Giant Enhancement of External Quantum Efficiency in Near‐UV Organic Light‐Emitting Diodes via Device Aging and Impedance Spectroscopy Analysis

Abstract

The issue of device durability is scarcely reported for state‐of‐the‐art UV organic light‐emitting diodes (UV OLEDs), but it is of theoretical interest and plays a key role in commercial products. Herein, the effects of aging time on current density and radiance of near‐UV OLEDs are investigated. Devices are fabricated with 9‐(4′‐(4,5‐diphenyl‐4H‐1,2,4‐triazol‐3‐yl)‐(1,1′‐biphenyl)‐4‐yl)‐9H‐carbazole as the fluorescent molecule, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) as the hole injection layer, and LiF as the electron injection layer. It is found that the current density decays much faster than radiance, and accordingly promotes the device external quantum efficiency from 2.5% initially to a giant value of 8.2% after a certain period of aging. Impedance spectroscopy is performed to study the carrier injection/transport capability by configuring a hole‐only cell (HOC) and an electron‐only cell (EOC). The transition curves of the impedance, phase, and capacitance as a function of voltage indicate that carrier injection/transport gradually decreases during aging, which accounts for the reduced current density and radiance. The electroluminescence spectra are independent of the decay process. The results provide a new approach for analyzing durability and advancing applications of UV OLEDs.

Facilely Solution‐Processed Lithium Carbonate for Tailoring Electron Injection in High‐Performance Inverted Near‐UV Organic Light‐Emitting Diodes

“…State‐of‐the‐art UV OLEDs are predominantly focused on normal structure for enhancing device efficiency and radiance. The typical external quantum efficiency (EQE) of UV OLEDs with conventional fluorescent molecules has been enhanced from initial 0.1% [ 8 ] to 4.6% [ 9 ] and soared to 8.2% [ 10 ] via device aging and was recently promoted to 9.3% from thermally activated delayed fluorescent material. [ 11 ] Inverted UV OLEDs are scarcely reported, [ 12,13 ] due to the fact of lacking a strong electron injection layer (EIL), which results in a high electron injection barrier between the commonly used transparent indium tin oxide (ITO) cathode and typical electron transport layer (ETL).…”