9,9-Diphenyl- thioxanthene derivatives as host materials for highly efficient blue phosphorescent organic light-emitting diodes

A series of 9,9-diphenyl-9H-thioxanthene derivatives with different valence states of sulfur atoms are reported as host materials in blue phosphorescent organic light-emitting diodes. Their photophysical, electrochemical and thermal properties, as well as device performance were thoroughly investigated to study their structure-property relationships, including the different carbazolyl linkage positions and valence states of sulfur atoms. Extremely low turn-on voltages of around 2.6 V for blue electrophosphorescence, which are already corresponding to the value of the emitted photon energy (hv)/electron charge (e), were achieved by utilizing the developed materials as hosts of the blue phosphor dopant iridium(III)bis(4,6-(difluorophenyl)-pyridinato-N,C-2') picolinate (FIrpic). Notably, a maximal power efficiency of 69.7 lm W-1 and an external quantum efficiency of 29.0% were achieved for an optimal device based on m-DCz-S consisting of the bivalent sulfur atom and meta-combined carbazolyl.