Donor-acceptor-donor type organic semiconductor containing quinoidal benzo[1,2-b:4,5-b′]dithiophene for high performance n-channel field-effect transistors

Article abstract of DOI:10.1039/c3cc47826b

A donor-acceptor-donor (D-A-D) type small molecule DHB-QDTB, with dicyano-substituted quinoidal benzo[1,2-b:4,5-b′]dithiophene as the acceptor, was designed and synthesized. A high electron mobility up to 0.88 cm² V^-1 s^-1 was observed for DHB-QDTB based transistors under ambient conditions, suggesting quinoidal benzo[1,2-b:4,5- b′]dithiophene is an excellent acceptor unit in D-A type n-channel organic semiconductors.

Full text of DOI:10.1039/c3cc47826b

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Donor–acceptor–donor type organic
semiconductor containing quinoidal benzo-
[1,2-b:4,5-b0]dithiophene for high performance
n-channel field-effect transistors†
Cite this:DOI: 10.1039/c3cc47826b
Received 11th October 2013,
Accepted 30th October 2013
Shitao Wang,‡a Mao Wang,‡a Xu Zhang,b Xiaodi Yang,b Qiuliu Huang,a
Xiaolan Qiao,a Haixia Zhang,c Qinghe Wu,a Yu Xiong,a Jianhua Gaoc and
Hongxiang Li*a
DOI: 10.1039/c3cc47826b
www.rsc.org/chemcomm
A donor–acceptor–donor (D–A–D) type small molecule DHB-QDTB, used in D–A type n-channel OSCs should have strong electron
with dicyano-substituted quinoidal benzo[1,2-b:4,5-b0]dithiophene as withdrawing properties. Currently, the common acceptor units are
the acceptor, was designed and synthesized. A high electron mobility aromatic diimides,8 benzobis(thiodiazole),9 benzothiodiazole,10
up to 0.88 cm2 Vꢀ1 sꢀ1 was observed for DHB-QDTB based transistors thioazole5c and aromatic rings with cyano, carbonyl and halogen
under ambient conditions, suggesting quinoidal benzo[1,2-b:4,5-b0]- substituents.11 However, the electron withdrawing capacities of most
dithiophene is an excellent acceptor unit in D–A type n-channel of these acceptors are not strong enough, and the corresponding
organic semiconductors.
OSCs do not display n-channel characteristics in ambient condi-
tions. Hence it is required to explore new types of strong acceptor
Organic field effect transistors (OFETs) have gained tremendous units. Quinoidal molecules are electron-deficient and possess very
interest because of their wide applications in flexible displays,1 low LUMO energy levels, suggesting their potential applications as
inexpensive radio frequency identification (RFID)2 and sensors.3 acceptor units in D–A type OSCs. Though high performance quino-
Organic semiconductors, as the key component of OFETs, deter- idal OSCs with ambient stability have been reported,12 D–A type
mine their performance. p-Channel organic semiconductors (OSCs) OSCs with quinoids as acceptors have rarely been reported. Herein,
have seen great progress.4 In contrast, the development of n-channel by using quinoidal benzo[1,2-b:4,5-b0]dithiophene as an example,
OSCs has largely lagged behind that of their p-channel counterparts we synthesized a quinoid containing D–A type n-channel OSC
in many aspects such as device performance, air stability, and DHB-QDTB. Experimental results revealed that DHB-QDTB exhibits a
material categories. Though some examples of n-channel transistors low LUMO energy level (ꢀ4.56 V) and strong self-assembly properties.
with electron mobility >1.0 cm2 Vꢀ1 sꢀ1 have been reported,5 most of Transistors based on single micrometre sized wires of DHB-QDTB
the devices were tested under vacuum or inert atmospheres which displayed high electron mobilities up to 0.88 cm2 Vꢀ1 sꢀ1 under
limits their practical applications. The lack of high performance and ambient conditions, one of the highest values for solution processable
ambient stable n-channel OSCs has become one of the bottlenecks quinoidal OSCs. These results suggested that quinoidal units are
for organic transistors. Considering the applications of n-channel excellent acceptor blocks in D–A type OSCs.
OSCs in complementary logic circuits and organic p–n junctions,6
the design and synthesis of ambient stable and high performance outlined in Scheme 1. Triple bonds were used to link the phenyl
n-channel OSCs is greatly needed.
rings (donor units) and the quinoidal benzo[1,2-b:4,5-b0]dithiophene
The chemical structure and synthetic route to DHB-QDTB are
Donor–acceptor (D–A) type conjugated molecules (small molecu- (acceptor unit) to eliminate steric repulsion between adjacent
lar weight compounds and polymers) are one of the most important aromatic rings and improve the planarity of the compound.
classes of OSCs.7 To achieve ambient stability, the acceptor units DHB-QDTB was synthesized through a palladium-catalyzed
Takahashi coupling reaction, followed by oxidation with 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) from the precursor com-
pound 1. DHB-QDTB has poor solubility in organic solvents
and its chemical structure was characterized by 1HNMR, MS
and elementary analysis.
To estimate the positions of the frontier orbitals of DHB-QDTB,
density functional theory (DFT) calculations were carried out
at the B3LYP/6-31G(d) level using the Gaussian 09 program
package. The calculated molecular orbital geometries and
a Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai,
200032, China. E-mail: lhx@sioc.ac.cn; Tel: +86-21-54925024
b Laboratory of Advanced Materials, Fudan University, Shanghai, 200438, China
c Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education, Hangzhou Normal University, Hangzhou 310012, China
† Electronic supplementary information (ESI) available: Experimental details,
1H and 13C NMR, MALDI-TOF and HRMS spectra, optical and AFM images of
micro/nanometre sized wires of DHB-QDTB. See DOI: 10.1039/c3cc47826b
‡ These authors contributed equally.
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Fig. 2 (a) Normalized UV-vis absorption spectra of DHB-QDTB in dilute
chloroform solution (5.5 ꢁ 10ꢀ6 mol Lꢀ1) and on thin film (quartz sub-
strate); (b) cyclic voltammogram of DHB-QDTB (0.1 M Bu4NPF6/CH2Cl2,
scan rate of 100 mV sꢀ1, Ag/AgNO3 as the reference electrode).
Scheme 1 Synthetic route to compound DHB-QDTB.
absorption at lmax = 625 nm, red shifted by B30 nm compared
with that of the solution, suggesting J-type molecular aggregation
was the dominant packing mode in the solid state.14 Cyclic
voltammetry of DHB-QDTB (Fig. 2b) showed two quasi-reversible
1/2
reduction waves in CH2Cl2 with the first half-wave potential (E
)
red1
at 0.29 V. The LUMO energy level calculated through the equation
1/2
red1
ELUMO = ꢀ(4.8 ꢀ EFerrocene + E ) eV (ref. 15) was ꢀ4.56 eV (the
half-wave potential of ferrocene was 0.53 V under the same condi-
tions, see ESI†), similar to that of C8-CNBDT (ꢀ4.6 eV), suggesting
that the donor group slightly affected the LUMO energy levels of the
quinone backbone, which was consistent with the calculated
results mentioned above.
DHB-QDTB displayed strong self-assembly properties. When a
solution of DHB-QDTB was drop cast on a substrate, micro/nanosize
wires instead of continuous thin films were formed. The morphology
of the wires showed nearly no dependence on solution concentration.
After careful optimization, high quality DHB-QDTB micro/nanosized
wires were prepared by drop casting a chlorobenzene solution
(0.5–1.0 mg Lꢀ1) on a hot octadecyltrichlorosilane (OTS)
modified SiO2 substrates. The lengths of the wires were in the
range of hundreds of nanometres to several micrometres, and
the widths were several hundreds of nanometres (Fig. 3b and
Fig. S9, ESI†). However, attempts to obtain selected area
electron diffraction (SAED) patterns of the wires failed.
Small angle X-ray diffraction (XRD) was employed to investigate
the molecular arrangement of DHB-QDTB in the wires (Fig. 3a). The
sharp diffraction peaks suggested the wires were highly crystallized.
The corresponding diffraction peaks were at 5.211, 8.111, 15.591
and 20.741, respectively, belonging to a single family diffrac-
tion. The d-spacing estimated from the diffraction peaks was
33.86 Å. This value is close to the calculated molecular length
Fig. 1 Calculated molecular orbital geometries and energy levels of
molecule DHB-QDTB obtained from DFT calculations at the B3LYP/6-
31G(d) level.
energy levels are shown in Fig. 1. The calculations predicated that
DHB-QDTB possessed a low LUMO energy level at ꢀ4.37 eV,
indicating its potential in ambient stable n-channel OSCs. The
LUMO electron density was distributed on the central quinoidal
benzo[1,2-b:4,5-b0]dithiophene block and carbon–carbon triple
bond, while the electrons on the HOMO orbital were fully
delocalized on the whole conjugated backbone, which indicated
that the donor groups (phenyl ring) affected only the HOMO
energy level of DHB-QDTB.
Fig. 2a illustrates the normalized UV-vis absorption spectra
of DHB-QDTB in dilute chloroform solution and in the solid state
(quartz substrate). The maximum absorption in dilute solution was
at 592 nm, which was red shifted by B20 nm compared with the
dicyano-substituted quinoidal benzo[1,2-b:4,5-b0]dithiophene deriva-
tive C8-CNBDT13 (chemical structure see ESI†). This was due to the
enlarged conjugation length. The optical band gap of DHB-QDTB
was 1.96 eV, as estimated from the initial absorption in
solution. The absorption in thin film exhibited a strong
Fig. 3 XRD pattern (a) and AFM image (b) of DHB-QDTB wires.
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This work was supported by the National Natural Sciences
Foundation of China (21190031, 51273212, 21103023) and the
National Basic Research Program of China (2011CB808405).
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In summary, D–A type n-channel organic semiconductor
DHB-QDTB, in which quinoidal benzo[1,2-b:4,5-b0]dithiophene
was an acceptor unit, was designed and synthesized. DHB-QDTB
displayed a low lying LUMO energy level at ꢀ4.56 eV and strong self-
assembly properties. The transistors based on single micrometre-
sized wires exhibited high performance with mobilities up to
0.88 cm2 Vꢀ1 sꢀ1 in ambient conditions, one of the highest
values for solution processable n-channel OSCs. These results
suggest that quinoidal units are promising acceptor blocks in
D–A type n-channel OSCs.
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