High electron mobility in solution-cast and vapor-deposited phenacyl-quaterthiophene-based field-effect transistors: Toward N-type polythiophenes

Article abstract of DOI:10.1021/ja054276o

New carbonyl-functionalized quaterthiophenes, 5,5?-diperfluorophenylcarbonyl-2,2′:5′,2″:5″,2?-quaterthiophene [DFCO-4T], 5,5?-diphenyl-2,2′:5′,2″:5″,2?-quaterthiophene [DPCO-4T], and a polymer having the same basic motif as DFCO-4T, poly{1,4-bis[(3′-n-oct

Full text of DOI:10.1021/ja054276o

Published on Web 09/03/2005
High Electron Mobility in Solution-Cast and Vapor-Deposited
Phenacyl-Quaterthiophene-Based Field-Effect Transistors: Toward N-Type
Polythiophenes
Joseph A. Letizia, Antonio Facchetti,* Charlotte L. Stern, Mark A. Ratner, and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208
Received June 28, 2005; E-mail: t-marks@northwestern.edu; a-facchetti@northwestern.edu
Realization of practical organic electronics has been impaired
by the paucity of high-mobility, solution-processable, electron-
transporting (n-type) semiconductors for organic field-effect transis-
tors (OFETs).1 The combination of such materials with hole
2,3
transporters (p-type) should enable inexpensive, high-throughput
organic CMOS fabrication via spin-coating, drop-casting, and/or
printing. There are two primary challenges to achieving such
properties: (i) obtaining favorable crystal packing while maintaining
useful solubility, and (ii) achieving n-type transport via appropriate
molecular electronic structure/orbital energetics and low-defect
4
density films. To date, postdeposition film processing and chem-
Figure 1. Crystal structures of semiconductors 1 (a) and 2 (b) viewed
perpendicular to the long axis of the unit cell (hydrogen atoms not shown).
Note the remarkably similar herringbone packing motif in 1 (c) and 2 (d).
istries have been explored to address these issues with various
outcomes since achieving high purity and highly regular film
morphology present significant challenges. This communication
3
describes the synthesis of a new class of soluble oligothiophenes
having high electron/hole mobilities both in solution-cast (µ up to
e
2
-1 -1
2
-1 -1
0
.25 cm ‚V ‚s ) and vapor-deposited (µ up to 0.51 cm ‚V ‚s )
e
5
8
films with very high current modulation (Ion:Ioff > 10 and 10 ,
respectively). Furthermore, the structure and energetics of these
molecular motifs are models for the synthesis/characteristics of new
n-type polythiophenes.
New quaterthiophenes DFCO-4T (1) and DPCO-4T (2) and
polythiophene P(COFCO-4T) (3) were synthesized according to
Scheme S1 (see Supporting Information) and characterized by
conventional chemical and physical methods. Crystals of 1 and 2
Figure 2. Transfer plots of OFET devices fabricated with DFCO-4T vapor-
deposited at TD ) 80 °C (a) and solution-cast (xylene) at TD ) 120 °C (b).
half the unit cell long axes (shown in Figure 1), indicating end-
on-substrate molecular orientation. As the substrate temperature
during vapor phase film deposition (T
from 25 to 90 °C, the films become more crystalline and minority
crystallite orientations, present at lower T , are no longer observable
by WAXRD. Diffraction patterns similar to those of the high T
D
) is increased progressively
D
D
vapor-deposited films are also observed for solution-deposited films
of 1 (Figure S4).
suitable for X-ray diffraction were obtained by sublimation. Both
crystallize in a herringbone motif (Figure 1), with the shortest inter-
core distance being 3.50 Å (C14-C15′) and 3.43 Å (C14-C16′)
for 1 and 2, respectively. The average dihedral angle between the
phenyl substituent and the adjacent thiophene subunit is ∼53° in 1
and ∼49° in 2. The quaterthiophene core of 2 is more planar than
that of 1 with a maximum inter-thiophene torsional angle of ∼4°
versus ∼13° in 1. However, the 1 carbonyl groups lie ∼6° out of
the plane of the adjacent thiophene ring, while in 2, this angle
increases considerably to ∼17°. Both semiconductors are thermally
stable and volatile, as indicated by differential scanning calorimetry
Top-contact OTFTs were fabricated by vapor-deposition/drop-
casting of 1-3 films onto temperature-controlled HMDS-treated
+
SiO
2
/p -Si substrates, followed by Au deposition through a shadow
mask to define the source and drain electrodes. OFET characteriza-
tion was preformed under argon. High electron mobilities (µ ) of
0.45 ( 0.07 cm ‚V ‚s are observed for vapor-deposited 1 films
e
2
-1 -1
(T
D
8
) 80 °C) with a threshold voltage (V
10 , Figure 2a and Figure S5).
T
) of ∼30 V (Ion:Ioff
>
This highly reproducible µ
e
value is one of the largest reported
to date, doubtless reflecting the favorable crystal packing of this
molecule. In solution-cast devices, µ is exceptionally high with a
e
2
-1 -1
5
(Figure S1) and thermogravimetric analysis (Figure S2). Films can
value of 0.21 ( 0.05 cm ‚V ‚s (Ion:Ioff ) 10 ; V ) 50-70 V,
T
be grown from the vapor phase and by solution-casting from
common solvents. Wide-angle X-ray diffraction (WAXRD) indi-
cates that vapor-deposited films are highly crystalline, having the
same phase as that observed in the crystal structures (Figure S3).
The progression of Bragg reflections corresponds to d-spacings of
from xylene, Figure 2b and Figure S6). This is the highest OFET
electron mobility for a solution-cast semiconductor reported to date,
1
n
surpassing that of the previous highest mobility n-type molecular
2
-1 -1
T
[0.01 cm ‚V ‚s (Ion:Ioff and V not reported) from trifluorotolu-
1l
2
-1 -1
5
ene] and polymeric [0.03 cm ‚V ‚s (Ion:Ioff ) 5 × 10 , V
T
2
-1 -1
27.62 Å (1) and 26.87 Å (2). These spacings are consistent with
(estimated from the reported data) ∼30 V)/0.1 cm ‚V ‚s (Ion
:
13476
9
J. AM. CHEM. SOC. 2005, 127, 13476-13477
10.1021/ja054276o CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
2
-1 -1 a
Table 1. Charge Carrier Mobilities (µ, cm ‚V ‚s ) and Current
4T), was synthesized, and blending 1 and 3 results in good n-type
transistor behavior in solution-cast films. The new n-type materials
developed in this work have solubility, processability, and perfor-
mance comparable to or greater than the most studied solution-
deposited p-type semiconductors (P3HT and F8T2 ), enabling
ready integration into CMOS fabrication. Variable temperature and
computational studies are in progress to fully understand this unique
class of organic semiconductors.
On/Off Ratios (Ion:Ioff) for OFETs Fabricated with Semiconductors
1
and 2 Measured at 25 °C
1
2
deposition
method
b
T
D
(°
C)
µ
e
I
on:Ioff
µ
h
I
on:Ioff
2l
4d
4
5
vapor
vapor
vapor
25
50
70
80
90
0.02
0.17
0.31
0.45
0.17
0.21
10
10
10
10
10
10
0.012
0.014
0.028
0.031
0.043
10
10
10
10
10
10
5
5
8
7
5
5
5
6
6
3
vapor
vapor
drop-castc
120
8 × 10
-4
Acknowledgment. We thank ONR (N00014-02-1-0909) and
the NSF-MRSEC program through the Northwestern Materials
Research Center (DMR-0076097) for support. We also thank Mr.
Myung-Han Yoon and Mr. Brooks Jones for helpful discussions,
and Ms. Smruti Amin for assistance with GPC characterization.
a
Calculated in the saturation regime. Typical standard deviations are
b
c
1
0-20%. Substrate deposition temperature. From xylenes.
I
off (estimated from the reported data) ∼5, V
methanesulfonic acid] solution processable semiconductors. Non-
fluorinated material 2 exhibits hole mobilities (µ ) in vapor-
T
not reported) from
Supporting Information Available: 1-3 syntheses, spectroscopic
data, crystal structures, and device fabrication details. This material is
available free of charge via the Internet at http://pubs.acs.org.
h
2
-1 -1
5
deposited films of 0.043 ( 0.008 cm ‚V ‚s (Ion:Ioff ) 10 ; V
T
∼
-20 V, Figure S7), however no electron conduction has been
observed. Drop-cast films of this material exhibit µ
cm ‚V ‚s . Similar dependencies of mobility on T
observed in both semiconductors (Table 1), consistent with the trend
h
) (8 ( 2) ×
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2
-1 -1
DFCO-4T exhibits very high µ
e
up to ∼0.51 cm ‚V ‚s (vapor-
2
-1 -1
deposited) and ∼0.25 cm ‚V ‚s (solution-cast) s the latter being
unprecedented for a solution-processed organic semiconductor. Any
minor air instability can be addressed by core substitution as
1c
previously demonstrated. The corresponding polymer, P(COFCO-
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