Highly efficient blue-green organic light-emitting diodes achieved by controlling the anionic migration of cationic iridium(iii) complexes

Abstract

We designed and synthesized a series of cationic iridium(III) complexes with the same coordinated iridium(III) cation but different-sized counter-ions, investigated their photophysical properties, electrochemical behaviours and thermal stability, then fabricated single-layer solution-processed organic light-emitting diodes (OLEDs) thereof, and demonstrated anionic migration in devices. By doping these cationic iridium(III) complexes at low concentrations (2 or 3 wt%) thus avoiding their anionic migration, we succeeded in the preparation of efficient blue-green OLEDs, achieving the highest current efficiency of 17.1 cd A⁻¹, an external quantum efficiency of 6.8%, a maximum luminance of 14.2 × 10³ cd m⁻² and colour coordinates of (0.21, 0.48). To our knowledge, these values are among the best reported OLEDs based on ionic transition metal complexes as phosphorescent emitters in the blue-green region. The impact of different-sized counter-ions on carrier transport characteristics was also examined by single carrier devices. Notably, by doping these cationic iridium(III) complexes at high concentrations (20 wt%) and thus wisely employing their anionic migration, we developed simple-constructed OLEDs with a temporary p–i–n junction instead of active n-type dopants in the cathode and obtained a high current efficiency of 8.3 cd A⁻¹ and a maximum luminance of 12.8 × 10³ cd m⁻², rather comparable to the corresponding OLEDs with the conventional Cs₂CO₃ active cathode.