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

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. The effects of aging time on current density, radiance, and external quantum efficiency (EQE) of near‐UV organic light‐emitting diodes (UV OLEDs) are demonstrated and analyzed using impedance spectroscopy. The EQE enhances continuously from an initial 2.5% on the first day to a giant value of 8.2% on the fourth day, and then decreases till device failure.