Maximizing field-effect mobility and solid-state luminescence in organic semiconductors

Article abstract of DOI:10.1002/anie.201108184

Conductive and emissive: Organic transistors made from a simple styrylanthracene derivative (see scheme) have high charge mobility and high luminescence quantum yields. These properties are attributed to the lack of singlet fission, and challenge the idea

Full text of DOI:10.1002/anie.201108184

Angewandte
Chemie
DOI: 10.1002/anie.201108184
Organic Electronics
Maximizing Field-Effect Mobility and Solid-State Luminescence in
Organic Semiconductors**
Afshin Dadvand, Andrey G. Moiseev, Kosuke Sawabe, Wei-Hsin Sun, Brandon Djukic,
Insik Chung, Taishi Takenobu, Federico Rosei, and Dmitrii F. Perepichka*
Tunable multifunctional properties that are accessible
through synthetic design and facilitated device fabrication
are the key advantages of organic semiconductors (OSCs) as
optoelectronic materials. A tandem of light-emitting and
charge-transporting properties in OSCs has been widely
explored, and already commercialized in organic light-emit-
ting diodes (OLEDs) and displays.[1,2] At the same time, it
proved to be very difficult to maximize the charge mobility
(m) and luminescence efficiency in the same OSCs. As a result
of a short electrical channel length, OLEDs do not require
a m value of more than 10À4–10À2 cm2 VÀ1 sÀ1, however,
a number of technological opportunities could arise if highly
emissive OSCs with a m value of more than 1 cm2 VÀ1 sÀ1 were
available. These include organic light-emitting transistors
(OLETs),[3] which combine the electroluminescence (EL)
with the current-control function of a field-effect transistor
(FET), and potentially allow simplifying the structure of
active matrix displays. Larger current densities that are
attainable in transistor configurations (versus that in
OLEDs[3d]) are highly desirable for the development of
electrically pumped organic lasers.[4] However, this demands
new materials with high m value.
efficient electroluminescent organic materials are amorphous.
There are several reasons for luminescence quenching in the
solid state, which include fission of a singlet exciton to give
two non-emissive triplets,[6] as well as exciton quenching on
the defect sites. On the other hand, some conjugated
molecules, for example, tetraarylethylenes,[7] show enhance-
ment of photoluminescence (PL) in the solid state (aggrega-
tion-enhanced emission).[8] One of the simplest blue emitters,
anthracene, has a PL quantum yield (PLQY) of 64% in
crystals,[9] but no thin-film transistors have been produced
from this material. The larger aromatic core in tetracene gives
rise to reasonable charge mobility of approximately
0.1 cm2 VÀ1 sÀ1 in thin-film FETs,[10] but the solid-state
PLQY drops down to 0.8%, mostly as a result of singlet
fission.[11] The tetraphenyl derivative of tetracene, rubrene,
has a large FET mobility of up to 10–20 cm2 VÀ1 sÀ1 in single
crystals[12] but a correspondingly very low (less than 1%)
PLQY in the solid state, which is also attributed to singlet
fission.[6b] Pentacene, a benchmark p-type OSC has a mobility
above 1.0 cm2 VÀ1 sÀ1[13] and does not fluoresce in the solid
state. A number of other high-mobility OSCs have been
reported,[14] including dinaphthothienothiophene and its
derivatives, with a m value of 8 cm2 VÀ1 sÀ1 or less in thin
films[15] and 16 cm2 VÀ1 sÀ1 in single crystals.[16] However, these
compounds all have low or no emission in the solid state. The
overall picture emerging from the literature is that max-
imizing the charge mobility in OSCs necessarily leads to
decreased emission efficiency.
Achieving a m value of greater than 1 cm2 VÀ1 sÀ1 in OSCs
generally requires crystalline materials with strong p–p
interactions. However, such interactions most often lead to
suppression of luminescence in the solid state.[5] Nearly all
[*] A. Dadvand, Dr. A. G. Moiseev, W.-H. Sun, Dr. B. Djukic, I. Chung,
Prof. D. F. Perepichka
Herein, we show that a charge mobility of greater than
1 cm2 VÀ1 sÀ1 can be achieved in a structurally simple OSC that
also produces blue EL and PL with a PLQY of 70% in the
crystalline state. Recently, styrylacenes have emerged as
a promising subclass of OSC, in which extending the
conjugation through the styrene group and keeping the less-
stable acene moiety short allows a high field-effect mobility
together with good stability to be achieved.[17] We reported
a green-emitting OLET that is based on 2-(4-pentylphenylvi-
nyl)tetracene (PPVTet).[17c] Although a reasonable hole
mobility (ca. 0.2 cm2 VÀ1 sÀ1) was measured in these devices,
PPVTet has a low solid-state PLQY of approximately 7%. In
this study, we report the surprising finding that shrinking the
acene moiety (from tetracene to anthracene) leads to a new
OSC, 2-(4-hexylphenylvinyl)anthracene (HPVAnt), that has
an even higher charge mobility, and a solid-state PLQY of
70%. These are the key properties for the development of
high-current light-emitting devices, and we demonstrate
a blue-emitting OLET based on HPVAnt.
Department of Chemistry and Centre for Self-Assembled Chemical
Structures, McGill University
801 Sherbrooke Street West, H3A 2K6 Montreal, QC (Canada)
E-mail: dmitrii.perepichka@mcgill.ca
A. Dadvand, Prof. F. Rosei
INRS—ꢀnergie, Matꢁriaux et Tꢁlꢁcommunications (Canada)
K. Sawabe
Department of Physics, Graduate School of Science
Tohoku University (Japan)
K. Sawabe, Prof. T. Takenobu
Graduate School of Advanced Science and Engineering
Waseda University (Japan)
[**] This work was funded by the NSERC of Canada through Discovery
Grants and a CRD grant supported by Plasmionique Inc. We are
grateful to the CFI and NSERC for infrastructure support. F.R.
acknowledges the Canada Research Chairs program for partial
salary support. We acknowledge Prof. G. Hanan and F. Belanger
(University of Montreal) for access to lifetime measurements and X-
ray analysis, respectively.
HPVAnt was synthesized from 2-hydroxyanthraquinone
(1) by reducing 1 with NaBH4, activating the hydroxy group of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108184.
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was measured in thin films, which is
likely a result of exciton quenching on
surface defects.[18] The same trend was
reported for anthracene, which has
a PLQY of 64%, 28%, and 18% in
single crystal, solution, and powder
forms, respectively.[9]
We suggest that the high PLQY of
HPVAnt crystals might result from the
cessation of the singlet-fission process.
Indeed, time-dependent DFT calcula-
tions (see the Supporting Information)
predict that the energy of singlet exci-
ton S1 of styrylanthracene (3.04 eV) is
insufficient for generating two triplets
Scheme 1. Synthesis of HPVAnt. Tf=trifluoromethansulfonyl; DMF=N,N-dimethylformamide.
anthracene (2) with triflic anhydride and, finally, a Suzuki
coupling of the resulting triflate 3 with 4-hexylphenylvinyl-
boronic ester 4 (Scheme 1). As a result of the small acene core
and lack of reactive moieties, HPVAnt has high thermal and
chemical stability.
(T1 = 1.74 eV). The calculations are in agreement with the
spectroscopically determined S1 state (2.94–3.15 eV[19]) and
the reported triplet energy of 1.77 eV for 2-(4-metylphenyl-
vinyl)anthracene,[20] which confirms the endothermic nature
of singlet fission and explains the high PLQY of HPVAnt
crystals. In contrast, the S1 energy (2.39 eV) of the tetracene
derivative PPVTet (discussed above) matches the energy of
two triplets (T1 = 1.20 eV), which enables single fission and
explains its low PLQY. Analysis of published photophysical
data and DFT calculations reveal the same trend (S1 ꢀ 2T1)
for other high-mobility acene OSCs, including tetracene,[6a]
pentacene,[6a] and rubrene.[21] Accordingly, all of these suffer
luminescence quenching in the solid state.
The transistor properties of HPVAnt were studied in thin
films (TF) and single crystals (SC) in top-contact configu-
ration (Figure 2). Typical output and transfer characteristics
of the TF-FET that is based on HPVAnt are shown in
Figure 2c and d. Remarkably, without any device optimiza-
tion, HPVAnt films grown on bare Si/SiO2 at room temper-
ature have an average hole mobility m+ value of (1.14 Æ
0.2) cm2 VÀ1 sÀ1 with a threshold voltage (VT) of À20 to
À24 Vand an on/off ratio of 106–107. The highest m+ value that
was measured in these devices was 1.5 cm2 VÀ1 sÀ1. We note
Density functional theory (DFT) calculations at the
B3LYP/6-31G(d) level in the gas phase predict the HOMO
and LUMO of HPVAnt at À5.05 eV and À1.78 eV, respec-
tively (see the Supporting Information). The HOMO and
LUMO of HPVAnt as measured electrochemically in solution
are À5.47 eV and À2.57 eV, respectively (assuming ferrocene
at À4.80 eV, see the Supporting Information). The absorption
spectrum of HPVAnt in solution has a vibronically structured
band with peaks at lmax = 362, 378, and 397 nm (Figure 1). The
optical gap determined from the onset of this band (3.0 eV,
410 nm) is in reasonable agreement with the value measured
electrochemically (2.9 eV) and DFT calculations (3.27 eV).
The PL spectrum is also vibronically structured (lmax = 422,
438, and 479 nm). Both absorption and emission are red-
shifted in the solid state, and the band gap determined from
an onset absorption is 2.8 eV. The high absolute PLQY of
70% measured in an integrating sphere in single crystals (up
from 55% in solution) makes HPVAnt a good candidate for
OLET applications; a lower PLQY of approximately 20%
Figure 2. Top-contact TF-FET fabricated with HPVAnt: a) Schematic
and b) optical image and EL of the biased device. c) Transfer character-
istics of the biased device. VDS =-75 V d) Output characteristics of the
biased device. Vg =-75 V (blue); -60 V (teal); -45 V (pink); -30 V
(black); -15 V (red); m=1.33 cm2 VÀ1 sÀ1; on/off=3ꢂ107; VT =À24 V.
IDS is drain-source current.
Figure 1. Absorption (broken lines) and PL (lex =410 nm, solid lines)
spectra of HPVAnt in solution (2ꢂ10À5 m in toluene, red), as a thin
film (on quartz, blue), and as a single crystal (teal).[28]
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that the mobility of HPVAnt slightly exceeded that of similar
pentacene-based devices fabricated by us, despite the higher
calculated reorganization energy of the HPVAnt-based
devices (l0/+ = 0.18 eV versus 0.09 eV for pentacene,[22] see
the Supporting Information). A somewhat lower mobility was
measured for TF-FETs that were fabricated on Si/SiO2
modified with hexamethyldisilazane (HMDS, (0.8 Æ
0.1) cm2 VÀ1 sÀ1) or poly(methyl methacrylate) (PMMA,
(0.45 Æ 0.08) cm2 VÀ1 sÀ1) layers. This can likely be ascribed[23]
to the rougher nature of these surfaces (root mean squared
(RMS) of 0.18 nm for bare Si/SiO2 and 0.45 nm for PMMA-
coated Si/SiO2).
The high mobility in TF-FET benefits from the almost
perfect layer-by-layer growth at room temperature, which
leads to continuous OSC films at a low nominal thickness, as
confirmed by atomic force microscopy (AFM, Figure 3). The
height profile suggests an upright orientation of molecules in
(1.01 Æ 0.09) cm2 VÀ1 sÀ1 see the Supporting Information),
but are less stable when operating in air. However, the
devices that were made on the PMMA-coated substrate
demonstrated an almost constant mobility of (0.5 Æ
0.05) cm2 VÀ1 sÀ1 for more than 20000 cycles in air (continuous
18 h run). The shelf-life stability of the TF-FETs after storage
of the unencapsulated devices under ambient conditions
(exposed to air and light) is excellent (the m value drops from
1 cm2 VÀ1 sÀ1 to 0.9 cm2 VÀ1 sÀ1 and 0.84 cm2 VÀ1 sÀ1 over 3 and
12 months storage, respectively). Besides excellent transistor
characteristics, almost all devices produce blue EL that is
modulated by a gate voltage (VG, Figure 2b). As in other
OLET devices with a single-carrier majority channel, the
emission zone is concentrated in the vicinity of the drain
electrode.[3b] Because of the top-contact configuration, most
of the emitted light must be buried under the drain; the
intense blue emission is nevertheless clearly visible at the
electrode edges.
Unlike in other single-layer OLETs, the emission of
HPVAnt is not subject to a “fatigue” effect, that is, the
emission does not fade out during continuous operation (see
the video in the Supporting Information). We note that only
a few blue-emitting OLETs have been reported to date;[26] the
highest mobility, which was measured for 4,4’-distyrylbi-
phenyl, was 0.01 cm2 VÀ1 sÀ1 (PLQY= 20%).[26b] We also
studied the performance of HPVAnt in single-crystal devices.
Crystals of HPVAnt were grown from the vapor phase in
a flow of Ar.[27] Single-crystal FET (SC-FET) structures were
prepared by laminating the crystals onto a PMMA-coated Si/
SiO2 substrate, and then the source and drain electrodes were
deposited by evaporating Au through a shadow mask. As
a result of the relatively large thickness of the crystals (a few
mm), the top metal contacts are located far from the charge
accumulation zone (HPVAnt/PMMA interface), which leads
to a large contact resistance.
Depending on crystal thickness, the buffer layer (MoO3,
CsF), and the electrode (Au or Ca), the m+ value was in the
range of 0.75–2.62 cm2 VÀ1 sÀ1. Remarkably, the current den-
sity in a saturation regime (more than 40 kAcmÀ2) exceeds
that of the rubrene-based SC-FET devices[3f] (see the
Supporting Information). Furthermore, an ambipolar behav-
ior with electron mobility in the range of 0.04–0.13 cm2 VÀ1 sÀ1
was measured for the HPVAnt/PMMA SC-FET (see the
Supporting Information).
We also investigated the light-emitting properties of
HPVAnt in SC-FET configuration. The optical image of the
device and PL of the single crystal are shown in Figure 4a,b.
The EL spectrum closely resembles the PL spectrum; the
emission band is somewhat broadened, but the lmax (490 nm)
is not shifted (Figure 4c). Figure 4d shows optical images of
EL that were taken of the surface of a device operating at
fixed gate voltage (À200 V) while sweeping the drain-source
voltage (VDS) between 0 V and À200 V. At a lower operating
bias, the EL is dominated by hole transport and the emission
zone is close to the drain electrode. However, increasing the
electron injection at higher VDS values progressively extends
the light emission zone toward the source electrode, which
confirms the ambipolarity of the device.
Figure 3. AFM images of a) submonolayer, b) monolayer, and c) multi-
layer films of HPVAnt grown on Si/SiO2. d) Height profile of image (c).
the layers. X-ray diffraction analysis of the vacuum-deposited
films corresponds well to that of bulk HPVAnt powder (see
the Supporting Information). Although the single-crystal X-
ray diffraction data are of insufficient quality to deduce the
exact interatomic distances, the orientation of the molecules
in the unit cell is clear (see the Supporting Information) and
closely resembles that of 2,6-bis(4-pentylphenylvinyl)anthra-
cene.[17a] The molecules pack in a usual herringbone fashion.
The anthracene groups of adjacent molecules align with one
another to form sheets that are perpendicular to the long
molecular axis (monoclinic unit cell). Such packing max-
imizes the two-dimensional p interactions between the mol-
ecules, and is slightly different to that of other long acenes, for
example, pentacene, which show slippage along the long
molecular axis.[24]
The bias-stress effects and the shelf stability of the device
are critical for the application of OSCs.[25] HPVAnt-based TF-
FETs that were made on bare SiO2/Si have excellent stability
after more than 20,000 operating cycles in vacuum (m+ =
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In conclusion, we have demonstrated that the structurally
simple hydrocarbon HPVAnt exhibits a unique combination
of high field-effect mobility of up to 1.5 cm2 VÀ1 sÀ1 in thin
films (2.6 cm2 VÀ1 sÀ1 in single crystals), strong solid-state
emission, and excellent operational stability in air. Both thin-
film and single-crystal transistors emit bright-blue EL that is
stable under continuous operation. To the best of our
knowledge, this is the highest mobility OLET prepared to
date. Importantly, the high PLQY of 70% that was achieved
for HPVAnt crystals disproves the common belief that the
strong p–p interactions that are required for high charge
mobility necessarily lead to quenching of the luminescence in
the solid state. This is likely attributable to the high energy of
the excited state T1 relative to S1 (2T1 > S1), which turns off
emission quenching through singlet fission and, thus, outlines
a new principle for the design of emissive crystalline OSCs.
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[18] The fluorescence of HPVAnt decay biexponentially with
approximate lifetimes of t1 = 6.7 ns (72%) and t2 = 27 ns
(28%) in solution, t1 = 9.2 ns (69%) and t2 = 20.5 ns (31%) in
crystals, and t1 = 1.4 ns (79%) and t2 = 6.3 ns (21%) in thin films
(see the Supporting Information). Similar lifetimes and biexpo-
nential decay was shown for 2-styrylanthracene: G. Bartocci, U.
Mazzucato, A. Spalletti, F. Elise, Spectrochim. Acta Part A 1990,
46, 413.
[19] The S1 state lies between the highest-energy absorption peak
(394 nm, 3.15 eV) and lowest-energy emission peak (423 nm,
2.94 eV). A cross-point of the absorption and emission spectra
occurs at 406 nm (3.06 eV).
[20] J. R. Shaw, R. T. Webb, R. H. Schmehl, J. Am. Chem. Soc. 1990,
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Received: November 22, 2011
Revised: January 11, 2012
Published online: March 2, 2012
Keywords: anthracenes · luminescence · semiconductors ·
.
singlet fission · transistors
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