Interfacial Charge Control for Stable Red Phosphorescent OLEDs

ABSTRACT Here, a composition‐tuned mixed hole‐blocking layer (HBL) is constructed by co‐evaporating high‐mobility ANT‐BIZ with deep‐HOMO B4PyMPM, and the resulting composition series is used to distinguish interfacial charge storage from static energy‐level alignment. Peak external quantum efficiency ( EQE ) increases monotonically with ANT‐BIZ content, whereas the measured LT 95 reaches a maximum at an intermediate composition, showing that the most efficient blend is not the most stable. The 75% ANT‐BIZ blend gives a higher peak EQE but a 17% shorter measured LT 95 than the 50% blend, despite their nearly identical frontier orbital energies. 50% blend shows a smaller forward‐bias capacitance drop (ΔC) and a shorter slow decay time (τ slow ) in transient electroluminescence (TREL). Its driving voltage also drifts less during aging. The optimized device delivers EQEs of 30.0% at 1000 cd m −2 and 25.3% at 10,000 cd m −2 , with a turn‐on voltage of 2.25 V. At 50 mA cm −2 , its measured LT 95 reaches 205 h, compared with 17 h for the neat B4PyMPM HBL device. In this blend series, reduced charge accumulation at the electronic side interface is associated with a longer operational life.