Systematic improvement in charge carrier mobility of air stable triarylamine copolymers

Article abstract of DOI:10.1021/ja9034818

(Figure Presented) The electrical performance of organic semiconducting polymers in field-effect transistor devices is now sufficient for initial low complexity circuit applications. To achieve high performance, either operation in an inert atmosphere or

Full text of DOI:10.1021/ja9034818

Published on Web 07/15/2009
Systematic Improvement in Charge Carrier Mobility of Air Stable Triarylamine
Copolymers
Weimin Zhang,*,†,‡ Jeremy Smith,† Rick Hamilton,† Martin Heeney,† James Kirkpatrick,†
Kigook Song,| Scott E. Watkins,§ Thomas Anthopoulos,† and Iain McCulloch†
Imperial College, London, SW7 2AZ, Guangxi UniVersity for Nationalities, Nanning 530006, PR China, Kyung Hee
UniVersity, Yongin, Gyeonggi-do 446-701, Korea, and CSIRO Molecular and Health Technologies,
VIC 3169, Australia
Received April 29, 2009; E-mail: weimin.zhang@imperial.ac.uk
A combination of improved semiconducting polymer molecular
design1-5 and fabrication techniques6 has resulted in recent demonstra-
tions of both p- and n-type charge carrier mobilities exceeding
application relevant values of 0.01 cm2/(V s) in field effect transistor
devices, fabricated and measured in air over extended periods.
However, even in instances where “reasonable” ambient stability is
reported, it is often the case that devices driven under high current
density (saturation) conditions or where the transistor architecture is
bottom gate, with the semiconductor film exposed to ambient condi-
tions, suffer from operational and even storage instabilities, leading to
large initial carrier mobilities deteriorating to significantly lower values
over short periods.7 Over typical operation lifetimes, the mobility often
degrades by over an order of magnitude.2 Additionally, high mobility
devices may also require both surface treatment and prolonged thermal
annealing steps to optimally orient and order at the molecular and
domain length scales.8 These additional processing steps compromise
the requirement of ease of fabrication. Clearly there is still demand
for high performance, fully air stable polymers that when deposited
from solution do not require further treatment to obtain optimal
performance and have the flexibility to be employed in any device
architecture.
In this report, we demonstrate that incorporation of bridged
phenyl units, specifically fluorene and indenofluorene,9,10 substan-
tially improves the charge carrier mobility of triarylamine polymers,
by almost an order of magnitude, while still retaining the excellent
stability of aryl amine semiconducting polymers.7 Planar, conju-
gated aromatic repeat units incorporated into semiconducting
polymer backbones have previously been shown to promote highly
ordered and oriented microstructures. In most cases, extensive
intermolecular π-stacking arises from the assembly of the coplanar
polymer conformation which contributes to enhancing in-plane
transport.11 A common molecular template for organization is the
thiophene or fused arylthiophene repeat unit. The incorporation of
conjugated thiophenes in a polymer, however, often leads to
sensitivities to ambient oxygen, moisture, or light, often attributed
to the high electron density, reactive diene functionality, and long
wavelength optical absorption exhibited by the thiophene units.12
The carbon bridged phenylene units fix coplanarity between adjacent
phenylene rings, optimizing backbone conjugation, as also shown
in previous work on oligomeric systems,13 as well as increasing
polymer persistence length, while reducing backbone conformation
rotational disorder. This improved coplanarity is believed to be a
primary reason for the observed improvements in charge carrier
mobility on incorporation of the bridged units.
Methyl substituted phenylamine polymers have been shown to
have excellent ambient photo and electrical stability.7 The presence
of the amine nitrogen in the backbone acts to prevent efficient
delocalization of electrons between adjacent phenyl units, thus
limiting the effective conjugation length and resulting in low lying
HOMO energy levels and excellent oxidative stability. DFT
calculations suggest an equilibrium torsion angle of 36° between
successive units. The nonplanar, rotationally free and large linkage
angle backbone however prevents optimal intermolecular π-electron
delocalization, leading to an amorphous microstructure and rela-
tively low charge carrier mobility in comparison to the polymers
referred to earlier. Further thermal treatment of the as-cast thin films
does not result in an improvement in charge carrier mobility, as
there is no microstructural ordering on annealing; thus a time-
consuming processing step can be circumvented. The design
strategy employed is to introduce low electron density, bridged
biphenyl and triphenyl units which will extend the backbone
conjugation without significantly affecting the HOMO energy level.
Both dialkylated fluorene and indenofluorene repeat units were
incorporated in a 1:1 stoichiometry. In fact DFT calculations suggest
that, since the fluorene and indenofluorene units have lower HOMOs
than TAA, their inclusion in the polymer backbone further stabilizes
positive charges. DFT calculations were carried out on the polymers
with geometries optimized using the hybrid density functional
B3LYP and a double split Pople basis set with extra polarization
(6-31 g*). Calculations were performed on oligomers of lengths 1,
2, 3, 4, and 5 (counting a TAA unit and a PF8 or PIF8 unit as a
unit, Figure 1) at which point the HOMO energy was found to
change by less than 10 meV. Aliphatic side chains were removed
from the polymer prior to computation.
All three polymers were synthesized via Suzuki polycondensation
protocol, optimized to obtain a number average molecular weight in
† Imperial College.
‡ Guangxi University for Nationalities.
| Kyung Hee University.
Figure 1. Polymer molecular structure and highest occupied molecular
orbital distribution.
§ CSIRO Molecular and Health Technologies.
9
10814 J. AM. CHEM. SOC. 2009, 131, 10814–10815
10.1021/ja9034818 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
the 25-100 kDa range. Although the molecular weight is not identical
for all polymers, we do not expect a strong molecular weight
dependence on electrical properties, as the number average molecular
weight is above 15 kDa in all cases, the end group density will be
low, and we should be in a plateau region for electrical properties.
The catalyst chosen was Pd2(dba)3/(o-tol)3P, a K3PO4 base, and Aliquat
336 was employed as the phase transfer catalyst in a toluene/1,4-
dioxane/water solvent. Full experimental and characterization details
are provided in the Supporting Information. PTAA was obtained by
the A-B polycondensation of N-(4-bromophenyl)-4-(4,4,5,5-tetram-
ethyl-1,3,2-dioxabo-rolan-2-yl)-N-(2,4-dimethylphenyl)benzenamine
monomer, whereas both copolymers were formed from an AA BB
Suzuki cross coupling condensation from the dibromotriarylamine and
the pinacol ester boronates of the bridged phenyl repeat units. All
polymerizations proceeded in good yield; the polymers were purified
by washing via Soxhlet extraction with methanol, acetone, and iso-
hexane. The trace amount of palladium salt was removed by treating
the polymer solution in chloroform with sodium diethyldithiocarbo-
mate. The polymer structures were characterized by 1H and 13C NMR
spectroscopy and element analysis. The weight-average molecular
weights (Mw) were determined by GPC versus polystyrene, and results
are summarized in Table 1. All polymers are soluble in organic solvents
such as chloroform, THF, and chlorobenzene, as well as more printing
friendly solvents such as toluene and xylene. The incorporation of the
dialkyl carbon bridging functionality and its out of plane projection
preserves the good solubility of the triaryl amine unit. No first-order
thermal transitions could be observed by differential scanning calo-
rimetry, and preliminary NEXAFS and GIXRD data indicate an
absence of measurable orientation or order in thin films. Additionally,
annealed thin films appear to exhibit no birefringence under crossed
polarizers.
Figure 2. (a) (Top) Periodic ON and OFF currents and (bottom) charge
carrier mobility measurements on ambient devices stored in air over a period
of ∼3 months. (b) Transfer characteristics of PIF8-TAA copolymer with L
) 30 µm, W ) 1000 µm, corresponding to a mobility of 3.8 × 10-2 cm2/
(V s).
measured over several weeks in a top gate device. Both top and bottom-
gate devices, where in the latter case the semiconductor is directly
exposed to ambient, showed that, within experimental measurement
accuracy, there was no change in electrical performance. As a
perspective, even the relatively stable polymer pBTTT would show a
two orders of magnitude drop in ON current under the same
conditions.2 In summary, incorporation of the bridged triphenyl repeat
unit, indenofluorene, as an alternating copolymer with triarylamine has
been shown to provide high performing amorphous, solution process-
able semiconducting polymers, suitable for printing large area electronics.
Acknowledgment. This work was in part carried out under
EPSRC Grants EP/FO56648/1 and EP/G031088/1.
Table 1. Polymer Properties
Supporting Information Available: Synthesis of PTAA, PF-TAA,
PIF-TAA; PESA data; transfer and output transistor characteristics; air
stability data, UV-vis spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
charge
carrier
mobility
(cm2/(V s))
HOMO
(PESA)
(-eV)
HOMO
(DFT)
(-eV)
Mw/Mn
(kDa)
polymer
PTAA
PF8-TAA
PIF8-TAA
96.4/45.5
52.9/27.9
25.4/15
5.2
5.4
5.5
4.6
4.8
4.7
4 × 10-3
2 × 10-2
4 × 10-2
References
(1) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler,
M.; Facchetti, A. Nature 2009, 457, 679–686.
(2) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.;
Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc,
M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5,
328–333.
An experimental estimation of the HOMO energy levels of the
polymers by Photo Electron Spectroscopy in Air (PESA),14
summarized in Table 1 (shown in full in the Supporting Informa-
tion), confirms high HOMO energy levels. This was expected to
enhance stability to electrochemical oxidation in the presence of
ambient oxygen and humidity.15
(3) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner, M. A.; Marks,
T. J. J. Am. Chem. Soc. 2008, 130, 7670–7685.
(4) Fong, H. H.; Pozdin, V. A.; Amassian, A.; Malliaras, G. G.; Smilgies, D.-M.;
He, M.; Gasper, S.; Zhang, F.; Sorensen, M. J. Am. Chem. Soc. 2008, 130,
13202–13203.
(5) Li, J.; Qin, F.; Li, C. M.; Bao, Q.; Chan-Park, M. B.; Zhang, W.; Qin, J.; Ong,
B. S. Chem. Mater. 2008, 20, 2057–2059.
Bottom-contact, top-gate (and bottom-gate) architecture field effect
transistor devices were fabricated with the polymer semiconductors
deposited from solution. As cast, hole mobilities were calculated from
the transfer characteristics using a standard thin film model. PTAA
devices showed a mobility of ∼4 × 10-3 cm2/(V s). This was improved
by a factor of 5 to 0.02 cm2/(V s) by the introduction of the fluorene
unit and further increased to 0.04 cm2/(V s), for the indenofluorene
copolymer (transfer characteristics shown in Figure 2b) with an ON/
OFF ratio in excess of 106. It is speculated that the increase in polymer
backbone planarity and persistence length in the copolymers leads to
improvement in the intramolecular π-orbital overlap as well as
improving the local structural organization, resulting in the measured
increase in hole mobilities. No evidence of thin film crystallinity could
be observed for any polymer semiconductor. Devices were stored in
ambient conditions and periodically measured over a period of several
months. Figure 2b illustrates ON and OFF currents, mobilities
(6) Tsao, H. N.; Cho, D.; Andreasen, J. W.; Rouhanipour, A; Breiby, D. W.; Pisula,
W.; Mu¨llen, K. AdV. Mater. 2009, 21, 209–212.
(7) Mathijssen, S. G. J.; Coelle, M.; Gomes, H.; Smits, E. C. P.; de Boer, B.;
McCulloch, I.; Bobbert, P. A.; de Leeuw, D. M. AdV. Mater. 2007, 19, 2785–
2789.
(8) Chang, J. F.; Sun, B.; Breiby, D. W.; Nielsen, M. M.; So¨lling, T. I.; Giles, M.;
McCulloch, I.; Sirringhaus, H. Chem. Mater. 2004, 16, 4772–4776.
(9) Allard, S.; Forster, M.; Souharce, B.; Thiem, H; Scherf, U. Angew. Chem.,
Int. Ed. 2008, 47, 4070–4098.
(10) Setayesh, S.; Marsitzky, D.; Mullen, K. Macromolecules 2000, 33, 2016–
2020.
(11) DeLongchamp, D. M.; Kline, R. J.; Jung, Y.; Lin, E. K.; Fischer, D. A.;
Gundlach, D. J.; Cotts, S. K.; Moad, A. J.; Richter, L. J.; Toney, M. F.; Heeney,
M.; McCulloch, I. Macromolecules 2008, 41, 5709–5715.
(12) Abdou, M. S. A.; Orfino, F. P.; Son, Y.; Holdcroft, S. J. Am. Chem. Soc.
1997, 119, 4518–4524.
(13) Usta, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 8580–
8581.
(14) Uda, M. J. J. Appl. Phys 1985, 24, 284–288.
(15) Leeuw, D. M. d.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth.
Metal. 1997, 87, 53–59.
JA9034818
9
J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009 10815