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Magnetic field influenced exciton generation in organic
semiconductors: an intermolecular quantum correlated effect
Baofu Ding, Yao Yao, Xunmin Ding, Changqin Wu * and Xiaoyuan Hou*
Surface Physics Laboratory (National Key Lab), Fudan University, Shanghai 200433, China
Background
1) Electroluminescence of organic light-emitting device can be
influenced by magnetic field, which is called
magnetoelectroluminescence (MEL). In the small magnetic
field region (typically <20mT), MEL appears to increase
rapidly with magnetic field . In the large magnetic field
region, MEL appears to saturate or decay.
2) Based on the above experimental phenomena, there are still
a lot of questions without answer: Why the larger the
voltage is, the smaller the MEL is? Why the insertion of LiF
leads to larger MEL? And why minority dominates organic
magnetic field effect?
Spin mixing is introduced for explaining the MEL. However
quantitative analysis depicted spin mixing is only
available in large magnetic field. It hardly covers the
whole process of MEL. Especially under small magnetic
field, MEL can not be explained only by the mechanism
of spin mixing.
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Dots with five kinds of
color are experimental
data chosen from part 2
with representative
doping ratios The solid
lines show the results
from our two-step model
by changing hopping
rates for electrons and
holes simultaneously.
Each curve in the figure has two parts. One in large magnetic
field (>25mT) is from “spin mixing”, and the other (<25mT) is
from “spin scattering”. Especially, as an example, we show
the whole behavior of “spin mixing” and “spin scattering” for
MEL with NPB doping ratio of 10% .
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The intensity of MEL depends on the mixing ratio of NPB
shown in left figure. We will, in our work, use our theoretical
model to simulate these measurements, and find the
essential role of hopping rate in organic magnetic field effect.
Hopping rate provide the answers to former questions
Prediction ⅠSufficiently
small intramolecular
Coulomb repulsion U
diminishes
the effect of spin
mixing thus benefits to
observe the decay,
predicted by spin
scattering. We choose to
dope fluorescent dye
DCM1 into host material
Alq3 with doping ratio 1%.
DCM1 has strong
trapping effect, which equivalently reduces U. As shown in right
figure, decay of MEL is observed in this structure (dots) which is
consistent with our theoretical prediction (lines).
Summary
Prediction Ⅱ: No change of triplet excitons. Intensity of spectrum
around 510nm under a field of 4mT appears to increase obviously
comparing to that without magnetic field. △EL/EL is about 1.5% which
is considered as relative increment of singlet excitons. For the
spectrum around 652nm, it is hardly affected by the magnetic field
which implies no change of triplet excitons in the system.
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We design devices aiming at tuning parameter, hopping
rate, and find it has a universal influence on both spin
scattering and spin mixing. This experiment indicates the
essential role of intermolecular quantum correlated effect
in MEL. Besides spin mixing, we propose firstly a
mechanism of spin scattering that plays a dominant role in
MEL under small field. There are two steps in exciton
generation. Two different mechanisms, Spin scattering
and spin mixing work in each step respectively. Our work
provides an overall description on MEL in both small and
large magnetic field. Observation on the two predictions in
experiments further prove the correctness of our
theoretical mode.
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