High efficiency warm white organic light-emitting diodes with precise confinement of charge carriers and excitons in the exciplex host system†
As one of the green and healthy illumination sources, white organic light-emitting diodes (WOLEDs) have attracted substantial attention in the application of solid-state lighting owing to their promising features. Under conditions of color rendering index (CRI) higher than 80, it is still a major challenge for WOLEDs to simultaneously realize high efficiency, color stability and low efficiency roll-off. In this work, we successfully fabricated high-performance warm WOLEDs, showing not only high efficiency and low efficiency roll-off, but also high CRI and stable spectrum emission. The turn-on voltage is as low as 2.4 V, and the maximum forward-viewing external quantum efficiency, power efficiency and current efficiency reach 22.9%, 89.0 lm W−1 and 68.0 cd A−1, and still remain at 21.7%, 41.5 lm W−1 and 51.1 cd A−1 at 1000 cd m−2, respectively. Furthermore, the CRI is also over 80 with stable Commission International de I’Eclairage coordinates ranging from (0.448, 0.436) to (0.436, 0.421) as the luminance increases from 1000 to 10 000 cd m−2. In our design, the blue, yellow and red phosphors are doped in an exciplex host consisting of an electron-transporting material (ETM) and a hole-transporting material (HTM) by blending, and the blue/yellow/red sequence three-emissive-layer structure is constructed. It can be seen that the charge carrier transport balance and effective confinement of excitons within emissive layers are well-achieved by precisely manipulating the ratio between the HTM and ETM; therefore, the performance of the resulting WOLEDs has been improved comprehensively.