Construction of thermally stable 3,6-disubstituted spiro-fluorene derivatives as host materials for blue phosphorescent organic light-emitting diodes

Abstract

Four novel thermally stable host materials, 9,9'-(9,9-diphenyl-9H-fluorene-3,6-diyl)bis(9H-carbazole) (BBDC), 3-(3,6-bis(diphenylphosphoryl)-9,9-diphenylfluorene (BBDP), 3,6-di(9H-carbazol-9-yl)-9,9'-spirobi[fluorene] (BSBDC), 3,6-di(9H-carbazol-9-yl)-9,9'-spirobi[fluorene] (BSBDP) were designed and synthesized by linking phosphine oxide or carbazole moieties to the 3,6-position of a spirofluorene core. In comparison to the conventional 2,7-disubstituted homologues, 3,6-disubstituted spiro-fluorene can offer more versatile and advanced properties in terms of higher triplet energy (>2.8 eV) and higher glass-transition temperatures (ca. 215 °C). Accordingly, doped blue-emitting devices were fabricated and the device with BSBDP as host exhibited a very low turn-on voltage of 2.8 V and maximum current efficiency of 34.2 cd A−1, external quantum efficiency of 18.7% and power efficiency of 34.4 lm W−1. These results demonstrate that 3,6-disubstituted spiro-fluorene derivatives are promising host materials for blue phosphorescent organic light-emitting devices (PhOLEDs).