Improving lifetime of phosphorescent organic light-emitting diodes by using a non-conjugated hybrid host

Three hybrid host materials, mCP-PhSiPh3, mCP-L-PhSiPh3, and tBu-mCP-L-PhSiPh3, have been synthesized and characterized for their thermal, morphological, electrochemical, fluorescence, phosphorescence, and electroluminescence properties. The flexible spacer in mCP-L-PhSiPh3 and tBu-mCP-L-PhSiPh3 makes them maintain high triplet energies at about 3.00 eV due to the interruption of π-conjugation. The introduction of the tert-butyl group at the electrochemically active C3 and C6 sites of the carbazole moiety greatly improves thermal and electrochemical stability of the host tBu-mCP-L-PhSiPh3. Blue and green phosphorescent organic light-emitting diodes (PhOLEDs) based on the non-conjugated hosts exhibit external quantum efficiencies at around 15 and 21%, respectively. Higher current densities in PhOLEDs hosted by mCP-PhSiPh3 produce current leakage, resulting in lower efficiencies, especially in green PhOLEDs. The green PhOLEDs using non-conjugated hosts show improved device lifetime in comparison with the mCP-PhSiPh3 based PhOLED because of enhanced morphological stability. The operational stability of the tBu-mCP-L-PhSiPh3 based device is further raised because of improved electrochemical stability by employing the tert-butyl group. Our results demonstrate that both the flexible linkage and the blocking of the electrochemically active sites are crucial measures to realize long-lived PhOLED devices.