A spray-processable, low bandgap, and ambipolar donor - Acceptor conjugatedpolymer

Article abstract of DOI:10.1021/ja809372u

(Figure Presented) Combining a strong donor, tris(dodecyloxy)phenyl)- dithieno[3,2-b:2',3'-d]pyrrole, with a strong acceptor, 4,8-dithien-2-yl-2L4M2- benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole, has yielded the lowest bandgap, soluble, spray-processable poly

Full text of DOI:10.1021/ja809372u

Published on Web 02/06/2009
A Spray-Processable, Low Bandgap, and Ambipolar Donor-Acceptor
Conjugated Polymer
Timothy T. Steckler,† Xuan Zhang,‡ Jungseek Hwang,§ Ryan Honeyager,§ Shino Ohira,‡
Xiao-Hong Zhang,¶ Adrian Grant,¶ Stefan Ellinger,† Susan A. Odom,‡ Daniel Sweat,‡
David B. Tanner,§ Andrew G. Rinzler,§ Stephen Barlow,‡ Jean-Luc Bre´das,‡ Bernard Kippelen,¶
Seth R. Marder,*,‡ and John R. Reynolds*,†
The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for
Macromolecular Science and Engineering, UniVersity of Florida, Box 117200, GainesVille, Florida 32611, School
of Chemistry & Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology,
901 Atlantic DriVe, Atlanta, Georgia 30332, Department of Physics, UniVersity of Florida, Box 118440, GainesVille,
Florida 32611, School of Electrical and Computer Engineering and Center for Organic Photonics and Electronics,
Georgia Institute of Technology, 777 Atlantic DriVe NW, Atlanta, Georgia 30332
Received December 7, 2008; E-mail: seth.marder@chemistry.gatech.edu; reynolds@chem.ufl.edu
Narrow bandgap conjugated polymers can provide a unique set
of redox and optoelectronic properties, including broad and long
wavelength light absorption, solid-state transport of both holes and
electrons, and access to multiple charge states in a small potential
window. In ground-breaking work, Wudl et al. demonstrated that
polyisothianaphthene exhibited a bandgap of ∼1 eV due to the
appended benzene ring forcing the thiophene into a quinoid-like
ground-state geometry.1 Multiple researchers have also sought to
reduce the band gap by using a donor-acceptor (DA) approach.2
This method has proven to be an effective strategy for tailoring
the properties of oligomers and polymers for applications in
photovoltaics,3 OLEDs,4 electrochromics,5 and OFETs.3a,4e,6
To enhance the D-A interaction and simultaneously produce
soluble low band gap polymers, we have utilized the strong donor
dithieno[3,2-b:2′,3′-d]pyrrole (DTP)6d,7 functionalized with a tri-
alkoxyphenyl group, combined with a strong acceptor based on
benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBT)8 which has been
previously used in soluble low band gap polymers for photovoltaic
applications.9 In investigating donor-acceptor-donor (DAD)
systems, we have previously used the electron-rich dioxythiophene-
based donors with this acceptor; however, the molecules suffer from
large dihedral angles which decrease the π conjugation relative to
Figure 1. (a) Synthesis of P(DTP-BThBBT); (b) HOMO/LUMO levels
molecules where unsubstituted thiophene flanks the acceptor. For
of an oligomer (DTP-BThBBT)n (n ) 2) used to calculate the theoretical
bandgap.
example, using 3,4-ethylenedioxythiophene (EDOT), the λmax for
BEDOT-BBT (4,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-
crude polymer was then purified by Soxhlet extraction with MeOH,
benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole) (643 nm) is blue-shifted
hexane, acetone, and CHCl3. The CHCl3 fraction was then reduced
about 0.16 eV compared to BTh-BBT (4,8-dithien-2-yl-2λ4δ2-
in volume, precipitated into methanol, and collected by filtration
benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole) (702 nm).8b,10 Turning to
yielding a black solid with a Mn of 14 K and a PDI of 3.6 (all
the fused-ring DTP, this donor provides a high-lying highest
synthetic and characterization details presented in the Supporting
occupied molecular orbital (HOMO), planarity for π stacking, and
Information).
solubility in the polymers due to the long-chain alkoxy substituents.
The geometric structure of P(DTP-BThBBT) and its electronic
As illustrated in Figure 1a, we report on the coupling polymer-
properties, including the lowest excited-state energies, were cal-
ization of the strong donor, N-(3,4,5-tris(dodecyloxy)phenyl)-
culated at the density functional theory (DFT) and time-dependent
dithieno[3,2-b:2′,3′-d]pyrrole with a strong acceptor, BThBBT, via
(TD)-DFT B3LYP/3-21G* levels using model oligomers (DTP-
Stille methodology,11 to obtain the lowest bandgap polymer P(DTP-
BThBBT)n with n ) 1, 2, 3, 4, and 6. The HOMO and lowest
BThBBT) prepared to date that is soluble and spray processable.
unoccupied molecular orbitals (LUMO) of (DTP-BThBBT)2 are
The polymerization was carried out in THF at reflux for ∼2 days,
illustrated in Figure 1b. As in several similar polymers with
followed by volume reduction and precipitation into methanol. The
alternating donor and acceptor units,12 while the HOMO wave
function is delocalized over the whole π system, the LUMO wave
† Department of Chemistry, University of Florida.
function is localized on the acceptor units. The calculated lowest
excited-state energy, obtained by extrapolation to n ) ∞, is 0.50
eV (Supporting Information, Figure S1). Since the lowest lying
transition is primarily of HOMO f LUMO character, this points
‡ School of Chemistry & Biochemistry and Center for Organic Photonics and
Electronics, Georgia Institute of Technology.
§ Department of Physics, University of Florida.
¶ School of Electrical and Computer Engineering and Center for Organic
Photonics and Electronics, Georgia Institute of Technology.
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2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Reflectance spectrum of P(DTP-BThBBT) film on gold (black
curve) and absorption spectrum on SWCNT (red curve).
Figure 3. Oxidative spectroelectrochemistry of P(DTP-BThBBT) spray
cast on to an ITO coated glass working electrode using a Ag wire
pseudoreference electrode calibrated to Fc/Fc+ and a Pt wire counterelec-
trode in 0.1 M TBAP/PC. The potential was switched from -0.13 to +0.97
V vs SCE in 100 mV increments. Inset shows a picture of the neutral and
oxidized polymer film.
to intramolecular charge-transfer and suggests that P(DTP-BTh-
BBT) will have an especially low onset for optical absorbance.
The theoretical results are confirmed for polymer films spray
cast from CHCl3 solutions onto indium tin oxide (ITO), single-
walled carbon nanotubes (SWCNT),13 and gold electrodes on glass
for spectral and redox characterization as shown in Figure 2. P(DTP-
BThBBT) sprayed onto a SWCNT electrode shows a λmax of 1208
nm with an absorption onset at ∼1967 nm corresponding to a
bandgap of ∼0.63 eV determined via the tangent lines (red
spectrum). Meanwhile the reflectance spectrum obtained on gold
and converted into absorption coefficient for direct comparison
(black), allows us to go deeper into the mid-IR for better baseline
characterization. The polymer shows an onset of absorption at
∼2260 nm giving a bandgap of 0.54 eV, which is close to the
calculated S1 excited-state energy of 0.50 eV. The reflectance
spectrum shows the C-H stretching vibrations (aromatic C-H
∼3100 to 3000 cm-1 and alkyl C-H ∼3000 to 2840 cm-1) at
∼3350 to 3550 nm along with the C-H bending modes (∼1450
and 1375 cm-1) between 7100 and 7400 nm. Thermal stability was
analyzed by TGA; the polymer showed a 5 wt % loss at 331 and
316 °C under N2 and air atmospheres, respectively. As another
demonstration of stability (Supporting Information Figure S6),
P(DTP-BThBBT) was exposed to outdoor (Supporting Information
Table S1) light/air conditions and showed only minor changes in
the UV-vis absorption spectrum over a period of 7 weeks.
Electrochemistry was carried out on drop-cast films (from CHCl3)
on a platinum button working electrode, using a Pt flag counter
electrode, and a Ag/Ag+ (0.01 M AgNO3, 0.1 M TBAP, CH3CN)
reference electrode. Films were repeatedly redox switched in a 0.1
M TBAP/PC (tetrabutylammonium perchlorate/propylene carbon-
ate) solution until reproducible electrochemistry was achieved.
nm makes the use of the SWCNTs less valuable for this experi-
ment). In neutral P(DTP-BThBBT), two absorption bands5d,14 are
observed with peaks at 1231 and 529 nm, corresponding to a purple
neutral polymer (see photograph in Figure 3). Upon incremental
oxidation, a bleaching of the peak at 529 nm occurs and a broad
absorption develops that extends into the IR with a maximum near
∼1400 nm that swamps the neutral polymer’s low-energy transition
suggesting a p-doped state.15 This results in a more visibly
transparent gray/blue polymer film.
Reductive spectroelectrochemistry (Supporting Information, Fig-
ure S3), with incremental potential stepping from the neutral state
(+0.05 V vs SCE) to the first reduced state (-0.95 V vs SCE),
induces a decrease in intensity of the long wavelength band of the
neutral polymer along with the formation of a less intense band at
∼1500 nm attributed to n-type doping.15 Concurrently, there is an
increase in the intensity of the high-energy band at 524 nm (with
a 10-15 nm red shift) along with the development of a slight
shoulder at 700 nm. This results in a darker purple film in the first
reduced state (Supporting InformationFigure S4). When the potential
is stepped to the second reduction, the band at ∼1500 nm is fully
bleached, while the shoulder from the first reduction develops into
a new well-defined peak at 770 nm. This results in a polymer with
peaks at ∼535 and ∼770 nm yielding a bright blue/purple film
with saturated color (Supporting Information, Figure S4).
Since the electrochemistry and spectroelectrochemistry of P(DTP-
BThBBT) showed both p- and n-type doping at moderate potentials,
it was though that this polymer might serve as a candidate ambipolar
charge-transport material. To investigate the transport properties
of P(DTP-BThBBT), top-contact OFETs were prepared on heavily
n-doped silicon (n+-Si) substrates with 200-nm-thick thermally
grown SiO2 as gate dielectric (εr ) 3.9). A 50 nm portion of Au
and 5 nm of Ti were deposited (using an e-beam system) on the
back of the n+-Si substrate as a layer to create a better contact of
the gate electrode (n+ doped Si) of the OFET. Ti was used as an
adhesion layer for the gold onto to the Si-substrate. The SiO2 layer
was then treated with either a self-assembled monolayer (SAM) of
octadecyltrichlorosilane (Q1 devices) or a thin buffer layer of
benzocyclobutene (Q2 devices). The P(DTP-BThBBT) layer was
then spin-coated on the treated SiO2 (Q1:OTS, Q2:BCB) at 500
rpm for 60 s in a N2-filled glovebox from a solution of 10 mg/mL
in chlorobenzene. The devices were then completed with 100 nm-
Cyclic voltammetry shows an E1/2 of 0.7 V vs SCE and two
ox
reductions at E1/2
) -0.50 and -1.05 V vs SCE (Supporting
red
Information Figure S2). On the basis of the onsets for oxidation
(ca. +0.3 V vs SCE, depending upon the method) and reduction
(ca. -0.3 V vs SCE) we estimate an electrochemical bandgap of
0.6-0.95 V consistent with the spectral and computational results.
Extending the energy levels to vacuum yields an estimated solid-
state ionization potential of ca. 5.0-5.1 eV and electron affinity of
ca. 4.4-4.2 eV. These values suggest the possibility of facile
injection of both holes and electrons from commonly used electrode
materials, and, therefore, of ambipolar charge transport.
Oxidative (Figure 3) and reductive spectroelectrochemistry was
performed on a film of P(DTP-BThBBT) on ITO after ca. 20 redox
cycles. ITO was used as the electrodes in these experiments (the
strong absorption of the solvent/electrolyte system beyond 2000
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C O M M U N I C A T I O N S
Program of the National Science Foundation under Agreement No.
DMR-0120967. The solid state device work was performed in part
at the Microelectronics Research Center at Georgia Institute of
Technology, a member of the National Nanotechnology Infrastruc-
ture Network, which is supported by NSF (Grant No. ECS- 03-
35765).
Supporting Information Available: Details of experimental results
(electrochemical data, theoretical calculations) and synthetic preparation.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 4. (a) Transfer characteristics of samples Q1 and (b) corresponding
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measured in a device annealed at 110 °C; n-channel operation (dash line in
panels a and b) is measured in a device annealed at 150 °C.
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geometry Q1 and Q2. One set was tested immediately after
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In conclusion, a new soluble, spray-processable, donor-acceptor
polymer has been synthesized and shows an optical bandgap of
0.5-0.6 eV, which is the lowest value reported for a soluble spray-
processable polymer. Four differently colored redox states of the
polymer can be accessed at moderate potentials and have good
stability. The polymer also shows potential for use in ambipolar
OFETs with respectable mobilities for a solution-processed device.
We are currently investigating other applications such as detectors
and infrared two-photon absorptivity.
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Acknowledgment. This work was funded in part by DARPA
(N00014-06-1-0897), the Office of Naval Research (at GIT and
N00014-08-1-0928), the AFOSR (FA9550-06-1-0192), and the STC
JA809372U
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