Recent progress on organic light-emitting diodes with phosphorescent ultrathin (<1nm) light-emitting layers

In recent years, phosphorescent dyes forming ultrathin light-emitting layers (<1 nm, UEMLs) have been widely applied to fabricate monochromatic and white organic light-emitting diodes (OLEDs) owing to its merits of simplified device structure and preparation process, more flexible design, lower material consumption, and complete exciton utilization. In addition, it was demonstrated that the OLEDs with UEMLs achieved high electroluminescence performance comparable to the conventional doping-based devices. Structurally, OLEDs were structured with phosphorescent UEMLs inserted into nonluminous materials, heterojunction interface as well as into luminescent materials including phosphorescent, conventional fluorescent, thermally activated delayed fluorescence, and exciplex emitters. We carefully reviewed the successful applications of UEMLs in OLEDs and underlying working mechanism of corresponding devices, and also emphasized the representative achievements about OLEDs with UEMLs, aimed at forming a comprehensive summary of the present research for UEMLs-based OLEDs. In the end, we also gave an outlook for the future development of UEMLs-based OLEDs.