Magnetic field-effects in bipolar, almost hole-only and almost electron-only
tris-(8-hydroxyquinoline) aluminum devices
T. D. Nguyen, Y. Sheng, J. Rybicki, and M. Wohlgenannt*
Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa,
Iowa City, Iowa 52242-1479, USA
Received 28 February 2008; revised manuscript received 1 May 2008; published 16 June 2008
We present magnetoconductivity and magnetoluminescence measurements in sandwich devices made from
the -conjugated molecule tris-8-hydroxyquinoline aluminum Alq
3
and demonstrate effects of more than
25% and 50% magnitude, respectively. These effects are known to be caused by hyperfine coupling in pairs of
paramagnetic species, and it is often assumed that these are electron-hole pairs. However, we show that the
very large magnitude of the effect contradicts present knowledge of the electron-hole pair recombination
processes in electroluminescent -conjugated molecules and that the effect persists even in almost hole-only
devices.
DOI: 10.1103/PhysRevB.77.235209 PACS numbers: 73.50.h, 73.43.Qt
I. INTRODUCTION
Organic magnetoresistance OMAR is a recently ob-
served
1–9
large, low-field magnetoresistive effect up to 10%
at 10 mT and 300 K in organic light-emitting diode OLED
structures. Similar effects have also been observed in various
measurements ranging from electroluminescence to photo-
conductivity.
10–15
OMAR poses a significant scientific puzzle
since it is the only known example of large room-temp-
erature magnetoresistance in nonmagnetic materials with the
exception of very-high-mobility materials.
16,17
The exact
mechanism causing OMAR is currently not known with cer-
tainty. Three kinds of models based on spin-dynamics in-
duced by hyperfine interaction have recently been suggested
as possible explanations of OMAR: i Electron-hole pair
EHP mechanism models
4–6,8,18
based on concepts bor-
rowed from the so-called magnetic field-effects in radical
pairs.
10,19
In this model the spin-dependent reaction between
oppositely charged polarons to form an exciton “recombina-
tion” is of central importance. ii The triplet-exciton po-
laron quenching TPQ model
7
that is based on the spin-
dependent reaction between a triplet exciton and a polaron to
give an excited singlet ground state i.e., the “quenching” of
the triplet exciton by the polaron. iii The bipolaron
mechanism
20
that treats the spin-dependent formation of
doubly occupied sites bipolarons during the hopping trans-
port through the organic film. Whereas mechanisms i and
ii are excitonic in nature, the bipolaron mechanism can
exist also in unipolar devices. We anticipate that the quanti-
tative modeling of OMAR will yield sensitive tests of our
understanding of organic semiconductor devices. At present,
however, any analysis of OMAR experiments is plagued by
ambiguity: experiments must be devised that will allow one
to distinguish between the three mechanism mentioned
above. Specifically, if model i is correct, then measure-
ments of OMAR allow determination of the singlet:triplet
ratio in OLEDs, whereas if ii is correct it will yield insights
into the physics of triplet excitons, and finally if iii is cor-
rect OMAR can be used to test our understanding of charge
and spin transport as well as bipolaron formation. In the
present paper we will study OMAR in tris-8-hydroxy-
quinoline aluminum Alq
3
devices with different electrode
materials to put the three models of OMAR to a test.
II. EXPERIMENT
Our devices used an undoped organic semiconductor
layer, and consequently the carriers that result in electrical
current must be injected from the electrodes. If both the an-
ode and cathode are chosen suitably, both form Ohmic con-
tacts and the device is bipolar and shows efficient electrolu-
minescence. If one of the electrodes is chosen to enforce a
large barrier to the injection of this carrier type, then the
device is almost unipolar and therefore shows ideally no
electroluminescence. With this in mind we have fabricated
devices with a large number of electrode material combina-
tions. The fabrication started with glass substrates coated
with either 30 nm of Al, 40 nm of Ag, 25 nm of Cr, 40 nm of
Au prepared by electron-beam evaporation at 10
-6
mbar,
40 nm of indium-tin-oxide ITO, purchased from Delta Tech-
nologies, or the conducting polymer poly3,4-ethylene-
dioxythiophene-polystyrenesulfonatePEDOT, purchased
from H C Starck spin-coated onto ITO as the anode. Since
we need to measure the electroluminescence output to assess
the carrier balance in the device, the thickness of the anode
electrode had to be carefully chosen; It has to be thick
enough to show high conductivity but has to be thin enough
to be optically semitransparent. The transmission spectrum
of the electrodes was measured and was used to correct the
externally measured electroluminescence intensity. The Alq
3
sublimed, HW Sands Corp. layer was thermally evaporated
in high vacuum 10
-6
mbar onto the bottom electrode,
yielding an organic semiconductor layer thickness of
100 nm, without breaking the vacuum. The cathode, either
Ca with an Al capping layer, Al, or Au was then deposited
by thermal Ca or electron-beam evaporation Al, Au on
top of the organic thin film. The device area was 1 mm
2
for
all devices.
The samples were operated in dynamic vacuum inside a
cryostat located between the poles of an electromagnet, al-
though the measurements were all taken at room tempera-
ture. The magnetoconductance MC ratio was determined
PHYSICAL REVIEW B 77, 235209 2008
1098-0121/2008/7723/2352095 ©2008 The American Physical Society 235209-1