Performance and defects in phosphorescent organic light-emitting diodes

Phosphorescent heavy metal complexes can utilize both singlet and triplet excitons and thus are interesting for doping polymer to obtain highly efficient organic light-emitting diodes. In this study, we have investigated devices using a new phosphorescent–metal complex containing fluorene and platinum added to a luminescent polymer blend, composed of 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-(1,3,4-oxadiazole) (PBD) and poly(9-vinylcarbazole) (PVK). The performance of devices (luminance and yield) is measured in indium tin oxide (ITO)/poly(3-4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/(PVK–PBD-complex)/Al diodes. The devices emit an orange light with a brightness of 607 cd/m2 and an external quantum efficiency of 0.28 cd/A at 25 V. In order to investigate the structural modifications of the polymer by the incorporation of phosphorescent–metal complex, we have studied the defect states in diodes by charge-based Deep Level Transient Spectroscopy (Q-DLTS). Analysis of Q-DLTS spectra obtained in undoped and doped devices, revealed at least three trap levels distributed in the range 0.2–0.5 eV within the band gap of the hybrid composite with trap density in the range around 1016 cm−3. Incorporation of Pt complex into the polymer blend modified the trap states by reducing the density of traps in the blend and by creating new trap levels in the band gap.

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