Ultrahigh energy gap hosts in deep blue organic electrophosphorescent devices

Four ultrahigh energy gap organosilicon compounds [diphenyldi(o-tolyl) silane (UGH1), p-bis(triphenylsilyl)benzene (UGH2), m-bis(triphenylsilyl)benzene (UGH3), and 9,9′-spiro-bisilaanthracene (UGH4)] were employed as host materials in the emissive layer of electrophosphorescent organic light-emitting diodes (OLEDs). The high singlet (∼4.5 eV) and triplet (∼3.5 eV) energies associated with these materials effectively suppress both the electron and energy transfer quenching pathways between the emissive dopant and the host material, leading to deep blue phosphorescent devices with high (∼10%) external quantum efficiencies. Furthermore, by direct charge injection from the adjacent hole and electron transport layers onto the phosphor doped into the UGH matrix, exciton formation occurs directly on the dopant, thereby eliminating exchange energy losses characteristic of guest-host energy transfer. We discuss the material design, and present device data for OLEDs employing UGHs. Among the four host materials, UGH2 and UGH3 have higher quantum efficiencies than UGH1 when used in OLEDs. Rapid device degradation was observed for the UGH4-based device due to electro- and/or photooxidation of the diphenylmethane moiety in UGH4. In addition to showing that UGH materials can be used to fabricate efficient blue OLEDs, we demonstrate that very high device efficiencies can be achieved in structures where the dopant transports both charge and excitons.