Direct Monitoring of Recombination Zone in Highly Efficiently Phosphorescent Organic Light-Emitting Diodes Based on High-Doping Concentration Emitting System

Four phosphorescent organic light-emitting diodes (PhOLEDs) (D1, D2, D3 and D4) employing two phosphorescent complexes (ppy)2Ir(acac) and (ppy)2Ir(dipba) doped in a common host material CBP with the low (7 wt%) and high (25 wt%) doping concentration respectively as the emitting layers (EMLs), were fabricated and characterized. To confirm the position of the recombination zones in these four devices and thus explain the different dependence of electroluminescence (EL) performance on two dopant phosphors’ doping concentration, a very thin sensing layer (2 nm) was inserted into the different location of the respective EMLs in D1-D4. The corresponding results showed that the similar EL performance between D1 and D3 are attributed to almost same recombination zone in both devices, where the EL processes are indeed dominated by CBP host because of their low doing concentration (7 wt%). For two high-concentration (25 wt%) devices, the remarkably higher efficiency/luminance levels of (ppy)2Ir(dipba)-based device (D4) than that of (ppy)2Ir(acac)-based case (D2) is attributed to the desirable bipolar charge-transporting property of (ppy)2Ir(dipba). The high-content bipolar phosphor can dominate the EL process and the recombination zone is almost broadened as wide as the whole EML in D4, instead of just distributed in the narrow area near the EML’s edge like that in D2.