University of Groningen
Effect of impurities on the mobility of single crystal pentacene
Jurchescu, O. D.; Baas, J.; Palstra, T. T. M.
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Applied Physics Letters
DOI:
10.1063/1.1704874
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Jurchescu, O. D., Baas, J., & Palstra, T. T. M. (2004). Effect of impurities on the mobility of single crystal
pentacene.
Applied Physics Letters
,
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(16), 3061-3063. https://doi.org/10.1063/1.1704874
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Effect of impurities on the mobility of single crystal pentacene
Oana D. Jurchescu, Jacob Baas, and Thomas T. M. Palstra
a)
Solid State Chemistry Laboratory, Materials Science Centre, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
共Received 10 December 2003; accepted 18 February 2004兲
We have obtained a hole mobility for the organic conductor pentacene of
⫽35 cm
2
/V s at room
temperature increasing to
⫽58 cm
2
/V s at 225 K. These high mobilities result from a purification
process in which 6,13-pentacenequinone was removed by vacuum sublimation. The number of traps
is reduced by two orders of magnitude compared with conventional methods. The temperature
dependence of the mobility is consistent with the band model for electronic transport. © 2004
American Institute of Physics. 关DOI: 10.1063/1.1704874兴
Organic materials are presently being investigated and
incorporated in semiconductor devices for the electronics in-
dustry. The understanding of the electrical conduction
mechanism in these materials and at their interfaces repre-
sents a challenge, for which various, often conflicting models
have been proposed. Organic conductors are conjugated ma-
terials where the
delocalized electrons are responsible for
the intramolecular conduction. Molecular crystals are formed
by relatively weak Van der Waals interaction between mol-
ecules, where the molecular packing determines the elec-
tronic behavior. Thus the charge carrier transport must be
described using completely different models than for co-
valently bonded semiconductors. Of the many molecular
conductors, pentacene is a promising candidate for future
electronic devices and an interesting model system. Recent
improvements in electronic applications showed that this ma-
terial exhibits mobilities higher than 1 cm
2
/V s for TFTs
made from highly ordered films.
1,2
A mobility up to 8
cm
2
/V s was measured in single crystals of rubrene using a
complete organic field-effect transistor.
3
The importance of
impurities for the limitations in device performance has been
emphasized during the last few years. However, little quan-
titative analysis concerning the consequences of impurities is
incorporated in recent studies.
4
We report a mobility of
⫽35 cm
2
/V s at room temperature increasing to
⫽58
cm
2
/V s at 225 K for purified pentacene single crystals. The
crystals were obtained by vapor transport growth in argon
flow after purification of the material by a vacuum sublima-
tion technique designed to remove pentacenequinone. The
content of the quinone impurity in pentacene was determined
using high pressure liquid chromatography technique
共HPLC兲, indicating a reduction by almost one order of mag-
nitude.
The starting material for the experiment was pentacene
obtained from Aldrich. Infrared absorption measurements
共Nicolet Nexus兲 show that 6,13-pentacenequinone is present
as impurity. The evidence for this is the presence of the ab-
sorption peak at 1697 cm
⫺ 1
which is assigned to a C⫽O
bond vibration. The pure material does not have significant
absorption in this region. The infrared experiments were
complemented by mass spectrometry analysis, which con-
firmed that the C⫽O vibration originates from a pentacene-
quinone molecule.
We have used vacuum sublimation under a temperature
gradient as purification method. This technique is effective
for separation of impurities from a solid if these impurities
have a vapor pressure that is sufficiently different from the
desired product.
5
The pentacene is placed in an alumina boat
inside a glass tube that is thoroughly cleaned chemically, and
then heated in a furnace under vacuum. The purification
takes place at 430 K for 70 h under a dynamic vacuum of a
membrane pump. Special attention is given to avoid con-
tamination due to vacuum connections. The sublimated mol-
ecules will condense in the cold zone of the tube. The entire
setup is in the dark to prevent UV degradation of the acene
molecules. The carbonyl groups at each side of the middle
ring reduce the sublimation enthalpy compared to the host
molecule, thus at 430 K pentacene will not sublime and only
quinone will be removed. This can be detected as a brown
powder on the walls of the tube. The violet powder that did
not sublime is purified pentacene that is used as the starting
material for the single crystal growth.
Pentacene single crystals were obtained using physical
vapor transport in a horizontal glass tube
6
under a stream of
argon. The use of ultrapure argon without hydrogen as the
transporting gas is motivated by the need to prevent the in-
troduction of other impurities, like 6,13-dihydropentacene in
the crystal, which can form by the hydrogenation of the
acene at the middle ring 共most reactive positions兲. The inner
tube was cleaned by heating it under argon gas flow to re-
move the solvents used for cleaning. The gas was obtained
from AGA, with purity of 99.999%. A drying column that
consists of aluminum oxide and hydrogen activated BTS
catalyst was inserted in the system for additional purification
of the gas. We paid attention to the quality of the transporting
gas because the quinone can be reintroduced by residual wa-
ter or oxygen as ppm impurities in the carrier gas during
growth. 30–40 mg of the source pretreated material was
placed at the end of the tube in an alumina crucible. A tem-
perature gradient was applied by resistive heating of two
heater coils around the tube. Without prepurification of the
pentacene, pentacenequinone will sublime together with the
host molecule. Part of it will be introduced in the pentacene
matrix in the crystallization process at the low temperature
part of the tube. After obtaining ultrapure crystals, they were
a兲
Electronic mail: palstra@chem.rug.nl
APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 16 19 APRIL 2004
30610003-6951/2004/84(16)/3061/3/$22.00 © 2004 American Institute of Physics
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annealed for 50 h at a lower temperature 共450 K兲 than the
crystallization temperature 共490 K兲. The crystals were stored
in vacuum and darkness, as exposure to air and light will
cause slow oxidation.
We used HPLC 共Agilent 1100 LC/MSD兲 to determine
the impurity concentration of the quinone. Pentacene and the
pentacenequinone were separated on a silica column with a
3:1 v/v mixture of 1,2,4-trichlorobenzene and cyclohexane at
80 °C. The amount of quinone was determined from the in-
tegrated intensity of the chromatogram using a diode array
UV-Vis detector tuned at an absorption line 共⫽390 nm兲.
The quinone concentration was reduced from the as-received
material containing 0.68% in two sublimation steps to
0.17%. Subsequent crystal growth reduces the quinone con-
centration to 0.028% 共⫾0.004兲 compared with 0.11%
共⫾0.006兲 in crystals grown from untreated powder 共Fig. 1兲.
The characteristic absorbtion of C⫽O is observable even for
the purest crystals 共stage 5 in Fig. 1兲
We determined the electrical properties of the pure pen-
tacene single crystals using space-charge-limited current
共SCLC兲 measurements 共Fig. 2兲. The samples were measured
in darkness and a vacuum of 2⫻ 10
⫺ 7
mbar. The mobility
was calculated from the trap-free region 共⌰⫽1兲 of the space-
charge-limited current regime, using the Mott–Gurney Eq.
共1兲. We note that this formula was derived for the opposed
electrode geometry, whereas we use a coplanar geometry:
J
SCLC,tf
⫽
9
8
⑀
0
⑀
r
⌰
V
2
L
3
, 共1兲
where J
SCLC,tf
is the current density in the trap-free regime,
V is the applied voltage across a length L,
⑀
r
is the dielectric
constant of the conductor 共for which we use the literature
value 3兲, and ⌰ the concentration of free carriers with re-
spect to the total numbers of carriers 关see Eq. 共3兲兴. The
SCLC measurements show that the pentacene single crystals
grown after precleaning of the starting material are very
pure, with N
t
⫽ 1.74⫻ 10
11
traps/cm
⫺ 3
. This is almost two
orders of magnitude lower than the number of traps obtained
for crystals grown with the conventional procedures.
7
If we assume a homogeneous current flow through the
sample, the mobility is
⫽11.2 cm
2
/V s. However, as the
mobility in the basal plane ab is much larger than the mo-
bility along the c
*
axis 共perpendicular to ab plane兲, the cur-
rent will be confined to the contact side of the crystal. The
measurements of the ohmic regime of the current–voltage
characteristic showed different values for the resistivity for
different directions (
a
⫽ 1.3⫻ 10
6
,
b
⫽ 4.7⫻ 10
5
, and
c
*
⫽ 2.1⫻ 10
8
⍀ m兲. We have used Montgomery’s method for
analyzing anisotropic materials, transforming an anisotropic
sample with resistivities
a
,
b
, and
c
*
and dimensions x,
y, and z, to an isotropic solid with dimensions x
⬘
, y
⬘
, and
z
⬘
.
8,9
For the isotropic solid, the normalized effective thick-
ness is determined to be z
eff
⬘
/(x
⬘
•y
⬘
)⫽0.7 for normalized
sample thickness z
⬘
(x
⬘
• y
⬘
)
1/2
⫽2.19 and the ratio for the in-
plane directions y
⬘
/x
⬘
⫽0.535. Using this procedure, the re-
lation between the effective thickness (z
eff
) and the real
thickness of the crystal (z) is calculated. Equation 共2兲 ex-
presses the conversion for the two dimensions:
z
eff
⫽ 0.32• z. 共2兲
With the effective thickness introduced in the current density
J
SCLC,tf
in Eq. 共1兲, the calculated mobility increases by more
than a factor of three 共see Fig. 3兲. Therefore, a more accurate
value for the mobility is
⫽35 cm
2
/V s at 290 K and
⫽58
FIG. 1. 6,13-pentacenequinone concentration in pentacene in different
stages of purification: 共1兲-as-received, 共2兲-single sublimation clean, 共3兲-
double sublimation clean, 共4兲-crystal grown from untreated powder, 共5兲-
crystal grown from doubly cleaned powder. The inset represents the Lewis
structure of pentacene and 6,13-pentacenequinone.
FIG. 2. Current–density vs electric field for pentacene single crystal at room
temperature. The solid lines represent the fits for the three different regimes.
The inset shows experimental configuration of the a, b, and c
*
axis, and the
contacts. The hole-injecting electrodes are 40–50 nm thick gold.
FIG. 3. Temperature dependence of the electrical hole mobility for penta-
cene single crystal using the actual crystal thickness 共䊊兲 and effective thick-
ness 共䊉兲.
3062 Appl. Phys. Lett., Vol. 84, No. 16, 19 April 2004 Jurchescu, Baas, and Palstra
Downloaded 21 Nov 2005 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
cm
2
/V s at 225 K. We note that the Montgomery method is
only valid in the linear part of the I–V regime. For I–V
2
the
effective thickness (z
eff
) should be considered as the upper
limit. Thus, this analysis provides a lower limit of the intrin-
sic mobility.
Figure 3 shows that below room temperature the mobil-
ity increases with decreasing temperature following the rela-
tion
⫽ C• T
⫺ n
with n⫽2.38. We notice this behavior in
several samples and it is consistent with a band model for
charge transport in pentacene,
10
with the interaction of the
delocalized carriers with the phonons, the main scattering
process. For electron–phonon coupling the mobility de-
creases with increasing temperature. This temperature depen-
dence was also found in acenes with smaller number of ben-
zene rings: naphthalene, anthracene, and tetracene 共Ref. 10
and references therein兲. Above room temperature a different
transport mechanism dominates the mobility.
In the first part of the SCLC regime 共Fig. 2兲, the injected
carriers are trapped and the current is reduced by a factor ⌰,
which represents the ratio between free and total number of
charge carriers introduced in the solid 关Eq. 共3兲兴:
⌰⫽
n
f
n
tot
⫽
n
f
n
f
⫹ n
t
, 共3兲
where n
f
and n
t
are the free and trapped carriers density,
respectively, and n
tot
is the total carrier density. In our crys-
tals ⌰ varies from 0.3 at 225 K to 0.82 at 340 K. With
increasing electric field the density of injected carriers will
increase, and above the trap-filled limit voltage (V
TFL
) the
mobility is not affected by impurity states and defects.
11
In the following, we will focus on the origin of the trap-
ping factor 共⌰兲 in Eq. 3. The traps in the crystal are mainly
caused by structural imperfections and chemical impurities.
Extended defects, such as edge dislocations or screw dislo-
cations modify the available energy levels in their vicinity,
often leading to the presence of accessible vacant orbitals in
the band gap. We minimized the number of traps by a careful
crystal growth and subsequent handling. Heating the as-
grown crystals in an inert argon atmosphere will reduce the
dislocations density. These defects are introduced during the
crystallization process and are thermodynamically unstable.
Thus their number decrease considerably by annealing. Dis-
locations will also enhance the chemical reactivity in their
vicinity. Under the influence of light and temperature, reac-
tions that oxidize pentacene to pentacenequinone will occur
preferentially at dislocations.
4
So, even if the quinone is not
present after crystal growth, it can be formed at defects after
exposure to air and/or light.
We found that 6,13-pentacenequinone is the dominant
chemical impurity. We did not observe C
22
H
15
and C
22
H
13
O
impurities that were calculated to form gap states in our pen-
tacene, C
22
H
14
.
12
Moreover, we argue that these molecules
are irrelevant as these radicals are highly reactive. We were
able to prevent the formation of the dihydropentacene C
22
H
16
by using argon as transport gas during the crystal growth. We
have shown that the reduction of 6,13-pentacenequinone
(C
22
H
12
O
2
) impurities in pentacene by a factor five reduces
the number of traps by almost two orders of magnitude.
These impurities have different energy levels from penta-
cene, but they are energetically inert as a hole trap because
their HOMO level is below that of the host molecule 共Ref.
12 for C
22
H
16
and Ref. 13 for C
22
H
12
O
2
). This is distinctly
different from experiments on smaller acenes, where the im-
purities yield stated in the gap.
4
For this reason, the number
of traps in our measurements can be different from the num-
ber of chemical impurities. Although the impurity molecules
do not act as trapping centers, they will induce a local defor-
mation by distorting the pentacene lattice locally and create a
scattering center. The quinone molecule is nonplanar and
larger than pentacene. The middle ring has a flattened-chair
shape with the C⫽O bond length of 1.216 Å and 典C⫽O
planar inclined at an angle of 3.1° to the molecular plane.
14
Thus, it will induce a local deformation leading to an in-
crease in potential energy because of changes in molecular
density. The quinone will strongly influence the number of
such scattering sites, and thus the charge transport through
organic single crystal.
In conclusion, we have reduced the impurity concentra-
tion of pentacenequinone in pentacene by a pretreatment
consisting of vacuum sublimation of the impurity under a
temperature gradient. The crystals exhibit a trap-free space
charge current limited behavior. The mobility increases with
decreasing temperature with a power law
⬃T
⫺ n
from
⫽35 cm
2
/V s at room temperature to
⫽58 cm
2
/Vsat225
K, indicating band transport. These results incorporate cor-
rections for the effective thickness of the crystal for the an-
isotropic resistivity, where the effective thickness is at least
three times smaller than the crystal thickness. Our results
emphasize the importance of the control of defects and im-
purity states in molecular organic crystals in order to obtain a
high electronic mobility, and allow studies of the band trans-
port regime.
15
The authors would like to acknowledge P. W. M. Blom,
C. R. van den Brom, A. F. England, R. A. de Groot, P. van’t
Hof, J. C. Hummelen, C. Tanase, and G. de Wijs for experi-
mental facilities and stimulating discussions. One of the au-
thors 共O.D.J.兲 would like to thank M. Mulder for constant
encouragement, support, and help.
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3063Appl. Phys. Lett., Vol. 84, No. 16, 19 April 2004 Jurchescu, Baas, and Palstra
Downloaded 21 Nov 2005 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp