Controlled host mixture and doping profile for ideal electrophosphorescent devices

• Host mixing profile was optimized for efficient electrophosphorescent device.
• Charge carrier mobility, energy level offset, and energy transfer were characterized.
• Narrow band-gap host mixture showed improved efficiency and lower driving voltage.
• Ideal mixed host was designed at confined layer mixing; TCTA/TCTA:Bepp2 mixture/Bepp2.

We report the high-efficiency and low-driving-voltage behavior of green and red phosphorescent devices with mixed and layered light emitting host profile. The pair of hosts, which are predominant for either hole- or electron-transporting, was selected based on the result of charge carrier mobility and initial screening test for device performance. Devices with hole- and electron-transporting mixed host profiles with narrower band-gap materials showed lower driving voltage behavior as well as improved efficiency, where the charge-trapping issue due to host-dopant energy level offset is not significant and efficient Förster energy transfer is satisfied. For the electron transporting host materials, either for green or red phosphorescence, beryllium complexes were efficient both for efficiency at high current (brightness) and low driving voltage. The uniform mixing of host was compared with the layered double host structure, where the thickness of hole- and electron-transporting host was varied. Precisely defined heterostructured mixed layer profile, composed of hole-transporting/mixed/electron-transporting host (three layer), showed further improvement of luminous efficiency at high brightness maintaining driving voltage as low as that of simple mixed host device.

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