Molecular Engineering of Nonhalogenated Solution-Processable Bithiazole-Based Electron-Transport Polymeric Semiconductors

TL;DR: In this article, 2,2′-bithiazole was synthesized in one step and copolymerized with dithienyldiketopyrrolopyrrole to afford poly(dithienymylldiketsopyrylopyrdrug-biomethane)-bithiaide, PDBTz, which exhibited electron mobility reaching 0.3 cm2 V 1 s−1 in organic field effect transistor (OFET) configuration.

Abstract: The electron deficiency and trans-planar conformation of bithiazole is potentially beneficial for the electron-transport performance of organic semiconductors. However, the incorporation of bithiazole into polymers through a facile synthetic strategy remains a challenge. Herein, 2,2′-bithiazole was synthesized in one step and copolymerized with dithienyldiketopyrrolopyrrole to afford poly(dithienyldiketopyrrolopyrrole-bithiazole), PDBTz. PDBTz exhibited electron mobility reaching 0.3 cm2 V–1 s–1 in organic field-effect transistor (OFET) configuration; this contrasts with a recently discussed isoelectronic conjugated polymer comprising an electron-rich bithiophene and dithienyldiketopyrrolopyrrole, which displays merely hole-transport characteristics. This inversion of charge-carrier transport characteristics confirms the significant potential for bithiazole in the development of electron-transport semiconducting materials. Branched 5-decylheptacyl side chains were incorporated into PDBTz to enhance polyme...

Summary

1. Introduction

• Side chain substitution on the bithiazole and dithienyl groups was avoided to minimize steric effects within the PDBTz backbone.
• Branched 5-decylheptadecyl (5-DH) side chains were utilized as they had been shown in their previous study to facilitate both polymer solubility and effective π-π inter-chain interactions by virtue of having a branch point remote from the polymeric main chain. [38].
• Such a side chain was incorporated into the TDPP unit to promote π-π inter-chain interactions and solubility in a wide range of solvents including non-halogenated options such as xylenes or 1,2,3,4-tetrahydronaphthalene (THN), which are more eco-friendly than halogenated alternatives.

2.1. Polymer Synthesis, Solubility, and Thermal Properties

• PDBTz is stable up to 417 °C (see TGA characterization, Table 2 and Figure S7 ), indicating high thermal stability.
• The polymer exhibits one endothermic (T h = -44 °C) and one exothermic transition (T c = -47 °C) upon heating and cooling, respectively (see DSC characterization, Table 2 and Figure S7 ).
• In light of prior results using the same substituent, this most likely corresponds to the disordering and reordering processes associated with the 5-DH side chains. [38].

.2. Photophysical Properties

• The spectroscopic features of PDBTz solutions in the different solvents are quite similar.
• The absorption spectra of PDBTz thin-films (Figure 2b ) are essentially identical to those of the solution spectra, suggesting a similar rigid polymer chain conformation in the solid state as in solution.
• Taking account of low PDBTz concentration in solution (10 -6 M in Figure 2a ), PDBTz has strong inter-chain interactions.
• Based on the absorption onset, the optical band gap (E g opt ) of PDBTz is estimated as 1.33 eV.

2.3. Characterization of Electronic Structure

• EV) [52] were selected as source and drain electrodes in TCBG and BCTG devices, respectively (Figure 2d ).
• Comparable electron transport performance was determined for PDBTz-based BCTG OFETs with CYTOP/Al 2 O 3 as dielectric and encapsulation bilayers.
• No appreciable changes were observed in I ON/OFF and only a small decrease in µ e was observed over this period (Figure 3d,h ).
• Further enhancement of device stability against O 2 and H 2 O is expected through inorganic/organic multilayer encapsulation. [53].
• PDBTz possesses similar solubility in non-halogenated o-xylene, p-xylene, and THN, as in DCB (vide supra).

2.5. Thin film morphology and microstructure

• After annealing, all PDBTz films exhibited a reduction in FWHM and corresponding improvement in coherence length for both <100> and <010> patterns (Figure 6b, f and c, g ), together with a 3-10% increase in rDoC, which correlate well with the observed increase in electron mobility upon annealing.
• In consideration of the subtle increase in orientation distribution within all four PDBTz films after thermal annealing, as shown by changes in S and ∆χ FWMH (Table 5 ), annealing primarily impacts PDBTz ordering and grain size, while crystallite growth likely proceeds equally along each orientation distribution direction.
• The microstructure and morphology analyses demonstrate that the processing solvents have similar effect on the morphology, molecular ordering, orientation, and rDoC of PDBTz films, in good accord with very similar electron transport properties for the respective samples.

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• Comparison of PDBTz with PDQT Several groups have reported the copolymerization of bithiophene-containing monomers and TDPP to afford poly(diketopyrrolopyrrole-quaterthiophene), PDQT, [35] [36] [37] whose chemical structure is shown in Figure 1d .
• Electrochemical studies of PDQT reveal reversible oxidation, and hole transport properties are observed in OFETs prepared from this material.
• In contrast, PDBTz, where the bithiophene segment of each PDQT repeat unit is replaced by bithiazole, displays reversible electrochemical reduction and electron transport behavior.
• Density functional theory (DFT) calculations using tuned-ωB97X/cc-pVDZ were utilized to first explore the torsional potentials of PDBTz and PDQT subunits; the results were compared to reference single-point SCS-MP2 calculations (Figure 1a , see S.I. for details). [56] [57] [58].
• The calculations indicate that within one PDBTz repeat unit, the bithiazole segment adopts a trans coplanar conformation with a dihedral angle (φ) of 180° between the two thiazole rings (Figure 1a ).

Full text

Molecular Engineering of Non-Halogenated
Solution-Processable Bithiazole based
Electron Transport Polymeric Semiconductors
Item Type Article
Authors Fu, Boyi; Wang, Cheng-Yin; Rose, Bradley Daniel; Jiang, Yundi;
Chang, Mincheol; Chu, Ping-Hsun; Yuan, Zhibo; Fuentes-
Hernandez, Canek; Bernard, Kippelen; Bredas, Jean-Luc;
Collard, David M.; Reichmanis, Elsa
Citation Molecular Engineering of Non-Halogenated Solution-Processable
Bithiazole based Electron Transport Polymeric Semiconductors
2015:150401105405000 Chemistry of Materials
Eprint version Post-print
DOI 10.1021/acs.chemmater.5b00173
Publisher American Chemical Society (ACS)
Journal Chemistry of Materials
Rights Archived with thanks to Chemistry of Materials
Download date 09/08/2022 22:51:34
Link to Item http://hdl.handle.net/10754/350202

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of their duties.
Article
Molecular Engineering of Non-Halogenated Solution-Processable
Bithiazole based Electron Transport Polymeric Semiconductors
Boyi Fu, Cheng-Yin Wang, Bradley D. Rose, Yundi Jiang, Mincheol Chang, Ping-Hsun Chu, Zhibo Yuan,
Canek Fuentes-Hernandez, Kippelen Bernard, Jean-Luc Bredas, David M. Collard, and Elsa Reichmanis
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00173 • Publication Date (Web): 01 Apr 2015
Downloaded from http://pubs.acs.org on April 7, 2015
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1
Molecular Engineering of Non-Halogenated Solution-Processable Bithiazole based Electron
Transport Polymeric Semiconductors
Boyi Fu,
1
Cheng-Yin Wang,
2
Bradley D. Rose,
3
Yundi Jiang,
1
Mincheol Chang,
1
Ping-Hsun
Chu,
1
Zhibo Yuan,
4
Canek Fuentes-Hernandez,
2
Bernard Kippelen,
2
Jean-Luc Brédas,
3
David M.
Collard,
4
and Elsa Reichmanis
1,4,5*
1
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst
Drive, Atlanta, GA 30332-0100, U.S.A
2
School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic
Dr NW, Atlanta, GA 30332-0250, U.S.A
3
Solar and Photovoltaics Engineering Research Center, King Abdullah University of Science and
Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
4
School of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive,
Atlanta, GA 30332-0400, U.S.A
5
School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive,
Atlanta, GA 30332-0245, U.S.A.
* Corresponding author: ereichmanis@chbe.gatech.edu
Keywords: electron transport polymeric semiconductors, n-channel organic field-effect
transistors, bithiazole, non-halogenated solvents
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Abstract:
The electron deficiency and trans planar conformation of bithiazole is potentially beneficial
for the electron transport performance of organic semiconductors. However, the incorporation of
bithiazole into polymers through a facile synthetic strategy remains a challenge. Herein, 2,2’-
bithiazole was synthesized in one step and copolymerized with dithienyldiketopyrrolopyrrole to
afford poly(dithienyldiketopyrrolopyrrole-bithiazole), PDBTz. PDBTz exhibited electron
mobility reaching 0.3 cm
2
V
-1
s
-1
in organic field-effect transistor (OFET) configuration; this
contrasts with a recently discussed isoelectronic conjugated polymer comprising an electron rich
bithiophene and dithienyldiketopyrrolopyrrole, which displays merely hole transport
characteristics. This inversion of charge carrier transport characteristics confirms the significant
potential for bithiazole in the development of electron transport semiconducting materials.
Branched 5-decylheptacyl side chains were incorporated into PDBTz to enhance polymer
solubility, particularly in non-halogenated, more environmentally compatible solvents. PDBTz
cast from a range of non-halogenated solvents exhibited film morphologies and field-effect
electron mobility similar to those cast from halogenated solvents.
1. Introduction
The development of high efficiency, air stable electron transport polymeric semiconductors
for organic electronic devices has attracted much attention due to their importance in the
fabrication of organic p-n junction devices, such as complementary-metal-oxide-semiconductor
(CMOS)-like logic circuits,
[1-2]
thermoelectrics,
[3]
hetero-junction photovoltaics,
[4-6]
and organic
light-emitting diodes.
[7-8]
For example, a combination of hole transport and electron transport
semiconductors with comparable mobility values is required to implement CMOS-like logic,
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which is widely used in digital integrated circuits including microprocessors, microcontrollers,
and static random access memory devices.
[1, 9-10]
Significant advances in the development of hole
transport polymeric semiconductors have led to materials that demonstrate field-effect hole
mobilities of up to 20 cm
2
V
-1
s
-1
.
[11-12]
However, less progress has been made towards the
development of electron transport counterparts.
[13-16]
The more limited advances in this instance
result from challenges associated with the stabilization and delocalization of the lowest
unoccupied molecular orbital (LUMO) of π-conjugated polymers.
[7, 13, 17-18]
Stabilization of the
LUMO means raising the electron affinity, which can be realized by materials that consist of
electron-deficient conjugated repeat units.
[13, 18-20]
The LUMO delocalization can be enhanced by
backbone planarization and inter-chain stacking.
[21]
The 2,2’-bithiazole unit exhibits a number of features that could be attractive in the search for
electron transport conjugated polymers. The presence of electronegative nitrogen atoms lowers
the LUMO energy in comparison to analogs that consist of electron rich units such as thienyl
derivatives.
[22-26]
The trans conformation of bithiazole (with a dihedral angle between the
thiazole rings close to 180°, as confirmed by density functional theory, DFT, in this study, vide
infra) can promote polymer backbone planarity, which extends intrachain π-conjugation and
interchain π-π stacking, in comparison to analogs such as biphenyl that is not coplanar.
[27-29]
Thiazole has a large dipole moment of 1.6 D;
[30-31]
an antiparallel alignment between the two
thiazole moieties within bithiazole leads to a net zero dipole, which is one driving force for
planarization of bithiazole. Additionally, the large dipole of the thiazole unit could impart strong
dipole-dipole interactions between bithiazole-based polymer chains.
[32]
The bithiazole unit has been primarily used to build hole transport donor-acceptor π-
conjugated copolymers; bithiazole was considered as a weak acceptor. Recent studies indicated
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