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Understanding charge transport in Ir(ppy)3:CBP OLED films
Stephen Sanderson1, Bronson Philippa1, George Vamvounis1
1College of Science and Engineering, James Cook University, Townsville, QLD 4811, Australia.
The Journal of Chemical Physics
|March 10, 2019
Summary
Charge transport in organic light-emitting diodes (OLEDs) is complex. This study reveals that iridium(iii) complexes create charge traps, impacting mobility and suggesting optimal device performance at specific concentrations.
Area of Science:
- Materials Science
- Organic Electronics
- Photophysics
Background:
- Iridium(iii) complex (Ir(ppy)3):CBP blends are key emissive layers in organic light-emitting diodes (OLEDs).
- Understanding charge transport mechanisms within these blends is crucial for device optimization.
- Molecular dynamics simulations indicate Ir(ppy)3 molecules form connected pathways at low concentrations (≥5 wt. %).
Purpose of the Study:
- To investigate the influence of guest concentration on charge transport and mobility in Ir(ppy)3:CBP blends.
- To elucidate the role of iridium(iii) complexes in facilitating or hindering long-range charge transport.
- To determine optimal guest concentrations for enhanced OLED performance by analyzing charge trapping effects.
Main Methods:
- Kinetic Monte Carlo (KMC) transport modeling was employed to simulate charge carrier mobility.
- The study analyzed the dependence of charge mobility on the guest (Ir(ppy)3) concentration.
- Simulations considered the impact of electric field strength and charge carrier type (electrons vs. holes).
Main Results:
- Distinct minima in charge mobility were observed around 10 wt. % Ir(ppy)3 concentration.
- These mobility minima are attributed to an increased number of trap states, including interconnected Ir(ppy)3 complexes.
- Charge trapping depths differ for electrons and holes due to variations in ionization potentials and electron affinities, leading to asymmetric transport behavior.
Conclusions:
- Optimal OLED performance is achieved when significant charge trapping occurs on iridium(iii) complex guest molecules.
- Minimizing interactions between emissive chromophores is essential to prevent triplet-triplet annihilation.
- The findings suggest that device efficiency is linked to a balance between charge trapping and minimizing detrimental excited-state interactions.
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