Silicon-based electron-transport materials with high thermal stability and triplet energy for efficient phosphorescent OLEDs

Abstract

A series of electron transporting materials was designed and used in organic light-emitting diodes (OLEDs), exhibiting green phosphorescence. We used the tetrahedral structural motif of silicon atom, which annulated with the 1,2-diphenyl-benzoimidazole (DBI) units in its periphery (1–4) and their thermal, photophysical, and electrochemical properties were investigated. Among the series, the X-ray crystal structure of compound 1 was obtained and investigated. Photophysical and electrochemical properties showed that their LUMO levels can be slightly tuned as increasing number of DBI units and enhancing the electron injection capability. Furthermore, thermal stability correlated well with an increase in the number of DBI units, showing a gradual increase in Tg values in the range of 100–141 °C. The electron-only devices (EOD) based on compounds 2 and 3 were fabricated; EOD device with compound 3 showed higher current densities at the same voltages, indicating higher electron transport (ET) capability compared to compound 2. The electron mobilities (μ) of compounds 2 and 3 were estimated as 1.93 × 10−5 cm2/V and 3.67 × 10−5 cm2/V at 1 MV/cm, respectively. We further investigated the excellent ET property of compound 3via the phosphorescent OLEDs in which the electron-transporting material (ETM) was coupled with the green emitter, Ir(ppy)3. Finally, we compared it with the device based on compound 2. The OLEDs device with compound 3 exhibited maximum current and external quantum efficiencies of 62.8 cd/A and 18.0%, respectively, with a small efficiency roll-off at high current densities.