Optimized structure of silane-core containing host materials for highly efficient blue TADF OLEDs

Three new derivatives containing silane cores, viz. 9,9',9"-(((4-(pyridin-3-yl) phenyl) silanetriyl) tris(benzene4,1- diyl)) tris(9H-carbazole) (SiCz3Py1), bis(4-(9H-carbazol-9-yl) phenyl) bis(4-(pyridin-3-yl) phenyl) silane (SiCz2Py2), and 9-(4-(tris(4-(pyridin-3-yl) phenyl) silyl) phenyl)-9H-carbazole (SiCz1Py3), were designed and synthesized. Carbazole as a donor and pyridine as an acceptor were tethered to tetraphenylsilane at different mole ratios. All three host materials showed high glass transition temperatures between 118 and 164 degrees C, which are different from those of the previous silane-based host materials (e.g., diphenyldio- tolylsilane (UGH-1), 1,4-bis(triphenylsilyl) benzene (UGH-2), and 1,3-bis(triphenylsilyl) benzene (UGH-3)). The triplet energies of these three hosts are observed at 2.85-2.90 eV, which is high enough for them to act as blue host materials in thermally activated delayed fluorescence organic light emitting diodes (TADF OLEDs). In particular, SiCz2Py2 and SiCz1Py3 hosted-TADF OLEDs demonstrated excellent performances, with a maximum external quantum efficiency (EQE(max)) of 18.7 and 18.8%, respectively. Such good performances of SiCz2Py2 and SiCz1Py3 are originated by suppressing the non-radiative triplet decay and high reverse intersystem crossing (RISC) rate constant for efficient triplet to singlet up-conversion. This work demonstrates that tetraphenylsilane is a promising non-conjugate (i.e., sp3-hybridized) linkage core for developing a variety of high T-g host materials, particularly for blue TADF OLEDs.