The effect of the exciplex heterojunction interlayer on efficiency roll-off in non-doped blue phosphorescent organic light-emitting diodes

High exciton density will cause triplet-triplet annihilation (TTA)/triplet-polaron annihilation (TPA) and result in low efficiency of blue phosphorescent organic light-emitting diodes (OLEDs) at high voltage. The carriers and excitons can be effectively localized through the interfacial effect generated by the energy level structure of heterojunction interlayer , which can extend the recombination zone (RZ) and disperse excitons in the emitting layer (EML). In order to enhance the local effect on excitons, we introduce exciplex material as the spacer layer (SPL) in heterojunction interlayer. When the thickness of SPL is 3 nm, device C1 with a exciplex SPL (TCTA:TPBi) obtains the optimal efficiency roll-off and the maximum luminance of 4434 cd/m2. The main reason can be attributed to the fact that the triplet energy level of SPL material in device C1 is lower than that in device C3 with a exciplex SPL (mCP:TPBi) as shown in the photoluminescence (PL) spectra of exciplexes, and device C1 has more balanced carrier transport than device C2 with a exciplex SPL (TAPC:TPBi). The optimization of SPL material and thickness can effectively improve the efficiency of OLEDs by localizing excitons while dispersing the exciton density and reducing efficiency roll-down.