Structure effect of carbazole-oxadiazole based organic small molecule hosts on the solution-processed phosphorescent OLEDs performance

Abstract

A group of solution-processed bipolar small molecular host materials, namely 1,4-bis(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (p-BCOB), 1,3-bis(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (m-BCOB) and 1,3,5-tris(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (TCOB) were synthesized by means of connecting carbazole-oxadiazole substituents on the para-, meta- and 1,3,5-meta- positions of benzene ring respectively. The influence of different linking positions of carbazole-oxadiazole substituents on the thermal, photophysical, electrochemical and charge transport properties of the compounds is comprehensively studied. For verifying the host configuration effect on the performance of phosphorescent organic light-emitting diodes (PHOLEDs), solution-processed green and red PHOLEDs with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(1-phenylisoquinolinato)(acetylacetonate) iridium (Ir(piq)2acac) as phosphorescent dopants were fabricated. The m-BCOB:Ir(ppy)3 and TOCB:Ir(ppy)3 PHOLEDs show excellent device performance with the maximum luminance efficiencies of 15.0cdA−1 and 13.7cdA−1, respectively, which are increased by 9.8 and 9.0 times compared with the p-BCOB:Ir(ppy)3 PHOLED because of effective Förster and Dexter energy transfer. However, the p-BCOB:Ir(piq)2acac PHOLED is the best among three hosts with a maximum external quantum efficiency of 7.9% due to the effective charge trapping. This work demonstrates that different host molecular structures have a big effect on the PHOLEDs performance.