Solution processed blue phosphorescent organic light emitting diodes using a Ge-based small molecular host

Abstract

Two kinds of host materials, 4,4′-(diphenylgermanediyl)bis(N,N-diphenylaniline) and bis(4-(9H-carbazol-9-yl)phenyl)diphenylgermane (DCzGe), for blue phosphorescent organic light emitting diodes (PhOLEDs) were designed by incorporating electron donating groups (carbazole and triphenylamine) into tetraphenylgermane, which is a new type of core moiety that has never been studied for use in this field. This molecular structure endows the compounds with a wide energy bandgap, high thermal/morphological stability and good solution processability. Based on the theoretic calculations, DCzGe was selected and synthesized as a host material which demonstrates a wide bandgap (E_g: 3.56 eV) and a high triplet energy (E_T: 3.02 eV). It also exhibits a high glass transition temperature (110 °C), which is beneficial for resisting the Joule heat in devices. All solution processed, blue emitting PhOLEDs were fabricated by using a mixed host combining DCzGe and an electron-transporting material, with a maximum luminance of 10 000 cd m⁻² and a maximum current efficiency of 15.2 cd A⁻¹. Furthermore, the devices showed a very low current efficiency roll-off, which remained as high as 15.2 cd A⁻¹ at the luminance of 1000 cd m⁻², and the roll-off is only 2.6% even at the higher luminance of 2000 cd m⁻².