Higher order effects in organic LEDs with sub-bandgap turn-on.

Area of Science:

• Organic electronics
• Materials science
• Photophysics

Background:

• Spin-dependent nonlinear optical processes, like singlet fission and triplet-triplet annihilation, are crucial for advancing organic electronic devices.
• Rubrene/C60 organic light-emitting diodes (OLEDs) display a notable low-voltage (half-bandgap) emission threshold, with proposed origins including Auger-assisted energy up-conversion or triplet-triplet annihilation.

Purpose of the Study:

• To investigate the mechanisms responsible for the low-voltage turn-on in rubrene/C60 OLEDs.
• To differentiate between Auger-assisted up-conversion and triplet-triplet annihilation as the primary low-voltage emission source.
• To explore interface engineering strategies for enhancing OLED performance.

Main Methods:

• Systematic modification of rubrene/C60 interface kinetics using thin interlayers.
• Quantitative analysis of unmodified rubrene/C60 devices.
• Performance evaluation of devices with inserted bathocuproine interlayers.

Main Results:

• Quantitative analysis ruled out higher-order processes as the origin of sub-bandgap turn-on in unmodified devices; band-to-band recombination is the likely radiative process.
• Insertion of a bathocuproine interlayer resulted in a threefold increase in luminance compared to the unmodified device.
• Interface modification effectively suppressed parasitic processes.

Conclusions:

• Band-to-band recombination is the dominant radiative process in unmodified rubrene/C60 OLEDs at low voltages.
• Suppression of parasitic interface processes through judicious interface modification enables the observation of the triplet-triplet annihilation channel.
• Interface engineering is a viable strategy to enhance luminance and harness efficient energy transfer pathways in organic light-emitting diodes.