the polaron also steps out of the potential well, which leads to a major setback of the overall device efficiency. Then, when the electric field reaches as high as 0.8 MV/cm, the original exciton is dissociated. For achieving highly efficient fluorescent PLEDs, it is seen that the appropriate electric field magnitude ranges from 5 × 10 2 to 2 × 10 4 V/cm.

A theoretical study of the activation of nitromethane under applied electric fields

“…13 In recent years many theoretical and experimental studies have been conducted to understand the effect of external electric elds on molecular structures and properties, and the applied elds have become a powerful tool for discovering new physical and chemical phenomena, and developing new materials. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] The quantum-mechanical theory of eld-induced chemistry was rstly developed by Kreuzer et al, 21 and have been used to address the kinetics of insulator and semiconductor clusters under electrostatic eld. [22][23][24][25] The variations of cluster geometries, HOMO-LUMO gaps, and fragmentation paths with external eld were studied with density functional theory (DFT) calculations.…”