High efficiency phosphorescent white organic light-emitting diodes with low efficiency roll-off achieved by strategic exciton management based on simple ultrathin emitting layer structures

White organic light-emitting diodes (WOLEDs) with ultrathin emitting layer (UEML) structures have vast potential in applications due to highly simplified processing. However, the efficiency and efficiency roll-off at high luminance require further improvement. In this paper, we successfully fabricated high efficiency and low roll-off phosphorescent WOLEDs by strategically controlling the location of red, green and blue UEMLs. The probability of exciton recombination was significantly enhanced, along with greatly suppressed exciton annihilation. The resulting WOLEDs exhibited a maximum external quantum efficiency (EQE) of 20.3%, a current efficiency (CE) of 44.2 cd A^-1, and a power efficiency (PE) of 39.0 lm W^-1, and remained as high as 18.8%, 41.9 cd A^-1, and 28.6 lm W^-1 at a luminance of 5000 cd m^-2, respectively. Additionally, the devices showed superior warm white emission with a small variation in the Commission Internationale de l'Eclairage (CIE), from (0.47, 0.43) to (0.43, 0.44) in the luminance range of 1000 cd m^-2 to 30 000 cd m^-2, and the color rendering index (CRI) was as high as 80. This should be among the best results reported so far for WOLEDs based on UEML structures, indicating the great potential of high-performance WOLEDs fabricated using a very simple technology.