Molecular Engineering of Cyano-Substituted Carbazole-Based Host Materials for Simultaneous Achievement of High Efficiency and Long Lifetime in Blue Phosphorescent Organic Light-Emitting Diodes

A series of isomeric high triplet energy host materials named 1CNCzBN, 2CNCzBN, 3CNCzBN, and 4CNCzBN containing a cyanocarbazole and a benzonitrile for blue phosphorescent organic light-emitting diodes (PhOLEDs) is presented. The effect of the substitution position of the cyano unit in the carbazole core on physicochemical and electroluminescence characteristics is described. All the compounds show high triplet energy (E_T) of above 2.85 eV and the 3CNCzBN compound containing the cyano unit at C3 position of carbazole exhibits superior E_T of 3.04 eV. Except for 1CNCzBN, all the compounds reveal good electron-transporting properties, evidenced by the single carrier device analysis. Furthermore, the applicability of these compounds as electron-transport-type host materials for blue PhOLEDs is tested by employing a blue phosphor and the blue PhOLEDs exhibit high EQE over 20%. Among the compounds, the 3CNCzBN-based device exhibits excellent performance with maximum external quantum efficiency (EQE) of 25.6% and current efficiency (CE) of 52.1 cd A⁻¹. Importantly, all the compounds reveal excellent operational stability with long lifetime and the 3CNCzBN device displays superior lifetime of 20 h up to 70% of its initial luminance (L₀ = 1000 cd m⁻²). It is believed that this study will provide fundamental insights to develop highly efficient cyano featured carbazole-based host materials for blue PhOLEDs with superior EQE and long lifetime.