Unveiling the Role of Dopant Polarity on the Recombination, and Performance of Organic Light-Emitting Diodes

TL;DR

This study reveals how the polarity of dopants influences charge recombination mechanisms and device performance in organic LEDs, highlighting the importance of dipole moments in charge trapping and emission efficiency.

Contribution

The paper introduces a combined dipole trap theory and drift-diffusion model to explain how dopant dipole moments affect recombination and device efficiency in organic LEDs.

Findings

Large dipole moment dopants cause high trapping and voltage.

Small dipole moment dopants lead to lower voltage and higher efficiency.

Recombination mechanisms depend on dopant dipole moments and trap depth.

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.