Deep-red excimer emission from Ir doped organic light-emitting devices

Abstract

Here we report on the concentration-dependent photoluminescence and electroluminescence emission from blends of a conjugated polymer and an Ir(III) organic complex. Energy transfer from the polymer host to the phosphor molecule leads to emission of the latter falling into two pools: the characteristic single molecule emission and a low-energy vibronically unstructured emission band. We provide complementary photophysical and device characterization of the low-energy emissive state. The unchanged absorption spectrum upon phosphor concentration changes, the emission enhancement of the low-energy band upon doping (observed both under electrical or photo-excitation), together with a decrease of its relative electroluminescence emission as the temperature is lowered, and, the strikingly different photoluminescent dynamics associated with both emission bands suggest the formation of excimers at high doping ratios. Owing to the non-planar coordination structure of Ir(III) complexes we conjecture that these excimer emissions must arise from π–π stacking of nearly parallel rings of the cyclometallated ligands. We highlight the significance of this low-energy emission in the development of deep-red electroluminescent devices as inferred from Commission International de l'Eclairage coordinates equal to (0.66; 0.34).