Improved efficiency roll-off at high brightness in simplified phosphorescent organic light emitting diodes with a crossfading-host

• Both light emitting hosts are used as transporting layers as well.
• Crossfading-host leads to better performance than double-host and co-host.
• The external quantum efficiency reaches 21% at 1000 cd/m2 and 19.3% at 10,000 cd/m2.
• Similar exciton lifetime in gradient-host and co-host means no impact on roll-off.
• Recombination zone is expanded with increasing current density in gradient-host.

We report a high efficiency and low efficiency roll-off green phosphorescent organic light emitting diode using both hole- and electron-transporting host materials in a crossfading profile. To eliminate the energy barrier and reduce the charge carrier accumulation, the host materials are used as transporting layers as well, which also simplifies the device fabrication. It is found out that the recombination zone of gradient doping host sample is not only wider but also extended at high current density, which contributes to the suppressed efficiency roll-off at high luminance. An external quantum efficiency of 21.0% at 1000 cd/m2 is obtained, and maintains to 19.3% at 10,000 cd/m2.

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