Complete polarization of electronic spins in OLEDs

Tobias Scharff,Wolfram Ratzke,John M Lupton,Jonas Zipfel,Philippe Klemm,Sebastian Bange

doi:10.1038/s41467-021-22191-3

Abstract

At low temperatures and high magnetic fields, electron and hole spins in an organic light-emitting diode become polarized so that recombination preferentially forms molecular triplet excited-state species. For low device currents, magnetoelectroluminescence perfectly follows Boltzmann activation, implying a virtually complete polarization outcome. As the current increases, the magnetoelectroluminescence effect is reduced because spin polarization is suppressed by the reduction in carrier residence time within the device. Under these conditions, an additional field-dependent process affecting the spin-dependent recombination emerges, possibly related to the build-up of triplet excitons and their interaction with free charge carriers. Suppression of the EL alone does not prove electronic spin polarization. We therefore probe changes in the spin statistics of recombination directly in a dual singlet-triplet emitting material, which shows a concomitant rise in phosphorescence intensity as fluorescence is suppressed. Finite spin-orbit coupling in these materials gives rise to a microscopic distribution in effective g-factors of electrons and holes, Δg, i.e., a distribution in Larmor frequencies. This Δg effect in the pair, which mixes singlet and triplet, further suppresses singlet-exciton formation at high fields in addition to thermal spin polarization of the individual carriers.

Highlights

At low temperatures and high magnetic fields, electron and hole spins in an organic lightemitting diode become polarized so that recombination preferentially forms molecular triplet excited-state species
We use a single-photon counting area detector, a scientific CMOS camera[27], which offers a detection quantum efficiency of almost 80% together with the ability to correct for any unwanted mechanical movement, provided that the image of the organic light-emitting diode (OLED) pixel projected onto the camera is smaller than the area of the camera chip
Wang et al.[24] appeared to observe an additional slow relaxation process at high magnetic fields which they attributed to the thermal spin polarization (TSP) mechanism

Summary

Introduction

At low temperatures and high magnetic fields, electron and hole spins in an organic lightemitting diode become polarized so that recombination preferentially forms molecular triplet excited-state species. Similar phenomenology appears at the heart of photochemical reactions triggered by photoinduced electron transfer[7] It has, for example, been proposed that retinal pigment-protein complexes of some bird species support the formation of spatially separated spincorrelated carrier-pair states, the singlet-triplet recombination yield of which can be influenced by the changes in spin precession arising on magnetic-field scales as small as geomagnetic-field strengths[8]. The effective g-factor can be measured by the electron paramagnetic resonance condition, the absorption of electromagnetic radiation of a frequency hν = gμBB In organic semiconductors, both electron and hole spin behave rather like free electrons in terms of their gfactors, with very subtle shifts arising from the combination of spin and angular momentum, i.e., SOC14. Spin-polarized photoelectron spectroscopy may be sensitive to the interface of the ferromagnetic electrode and the semiconductor[20], whereas muon scattering provides a somewhat indirect measure of electronic spin polarization and requires major research infrastructure[21]

Methods

Results

Conclusion