High-Efficiency Nondoped Blue Organic Light-Emitting Devices with Reduced Efficiency Roll-Off

Nondoped blue organic light-emitting devices with a structure of ITO/hole transporting layer (HTL)/2-diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′- spirobifluorene/electronic transporting layer (ETL)/LiF/Al are fabricated. The performances of the devices are dependent on the charge mobility, charge injection, and energy level characteristics of HTL and ETL. The device with 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine and 4,7-diphenyl-1,10- phenanthroline as HTL and ETL shows high efficiency and brightness at low voltage (5.8 cd/A and 1000 cd/m 2 at 4.6 V). Furthermore, the device shows a slight efficiency roll-off of 16% from the brightness at maximum current efficiency to 10 000 cd/m 2 . We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device.