Organic Thin Film Transistors from a Soluble Oligothiophene Derivative Containing Thermally Removable Solubilizing Groups

Article abstract of DOI:10.1021/ja039529x

A symmetrical α,ω-substituted sexithiophene derivative containing thermally removable branched ester solubilizing groups has been prepared. These oligomers can be solution cast into thin films and then thermolyzed to remove the solubilizing group, leaving short pendant alkene groups on the oligomer. Device testing of thin film transistors shows an increase in hole mobility from 1 × 10^-5 cm²/(V s) with on/off ratios of ～100 before thermolysis to 5 × 10^-2 cm²/(V s) with on/off ratios >10⁵ after thermolysis. This method offers an attractive route to easily processed and highly performing thiophene oligomers. Copyright

Full text of DOI:10.1021/ja039529x

Published on Web 01/23/2004
Organic Thin Film Transistors from a Soluble Oligothiophene Derivative
Containing Thermally Removable Solubilizing Groups
Amanda R. Murphy,† Jean M. J. Fre´chet,*,† Paul Chang,‡ Josephine Lee,‡ and Vivek Subramanian‡
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460, and Department of Electrical
Engineering and Computer Science, UniVersity of California, Berkeley, California 94720
Received November 10, 2003; E-mail: frechet@cchem.berkeley.edu
Scheme 1. Thermolysis of the Ester-Substituted Sexithiophene.
The solubility of organic semiconducting materials is a particu-
larly important consideration when evaluating candidates for use
in low-cost electronic devices such as organic thin film transistors
(OTFTs),1 since the desired processing of these materials includes
solution-based methods such as spin coating,2 stamping,3 or ink-
jet printing.4 These methods have the advantage of being conducted
at ambient temperatures, allowing for the use of a great variety of
substrates such as plastic or fabric, and they are also amenable to
large-area or continuous applications. However, highly conjugated
organic materials are notoriously difficult to manipulate due to their
low solubility. Therefore, our focus in designing the next generation
of organic semiconductors has been to impart solubility to the
materials through the use of strategically placed substituents.5 Herein
we report the synthesis and characterization of a symmetrical R,ω-
substituted sexithiophene derivative containing thermally removable
solubilizing groups for use in solution-processed OTFTs.
Scheme 2. Synthesis of Ester-Terminated Sexithiophene 6
Thiophene oligomers, particularly R,ω-dihexyl-sexithiophene
(DH6T), have recently been reported to exhibit thin film charge
mobilities as high as 1.0 cm2/(V s),6 but due to limited solubility
they need to undergo vacuum deposition to form thin films.7
Solution processing has been attempted with R,ω-dihexyltetra-,8
penta-,9 and sexithiophene,10 giving OTFT devices with mobilities
on the order of 10-2 cm2/(V s) with on/off ratios of 103. However,
high temperatures are required of both the substrate and the spinning
solvent. Improvements in solubility of sexithiophene oligomers has
been demonstrated by incorporating polar functionalities such as
alkyl phosphonates11 and poly(ethylene oxide)12 at the R- and
ω-positions at the cost of decreasing mobilities to 10-3 cm2/(V s)
and 10-4 cm2/(V s), respectively. Substituents impart solubility by
interrupting the natural tendency of a material to π-stack; therefore,
semiconducting materials that rely heavily on π-orbital overlap to
achieve high charge mobility suffer as a result of functionalization.
Furthermore, typical substituents consist of insulating saturated
alkane derivatives that may lower the intrinsic electronic properties
of a material.
To overcome the negative effects of these necessary solubilizing
groups, it would be advantageous to be able to remove these groups
in a postprocessing step. The thermal removal of solubilizing
groups, key to our design of chemically amplified photoresists,13
was also used in the Durham polyacetylene14 and more recently
for solution-processible pentacene.15 Our specific target was the
diester-functionalized sexithiophene derivative 6 (Scheme 1). This
target contains activated secondary esters at the R- and ω-positions
which are known to undergo thermolysis between 150 and 300 °C.
The branched nature of the ester renders the oligomer soluble in
common organic solvents, facilitating synthesis, purification, and
processing. These oligomers can be solution cast into thin films
and then thermolyzed with a relatively low-temperature heating
cycle, leaving pendant alkene groups on the oligomer.
Compound 6 was synthesized by linking a central bis-stannylated
bithiophene with terminal R-functionalized bithiophene units
through a Stille cross-coupling reaction (Scheme 2). The R-func-
tionalized bithiophene 2 is prepared by a Friedel-Crafts reaction
between bithiophene and propionyl chloride, followed by a reduc-
tion of the carbonyl with LAH. Following esterification of 2 with
2-butyloctanoyl chloride to form ester 4 and bromination to 5, Stille
coupling was carried out with 5,5′-di(tributylstannyl)-2,2′-bithio-
phene16 to afford compound 6. As a result of the presence of its
branched ester side chains, the orange semicrystalline product is
highly soluble in most organic solvents.
Thermogravimetric analysis (TGA) of the product verified the
expected ester thermolysis reaction (Figure 1). A 41% loss of the
total mass, corresponding to a loss of the carboxylic acid, initiates
at 120 °C and is complete by 210 °C. The thermolysis reaction
was also monitored by MALDI-TOF showing the expected mass
loss upon heating the sample of 6 (Figure 2).
The morphologies of thin films of 6 on Si/SiO2 were imaged
with AFM before and after heating. The initial unheated films are
microcrystalline and show only local aggregation (Figure 3a).
However, after thermolysis the films reorganize into large crystalline
terraces, which increase in height with increasing annealing
† Department of Chemistry.
‡ Department of Electrical Engineering and Computer Science.
9
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J. AM. CHEM. SOC. 2004, 126, 1596-1597
10.1021/ja039529x CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Figure 1. Thermogravimetric analysis of 6 heating at 5 °C/min. The
expected loss of 41% of the mass occurs between 120 and 210 °C.
Figure 4. Electrical characterization of OTFTs after heating to 200 °C.
(a) Drain current ID vs drain voltage VD as a function of gate voltage for a
top-contact device with L ) 200 µm and W ) 20 µm. (b) ID and ID vs
VG at VD ) -22 V.
1/2
of the substituents may allow even further enhancement in
properties. These materials show great promise for use in solution-
processed low-cost electronics.
Acknowledgment. We thank the DOE-BES, AFOSR, NSF, the
Eastman-Kodak Co., and MARCO for funding.
Supporting Information Available: Complete experimental pro-
cedures and characterization of 1-6 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 2. MALDI-TOF MS of 6 (a) before heating (calc MW ) 975.5)
and (b) after heating to 150 °C for 10 min (calc MW ) 574.9).
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