Title: Unveiling the Role of Dopant Polarity on the Recombination, and Performance of Organic Light-Emitting Diodes,
Abstract: The recombination of charges is an important process in organic photonic
devices because the process influences the device characteristics such as the
driving voltage, efficiency and lifetime. By combining the dipole trap theory
with the drift-diffusion model, we report that the stationary dipole moment
({\mu}0) of the dopant is a major factor determining the recombination
mechanism in the dye-doped organic light emitting diodes when the trap depth
({\Delta}Et) is larger than 0.3 eV where any de-trapping effect becomes
negligible. Dopants with large {\mu}0 (e.g., homoleptic Ir(III) dyes) induce
large charge trapping on them, resulting in high driving voltage and
trap-assisted-recombination dominated emission. On the other hand, dyes with
small {\mu}0 (e.g., heteroleptic Ir(III) dyes) show much less trapping on them
no matter what {\Delta}Et is, leading to lower driving voltage, higher
efficiencies and Langevin recombination dominated emission characteristics.
This finding will be useful in any organic photonic devices where trapping and
recombination sites play key roles.