High-performance blue and green electrophosphorescence achieved by using carbazole-containing bipolar tetraarylsilanes as host materials

A series of carbazole-containing tetraarylsilane compounds, namely p-BISiPCz (1), m-BISiPCz (2), p-OXDSiPCz (3) and m-OXDSiPCz (4) were designed and synthesized by incorporating electron-donating carbazole and electron-accepting benzimidazole or oxadiazole into one molecule via a silicon-bridge linkage mode. Their thermal, photophysical and electrochemical properties can be finely tuned through the different groups and linking topologies. The di-para-position compounds 1 and 3 display higher glass transition temperatures and slightly lower triplet energies than their di-meta-position isomers 2 and 4, respectively. The four compounds exhibit similar HOMO levels (5.60-5.63 eV), while the LUMO level of 3 (2.36 eV) is slightly lower than that of 4 (2.28 eV). The silicon-interrupted conjugation of the electron-donating and electron-accepting segments endows these materials with relative high triplet energies, good thermal and morphological stability, and bipolar transporting ability. For FIrpic-based blue PhOLEDs, the di-meta-position compounds 2 and 4 display better device performances than their di-para-position analogues 1 and 3, respectively. Device B using 2 as the host exhibits the best performance with a maximum current efficiency of 29.3 cd A^-1, a maximum power efficiency of 19.8 lm W^-1, and a maximum external quantum efficiency of 11.4%. Green phosphorescent devices using (ppy)₂Ir(acac) as guest and 1-4 as hosts show excellent EL performances with maximum external quantum efficiencies of 18.3-22.2%. Remarkably, device H hosted by 4 still exhibits an external quantum efficiency of 19.4% at the extremely high luminance of 10000 cd m^-2. These efficiencies are significantly higher than those of blue and green control devices using mCP as host, respectively.