High performance, acene-based organic thin film transistors

Article abstract of DOI:10.1039/b901448a

1,4,8,11-Methyl-substituted 6,13-triethylsilylethynylpentacene shows extended π-π overlap when deposited from solution, yielding organic thin film transistors with high and reproducible hole mobility with negligible hysteresis. The Royal Society of Chemis

Full text of DOI:10.1039/b901448a

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High performance, acene-based organic thin film transistorsw
Gonzalo Rincon Llorente,a Marie-Beatrice Dufourg-Madec,a David J. Crouch,a
Robin G. Pritchard,b Simon Ogierc and Stephen G. Yeates*a
Received (in Cambridge, UK) 22nd January 2009, Accepted 23rd March 2009
First published as an Advance Article on the web 9th April 2009
DOI: 10.1039/b901448a
1,4,8,11-Methyl-substituted 6,13-triethylsilylethynylpentacene
shows extended p–p overlap when deposited from solution,
yielding organic thin film transistors with high and reproducible
hole mobility with negligible hysteresis.
annealing. Importantly the variability in mobility values
across a single substrate is lower than for previously reported
6,13-substituted pentacene derivatives, which is important in
the commercialisation of such technology.
Scheme 1 shows the synthetic strategy used. Reduction of
1,4-dimethyl-7-oxobicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic
anhydride, 3, resulted in 50–60% yields similar to those
reported in the literature9 with the further reduction of
3,6-dimethylphthalic anhydride, 4, by standard methods10 to
give 3,6-dimethylphthalyl alcohol, 5. A high yielding route to
6,13-pentacenequinone has been reported via 1,2-benzenedi-
carboxaldehyde.11 We attempted the oxidation of 5 to
3,6-dimethylphthalaldehyde by several literature methods12–14
but in all cases a mixture of 5 and 4,7-dimethyl-isobenzofuranone
was obtained. Therefore we proceeded via the reaction of
3,6-dimethyl-1,2-bis(bromomethyl)benzene, 6, which was
obtained in high yield from 5, with p-benzenequinone to give
1,4,8,11-tetramethyl-6,13-pentacenequinone, 7, in low yield. 8
and 9 were synthesised by reported methods6 and purified
using a silica plug.
New molecular structure plays a key role in optimising the
performance of organic electronics whether they are small
molecules or oligomeric semiconductors. In the case of
acene-based small molecules, the effects of conjugation length
and side chain type on solubility and thin film electronic
properties have been extensively studied.1,2 Organic thin film
transistors (OTFTs) based on pentacene exhibit some of the
best performance to date with field effect hole mobilities (mFET
)
of 3 cm2 Vꢀ1 sꢀ1 for vacuum deposited3 and 35 cm2 Vꢀ1 sꢀ1
for single crystal devices,4 respectively. 6,13-substituted
pentacene derivatives have been the subject of intensive recent
investigation attempting to combine good low cost solution
processability with high performance.5 Substitution with
either alkylethynyl or trialkylsilylethynyl has been shown to
both improve solvent solubility whilst disrupting the
herringbone packing in pentacene resulting in improved p–p
interaction and improved hole mobilities.6
Single crystals of 8 and 9 were obtained showing the
ꢀ
molecules crystallise in a low symmetry P1 groupz with
the molecules packed in the highly desirable 2D slip stack
Although 6,13-triisopropylsilyletynylpentacene (TIPSP) has
been shown to exhibit hole mobility up to 1.8 cm2 Vꢀ1 sꢀ1
from solution, extreme care has to be taken with respect to
thin film processing conditions with variability in measured
mobility values across a single substrate.5 Other processes
including nanoribbon7 or evaporation8 permit high mobility
devices but move further away from the concept of cheap and
large area device fabrication.
structure with an average p–p stacking distance between the
In this work we present the first reported study on the
synthesis and characterisation of 1,4,8,11-methyl-substituted
6,13-trialkylsilylethynylpentacene. We present two molecules
1,4,8,11-tetramethyl-6,13-triethylsilylethynylpentacene 8 and
1,4,8,11-tetramethyl-6,13-triisopropyllsilylethynylpentacene 9
where 8 has a maximum mFET of 2.5 cm2 Vꢀ1 sꢀ1 in air
when processed from solution at 70 1C with no post-thermal
a Organic Materials Innovation Centre, School of Chemistry,
University of Manchester, Oxford Road, Manchester, UK M13 9PL.
E-mail: Stephen.yeates@manchester.ac.uk;
Fax: +44 (0)161 275 4273; Tel: +44 (0)161 275 1421
b School of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL. Tel: +44 (0)161 276 4516
c PETeC, Wilton Centre, WiltonRedcar, Cleveland, UK TS10 4RF.
Tel: +44 (0)1740 625 700
w Electronic supplementary information (ESI) available: Details of
experimental procedures, UV-vis, X-ray OOP d-spacings, X-ray dif-
fraction, graphs, transfer curves, and AFM images. CCDC 717661
and 717662. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b901448a
Scheme 1 Synthetic route to molecules 8 and 9. Reagents and
conditions: (a) Et2O, rt, 60%; (b) H2SO4, ꢀ6 1C, 50%; (c) LiAlH4,
THF, 70%; (d) PBr3, toluene, 95%; (e) NaI, p-benzoquinone, 90 1C,
DMA 25%; (f) (1) BuLi, trialkylsilylacetylene, (2) SnCl2, HCl,
THF, 50%.
ꢁc
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Chem. Commun., 2009, 3059–3061 | 3059

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Fig. 2 Typical (a) output and (b) transfer curves for 8 solution
deposited from 0.5% w/w toluene at RT on OTS-treated SiO2.
Fig. 1 Molecular packing of 8 (a) and 9 (b) illustrating the packing
and p–p overlap.
long, were thermally evaporated, at 10ꢀ6 mBar, until a layer
thickness of 50 nm was obtained, yielding 9 transistors per
substrate. Gold was chosen since its work function, 5.1 eV,
closely matches the HOMO level of 8 and 9.
conjugated pentacene backbones of 3.48
A similar to
TIPSP.5,6 Although the interplanar distances are similar for
both 8 and 9, there is significant difference in the extent of
fused rings backbone overlap, with 9 being similar to TIPSP
whereas 8 shows a significantly greater overlap (Fig. 1). AFM
confirms the large difference in packing between 8 and 9
(ESIw). 8 is able to form a nearly continuous film when drop
cast from toluene solution onto an OTS modified silica
surface, whereas 9 wets only partially. Tapping mode of a
thin film of 8 reveals atomic steps of about 1.77 nm, indicating
that the molecule packs almost vertically to the substrate in an
atomic layer.
Compound 8 shows very high on/off ratios in excess of 107,
with maximum mSAT = 2.5 cm2 Vꢀ1 sꢀ1 (Fig. 2) and with
negligible hysteresis and stable low threshold voltage. This
device was one of the group of 9 OTFTs fabricated, the
mobility range being 0.6 to 2.5 cm2 Vꢀ1
mobility of 1.3 cm2 Vꢀ1 sꢀ1 and a standard deviation of
0.4 cm2 Vꢀ1
s
ꢀ1, with an average
s
ꢀ1. Compound 9 exhibits a maximum mSAT of
about 4 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1, due to poor film quality and less
favourable charge injection. Under equivalent conditions
TIPSP18 drop cast devices exhibit discontinuous film
Molecular orbital levels of 8 and 9 were measured using
cyclic voltammetry in acetonitrile, and UV-vis performed in
solution and of thin films (ESIw). Whilst the limitations of this
methodology are recognised, we are unable to determine the
reduction onset by CV since both materials undergo decom-
position on reduction. Results are summarised in Table 1.
It is known that charge transport in disordered crystalline
semiconductor obeys a semi-classical hopping charge transfer
theory,15 with factors such as translation along the short axis
and ring orientation influencing dramatically the electronic
performance of the electronic semiconductor.16 It has been
shown that increasing the extent of p–p overlap can result in
an increase in hole mobility at the same p–p stacking
distance.17 Since 8 shows a higher degree of p–p overlap and
the same p–p stacking distance of the conjugated backbone as
both 9 and TIPSP then a higher hole mobility would be
anticipated.
structures and gave maximum mSAT of B10ꢀ3 cm2 Vꢀ1 sꢀ1
.
To conclude, we have shown that 1,4,8,11-tetramethyl-6,13-
triethylsilylethynylpentacene can make excellent OFET’s, with
high mobilities and on/off ratios when solution processed
in air with improved device to device reproducibility as a
consequence of the extended p–p overlap. With optimisation
of formulation18 and device architecture it is believed that this
new family of compounds will enable all solution processed
devices with high performance and excellent reproducibility to
be fabricated.
The authors would like to acknowledge the UK Home
Office and the University of Manchester for funding and
W. Eccleston and M. Raja of the University of Liverpool
for access to clean room facilities.
Notes and references
Discrete bottom-gate, top-contact organic field effect
transistor (OFET) devices were fabricated on OTS treated
heavily n-doped silicon wafers comprising a 3000 A thermally
grown gate oxide layer. 8 and 9 were drop cast from 0.5 wt%
toluene solution and dried under ambient atmosphere at 70 1C
in an open laboratory with no post-thermal annealing. Gold
source and drain channel electrodes, 60 mm width and 2 mm
z Single crystals of compounds 8 and 9 were obtained by solvent
diffusion (solvent was 1,2,4-trichlorobenzene and antisolvent was
acetonitrile), mounted in inert oil and transferred to the cold gas
stream (150 K) diffractometer.
Crystal determination of compound 8: C42H50Si2, M = 611.00,
triclinic, a = 7.3024(2), b = 10.9762(3), c = 12.1090(4) A, a =
74.402(3)1, b = 89.953(2)1, g = 73.746(2)1, U = 894.55(5) A3, T =
ꢀ
100 K, space group P1 (no. 2), Z = 1, 13 549 reflections measured,
3297 unique (Rint = 0.077) which were all used in the calculations. The
final wR(F2) was 0.1991 (all data) and R1 0.0771 (I 4 3s(I)).
Crystal determination of compound 9: C48H62Si2, M = 695.16, a =
9.0135(6), b = 10.2107(5), c = 11.7743(8) A, a = 70.694(2)1, b =
84.900(2)1, g = 82.342(5)1, U = 1012.41(11) A3, T = 100 K, space
Table 1 Oxidation potentials of 8 and 9 versus SCE reference by
comparison to ferrocene and their optical band gap from thin film,
determined at an onset absorption peak. Gap is determined from UV
absorption edge in liquid state
ꢀ
group P1 (no. 2), Z = 1, 5600 reflections measured, 3548 unique
(Rint = 0.098) which were all used in the calculations. The final wR(F2)
was 0.2784 (all data) and R1 0.1180 (I 4 3s(I)).
Compound
8
9
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