Precise manipulation of the carrier recombination zone: A universal novel device structure for highly efficient monochrome and white phosphorescent organic light-emitting diodes with extremely small efficiency roll-off

Achieving superior device efficiency and very small efficiency roll-off simultaneously for all phosphorescent OLEDs (PHOLEDs) is still an open challenge. In this work, a universal novel device structure, having mixed hosts sandwiched between hole- and electron-transporting hosts, was proposed, and a series of monochrome and white PHOLEDs based on the proposed novel device structure were developed. All the resulting PHOLEDs achieve a maximum external quantum efficiency (EQE) exceeding the theoretical limit, reaching 21.71%, 23.85%, 23.99%, 21.79%, and 23.15% for green, yellow, red, blue, and white PHOLEDs, respectively. Moreover, apart from blue PHOLEDs using inefficient blue phosphor, other monochrome and white PHOLEDs show extremely small efficiency roll-off. At a practical luminance of 5000 cd m^-2, the EQE is still up to 20.34%, 20.95%, 20.07%, and 16.06% for green, yellow, red, and white PHOLEDs, respectively. Such high device performance is testified from the precise manipulation of the carrier recombination zone via a novel device structure, which contributes to a strictly limited and broadened carrier recombination zone, a balanced distribution of electrons and holes, as well as consequentially reduced triplet exciton aggregation and polaron formation, thus effectively boosting the device efficiency and suppressing the notorious triplet-triplet annihilation and triplet-polaron quenching.