Efficient donor–acceptor emitter based nonsymmetrical connection for organic emitting diodes with improving exciton utilization

Abstract

A new strategy developed to construct blue emissive materials having an unsymmetrical connection with identical conjugated phenanthrimidazole groups results in the separation of the frontier orbitals and leads to donor–acceptor (D–A) architecture. The blue device with 2-(naphthalen-1-yl)-1-(4-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (p-PPI)/2-(naphthalen-1-yl)-1-(3-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (m-PPI) emissive layer (λ_EL – 434/420 nm) shows high efficiencies: current efficiency (η_c) – 5.83/3.56 cd A⁻¹; power efficiency (η_p) – 5.73/3.48 lm W⁻¹; external quantum efficiency (η_ex) – 8.98/6.48% at 3.0 V. Their cyano derivatives, p-CNPPI/m-CNPPI (λ_EL – 422/406 nm) exhibit maximum efficiencies (η_c – 6.28/4.38 cd A⁻¹; η_p – 6.14/4.01 lm W⁻¹; η_ex – 9.01/6.72%) at 2.9 V compared to the p-PPI/m-PPI devices. The weak charge transfer in the D–A emitters results in deep blue emission. The anisotropic structural characteristics of p-PPI, p-CNPPI, m-PPI and m-CNPPI induced horizontal dipole orientation in films and enhanced EL efficiency. These bipolar materials with suitable triplet energy can be used as hosts in green as well as red phosphorescent organic light emitting devices (PHOLEDs). The green/red device (λ_EL – 504/618 nm) with p-CNPPI: Ir(ppy)₃/Ir(MDQ)₂ (acac) exhibits a maximum L – 8823/38418 cd m⁻²; η_ex – 24.56/17.31%; η_c – 84.30/18.09 cd A ⁻¹; η_p – 86.43/21.43 lm W ⁻¹with CIE (0.33, 0.61)/(0.65, 0.34).