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Addressing the Efficiency Roll-off in a Fluorescent OLED by Facile Electron Transport Layer Doping and Carrier Confinement

Fig. 1. Device structures used in this study


•The combination of the optimized ETL doping and a EBL effectively controls the efficiency roll-off in a fluorescent OLED.
•A judicious choice of EBL introduces a mobility barrier for holes at the EML/EBL interface, increasing the charge balance.
•Hole mobility of the EBL increasing with the electric field could be the reason for the improved performance at high fields.
Organic light emitting diodes (OLEDs) often face the issue of decreasing power efficiency with increasing brightness. Loss of charge carrier balance is one of the factors contributing to the efficiency roll-off. We demonstrate that by using a combination of doped electron transport layer (ETL) and a specially chosen electron blocking layer (EBL) having high hole mobility, this efficiency roll-off can be effectively suppressed. A tris-(8-hydroxyquinoline) aluminium (Alq3) based OLED has been fabricated with 2,3,6,7-Tetrahydro-1,1,7,7,-tetramethyl-1H, 5H,11H-10-(2-benzothiazolyl) quinolizino-[9,9a, 1n gh]coumarin (C545T) as the emissive dopant. Bulk doping of the ETL with lithium fluoride (LiF) was optimized to increase the luminous intensity as well as the current efficiency. An EBL with high hole mobility introduced between the EML and the hole transport layer (HTL) improved the performance drastically, and the device brightness at 9?V got improved by a factor of 2.5 compared to that of the control device. While increasing the brightness from 100?cd/m2 to 1000?cd/m2, the power efficiency drop was 47% for the control device whereas only a drop of 15% was observed for the modified device. The possible mechanisms for the enhanced performance are discussed.