Highly efficient deep-red organic electrophosphorescent devices with excellent operational stability using bis(indoloquinoxalinyl) derivatives as the host materials

Abstract

Three new bis(indoloquinoxalinyl) derivatives, BIQF (6,6′-(4,4′-(9H-fluorene-9,9-diyl)bis(4,1-phenylene))bis(6H-indolo[2,3-b]quinoxaline)), BIQTP (9,9′-(1,1′:4′,1′′-terphenyl-4,4′′-diyl)bis(6H-indolo[2,3-b]quinoxaline), and BIQMCz (6,6′-(9-p-tolyl-9H-carbazole-3,6-diyl)bis(6H-indolo[2,3-b]quinoxaline)) linked by the fluorenyl, terphenyl and carbazolyl groups, respectively, were synthesized, characterized and used as the hosts for deep-red phosphorescent organic light-emitting diodes (PhOLEDs). The central linkers provide a way to reach higher molecular weights with little change of the energy gap of the molecules relative to the parent indoloquinoxaline molecule. These materials are bipolar and show high glass transition temperatures ∼200 °C and triplet energy gaps 2.25–2.37 eV. The electroluminescent (EL) devices based on these host materials and iridium deep-red emitters exhibit very high device efficiency with a EQE_max mostly above 20%. For example, the device utilizing (tmq)₂Ir(acac) as the dopant and BIQMCz as the host shows a maximum current efficiency of 32.4 cd A⁻¹ and a maximum power efficiency of 30.9 lm W⁻¹. The operational lifetimes (T₅₀) of these devices were also measured. The results show that the BIQMCz/Ir(piq)₃-based device gave T₅₀ of more than 20 000 h at an initial luminance of 500 cd m⁻² with a current efficiency (CE) of 23.2 cd A⁻¹ and Commission Internationale de l'Éclairage (CIE) coordinates of (0.67, 0.33).