Easily processable phenylene-thiophene-based organic field-effect transistors and solution-fabricated nonvolatile transistor memory elements

Article abstract of DOI:10.1021/ja035143a

The synthesis of a new series of mixed phenylene - thiophene oligomers is reported; 2,5-bis(4-n-hexylphenyl)thiophene (dH-PTP, 1), 5,5′-bis(4-n- hexylphenyl)-2,2′-bithiophene (dH-PTTP, 2), 5,5″-bis-(4-n-hexylphenyl)-2,2′:5′,2″-terthiophene (dH-PT₃P, 3), 5,5?-bis(4-n-hexylphenyl)-2,2′:5′, 2″:5′,2?-quater-thiophene (dH-PT₄P, 4), 1,4-bis[5-(4-n-hexylphenyl)-2-thienyl]benzene (dH-PTPTP, 5), and 2,5-bis[4(4′-n-hexylphenyl)phenyl]thiophene (dH-PPTPP, 6) were characterized by ¹H NMR, elemental analysis, UV-visible spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Vacuum-evaporated and solution-cast films were characterized by X-ray diffraction and scanning electron microscopy. All compounds display high p-type carrier mobilities as evaporated (up to 0.09 cm²/Vs) and as solution-cast (up to 0.03 cm²/Vs) films on both Si/SiO₂ and ITO/GR (glass resin) substrates. The straightforwardly synthesized dH-PTTP (2) displays an unprecedented combination of mobility, on/off ratio, stability, and processability. Both dH-PTTP (2) and dH-PPTPP (6) display a reversible, tunable, and stable memory effect even as solution-cast devices, with turn-on characteristics shifting from accumulation mode to zero or depletion mode after a writing voltage V_w is applied. The charge storage is distributed over the gate dielectric structure and is concentrated near the dielectric-semiconductor interface, as evidenced by the response of floating gate configuration devices. Simple nonvolatile elements have been fabricated by solution-only techniques on ITO substrates using spin-coated glass resin, solution-cast oligomeric semiconductors, and painted graphite paste electrodes.

Full text of DOI:10.1021/ja035143a

Published on Web 07/15/2003
Easily Processable Phenylene-Thiophene-Based Organic
Field-Effect Transistors and Solution-Fabricated Nonvolatile
Transistor Memory Elements
Melissa Mushrush,† Antonio Facchetti,† Michael Lefenfeld,‡ Howard E. Katz,*,‡ and
Tobin J. Marks*,†
Contribution from the Department of Chemistry and the Materials Research Center,
Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 and Bell
Laboratories, Lucent Technologies, 600 Mountain AVenue, Murray Hill, New Jersey 07974
Received March 13, 2003; E-mail: hek@lucent.com; t-marks@northwestern.edu
Abstract: The synthesis of a new series of mixed phenylene-thiophene oligomers is reported; 2,5-bis(4-
n-hexylphenyl)thiophene (dH-PTP, 1), 5,5′-bis(4-n-hexylphenyl)-2,2′-bithiophene (dH-PTTP, 2), 5,5′′-bis-
(4-n-hexylphenyl)-2,2′:5′,2′′-terthiophene (dH-PT3P, 3), 5,5′′′-bis(4-n-hexylphenyl)-2,2′:5′,2′′:5′′,2′′′-quater-
thiophene (dH-PT4P, 4), 1,4-bis[5-(4-n-hexylphenyl)-2-thienyl]benzene (dH-PTPTP, 5), and 2,5-bis[4(4′-n-
1
hexylphenyl)phenyl]thiophene (dH-PPTPP, 6) were characterized by H NMR, elemental analysis, UV-
visible spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Vacuum-evaporated
and solution-cast films were characterized by X-ray diffraction and scanning electron microscopy. All
compounds display high p-type carrier mobilities as evaporated (up to 0.09 cm2/Vs) and as solution-cast
(up to 0.03 cm2/Vs) films on both Si/SiO2 and ITO/GR (glass resin) substrates. The straightforwardly
synthesized dH-PTTP (2) displays an unprecedented combination of mobility, on/off ratio, stability, and
processability. Both dH-PTTP (2) and dH-PPTPP (6) display a reversible, tunable, and stable memory
effect even as solution-cast devices, with turn-on characteristics shifting from accumulation mode to zero
or depletion mode after a writing voltage Vw is applied. The charge storage is distributed over the gate
dielectric structure and is concentrated near the dielectric-semiconductor interface, as evidenced by the
response of “floating gate” configuration devices. Simple nonvolatile elements have been fabricated by
solution-only techniques on ITO substrates using spin-coated glass resin, solution-cast oligomeric
semiconductors, and painted graphite paste electrodes.
Introduction
adaptive elements or information storage buffers, while attrac-
tive, has not been extensively pursued.
Organic semiconductors employed as active layers in field-
effect transistors (FETs) are of great current interest because
such FETs can potentially be fabricated at low cost, over large
areas, and on flexible substrates.1 Such facile fabrication
approaches offer a significant advantage over silicon technology
in numerous applications where the performance level of silicon
is not essential. Organic semiconductors have now reached
impressive levels of performance and depth of understanding.1,2
Concurrently, substantial progress has been made in various
organic-based electronic circuits, such as displays,3 inverters
and logic elements,4 sensors,5 and photovoltaic cells.6 Neverthe-
less, very few current-generation organic semiconductors si-
multaneously meet the criteria of high carrier mobility, low
leakage current, straightforward synthesis, facile film deposi-
tion,7 and chemical stability. Furthermore, the use of organic
transistors as nonvolatile devices that could function as tunable/
While the nonvolatile transistor memory element is a well-
known device in conventional silicon electronics,8 it has only
(2) (a) Marks, T. J.; Facchetti, A. WO Patent 0,310,778, 2003. (b) Facchetti,
A.; Mushrush, M.; Katz, H. E.; Marks, T. J. AdV. Mater. 2003, 15 (1), 33.
(c) Katz, H. E.; Siegrist, T.; Scho¨n, J. H.; Kloc, C.; Batlogg, B.; Lovinger,
A. J.; Johnson, J. ChemPhysChem 2001, 3, 167. (d) Malenfant, P. R. L.;
Dimitrakopoulos, C. D.; Gelorme, J. D.; Kosbar, L. L.; Graham, T. O.
Appl. Phys. Lett. 2002, 80 (4), 2517. (e) Marks, T. J.; Facchetti, A.;
Sirringhaus, H.; Friend, R. H. WO Patent 0,209,201, 2002. (f) Babel, A.;
Jenekhe, S. A. AdV. Mater. 2002, 145, 371. (g) Facchetti, A.; Deng, Y.;
Wang, A.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend, R. H. Angew.
Chem., Int. Ed. 2000, 39 (24), 4547. (h) Facchetti, A.; Wang, A.; Marks,
T. J.; Sirringhaus, H.; Deng, Y.; Friend, R. H. Polym. Mater. Sci. Eng.
2000, 83, 290. (i) Katz, H. E.; Johnson, J.; Lovinger, A. J.; Li, W. J. Am.
Chem. Soc. 2000, 122, 7787.
(3) (a) Sheraw, C. D.; Zhou, L.; Huang, J. R.; Gundlach, D. J.; Jackson, T. N.;
Kane, M. G.; Hill, I. G.; Hammond, M. S.; Campi, J.; Greening, B. K.;
Francl, J.; West, J. Appl. Phys. Lett. 2002, 80 (6), 1088. (b) Mitschke, U.;
Bauerle, P. J. Mater. Chem. 2000, 10 (7), 1471. (c) Wisnieff, R. Nature
1998, 394, 225. (d) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson,
J. Nature 1998, 394, 253.
(4) (a) Crone, B. K.; Dodabalapur, A.; Sarpeshkar, R.; Filas, R. W.; Lin, Y.-
Y.; Bao, Z.; O’Neill, J. H.; Li, W.; Katz, H. E. J. Appl. Phys. 2001, 89 (9),
5125. (b) Crone, B.; Dodabalapur, A.; Lin, Y.-Y.; Filas, R. W.; Bao, Z.;
LaDuca, A.; Sarpeshkar, R.; Katz, H. E.; Li, W. Nature 2000, 403, 521.
(c) Lin, Y.-Y.; Dodabalapur, A.; Sarpeshkar, R.; Bao, Z.; Li, W.; Baldwin,
K.; Raju, V. R.; Katz, H. E. Appl. Phys. Lett. 1999, 74 (18), 2714.
(5) (a) Bartic, C.; Palan, B.; Campitelli, A.; Borghs, G. Sens. Actuators, B
2002, 83, 115, (b) Crone, B.; Dodabalapur, A.; Gelperin, A.; Torsi, L.;
Katz, H. E.; Lovinger, A. J.; Bao, Z. Appl. Phys. Lett. 2001, 78 (15), 2229.
† Northwestern University.
‡ Lucent Technologies.
(1) (a) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals
and Polymers; Oxford University Press: Oxford, 1999. (b) Dimitrakopoulos,
C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14 (2), 99. (c) Horowitz, G.
J. Mater. Chem. 1999, 9, 2021. (d) Horowitz, G. AdV. Mater. 1998, 10
(5), 365.
9
9414
J. AM. CHEM. SOC. 2003, 125, 9414-9423
10.1021/ja035143a CCC: $25.00 © 2003 American Chemical Society

Phenylene−Thiophene-Based Field-Effect Transistors
A R T I C L E S
recently been shown to have potential in organic transistors.9
In a nonvolatile transistor, an added voltage is introduced be-
tween the gate and the semiconducting channel, and via charge
storage, the effective gate voltage is then altered from the applied
gate voltage Vg. The resulting polarization creates additional
electronic states within the device, and the effective threshold
voltage can be brought close to zero (or some other arbitrary
value) for real-world applications where available gate voltage
is limited. Previously, a memory effect in organic-based devices
was only observed as a hysteresis in the current/voltage char-
acteristics10 or as changes in conductivity or photocurrent in
two-terminal devices.11 The advantages of a three-terminal
memory device include more ready incorporation into amplify-
ing and adaptive circuits and use as a tunable element in a
multitransistor circuit block.
Unsubstituted phenylene (P)-thiophene (T) oligomers with
various P:T ratios are a promising class of semiconductors; many
have been employed in light-emitting diodes12 as well as in
p-channel FETs.13,14 We have recently demonstrated a substan-
tial memory effect in vapor-deposited 1,4-bis(5-phenyl-2-
thienyl)benzene (PTPTP)- and N,N′-bis(1H,1H-perfluorooctyl)-
naphthalene-1,4,5,8-tetracarboxylic diimide (F15-NTCDI)-based
devices, exhibiting p- and n-type behavior, respectively, with
threshold voltage shifts of up to 40 V, depending on the
dielectric material and polarization conditions employed.9 It was
suggested that at least some of the charge storage must occur
in the bulk of the dielectric since the incorporation into the
dielectric of small amounts of additives has a significant effect
on the polarization.
Also, the incorporation of phenylene units into the thiophene
backbone results in a small lowering of the HOMO energy, and
it has been shown that this lowering is accompanied by a
reduction in the off current, thus increasing the device on/off
ratio.13 This effect is also apparent as a shift of the threshold
voltage toward the accumulation regime.
The above observations and the questions they raise about
memory function, solution processability, and device current
on/off ratio tuning motivate the present study: the synthesis
and characterization of a new series of n-hexyl-substituted
thiophene-phenylene oligomers (1-6) designed for solution-
processable FETs and simple nonvolatile elements. The work
reported here focuses on compounds that are particularly easy
to synthesize and may be readily deposited by both sublimation
at moderate temperatures and casting from solution in xylenes.
We demonstrate here the first example (to our knowledge) of a
tunable and stable memory effect in OFETs fabricated using
simple solution-casting methods. We also report more definitive
experiments on a floating gate structure which support the idea
that charging of the dielectric, especially near the dielectric-
semiconductor interface, rather than solely at the semiconduc-
tor-metal interfaces, plays a key role.
It will be seen that compounds 1 and 2 exhibit remarkably
high carrier mobilities for such small and readily prepared
molecules. For compounds 2 and 6 in this series, with large
threshold voltages (substantially in the accumulation regime),
a large memory effect is observed on both Si/SiO2 and ITO/
GR substrates. The stability, reversibility, and tunability of this
memory effect are defined, and the mechanism of charge storage
Previous work with FETs fabricated from thiophene-
phenylene oligomers functionalized with terminal n-hexyl
groups13 suggests that the solubilizing effect is more substantial
than in all-thiophene backbones.15 These observations raise the
interesting question of whether this effect is general and whether
it can lead to enhanced film-forming properties from solution.
(6) (a) Otsubo, T.; Aso, Y.; Takimiya, K. J. Mater. Chem. 2002, 12, 2565. (b)
Harrison, M. G.; Friend, R. H. In Electronic Materials: The Oligomer
Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998;
pp 515-558.
(7) (a) Afzali, A.; Breen, T. L.; Kagan, C. R. Chem. Mater. 2002, 14, 1742.
(b) Halik, M.; Klauk, H.; Zschieschang, U.; Kriem, T.; Schmid, G.; Radlik,
W. Appl. Phys. Lett. 2002, 81 (2), 289. (c) Afzali, A.; Dimitrakopoulos,
D.; Breen, T. L. J. Am. Chem. Soc. 2002, 124, 8812. (d) Kawase, T.;
Sirringhaus, H.; Friend, R. H.; Shimoda, T. AdV. Mater. 2001, 13 (21),
1601. (e) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.;
Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123.
(8) Sze, S. M. Semiconductor DeVices: Physics and Technology; Wiley: New
York, 1985; pp 6-7, 216-218, 507-510.
(12) (a) Yanagi, H.; Morikawa, T.; Hotta, S. Appl. Phys. Lett. 2002, 81 (8),
1512. (b) Yanagi, H.; Morikawa, T.; Hotta, S.; Yase, K. AdV. Mater. 2001,
13 (5), 313. (c) Dingemans, T. J.; Bacher, A.; Thelakkat, M.; Pederson, L.
G.; Samulski, E. T.; Schmidt, H.-W. Synth. Met. 1999, 105 (3), 171.
(13) Hong, X. M.; Katz, H. E.; Lovinger, A. J.; Wang, B.-C.; Raghavachari, K.
Chem. Mater. 2001, 13, 4686.
(9) Katz, H. E.; Hong, X. M.; Dodabalapur, A.; Sarpeshkar, R. J. Appl. Phys.
2002, 91 (3), 1572.
(10) (a) Majumdar, H. S.; Bandyopadhyay, A.; Bolognesi, A.; Pal, A. J. J. Appl.
Phys. 2002, 91 (4), 2433. (b) Velu, G.; Legrand, C.; Tharaud, O.; Chapoton,
A.; Remiens, D.; Horowitz, G. Appl. Phys. Lett. 2001, 79 (5), 659. (c)
Sentein, C.; Fiorini, C.; Lorin, A.; Nunzi, J. M. AdV. Mater. 1997, 9 (10),
809.
(11) (a) Ma, L.; Liu, J.; Pyo, S.; Yang, Y. Appl. Phys. Lett. 2002, 80 (3), 362.
(b) Liu, C.-Y.; Bard, A. J. Acc. Chem. Res. 1999, 32, 235.
(14) (a) Hotta, S.; Goto, M. AdV. Mater. 2002, 14 (7), 498. (b) Lee, S. A.; Hotta,
S.; Nakanishi, F. J. Phys. Chem. A 2000, 104, 1827. (c) Hotta, S.; Lee, S.
A. Synth. Met. 1999, 101, 551.
(15) (a) Horowitz, G.; Fichou, D.; Peng, X.; Xu, Z.; Garnier, F. Solid State
Commun. 1989, 72, 381. (b) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz,
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115, 8716.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9415

A R T I C L E S
Mushrush et al.
dibromobenzene.21,22 The reagent 1,4-bis(5-bromo-2-thienyl)benzene
was synthesized by bromination of 1,4-bis(2-thienyl)benzene in
DMF with N-bromosuccinimide,18,21 and 2,5-bis(4-bromophenyl)-
thiophene was synthesized by the Pd(0)-catalyzed coupling of 2,5-
dibromothiophene with 4-bromophenylboronic acid according to a
known procedure.23 Other chemicals and solvents are commercially
available.
is further explored through devices in a “floating gate” transistor
configuration. The first all-solution-processed nonvolatile de-
vices are presented and evaluated. Finally, we propose a means
to optimize memory response using dielectric materials with
improved charge storage capability.
Experimental Section
Preparation of 2-(Tri-n-butylstannyl)-5-(4-n-hexylphenyl)thio-
phene (9) and 1-(Tri-n-butylstannyl)-4-n-hexylbenzene (10). To a
solution of 5-(4-n-hexylphenyl)thiophene (1.52 g, 6.22 mmol) in dry
THF (30 mL) in a bath at -78 °C was slowly added a hexanes solution
of n-BuLi (3.9 mL, 6.2 mmol). After 1 h of stirring at this temperature,
tri-n-butyltin chloride (2.0 g, 6.2 mmol) was added, and the reaction
mixture was allowed to warm slowly to room temperature and stir
overnight. Hexane (50 mL) was then added. The reaction mixture was
washed with 5% NaHCO3 (20 mL) and water (2 × 20 mL), dried over
MgSO4, filtered, and concentrated in vacuo to give 3.2 g of clear, light
yellow liquid (97%). The same procedure was followed for compound
10, and both 9 and 10 were used as crude materials in subsequent
Materials and General Methods. Prime grade silicon wafers with
300 nm thermally grown oxide (Process Specialties) and indium-tin-
oxide (ITO)/glass spin-coated with GR-720 or GR-720P glass resin
(Techneglas, Inc.) were used as device substrates. Glass resin layers
were ∼2 µm. The ITO/glass substrates were polished with fine Celite
and rinsed with water, methanol, and acetone before spin-coating the
glass resin. The organic semiconductors were deposited by vacuum
evaporation (pressures < 10-5 Torr) and by casting from xylenes
(distilled from sodium) solutions. For vacuum-evaporated films, silicon
substrates were rinsed with water, methanol, and acetone before use.
Trimethylsilyl functionalization of the Si/SiO2 surface was carried out
by exposing the silicon wafers to hexamethyldisilazane (HMDS) vapor
in a closed container overnight. For solution depositions, a region of
the substrate surface (∼1-2 cm2) was defined using 3M Novec EGC-
1700 fluorocarbon electronic coating (comparable in properties to the
previously used FC-722 product) before casting.16 Solution concentra-
tions were 200-400 ppm, and substrate temperatures during casting
were approximately 110-115 °C. The room temperature or warm
solution was applied inside the defined area and allowed to evaporate,
with no special care taken to avoid dust in the environment. Top-contact
electrodes were deposited by evaporating gold (pressures <10-5 Torr)
or by simply painting contacts with conducting carbon paste (Structure
Probe, Inc.). All devices referred to in this study employed gold
electrodes unless otherwise mentioned. The additional gold electrode
for “floating gate” devices was evaporated on top of the glass resin
dielectric. An additional layer of glass resin was then spin-coated on
this floating gate electrode before deposition of the semiconductor.
3
reactions. Compound 9: 1H NMR (CDCl3) δ 7.51 (d, 2H, J ) 8.2
Hz), 7.37 (d, 1H, 3J ) 3.4 Hz), 7.15 (d, 2H), 7.10 (d, 1H), 2.58 (t, 2H,
3J ) 8.0 Hz), 1.59-1.53 (m, 8H), 1.36-1.27 (m, 12H), 1.10 (t, 6H, 3J
) 8.4 Hz), 0.91-0.87 (m, 12H). HRMS (EI, 70 eV) m/z: calcd for
C28H46SSn, 534.2341; found, 534.5131. Compound 10: 1H NMR
3
3
(CDCl3) δ 7.39 (d, 2H, J ) 7.6 Hz), 7.14 (d, 2H), 2.57 (t, 2H, J )
3
7.8 Hz), 1.62-1.48 (m, 12H), 1.03 (t, 6H, J ) 8.0 Hz), 0.91-0.86
(m, 12H). HRMS (EI, 70 eV) m/z: (M+) calcd for C24H44Sn, 452.2465;
found, 451.2426.
Preparation of 2-(4-n-Hexylphenyl)thiophene (8). A mixture of
4-bromo-n-hexylbenzene (7; 5.68 g, 23.5 mmol), 2-tri-n-butylstan-
nylthiophene (8.84 g, 23.7 mmol), and tetrakis(triphenylphosphine)-
palladium(0) (0.4 g, 0.4 mmol) in dry DMF (90 mL) was heated under
nitrogen at 90 °C overnight. After the reaction was cooled to room
temperature, water (250 mL) and hexane (100 mL) were added. The
organic layer was separated, and the aqueous layer was extracted with
hexane (2 × 100 mL). The organic layers were combined, washed with
water (2 × 100 mL), dried over MgSO4, filtered, and concentrated in
vacuo to give a clear orange liquid. After purification by column
chromatography on silica gel (hexane), 2-(4-n-hexylphenyl)thiophene
(8) is obtained as a clear, colorless liquid: 1H NMR (CDCl3) δ 7.50
(d, 2H, 3J ) 8.0 Hz), 7.25 (d, 1H, 3J ) 3.6 Hz), 7.23 (d, 1H, 3J ) 5.2
Hz), 7.17 (d, 2H), 7.05 (dd, 1H), 2.59 (t, 2H, 3J ) 8.0 Hz), 1.63-1.57
(m, 2H), 1.36-1.28 (m, 6H), 0.87 (t, 3H, 3J ) 7.2 Hz). HRMS (EI, 70
eV) m/z: (M+) calcd for C16H20S, 244.1286; found, 244.1245.
Preparation of 2,5-Bis(4-n-hexylphenyl)thiophene (dH-PTP, 1).
A mixture of 4-bromo-n-hexylbenzene (7; 3.01 g, 12.5 mmol), 2,5-
bis(tri-n-butylstannyl)thiophene (3.81 g, 5.75 mmol), and tetrakis-
(triphenylphosphine)palladium(0) (0.14 g, 0.12 mmol) in dry dimeth-
ylformamide (20 mL) was heated under nitrogen at 80 °C overnight.
The reaction mixture was then poured into water (50 mL) and extracted
with hexanes (3 × 30 mL). The organic phase was then washed with
water (2 × 30 mL), dried over MgSO4, filtered, and concentrated in
vacuo to give a pale yellow solid 1, which was recrystallized from
hexane (1.44 g, 62%): mp 144 °C; 1H NMR (CDCl3) δ 7.52 (d,
4H, 3J ) 8.0 Hz), 7.21 (s, 2H), 7.17 (d, 4H), 2.59 (t, 4H, 3J )
All synthetic experiments were performed under a nitrogen atmo-
sphere using conventional Schlenk line techniques. 1H NMR (400 MHz)
spectra were measured with a Varian Mercury 400 (room temperature)
or a Varian VXR 300 (high temperature) instrument. Thermal analyses
were performed with a TA Instruments DSC 2920 differential scanning
calorimeter (N2 atmosphere) and a TA Instruments SDT 2960 simul-
taneous DTA-TGA instrument (10-2 Torr). Emission and absorption
spectra were obtained with a Cary 1 ultraviolet/visible spectrometer
and a PTI QM2 fluorescence instrument. Electrical measurements were
performed using a home-built coaxial probe station and a Hewlett-
Packard 4155A semiconductor parametric analyzer. Fabricated devices
were analyzed in situ by X-ray film diffractometry (XRD), using
standard θ-2θ scanning techniques [calibration to Si(100)], with Cu
KR radiation and a monochromator, and by scanning electron micros-
copy (SEM), using a Hitachi S4500 FE microscope. The reagents 5,5′-
bis(tri-n-butylstannyl)-2,2′-bithiophene,17 2,5-bis(tri-n-butylstannyl)-
thiophene,17 and 5,5′-dibromo-2,2′-bithiophene18 were prepared according
to known procedures. General procedures for preparation of tributyl-
stannyl derivatives19 were employed with slight modifications for new
compounds 9 and 10. The reagent 1,4-bis(2-thienyl)benzene was
prepared via the Grignard coupling20 of 2-thienylmagnesium bromide
with 1,4-diiodobenzene. Alternatively, it can be synthesized by transi-
tion-metal-catalyzed coupling of 2-thienylzinc chloride with 1,4-
3
7.6 Hz), 1.63-1.57 (m, 4H), 1.34-1.27 (m, 12H), 0.87 (t, 6H, J )
6.6 Hz). Anal. Calcd for C28H36S: C, 83.11; H, 8.97. Found: C, 82.61;
H, 8.70.
Preparation of 5,5′-Bis(4-n-hexylphenyl)-2,2′-bithiophene (dH-
PTTP, 2). A mixture of 4-bromo-n-hexylbenzene (7; 1.21 g, 5.02
mmol), 5,5′-bis(tri-n-butylstannyl)-2,2′-dithiophene (1.69 g, 2.27 mmol),
(16) Katz, H. E. U.S. Patent 6,403,397, 2002.
(17) Wei, Y.; Yang, Y.; Yeh, J.-M. Chem. Mater. 1996, 8, 2659.
(18) Ba¨uerle, P.; Wu¨rthner, F.; Go¨tz, G.; Effenberger, F. Synthesis 1993, 11,
1099-1103.
(21) Pelter, A.; Jenkins, I.; Jones, D. E. Tetrahedron 1997, 53 (30), 10357-
10400.
(19) Peters, D.; Ho¨rnfeldt, A.-B.; Gronowitz, S. J. Heterocycl. Chem. 1991, 28,
1613-1617.
(22) Sarker, H.; Gofer, Y.; Killian, J. G.; Poehler, T. O.; Searson, P. C. Synth.
Met. 1998, 97, 1-6.
(20) Tamao, K.; Kodama, S.; Nakajima, I.; Kumada, M.; Minato, A.; Suzuki,
K. Tetrahedron 1982, 38 (22), 3347-3354.
(23) Alo, B. I.; Kandil, A.; Patil, P. A.; Sharp, M. J.; Siddiqui, M. A.; Snieckus,
V.; Josephy, P. D. J. Org. Chem. 1991, 56 (12), 3763-3768.
9
9416 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Phenylene−Thiophene-Based Field-Effect Transistors
A R T I C L E S
and Pd(PPh3)4 (0.07 g, 0.06 mmol) in DMF (35 mL) was heated under
nitrogen at 80 °C overnight. The dark yellow precipitate was then
collected, washed several times with methanol, diethyl ether, and
hexane, and dried under reduced pressure (0.75 g, 68%). The crude
solid was recrystallized from toluene/hexane to afford an analytically
pure yellow-orange solid 2: mp 232 °C; 1H NMR (CDCl3) δ 7.50 (d,
4H, 3J ) 8.0 Hz), 7.19-7.17 (m, 6H), 7.12 (d, 2H, 3J ) 3.6 Hz), 2.60
(t, 4H, 3J ) 7.6 Hz), 1.62-1.58 (m, 4H), 1.36-1.27 (m, 12H), 0.87 (t,
6H, 3J ) 6.4 Hz). Anal. Calcd for C32H38S2: C, 78.96; H, 7.87.
Found: C, 79.06; H, 7.80.
with the improved solubility/film processability and close
packing tendencies associated with R,ω alkyl chain substi-
tution.15b,24 Straightforward syntheses, thermal stability, and
solution processability make these oligomers very attractive
candidates for low-cost molecular electronics applications. The
phenylene-thiophene oligomers 1-6 were prepared via pal-
ladium(0)-catalyzed coupling of the appropriate bromo deriva-
tives with the corresponding tri-n-butylstannyl derivatives, as
shown in Scheme 1. The smallest oligomers 1 and 2 require
only one and two steps, respectively. For all coupling reactions,
tri-n-butylstannyl derivatives were not purified before use in
subsequent reactions. All of oligomers 1-6 are readily purified
by recrystallization rather than high-vacuum sublimation and
can be stored for at least several months in air with no apparent
decomposition or discoloration.
Optical absorption and emission maxima, extinction coef-
ficients, optical band gaps, melting points, and thermal transition
temperatures (DSC) for compounds 1-6 are summarized in
Table 1. The absorption and emission characteristics, as
expected, correlate very well with those of the analogous
unsubstituted systems.14 The two largest band gaps, 3.22 and
3.16 eV, correspond to compounds 1 and 6, which have the
largest ratios of phenylene to thiophene units in the backbone
structure. Presumably 1 and 6 exhibit additional twisting of the
conjugated backbone due to the phenylene rings, which ef-
fectively decreases the conjugation.
Preparation of 5,5′′-Bis(4-n-hexylphenyl)-2,2′:5′,2′′-terthiophene
(dH-PT3P, 3). A mixture of 2,5-dibromothiophene (0.39 g, 1.6 mmol),
2-(tri-n-butylstannyl)-5-(4-n-hexylphenyl)thiophene (9; 3.97 mmol), and
Pd(PPh3)4 (0.07 g, 0.06 mmol) in DMF (15 mL) was heated under
nitrogen at 90 °C overnight. The precipitate was then collected, washed
several times with methanol, acetone, diethyl ether, and hexanes, and
dried under reduced pressure (0.70 g, 76%). The crude product was
then recrystallized from toluene/1,1,2-trichloroethane to give an analyti-
cally pure light orange solid 3: mp 284 °C; 1H NMR (C2D2Cl4, 80 °C)
δ 7.53 (d, 4H, 3J ) 8.4 Hz), 7.23-7.21 (m, 6H), 7.17 (s, 2H), 7.13 (s,
3
2H), 2.65 (t, 4H, J ) 7.6 Hz), 1.70-1.63 (m, 4H), 1.41-1.34 (m,
3
12H), 0.93 (t, 6H, J ) 6.4 Hz). Anal. Calcd for C36H40S3: C, 76.00;
H, 7.09. Found: C, 75.88; H, 6.88.
Preparation of 5,5′′′-Bis(4-n-hexylphenyl)-2,2′:5′,2′′:5′′,2′′′-qua-
terthiophene (dH-PT4P, 4). A mixture of 2-(tri-n-butylstannyl)-5-(4-
n-hexylphenyl)thiophene (9) (4.49 mmol), 5,5′-dibromo-2,2′-bithiophene
(0.58 g, 1.79 mmol), and Pd(PPh3)4 (0.08 g, 0.07 mmol) in DMF (30
mL) was heated under nitrogen at 90 °C overnight. The precipitate
was then collected, washed several times with methanol, diethyl ether,
and hexanes, and dried under reduced pressure (0.87 g, 75%). The crude
product was then recrystallized from toluene/1,1,2-trichloroethane to
Differential scanning calorimetry reveals reversible melting
features for oligomers 1-6, with each oligomer exhibiting one
or more thermal transitions before melting. Figure 1 shows the
DSC trace for compound 2, which is typical of the series. This
compound exhibits three liquid crystalline transitions: an SF/
1
give an analytically pure dark orange solid 4: mp 348 °C; H NMR
3
(C2D2Cl4, 100 °C) δ 7.54 (d, 4H, J ) 7.6 Hz), 7.24-7.22 (m, 6H),
3
7.18 (s, 2H), 7.14 (s, 4H), 2.67 (t, 4H, J ) 7.6 Hz), 1.73-1.66 (m,
3
4H), 1.42-1.37 (m, 12H), 0.94 (t, 6H, J ) 6.4 Hz). Anal. Calcd for
SB to SC followed by an SA/nematic phase transition. Correlation
C40H42S4: C, 73.80; H, 6.50. Found: C, 73.43; H, 6.29.
between film quality, LC phase, and device performance for
films of 1-6 is currently under investigation. In addition to
reversible melting features, clean and quantitative sublimation
is observed in thermogravimetric analysis of compounds 1-5.
Compound 6, however, decomposes to some extent upon
sublimation, leaving about 10% residue by weight. The TGA
plots for 1-6 are compared in Figure 2. Interestingly, for
compounds with a highly exothermic LC transition (4, 5, and
6) before melting, sublimation (as observed by TGA) is
complete at temperatures much lower than the melting point.
Compounds 4 and 5 (mp’s 358 and 324 °C, respectively)
completely sublime before 330 and 275 °C, respectively, while
compounds 1, 2, and 3 (mp’s 144, 232, and 284 °C, respectively)
are not fully sublimed until temperatures 10-35° higher than
their respective melting points.
Film Growth and Characterization. Without exception, the
smoothest and most continuous films of 1-6 from solution are
obtained by casting from xylenes solutions. Other solvents
(benzene, toluene, chlorobenzene, choroform, 1,2,5-trichlo-
robenzene, 1,1,2,2-tetrachloroethane, and 1,2-dichloroethane)
typically result in uneven films with areas of heavy deposition,
distinguishable by eye or optical microscopy as islands of
crystalline material, and areas of very light or no deposition.
The occasional smooth film obtained from casting with one of
these solvents is not easily reproducible. All solvents were
explored under a range of deposition temperatures, from
substantially below the boiling point of the solvent to just above
Preparation of 1,4-Bis[5-(4-n-hexylphenyl)-2-thienyl]benzene (dH-
PTPTP, 5). A mixture of 1-(tri-n-butylstannyl)-4-n-hexylbenzene (10)
(2.6 mmol), 1,4-bis(5-bromo-2-thienyl)benzene (0.40 g, 1.0 mmol), and
Pd(PPh3)4 (0.06 g, 0.05 mmol) in dry DMF (20 mL) was heated under
nitrogen at 90 °C overnight. The precipitate was then collected and
washed several times with methanol, ether, and hexane to give a yellow-
brown solid (0.46 g, 82%), which was then recrystallized from toluene/
1,1,2-trichloroethane: mp 324 °C; 1H NMR (C2D2Cl4, 100 °C): δ 7.67
(s, 4H), 7.57 (d, 4H, 3J ) 7.6 Hz), 7.33 (d, 2H, 3J ) 3.2 Hz), 7.28 (d,
2H), 7.24 (d, 4H), 2.67 (t, 4H, 3J ) 7.6 Hz), 1.73-1.66 (m, 4H), 1.43-
3
1.36 (m, 12H), 0.94 (t, 6H, J ) 6.0 Hz). Anal. Calcd for C38H42S2:
C, 81.09; H, 7.52. Found: C, 80.97; H, 7.42.
Preparation of 2,5-Bis[4(4′-n-hexylphenyl)phenyl]thiophene (dH-
PPTPP, 6). A mixture of 1-(tri-n-butylstannyl)-4-n-hexylbenzene (10)
(5.1 mmol), 2,5-bis(4-bromophenyl)thiophene (0.80 g, 2.03 mmol), and
Pd(PPh3)4 (0.1 g, 0.08 mmol) in dry DMF (35 mL) was heated under
nitrogen at 90 °C overnight. After the reaction mixture had cooled, it
was filtered, and the precipitate was washed several times with
methanol, ether, and hexane to give a tan-yellow solid (0.69 g, 69%),
which was recrystallized from toluene/1,1,2-trichloroethane: mp > 360
1
3
°C; H NMR (C2D2Cl4, 100 °C) δ 7.72 (d, 4H, J ) 8.4 Hz), 7.65 (d,
4H), 7.57 (d, 4H, 3J ) 7.6 Hz), 7.36 (s, 2H), 7.29 (d, 4H), 2.70 (t, 4H,
3J ) 7.6 Hz), 1.75-1.68 (m, 4H), 1.43-1.37 (m, 12H), 0.95 (t, 6H, 3J
) 6.4 Hz). HRMS (EI, 70 eV) m/z (M+): calcd for C40H44S, 556.3154;
found, 556.3164.
Results and Discussion
Synthetic Strategies. Structures 1-6 were targeted because
they were anticipated to combine the high mobility character-
istics typically exhibited by thiophene-phenylene oligomers1,9,13
(24) Fichou, D. J. Mater. Chem. 2000, 10, 571.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9417

A R T I C L E S
Mushrush et al.
Scheme 1. Synthetic Scheme for Thiophene-Phenylene Oligomers 1-6a
a (i) Pd(PPh3)4, DMF, 80-90 °C. (ii) (1) BuLi, THF, -78 °C, (2) Bu3SnCl.
Table 1. Optical Absorption and Emission Maxima in THF
Solution, Extinction Coefficients, Optical Band Gaps, Capillary
Tube Melting Points, and DSC Transition Temperatures (2nd
Heating Cycle) of Phenylene-Thiophene Oligomers 1-6
a
b
λabs
(nm) log ꢀ
λem
(nm)
E
(eV)
mp
(°C)
TDSC
(°C)
g
compound
dH-PTP (1)
dH-PTTP (2) 373 4.60 436, 460* 2.97
dH-PT3P (3)
dH-PT4P (4)
dH-PTPTP (5) 378 4.78 432*, 458 3.01
366 4.51 385
3.22
144 92, 150*
232 139, 150, 235*, 239
284 278*, 295
358 328*, 356, 362
324 83, 267*, 323, 328
408 4.71 472*, 503 2.77
432 4.73 498*, 536 2.62
dH-PPTPP (6) 356 4.16 414, 437* 3.16 >360 299*, 365, 369
a Absolute maximum indicated by asterisk. b Largest thermal transition
indicated by asterisk.
Figure 1. Differential scanning calorimetry trace (second cycle) for dH-
PTTP (2) showing reversible melting and liquid crystalline transitions (ramp
rate, 10 °C/min).
it. Higher-boiling solvents typically produce smoother films at
temperatures in the range of 5-30 °C below their boiling points.
All solution-cast films characterized in this study were cast from
xylenes solutions at a substrate temperature of 115 ( 10 °C on
ITO/glass resin, due to the high reproducibility of film quality
under these conditions. As an example, the FET performance
of 2 is highly dependent on casting temperature; devices cast
at substrate temperatures outside of a range of ∼110-120 °C
showed significantly lower carrier mobilities. The capability of
casting highly reproducible, smooth, and continuous films from
a non-chlorinated solvent is a significant step toward low-cost
and low-toxicity solution fabrication techniques. The mechanism
by which an oligothiophene film forms on a substrate from the
overlying solution is poorly understood and is under investiga-
tion.
The phenylene-thiophene oligomers prepared in this study
all yield highly ordered films on Si/SiO2 and ITO/GR substrates,
both by evaporation under high vacuum and by deposition from
cast xylenes solutions, as assessed by X-ray diffraction. The
observed intense reflections and calculated interplanar d spacings
for solution-cast and vacuum-evaporated films of oligomers 1-6
are summarized in Table 2. In all films, the first-order reflection
is very intense, and many films exhibit well-resolved higher-
order reflections as well, indicating well-ordered, layered
microstructures. Vacuum deposition at a substrate temperature
of 50 °C enhances XRD peak sharpness, but since there is not
a substantial difference in device performance, the substrate
temperature during growth was not further optimized in this
9
9418 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Phenylene−Thiophene-Based Field-Effect Transistors
A R T I C L E S
Previous diffraction studies on films of analogous phenylene-
thiophene oligomers without R,ω-alkyl substituents indicate that
smaller oligomers (<5 thiophene or phenyl rings) favor vertical
molecular alignment (long molecular axis roughly perpendicular
to the substrate surface) over parallel alignment (long molecular
axis roughly parallel to the substrate surface) when compared
to larger oligomers (g5 rings) deposited under identical
conditions (casting from methylene chloride solution or evapo-
rating at ∼10-6 Torr and a growth rate of 1-2 Å/sec).26 Previous
work also showed that exclusively vertical molecular alignment
(no detectable presence of reflections corresponding to parallel
alignment structures) can be achieved for some compounds only
under higher vacuum (∼10-9 Torr) and at extremely low film
deposition rates (<0.01 Å/sec).26 As seen in Table 2, we observe
for 1-6 that vertical molecular alignment is predominant for
all films grown by either solution or vacuum techniques
(deposition rates of <3 Å/sec and pressures < 10-6 Torr) but
that, generally, the longer oligomers exhibit smaller apparent
tilt angles of the molecular long axes with respect to the substrate
normal, contrary to what was observed for the unsubstituted
analogues.26 That is, the present experimental interplanar d
spacings correlate very well with the calculated long molecular
axis lengths of the oligomers, taking into account reasonable
tilt angles. Simple addition of the molecular lengths of the
analogous unsubstituted oligomers (13, 17, 21, 24, 21, and 22
Å, in analogous order to 1-6)26 and the length estimated for
two hexyl chains (∼9.3 Å each)15b affords values 2 to 6 Å larger
than the experimentally determined d spacings for vacuum-
evaporated films of 2-6 shown in Table 2. Assuming molecular
packing motifs similar to those of the unsubstituted oligomers,26
this implies tilt angles of approximately 25-40° from the
substrate normal (except for the shortest oligomers 1 and 2,
with apparent tilt angles of 50-60°), with solution-cast films
having slightly larger apparent tilt angles (10-15° greater) than
evaporated films of the same compound (except for the shortest,
1, for which they are identical). The large difference in growth
rate between solution-casting and vacuum evaporation may give
rise to different growth directions or packing motifs, which is
a reasonable explanation for the larger apparent tilt angles seen
in solution-cast films. As noted above, the growth rate has a
significant effect on molecular alignment for the unsubstituted
phenylene-thiophene oligomers.26
Figure 2. Thermogravimetric analysis data (10-2 Torr, 1.5 °C/min) for
phenylene-thiophene oligomers 1-6.
Table 2. X-ray Diffraction Data for Vacuum-Evaporated and
Solution-Cast Films of Phenylene-Thiophene Oligomers: Major
Reflections and Derived Interplanar Distances
vacuum-deposited films
(25 °C substrate temperature)
solution-cast films
(xylenes solution, 110−115 °C)
other observable
other observable
compound
d (Å)
25 none
30 2, 3, 4, 5, 6b
reflections (order)
d (Å)
reflections (order)
dH-PTP (1)
dH-PTTP (2)
dH-PT3P (3)
dH-PT4P (4)
25
none
30 (26)a
31
3, 4, 5, 6b
2 (weak)
2
35
39
2
2
37
dH-PTPTP (5) 35 2, 4 (weak), 6 (weak) 33
dH-PPTPP (6) 38 35
2 (weak)
none
2
a Peaks corresponding to a second phase are also observed. b Scans did
not extend higher than 6th order peaks.
Figure 3. X-ray diffraction θ-2θ scans of solution-cast (from xylenes
solution, 110-115 °C; bottom) and vacuum-evaporated (ambient temper-
ature; top) films of dH-PTTP (2) on Si/SiO2 substrates.
Scanning electron microscopy (SEM) reveals very smooth
morphologies for evaporated films of all phenylene-thiophene
compounds 2-6 grown both at ambient temperature and at 50
°C (the lower-melting 1 was not examined by SEM). Much
greater variation in morphology is observed for solution-cast
films, with the larger oligomers 3-6 displaying overlapping
crystallites having dimensions as large as several tens of
micrometers. The smaller molecule 2, however, adopts quite a
different film morphology (as cast from solution) from the
others; it is much more smooth and has much smaller features.
Figure 4 compares typical micrographs of 2 and 3 to illustrate
this difference. In general, the interconnectedness of crystallites
in the solution-cast films imaged by SEM combined with the
high order of crystallinity and vertical molecular alignment seen
in the XRD provide strong support for the efficient hole-
transporting capabilities of 1-6.
study. Pretreatment of the silicon substrates with HMDS vapor
to create lipophilic monolayer coatings has negligible effects
on the XRD results. Figure 3 compares θ-2θ diffraction scans
of solution-cast (bottom) versus vacuum-evaporated (top) films
of 2. Unlike films of the other oligomers, films of 2 contain
two major crystallite orientations or phases when cast from
solution. Because of the non-negligible difference in the
measured d spacings (4 Å) from the first (low-angle) reflections
(30 and 26 Å), it appears likely that these two distinct lattice
spacings correspond to distinct growth orientations or poly-
morphs of compound 2. This phenomenon has been observed
in various substituted oligothiophenes as well,24 including the
all-thiophene analogue of 2, R,ω-dihexylquaterthiophene.25
(25) Garnier, F.; Hajlaoui, R.; El Kassmi, A.; Horowitz, G.; Laigre, L.; Porzio,
(26) Hotta, S.; Ichino, Y.; Yoshida, Y.; Yoshida, M. J. Phys. Chem. B 2000,
104, 10316.
W.; Armanini, M.; Provasoli, F. Chem. Mater. 1998, 10, 3334.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9419

A R T I C L E S
Mushrush et al.
Solution-cast films on ITO/GR substrates were used for all
of the following memory effect experiments; Si/SiO2 devices
are briefly compared later. Writing voltages Vw were applied at
the gate with a common voltage at the source and drain.
0.5
Threshold voltages Vt were calculated from the plot of Id
versus Vg, where the x-intercept of the linear portion of the plot
is Vt.9 Before and after the writing voltages were applied, the
FET characteristics were measured and the characteristics
compared. Both dH-PTTP (2) and dH-PPTPP (6) show a
substantial memory effect; high initial device turn-on voltages
for 2 and 6 can be brought from accumulation mode turn-on to
zero or depletion mode by applying a writing voltage. Of the
two, the better-behaved system is dH-PTTP (2) because of the
reversibility and predictability of the effects of the applied
writing voltage. Figure 6 shows representative current-voltage
plots for a solution-cast device of dH-PTTP (2) (ITO/GR with
top contact Au electrodes) before and after the application of a
writing voltage of +100 V for 1.0 min. The plots include only
the current-voltage curves for gate values of 0 V, -20 V, and
-40 V, to show more clearly the magnitude of the effect of
writing.
An informative measure of the memory effect is the ratio of
the maximum current after writing to the maximum current
before writing, at a gate voltage of -20 V (Imax, where Vsd
)
-100 V and Vg ) -20 V). In accord with a mechanism in
which the stored charge from writing affects the accumulation
of majority carriers in the channel by prefilling traps (discussed
below),9 the largest effects in the current-voltage characteristics
are observed at lower Vg values, where traps have a greater
effect. The ratio of maximum current (Vsd ) -100 V, Vg )
-20 V) after writing to that before writing is about 12 for the
device shown in Figure 6, and the turn-on voltage shift is from
about -50 V to about -30 V. Typical current ratios for ITO/
GR devices fabricated from solution-cast 2 for this writing
voltage and time are in the range of 5-15, although ratios as
high as 25 are sometimes observed. Typically these ratios drop
upon measurement, and after just a few cycles, where a cycle
is defined as a sweep of Vsd from 0 to -100 V for Vg values of
0, -20, -40, -60, -80, and -100 V, the ratio of Imax after
writing to before writing drops by more than 50%, although it
is still easily detectable. If, however, lower voltages are used
in measuring the device between writing voltages, the effect
persists much longer. If Vsd and Vg are each swept only to -40
V, for example, the ratio of maximum currents at Vg ) -20 V
(now for Vsd ) -40 V) after writing to before writing remains
within 75% of its initial value after more than 15 cycles.
Figure 4. Scanning electron micrographs of films cast from xylenes solution
on Si/SiO2 at 110-115 °C of (a) dH-PTTP (2) (400 ppm concentration)
and (b) dH-PT3P (3) (300 ppm concentration).
Transistor and Nonvolatile Memory Characteristics. In
p-channel operation, compounds 2-6 display hole mobilities
between 0.01 and 0.09 cm2/Vs as vacuum-evaporated films (on
Si/SiO2) and between 0.001 and 0.03 cm2/Vs as solution-cast
films (on ITO/GR). The response of compound 1 varies sub-
stantially from device to device and shows a significant mobility
as an evaporated film only on HMDS-treated SiO2. These data
are summarized in Table 3 and shown for dH-PTTP (2) in Figure
5. Pretreatment of substrates with HMDS before vacuum evap-
oration has negligible effects on the FET response of 2, 4, and
6 but increases the current on/off ratios of room-temperature-
evaporated films of 3 and 5 by 1 or 2 orders of magnitude. In
several cases, devices in fact display greater performance on
untreated silicon substrates than on HMDS-treated ones (refer
to Table 3). Regardless of the surface pretreatment, FET
characteristics for both evaporated and solution-cast films are
highly reproducible, and on/off ratios of 104 and higher are
readily obtainable. It is particularly notable that useful mobilities,
on/off ratios, and resistance to doping (as indicated by off
currents of ∼10-10 A) are obtained from 1 and 2, which can be
synthesized in so few steps from readily available starting
materials. Compound 2 in particular is the most reliable
thiophene derivative yet identified for rapidly producing an
undoped, air-stable, semiconducting film from solution. Fur-
thermore, vacuum sublimation is not required to purify these
two semiconductors to a level that exhibits large current on/off
ratios.
The memory effect (measured in terms of the ratio of
maximum currents at Vg ) -20 V before and after writing)
can be tuned by writing at various voltages and for various times.
It can also be selectively erased and restored multiple times
without a change in magnitude and without breakdown of the
device. Figure 7 shows an example of this tuning for an ITO/
GR device with solution-cast 2. Taking the original ratio as 1.0
(effectively, application of Vw ) 0 V) and then applying some
positive writing voltage, we can tune the current for smaller or
larger effects and subsequently erase it by writing with the
appropriate negative (opposite) voltage. In this example, a
writing voltage of +50 V applied for 10 min increases the
current ratio to about 8, as does a writing voltage of +100 V
applied for 3 min. The negative writing voltages applied to
9
9420 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Phenylene−Thiophene-Based Field-Effect Transistors
A R T I C L E S
Table 3. FET Mobilities and On/Off Ratios for Vacuum-Evaporated and Solution-Deposited Phenylene-Thiophene Semiconductor Films
vacuum-deposited filmsb
solution-cast filmsa (xylenes, 110−115 °C)
room temperature
50 °C
2
2
2
compd
mobility (cm /Vs)
ION/IOFF
mobility (cm /Vs)
ION/IOFF
mobility (cm /Vs)
ION/IOFF
1
2
3
4
5
6
10-5
60
∼10-5 (0.01)
30 (2500)
105 (105)
∼10-4 (0.01)
200 (3000)
0.02 [0.006]
2 × 104 [3 × 103]
100 [10]
1000 [10]
1700
0.07 (0.09)
0.09 (0.09)
4 × 104 (1.5 × 105)
0.004 [3 × 10-4
]
0.054 (0.028)
0.09 (0.054)
0.054 (0.054)
0.001 (7 × 10-4
7700 (2 × 104)
900 (3500)
900 (4 × 104)
1100 (600)
0.054 (0.028)
0.07 (0.09)
0.028 (0.054)
0.018 (0.018)
6000 (2800)
0.028 [10-3
0.01
]
1400 (1100)
5 × 104 (6 × 104)
0.001
600
)
104 (1.5 × 104)
a Values in brackets refer to all-solution devices fabricated on ITO with spin-coated glass resin, solution-cast semiconductors, and painted graphite electrodes.
b Values in parentheses refer to devices on silicon substrates that were treated with hexamethyldisilazane (HMDS) before film growth.
Figure 5. Current-voltage characteristics for devices with evaporated Au
top-contact electrodes and dH-PTTP (2) films: (a) vacuum-deposited onto
Si/SiO2 and (b) cast from 400 ppm xylene solution at 115 °C onto ITO/
GR. Gate voltages are 0 to -100 V in increments of 20 V, bottom to top
(on bottom plot Vg 0 to -40 V all overlap at 0 V).
Figure 6. Current-voltage characteristics for solution-cast dH-PTTP (2)
on ITO/GR with top-contact evaporated Au electrodes: (a) before and (b)
after writing with Vw ) +100 V for 1.0 min. Gate voltages are 0, -20,
and -40 V, from bottom to top.
“erase” the memory effect, or remove the stored charge,
consistently bring the maximum current value (at Vg ) -20
V) to less than 2.5 times its value before writing. By slightly
increasing the writing time for the opposite Vw, we can bring
the maximum current to within less than 1.2 times its original
value.
In comparison with ITO/GR, devices of Si/SiO2 with solution-
cast 2 exhibit a much larger ratio of maximum currents, but
the effect disappears almost instantaneously, even before the
first cycle is complete. Thus, the ratio of maximum currents
(Vsd ) -100 V) must be compared at Vg ) 0 V in order to
detect a large effect, and after one full cycle of measurements
up to Vg ) -100 V, the effect is essentially undetectable. Ratios
are typically in the range of 225-500, but values as high as
650 are observed. Glass resin has a much higher charge storage
capability than SiO2; the observed difference in the effect is as
expected.9 Optimization of the dielectric should enable far larger
and more stable writing effects.
Figure 7. Histogram showing the tunability of the memory effect for a
solution-cast dH-PTTP (2)-based device on ITO/GR with various applied
voltages and times. Each bar represents the ratio of the maximum current
at Vg ) -20 V after the indicated writing voltage is applied to the same
maximum current before the writing voltage is applied.
somewhere between the semiconductor and gate electrode, either
in the bulk of the dielectric and/or at one or both dielectric
interfaces. The charge may be in the form of isolated monopoles,
or as oriented dipoles; indeed, the exact description is still
incompletely defined. This phenomenon results in a shift in the
electrical potential between the gate and the semiconductor and
A physical mechanism for the memory effect has been
suggested previously.9 Briefly, holding a voltage across the gate
dielectric (“writing”) causes the injection of static charge
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9421

A R T I C L E S
Mushrush et al.
additional Au electrode deposited between two separate layers
of glass resin. In a standard inorganic memory device with a
floating gate structure, this fourth electrode actually stores
charge.8 The purpose of this configuration here, however, is
merely to determine whether the stored charge is distributed
evenly throughout the dielectric (i.e., identical memory effect
observations in floating gate and standard FET devices) or
concentrated at the dielectric-semiconductor interface (i.e.,
greatly reduced or absent memory effect in floating gate
devices). The “floating gate” electrode should not affect normal
TFT operation. When a writing voltage is applied at the region
of the dielectric nearer to the ITO substrate (applied at the gate
with respect to a constant voltage at the floating gate), the
maximum current increases by only a factor of 3 or less. Such
a writing procedure is therefore less effective and less reproduc-
ible than the standard procedure using the source and drain
electrodes to affect threshold voltage. In addition, current/ratio
changes are not monotonic for all gate voltages used to test the
device. Nevertheless, these results demonstrate that memory
effects must arise primarily from charge stored near the
semiconductor-dielectric interface.
In a nonvolatile memory element, the required writing voltage
depends on the efficiency of the charge storage and the capaci-
tance of the dielectric. In particular, a thinner dielectric would
enable writing at a lower voltage, since it is actually the electric
field across the dielectric that determines the magnitude of
charge storage. Also, use of more hydrophobic and/or ferroelec-
tric dielectric composites should increase the stability of charge
storage and lower the reversion rate of the transistor. Such
materials, including highly fluorinated polymers, are currently
under investigation as alternative gate dielectrics for these
“nonvolatile” devices. Preliminary results suggest that these
polymers do indeed stabilize the adjusted states of the transistors.
Figure 8. Schematic of an organic transistor, showing the injected static
charge after application of a writing voltage as isolated monopoles (top) or
as oriented dipoles (bottom).
Devices were also fabricated using solution-only techniques
on ITO substrates, by first applying a spin-coated glass resin
and then a solution-cast phenylene-thiophene oligomer and
finally painting graphite paste electrodes. Typical channel
dimensions for painted electrodes were about 0.5-1 mm (L)
and about 5 mm (W). Several devices were fabricated in this
way using dH-PTTP (2); observed mobilities reached 0.006 cm2
V-1 s-1, and on/off ratios for these devices exceeded 103. These
data are only a factor of 3 lower than those for the analogous
devices employing gold electrodes (see Table 3). Only two
substrates were prepared from all-solution techniques for
compounds 3 and 4, and the mobilities were within 1 or 2 orders
of magnitude of the analogous devices with gold electrodes.
One limitation of the graphite electrodes is that they tend to
break down more readily, possibly due to delamination from
the hydrophobic semiconductor surface. A typical device with
painted electrodes could be measured at least several times with
no observable deterioration of the electrode contacts, but writing
voltages could only be applied 1 or 2 times before the observed
current dropped substantially. These preliminary results on ITO/
GR using graphite paste are very promising, and alternatives
such as polyaniline and PEDOT, which should be less suscep-
tible to delamination, are under investigation.
Figure 9. Schematic of a “floating gate” transistor.
alters the charge distribution in the transistor, as illustrated in
Figure 8. It appears that the applied depletion voltage, or writing
voltage Vw, results in the filling of traps due to the electrostatic
attraction of holes to the negatively polarized semiconductor-
dielectric interface. This allows for more efficient establishment
of the semiconducting channel by majority carriers during
operation. Thus, the threshold voltage shifts from accumulation
mode (negative for p-type semiconductors, positive for n-type)
to zero or even to the depletion mode.
ITO/GR and Si/SiO2 devices with the same semiconductor
behave very differently upon writing; thus, it can be concluded
that metal-semiconductor barrier modulations do not play a
role. As an example, the effect for dH-PTTP in an Si/SiO2 device
is at least an order of magnitude larger than that in an ITO/GR
device. Furthermore, with all Au-semiconductor interfaces
identical, the stability of the effect in ITO/GR devices is much
greater than that in Si/SiO2 devices. These observations clearly
indicate that the effect involves the dielectric and not the gold-
semiconductor interfaces.
To further bound the distribution of the stored charge within
the dielectric, ITO/GR devices with a “floating gate” electrode
configuration were fabricated.8 Figure 9 shows a diagram of
the floating gate device structure; the only unique feature of
such a device (compared to the typical TFT structure) is an
Conclusions
A new series of mixed phenylene-thiophene oligomers with
terminal n-hexyl groups has been synthesized and characterized
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9422 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Phenylene−Thiophene-Based Field-Effect Transistors
A R T I C L E S
both as bulk solids and as thin films. The compounds are
thermally stable, and the films adopt microstructures with lattice
spacings suggestive of molecular alignment along the substrate
normal. Morphologies of the vacuum-evaporated films are
featureless; however, the morphologies of solution-cast films
vary considerably, with highly interconnected crystallites of
various shapes and sizes. Both vacuum-evaporated (on Si/SiO2)
and solution-cast (on ITO/GR) films of the compounds exhibit
high p-type mobilities, including the straightforwardly prepared
compounds 1 and 2. For compounds 2 and 6, a significant
nonvolatile adjustment of the device characteristics is possible.
This memory effect is reversible and tunable by selecting the
appropriate writing voltages and writing times. For the first time,
evidence demonstrating that most of the effect arises from charge
stored near the semiconductor-dielectric interface is deduced
from floating gate device experiments. The first all-solution-
processed simple nonvolatile elements have been fabricated on
ITO substrates, with spin-coated glass resin, solution-cast semi-
conductors, and conducting graphite paste components; initial
results are quite promising.
Acknowledgment. We thank ONR (N00014-02-1-0909) and
the NSF-MRSEC program through the Northwestern Materials
Research Center (DMR-0076097) for support of this research.
JA035143A
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J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9423