Plasmonic Purcell effect reveals obliquely ordered phosphorescent emitters in Organic LEDs

Area of Science:

• Optoelectronics
• Materials Science
• Physical Chemistry

Background:

• Non-isotropic molecular alignment enhances light-matter interactions in emissive and absorptive systems.
• This principle is crucial for improving the efficiency of organic optoelectronic devices, including organic light-emitting diodes (OLEDs).
• Existing methods for determining molecular orientation in phosphorescent emitters typically yield only average values due to reliance on second-moment emission patterns.

Purpose of the Study:

• To develop a method for resolving finer details of molecular orientation distributions beyond simple averages.
• To introduce and analyze polarization-dependent emission characteristics to differentiate between parallel and perpendicular molecular orientations.
• To measure the transient emission decay lifetimes of specifically oriented molecular ensembles within an OLED.

Main Methods:

• Incorporation of a thin metal layer near phosphorescent emitters to induce plasmon-mediated losses, primarily affecting perpendicular emitters.
• Analysis of emission at different polarizations to measure distinct emission lifetimes for parallel and perpendicular molecular orientations.
• Observation of temporal phosphorescence decay under varying directions and polarizations to detect alterations caused by lifetime splitting.

Main Results:

• The study successfully measured differential emission lifetimes for parallel and perpendicularly oriented emitters.
• The degree of lifetime splitting allowed for the derivation of the angular width of the molecular orientation distribution.
• Results indicate a narrow, obliquely oriented molecular ensemble of Ir(MDQ)₂(acac) within an α-NPD host in an OLED stack.

Conclusions:

• The developed optical analysis method, utilizing plasmonic effects and polarization-dependent lifetime measurements, provides a more detailed understanding of molecular orientation.
• This technique enables the quantification of the angular width of molecular orientation distributions, moving beyond average values.
• The findings demonstrate a specific, non-uniform molecular alignment in the investigated OLED, offering insights for device optimization.