Variation of Electrical Transport Properties in Dense Alq3
Feng Ke,a Bin Chenb and Chunxiao Gao*a
a
State Key Lab for Superhard Materials, Institute of Atomic and Molecular Physics, Jilin University,
Changchun 130012, China.
b
Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China.
We report intriguing electrical and structural
properties on compressed Alq3, an extensively used
electron transport material in OLED. The experimental
results reveal that except for the π-π orbital overlaps, the
Al-oxine interaction is also significant on the transport
behavior of Alq3. Pressure-induced amorphization is
found to be reversible in low-density amorphous state
(L-DAS) but irreversible in H-DAS.
The impedance spectroscopy data collected from
Alq3 powders with pressure up to 19.4 GPa are presented
in Fig. 1 as the imaginary part of complex impedance (Z″)
plotted vs that of the real part (Z′).
Figure 1. (a)-(c) Complex impedance plots of Z″ vs Z′ of
Alq3 under compression. Lines are the fitted results with the
equivalent circuit models describing the bulk (R1-CPE1) and
grain boundary (R2-CPE2) effects shown in inset of Fig. 1b.
The fitting error is less than 5%. (d) Pressure dependence of
bulk and grain boundary resistances of Alq3.
The impedance spectra were fitted using a common
equivalent circuit model, consisting of two parallel
resistors (R) and constant-phase element (CPE) elements
(Fig. 1) to describe the bulk and grain boundary
relaxation processes, respectively, on Zview2 impedance
analysis software. The obtained bulk (Rb) and grain
boundary resistances (Rgb) are plotted in Fig. 1d. Rb of
Alq3 increases markedly at low pressures (< 3.1 GPa),
followed by a shoulder from 3.1 to 5.7 GPa, and
continues the increasing tendency up to 7.9 GPa.
However, it changes the pressure dependence with
continuous compression and drops rapidly up to 15.9
GPa. Similar shoulder has also been observed on the
photoluminescence spectra of Alq3 at pressure range of
3.1~6.1 GPa.1,2
High pressure XRD measurements7 were performed
on Alq3 up to 17.4 GPa for in situ observation of the
structural modification. The selected patterns are shown
in Fig. 2. At room conditions, triclinic crystal structure
with P-1 symmetry (β-Alq3) seems more suitable for the
structure of Alq3. The inset of Fig. 2 shows the structure
model of Alq3. Above 16.1 GPa, all the diffraction peaks
lose their intensity, suggesting that Alq3 becomes
amorphous. After quenching to ambient pressure, all of
the reflection peaks re-emerge (Fig. 2), indicating that
the amorphization process is reversible. However, when
uploading pressure up to 23.8 GPa and then quenching to
ambient pressure, it is found that the pattern remains
disappeared, showing the irreversible character.
Figure 2. Representative XRD patterns of Alq3 at various
pressures. The XRD experiments were conducted on
beamline 4W2 of Beijing Synchrotron Radiation Facility
(BSRF) and beamline 15U1 of Shanghai Synchrotron
Radiation Facility (SSRF) using angle-dispersive XRD
source (λ = 0.6199 Å). The inset shows the structural model
of Alq3.
Raman measurements on Alq3 up to 17.7 GPa are
shown in Figure 3 and 4. At pressure of 8.6 GPa and
above, the low-frequency vibration modes ranged from
50 to 200 cm-1 lose their intensity, and the disappearance
tendency expands to higher-frequency vibration modes
with continuous compression. Finally, all of the Raman
vibration modes disappear at ~17.7 GPa. Under
quenching process, the Raman vibration modes of Alq3
reappear when the sample was decompressed from 17.7
GPa, but remain disappeared when decompressed from
23.4 GPa. The Raman results show better agreement
with the XRD observation that Alq3 becomes amorphous
above 16.1 GPa, and the amorphous process is reversible
at low pressure (< 17.4 GPa) but irreversible upon
further increasing pressure up to 23.8 GPa. At low
pressure, the crystalline topology (including the atomic
coordination and bonding) is preserved, which retain a
“memory” of its original crystal structure and can revert
to it. With continuous compression, the relative higher
density amorphous state involves some bonding breaking,
in which there is insufficient thermal energy to
re-establish these broken bonds and recover to the
original state, and consequently the pressure-induced
amorphization will be irreversible.
Figure 3. High-pressure Raman spectra of Alq3 with the
Raman peaks of diamond at ~1331 cm-1 subtracted. Here
the def., wag., str., and bre. represent the deformation,
wagging, stretching, and breathing vibration modes,
respectively.
As clearly seen from Fig. 4, the Raman active
modes lose their intensity gradually from low- to
high-frequency above 8.0 GPa, that is the Al-oxine
bonds are firstly tuned, and then the oxine ligands lose
their long-term ordering, followed by the modification of
the ring and C-H bonds with increasing pressure. The
relative Raman intensity changes of the Al-oxine
vibration modes (Fig. 4) should be caused by the
pressure-induced modification of the Al-oxine bonds and
the non-planar conformation of the Alq3 molecules.
Figure 4. (a) Pressure dependence of Raman peaks
positions of Alq3. (b) and (c) The ratio of the intensity of
the ~65 cm-1, ~90 cm-1, and ~170 cm-1 Raman vibration
modes to the ~1400 cm-1 mode in Alq3 as a function of
pressure. (d) The variation of the Raman peak width (~1400
cm-1) under pressure.
Under compression, the π-π interaction of Alq3 is
greatly enhanced, which narrows the LUMO and HOMO
gap,3 and hence will facilitate the electrical transport of
the π-π orbital interaction. That is the conduction of Alq3
should be improved under compression, which is not
consistent with the behavior of Rb that increases
substantially within pressure range of 1.6~7.9 GPa. It
implies that in addition to the significance of the π-π
interaction on the transport properties of Alq3, some
other factors play important roles as well. The similar
change tendency between the Al-oxine deformation
mode (at ~90 cm-1, shown in Fig. 4) and Rb below 8.0
GPa easily reminds one of their correlation. At ambient
pressure, resulting from the low symmetry of crystal
structure that those three Al-oxine ligands (including the
Al-O and Al-N bonds) each have different bond lengths
and angles, the charges are not localized uniformly on
three ligands.3b Such conformation makes the Al-oxine
interaction sensitive with pressure, and causes the
pronounced enhancement of the Al-oxine deformation
modes, which describe the variation of the bond angles
and torsion angles. The strengthened Al-oxine bonds,
especially the Al-N and Al-O bonds, should localize the
charge carriers, and hence hinder the conduction of Alq3.
Moreover, the Al-oxine bonds angle changes can result
in defects and increase the degree of structural disorder,
accounting for the broadening in the Raman spectra and
XRD patterns of Alq3 under compression, which can
also capture the charge carriers and limit the electrical
transport. Thus, Rb of Alq3 increases with uploading
pressure. At higher pressure (>8.0 GPa), the essential
changes of Al-oxine interaction fundamentally modify
the electronic structure of Alq3, driving the redistribution
of electron density between adjacent oxine ligands and
the localized-delocalized charger states transfer, which
combine with the improved conduction caused by
enhanced π-π orbital overlaps and non-planar to
more-planar
conformation
modification
under
compression compensate the hindering effect of
structural disorder or defects, and cause Alq3 better
conductive. The dramatic increase of Rb above 16.4 GPa
should be induced by the amorphization process, which
bring about a higher degree of structural disorder and kill
the charge carrier of Alq3. It suggests that the Al-oxine
interaction also play an important role on the electrical
transport properties of Alq3 under compression.
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