The role of triplet states in the emission mechanism of polymer light-emitting diodes

TL;DR

This study reveals that triplet states significantly influence the emission mechanism in polyfluorene-based LEDs, especially under oxygen exposure and in the presence of paramagnetic impurities, affecting the device's blue emission stability.

Contribution

It demonstrates the crucial role of triplet states in the emission process of PF LEDs through experimental and theoretical analysis involving paramagnetic impurities and oxygen effects.

Findings

Triplet states contribute to low energy emission in PF LEDs.

Paramagnetic impurities increase triplet formation during electroluminescence.

Oxygen presence affects the emission mechanism via triplet state interactions.

Abstract

The blue emission of polyfluorene (PF) based light-emitting diodes (LEDs) is known to degrade due to a low energy green emission, which hitherto has been attributed to oxidative defects. By studying the electroluminescence from ethyl-hexyl substituted PF LEDs in the presence of oxygen and in an inert atmosphere, and by using trace quantities of paramagnetic impurities (PM) in the polymer, we show that the triplet states play a major role in the low energy emission mechanism. Our time-dependent many-body studies show that there is a large cross-section for the triplet formation in the electron-hole recombination process in presence of PM, and intersystem crossing from excited singlet to triplet states.