Air-stable ambipolar organic field-effect transistors based on naphthalenediimide-diketopyrrolopyrrole copolymers

Article abstract of DOI:10.1039/c5ra00391a

Two air-stable polymeric semiconductors were rationally designed and synthesized, namely PNDI-DPP and PNDI-T(DPP)T, containing naphthalenediimide (NDI) units and diketopyrrolopyrrole (DPP). The coplanar thiophene-substituted DPP moieties act as donors rather than acceptors, even though DPP is an electron-deficient core. In a bottom-gate/top-contact device architecture, the effect of changing the number of thiophene linkers on the performance of the two completely different OFETs was investigated. PNDI-T(DPP)T presented unipolar p-type behaviour with an average hole mobility of 0.02 cm² V^-1 s^-1, while PNDI-DPP exhibited ambipolar transport with average electron and hole mobilities of 5.7 × 10^-3 and 1.6 × 10^-3 cm² V^-1 s^-1, respectively. Moreover, OFETs based on the two polymers showed good air-stability with negligible changes after being stored under ambient conditions for over 3 months. This journal is

Full text of DOI:10.1039/c5ra00391a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Air-stable ambipolar organic field-effect transistors
based on naphthalenediimide–
Cite this: RSC Adv., 2015, 5, 19520
diketopyrrolopyrrole copolymers†
a
b
b
b
a
ab
Ping Wang, Hui Li, Chunling Gu, Huanli Dong, Zhenzhen Xu and Hongbing Fu*
Two air-stable polymeric semiconductors were rationally designed and synthesized, namely PNDI-DPP and
PNDI-T(DPP)T, containing naphthalenediimide (NDI) units and diketopyrrolopyrrole (DPP). The coplanar
thiophene-substituted DPP moieties act as donors rather than acceptors, even though DPP is an
electron-deficient core. In a bottom-gate/top-contact device architecture, the effect of changing the
number of thiophene linkers on the performance of the two completely different OFETs was
investigated. PNDI-T(DPP)T presented unipolar p-type behaviour with an average hole mobility of 0.02
Received 8th January 2015
Accepted 9th February 2015
2
ꢀ1 ꢀ1
cm
V
s
, while PNDI-DPP exhibited ambipolar transport with average electron and hole mobilities of
ꢀ
3
ꢀ3
2
ꢀ1 ꢀ1
5
.7 ꢁ 10 and 1.6 ꢁ 10 cm
V s , respectively. Moreover, OFETs based on the two polymers
DOI: 10.1039/c5ra00391a
showed good air-stability with negligible changes after being stored under ambient conditions for over
3 months.
www.rsc.org/advances
13–15
moieties to achieve efficient electron and hole transport.
On
Introduction
this basis, appropriate selection of electron-rich and electron-
Polymeric semiconductors are very popular in organic eld- decient components is a key step in order to obtain an ideal
16,17
effect transistors (OFETs) because they have the following ambipolar semiconductor.
In addition, the introduction of
advantages: designability and tailorability, functional diversity, appropriate alkyl side chains also plays an important role, not
1–4
low cost and the availability of a wide variety of sources. High only increasing the solubility of polymers to result in facile
performance OFETs are mostly fabricated using a single- solution processing but also inuencing intermolecular inter-
1
8,19
component layer of donor–acceptor (D–A) conjugated poly- actions and molecular packing.
mers, because of strong interchain interaction and large
5
Diketopyrrolopyrrole (DPP) core is excellent structural unit
p-delocalization. In recent years, the progress in p-type D–A for the D–A polymers which have been demonstrated high hole
2
0–24
polymers as active layers for OFETs is notable, with mobilities or electron transporting motilities in thin-lm devices.
2
ꢀ1 ꢀ1
surpassing 10 cm
applications in complementary-like circuits,
V
s
. However, in order to achieve Ambipolar DPP-based co-polymers have been reported by Yang
6–8
25–27 28–30
and light- group
and Sirringhaus group,
exhibiting high perfor-
9–12
emitting transistors,
etc., the development of ambipolar mance even with balanced hole and electron mobilities, and
OFETs is particularly important. So far, most of the high mostly are applied to complementary metal oxide semi-
performance ambipolar OFETs are obtained under nitrogen conductor (CMOS), exhibiting high switching speed and excel-
rather than in ambient conditions. Therefore, the development lent signal robustness. However, in these reported works, DPP is
of air-stable ambipolar polymeric semiconductors is an urgent generally regarded as an acceptor moiety. DPP as the donor
requirement. To achieve ambipolar transport, one of the moiety is not universally investigated in D–A polymers.
requirements is that the LUMO energy levels of the D–A poly-
In our article, the D–A polymers were designed and synthe-
mers are low enough. The ne-tuning of energy levels is realized sized by combining NDI core and DPP core through thiophene
through the appropriate combination of donor and acceptor (T) linkers, namely PNDI-DPP and PNDI-T(DPP)T. The polymer
of PNDI-DPP exhibited air-stable and ambipolar transport with
ꢀ
3
2
ꢀ1
electron and hole mobilities up to 5.7 ꢁ 10 ꢂ 0.001 cm V
ꢀ
1
ꢀ3
2
ꢀ1 ꢀ1
a
s
and 1.6 ꢁ 10 ꢂ 0.002 cm V
s
, respectively. Compared
Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of
Chemistry, Capital Normal University, Beijing 100048, P. R. China. E-mail: with PNDI-DPP, PNDI-T(DPP)T only presented unipolar p-type
2
ꢀ1 ꢀ1
hongbing.fu@iccas.ac.cn; Fax: +86 10 6890 7107; Tel: +86 10 6890 7107
b
with hole mobility up to 0.02 ꢂ 0.007 cm V
s . As we all
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry,
know, DPP core is electron decient group and generally acts as
acceptor in D–A polymers. However, thiophene-substituted DPP
moieties serve as donors in our work illustrated by Density
functional theory calculations. The decrease of thiophene
Chinese Academy of Sciences, Beijing 100190, P. R. China. Fax: +86-10-82616517; Tel:
+
†
2
86-10-62526801
Electronic supplementary information (ESI) available: GPC, TGA, DSC,
D-GIXRD pattern of polymers. See DOI: 10.1039/c5ra00391a
19520 | RSC Adv., 2015, 5, 19520–19527
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
ꢄ
linkers from 2 to 1 in the polymeric backbone of PNDI-mT(DPP)mT of concentrated sulfuric acid and hydrogen peroxide in 140 C,
(
4
m ¼ 0, 1) leads to the LUMO energy level down from 3.95 to and then treated with octadecyltrichlorosilane (OTS) to form a
.18. In addition, the ambipolar PNDI-DPP OFETs were inte- self-assembled monolayer. Polymer lms (40 ꢂ 3 nm) were
grated into organic complementary inverters, resulting in spin-coated from Chlorobenzene solution with a concentra-
ꢀ
1
effective inversion characteristics with gains of ꢃ8.6 as a result tion of 8 mg mL and subsequently annealed for 30 min
of balanced transport properties. Microstructures of both under nitrogen. Gold source and drain electrodes (60 nm) were
polymers were investigated by two dimensional grazing inci- deposited through vacuum evaporation on the organic layer
dence X-ray diffraction (2D-GIXRD) to gain a further under- using a shadow mask. The channel length (L) and width (W)
standing of the charge transport properties.
were 200 mm and 500 mm, respectively. CMOS-like inverters
were fabricated on a substrate with larger channel dimensions
(
W ¼ 500 mm and L ¼ 30 mm). Electrical characterization was
Experimental section
Materials
performed on a Keithley 4200 SCS (semiconductor character-
ization system) with a micromanipulator 6150 probe station
All the starting materials were purchased from Aldrich, Alfa, under an ambient atmosphere for the OFETs and inverter
Acros, or TCI and used directly without further purication. All characterizations. The mobility of the OFETs in the saturation
the common reagents and solvents were obtained from Beijing region was extracted from the equation IDS ¼ C
i
m(W/2L) (V
G
ꢀ
2
Chemical Works. Toluene was distilled from sodium under an
argon atmosphere, and benzophenone was acted as the unit area of the SiO
V
T
) , where IDS is the drain current, C
i
is the capacitance per
ꢀ
2
dielectric layer (C
eld-effect carrier mobility, W and L are the channel width and
¼ 10 nF cm ), m is
2
i
indicator.
length, VG is the gate voltage, and V is the threshold voltage.
T
The mobility data were collected from more than seven
different devices.
Instruments
NMR spectra were recorded on a Bruker ANACE 400 MHz
spectrometer in CDCl solutions with tetramethylsilane (TMS)
3
(
TMS; d ¼ 0 ppm) as internal standard. Mass spectra were
Syntheses of the monomers
obtained on a Bruker Daltonics BIFLEX III MALDI-TOF analyzer
using MALDI mode. Molecular weights of polymers was deter-
mined by gel permeation chromatography (GPC) with a Waters
3
,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione 2.
A 500 mL double-neck ask was charged with tert-amyl alcohol
350 mL). Sodium metal (3.84 g, 162 mmol) immersed in mineral
(
5
15 chromatograph connected to a Waters 2410 refractive index
oil was washed with hexanes and cut into small pieces. The
sodium metal pieces was slowly added to the tert-amyl alcohol
within 1.5 h and the temperature of the reaction mixture was
detector, using THF as eluent and polystyrene standards as
calibrants. Thermogravimetric analysis (TGA) measurements
were performed on Shimadzu thermogravimetric analyzer
ꢄ
increased to 120 C at the same time. All the reaction mixture was
(model DTG-60) under a nitrogen ow with a heating rate of
stirred under an argon atmosphere. Compound 1 (17.85 g, 162
mmol) was added to the reaction aer all the sodium metal pieces
were dissolved. The solution turned dark red gradually when
dimethyl succinate (7.94 g, 54.3 mmol) was added dropwise to the
ꢄ
ꢀ1
1
0 C min . Differential scanning calorimetry (DSC) analyses
were carried out with a METTLER TOLEDO Instrument DSC822
calorimeter. UV-vis absorption spectra were measured on a
Shimidazu UV-3600 UV-vis-NIR spectrophotometer. Cyclic vol-
tammograms were obtained by an electrochemistry workstation
reaction mixture over 1 h period. The resulting mixture was at
ꢄ
1
20 C for 2 h, and then precipitated into acidic group (600 mL
ꢀ1
with a CHI voltammetric analyzer at a scan rate of 50 mV s
.
methanol and 30 mL concentrated hydrochloric acid). The
precipitate was ltered with a Buchner funnel to yield 13.20 g of 2
as a dark red solid, which was used for next step without further
purication (81% yield).
The conventional three-electrode conguration consisted of a
platinum working electrode coated with a thin lm layer of
polymer, a platinum wire auxiliary electrode, and an Ag/AgCl
reference electrode with ferrocene–ferrocenium (Fc –Fc) as the
internal standard; the absolute energy level of ferrocene–ferro-
+
2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione 3. Compound 2 (9.0 g, 30 mmol), potassium
cenium is 4.8 eV below vacuum. Tetrabutylammonium hexa-

carbonate and dry DMF (120 mL) were added to a 250 mL of
round bottom ask. The reaction mixture were stirred at 120 C
for 3 h before 3-(bromomethyl)heptane (14.6 g, 75 mmol) was
added to the mixture under an argon atmosphere. Aer reaction
6
uorophosphate (TBAPF ) (0.1 M) was used as the supporting
ꢄ
electrolyte. DFT calculations are performed at the B3LYP/6-31G
level. For grazing incidence X-ray diffraction (GIXRD), the spin-
coated lms were illuminated at a constant incidence angle of
ꢄ
mixture was subsequently stirred overnight at 140 C, it was
ꢄ
˚
0
.2 (l ¼ 2d sin q ¼ 1.54 A).
ltered through Buchner funnel into a 500 mL eggplant-shaped
bottle to remove salt byproducts. Most of solvent was removed
in vacuo, and the residue was puried by column chromatog-
Device fabrication and characterization
OFET devices were manufactured with a top-contact bottom- raphy on silica gel with a mixture of ethyl acetate/petroleum ether
gate structure. An n-type heavily doped Si wafer with 300 nm (1 : 100, v/v) as eluent, obtaining a dark red solid as the product 3
ꢀ
2
1
thick thermally grown SiO (C ¼ 10 nF cm ) was used as gate (2.36 g, yield 15%). H NMR (400 MHz, CDCl ): d ¼ 8.88 (d, 2H),
2
i
3
electrode and dielectric layer. The Si/SiO
2
substrate was cleaned 7.61 (d, 2H), 7.25 (d, 2H), 4.01 (m, 4H), 1.85 (d, 2H), 1.35–1.23 (m,
carefully by pure water, ethanol, acetone, and a mixed solution 17H), 0.87–0.85 (m, 13H). C NMR (100 MHz, CDCl
1
3
3
): d ¼ 161.8,
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 19520–19527 | 19521

View Article Online
RSC Advances
Paper
0
1
2
40.5, 135.4130.6, 129.9, 128.5, 108.0, 45.9, 39.2, 30.3, 28.4, 23.6,
3.1, 14.1, 10.6. MALDI-TOF-MS [M] : 524.6 (m/z).
N,N -bis(2-octyldodecyl)-2,6-bis(5-hexyl-thiophen-2-yl)-1,4,5,8-
naphthalene diimide 10. A mixture of compound 9 (4.3 g, 10
+
3
,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo- mmol), 2-octyldodecan-1-amine (8.9 g, 30 mmol) and acetic acid
ꢄ
[
3,4-c]pyrrole-1,4(2H,5H)-dione 4. Compound 3 (1.57 g, 3 (120 mL) was stirred overnight at 120 C. The reaction mixture
mmol) and chloroform (60 mL) was added in a 250 mL single- was then cooled to room temperature, and the solvents were
neck round-bottom, stirred under an argon conditions. NBS removed in vacuo. The residue was puried by column chroma-
(
1.16 g, 9 mmol) was added to the mixture aer the reaction tography with a mixture of ethyl acetate/petroleum ether (1 : 150,
ꢄ
1
contents was stirred in an ice bath at 0 C for 30 min. The v/v) as eluent to give a yellow solid 10 (1.9 g, 19%). H NMR (400
mixture was stirred overnight, and then the solvent was MHz, CDCl ): d ¼ 8.99 (s, 2H), 4.13 (d, 4H), 2.00–1.96 (m, 2H),
3
removed under reduced pressure. The product was puried by 1.30–1.23 (m, 64H), 0.89–0.83 (m, 13H).
column chromatography using an solvent mixture of
2,7-bis(2-octyldodecyl)-4,9-bis(tributylstannyl)benzo[lmn]-
dichloromethane/petroleum ether (1 : 10, v/v). The pure [3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone 11. A mixture of
product 4 was collected as a purple solid (1.69 g, 83% yield). compound 10 (492 mg, 0.50 mmol), 1,1,1,2,2,2-hex-
1
H NMR (400 MHz, CDCl
3
): d ¼ 8.63 (d, 2H), 7.22 (d, 2H), 3.89 abutyldistannane (696 mg, 1.20 mmol), Pd (dba)3 (46 mg,
2
(
m, 4H), 1.83 (d, 2H), 1.34–1.25 (m, 19H), 0.90–0.84 (m, 13H). 0.05 mmol), P(o-tolyl) (61 mg, 0.20 mmol), and toluene (20
3
+
ꢄ
MALDI-TOF-MS [M] : 682.0 (m/z).
mL) were stirred and reuxed at 110 C under argon for 5 h.
0
3
,6-di(2,2 -bithiophen-5-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4- Upon cooling to room temperature, the solvent was removed
c]pyrrole-1,4(2H,5H)-dione 5. Compound 4 (682 mg, 1 mmol), under reduced pressure, the product was puried by column
tributyl(thiophen-2-yl)stannane (970 mg, 2.6 mmol), chromatography
using an solvent mixture of
Pd(PPh ) Cl (70 mg, 0.1 mmol), and 20 mL of anhydrous dichloromethane/petroleum ether (1 : 150, v/v). The puried
3
2
2
1
toluene were added to a double-neck ask under argon. The product 11 (610 mg, 87%) was slightly yellow liquid. H NMR
ꢄ
mixture was reuxed overnight at 110 C and then cooled (400 MHz, CDCl ): d ¼ 8.89 (t, 2H), 4.11 (d, 4H), 1.96 (s, 2H),
3
down to room temperature. Aer removing the solvent under 1.56–1.48 (m, 14H), 1.36–1.18 (m, 96H), 0.89–0.81 (m, 33H).
reduced pressure, the product was puried by column chro-
matography using an solvent mixture of dichloromethane/
Syntheses of the polymers
petroleum ether (1 : 5, v/v) to get the nal product 5 as a
1
Poly[4-(5-(2,5-bis(2-ethylhexyl)-3,6-dioxo-4-(thiophen-2-yl)-
mazarine solid (158 mg, 23% yield). H NMR (400 MHz,
2
,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thiophen-2-yl)-2,7-
CDCl ): d ¼ 8.92 (s, 2H), 7.31 (d, 6H), 7.06 (t, 2H), 4.10–3.99 (m,
3
bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-
tetraone] (PNDI-DPP). 4 (69 mg, 0.1 mmol), 11 (140 mg, 0.1 mmol),
Pd(PPh ) (12 mg, 0.01 mmol), CuI (4 mg, 0.02 mmol) and 3 mL
3 4
of anhydrous toluene were added to a Schlenk tube. The tube was
charged with nitrogen through a freeze–pump–thaw cycle for three
4
H), 1.92 (d, 2H), 1.38–1.27 (m, 18H), 0.93–0.85 (m, 13H).
+
MALDI-TOF-MS [M] : 688.0 (m/z).
31,32
0
0
3
,6-bis(5 -bromo-2,2 -bithiophen-5-yl)-2,5-bis(2-ethylhexyl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione 6. Compound 5 (276
mg, 0.4 mmol) and chloroform (30 mL) was added in a 100 mL
single-neck round-bottom, stirred under an argon conditions.
NBS (160 mg, 0.9 mmol) was added to the mixture aer the
ꢄ
times. The mixture was stirred for 48 h at 130 C. Aer having been
cooled to room temperature, the mixture was precipitate into
methanol (100 mL). The precipitate was collected by ltration and
puried via Soxhlet extraction for 12 h with methanol, acetone,
and hexane, and chloroform, and nally collected with chloro-
form. Then the chloroform solution was removed under vacuum
ꢄ
reaction contents was stirred in an ice bath at 0 C for 30 min.
The mixture was allowed to room temperature and stirred for 3
h, and then the solvent was removed under reduced pressure.
The product was puried by column chromatography using an
solvent mixture of dichloromethane/petroleum ether (1 : 8, v/
1
to obtain a dark blue solid (58 mg, yield 43%). H NMR (400 MHz,
CDCl
3
): d ¼ 9.07 (t, 2H), 8.83 (d, 2H), 7.51 (s, 2H), 4.09 (t, 8H), 2.03
v). The pure product 6 was collected as a black solid (277 mg,
1
(s, 5H), 1.41–1.25 (m, 84H), 0.94–0.85 (m, 25H).
8
2
1
2% yield). H NMR (400 MHz, CDCl ): d ¼ 8.86 (s, 2H), 7.24 (d,
3
0
0
Poly[4-(5 -(4-(2,2 -bithiophen-5-yl)-2,5-bis(2-ethylhexyl)-3,6-
H), 7.03 (d, 4H), 4.07–3.97 (m, 4H), 1.89 (s, 2H), 1.39–1.26 (m,
6H), 0.92–0.85 (m, 12H). MALDI-TOF-MS [M] : 845.9 (m/z).
0
+
dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)-2,2 -bithiophen-
5
1
-yl)-2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-
,3,6,8(2H,7H)-tetraone] (PNDI-T(DPP)T). The same poly-
2
,6-Dibromonaphthalene-1,4,5,8-tetracarboxydianhydride 9.
,4,5,8-Naphthalenetetracarboxylic dianhydride (7) (9.4 g, 35
, 350 mL) was stirred for 2 h at room
temperature. A solution of dibromoisocyanuric acid (8) (10.5 g,
6.75 mmol) in oleum (20% SO , 175 mL) was added dropwise
1
merization and purication routes as those described for
mmol) and oleum (20% SO
3
PNDI-DPP, PNDI-T(DPP)T was collected as a dark blue solid
1
(
62 mg, yield 41%). H NMR (400 MHz, CDCl
3
): d ¼ 9.02 (d,
3
3
2
H), 8.82 (d, 2H), 8.55 (d, 2H), 7.51 (d, 4H), 4.09 (t, 8H), 1.98
to the reaction mixture over 4 h. The resulting mixture was
stirred at room temperature for 1 h and then poured onto
crushed ices (2000 g) carefully to get a bright yellow precipitate.
The mixture was diluted with water (6 L) and allowed to stand
for 3 h. The yellow solid was collected by Buchner funnel,
washed with water and dried. Yield: 11.8 g (79%). The product 9
was used without further purication.
(
s, 4H), 1.25–1.09 (m, 83H), 0.94–0.85 (m, 25H).
Results and discussion
Synthesis and thermal stability
The synthesis of PNDI-DPP and PNDI-T(DPP)T is depicted in
Scheme 1. Compound 2 was started from thiophene-2-
19522 | RSC Adv., 2015, 5, 19520–19527
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
carbonitrile, followed by alkylation and dibrominated to give
33
monomer 4. Compound 5 was synthesized by Stille coupling of
compound 4 with tributyl(thiophen-2-yl)stannane, and then
bromized with N-bromosuccinimide to get the monomer 6. The
polymers PNDI-DPP, PNDI-T(DPP)T were synthesized via Stille
coupling between compound 11 and monomer 4, and monomer
6, respectively. Both polymers were puried by Soxhlet extrac-
tion with methanol, acetone, and hexane and nally collected
from chloroform. PNDI-DPP and PNDI-T(DPP)T were soluble in
chloro-solvents, such as chloroform and chlorobenzene.
The molecular weights of both polymers were evaluated by
gel permeation chromatography (GPC) with THF solution as
Fig. 1 Normalized absorption spectra of both polymers in solution
eluent relative to polystyrene standard. The number average (chloroform) and in thin films spin-coated on glass.
molecular weight (M ) of PNDI-DPP was 16.6 kDa, and its
weight average molecular weight (M ) was 20.8 kDa. For PNDI-
T(DPP)T, it had a relatively lower M of 9.7 kDa and M (16.5
n
w
n
w
solutions, the absorption spectra of both polymers show the
single and structureless bands, with the maximum at 750 nm
for PNDI-DPP and 663 nm for PNDI-T(DPP)T. Remarkably, the
absorption band of PNDI-T(DPP)T is obviously blue-shied
relative to that of PNDI-DPP due to the introduction of thio-
phene units. This blue-shi is likely to be the result of steric
effect and will be discussed in following part. In solid state, both
the 0–0 peaks and the 0–1 peaks of polymers show red-shis
and the enhanced, which are attributed to solid-state packing
kDa). The lower molecule weight of PNDI-T(DPP)T is probably
due to its poor solubility in THF. Both polymers showed narrow
polydispersity index (PDI: PNDI-DPP, 1.1; PNDI-T(DPP)T, 1.3).
To determine the thermal properties of polymers, thermal
gravimetric analysis (TGA) and differential scanning calorim-
etry (DSC) analysis were performed under nitrogen. Both poly-
mers exhibited excellent thermal stability: PNDI-DPP with 5%
ꢄ
weight loss at 409.0 C and PNDI-T(DPP)T with 5% weight loss
at 396.9 C. DSC thermographs of polymers did not reveal
ꢄ
34
effects. The PNDI-DPP lm shows dual absorption bands, with
obvious endothermic or exothermic peaks in the range of
the 0–0 vibrational peak at 906 nm and 0–1 vibrational peak at
ꢄ
0–300 C, suggesting no evidence of melting or crystallization
834 nm. The absorption spectrum of PNDI-T(DPP)T lm pres-
transitions.
ents the 0–0 vibrational peak at 709 nm and 0–1 vibrational
peak at 660 nm. Furthermore, the optical energy bandgap
estimated from the absorption onsets in thin lms are 1.15 eV
for PNDI-DPP and 1.25 eV for PNDI-T(DPP)T. The increase in
the energy gap for PNDI-T(DPP)T compared with PNDI-DPP is
Optical properties
The UV-vis absorption spectra of both polymers were measured
in chloroform solutions (ca. 10 M) and in thin lms (Fig. 1). In
ꢀ
6
Scheme 1 Synthetic route of monomers and polymers.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 19520–19527 | 19523

View Article Online
RSC Advances
Paper
different from other polymers whose spectra would red-shi
when incorporation more thiophene units. This is probably
because the large torsion conguration of polymer chains in
35
PNDI-T(DPP)T which reduce the efficient conjugation length.
DFT-calculated molecular geometries and orbitals
To predict the minimum-energy conformation of the polymer
backbone and the frontier orbital energy levels (LUMO/HOMO),
quantum-chemical calculations (B3LYP/6-31G) were performed
for the trimmers of both polymers. Long alkyl chains were
replaced by methyl groups in order to reduce the calculation
time. This treatment does not affect the results because the
inuence of alkyl chain on the electronic structure of the poly-
Fig. 3 Cyclic voltammetry curves of polymers films drop-casted on a
platinum electrode.
36
mers can be ignored. As shown in Fig. 2, the dihedral angle
between NDI core and the neighboring thiophene units is about
ꢄ
4
4 in the trimmers of PNDI-DPP, which is much smaller than
ꢄ
that of PNDI-T(DPP)T (about 64 ). A more intuitive diagram of
molecular conformations is showed in Fig. S5 in the (ESI†).
From this, it can be seen that the introduction of thiophene
units to the polymer backbone reduces the planarity of the
PNDI-T(DPP)T conguration and impairs delocalization along
polymer chains. The result is consistent with the fact that blue-
shi spectra are observed both in solution and lm of PNDI-
T(DPP)T compared with those of PNDI-DPP. The HOMO
orbitals of both polymers are mainly delocalized over the DPP
moieties, and the LUMO orbitals are mainly located in NDI
parts. In both polymers, thiophene-substituted DPP moieties
serve as donors, though DPP is generally considered as the
electron-decient core.
is approximately same with the oxidation peak. However, the
oxidation peak of PNDI-T(DPP)T is stronger than the reduction
peak. It is worth mentioning that both the oxidation peaks and
reduction peaks for both polymers are reversible. The HOMO
and LUMO values are calculated from the oxidation and
reduction onsets of CV curves, and all data are summarized in
Table 1. The HOMO value of ꢀ5.60 eV and LUMO value of ꢀ4.18
eV for PNDI-DPP just locate in the energy ranges that ensure the
37
injection of both hole and electron from the Au electrodes.
Whereas, the HOMO and LUMO levels of polymer PNDI-T(DPP)T
are estimated to be ꢀ5.55/ꢀ3.95 eV, indicating that PNDI-T(DPP)T
is possibly more suitable for the p-type semiconductor. The elec-
trochemical energy bandgaps of polymers are 1.42 and 1.60 eV,
respectively, larger than the optical bandgaps. The LUMO levels of
both polymers are low enough for achieving air-stable charge
Electrochemical properties
38
transport.
The electrochemical properties of both polymers are investi-
gated by cyclic voltammetry (CV). The highest occupied molec-
ular orbital (HOMO) and the lowest unoccupied molecular Characteristics of organic eld-effect transistors
orbital (LUMO) were estimated through this measurement. As
depicted in Fig. 3, for PNDI-DPP, the intensity of reduction peak
To test the transistor performance of both polymers, typical
bottom-gate/top-contact devices with a channel length of 200
mm and a width of 500 mm were fabricated by spin-coating
ꢀ
1
polymer solutions (8 mg mL in chlorobenzene) onto octade-
+
+
cyltrichlorosilance (OTS) treated SiO
2
(300 nm)/n -Si
substrates. Aer thermal annealing at optimized temperature of
ꢄ
1
60 C for 30 min under nitrogen, 60 nm thick gold electrodes
were deposited on the polymer lms through a shadow mask.
Fig. 4 shows the typical output and transfer curves of the OFETs
tested in ambient air conditions without any special precau-
tions, and the performance data are listed in Table 2. It can be
obviously observed that OFET devices based on PNDI-T(DPP)T
exhibit unipolar p-type transport characteristics while PNDI-
DPP devices show balanced ambipolar transport characteristics.
Calculated at saturated region, PNDI-T(DPP)T presents an hole
2
ꢀ1 ꢀ1
mobility of 0.02 cm
V
s , while PNDI-DPP exhibits the
ꢀ
3
ꢀ3
electron and hole mobilities of 5.7 ꢁ 10 (m
e
) and 1.6 ꢁ 10
2
ꢀ1 ꢀ1
(
m ) cm V
s , respectively.
h
A transition from ambipolar (both hole and electron trans-
port) for PNDI-DPP to p-channel (hole transport) for PNDI-
T(DPP)T can be attributed to increasing the thiophene units
Fig. 2 LUMO/HOMO orbitals of the trimmers of PNDI-DPP and PNDI-
T(DPP)T obtained with DFT-calculations (B3LYP/6-31G).
19524 | RSC Adv., 2015, 5, 19520–19527
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 1 Optical and electrochemical properties of polymers
a
optb
g
CV
c
CV
c
CVd
g
Polymers
l
max/onset (nm)
E
(eV)
E
HOMO (eV)
E
LUMO (eV)
E
(eV)
PNDI-DPP
PNDI-T(DPP)T
834, 906/1079
660, 709/995
1.15
1.25
ꢀ5.60
ꢀ5.55
ꢀ4.18
ꢀ3.95
1.42
1.60
a
b
c
Maximum absorption and absorption onset in thin lm. Optical band gap estimated from the onset of lm absorption. Electrochemically
+
ox
ox
Fc=Fc
red
ox
d
determined vs. Fc/Fc , EHOMO ¼ ꢀðE
þ 4:8 ꢀ E
þ
Þ, ELUMO ¼ ꢀðE
þ 4:8 ꢀ E
þ
Þ. Energy band gap calculated from the difference
onset
onset
Fc=Fc
between EHOMO and ELUMO
.
Table 2 OFET devices performance of both polymers
a
2
ꢀ1 ꢀ1
b
a
e
2
ꢀ1 ꢀ1
c
Polymers
m
h
(cm V
s
)
(Ion/Ioff
)
m
(cm V
s
)
(Ion/Ioff)
ꢀ
ꢀ
3
2
2
3
5
ꢀ3
2
3
PNDI-DPP
PNDI-T(DPP)T
1.6 ꢁ 10
2.3 ꢁ 10
10 –10
5.7 ꢁ 10
10 –10
—
4
10 –10
—
a
b
c
Average values measured over 10 devices in ambient conditions (R
H
¼ 50–60%). Evaluated at VDS ¼ ꢀ80 V. Evaluated at VDS ¼ 80 V.
Fig. 4 Output characteristics at different gate bias (left) and transfer characteristics at VDS ¼ ꢀ80 V (right) for devices (W ¼ 500 mm, L ¼ 200 mm)
ꢄ
using PNDI-DPP (a–d) and PNDI-T(DPP)T (e and f) as semiconductors at optimized annealing temperature of 160 C.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 19520–19527 | 19525

View Article Online
RSC Advances
Paper
from 1 to 2 in the polymer backbone. This indicated that the
thiophene units have a profound effect on their charge carrier
transport behavior. Additionally, the OFET devices of both
polymers show good stability. For instance, even though stored
in ambient for 3 months without encapsulation, the charge
mobilities of both polymers were almost unchanged. OFETs
transfer curves for fresh devices and aer 3 months storage in
ambient based on polymers were showed in Fig. S6.†
Characteristics of complementary inverters
The balanced carrier mobilities of polymer PNDI-DPP was
observed in OFETs, therefore, CMOS-like inverters based on two
identical ambipolar PNDI-DPP OFETs (W ¼ 500 mm, L ¼ 30 mm)
were fabricated using a common gate electrode as the input
voltage (V ). The output voltage (Vout) was monitored as a
in
function of Vin at constant supply bias (Vdd). A efficiently
inversion of Vout from low voltage about ꢂ14 V to high voltage
about ꢂ50 V is shown in Fig. 5 with Vdd ¼ ꢂ 60 V. The steepness
of the inverter curves indicate that PNDI-DPP ambipolar semi-
conductors yield the voltage gains (ꢀdVout/dVin) of ꢃ8.6. The
inversion of V is observed at about ꢂ30 V, half of the control
in
39
Vdd, which is ideal for ambipolar inverters.
Fig. 6 (a) Out of plane X-ray diffractograms and (b) in-plane grazing
incidence X-ray diffractograms of polymer films annealed at 160 C.
ꢄ
Microstructural study of the polymer thin lms
The morphology and crystallinity of polymer lms were key
parameters for device performance. The thin lm microstruc-
tures of both polymers were studied using grazing incidence X-
ray diffraction (GIXRD). The two-dimensional GIXRD intensity
proles of the polymer thin lms are shown in Fig. 6. For
diffraction peaks are observed not only in the out-of-plane
pattern at 2q ¼ 4.3ꢄ but also in the in-plane pattern, corre-
˚
sponding to a interlayer distance of 20.5 A. Thus, the thin lm
of PNDI-DPP has not just an edge-on conguration, but also a
face-on conguration. These ndings reveal that the molecular
arrangement of PNDI-T(DPP)T is more ordered than PNDI-DPP
in the thin lms. The edge-on packing would be more conducive
polymer thin lm of PNDI-T(DPP)T, a strong diffraction peak at
ꢄ
2
(
q ¼ 4.5 is observed in the out-of-plane pattern and ascribed to
100) diffraction of lamellar structure, corresponding to a chain-
40,41
to charge carrier transport throughout the polymer lm,
˚
to-chain distance of 19.6 A. The (200) diffraction peak also
appears, indicating long-range order in PNDI-T(DPP)T. In the
in-plane pattern, the lack of obvious peaks imply that there is no
p–p stacking. These results suggest that PNDI-T(DPP)T may
solely adopt the edge-on packing model in thin lms.
Compared with PNDI-T(DPP)T, PNDI-DPP exhibits different
molecular packing. For PNDI-DPP thin lm, the (100)
which may be the reason of higher hole mobilities of PNDI-
T(DPP)T than that of PNDI-DPP. AFM height images of polymer
ꢄ
thin lms aer annealing at 160 C for 30 min are showed in
Fig. S7.† The intertwined polymer nanobers were observed for
both PNDI-DPP and PNDI-T(DPP)T.
Conclusions
We have designed, synthesized and characterized two donor–
acceptor polymers PNDI-DPP and PNDI-T(DPP)T. DFT calcula-
tion results showed that the thiophene-substituted DPP moie-
2
ties served as donors. Unipolar p-type transport (m ¼ 0.02 cm
h
ꢀ1
ꢀ1
V
s ) was observed for OFETs based on PNDI-T(DPP)T. By
simply reducing the thiophene in the repeat unit, balanced
ambipolarity was achieved in PNDI-DPP under ambient condi-
ꢀ3
tion with electron and hole mobilities of 5.7 ꢁ 10 and 1.6 ꢁ
ꢀ3
2
ꢀ1 ꢀ1
10
cm V
s , respectively. Our results give an example that
a transition from unipolar to ambipolar transport could be
achieved through adjusting number of thiophene units in the
backbone, and demonstrated an efficient approach to achieve
balanced hole and electron mobilities of NDI-based co-polymer,
which is applied in CMOS-like circuits.
Fig. 5 Transfer characteristics of the complementary inverter based
on ambipolar PNDI-DPP OFETs.
19526 | RSC Adv., 2015, 5, 19520–19527
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
2
0 J. D. Yuen, R. Kumar, D. Zakhidov, J. Seier, B. Lim,
A. J. Heeger and F. Wudl, Adv. Mater., 2011, 23, 3780.
Acknowledgements
This work was supported by the National Natural Science 21 X. Zhang, L. J. Richter, D. M. DeLongchamp, R. J. Kline,
Foundation of China (nos 21073200, 21273251, 91333111,
1190034, 21221002), Beijing Municipal Science & Technology
Commission (no. Z131103002813097), project of Construction
of Innovative Teams and Teacher Career Development for
M. R. Hammond, I. McCulloch, M. Heeney, R. S. Ashraf,
J. N. Smith, T. D. Anthopoulos, B. Schroeder, Y. H. Geerts,
D. A. Fischer and M. F. Toney, J. Am. Chem. Soc., 2011, 133,
15073.
2
Universities and Colleges Under Beijing Municipality 22 P.-T. Wu, F. S. Kim and S. A. Jenekhe, Chem. Mater., 2011, 23,
(
(
IDHT20140512), the National Basic Research Program of China
973) 2011CB808402, 2013CB933500, and the Chinese Academy 23 B. Sun, W. Hong, H. Aziz and Y. Li, Polym. Chem., 2015, 6,
938.
Synchrotron Radiation Facility. The authors gratefully 24 C. Kanimozhi, N. Yaacobi-Gross, K. W. Chou, A. Amassian,
4618.
of Sciences. The GIXRD data were obtained at 1W1A, Beijing
acknowledge the assistance of scientists of the Diffuse X-ray
Scattering Station during the experiments.
T. D. Anthopoulos and S. Patil, J. Am. Chem. Soc., 2012,
134, 16532.
2
2
5 S. Cho, J. Lee, M. Tong, J. H. Seo and C. Yang, Adv. Funct.
Mater., 2011, 21, 1910.
6 J. Lee, A. R. Han, J. Kim, Y. Kim, J. H. Oh and C. Yang, J. Am.
Chem. Soc., 2012, 134, 20713.
Notes and references
1
H. Yan, Z. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dotz,
M. Kastler and A. Facchetti, Nature, 2009, 457, 679.
H. Sirringhaus, Science, 2000, 290, 2123.
27 J. Lee, A. R. Han, H. Yu, T. J. Shin, C. Yang and J. H. Oh, J. Am.
Chem. Soc., 2013, 135, 9540.
J. H. Sch ¨o n, S. Berg, C. Kloc and B. Batlogg, Science, 2000, 28 Z. Chen, M. J. Lee, R. Shahid Ashraf, Y. Gu, S. Albert-Seifried,
2
3
2
87, 1022.
Z. Bao, W. K. Chan and L. Yu, J. Am. Chem. Soc., 1995, 117,
2426.
M. Meedom Nielsen, B. Schroeder, T. D. Anthopoulos,
M. Heeney, I. McCulloch and H. Sirringhaus, Adv. Mater.,
2012, 24, 647.
4
1
5
6
M. Muccini, Nat. Mater., 2006, 5, 605.
J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen,
29 A. J. Kronemeijer, E. Gili, M. Shahid, J. Rivnay, A. Salleo,
M. Heeney and H. Sirringhaus, Adv. Mater., 2012, 24, 1558.
M. M. Wienk, M. Turbiez, D. M. de Leeuw and 30 M. Shahid, R. S. Ashraf, Z. Huang, A. J. Kronemeijer,
R. A. J. Janssen, J. Am. Chem. Soc., 2009, 131, 16616.
J. Fan, J. D. Yuen, M. F. Wang, J. Seier, J. H. Seo,
A. R. Mohebbi, D. Zakhidov, A. Heeger and F. Wudl, Adv.
Mater., 2012, 24, 2186.
M. L. Tang, A. D. Reichardt, N. Miyaki, R. M. Stoltenberg and
Z. Bao, J. Am. Chem. Soc., 2008, 130, 6064.
J. Zaumseil, C. L. Donley, J. S. Kim, R. H. Friend and
H. Sirringhaus, Adv. Mater., 2006, 18, 2708.
0 L. B u¨ rgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H.-J. Kirner
and C. Winnewisser, Adv. Mater., 2008, 20, 2217.
T. McCarthy-Ward, I. McCulloch, J. R. Durrant,
H. Sirringhaus and M. Heeney, J. Mater. Chem., 2012, 22,
12817.
7
31 A. D. Lebsack, J. T. Link, L. E. Overman and B. A. Stearns, J.
Am. Chem. Soc., 2002, 124, 9008.
32 L. S. Liebeskind and R. W. Fengl, J. Org. Chem., 1990, 55,
5359.
33 C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and
J. M. J. Fr ´e chet, J. Am. Chem. Soc., 2010, 132, 15547.
34 H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J. Du,
J. R. Durrant, P. Shakya Tuladhar, K. Song, S. E. Watkins,
Y. Geerts, M. M. Wienk, R. A. J. Janssen, T. Anthopoulos,
H. Sirringhaus, M. Heeney and I. McCulloch, J. Am. Chem.
Soc., 2011, 133, 3272.
8
9
1
1
1
1 S. N. Bhat, R. D. Pietro and H. Sirringhaus, Chem. Mater.,
2012, 24, 4060.
2 M. C. Gwinner, D. Kabra, M. Roberts, T. J. K. Brenner,
B. H. Wallikewitz, C. R. McNeill, R. H. Friend and
H. Sirringhaus, Adv. Mater., 2012, 24, 2728.
3 P. Sonar, S. P. Singh, Y. Li, M. S. Soh and A. Dodabalapur,
Adv. Mater., 2010, 22, 5409.
35 H. Li, C. Gu, L. Jiang, L. Wei, W. Hu and H. Fu, J. Mater.
Chem. C, 2013, 1, 2021.
36 L. Tan, Y. Guo, G. Zhang, Y. Yang, D. Zhang, G. Yu, W. Xu
and Y. Liu, J. Mater. Chem., 2011, 21, 18042.
1
1
4 A. Luzio, D. Fazzi, D. Natali, E. Giussani, K.-J. Baeg, Z. Chen,
Y.-Y. Noh, A. Facchetti and M. Caironi, Adv. Funct. Mater., 37 J. L. Kan, Y. L. Chen, D. D. Qi, Y. Q. Liu and J. Z. Jiang, Adv.
014, 24, 1151. Mater., 2012, 24, 1755.
2
15 M. M. Szumilo, E. H. Gann, C. R. McNeill, V. Lemaur, 38 C. Gu, W. Hu, J. Yao and H. Fu, Chem. Mater., 2013, 25, 2178.
Y. Oliver, L. Thomsen, Y. Vaynzof, M. Sommer and 39 Y. Zhang, C. Kim, J. Lin and T.-Q. Nguyen, Adv. Funct. Mater.,
H. Sirringhaus, Chem. Mater., 2014, 26, 6796.
2012, 22, 97.
1
1
1
1
6 J. Wang, X. Chen, Z. Cai, H. Luo, Y. Li, Z. Liu, G. Zhang and 40 H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen,
D. Zhang, Polym. Chem., 2013, 4, 5283.
7 J. D. Yuen, J. Fan, J. Seier, B. Lim, R. Hufschmid,
A. J. Heeger and F. Wudl, J. Am. Chem. Soc., 2011, 133, 20799.
K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering,
R. A. J. Janssen, E. W. Meijer, P. Herwig and D. M. de
Leeuw, Nature, 1999, 401, 685.
8 J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney and Z. Bao, J. Am. 41 J. E. Donaghey, E.-H. Sohn, R. S. Ashraf, T. D. Anthopoulos,
Chem. Soc., 2011, 133, 20130.
S. E. Watkins, K. Song, C. K. Williams and I. McCulloch,
9 J. D. Yuen and F. Wudl, Energy Environ. Sci., 2013, 6, 392.
Polym. Chem., 2013, 4, 3537.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 19520–19527 | 19527