Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation

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Abstract
The authors investigate phosphorescent organic light emitting diodes comprising mixed films of fac tris(2-phenylpyridine) iridium dispersed in 4 , 4 ′ , 4 ′ -tris( N -carbazolyl)-triphenylamine (TCTA) as emission layer (EML). Based on the results of photoluminescence experiments, they intermit the EML with thin neat layers of TCTA acting as an exciton blocking layer inside the EML which suppresses triplet-triplet annihilation. They show that this EML structure leads to an improved roll-off behavior: Starting at the initial external quantum efficiencies (IEQE) of 15.8% and 14.4% at low brightness for the reference and interlayer device, respectively, those structures yield critical current densities j c of 140 mA ∕ cm 2 and 270 mA ∕ cm 2 defining the current density of half-value IEQE.