Solution-processable highly efficient yellow- and red-emitting phosphorescent organic light emitting devices from a small molecule bipolar host and iridium complexes

A bipolar transport compound, 2,5-bis(4-(9-(2-ethylhexyl)-9H-carbazol-3-yl) phenyl)-1,3,4-oxadiazole (CzOXD), incorporating both electron- and hole-transport functionalities, was synthesized and fully characterized by ¹H NMR, ¹³C NMR, elemental analysis and mass spectrometry. Its thermal, electrochemical, electronic absorption and photoluminescent properties were studied. The atomic force microscopy images indicate that smooth and homogeneous films can be obtained by spin-coating from a chloroform solution of CzOXD/iridium complex blends. Highly efficient small-molecule-based organic light emitting devices were fabricated by a wet process. With the device structure of ITO/PEDOT:PSS/Ir complex: CzOXD/BCP/Alq₃/LiF/Al, a maximum luminance of 15232 cd m^-2 and current efficiency of 20 cd A^-1 for a yellow-emitting OLED, and 4896 cd m^-2, 4.6 cd A^-1 for a red-emitting OLED were achieved under ambient conditions.