Magneto-electroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

TL;DR

This paper develops an analytical model for magneto-electroluminescence in organic heterostructures, explaining how magnetic fields influence light emission through hyperfine interactions and validating it with spectrally-resolved measurements.

Contribution

The paper introduces a new analytical theory for magneto-electroluminescence in organic heterostructures, incorporating hyperfine and exchange interactions, supported by experimental validation.

Findings

Magnetic field modulates electroluminescence spectra.

Hyperfine interaction affects exciplex singlet/triplet ratio.

Model explains wavelength-dependent magnetic field effects.

Abstract

The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magneto-electroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solution using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. To validate the theory, spectrally-resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. These observations are consistent with the model. Finally, the analytical theory is extended to account for an additional low-field structure due to the exchange interaction in the weakly bound polaron-pair states.