Effect of Donor Building Blocks on the Charge-Transfer Characteristics

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Effect of Donor Building Blocks on the Charge Transfer Characteristics of Diketopyrrolopyrrole-Based Donor–Acceptor-Type Semiconducting Copolymers Gyu Bok Yoon, Ho-Young Kwon, Seok-Heon Jung, Jin-Kyun Lee, and Jiyoul Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11897 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Applied Materials & Interfaces

Effect of Donor Building Blocks on the Charge Transfer Characteristics of DiketopyrrolopyrroleBased Donor–Acceptor-Type Semiconducting Copolymers

Gyu Bok Yoon†,§, Ho-Young Kwon‡,§, Seok-Heon Jung‡, Jin-Kyun Lee‡, and Jiyoul Lee*,†

†

Department of Graphic Arts Information Engineering, Pukyong National University, Sinseon-ro

365, Nam-gu, Busan 608-739, Republic of Korea ‡

Department of Polymer Science Engineering, Inha University, Inha-ro 100, Nam-gu, Incheon

402-751, Republic of Korea

KEYWORDS: Polymer semiconductor, Field-effect transistor, Donor–Acceptor-type copolymer, Charge transfer characteristic, X-ray diffraction

ACS Paragon Plus Environment

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ABSTRACT We investigate the effect of donor building blocks on the charge transportation characteristics of donor (D)–acceptor (A)-type semiconducting copolymers with alternating electron-donating and electron-accepting units in order to provide a basis for the rational design of high-performance semiconducting polymers. For this purpose, we studied three different diketopyrrolopyrrole (DPP)-based semiconducting copolymers comprising a common dithienyl-DPP [3,6-dithienyl-2,5diketopyrrolo(3,4-c)pyrrole] and variable donor moieties: phenylene (P)–PDPPTPT, thiophene (T)–PDPP3T, and thienothiophene (TT)–PDPP2T-TT. Structural analysis using grazing incidence X-ray diffraction (GIXRD) indicates that all three DPP-based copolymer films have edge-on phases but poor crystallinity of the films, except the PDPP2T-TT copolymer with branched alkyl side chains that are relatively long. The electrical measurements show that the DPP-based copolymer with a TT donor unit has the highest field-effect mobility value of 0.30 cm2V−1s−1. To understand the role of the donor units in DPP-based D–A copolymers, further insight into the charge transportation behavior is realized by analyzing the temperature-dependent transfer curves of the DPP semiconducting copolymer-based field-effect transistors (FETs) using the Gaussian disorder model (GDM). Compared to the DPP-based D–A-type semiconducting copolymer with a P-moiety and shorter-branched alkyl side chains that exhibits a broad distribution in the density of localized states (DOS) and a higher thermal activated energy for charge hopping, the DPP copolymers with a TT-moiety and longer-branched side-chains have the narrowest DOS, the lowest activation energy, and thus the highest hole mobility. These results suggest that the higher mobilities obtained from PDPP2T-TT with a TT donor unit can be attributed to the suppressed DOS distribution near the transport level, which mainly originates from the narrowest energy band gap tuned with the orbital couplings of the DPP acceptor and TT donor units.


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INTRODUCTION Since the introduction of the first polymer-based field-effect transistors (PFETs) by Ebisawa et al. in 1983 and later by Tsumura et al. in 1986, considerable progress has been made in the PFET technology over the last three decades. The simultaneous development of appropriate electrodes, semiconductors, and dielectric materials was the most influential factor in achieving high-performance PFET devices.1–3 In particular, recent reports on donor (D)–acceptor (A)-type semiconducting copolymers with an alternating electron-donating unit and an electron-accepting unit along the polymer backbone confirm that it is possible to obtain a high charge-carrier mobility value of up to 30 cm2V−1s−1.4–8 The D–A type conjugated copolymers are intriguing materials because they exhibit high charge-carrier mobility as well as ambipolar characteristics despite their highly disordered microstructure that is in some cases close to an amorphous state. This is counterintuitive to conventionally well-known high-mobility polymers, such as liquid crystalline or semicrystalline thiophene-based semiconductors.9–13 For a D–A-type conjugated copolymer, it is known that the extent of the ground-state charge transfer can be modulated by changing the relative donor and acceptor strengths. However, to maintain the semiconducting characteristics of a synthesized D–A conjugated copolymer, the donor and acceptor strengths need to be appropriately controlled to prevent extensive ground-state charge transfer to the biradical anion–cation interaction where formal charge separation occurs.12,14,15 Moreover, in such D–A-type conjugated copolymer systems, the highest occupied molecular orbital (HOMO) is generated from the donor segment, whereas the lowest unoccupied molecular orbital (LUMO) is mainly generated from the acceptor segment. Thus, the redistribution of frontier molecular orbitals in the entire conjugated backbone can be tuned by the selection of the donor and acceptor building blocks. Accordingly, this leads to the narrowing of the energy


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band gap of D–A-type semiconducting copolymers.14,15 In this regard, the rational selection of the donor and acceptor building blocks is crucial in the development of high-performance D–A-type semiconducting copolymers. The emerging class of D–A-type semiconducting copolymers that have recently gained attention is based on an electron-accepting building block modified with dye and pigment molecules, such as diketopyrrolopyrrole (DPP) and isoindigo (IIG). In particular, DPP is a synthetic red pigment with remarkable thermal stability and excellent photostability.6–8 It is most popularly known as “Ferrari Red” as it is originally used in automotive coatings. Herein, we investigate the influence of donor building blocks on the charge transportation characteristics of DPP-based D–A-type semiconducting copolymer thin films. This study mainly aims to provide a guideline for the rational design of high-performance D–A-type semiconducting polymers by investigating the impact of the donor building blocks in DPP-based D–A semiconducting copolymers on the energy states and charge transfer characteristics of the copolymers. Three different DPP-based semiconducting copolymers comprising two main moieties were prepared for use in the active channel layer of PFETs: a common dithienyl-DPP [3,6-dithienyl-2,5-diketopyrrolo(3,4-c)pyrrole] unit and a variable additional donor moiety that can be phenylene (P)–PDPPTPT, thiophene (T)–PDPP3T, or thienothiophene (TT)–PDPP2T-TT. Figure 1(a) shows the chemical structure of the dithienyl-DPP unit and the three different additional donor units. The crystal structures of the DPP-based copolymer films were studied using grazing incidence X-ray diffraction (GIXRD). The GIXRD spectra revealed that all three DPPbased copolymer films have an edge-on phase but less crystalline ordering of the films, except the DPP copolymer with a TT moiety and relatively long branched alkyl side chains. For further investigation, electrical measurements were performed to obtain basic device parameters, such as the charge-carrier mobility. These were performed using the PFET platform, and the temperature-


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dependent field-effect mobility values were analyzed using the Gaussian disorder model (GDM). Analysis of the experimental results using GDM provides a reasonable explanation of the role of the donor unit on the charge transportation properties of the DPP-based D–A copolymers.

Figure 1. (a) Chemical structure of a dithienyl-diketopyrrolopyrrole unit and variable additional donor moieties: phenylene in PDPPTPT, thiophene in PDPP3T, and thienothiophene in PDPP2TTT. The energy states estimated using the results of cyclic voltammetry and ultraviolet–visible spectroscopy for each DPP-based semiconducting copolymer are also shown. (b) Device configurations of the polymer-based field-effect transistors (PFETs) used in this study. (c) Optical images of the source–drain electrodes of the PFETs.

EXPERIMENTAL METHODS Sample Preparation. For PFET fabrication, staggered [top-gate and bottom-contact (TGBC)] structural devices, as shown in Figure 1(b), were employed. First, soda-lime glass substrates (Corning EAGLE Glass) with photolithographically patterned 35-nm-thick Au/Ti source–drain


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electrodes were cleaned using a general cleaning procedure: sonication in acetone, isopropanol, and deionized water in sequence for 10 min each. The width and length of interdigitated S/D electrodes in a comb shape were 1 mm and 10 m, respectively, as shown in Figure 1(c). Then, a 5-mg/mL solution of DPP-based D–A conjugated copolymers dissolved in chloroform was spincoated and the semiconducting copolymer films were annealed at 200 °C for 30 min in a glove box with an atmosphere containing less than 0.1 ppm of oxygen and water. Among the used DPPbased semiconducting copolymers, PDPPTPT [Mw (~37,000) with a PDI of 2.5] and PDPP3T [Mw (~39,000) with a PDI of 2.7] were synthesized following a procedure described in the literature16–19 [see the Supporting Information] and PDPP2T-TT [Mw (~75,000) with a PDI of 2.5] was purchased from ONE-Materials Inc. The previously reported energy states of the DPP-based semiconducting copolymers (PDPPTPT, PDPP3T, and PDPP2T-TT), as measured using cyclic voltammetry (CV) and ultraviolet–visible spectroscopy, are shown in Figure 1(a).16–20 The average thickness of the DPP-based semiconducting copolymer films estimated using an atomic force microscope (AFM) was in the range of 50–55 nm. Subsequently, poly(methyl methacrylate) (PMMA) dissolved in n-butylacetate (80 mg/mL) was directly spin-coated onto the DPP-based semiconducting copolymer films to form an insulating layer and baked at 80 °C for 2 h. This yielded a film thickness of approximately 350 nm and a specific capacitance of approximately 8.7 nFcm−2. Finally, the top-gate electrode was formed by thermally evaporating Al (40 nm) through a shadow mask to complete the fabrication of DPP semiconducting copolymer-based PFETs. Device Characterization. The current (I)–voltage (V) characteristics of the PFETs were recorded using a Keithley 236 Source Measure Unit in combination with a Keithley 2635 Source Meter controlled using the LabVIEW code. The electrical measurement setup was connected to MS-Tech Vacuum Probe Station with a chamber pressure lower than 10−3 Torr at room temperature (RT).


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Liquid nitrogen was introduced into the cryogenic probe station to enable electrical measurements under different substrate temperatures in the range of 170 K–260 K. GIXRD. For GIXRD measurements, the DPP-based semiconducting copolymer films were spincoated onto the same glass substrates used for the fabrication of devices. The GIXRD images were collected in grazing incidence refection mode with a two-dimensional (2D) area detector in a helium chamber at beamline 11-3 of the Stanford Synchrotron Radiation Lightsource (SSRL). The sample-to-detector distance was 400 mm, and the incidence angle was 0.12°. The X-ray wavelength was 0.9758 Å , corresponding to a beam energy of 12.7 keV.

Figure 2. Crystalline characteristics of the DPP-based semiconducting copolymer films spincoated onto glass substrates: two-dimensional (2D) grazing incidence X-ray diffraction (GIXRD) pattern images were collected from (a) PDPPTPT, (b) PDPP3T, and (c) PDPP2T-TT and onedimensional (1D) X-ray diffractograms were obtained from (d) out-of-plane and (e) in-plane directions of the corresponding 2D GIXRD patterns.


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RESULTS Crystalline Characteristics of DPP-Based Conjugated Copolymer Films. The crystalline microstructures of the DPP-based conjugated copolymer films were studied using GIXRD. Figures 2(a)–2(c) show the 2D GIXRD patterns of PDPPTPT, PDPP3T, and PDPP2T-TT, respectively. As shown in Figures 2(a) and 2(b), the GIXRD images of the PDPPTPT and PDPP3T films exhibit a diffused first-order (100) peak and dim spots along the qz axis (out of plane), indicating that they have a low crystallinity, while the PDPP2T-TT film shows stronger out-of-plane ordering, (h00), with little arcing. This means that the PDPP2T-TT films contain domains of macromolecules in edge-on chain orientations [Figure 2(c)] with a certain degree of disorder. In contrast, from the high-resolution specular diffraction patterns shown in Figure 2(d), the d-spacings along the outof-plane direction can be estimated to be 18.2 Å , 19.0 Å , and 21.9 Å for PDPPTPT, PDPP3T, and PDPP2T-TT, respectively. For the molecular chains with edge-on orientations, the conjugated πorbital planes are vertical to the xy surface plane, forming face-to-face π–π stacking in the domains tilted with respect to each other. Thus, the d-spacings along the out-of-plane direction for the DPPbased copolymers are significantly affected by the length of the branched alkyl side chains. The PDPPTPT and PDPP3T copolymers with C6 and C8 branched alkyl chains showed fewer ordered peaks, whereas PDPP2T-TT with C8 and C10 branched alkyl chains provided at least three ordered peaks with little arcing, indicating more well-ordered edge-on orientations on the surface of the substrates. Moreover, the length of the branched alkyl chains may also affect the degree of the π– π stack. As shown in Figure 2, the GIXRD images of the PDPPTPT and PDPP3T films display more diffuse spots. This suggests that the PDPPTPT and PDPP3T films may have less crystalline ordering compared to the PDPP2T-TT films. The one-dimensional (1D) profile of the X-ray intensity obtained from the qxy directions shown in Figure 2(e) provide more information on the π–π stacking plane distance. According to the 1D profile of the X-ray intensity, the (010)


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diffraction peak in the qxy direction of the PDPP3T and PDPP2T-TT copolymer films were identified. The peaks correspond to π–π stacking plane distances of 3.85 Å and 3.82 Å , respectively. However, it was difficult to identify the (010) diffraction profile in the qxy direction of the PDPPTPT copolymer film as it was considerably broad. This made it difficult to reliably calculate the π–π stacking distance between the PDPPTPT molecules. Considering that both PDPPTPT and PDPP3T have the same branched alkyl side chains, it is believed that the π–π stacking plane distance is mainly affected by the donor unit in the DPP-based copolymer backbone rather than the branched alkyl side chain. Therefore, these results may infer that the P-moiety in PDPPTPT induces a relatively weak π–π interaction compared to the T-moiety in PDPP3T and the TT-moiety in PDPP2T-TT. In addition, the results may infer that the TT-moiety in PDPP2T-TT induces a stronger π–π interaction than the T-moiety in PDPP3T.

Figure 3. Chemical structures of (a) PDPPTPT, (b) PDPP3T, and (c) PDPP2T-TT and the electrical characteristics of the PFETs based on the corresponding DPP-based semiconducting copolymer films.


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Electrical Properties of PFETs with DPP-Based Semiconducting Copolymer Films. Figure 3 shows the chemical structures of the DPP-based semiconducting copolymer films used in this study (upper panel) and their corresponding electrical properties, including the transfer and output curves of the PFETs with the corresponding DPP-based semiconducting copolymer films (lower panel). The transfer curves of all three PFETs with different DPP-based D–A-type semiconducting polymer films shown in the Figure 3 exhibited a symmetric V-shaped ambipolar behavior as the charge transport changed from electron to hole accumulation upon the sweeping of the gate bias from positive to negative and vice versa. The output curves in Figure 3 show appropriate saturation at high gate biases. However, we can also observe a pronounced increase in the current at low gate biases, particularly for the electron accumulation mode. These are typical for transistors with ambipolar characteristics and are not present in unipolar devices.21,22 In contrast, from the slope of −VG vs. I D of the transfer curves, the hole mobility of the PFETs with different DPP-based D– A-type semiconducting polymer films in the saturation region was calculated to be approximately 0.11, 0.20, and 0.30 cm2/Vs for the PDPPTPT, PDPP3T, and PDPP2T-TT copolymer films, respectively. The electron mobilities obtained from the slope of +VG vs. I D were approximately 0.27, 0.28, and 0.37 cm2/Vs for the PDPPTPT, PDPP3T, and PDPP2T-TT copolymer films, respectively. Although the DPP-based semiconducting copolymer films have relatively poor crystallinities, as shown in Figure 2, the mobility values obtained from the transfer curves of the DPP-based PFETs are reasonably high. In particular, the mobility of PDPP2T-TT containing a TT donor building block with a relatively strong electron-donating property is higher than that of PDPPTPT and PDPP3T.


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Figure 4. Temperature dependence of the transfer curves in the linear regime (VD = −20 V) for PFETs with (a) PDPPTPT, (b) PDPP3T, and (c) PDPP2T-TT.

Temperature- and Gate Bias-Dependent Charge Transports in DPP-Based Copolymer Films. To examine the charge transport in the channel of PFETs with different DPP-based semiconducting copolymers, temperature-dependent transfer curve measurements were performed over a temperature (T) range of 170 K–260 K. Figure 4 displays the temperature-dependence of the transfer characteristics in the linear regime (VD = −20 V). As shown in Figure 4, all three PFETs display similar trends wherein the electrical current increases at elevated temperatures, indicating the thermally activated transport of hole carriers. The thermally activated charge-hopping process can be explained using the multiple trapping and release (MTR) model, which claims that the charge carriers can easily hop only after filling the shallow trap sites below the band edge.23–25 The activation energy of charge hopping was extracted from the plot of the linear mobility (μlin) of PFETs with different DPP-based semiconducting copolymer films against the inverse of the temperature. The obtained values were 81.7, 59.6, and 40.4 meV for PDPPTPT, PDPP3T, and PDPP2T-TT copolymers, respectively, as shown in Figure 5(a).


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Figure 5. (a) Plots of linear mobility vs. 1/T for PFETs with PDPPTPT, PDPP3T, and PDPP2TTT copolymers. (b) Gate voltage-dependent activation energy for PFETs with three different DPPbased semiconducting polymers. (c) The extracted charge trap density of states (DOS) of PFETs with DPP based semiconducting polymers above the valence band EVB. The DOS were fitted with to the Gaussian distribution model (GDM).

Generally, the thermally activated charge transport described in terms of the MTR model shows the gate bias dependence of the activation energy and leads to the following equation (known as the Meyer–Neldel rule):25–28

  1 1   , I (VG )  I ' exp Ea VG       kT EMN 

(1)

where I′ is the characteristic prefactor and EMN is defined as the Myer–Neldel energy, i.e., a crossing point for different activation energies at a specific temperature, T = EMN/k.25–29 Figure 5(b) shows the dependency of the activation energy on the gate bias applied to PFETs with DPPbased semiconducting copolymer films in which the activation energy is extracted from the slopes of the thermally activated currents to the inverse of the temperature at the applied gate voltages. The activation energy is also related to the Fermi level EF of the DPP polymer semiconductors.


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This can be controlled by the applied gate bias, regardless of the presence of the trap states in the bulk of the semiconducting copolymer. Indeed, the energy difference between EF of the semiconducting copolymer and the HOMO level (i.e., the valence band edge EVB) at the semiconducting copolymer/dielectric interface is approximated using the measured Ea value. The effective potential at the dielectrics in contact with the DPP-based semiconducting copolymer is nearly zero because the flat-band voltage is close to zero for the polymer semiconductor. Thus, this relation can be simply expressed as Ea = EVB − EF. On the other hand, as the applied gate bias increases, the accumulated charges in the channel layer increase, filling the localized trap states from a lower energy level. Thus, the increased number of charges decreases the activation energy, as shown in Figure 5(b). For the field-effect devices, the charges induced by the bias applied to the dielectric layer are confined in the 2D charge sheets formed just beneath the semiconductor/dielectric interface. The number of the charges in the 2D charge sheets corresponds to n2D = (Ci·ΔVG)/q, and the 2D charge density can be converted to the volume density n3D using the relation n2D = n3D·t, where t is the thickness of the accumulation charge sheets, assuming that the charges are uniformly distributed within the charge sheets. From the above relations between the activation energy and the charge density, the spectral DOS of the DPP-based semiconducting copolymer films can be estimated using the following equation:

C  dE n N (E)   i   a Ea qt  dVG

1

  . 

(2)

The spectral trap DOS as a function of the energy above EVB in Figure 5(c) was extracted based on Equation (2), and the extracted DOS profiles of the DPP semiconducting copolymer films were fitted to the Gaussian distribution model of sub-band gap DOS expressed as follows:


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 E2   g E   exp   2, 2   2   N

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(3)

with the width of the distribution being σ and the total concentration of the charge traps being N. The fitted parameters of the Gaussian distribution model for N (E) were N = 8.24 × 1018 #/cm3 and σ = 44.3 meV for PDPPTPT, N = 2.21 × 1018 #/cm3 and σ = 37.6 meV for PDPP3T, and N = 2.16 × 1018 #/cm3 and σ = 34.9 meV for PDPP2T-TT copolymers. Although the extracted DOS profiles of the PDPPTPT polymer film are only represented in considerably limited ranges (0.078–0.107 meV), these results clearly indicate that the trap density of the DPP copolymers including a Tmoiety or TT-moiety is lower than that of the P-donor-contained DPP copolymers. Temperature-Dependent Mobility Interpreted Using the GDM. To further investigate the difference in the charge transport of the DPP-based D–A-type semiconducting copolymers containing different donor units in the polymer backbone, the temperature-dependent mobilities at different VG values were analyzed using the GDM, which explains the charge transport in disordered organic materials with the hopping process of the charges in energy states that follow a Gaussian distribution. Under small electric fields, the temperature-dependent mobility is described as follows:23,25,30–32

  2  2     o exp      3kT  

(4)

where 0 is the mobility prefactor related to the amount of overlap between the molecular orbitals and may be regarded as the mobility in the absence of the disorder and σ is the disorder parameter corresponding to the width of the Gaussian DOS. To extract the GDM parameters, the measured


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mobility values of PFETs with different DPP-based semiconducting copolymer films at different VG values are plotted as a function of the inverse square of the temperature in Figure 6.

Figure 6. Plots of mobility vs. 1/T2 at different gate voltages for PFETs with (a) PDPPTPT, (b) PDPP3T, and (c) PDPP2T-TT copolymers. Dashed lines are fitted according to the GDM described in Equation (4).


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Table 1. Summary of the extracted Gaussian distribution model (GDM) parameters for polymerbased field-effect transistors (PFETs) using diketopyrrolopyrrole (DPP)-based donor (D)–acceptor (A)-type semiconducting copolymers containing different donor units in the polymer backbone.

VG

PDPPTPT

PDPP3T

PDPP2T-TT

0

σ

0

σ

0

σ

(cm2/Vs)

(meV)

(cm2/Vs)

(meV)

(cm2/Vs)

(meV)

−10

0.015

46.07

1.243

60.56

2.391

57.97

−20

0.045

42.79

0.494

46.82

0.806

44.31

−30

0.083

40.66

0.271

38.88

0.384

35.90

−40

0.141

40.33

0.205

34.57

0.267

31.62

(V)

The extracted GDM parameters and their values for PFETs with PDPPTPT, PDPP3T, and PDPP2T-TT copolymers are listed in Table 1. From the extracted parameters, it can be confirmed that σ decreases for all PFETs with increasing VG, which can be understood as a consequence of the filling of energy states. The charge carriers accumulated by the applied VG will occupy the lowest energy states in the first location that lies on the far side of the tail of the distribution. With these states filled, the distribution of the remaining unoccupied states involved in the charge transport will be narrowed.32 However, unlike the σ parameter of PFETs with DPP-based copolymers, the changes in the μ0 value varied depending on the material. As the applied VG increases, the μ0 value of PFETs with PDPPTPT increases, while that of PFETs with PDPP3T and PDPP2T-TT decreases. A clue to explain this trend can be found from the GIXRD results. According to the above GIXRD results, the T-moiety of the PDPP3T copolymer has a stronger π– π interaction than the P-moiety of the PDPPTPT copolymer and the TT-moiety of the PDPP2T-


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TT copolymer has a stronger π–π interaction than the T-moiety of the PDPP3T copolymer. The strong π–π interactions between the polymer backbones enable charge delocalization between the molecules by further decreasing the intermolecular spacing. In inorganic semiconductor systems, such as silicon (Si), germanium (Ge), or III–V compound semiconductors (e.g., GaAs and GaN), wherein the charges are delocalized over the lattice, the field-effect mobility of the charge decreases as the applied electric field increases (i.e., the number of charges induced by the field effect increases) in the field-effect device due to the charge repulsion effect and collisions between the charges.33,34 In the same manner, the different changes in the μ0 value upon the increase in the applied gate bias to the different DPP copolymer-based PFETs can be explained as follows: the molecular orbital interaction becomes weaker as the applied gate bias increases for PDPP3T and PDPP2T-TT wherein π-delocalization between the molecules is relatively strong, while the molecular orbital interaction gradually increases because the repulsion effect between charges is insignificant even when the number of induced charges increases in the case of PDPPTPT in which delocalization of the charges is not sufficiently achieved.

Figure 7. Schematic of the trap DOS in accordance with the energy band gap of the DPP-based semiconducting copolymers.


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DISCUSSION It is generally known that the electron-donating property of the conjugated backbone of D– A-type semiconducting copolymers is strong in the order of P-moiety < T-moiety < TT-moiety.14,15 Therefore, it is expected that the energy band gap EG of the DPP-based copolymer with the TTmoiety (PDPP2T-TT) is smaller than that of the DPP copolymer with the T-moiety (PDPP3T) and that the EG value of PDPP3T is smaller than that of PDPPTPT with the P-moiety. Indeed, the estimated EG values obtained from CV and ultraviolet–visible spectroscopy were reported to be 1.50, 1.30, and 1.20 eV for PDPPTPT, PDPP3T, and PDPP2T-TT, respectively, as shown in Figure 1.16–20 Furthermore, as the energy band gap becomes narrower when the molecules are placed in an interacting polarizable environment, the DPP-based copolymer with a stronger π–π interaction (i.e., in the order of PDPPTPT < PDPP3T < PDPP2T-TT) will have a narrower energy band gap than the molecular state when the molecules are aggregated, and thus, the actual band gap of each DPP polymer is expected to be narrower with a larger difference than the aforementioned values.35 Conversely, for DPP-based semiconducting copolymers with a narrower band gap, the trap distribution width should become narrow, as shown in Figure 7. The narrow trap distribution width leads to a shallower depth of the deep trap states in order to reduce the number of deep trap states, resulting in a lower activation energy for the charge-hopping process. The results of the fitted Gaussian distribution parameters can also be correlated to that of the activation energy, i.e., the Ea value of PDPP2T-TT is smaller than that of PDPP3T, and the Ea of PDPP3T is smaller than that of PDPPTPT (Ea follows the order of PDPP2T-TT < PDPP3T < PDPPTPT). This order is the reverse of the mobility values of DPP-based copolymers (PDPP2T-TT > PDPP3T > PDPPTPT). Therefore, the higher mobilities obtained using PDPP2T-TT can be attributed to the suppressed


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DOS distribution near the transport level, which mainly originated from the narrowest energy band gap tuned with the orbital couplings of the DPP acceptor and the TT donor moiety.

CONCLUSIONS Herein, the role of the donor moiety in DPP-based D–A-type semiconducting copolymers on the charge transport properties was investigated in order to provide a guideline for the rational design of high-performance semiconducting polymers. In accordance with our analysis of the experimental results, the advantage of selecting donor units with a stronger electron-donating property in the conjugated backbone of a D–A-type semiconducting copolymer is that they induce a stronger π–π interaction and lead to a narrower energy band gap. This results in a lower trap DOS, lower thermal activation energy for the charge-hopping process, and thus a higher mobility for the charge carrier. Thus, among the three different donor units (P-moiety, T-moiety, and TTmoiety) that we tested, the TT-moiety is conclusively regarded as the best choice as a donor-unit for a DPP-based D–A-type semiconducting copolymer.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. ; Gel permeation chromatography (GPC) results of the synthesized DPP based D-A copolymers and electrical characterization of inverted coplanar [bottom-gate and bottom-contact (BGBC)] structural PFETs for the initial evaluation of the synthesized DPP copolymer films.


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AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (J. Lee)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning of Korea (Code No. 2015R1C1A1A02037534). We gratefully acknowledge to Dr. Jong Won Chung at Samsung Advanced Institute of Technology for helpful technical support of synchrotron X-ray diffraction.

ABBREVIATIONS D, donor; A, acceptor; DPP, diketopyrrolopyrrole; P, phenylene; T, thiophene; TT, thienothiophene; GIXRD, grazing incidence X-ray diffraction; FETs, field-effect transistors; PFETs, polymer-based FETs; GDM, Gaussian disorder model; DOS, density of states; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; IIG, isoindigo; TGBC, top-gate and bottom-contact; PMMA, poly(methyl methacrylate); I, current; V, voltage; SSRL, Stanford Synchrotron Radiation Lightsource; MTR, multiple trapping and release; 1D, one-dimensional; 2D, two-dimensional


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Effect of Donor Building Blocks on the Charge-Transfer Characteristics

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