Improvement of charge balance, recombination zone confinement, and low efficiency roll-off in green phosphorescent OLEDs by altering electron transport layer thickness

Recombination zone (RZ) confinement and charge balance are the most important factors for realizing the enhanced efficiency in phosphorescent organic light emitting devices (Ph-OLEDs). Here, we demonstrated the RZ movement and improved Ph-OLED efficiency by varying the electron transport layer 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) thickness. The thickness increment of TPBi not only control the electron transport on to the emission layer but also shifting the RZ towards cathode. The RZ movement in Ph-OLED with different thick TPBi is extracted from the exciplex peak generated at exciton blocking layer/emission layer (EML) interface. The optimized electron to hole ratio at EML have occurred with 40 nm TPBi in which the Ph-OLED exhibits superior current efficiency of 80 cd A⁻¹. The Ph-OLED efficiency roll-off is also very low in 40 nm TPBi based devices. This efficiency roll-off and RZ shift were simply estimated by interface exciplex peak in electroluminescence intensity without extra sensing layers. Further, the electron only devices made with various TPBi thickness also demonstrated the possible changes in the electron transport and supports the RZ and charge balance tactics. (paper)