Electrochromic devices and thin film transistors from a new family of ethylenedioxythiophene based conjugated polymers

Article abstract of DOI:10.1039/c0nj00837k

New electrochromic conjugated polymers and their corresponding devices based on EDOT (ethylenedioxythiophene) are described. The best of these polymers display response times on the order of 1s and high switchable contrast in the visible and near-infrared

Full text of DOI:10.1039/c0nj00837k

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PAPER
Electrochromic devices and thin film transistors from a new family of
ethylenedioxythiophene based conjugated polymerswz
Zhongtao Li, Yuan Zhang, Amanda L. Holt, Borys P. Kolasa,
a
c
a
b
b
b
c
Justin G. Wehner, Andreas Hampp, Guillermo C. Bazan,
c
a
Thuc-Quyen Nguyen and Daniel E. Morse*
Received (in Gainesville, FL, USA) 28th October 2010, Accepted 7th January 2011
DOI: 10.1039/c0nj00837k
New electrochromic conjugated polymers and their corresponding devices based on EDOT
(
ethylenedioxythiophene) are described. The best of these polymers display response times on the order
of 1s and high switchable contrast in the visible and near-infrared (Vis-NIR) spectral regions. Thin films
70 nm) of these new polymers displayed optical band gaps on the order of 1.73 eV (7a) o 2.19 eV (7b)
o 2.23 eV (7c) o 2.31 eV (4) o 2.34 eV (2) as calculated from their extrapolation of the
(
ꢁ
5
2
ꢁ1 ꢁ1
s
absorption edges. Polymers 4 and 7a show field effect hole mobilities of ca. 6.7 ꢀ 10 cm V
4
ꢁ5
2
ꢁ1 ꢁ1
3
(on/off ratio10 ), respectively, related to their highly
(
on/off ratio 10 ) and 2.5 ꢀ 10 cm V
s
ordered inter-chain packing as confirmed by XRD analyses of polymer 4. Electrochromic
characterizations show that polymers 7a–c exhibit significant absorption changes in the infrared at
low voltage. The resulting solid-state devices offer promise for electrochromic shutters and filters
in the IR, since their high charge transfer mobility and ion injection efficiency permits relatively
rapid switching and good switchable contrast, while their robustness exceeds that of aqueous devices.
Research on conjugated polymer electrochromics has focused
Introduction
on increasing switching speed and optical contrast in the visible,
4
as well as on device design. These materials can be induced by
Electrochromic materials were brought to public attention
3
5 years ago in seminal work on tungsten oxide films by
modest voltages to change reversibly between a colored neutral
5
state and a transparent doped state. Although relatively few
1
Deb. In essence, the optical absorption changes between
widely separated extrema as charge is inserted or extracted.
Electrochemical manipulation of the redox processes in thin
films of these materials thus allows modulation of their
transmittance spectra. Over the past few years, electrochromic
devices (ECDs) utilizing electroactive conjugated polymers
have received increasing attention owing to their fast response
time, high optical contrasts, low fabrication cost and easy
investigations have explored the electrochromic properties of
conjugated polymers in the infrared, these materials are known
to offer potential for device applications in these wavelengths
ranging from variable attenuators for optical information
6
technology, and infrared camouflage in low-light environ-
7
8
ments, to shutters or filters for infrared detection and imaging.
Our group previously designed and fabricated transmissive
2
functional modifications. Conjugated polymers, such as
infrared electrochromic devices (ECDs) based on poly-
8
3-hexylthiophene) (P3HT) and solid gel electrolytes. We
polyanilines, polypyridines, polypyrroles, polythiophenes,
and in particular, poly(3,4-ethylenedioxythiophene) (PEDOT)
and its derivatives have thus far proved most useful. Reynolds
and some other groups have investigated the electro-
chromic properties of PEDOT and other EDOT containing
(
varied the salt type, salt concentration and polymer film
thicknesses and discussed the effects these changes made on
the devices’ optical contrast and response time. Although
those devices exhibited high optical contrast in the infrared,
their switching speeds were on the order of tens of seconds.
We report here the synthesis and use of a series of
conjugated polymers (Scheme 1), namely poly(5-(9,9-didodecyl-
2
,3
polymers.
a
Institute for Collaborative Biotechnologies, University of California,
Santa Barbara, CA 93106, USA. E-mail: dan.morse@lifesci.ucsb.edu
Raytheon Vision Systems, 75 Coromar Dr., Goleta, CA 93117, USA
Department of Chemistry and Biochemistry, Institute for Polymers
and Organic Solids, University of California, Santa Barbara,
CA 93106, USA
7
-methyl-9H-fluoren-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxine)
2, poly((2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-(heptadecan-
-yl)-9H-carbazole) 4, poly((4,7-(2,3-dihydrothieno[3,4-b][1,4]-
b
c
9
dioxin-5-yl)benzo[c][1,2,5]thiadiazole)-9-(heptadecan-9-yl)-9H-
carbazole) 7a, poly((5,6-bis(octyloxy)-4,7-(2,3-dihydrothieno-
w This article is part of a themed issue on Molecular Materials: from
Molecules to Materials, commissioned from the MolMat2010 conference.
z Electronic supplementary information (ESI) available. See DOI:
[3,4-b][1,4]dioxin-5-yl)benzo[c][1,2,5]thiadiazole)-9-(heptadecan-
10.1039/c0nj00837k.
9-yl)-9H-carbazole) 7b and poly((5,6-bis(dodecyloxy)-4,7-
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 1327–1334 1327

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Scheme 1 Synthetic routes to polymers 2, 4 and 7a–c.
(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[c][1,2,5]thiadiazole)-
Experiment
9
-(heptadecan-9-yl)-9H-carbazole) 7c, with different functional
Materials
groups that enable us to further explore the relationship
between electrochromic properties and chemical structures.
The charge transfer mobility of these polymers was also
measured to determine the relationship between the structure
and electrochromic switching. These polymers contain
ethylenedioxythiophene (EDOT) units, which provide
exceptional stability in the oxidation state combined with a
All reagents were purchased from commercial sources and
were used without further purification, unless otherwise noted.
1
2
The solvents were distilled and dried using known methods.
All reactions were carried in an air atmosphere unless otherwise
mentioned. The following acceptor and donor blocks were
prepared according to literature procedures: 4,7-dibromo-
9
13
low energy requirement for oxidation. 9,9-Didodecyl-9H-
2,1,3-benzothiodiazole (DB-BT), tributyl(2,3-dihydrothieno-
1
[3,4-b][1,4]dioxin-5-yl)stannane (EDOT-T),
2,3-dihydrothieno[3,4-b][1,4]dioxine (DB-EDOT), 4,7-dibromo-
4
fluorene and 9-(heptadecan-9-yl)-9H-carbazole are introduced
into the polymer chains to improve ECDs’ performance
through the adjustment of spacing between the polymer chains
5,7-dibromo-
1
5
1
1
5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole
(DB-BTC8),
1
0
to facilitate ion injection. The introduction of the acceptor
unit benzo[c][1,2,5]thiadiazole (BT) and its derivations was
used to decrease the lowest unoccupied molecular orbital
4,7-dibromo-5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole
1
(DB-BTC12),
1
0
2,2 -(9,9-didodecyl-9H-fluorene-2,7-diyl)bis-
16
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) 1 and 9-(heptadecan-
(
LUMO) level and to narrow the band gap, thereby extending
1
absorbance to longer wavelengths.
9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-
17
carbazole 3. ITO/glass electrodes were obtained from Delta
1
1
328 New J. Chem., 2011, 35, 1327–1334
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Technologies, Ltd., and rinsed successively with distilled
water, acetone and isopropanol before use.
1.80 (br, 4H), 1.41–1.19 (m, 48H), 0.83 (tr, 12H). Anal. calcd
for C49
55 3 6 3
H N O S (%): C, 66.86; H, 6.53. Found: C,
6
7.03, H, 6.81.
General procedures of polymerization through Suzuki
cross-coupling
Poly((5,6-bis(dodecyloxy)-4,7-(2,3-dihydrothieno[3,4-b][1,4]-
dioxin-5-yl)benzo[c][1,2,5]thiadiazole)-9-(heptadecan-9-yl)-9H-
carbazole) 7c
The general synthesis protocols used to produce the
conjugated copolymers are shown in Scheme 1. The dibromo-
monomer (1.0 mol ratio), bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
150 mg 1 (0.16 mmol), 104 mg 3 (0.16 mmol), 5 ml toluene and
borolane) (1.0 mol ratio), Pd(PPh3) (0.02 mol ratio), and
4
0.5 ml 2 M K CO solution were used to afford 100 mg red solid
3
2
1
yield 63%). M , 18 836; M , 34 286; PDI, 1.82. H NMR
toluene were placed into a two neck-bottle under argon. After
complete dissolution of the reagents, a degassed 2 M K CO
(
n w
1
2
3
(500 MHz CDCl3), H NMR (500 MHz CDCl ), d = 8.08
3
solution was added under argon to the mixture. The reaction
was refluxed under stirring for 48 h. The mixture was poured
into methanol. The precipitates were collected, extracted with
methanol, acetone and hexane using a Soxhlet apparatus, and
then dried under vacuum.
(
s, 3H), 7.85 (s, 1H), 7.68 (br, 2H), 4.65 (s, 1H), 4.47 (s, 4H), 4.37
s, 4H), 4.16 (s, 4H), 2.40 (br, 2H), 1.98 (br, 2H), 1.79 (br, 4H),
.43–1.17 (m, 64H), 0.88–0.83 (tr, 12H). Anal. calcd for
(%): C, 68.99; H, 7.41. Found: C, 68.79, H, 7.05.
(
1
57 71 3 6 3
C H N O S
Characterization
Poly(5-(9,9-didodecyl-7-methyl-9H-fluoren-2-yl)-2,3-
dihydrothieno[3,4-b][1,4]dioxine) 2
1
13
H NMR and C NMR spectra were collected on a Mercury
5
00 MHz NMR spectrometer using CDCl and referenced to
3
1
50 mg 1 (0.20 mmol), 60 mg DB-EDOT (0.20 mmol), 5 ml
toluene and 0.5 ml 2 M K CO solution were used to afford
0 mg yellow solid (yield 61%). M , 16 620; M , 31 445; PDI,
), d = 7.85–7.77 (m, 6H),
.47 (s, 4H), 2.08 (br, 4H), 1.22–1.13 (m, 40H), 0.79 (s, 6H);
S (%): C, 80.32; H, 9.72. Found: C,
1
13
the solvent residual peak (CDCl : H: d = 7.26 ppm, C:
3
2
3
d = 77.23 ppm). Gel permeation chromatography (GPC) was
performed using a Waters Associates GPCV2000 liquid
chromatography system with its internal differential refractive
index detector (DRI) at 150 1C. Cyclic voltammetry (CV) was
performed using a CIH 500/A electrochemical workstation
from CH Instruments, Inc. These analyses were carried out
using a three-electrode cell with a Pt wire as the counter
electrode, a Ag/AgCl reference electrode calibrated with a
8
1
4
n
w
1
.89. H NMR (500 MHz CDCl
3
Anal. calcd for C43
0.16, H, 9.58.
60 2
H O
8
Poly((2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-
heptadecan-9-yl)-9H-carbazole) 4
(
5
mM solution of Fc/Fc+ in 0.1 M TBAP/acetonitrile
1
50 mg 3 (0.23 mmol), 68 mg DB-EDOT (0.23 mmol), 5 ml
toluene and 0.5 ml 2 M K CO solution were used to afford
5 mg yellow solid (yield 56%). M , 32 649; M , 73 570; PDI,
(ACN) as electrolyte solution and a glassy carbon working
electrode. All absorption spectra were collected with a Jasco
670 UV-vis-NIR spectrophotometer at room temperature
using a quartz cuvette with a path length of 1 cm and
chlorobenzene as solvent. For the thin film spectra, polymers
2
3
7
2
n
w
1
.25. H NMR (500 MHz CDCl ), d = 8.10 (s, 3H), 7.87
3
(
s, 1H), 7.67 (s, 2H), 4.68 (s, 1H), 4.51 (s, 4H), 2.43 (br, 2H),
.02 (br, 2H), 1.30–1.19 (m, 28H), 0.84 (tr, 6H). Anal. calcd for
S (%): C, 77.02; H, 8.68. Found: C, 77.26, H, 8.43.
ꢁ
1
2
were first dissolved in chlorobenzene (1 mg mL ), and
spin-coated at a speed rate of 1500 rpm for 30 s onto the glass
slides. Optical bandgaps were calculated from the edge of the
visible-absorption bands in the thin films spectra. X-ray
diffraction (XRD) was performed by Philips X’PERT MPD
35 2
C H45NO
Poly((4,7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[c]-
1,2,5]thiadiazole)-9-(heptadecan-9-yl)-9H-carbazole) 7a
[
˚
X-ray diffractometer, using Cu Ka radiation (l = 1.5405 A)
150 mg 6a (0.26 mmol), 170 mg 3 (0.26 mmol), 5 ml toluene and
with a typical scan range from 21 to 351, 11 step size, and 120 s
per step on the drop-cast polymer film.
0
.5 ml 2 M K CO solution were used to afford 105 mg black
2
3
solid (yield 65%). M
n
,
20 800; M
w
, 48 117; PDI, 2.31.
), H NMR (500 MHz CDCl ),
d = 8.53 (s, 2H), 8.17–8.11 (m, 3H), 7.92 (br, 1H), 7.77–7.75
br, 2H), 4.71 (s, 1H), 4.56–4.53 (d, 8H), 2.44 (br, 2H), 2.03
br, 2H), 1.31–1.20 (m, 28H), 0.82 (tr, 6H). Anal. calcd
for C33 (%): C, 63.54; H, 4.04. Found: C, 63.76,
H, 4.52.
1
1
H NMR (500 MHz CDCl
3
3
Fabrication and characterization of field effect transistors
(
Transistors were fabricated using a bottom gate–top electrode
(
device structure. 150 nm thick SiO with an area capacitance
2
ꢁ
4
2
25 3 4 3
H N O S
of B2.33 ꢀ 10 F m was used as the gate dielectric under
which a highly n-type doped Si layer functions as the gate. All
the materials studied here were dissolved in chlorobenzene
Poly((5,6-bis(octyloxy)-4,7-(2,3-dihydrothieno[3,4-b][1,4]-
dioxin-5-yl)benzo[c][1,2,5]thiadiazole)-9-(heptadecan-9-yl)-9H-
carbazole) 7b
ꢁ1
with a concentration of 8 mg ml and the solution was
2
sufficiently stirred at 110 1C on a hotplate. The SiO substrates
were sonicated with acetone and 2-isopropanol for 30 min and
dried at 140 1C, then immediately transferred in a UV–ozone
oven for 1 hour to remove surface impurities. Subsequently the
1
50 mg 6b (0.18 mmol), 118 mg 3 (0.18 mmol), 5 ml toluene
and 0.5 ml 2 M K CO solution were used to afford 114 mg red
solid (yield 71%). M , 16 128; M , 26 648; PDI, 1.65.
H NMR (500 MHz CDCl ), H NMR (500 MHz CDCl ),
2
3
n
w
2
SiO substrates were soaked in octadecyltrichlorosilane/
1
1
hexane dilute solution (1 : 750 vol. ratio) for 30 min to
enhance surface hydrophobicity. The active layers were
spun-coat from chlorobenzene with a spin-speed of 2000 RPM
3
3
d = 8.08 (s, 3H), 7.85 (s, 1H), 7.67 (br, 2H), 4.65 (s, 1H),
.47 (s, 4H), 4.38 (s, 4H), 4.17 (s, 4H), 2.39 (br, 2H), 2.00 (br, 2H),
4
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New J. Chem., 2011, 35, 1327–1334 1329

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leading to film thickness typically of B40 nm. A thermal
annealing of 140 1C for 20 min was also applied to the cast
films. At last 90 nm of the Au top electrode was evaporated
through a shadow mask on the active layers under a pressure
of 2 ꢀ 10ꢁ Torr resulting in a channel width of 1 mm and
length of 40 mm.
7
Transistor characteristics were analyzed with in a Lakeshore
ꢁ
4
probe station under a pressure of B10 Torr and electrical
measurements taken with a Keithley 4200 semiconductor
parameter analyzer.
Fabrication and characterization of electrochromic devices
The polymer films (thicknesses ca. 70 nm) were spun-cast from
chlorobenzene (1% wt.) solution onto the ITO/glass electrode at
1500 rpm for 30 s and vacuum annealed at 120 1C overnight. The
gel electrolyte base prepared from an acetonitrile solution of
poly(methyl methacrylate) (PMMA), propylene carbonate and
ethylene carbonate in weight ratios of 1 : 0.1 : 0.4 : 0.4, and
tetrabutyl ammonium tetrafluoroborate TBABF4 was added
in a 0.9 M concentration relative to the acetonitrile. After
spin-coating a given polymer and the gel electrolyte on
two separate ITO/glass substrates, the coated surfaces were
manually sandwiched together and sealed with epoxy.
Result and discussion
Optical properties
The normalized (to the maximum absorption value) UV-visible
absorption spectra of the studied copolymers in dilute
Fig. 1 Normalized optical absorption spectra of polymers 2, 4, 7a–c
in (a) dilute chlorobenzene solutions and (b) thin films on a glass plate.
ꢁ
3
ꢁ1
chlorobenzene (1.0 ꢀ 10 mg ml ) and thin films are shown
in Fig. 1a and b, respectively. The corresponding maximum
opt
energy level of the EDOT based polymers can be efficiently
tuned by combination with different subunits and polymer
backbone planarity.
absorption wavelengths (lmax) and optical band gaps (E
g
)
are summarized in Table 1. The lmax of polymers 2, 4, and
7
a–7c in solution are observed at 442 nm, 468 nm, 591 nm,
4
65 nm and 460 nm, respectively, while those of the thin films
Electronic properties
are at 450 nm, 475 nm, 600 nm, 473 nm and 468 nm. The solid
films show similar absorption spectra to those in chloro-
benzene solutions, but the peaks are broadened. The lmax of
the polymer films are slightly red-shifted relative to those of
the same polymers in solution, suggesting p–p inter-chain
Cyclic voltammetry (CV) curves of the polymers were
obtained with drop-cast polymer films under 0.1 M TBAP/
ꢁ
1
acetonitrile electrolyte solutions scanned at a rate of 100 mV s
.
The HOMO and LUMO energy levels and electrochemical
ec
association and aggregation in the solid state.
opt
g
band gaps (E ) were estimated from the onset oxidation
ox
red
The optical band gaps (E
g
, eV) obtained from extra-
potentials (Eonset) and onset reduction potentials (Eonset) based
ox
ferrocene
polation of the absorption edges of the polymer film spectra
are in the order of 7a (1.73 eV) o 7b (2.19 eV) o 7c (2.23 eV)
o 4 (2.31 eV) o 2 (2.34 eV). Lacking the acceptor–donor
on the following equations: HOMO = ꢁ[Eonset ꢁ E1/2
+
red
ferrocene
4.8] eV and LUMO = ꢁ[Eonset ꢁ E1/2
+ 4.8] eV, with the
potentials referenced to an Ag/AgCl reference electrode.
ferrocene
structure, polymers 2 and 4 have shorter l
absorbances
E
= 0.26 V versus Ag/AgCl. The results are
max
1/2
relative to those of polymers 7a–c. Introduction of the
acceptor benzothiodiazole (BT) also effectively reduces the
polymer band gap. Polymer 7a exhibits the longest lmax
absorbance, largely as a result of the intramolecular charge
transfer effect between the donor and acceptor and the
extension of the conjugated system. On the other hand, the
incorporated bulky side chains (dodecyloxy or octyloxy) on
acceptors in polymers 7b and 7c lead to a blue shift of
absorption. Even though there are donor–acceptor structures
in these polymer backbones, the non-planarity of the extended
summarized in Table 1. All the polymers exhibited both
reversible p-doping/dedoping (oxidation/rereduction) and
n-doping/dedoping (reduction/reoxidation) processes. The
corresponding HOMO energy levels of 2, 4, and 7a–c are
ꢁ5.52, ꢁ5.04, ꢁ5.28, and ꢁ5.17 eV, respectively, while their
LUMO energy levels are ꢁ3.24, ꢁ3.38, ꢁ3.22, and ꢁ3.62 eV.
Although the band gap values show the same trend as a
ec
g
function of polymer structure, the E
opt
values are up to
0.3 eV larger than the E
g
values, an effect probably due to
1
the exciton binding energy of the conjugated polymers.
9
1
8
conjugated system also leads to a smaller band gap. The
above results indicate that the band gap and the molecular
Polymer 7a exhibits the lowest HOMO and highest LOMO.
Apparently, intramolecular charge transfer renders this polymer
1
330 New J. Chem., 2011, 35, 1327–1334
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Table 1 Optical and electrochemical properties of polymers 2, 4, 7a–c
UV-visible absorption spectra
Cyclic voltammetry
a
a
opt
red
E
ox
ec
g
E /ev
Polymer
l
max (solution)/nm
l
max (film)/nm
E
g
/ev
/LUMO/ev
E
/HOMO/ev
2
4
442
468
591
465
460
450
475
600
473
468
2.31
2.34
1.73
2.19
2.23
ꢁ1.41/ꢁ3.13
ꢁ1.43/ꢁ3.12
ꢁ1.18/ꢁ3.36
ꢁ1.37/ꢁ3.17
ꢁ1.43/ꢁ3.12
0.99/ꢁ5.53
0.98/ꢁ5.52
0.56/ꢁ5.10
1.01/ꢁ5.55
1.02/ꢁ5.56
2.40
2.41
1.74
2.38
2.45
7a
7b
7c
a
Chlorobenzene as processing solution.
Fig. 2 XRD pattern of a polymer 4 film drop-cast on glass from
chlorobenzene.
more able to capture electrons at higher reduced potential and
lose electrons at lower oxidized potential, resulting in a narrower
band gap. Compared to the HOMO/LUMO energy levels of 7a
(
ꢁ5.52/ꢁ3.24 eV), those of 7b (ꢁ5.04/ꢁ3.38 eV) and 7c exhibit a
higher HOMO level by 0.48 eV, and a lower LUMO level by
.14 eV, apparently the result of the reduced electron withdrawing
0
activity of the BT units. The stabilization of the HOMO level
and destabilization of the LUMO level thus increase the band
2
0
gaps of 7b and 7c. Polymers 2 and 4 exhibit similar HOMO/
LOMO levels to those of polymers 7b and 7c. Even though
there are no donor–acceptor structures in these polymer back-
bones, the co-planarity of the extended conjugated system also
leads to a smaller band gap. The above results indicate that the
band gap and the molecular energy level of the EDOT
based polymers can be efficiently tuned by combination with
different subunits and polymer backbone planarity.
Fig. 3 p-Type output characteristics of FETs and transfer character-
istics of polymer 4 prepared by spin-coating from chlorobenzene onto
the ODTS modified substrates.
diffraction peak at 2y E 5.41 corresponds to the d spacing value
1
˚
of 16.3 A, which is assigned to the interchain spacing between
polymer main chains, where the alkylcarbazole substituents are
segregated similar to those of p-conjugated polymers with long
side chains. The d , d and d spacings exhibit a proportionality
1 2 3
X-Ray diffraction analysis
21
To further understand the molecular organization and
morphology in the film, X-ray diffraction (XRD) was used
to analyze polymer 4 (in Fig. 2). Films were drop-cast from
chlorobenzene onto glass plates and annealed at 180 1C
overnight for X-ray analysis, as shown in Fig. 2. The higher
1 2 3
(d : d : d E 4 : 2 : 1) that suggests a highly organized packing
˚
of polymer chains. Hence, the 16.3 A out-of-plane spacing (100)
in the polymer 4 film suggests partial interdigitation of the side
chains, indicative of the formation of a lamellar structure that
may enhance charge transport. XRD analyses of the other
polymers failed to detect any peaks.
3
angle peak of the d spacing at 22.21 corresponds to a distance
˚
of approximately 4.03 A that can be assigned to the p–p
stacking between aromatic rings. The higher intensity 11.11
peak of the second-order diffraction peak d corresponds to a
Field-effect transistor (FET) characteristics
2
˚
distance of 8.03 A, which may be related to the spacing between
Fig. 3a shows the output characteristics of the FETs in terms
co-planar chains caused by the solubilizing groups. The distinct
of the hole transport in polymer 4. The steep curves in the
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Fig. 5 Spectroelectrochemistry of polymer 2 and 4 films on the
ITO-coated glass substrate; UV-vis-NIR absorption spectra were
monitored while different potentials were applied to the films:
(a) polymer 2; (b) polymer 4; a 0 V, b 2.0 V, c 2.5 V, d 2.8 V,
e 3.0 V, f 0 V, g 2.0 V, h 2.5 V, i 2.7 V, j 3.0 V.
Fig. 4 p-Type output characteristics of FETs and transfer character-
istics of polymer 7a prepared by spin-coating from chlorobenzene onto
the ODTS modified substrates.
linear regime are an indication of a relatively low contact
resistance between the Au and polymer. Upon application of
Electrochromic properties
Oxidation and reduction of the polymer films are accompanied
by remarkable color changes depending on the chemical
structure of the polymers. To analyze these color changes
in detail, we carried out a spectro-electrochemical study,
monitoring the absorption spectra while different potentials
were applied to the films.
–
60 V, the drain current becomes saturated. Fig. 3b represents
the transfer curves of the FETs. Varying the drain voltage
from ꢁ40 V to ꢁ60 V results in identical drain currents,
suggesting that the channel is totally pinched off under at this
bias. The current when the FETs are fully turned on and off
4
exhibits an on/off ratio of 10 . From the extrapolation of
Polymers 2 and 4 display similar electrochromic spectral
properties (Fig. 5). The films change color from yellow to blue
when the applied bias drives oxidation. The first doping states
occur at 2.5 V. The color changes in these two polymers are
marked by a decrease in the bands at around 450 nm and
OIdrain vs. V
g
a threshold voltage of ꢁ23 V is obtained. Using
the standard FET equation as follows:
pffiffiffiffiffiffiffiffiffiffi
2
L @ Idrain
msatðFETÞ ¼
ð1Þ
WCi @Vg
4
70 nm, respectively, and the formation of new peaks at
where W and L are the width and length of the FET channels,
710 nm and 700 nm. At 3.0 V, oxidation intensifies and the
films become light blue and more transparent. The different
colors of the films can be ascribed to different oxidation states,
radical delocalization, and formation of a quinoid structure in
and C is the area capacitance, the estimated hole mobility of
i
ꢁ
5
2
ꢁ1 ꢁ1
polymer 4 amounts to 6.7 ꢀ 10 cm V
s . This value
negligibly varies with device and dimensions due to slight
differences in the gate-source leakage.
2
2
the polymer backbone.
The output characteristics and transfer curves of the FETs
based on polymer 7a are shown in Fig. 4. This device shows a
The color transitions in both polymers are accompanied by
an absorption decrease at around 700 nm and the appearance
of a very broad absorption starting at 900 nm with a long
wavelength tail extending to the infrared. Absorption also
decreases below 500 nm upon oxidation. These spectral
changes are highly reversible and do not necessitate special
precautions to protect the films against oxygen or humidity.
ꢁ
5
2
ꢁ1 ꢁ1
hole mobility of 2.5 ꢀ 10 cm V
s
and an on/off ratio of
3
0 , roughly similar to of the values for polymer 4. Polymers 2
1
and 7b and 7c did not exhibit FET behavior, possibly due
to the weak intermolecular interactions and decreased
crystallinity in their thin films.
1
332 New J. Chem., 2011, 35, 1327–1334
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Fig. 6 Switching property of EC devices based on polymers 2 and 4.
(a) polymer 2; (b) polymer 4.
Fig. 6 shows the NIR switching behavior of devices based
on polymers 2 and 4. Switching times are determined by
monitoring the absorption intensity changes at 700 nm with
the applied voltage switching between ꢁ2.5 V and 2.5 V. By
expanding the time scale of a single operating cycle, the
switching time could be determined. Devices based on polymer
2
need over 10 s for turning on and approximately 1–2 s for
Fig. 7 Spectroelectrochemistry of coordination polymer 7a and 7b
films on the ITO-coated glass substrate; UV-vis-NIR absorption
spectra were monitored while different potentials were applied to the
films: (a) polymer 7a; (b) polymer 7b; a 0 V, b 2.0 V, c 2.25 V, d 2.5 V,
e 0 V, f 1.5 V, g 2.0 V, h 2.5 V.
turning off (85% of total transformation). The response time
of polymer 4 is much quicker, requiring only 1.8 s on and 0.6 s
,23
off; this is considerably faster than previously reported.
8
The dynamics of color switching of an electrochromic device
are typically limited by the ionic conductivities of the electrolyte
and polymer film, electron and hole transfer at the electrodes and
charge mobility in the polymer film. In our case, when the EC
device turns on, the polymers lose their electrons and become
electropositive as a result of electron transfer from the polymer
layer into the contacting electrode. At the same time, oppositely
charged ions in the electrolyte gel are separated under the
influence of the external field, and these anions are injected into
the polymer, oxidizing it and maintaining electrical neutrality.
Polymer 4 has much higher charge transfer mobility than
polymer 2. When turning on the devices, electrons are more
easily transferred into the electrode from polymer 4 as a result of
this high charge transfer mobility. The carbazole in polymer 4
also plays an important role in stabilizing polymer oxidization.
However, this oxidation proceeds through the a positions of the
external heterocyclic rings since the 2, 7, and 9 positions of the
carbazole are blocked. The external rings contain a higher
24
electron density and thus result in rapid oxidation.
When the device is turned off, the process is reversed. Electrons
flow back from the electrode into the polymer and anions flow
from the polymer into the electrolyte. The injection of the smaller
electrons from the electrode into the polymer layer is much faster
than of the movement of the bulk ions from the electrolyte,
leading to a higher turn-off speed.
Fig. 8 Switching property of EC devices based on polymers 7a and
7b. (a) polymer 7a; (b) polymer 7b.
oxidized state. The depletion of both of these transitions
occurs with a decrease in the absorbance at 600 and 410 nm.
The doping-induced infrared absorption bands begin to
emerge at intermediate oxidation levels with an absorbance
centered at 800 nm and a broad absorbance beginning at
950 nm and extending into the IR (Fig. 7a).
As shown in Fig. 7a, when positive voltage is applied to
devices based on polymer 7a, the polymer switches from a
dark teal-green in the neutral state to a pearl blue in the
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New J. Chem., 2011, 35, 1327–1334 1333

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Polymers 7b and 7c (see ESIz) exhibit similar EC properties.
Decreases in absorbance occur at 473 nm and 390 nm and an
increased broad absorbance starts at 800 nm. Unlike polymers
Laboratory (grant W911NF-09-D-0001) and the University
of California Discovery Program (IUCRP, grant gcp07-10260).
2
and 4, polymers 7a–c show significant absorption changes
Notes and references
in the IR at low voltage related to their donor–acceptor
properties (Fig. 8).
1
(a) S. K. Deb, Appl. Opt., Suppl., 1969, 3, 192; (b) S. K. Deb,
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The switching times of polymers 7a and 7b were determined
by monitoring the absorption intensity changes at 1100 nm
with the applied voltage switching between ꢁ2.5 V and
2
(a) R. J. Mortimer, A. L. Dyer and J. R. Reynolds, Displays, 2006,
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2
2
.5 V. The response time of devices based on polymer 7a is
s for turning on and 0.7 s for turning off. The color change
3
(a) G. Sonmez, Chem. Commun., 2005, 5251; (b) G. Sonmez,
H. B. Sonmez, C. K. F. Shen and F. Wudl, Adv. Mater., 2004,
time of polymer 7b is much longer, requiring tens of seconds
to turn on and several seconds to turn off. This marked
difference in behavior probably stems from the introduction
of the long side chains in 7b. Polymer 7a has similar charge
transfer mobility to that of 4, and also contains the carbazole
unit to accelerate ion injection, leading (as described above)
to high switching speeds. But while the substitutions on
BT in polymers 7b and 7c improved the polymers’ solubility
1
6, 1905; (c) I. Schwendeman, J. Hewang, D. M. Welsh,
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4
5
6
7
(
(
making them readily soluble in common solvents at
room temperature), the flexible bulky side-chains evidently
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2
008, 20, 2328.
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`
Conclusion
We have synthesized and investigated the optical properties of
a new series of electrochromic polymers. These polymers
contain EDOT units and fluorene (2), carbazole (4), carbazole–
benzothiadiazole (7a), carbazole–benzothiadiazoles (7b and 7c)
formed by cross-linking though the Suzuki reaction. The
experimental data reveal that the materials’ optical and
electrical properties can be tuned by copolymerization with
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1
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2
ꢁ1 ꢁ1
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Acknowledgements
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We gratefully acknowledge support from DARPA through
ARO (grant W911NF-08-1-0494), the Army Research
2
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334 New J. Chem., 2011, 35, 1327–1334
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011