Discrimination between spin-dependent charge transport and spin dependent recombination in {\pi}-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

TL;DR

This study distinguishes between spin-dependent charge transport and recombination in conjugated polymers by correlating current and electroluminescence magnetic resonance, revealing recombination as the dominant process at room temperature.

Contribution

The paper introduces a correlated measurement approach combining pulsed electrically and optically detected magnetic resonance to differentiate spin-dependent transport from recombination.

Findings

Correlation between pEDMR and pODMR at room temperature indicates recombination dominates.

No evidence found for spin-dependent transport influencing room temperature magnetoresistance.

At low temperatures, multiple spin-dependent processes affect the material properties.

Abstract

Spin-dependent processes play a crucial role in organic electronic devices. Spin coherence can give rise to spin mixing due to a number of processes such as hyperfine coupling, and leads to a range of magnetic field effects. However, it is not straightforward to differentiate between pure single-carrier spin-dependent transport processes which control the current and therefore the electroluminescence, and spin-dependent electron-hole recombination which determines the electroluminescence yield and in turn modulates the current. We therefore investigate the correlation between the dynamics of spin-dependent electric current and spin-dependent electroluminescence in two derivatives of the conjugated polymer poly(phenylene-vinylene) using simultaneously measured pulsed electrically detected (pEDMR) and optically detected (pODMR) magnetic resonance spectroscopy. This experimental approach requires careful analysis of the transient response functions under optical and electrical detection. At room temperature and under bipolar charge-carrier injection conditions, a correlation of the pEDMR and the pODMR signals is observed, consistent with the hypothesis that the recombination currents involve spin-dependent electronic transitions. This observation is inconsistent with the hypothesis that these signals are caused by spin-dependent charge carrier transport. These results therefore provide no evidence that supports earlier claims that spindependent transport plays a role for room temperature magnetoresistance effects. At low temperatures, however, the correlation between pEDMR and pODMR is weakened, demonstrating that more than one spin-dependent process influences the optoelectronic materials properties.