Increasing electron transporting properties and horizontal molecular orientation via meta-position of nitrogen for “(A)n–D–(A)n” structured terpyridine electron-transporting material

Abstract

The electron-transporting materials (ETMs) with excellent electron injection (EI) and electron transporting properties are prerequisites for highly efficient organic light-emitting diodes (OLEDs). In this work, we report a novel ETM, 2,7-di([3,2′:6′,3′′-terpyridin]-4′-yl)-9,9′-spirobifluorene (27-mTPSF), which is synthesized by combining electron-withdrawing terpyridine (TPY) moieties with rigid twisted spirobifluorene. This rigid twisted structure helps to maintain the morphological stability of the amorphous film and contributes to the enhancement of the device lifetime. The nitrogen atom at the meta-position on the peripheral pyridine in 27-mTPSF can enhance the horizontal molecular orientation and the electron-transporting property. A green phosphorescent OLED (PhOLED) based on tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)₃) as the emitter and 27-mTPSF as ETM displayed a maximum external quantum efficiency (EQE) of 23.1%, and a half-life (T₅₀) of 77, 4330 and 243 495 h at an initial luminance of 10 000, 1000 and 100 cd m⁻², respectively, which are significantly superior to those of the device based on the conventional ETM 1,3,5-tris(N-phenylbenzimid azol-2-yl-benzene (TPBi). These results indicate a potential application for the “(A)_n–D–(A)_n” structured terpyridine ETMs.