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 11479

dx.doi.org/10.1021/jp400664r | J. Phys. Chem. C 2013, 117, 11479−11486

The Journal of Physical Chemistry C

Article

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 11480



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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 11481



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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 11483



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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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(1) Chua, L. L.; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. General Observation of n-Type FieldEffect Behaviour in Organic Semiconductors. Nature 2005, 434, 194− 199. (2) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (3) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296−1323. (4) Zhang, W.; Smith, J.; Watkins, S. E.; Gysel, R.; McGehee, M.; Salleo, A.; Kirkpatrick, J.; Ashraf, S.; Anthopoulos, T.; Heeney, M.; et al. Indacenodithiophene Semiconducting Polymers for HighPerformance, Air-Stable Transistors. J. Am. Chem. Soc. 2010, 132, 11437−11439. (5) de Gans, B. J.; Duineveld, P. C.; Schubert, U. S. Inkjet Printing of Polymers: State of the Art and Future Developments. Adv. Mater. 2004, 16, 203−213. (6) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated Optoelectronic Devices Based on Conjugated Polymers. Science 1998, 280, 1741−1744. (7) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; et al. Liquid-Crystalline Semiconducting Polymers with High Charge-Carrier Mobility. Nat. Mater. 2006, 5, 328−333. (8) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Organic Thin Film Transistors for Large Area Electronics. Adv. Mater. 2002, 14, 99−117. (9) Beaujuge, P. M.; Pisula, W.; Tsao, H. N.; Ellinger, S.; Müllen, K.; Reynolds, J. R. Tailoring Structure−Property Relationships in Dithienosilole−Benzothiadiazole Donor−Acceptor Copolymers. J. Am. Chem. Soc. 2009, 131, 7514−7515. (10) Cho, J. H.; Lee, J.; Xia, Y.; Kim, B. S.; He, Y.; Renn, M. J.; Lodge, T. P.; Frisbie, C. D. Printable Ion-Gel Gate Dielectrics for LowVoltage Polymer Thin-Film Transistors on Plastic. Nat. Mater. 2008, 7, 900−906. (11) Usta, H.; Facchetti, A.; Marks, T. J. n-Channel Semiconductor Materials Design for Organic Complementary Circuits. Acc. Chem. Res. 2011, 44, 501−510. (12) Sirringhaus, H.; Kawase, T.; Friend, R.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits. Science 2000, 290, 2123−2126. (13) Braga, D.; Erickson, N. C.; Renn, M. J.; Holmes, R. J.; Frisbie, C. D. High-Transconductance Organic Thin-Film Electrochemical Transistors for Driving Low-Voltage Red-Green-Blue Active Matrix Organic Light-Emitting Devices. Adv. Funct. Mater. 2012, 22, 1623− 1631. (14) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (15) Meijer, E.; De Leeuw, D.; Setayesh, S.; Van Veenendaal, E.; Huisman, B.; Blom, P.; Hummelen, J.; Scherf, U.; Klapwijk, T. Corrigendum: Solution-processed Ambipolar Organic Field-effect Transistors and Inverters. Nat. Mater. 2003, 2, 678−682. (16) Crone, B.; Dodabalapur, A.; Lin, Y. Y.; Filas, R.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H.; Li, W. Large-Scale Complementary Integrated Circuits Based on Organic Transistors. Nature 2000, 403, 521−523. (17) Ha, M.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Printed, Sub-3V Digital Circuits on Plastic from Aqueous Carbon Nanotube Inks. ACS Nano 2010, 4, 4388−4395. (18) Xia, Y.; Zhang, W.; Ha, M.; Cho, J. H.; Renn, M. J.; Kim, C. H.; Frisbie, C. D. Printed Sub-2 V Gel-Electrolyte-Gated Polymer Transistors and Circuits. Adv. Funct. Mater. 2010, 20, 587−594. (19) Smith, J.; Bashir, A.; Adamopoulos, G.; Anthony, J. E.; Bradley, D. D. C.; Heeney, M.; McCulloch, I.; Anthopoulos, T. D. Air-Stable

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.

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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.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.K.) and [email protected] (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.

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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



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Solution-Processed Hybrid Transistors with Hole and Electron Mobilities Exceeding 2 cm2 V−1 s−1. Adv. Mater. 2010, 22, 3598−3602. (20) Kim, F. S.; Guo, X.; Watson, M. D.; Jenekhe, S. A. HighMobility Ambipolar Transistors and High-Gain Inverters from a Donor−Acceptor Copolymer Semiconductor. Adv. Mater. 2010, 22, 478−482. (21) Dodabalapur, A.; Katz, H.; Torsi, L.; Haddon, R. Organic Heterostructure Field-Effect Transistors. Science 1995, 269, 1560. (22) Steckler, T. T.; Zhang, X.; Hwang, J.; Honeyager, R.; Ohira, S.; Zhang, X. H.; Grant, A.; Ellinger, S.; Odom, S. A.; Sweat, D.; et al. A Spray-Processable, Low Bandgap, and Ambipolar Donor−Acceptor Conjugated Polymer. J. Am. Chem. Soc. 2009, 131, 2824−2826. (23) Thompson, B. C.; Fréchet, J. M. J. Polymer−Fullerene Composite Solar Cells. Angew. Chem., Int. Ed. 2007, 47, 58−77. (24) Biniek, L.; Schroeder, B. C.; Nielsen, C. B.; McCulloch, I. Recent Advances in High Mobility Donor−Acceptor Semiconducting Polymers. J. Mater. Chem. 2012, 22, 14803−14813. (25) Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009−20029. (26) Guo, X.; Kim, F. S.; Seger, M. J.; Jenekhe, S. A.; Watson, M. D. Naphthalene Diimide-Based Polymer Semiconductors: Synthesis, Structure−Property Correlations, and n-Channel and Ambipolar Field-Effect Transistors. Chem. Mater. 2012, 24, 1434−1442. (27) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. Poly(diketopyrrolopyrrole−terthiophene) for Ambipolar Logic and Photovoltaics. J. Am. Chem. Soc. 2009, 131, 16616−16617. (28) Lee, H. S.; Lee, J. S.; Cho, S.; Kim, H.; Kwak, K. W.; Yoon, Y.; Son, S. K.; Kim, H.; Ko, M. J.; Lee, D. K.; et al. CrystallinityControlled Naphthalene-alt-diketopyrrolopyrrole Copolymers for High-Performance Ambipolar Field Effect Transistors. J. Phys. Chem. C 2012, 116, 26204−26213. (29) Bronstein, H.; Chen, Z. Y.; Ashraf, R. S.; Zhang, W. M.; Du, J. P.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; et al. Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices. J. Am. Chem. Soc. 2011, 133, 3272− 3275. (30) Lee, J. S.; Son, S. K.; Song, S.; Kim, H.; Lee, D. R.; Kim, K.; Ko, M. J.; Choi, D. H.; Kim, B. S.; Cho, J. H. Importance of Solubilizing Group and Backbone Planarity in Low Band Gap Polymers for High Performance Ambipolar Field-Effect Transistors. Chem. Mater. 2012, 24, 1316−1323. (31) Lee, J.; Han, A. R.; Kim, J.; Kim, Y.; Oh, J. H.; Yang, C. Solution-Processable Ambipolar Diketopyrrolopyrrole−Selenophene Polymer with Unprecedentedly High Hole and Electron Mobilities. J. Am. Chem. Soc. 2012, 134, 20713−20721. (32) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; et al. A Stable Solution-Processed Polymer Semiconductor with Record High-mobility for Printed Transistors. Sci. Rep. 2012, 2, 754. (33) Sirringhaus, H. Device Physics of Solution-Processed Organic Field-Effect Transistors. Adv. Mater. 2005, 17, 2411−2425. (34) Sirringhaus, H. Reliability of Organic Field-Effect Transistors. Adv. Mater. 2009, 21, 3859−3873. (35) Kim, D. H.; Lee, B. L.; Moon, H.; Kang, H. M.; Jeong, E. J.; Park, J. I.; Han, K. M.; Lee, S.; Yoo, B. W.; Koo, B. W.; et al. LiquidCrystalline Semiconducting Copolymers with Intramolecular Donor− Acceptor Building Blocks for High-Stability Polymer Transistors. J. Am. Chem. Soc. 2009, 131, 6124−6132. (36) Street, R.; Chabinyc, M.; Endicott, F.; Ong, B. Extended Time Bias Stress Effects in Polymer Transistors. J. Appl. Phys. 2006, 100, 114518−114518−10. (37) Salleo, A.; Street, R. Light-induced Bias Stress Reversal in Polyfluorene Thin-film Transistors. J. Appl. Phys. 2003, 94, 471−479. (38) Salleo, A.; Endicott, F.; Street, R. Reversible and Irreversible Trapping at Room Temperature in Poly(thiophene) Thin-Film Transistors. Appl. Phys. Lett. 2005, 86, 263505.

(39) Kim, S.-O.; Chung, D. S.; Cha, H.; Hwang, M. C.; Park, J.-W.; Kim, Y.-H.; Park, C. E.; Kwon, S.-K. Efficient Polymer Solar Cells based on Dialkoxynaphthalene and Benzo[c][1,2,5]thiadiazole: A New Approach for Simple Donor−acceptor Pair. Solar Energy Mater. Solar Cells 2011, 95, 1678−1685. (40) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q. Design, Synthesis, and Self-Assembly of Oligothiophene Derivatives with a Diketopyrrolopyrrole Core. J. Phys. Chem. C 2008, 112, 15543− 15552. (41) Comedi, D.; Zalloum, O.; Irving, E.; Wojcik, J.; Roschuk, T.; Flynn, M.; Mascher, P. X-ray-Diffraction Study of Crystalline Si Nanocluster Formation in Annealed Silicon-rich Silicon Oxides. J. Appl. Phys. 2006, 99, 023518. (42) Sirringhaus, H.; Wilson, R.; Friend, R.; Inbasekaran, M.; Wu, W.; Woo, E.; Grell, M.; Bradley, D. Mobility Enhancement in Conjugated Polymer Field-Effect Transistors through Chain Alignment in a Liquid-Crystalline Phase. Appl. Phys. Lett. 2000, 77, 406− 408. (43) Kang, E. H. S.; Yuen, J. D.; Walker, W.; Coates, N. E.; Cho, S.; Kim, E. S.; Wudl, F. Amorphous Dithenylcyclopentadienone-carbazole Copolymer for Organic Thin-Film Transistors. J. Mater. Chem. 2010, 20, 2759−2765. (44) Jackson, W.; Marshall, J.; Moyer, M. Role of Hydrogen in the Formation of Metastable Defects in Hydrogenated Amorphous Silicon. Phys. Rev. B 1989, 39, 1164. (45) Lee, J. M.; Cho, I. T.; Lee, J. H.; Kwon, H. I. Bias-Stress-Induced Stretched-Exponential Time Dependence of Threshold Voltage Shift in InGaZnO Thin Film Transistors. Appl. Phys. Lett. 2008, 93, 093504. (46) Mathijssen, S. G. J.; Cölle, M.; Gomes, H.; Smits, E. C. P.; de Boer, B.; McCulloch, I.; Bobbert, P. A.; de Leeuw, D. M. Dynamics of Threshold Voltage Shifts in Organic and Amorphous Silicon FieldEffect Transistors. Adv. Mater. 2007, 19, 2785−2789. (47) Miyadera, T.; Wang, S.; Minari, T.; Tsukagoshi, K.; Aoyagi, Y. Charge Trapping Induced Current Instability in Pentacene Thin Film Transistors: Trapping Barrier and Effect of Surface Treatment. Appl. Phys. Lett. 2008, 93, 033304.

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Correlation between Crystallinity, Charge Transport, and Electrical

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