Abstract
A simple way of tuning the emission color and electroluminescence efficiency from vacuum-deposited emitters of phosphorescent organic light emitting diodes (LEDs) is demonstrated. For each color a single-emissive layer consisting of a blend of two materials, one of Pt(N∧C∧N) complex series [where (N∧C∧N)=di(2-pyridinyl)benzene-based tridentate ligands] as either the low-concentration bluish green (molecular) phosphorescence emitter or high-concentration red (excimer) phosphorescence emitter, and (4,4′,4″-tris(N-carbazolyl-triphenylamine) as the host was employed. By adjusting the relative amount of blue and red emissive species, the color of the light emission was tuned from bluish green through green and white up to red. Very high external quantum efficiency (up to 18.3±0.5%) and current efficiency (up to 44.8±0.5 cd/A) at ∼500 cd/m2 four-layer devices were achieved with white and greenish light emitting layers, respectively. It is found that the introduction of electron-withdrawing fluorine atoms at the central ring and electron-donating groups at the lateral rings of the Pt complex leads to a blueshift in the molecular and excimer emissions, respectively. This allows to refine colors and optimize the efficiency of the LEDs by selecting suitable substituents.
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