Phosphorescent ultrathin emitting layers sensitized by TADF interfacial exciplex enables simple and efficient monochrome/white organic light-emitting diodes

In this work, we proposed a novel scheme enabling simple and efficient monochrome/white organic light-emitting diodes (OLEDs) by simultaneously combining thermally activated delayed fluorescence (TADF) interfacial exciplex and phosphorescent ultrathin emitting layers (Ph-UEMLs). Specifically, monochrome/white OLEDs were structured by directly inserting single or complementary Ph-UEMLs into the interface of TADF interfacial exciplex (m-bis(N-carbazolyl)-benzene (mCP)/2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T)) using simple doping-free technology. Consequently, all monochrome (blue, green, orange and red) OLEDs exhibit a low turn-on voltage of 2.4–2.5 V and realize ultra-high device performance with maximum external quantum efficiency (EQE) exceeding 20%. Therein, the Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic)-based blue OLED achieves the forward-viewing EQE reaching 30.21%, which is among highest values ​reported in literatures. Moreover, the fabricated two-, three- and four-colors white OLEDs also obtain the same excellent performance with a low turn-on voltage of 2.5 V and high EQEs of 23.47, 22.70 and 23.88% (corresponding to 78.80, 76.70 and 70.89 lm/W). All white devices also exhibit extremely high color stability at a practical luminance range from about 5000 to 12000 cd/m2, indicating a huge potential application. It is demonstrated that Ph-UEMLs sensitized by TADF interfacial exciplex ensures a complete exciton utilization by multi-channels energy transfer from TADF interfacial exciplex to Ph-UEMLs, which is responsible for the main reason for achieving high device performance. We believe our proposed scheme paves a new path for developing ultra-simple and high-performance monochrome/white OLEDs.