Correlation between Crystallinity, Charge Transport, and Electrical

Article
pubs.acs.org/JPCC
Correlation between Crystallinity, Charge Transport, and Electrical
Stability in an Ambipolar Polymer Field-Effect Transistor Based on
Poly(naphthalene-alt-diketopyrrolopyrrole)
Beom Joon Kim,†,○ Hyo-Sang Lee,‡,▽,○ Joong Seok Lee,§ Sanghyeok Cho,⊥ Hyunjung Kim,⊥
Hae Jung Son,‡ Honggon Kim,‡ Min Jae Ko,‡ Sungnam Park,▽ Moon Sung Kang,∥ Se Young Oh,#
BongSoo Kim,*,‡ and Jeong Ho Cho*,†
†
SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), School of
Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
‡
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
§
Department of Organic Materials and Fiber Engineering and ∥Department of Chemical Engineering, Soongsil University, Seoul
156-743, Republic of Korea
⊥
Department of Physics and #Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of
Korea
▽
Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea
S Supporting Information
*
ABSTRACT: We characterized the electrical properties of
ambipolar polymer field-effect transistors (PFETs) based on the
low-band-gap polymer, pNAPDO-DPP-EH. The polymer consisted of electron-rich 2,6-di(thienyl)naphthalene units with
decyloxy chains (NAPDO) and electron-deficient diketopyrrolopyrrole units with 2-ethylhexyl chains (DPP-EH). The as-spun
pNAPDO-DPP-EH PFET device exhibited ambipolar transport
properties with a hole mobility of 3.64 × 10−3 cm2/(V s) and an
electron mobility of 0.37 × 10−3 cm2/(V s). Thermal annealing of
the polymer film resulted in a dramatic increase in the carrier
mobility. Annealing at 200 °C yielded hole and electron
mobilities of 0.078 and 0.002 cm2/(V s), respectively. The
mechanism by which the mobility had improved was investigated
via grazing incidence X-ray diffraction studies, atomic force
microscopy, and temperature-dependent transport measurements. These results indicated that thermal annealing improved the
polymer film crystallinity and promoted the formation of a longer-range lamellar structure that lowered the thermal activation
energy for charge hopping. Thermal annealing, moreover, reduced charge trapping in the films and thus improved the electrical
stability of the PFET device. This work underscores the fact that long-range ordering in a crystalline polymer is of great
importance for efficient charge transport and high electrical stability.
1. INTRODUCTION
Polymer field-effect transistors (PFETs) have attracted much
attention in recent decades due to the advantages of low-cost
preparation, large-area fabrication, and compatibility with
flexible substrates.1−11 Organic complementary circuits, including inverters, ring oscillators, logic NAND gates, and D flipflops, have been developed toward realizing electronic products
based on flexible integrated circuits, such as static random
access memory (SRAM), amplifiers, image sensors, and
radiofrequency identification tags.12−16 Complementary logic
circuits, in which the n-channel and p-channel TFTs function in
concert, have several advantages, including low power
dissipation, a high noise tolerance margin, greater operating
speeds, and excellent robustness.11,16−18 For the construction
© 2013 American Chemical Society
of complementary metal oxide semiconductor (CMOS)-like
inverters using PFETs, it is more desirable to use singlecomponent ambipolar transistors rather than a combination of
p- and n-channel transistors, which simplifies the circuit design
and reduces the number of steps involved in device
fabrication.15,19−21
High-performance low-band-gap polymers may offer a path
toward such single-component ambipolar transistors. Many
research groups have developed donor−acceptor-type low-band
gap polymers composed of alternating π-electron rich donor
Received: January 20, 2013
Revised: April 20, 2013
Published: May 13, 2013
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Scheme 1. Synthesis of the pNAPDO-DPP-EH Polymer
blocks and π-electron deficient acceptor blocks.22−26 The
diketopyrrolopyrrole (DPP)-based conjugated copolymers have
shown particularly good carrier mobilities exceeding 1 cm2/(V
s) because the planar electron-deficient DPP moieties form
strong π−π interactions.27−32 In addition to requiring a high
field-effect mobility, practical applications of PFETs require
device stability.7,32−34 For example, gate-bias stress can induce a
considerable threshold voltage (Vth) shift in a PFET. This
instability can arise from charge trapping at the dielectric/
semiconductor interface, inside the dielectric material, or in the
polymer-active channel.35−38 Despite its importance for
practical applications, few studies have characterized the
electrical stabilities of PFET based on recently developed
donor−acceptor-type high-performance polymers.
We report the synthesis, characterization, and transistor
properties of a low-band gap copolymer, pNAPDO-DPP-EH,
containing electron-rich 2,6-di(thienyl)naphthalene with decyloxy groups (NAPDO) and electron-deficient DPP units with
2-ethylhexyl groups (DPP-EH). The synthetic routes are shown
in Scheme 1. This polymer exhibited ambipolar transport
characteristics and a dramatic increase in carrier mobility after
thermal annealing. The thermal annealing effects of the
ambipolar pNAPDO-DPP-EH polymer were thoroughly
investigated by grazing incidence X-ray diffraction (GIXD),
atomic force microscopy (AFM), temperature-dependent
charge-transport measurements, and gate-bias stress experiments. The GIXD and AFM results revealed that thermal
annealing improved the polymer film crystallinity and
promoted the formation of long-range lamellar structures.
Thermally annealed pNAPDO-DPP-EH required a thermal
activation energy for hopping that was lower than the activation
energy in as-spun pNAPDO-DPP-EH due to the improved
polymer-chain-packing structures and interchain connectivity.
The good crystallinity and connectivity in the pNAPDO-DPPEH layers submitted to thermal annealing reduced the number
of hole trapping sites in the films, thereby improving the
electrical stability of the device. These results demonstrated
that control over the interchain stacking among polymer chains
is important for the transistor performance, carrier transport,
and electrical stability of the devices.
2. EXPERIMENTAL SECTION
2.1. Materials and Synthesis. Pd2dba3, P(o-tolyl)3, Aliquat
336, n-BuLi (1.6 M in hexane), thiophene-2-carbonitrile,
dibutyl succinate, N-bromosuccinimide (NBS), 1-bromodecane, and bromine were purchased from Sigma-Aldrich, Acros,
and TCI. Common organic solvents were purchased from
Daejung CMI and J. T. Baker. Tetrahydrofuran (THF) was
dried over sodium and benzophenone prior to use. All other
reagents were used as received without further purification. The
synthetic routes to and chemical structures of the polymers
used in this study are shown in Scheme 1. Compounds 1−4
were synthesized according to procedures reported in the
literature.39,40
Synthesis of pNAPDO-DPP-EH. To a degassed 9 mL toluene
solution of 2,2′-(1,5-bis(decyloxy)naphthalene-2,6-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.290 g, 0.419
mmol), monomer 5 (0.285 g, 0.419 mmol), K3PO4 (0.444 g,
2.09 mmol), and three drops of Aliquat 336 was added 1 mL of
degassed demineralized water. Pd2dba3 (7.7 mg, 0.008 mmol)
and tri(o-tolyl)phosphine (7.0 mg, 0.0268 mmol) were then
added to the reaction mixture. The reaction solution was stirred
for 1 h at 60 °C under an argon atmosphere. The reaction
mixture was poured in methanol/H2O (4/1 v/v) solution. The
precipitated polymer was redissolved in chloroform and
reprecipited in MeOH. The collected polymer was further
purified by Soxhlet extraction using methanol, hexane, acetone,
and chloroform. The chloroform fraction was collected and
reprecipitated in methanol and filtered. The polymer was dried
under vacuum, yielding 0.319 g (79.1%). 1H NMR (CDCl3,
400 MHz) δ: 9.13 (b, 2H), 8.10−7.95 (b, 6H), 4.32−3.89 (b,
8H), 2.13−1.87 (b, 4H), 1.75−0.82 (b, 64 H). GPC Mw = 11
600 Da. PDI = 1.73.
2.2. Material Characterization. 1H NMR spectra were
recorded on a Bruker Advance 400 spectrometer (400 MHz).
The molecular weights of the polymers were measured by gel
permeation chromatography (GPC) using chloroform as the
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Table 1. Redox Potentials, Energy Levels, and Band Gaps of pNAPDO-DPP-EH
pNAPDO-DPP-EH
Eonset,ox (V)
Eonset,red (V)
HOMO (eV)a
LUMO (eV)b
Eg,opt (eV)c
Eg,cv (eV)d
0.27
−1.35
−5.07
−3.45
1.59
1.62
HOMO = −(Eonset,ox + 4.8) eV. bLUMO = −(Eonset,red + 4.8) eV. cEg,opt was determined from the onset of the UV−visible absorption spectra. dEg,cv
= (LUMO − HOMO) eV.
a
Figure 1. (a) UV−visible absorption spectra of the pNAPDO-DPP-EH films at 25, 100, 150, and 200 °C and (b) CV curves for the pNAPDO-DPPEH film.
annealed for 30 min in a vacuum chamber at various
temperatures, 25, 100, 150, and 200 °C. The Au source/drain
electrodes (50 nm) were vacuum-deposited through a shadow
mask on the polymer film to form channels 50 μm in length
and 800 μm in width.
eluent and polystyrene as the standard. Thermogravimetric
analysis (TGA) was determined by heating in a TA Q10 from
30 to 700 °C at a heating rate of 20 °C/min under nitrogen.
Differential scanning calorimetry (DSC) curves were recorded
on a Perkin-Elmer Pyris 1 DSC instrument from 20 to 300 °C
at a heating rate of 10 °C/min under nitrogen. UV−visible
spectra were collected on a Perkin-Elmer Lamb 9 UV−visible
spectrophotometer. Cyclic voltammetry (CV) was performed
using a CH Instruments electrochemical analyzer, and the
degassed acetonitrile solutions contained 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte.
The voltage sweep rate was 50 mV/s. A Pt wire electrode
coated with a thin film of the polymer was used as the working
electrode, a Pt wire was the counter electrode, Ag/Ag+ was the
reference electrode, and ferrocene was used as the internal
standard. The crystalline nanostructure of the pNAPDO-DPPEH films was characterized by GIXD measurements at the 8ID-E beamline of Advanced Photon Source (APS) at Argonne
National Laboratory, USA. The surface morphologies of the
samples were investigated by tapping mode AFM (D3100
Nanoscope V, Veeco). Transistor current−voltage characteristics were measured using Keithley 2400 and 236 source/
measure units at room temperature under vacuum conditions
of 10−5 Torr in a dark environment.
2.3. Device Fabrication. PFETs based on pNAPDO-DPPEH were fabricated on a highly doped n-type Si wafer with a
thermally grown 300 nm thick silicon oxide (SiO2) layer as the
substrate. The wafer served as the gate electrode, whereas the
thermally grown 300 nm thick SiO2 layer acted as a gate
insulator. Prior to treating the silicon oxide surface, the wafer
was cleaned in piranha solution for 30 min at 100 °C and
washed with copious amounts of distilled water. The SiO2 layer
was modified using an octadecyltrichlorosilane (ODTS, Gelest)
to reduce electron trapping by the silanol groups on SiO2. A 40
nm thick pNAPDO-DPP-EH ambipolar semiconducting film
was deposited by spin-coating a 0.5 wt % chloroform solution
onto the ODTS-treated substrates. After spin-coating, the
samples were dried in a vacuum chamber for 24 h. The
pNAPDO-DPP-EH ambipolar polymer films were thermally
3. RESULTS AND DISCUSSION
The low-band gap naphthalene-alt-diketopyrrolopyrrole ambipolar semiconducting polymer (pNAPDO-DPP-EH) was
synthesized as shown in Scheme 1. The optical electronic
properties of pNAPDO-DPP-EH were characterized using
UV−visible absorption spectroscopy and CV. Redox potentials,
energy levels, and band gaps of pNAPDO-DPP-EH are
summarized in Table 1. An optical band gap (Eg,opt) of 1.59
eV was obtained from the onset of the UV−visible absorption
spectra (Figure 1a). The absorption peak at 711 nm gradually
shifted toward the red, to 721 nm, as the annealing temperature
increased, indicating that the thermal energy promoted a selforganized stacking rearrangement among the polymer chains
via strong intermolecular interactions. The HOMO and LUMO
levels were estimated to be −5.07 and −3.45 eV, respectively,
using CV measurements (Figure 1b). These results suggested
that the pNAPDO-DPP-EH polymer would display a low hole
injection barrier but a rather high electron injection barrier in
the presence of Au source−drain electrodes. DSC measurements revealed no clear thermal transitions over the range 30−
300 °C, and thermogravimetric analysis revealed a decomposition temperature of 328 °C (Figure S1 in the Supporting
Information), suggesting that the pNAPDO-DPP-EH polymer
is thermally stable.
Bottom-gate top-contact PFETs were employed to evaluate
the electrical properties of the pNAPDO-DPP-EH. Figure 2a
shows the drain current (ID)−gate voltage (VG) characteristics
at fixed drain voltages (VD) of −80 or +80 V for PFETs based
on the pNAPDO-DPP-EH annealed at various temperatures:
25 (as-spun), 100, 150, and 200 °C. V-shape ambipolar
characteristics were clearly observed in the hole-enhancement
(VD = −80 V) and electron-enhancement (VD = +80 V)
operational modes. The carrier mobilities of each PFET were
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which then decreased ID. By contrast, an increase in VG from 0
to 100 V was accompanied by a slow increase in ID due to
electron accumulation.
The molecular packing and ordering in the pNAPDO-DPPEH semiconducting film were investigated by synchrotron 2D
GIXD measurements (Figure 3a). The as-spun thin films
exhibited an intense (100) reflection with a second-order (200)
peak and a weak and broad (010) reflection on top of the
amorphous silicon oxide powder ring (q = 1.2 to 1.8 Å−1)41
along the qz direction, indicating that the pNAPDO-DPP-EH
chains in the film mainly preferred an edge-on orientation with
a partial radial distribution of the crystalline structures. The outof-plane spacings and π−π stacked interchain spacings in the
ordered pNAPDO-DPP-EH phases were calculated to be 19.1
and 4.0 Å, respectively. A lamellar stacking spacing of 19.1 Å
suggested that the interdigitation of the side alkyl chains in the
polymer was limited; the slightly larger π−π interchain stacking
spacing of 4.0 Å was caused by a twist in the polymer backbone
due to steric hindrance between the (1,5-decyloxy)naphthalene
and the neighboring thiophene rings. (See Figure S2 in the
Supporting Information). Thermal annealing at 100, 150, and
200 °C progressively intensified the primary (100) peak, and
the second-, third-, and fourth-order peaks became visible along
the qz direction.
Quantitative analysis of the GIXD patterns was performed by
the extracting 1D profiles along the out-of-plane direction.
Figure 3b shows the 1D out-of-plane profiles extracted along
the (h00) direction of the GIXD patterns of the pNAPDODPP-EH films. As the annealing temperature increased to 200
°C, both the (h00) and (010) diffraction peaks became more
pronounced; however, the intensities of the (h00) diffraction
peaks (the inset of Figure 3b, on the linear scale) increased
significantly compared with the (010) diffraction peak. Note
that the intensity ratio of the primary (100) peak to the (010)
peak increased from 15.1 (as-spun) to 61.5 (200 °C-annealed).
This asymmetric enhancement in the peak intensities upon
thermal annealing indicated that the self-assembly process
during heating induced the semiconducting polymers to adopt
an edge-on orientation. Considering that the edge-on molecular
orientation is particularly beneficial for charge transport
through the π−π stacks of conducting polymer chains in the
PFETs, because the drain current flows along the semiconducting channel parallel to the substrate, the observed
enhancement in carrier mobility upon thermal annealing arose
from the efficient π−π stacking and high intermolecular
ordering along the direction parallel to the substrate.
The full width at half-maximum (fwhm) in the azimuthal
angle (χ) through the (h00) peak provided additional
information about the angular distribution of the crystalline
plane orientation with respect to the substrate surface. Figure
3c shows the intensity along the azimuthal angle through the
(200) peak for pNAPDO-DPP-EH films annealed at various
temperatures. The fwhm of the (200) peak decreased gradually
with the annealing temperatures from 8.4 (as-spun) to 3.6°
(200 °C-annealed). Therefore, the carrier mobility was
improved during thermal annealing by the development of
Figure 2. (a) Transfer characteristics at a fixed VD of −80 and +80 V
for pNAPDO-DPP-EH PFETs annealed at various temperatures: 25,
100, 150, and 200 °C. (b) Hole and electron mobilities of the
pNAPDO-DPP-EH PFETs as a function of the annealing temperature.
(c) Output characteristics of the PFETs based on the 200 °C-annealed
pNAPDO-DPP-EH films.
calculated in the respective saturation regimes according to the
relationship ID = CiμW(VG − Vth)2/2L, where W and L are the
channel width and length, respectively, Ci is the specific
capacitance of the gate dielectric (11 nF/cm2), Vth is the
threshold voltage, and μ is the carrier mobility. The PFET of
the as-spun pNAPDO-DPP-EH exhibited maximum hole and
electron mobilities of (3.64 and 0.37) × 10−3 cm2/(V s),
respectively. The asymmetry in the hole and electron mobilities
presumably arose from the high electron injection barrier and
the low density of electron-deficient DPP units in a given
volume compared with the electron-rich donor units.28
Thermal annealing of the polymer films dramatically increased
the carrier mobilities, as shown in Figure 2b. (The values are
summarized in Table 2.) The hole and electron mobilities of
the PFETs based on 200 °C-annealed pNAPDO-DPP-EH were
0.078 and 0.002 cm2/(V s), respectively. These enhanced
electrical properties were ascribed to the more highly developed
crystallinity of the polymers, as confirmed by GIXD and AFM
measurements (see below). Figure 2c shows the ID−VD curves
at 11 different VG values for the 200 °C-annealed pNAPDODPP-EH PFETs. The decrease in VG from −100 to −20 V
reduced the hole accumulation in the semiconducting channel,
Table 2. Field-Effect Mobilities of PFETs Based on pNAPDO-DPP-EH Annealed at Various Temperatures
hole
electron
25 °C
100 °C
150 °C
200 °C
0.0036 (±0.0012)
0.0003 (±0.0001)
0.0177 (±0.0029)
0.0013 (±0.0001)
0.0529 (±0.008)
0.0018 (±0.0002)
0.0781 (±0.0154)
0.0020 (±0.0005)
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Figure 3. GIXD patterns for the pNAPDO-DPP-EH films annealed at various temperatures: 25, 100, 150, and 200 °C. (b) 1D out-of-plane X-ray
diffraction profiles extracted along the qz direction from the GIXD pattern annealed at 25 (black), 100 (red), 150 (green), and 200 °C (blue). The
inset shows enlarged (100) peaks in the X-ray diffraction pattern on the linear scale. (c) Intensity of the (200) reflection as a function of the
azimuthal angle (χ).
Figure 4. AFM images of the pNAPDO-DPP-EH films at various temperatures: 25, 100, 150, and 200 °C.
both a higher degree of crystalline ordering and better
alignment of the (h00) direction of the conjugated plane on
the substrate.
The film morphology as a function of thermal annealing was
examined. AFM images of the pNAPDO-DPP-EH semiconductor films at various annealing temperatures are shown
in Figure 4. The as-spun pNAPDO-DPP-EH films displayed
randomly oriented fibrous crystalline nanostructures. Thermal
annealing at higher temperatures induced the progressive
development of these crystalline nanostructures. The evolution
of the crystalline nanostructures slightly increased the surface
roughness (rms values = 0.92, 0.95, 1.01, and 1.17 nm for
pNAPDO-DPP-EH films annealed at 25, 100, 150, and 200 °C,
respectively), and thus both the GIXD and AFM images
indicated the development of crystalline features during thermal
annealing.
The activation energies for hole and electron transport were
measured by examining the temperature dependence of the
transfer characteristics. Figure 5 shows the Arrhenius plots for
the hole (black) and electron (red) mobilities in as-spun (open
squares) and 200 °C-annealed (solid squares) pNAPDO-DPPEH PFETs. The slopes of the plots indicated the activation
energies for charge transport. The activation energies for
electron transport were found to be higher than those obtained
for hole transport in both the as-spun and annealed samples.
Figure 5. Hole and electron saturation mobilities of as-spun and 200
°C-annealed pNAPDO-DPP-EH FETs as a function of the inverse
temperature.
Such results supported the higher hole (compared with
electron) mobility in pNAPDO-DPP-EH PFETs. The slope
of the Arrhenius plots for both hole and electron transport
decreased upon thermal annealing; the activation energy
associated with hole transport decreased from 91.3 to 57.0
meV, and the activation energy for electron transport decreased
from 124.5 to 60.3 meV. This result suggested that the
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exponentials are applicable to a variety of disordered
systems.44−47
improved polymer chain packing, as revealed by the GIXD and
AFM studies, facilitated thermal hopping by the carriers.42,43
The effects of thermal treatment of pNAPDO-DPP-EH on
the electrical stability of a PFET were also investigated by
monitoring the threshold voltage shift (ΔVth) as a function of
time under bias stress. First, the bias stress stability of hole
transport was examined by measuring the threshold voltage
(Vth,h) over a period of 3600 s under a sustained gate voltage of
−50 V. Figure 6a reveals negative Vth,h shifts for the 25, 100,
Vth − Vth,i
VG − Vth,i
⎡ ⎛ t ⎞β⎤
= 1 − exp⎢ −⎜ ⎟ ⎥
⎣ ⎝τ⎠ ⎦
Here Vth,i is the initial Vth at t = 0, β is the dispersion parameter
of the barrier energy height for charge trapping, and τ is the
characteristic time constant associated with the rate of charge
trapping. The fits (Figures 6b,d) yielded the values of β and τ
for the as-spun and annealed pNAPDO-DPP-EH devices, as
shown in Table 3. We observed a significant enhancement in
the τ values for hole trapping in pNAPDO-DPP-EH upon
thermal annealing from 1876 (as-spun) to 12 500 s (200 °Cannealed). A similar trend was obtained for electron transport,
that is, the rate of charge trapping decreased as the annealing
temperature increased. The reduced trapping rate upon thermal
annealing was correlated with a higher degree of crystallinity in
the pNAPDO-DPP-EH layer. It should be noted that although
the carrier trapping decreased with increasing annealing
temperatures for both hole and electron transport, τ increased
more significantly for hole transport than for electron transport.
We postulate that the greater increase in τ for hole transport
may reflect more intimate contact between the hole transporting sites in the polymer film. This idea is supported by the
fact that thermal annealing can promote the stacking among
donor moieties (thiophene-NAP-DO-thiophene), which supports hole hopping, thereby facilitating hole transport. The
intimate contact between the polymer chains was evidenced by
an increase in the crystallinity (the GIXD images) as well as the
red shift (the UV−visible absorption spectra). Interestingly, the
β values did not vary with thermal annealing, indicating that the
energy distribution of the charge trapping barrier was not
altered significantly.
Complementary inverter devices were successfully fabricated
by connecting two identical ambipolar PFETs based on the 200
°C-annealed pNAPDO-DPP-EH. The circuit diagram of an
inverter is displayed in the inset of Figure 7. Figure 7 shows the
output voltage (VOUT) as a function of the input voltage (VIN)
at a constant supply voltage (VDD). Ideal inverter action was
observed as the VIN was swept. The signal inversion could be
obtained at both positive (+80 V) and negative VDD values
(−80 V) due to the ambipolar nature of the constituent
transistors. The signal inverter gain, defined as the absolute
value of dVOUT/dVIN, was 2.8 at VDD = −80 V. The good
inverter behavior could be further improved by matching the
hole and electron currents with a variation in the channel
length/width ratio.
Figure 6. (a) Relative ΔVth,h values of 25, 100, and 200 °C-annealed
pNAPDO-DPP-EH PFETs under a sustained VG of −50 V as a
function of stress time. (b) Plot of ΔVth,h/ΔV0 vs bias stress time for
the pNAPDO-DPP-EH PFETs. The solid curves indicate fits to a
stretched exponential equation. (c) Relative ΔVth,e values of the
pNAPDO-DPP-EH PFETs under a sustained VG of +50 V as a
function of stress time. (d) Plot of ΔVth,e/ΔV0 versus bias stress time
for the pNAPDO-DPP-EH PFETs. ΔVth,h = Vth,h − Vth,h,i, ΔVth,e = Vth,e
− Vth,e,i, and ΔV0 = VG − Vth,i.
and 200 °C-annealed pNAPDO-DPP-EH PFETs. Thermal
annealing clearly mitigated the threshold voltage shifts. That is,
ΔVth,h was much lower for the annealed PFET than for the asspun PFET. Positive ΔVth,e shifts were observed for electron
transport under a sustained gate voltage of +50 V (Figure 6c).
These results indicated that less charge trapping occurred in the
annealed pNAPDO-DPP-EH films than in the as-spun
pNAPDO-DPP-EH films, for both hole and electron transport.
The threshold voltage shifts in Figure 6a,c were fit to a
stretched exponential model as a function of time (t). Stretched
4. CONCLUSIONS
We report the synthesis and characterization of a low-band-gap
naphthalene-alt-diketopyrrolopyrrole semiconducting polymer,
pNAPDO-DPP-EH. The as-spun pNAPDO-DPP-EH exhibited
a low carrier mobility; however, thermal annealing improved
the carrier mobility greatly. The hole and electron mobilities of
Table 3. β and τ Values Determined from the 25, 100, and 200 °C-Annealed pNAPDO-DPP-EH PFETs
as-spun (25 °C)
β
τ (s)
100 °C-annealed
200 °C-annealed
hole
electron
hole
electron
hole
electron
0.772 (±0.034)
1876 (±43)
0.407 (±0.027)
860 (±45)
0.599 (±0.042)
4680 (±340)
0.286 (±0.024)
3830 (±320)
0.685 (±0.053)
12500 (±1800)
0.408 (±0.056)
4440 (±670)
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ment of Energy, Office of Basic Energy Science, under contract
no. DE-AC02-06CH11357.
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Figure 7. Output voltage versus input voltage plots for an inverter
prepared using two PFETs based on pNAPDO-DPP-EH films
annealed at 200 °C at a constant supply voltage.
devices prepared using 200 °C-annealed pNAPDO-DPP-EH
films were found to be 0.078 and 0.002 cm2/(V s), respectively.
The temperature-dependent carrier mobility was strongly
correlated with the crystalline nanostructures and film
morphologies of the polymer films, as revealed by GIXD and
AFM experiments. The activation energy for carrier hopping
transport decreased significantly and the electrical stability
improved dramatically upon thermal annealing. Taken together,
this analysis demonstrates that thermal annealing is a powerful
method for modulating film crystallinity and morphology,
through which the charge transport and electrical stability may
be improved.
■
ASSOCIATED CONTENT
S Supporting Information
*
Thermal properties of pNAPDO-DPP−EH, and density
functional theory calculation result of model compound
(thiophene-dimethylDPP-thiophene-dimethoxynaphthalenethiophene). This material is available free of charge via the
Internet at http://pubs.acs.org.
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail: bongsoo@kist.re.kr (B.K.) and jhcho94@skku.edu
(J.H.C.).
Author Contributions
○
B. J. Kim and H.-S. Lee equally contributed to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by New and Renewable Energy
Program of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) grant funded by the
Ministry of Knowledge Economy (MKE) (20113030010060
and 20113010010030) and by Korea Research Council of
Fundamental Science and Technology (KRCF) and Korea
Institute of Science and Technology (KIST) for “NAP National
Agenda Project Program” and Basic Science Research Program
(2009-0083540 and 2010-0026294, 2011-0012251) through
the National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology. Use of the
Advanced Photon Source was supported by the U.S. Depart11485
dx.doi.org/10.1021/jp400664r | J. Phys. Chem. C 2013, 117, 11479−11486
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Supporting Information
Correlation between Crystallinity, Charge transport, and Electrical
Stability in an Ambipolar Polymer Field-effect Transistor based on
Poly(naphthalene-alt-diketopyrrolopyrrole)
Beom Joon Kim,1† Hyo-Sang Lee,2,5† Joong Seok Lee,3 Sanghyeok Cho,4 Hyunjung Kim,4
Hae Jung Son,2 Honggon Kim,2 Min Jae Ko,2 Sungnam Park,5 Moon Sung Kang,6 Se Young
Oh, 7 BongSoo Kim,2* and Jeong Ho Cho1*
1
SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface
Nano Technology (HINT), School of Chemical Engineering, Sungkyunkwan University,
Suwon 440-746, Republic of Korea
2
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology
(KIST), Seoul 136-791, Republic of Korea.
3
Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 156-743,
Republic of Korea.
4
Department of Physics, Sogang University, Seoul 121-742, Republic of Korea.
5
Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea
6
Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of
Korea.
7
Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742,
Republic of Korea.
†
B. J. Kim and H. S. Lee equally contribute to this work.
*Corresponding author: E-mail: bongsoo@kist.re.kr and jhcho94@skku.edu
1
Figure S1. (a) DSC curve and (b) TGA thermogram of pNAPDO-DPP-EH.
2
Figure S2. Energy levels and surface plots of frontier molecular orbitals and conformation
with
dihedral
angles
of
model
compound
(Thiophene-DimethylDPP-Thiophene-
Dimethoxynaphthalene-Thiophene) that is composed of the backbone aromatic rings of the
pNAPDO-DPP-EH. Density functional theory calculation was conducted as following: the
model compound was geometrically optimized to an energy minimum using Gaussian 09 at
the DFT B3LYP level with the 6-31+G(d,p) basis set. To expedite the calculation, alkyl
chains were shortened to methyl groups on the DPP nitrogens and alkoxy groups.
3