Spiro-configured bipolar hosts incorporating 4,5-diazafluroene as the electron transport moiety for highly efficient red and green phosphorescent OLEDs

Abstract

We designed and synthesized a series of spiro-configured bipolar hosts (CSC, DSD, TST, and NSN) comprising electron-transporting 4,5-diazafluorene and hole-transporting carbazole or diarylamine with thorough characterizations of their thermal, photophysical, electrochemical properties, and carrier mobilities. Time-of-flight (TOF) measurements and single-carrier devices revealed these compounds had good hole and electron mobilities (10⁻⁵ − 10⁻⁷ cm² V⁻¹ s⁻¹) together with promising physical properties, rendering them good host materials for efficient electrophosphorescence. The green PhOLED with (PPy)₂Ir(acac) as the emitter incorporating CSC as the host exhibited a remarkable high brightness of 260 000 cd m⁻² at 2400 mA cm⁻² (11 V) with the maximum external quantum efficiency (η_ext) of 17.1%, the η_ext remained as high as 15.4% under a high brightness of 10 000 cd m⁻² at a low operating voltage of 5.1 V even without p–i–n device configuration. The NSN-based PhOLED with the red dopant Os(bpftz)₂(PPhMe₂)₂ has a maximum brightness of 77 900 cd m⁻² at 1100 mA cm⁻² (10 V) with the maximum η_ext of 19.5%. The η_ext remained as high as 19.1% under a practical brightness of 1000 cd m⁻² (4.6 V). These results suggest that the spiro-configured D–A hosts are well-suited for achieving highly efficient PhOLED with limited efficiencies roll-off.