Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers.

Daniel Ping-Kuen Tsang,Toshinori Matsushima,Chihaya Adachi

doi:10.1038/srep22463

Daniel Ping-Kuen Tsang, Toshinori Matsushima + Show 1 more

Open Access

https://doi.org/10.1038/srep22463

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Journal: Scientific Reports	Publication Date: Mar 1, 2016
Citations: 107	License type: CC BY 4.0

Affiliation: Kyushu University

Abstract

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m2, the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.

Highlights

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage
Green phosphorescent OLEDs (PHOLEDs) have reached very long operational lifetimes of LT50 = 100,000 h, where LTx is defined as the time to reach x% of the initial luminance L0 = 1,000 cd/m2 under constant current operation[4]
We found that inserting an ultrathin Liq interlayer between the emissive layer (EML) and hole-blocking layer (HBL), as shown in Fig. 1, significantly improves the operational lifetime of 4CzIPN-based OLEDs

Summary

Introduction

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We found that inserting an ultrathin Liq interlayer between the EML and HBL, as shown, significantly improves the operational lifetime of 4CzIPN-based OLEDs. By including Liq interlayers with thicknesses of 3 and 2 nm, respectively, at both the EML/HBL and HBL/ETL interfaces, LT90 for an initial luminance of 1,000 cd/m2 was enhanced eight-fold.

Results

Conclusion