Silicon-Based Blue Phosphorescence Host Materials: Structure and Photophysical Property Relationship on Methyl/Phenylsilanes Adorned with 4-(<i>N</i>-Carbazolyl)phenyl Groups and Optimization of Their Electroluminescence by Peripheral 4-(<i>N</i>-Carbazolyl)phenyl Numbers

Abstract

Wide-band-gap phosphorescent organic light-emitting diode (OLED) host materials were developed employing the tetrahedral structural motif of the silicon atom, which functioned further to connect the 4-(N-carbazolyl)phenyl (CP) units in its periphery (2a−2d). The thermal, photophysical, and electrochemical properties, and X-ray structural studies as well as OLED device characteristics were fully investigated. Silicon effectively disconnected the charge delocalization between the CP units. By low temperature PL study, the triplet energy was measured to be around 3.0 eV for all prepared host materials. The thermal stability correlated well with the number of CP units adorned at the silicon atom, showing a gradual increase in Tg values of 99−214 °C as the number of CP units increased in the 2a−d series. Further structural modification was carried out with the phenylsilane series (3a−c). Hole mobility measurements for the silicon-tuned host materials of the methyl and phenyl series, 2b−c and 3b−c showed mobility values in the range of 2.7−6.5 × 10−4 cm2/(V s). A series of OLED device fabrication was attempted with these two series. The best performance was obtained with 2b exhibiting external quantum and power efficiencies of 12.1% and 13.6 lm/W with CIE x and y coordinates of 0.15 and 0.28, respectively.