Naphthalene Diimide-Based Polymer Semiconductors: Synthesis

Apr 2, 2012 - Steady progress in the development of p-type (hole transport) polymer semiconductors in the past decade has resulted in high performance...
5 downloads 10 Views 552KB Size
Download PDF
Article pubs.acs.org/cm

Naphthalene Diimide-Based Polymer Semiconductors: Synthesis, Structure−Property Correlations, and n-Channel and Ambipolar Field-Effect Transistors Xugang Guo,†,§ Felix Sunjoo Kim,‡ Mark J. Seger,† Samson A. Jenekhe,*,‡ and Mark D. Watson*,† †

Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, United States

‡

S Supporting Information *

ABSTRACT: A series of nine alternating donor−acceptor copolymer semiconductors based on naphthalene diimide (NDI) acceptor and seven different thiophene moieties with varied electron-donating strength and conformations has been synthesized, characterized, and used in n-channel and ambipolar organic field-effect transistors (OFETs). The NDI copolymers had moderate to high molecular weights, and most of them exhibited moderate crystallinity in thin films and fibers. The LUMO energy levels of the NDI copolymers, at −3.9 to −3.8 eV, were constant as the donor moiety was varied. However, the HOMO energy levels could be tuned over a wide range from −5.3 eV in P8 to −5.9 eV in P1 and P3. As semiconductors in n-channel OFETs with gold source/drain electrodes, the NDI copolymers exhibited good electron transport with maximum electron mobility of 0.07 cm2/(V s) in P5. Although head-to-head (HH) linkage induced backbone torsion, polymer P4 showed substantial electron mobility of 0.012 cm2/(V s) in bottom-gate/top-contact device geometry. Some of the copolymers with high-lying HOMO levels (P7 and P8) exhibited ambipolar charge transport in OFETs with high electron mobilities (0.006−0.02 cm2/(V s)) and significant hole mobilities (>10−3 cm2/(V s)). Varying the device geometry from top-contact to bottom-contact leads to the appearance or enhancement of hole transport in P4, P6, P7, and P8. Copolymers with smaller alkyl side chains on the imide group of NDI have enhanced carrier mobilities than those with bulkier alkyl side chains. These results show underlying structure−property relationships in NDI-based copolymer semiconductors while demonstrating their promise in n-channel and ambipolar transistors. KEYWORDS: naphthalene diimide copolymer, n-type polymer semiconductor, electron transport, n-channel organic transistor, ambipolar charge transport, donor−acceptor conjugated copolymer

■

INTRODUCTION Steady progress in the development of p-type (hole transport) polymer semiconductors in the past decade has resulted in high performance p-channel organic field-effect transistors (OFETs) with hole mobilities approaching or surpassing 1 cm2/(V s).1−5 However, the OFET performance of n-type (electron transport) polymer semiconductors significantly lags behind their ptype counterparts.6−8 The inferior device performance of ntype polymer semiconductors is mainly due to the scarcity of materials,6−8 poor solubility, and instability of charge carriers (organic anions) in ambient conditions9,10 as well as the existence of traps at the semiconductor/dielectric interface.11 In order to function as n-type semiconductors, the organic materials should possess high electron affinities, which can be realized by introducing electron-withdrawing groups, such as fluorines,12 cyanos,13 and imide groups.14,15 On the other hand, introducing such groups usually leads to deactivation for further chemical modifications, such as bromination, which are © 2012 American Chemical Society

essential for the subsequent polymerization. Furthermore, even polymer semiconductors incorporating such strong electron-withdrawing groups may still not have sufficiently large electron affinity or low-lying LUMO energy level, which can lead to a substantial energy barrier for electron injection and high sensitivity of the charge carrier to ambient species when the active channels are exposed to air.16 Among the different electron-deficient units, imide-functionalized arenes are among the most important building blocks for n-type polymer semiconductors due to their chemical accessibility,14 highly electron-deficient character,17 and the solubilizing ability provided by N-alkylation.18 Jenekhe et al. reported the first n-channel polymer OFETs19 based on a ladder polymer BBL (Figure 1), which has imide-like structures Received: November 17, 2011 Revised: April 2, 2012 Published: April 2, 2012 1434

dx.doi.org/10.1021/cm2034273 | Chem. Mater. 2012, 24, 1434−1442

Chemistry of Materials

Article

Figure 1. Structures of representative imide-based (or imide-like) n-type polymer semiconductors for OFETs and OPVs.

and an electron mobility (μe) as high as 0.1 cm2/(V s) was measured.20 The n-type polymer BBL could also be used to fabricate all-polymer solar cells with a power conversion efficiency (PCE) of 1.5%.21,22 The perylene diimide (PDI)based polymer (Figure 1) was first reported by Marder et al. for application as n-type semiconductors in OFETs and organic photovoltaics (OPVs),23 achieving electron mobility as high as 0.013 cm2/(V s) and PCE greater than 1%, respectively.24 Thereafter, a variety of PDI-based donor−acceptor (D-A) copolymers have been synthesized with promising device performance, especially as the electron-transport component in OPVs.18 Efficiency greater than 2% has been achieved in allpolymer solar cells using PC-PDI (Figure 1) as electron transport component in bulk heterojunction devices,25 which is among the highest in all-polymer solar cells.26,27 Marks et al. reported a bithiophene imide (BTI)-based homopolymers P(BTim-R) as n-type semiconductors for OFETs.28 The polymers showed a highly crystalline microstructure and μe up to 0.2 cm2/(V s) could be achieved by optimizing the polymer molecular weight and OFET fabrication.29 Although high μe and current modulation (Ion/Ioff) were achieved from P(BTim-R), other device characteristics (Vt and device stability) were not promising due to the low electron affinity. As a structural analogue to PDI, naphthalene diimide (NDI) could be a more attractive alternative for constructing n-type polymer semiconductors. Compared to dibrominated PDI,30 dibrominated NDI can be easily obtained as regioisomerically pure form,31 and the smaller naphthalene unit could lead to polymers with greater solubility/processability. More importantly, NDI should allow for a more coplanar backbone conformation in polymers when bridging aryl units are connected to the 2,6-positions of NDI than bridging via the sterically congested bay regions of PDI. The studies of NDIbased polymers for application in OFETs have been pioneered by Watson et al.32,33 Independently, Facchetti et al. developed a NDI-based polymer P(NDI2OD-T2) for application in OFETs,34 demonstrating unprecedented OFET characteristics with μe up to 0.85 cm2/(V s) in top-gate/bottom-contact device architecture.35 By varying the number of thiophene units in the donor portion within the polymer backbone, Luscombe

reported a series of NDI-oligothiophene copolymers for application in OFETs and a maximum μe as high as 0.076 cm2/(V s) was measured for PNDI-3Ph (Figure 1) in bottomgate/top-contact OFET architecture.36 In a previous communication,32 we reported the synthesis and characterization of a series of five NDI-based donor− acceptor copolymers. Initial studies of their optical, thermal, and electrochemical properties as well as self-assembly of polymer fibers indicated that they were promising materials for applications in n-channel and ambipolar OFETs. Investigation of one of the NDI-based copolymers in devices led to the demonstration of high-performance ambipolar OFETs and complementary-like inverters with sharp switching and a voltage gain of 30.33 We report herein the synthesis and characterization of a series of nine NDI-based polymer semiconductors containing an acceptor−donor architecture and seven different donor moieties of varying electron donating strength. The electrochemical redox properties and related electronic structures (HOMO/LUMO energy levels) were systematically investigated by cyclic voltammetry. The solid state morphology of the NDI copolymers was investigated by 2D wide-angle X-ray diffraction (WAXD) on fibers and by Xray diffraction (XRD) and atomic force microscopy (AFM) imaging on thin films. The charge transport properties were systematically investigated by n-channel and ambipolar OFETs. The LUMO energy levels of all nine NDI copolymers, at −3.9 to −3.8 eV, were nearly identical, reflecting the constant NDI moiety. The low-lying LUMO energy level and efficient molecular packing of the polymer chains facilitated good field-effect electron transport with average μe higher than 0.03 cm2/(V s) for P5. Although head-to-head linkage induced polymer backbone torsion was present in P4, substantial electron mobility (0.012 cm2/(V s)) was measured from P4based OFETs. Variation of the donor portions led to changes of the HOMO levels and thus optical band gaps. For example, cyclopentadithiophene and dialkyloxy bithiophene donor counits in polymers P7 and P8a, respectively, led to high mobility ambipolar charge transport in OFETs. The device geometry variation from top-contact to bottom-contact leads to the appearance or enhancement of hole transport in P4, P6, P7, 1435

dx.doi.org/10.1021/cm2034273 | Chem. Mater. 2012, 24, 1434−1442

Chemistry of Materials

Article

all polymerizations produced polymers with moderate to high molecular weights (24−252 kDa); the rather high molecular weights for some polymers could be due to aggregationinduced overestimation of molecular weight in THF. The identity and purity of the polymers were supported by NMR and elemental analysis (see the Supporting Information). Due to the extensive aggregation of P5−P8 in solution, resolved 1H NMR could only be obtained at elevated temperature, and no 13 C signals corresponding to the backbone carbons could be collected at practically accessible measurement temperatures (up to 130 °C) except P1c, P3, and P4. Optical Properties of Polymers. Optical properties of NDI polymers P1−P8 were investigated by measuring their absorption spectra in solution as well as thin films; the relevant data are collected in Table 1, among them are some polymers (P1c, P3, P4, P5, P8a, and P8b) whose absorption spectra we have reported previously.32,33 Representative solution and thin film absorption spectra of groups of the NDI copolymers are shown in Figures 2 and 3. Across the series, the absorption spectra of the polymers extended to longer wavelengths as the electron donating ability of the donor counit increases. Increasing the electron-donating ability of the donor counit results in higher-lying HOMO energy level. As the donor monomer was varied from thiophene, to thienothiophene, to 2,2′-bithiophene, to dithienothiophene, and finally to 3,3′didodecyloxy-2,2′-bithiophene, the electron donating ability of the donor increases. The absorption maxima were shifted from 600 nm (P1c), to 661 nm (P2), to 703 nm (P5), to 716 nm (P6), and finally to 1005 nm (P8b) in the film states; the absorption in solution followed the same trend (Figure 2). Optical band gaps (Egopts), estimated from the absorption onset of polymer films, are listed in Table 1 as follows: 1.66 eV for P1c, 1.64 eV for P2, 1.48 eV for P5, 1.42 eV for P6, and 1.08 eV for P8b, which indicates the band gap of polymers is a function of electron-donating ability of the donor counits in these NDI-based D−A polymers. It is worthwhile to compare the absorption spectra of polymer P6 with that of the PDIbased polymer reported by Marder et al.23 Both polymers share dithienothiophene as the donor counit. The PDI-based polymer has an absorption λmax at 630 nm, while the NDIbased polymer P6 has an absorption λmax at 716 nm. The optical band gap of P6 is ∼0.11 eV smaller than that of the PDI-based polymer. The decrease in band gap of P6 indicates its extended conjugation. Therefore, connection of the dithienothiophene units to the sterically hindered “bay” positions of PDI likely limits conjugation along the backbone due to torsion about the connecting bonds. Higher conjugation in P6 might be somewhat enhanced by attractive interactions between the (thienyl)S···O(carbonyl) interactions.40,41 The regioisomeric purity of P6 might be another reason accounting for its lower band gap.34 A common strategy to increase the solubility/processability of polymer semiconductors is to functionalize the polymer backbone with solubilizing side chains. Substituents can also affect film microstructure and morphology. Study of the optical properties of the NDI copolymers with bithiophene derivatives carrying different side chains provides valuable information. P3 shows excellent solubility even in hexane due to the bulky 2ethylhexyl chains on both NDI and bithiophene units, while polymer P5 without any side chain on bithiophene units can only be dissolved in chlorinated solvents such as chloroform and dichlorobenzene. These alkyl chains limit backbone planarization due to head-to-head (HH) linkages induced

and P8. Variation of side-chains on the polymer backbone was found to affect solubility, extent of conjugation, optical properties, energy levels of the frontier molecular orbitals (FMOs), morphology, and charge transport properties, which will be detailed in the following discussion.

■

RESULTS AND DISCUSSION Synthesis of Monomers and Polymers. The NDI monomers, 2,6-dibromonaphthalene diimides (Br2−NDI), were synthesized by following the published procedure,31 and they were purified by column chromatography. Before polymerizations, they were further purified by recrystallization in order to maximize purity beyond detection limits. Different amines were used for condensation with 2,6-dibromonaphthalene dianhydride to produce different Br2−NDIs in order to ensure sufficient polymer processability. Eight thiophene-based derivatives with different conjugation lengths, different conformation/geometry/size, and different electron donating ability were synthesized for copolymerization with Br2−NDIs. The purpose of the synthesis of different monomers is to enable fine-tuning of the band gap, frontier molecular orbital (FMO) energy levels, solubility/processability, and film morphology of the resulting polymers. The monomer syntheses were straightforward via modified published procedures.37−39 Since monomer purity significantly affects molecular weight and quality of the resulting polymers, distannylated thiophene comonomers were rigorously purified via column chromatography using basic alumina as the stationary phase and hexane/ triethylamine (19/1) as the mobile phase (Supporting Information).32 1H, 13C NMR spectra, and elemental analysis showed that the monomers were obtained in high purity. Polymers P1−P8 were synthesized under Stille conditions as shown in Scheme 1, and all polymerizations were highly Scheme 1.

a

Reagents and conditions: i: Pd2(dba)3, P(o-tolyl)3, THF, 80 °C (2EH = 2-ethylhexyl; 2-BO = 2-butyloctyl; n-DO = n-dodecyl; 2-DT = 2decyltetradecyl; n-OC = n-octyl).

a

effective due to the electronic properties of the monomers, which favor the Pd-mediated coupling reactions. Polymer P1a and P1b were insoluble due to the relatively small alkyl side chains and thus could not be characterized by solution techniques. Decent solubility and high molecular weight were achieved by choosing appropriate side chains in the other polymers (P1c and P2−P8). GPC measurements indicate that 1436

dx.doi.org/10.1021/cm2034273 | Chem. Mater. 2012, 24, 1434−1442

Chemistry of Materials

Article

Table 1. Molecular Weight, Thermal Properties, and Optical Properties of the NDI-Based Polymers P1−P8 yield (%) P1c P2 P3 P4 P5 P6 P7 P8a P8b

94 89 91 99 98 99 88 89 95

Mn[PDI]a (kDa) 23.6 [2.2] 61.2 [3.6] 78.7 [1.6] 73.0 [1.6] 252 [2.5] 71.2 [3.6] 188 [2.5] 120 [3.3] 133 [3.5]

Tmb (°C) 235 N/A 269 222 285 N/A N/A N/A 252

λmax (soln, nm) c

λmax (film, nm) e

568 633c 534c 539c 693c 714d 839c 860c 985c

600 661e 576e 610e 703e 716e 854e 867f 1005e

Δ λmaxg (nm) 32 28 42 71 10 2 15 7 20

λonset (film, nm) e

747 756e 752e 795e 838e 873e 1033e 1146f 1149e

Egopth (eV) 1.66 1.64 1.65 1.56 1.48 1.42 1.20 1.08 1.08

Measured by GPC (THF as eluent; vs polystyrene standards). bPeak melting point from DSC. c5 × 10−6 M in THF. d5 × 10−6 M in CHCl3. Pristine film spun-cast from 2% (w/w) toluene solution. fPristine film spun-cast from 2% (w/w) chlorobenzene solution. gShift in absorption λmax on going from solution to film. hEgopt = 1240/λonset. a e

Although P8b and P4 both have HH linkages, the insertion of oxygen between the alkyl substituent and the thiophene core in P8b leads to 309 nm red-shifted absorption maximum in comparison to that of P4 in thin film. Two factors contribute to this dramatic red-shift of absorption maximum in P8b. First, it is due to the greater electron donating ability of the dialkyloxy bithiophene monomer in P8b than that of the dialkyl bithiophene monomer in P4. Second, sulfur−oxygen interactions40 between the thienyl sulfur atom and pendant oxygen atom of alkyloxy chains enhance backbone planarity in P8b. The structural absorption spectra of P8b indicates that its backbone has higher ordered structure than the other polymers.42 Another strategy to increase solubility of polymers and maintain the coplanar structure is conformational locking with an adjoining dialkyl carbon atom. 4,4-Dioctyl-cyclopenta[2,1-b:3,4-b′]dithiophene was incorporated into polymer P7. The absorption maxima of P7 in both solution and thin film are red-shifted by approximately 150 nm relative to those of P5. Polymers P8a and P8b, which share common backbone structure and alkyloxy side chains on bithiophene counits, have different substituents on the imide groups. As expected from their structures, both polymers have similar absorption profiles (Figure S4). However the relative contribution of two absorption features (absorption maxima and absorption shoulders) to their overall absorption profiles are different. The optical band gaps from the onset absorption for both polymers are identical (1.08 eV), which reflects their similar backbone arrangements. The difference of absorption profile between P8a and P8b is possibly due to difference in degree of electronic delocalization caused by differing modes of selfassembly into aggregates in solution and their packing in the solid state. Electrochemical Properties and Electronic Structures of Polymers. The electrochemical redox properties and associated electronic structures (HOMO/LUMO energy levels) of the NDI-based polymers were investigated by cyclic voltammetry (CV). Figure 4 shows the cyclic voltammograms of all the NDI-based polymers, and the associated numerical results are summarized in Table 2. All polymers and the parent NDI small molecule show two reversible reduction waves. The first reduction peak corresponds to the formation of the NDIpolymer anion, and the second reduction peak is the formation of the NDI-polymer dianion. The oxidation CV peaks of the polymers vary significantly, but basically there is a trend for the oxidation peaks of the polymers. As the electron donating ability of donor counits was increased, the oxidation peak becomes stronger and more reversible, which indicates that p-

Figure 2. Absorption spectra of polymers P1c, P2, P5, P6, and P8b as the function of electron donating ability of donor counits in solutions (dashed lines; 5 × 10−6 M in THF, except P6 in CHCl3) and from ascast films (solid lines).

Figure 3. Absorption spectra of polymers P3, P4, P5, P7, and P8b having different substituents on bithiophene units in solutions (5 × 10−6 M in THF; dashed lines) and from as-cast films (solid lines).

steric barrier: P3 and P4 have absorption maxima at 576 and 610 nm in the film states, which are blue-shifted by 127 and 93 nm, respectively, relative to P5 containing unsubstituted bithiophene units (Figure 3). We note that, although the alkyl groups on the NDI unit are different between P3/P4 and P5, this interpretation is still valid because the thin film absorption maxima of P3 and P4 are in higher energy regions than that of P5 despite the fact that P5 has bulkier R1 groups which likely results in smaller degree of charge delocalization as well as solid-state optical absorption in higher energy band. 1437

dx.doi.org/10.1021/cm2034273 | Chem. Mater. 2012, 24, 1434−1442

Chemistry of Materials

Article

Figure 5. (a) 2D wide-angle X-ray (WAXD) diffraction patterns of NDI-polymer fibers and (b) X-ray diffraction (XRD) patterns of NDIpolymer thin films. Figure 4. Cyclic voltammograms of NDI copolymers as thin film in 0.1 M (n-Bu)4N.PF6 acetonitrile solution. The cyclic voltammogram of NDI is included for comparison.

spacing of 3.9−4.0 Å was seen except P3 and P4. The head-tohead (HH) dialkyl bithiophene linkages in P3 and P4 lead to less crystalline structures, showing no evidence of π−π stacking diffractions in P3 and P4. XRD of the NDI polymer films showed a peak at 2θ of 3.8−6.0° which represents the (100) diffraction of lamellar planes consisting of edge-on molecular backbones (Figure 5b). Polymers with the shorter 2-ethylhexyl and n-octyl chains, such as P3 and P7, tend to have a shorter lamellar d-spacing distance (∼15 Å) as shown in Table 3. In

doping becomes easier. The easier p-doping of P7 and P8 can be expected to lead to more efficient injection of holes and better performance in p-channel OFETs. The LUMO energy levels of all the NDI-based copolymers are in the range from −3.88 eV to −3.79 eV (Table 2), which is comparable to the LUMO level of the parent NDI small molecule (−3.85 eV). However the HOMO energy levels varied from −5.90 eV to −5.27 eV as a function of electron donating ability of donor monomer and polymer backbone conformation. These CV results indicate that the band gaps of NDI copolymers are determined largely by the donor counits, while the LUMO energy level remains a constant due to the NDI unit. The two quasi-reversible reduction waves and lowlying LUMO levels (19 Å). The lamellar structures of the polymer films and fibers imply that these polymer semiconductors have a large degree of backbone planarity, except P3 and P4, which is critical for efficient charge transport.43 A peak corresponding to π−π stacking was not observed in the XRD patterns of polymer thin films, suggesting a large degree of edge-on orientation of polymer backbone planes in the films. Our results are similar to a recent report on related NDI-based polymers36 but are somewhat different from another report which observed dominant face-on orientation.44 The discrepancy between our observation and a previous report could be due to the difference in device fabrication and film processing conditions. Salleo et al. found that the orientation of conjugated polymer chains can be dramatically changed from face-on to edge-on by annealing the polymer film up to melting for P(NDI2ODT2).45 We also note that the weak intensity or lack of diffraction peaks might not have come from the absence of crystalline structures in a specific direction but from the limited sensitivity of the WAXD/XRD experiments.35,44 The recently reported results of X-ray single-crystal structures of oligothiophene-NDI donor−acceptor cooligomers46 can provide insights into the crystalline structure and molecular packing of NDI-based copolymer semiconductors. For example, the single crystal structures of such model compounds revealed a monoclinic primitive lattice with slipped face-to-face π-stacking of molecular planes. One of the striking features is that the intermolecular distance is very small (3.2− 3.3 Å), while intermolecular orbital overlap between NDI in one molecule and thiophene rings in the adjacent molecule was observed.46 The close intermolecular distance is driven by strong π−π interaction which is also expected in the present NDI-based copolymer semiconductors, with implications for the nature (p- or n-channel or ambipolar) of charge transport in thin films. AFM imaging of the surfaces of the polymer (P1c, P2, P5, P7, P8a, and P8b) thin films, exemplified by the topographic images of Figure 6, revealed diffused nanofibrillar morphology. The root-mean-square surface roughness of the polymer thin

Figure 6. AFM topographical images of polymer thin films. Image size: 5 μm × 5 μm. Vertical scale: 20 nm. 1439

dx.doi.org/10.1021/cm2034273 | Chem. Mater. 2012, 24, 1434−1442

Chemistry of Materials

Article

Figure 7. (a-c) Output and (d-f) transfer characteristics for NDI-based polymers, P4 (a,d), P5 (b,e), and P7 (c,f), in bottom-gate/top-contact (BG/ TC) OFETs annealed at 150 °C.

Table 4. Electrical Parameters of Bottom-Gate OFETs Based on NDI-Copolymers P1−P8 polymer P1c P2 P3 P4 P5 P6 P7 P8a P8b

device geometrya TC BC TC BC TC BC TC BC TC BC TC BC TC BC TC BC TC BC

μeavg (cm2/(V s))b 8.6 × 1.5 × 3.6 × 1.2 × 8.6 × 2.3 × 0.012 5.3 × 0.033 0.023 9.6 × 8.0 × 4.4 × 5.6 × 0.023 0.014 9.9 × 8.0 ×

−5

10 10−3 10−3 10−3 10−5 10−4 10−3

10−3 10−3 10−3 10−3

10−4 10−4

μhavg (cm2/(V s))b

μe/μh

Vt,e (V)b

Vt,h (V)b

Ion/Ioff

− − − (3.3 × 10−5)c − − − 8.9 × 10−6 (3.6 × 10−4)c (2.1 × 10−3)c − 7.6 × 10−4 6.6 × 10−4 1.0 × 10−3 2.8 × 10−3 3.3 × 10−3 7.4 × 10−6 8.5 × 10−5

− − − (35.3)c − − − 590.3 (90.7)c (11.2)c − 10.4 6.6 5.6 8.3 4.2 134.0 9.4

9.4 25.9 13.9 0.3 12.0 20.2 24.9 11.6 5.5 −0.8 −0.8 −12.1 0.2 9.6 3.1 25.8 0.46 16.5

− − − (−39.9)c − − − −37.5 (−56.1)c (−40.1)c − −43.4 −34.8 −22.8 −30.3 −13.4 −19.8 −27.0

102−103 103 104 103−104 103 103 104−105 102−104 103−104 102−103 104−105 102−103 102 102 102 102 102 102

a

TC: Top-contact. BC: Bottom-contact. Both cases have a bottom-gate structure. bAverage of 4−10 devices. cHole transport was occasionally observed in some devices; the hole mobility (μh) and threshold voltage (Vt,h) of p-channel mode are taken from those devices.

in P3 and P4 was not detected from our experiments. Topcontact devices typically showed 1.2−3-fold higher electron mobilities than bottom-contact devices, except in the cases of P1c, P3, and P7. The increase in the electron mobility is likely from lower contact resistance in the top-contact/bottom-gate OFETs compared to the bottom-contact/bottom-gate devices.50 By incorporating various electron-donating units in the D−A copolymers, the HOMO levels of NDI-based polymers can be tuned over a wide range, while keeping the LUMO energy levels constant. The change of HOMO energy level resulted in transition from n-type to ambipolar charge transport, although the majority charge-carriers are still electrons (μe/μh > 1). Figure 8 summarizes the HOMO/LUMO energy levels of the present NDI copolymer semiconductors and the observed average field-effect electron and hole mobilities. The polymers with weak electron-donating counits (P1c, P2, P3, P4, P5, and P6) had low-lying HOMO energy levels (−5.9 to −5.7 eV) and exhibited dominant electron-transport properties in OFETs. On the other hand, polymers with strong electron-donating

counits (P7, P8a, and P8b) showed high-lying HOMO energy levels (−5.5 to −5.3 eV), which resulted in substantial ambipolar charge transport in OFETs. Well-matched electronic energy levels of organic semiconductors with the work-function of electrodes are considered to be necessary conditions for efficient charge injection/extraction by reducing energetic barriers in both unipolar and ambipolar OFETs.6 Similarly, the previously reported OFETs based on NDI-thiophene oligomers showed that as the electron-donating strength of counits increased, which was accompanied by higher-lying HOMO energy level, the hole mobility became observable or increased.46 In the present cases of P7, P8a, and P8b, the HOMO energy levels are sufficiently high-lying to result in good hole injection and thus substantial ambipolar charge transport in OFETs. These results demonstrate the effectiveness of D−A copolymer approach with various moieties to tune electronic energy levels and therefore control the polarity of the majority charge carriers in the materials. In the case of NDI copolymers which showed ambipolar OFET characteristics (P7, P8a, and P8b), the electron mobility 1440

dx.doi.org/10.1021/cm2034273 | Chem. Mater. 2012, 24, 1434−1442

Chemistry of Materials

Article

ambipolarity in NDI-based polymers has also been reported by others, although the origin is not yet understood.35 The OFETs based on the present series of NDI copolymer semiconductors showed good shelf stability with the carrier mobility decreasing by a factor of 1−3 when stored in ambient air for 2−4 weeks. However, the carrier mobility typically dropped by 2−3 orders of magnitude when the OFETs were tested in air without any environmental control (Table S1), indicating that they are not operationally stable in ambient conditions. Charge carriers in n-type organic semiconductors are anions, which are generally vulnerable to ambient air species, especially to moisture and oxygen,9,10,53 and only a few polymer-based n-channel OFETs have shown good operational stability in air.20,54−57 Nevertheless, the present NDI-based copolymers represent a big step toward high shelf stability of polymer semiconductor devices. The top-gate OFET structure offers enhanced device stability because of the encapsulating nature of multilayers of the gate dielectric and electrode.35,58,59 Finally, we note that the OFETs based on the class of polymer semiconductors presented here could be printable, as recently demonstrated by others,35,60and applicable to real electronic devices by taking advantage of their good solubility in organic solvents.

■

CONCLUSIONS We have synthesized and characterized a series of nine NDIbased donor−acceptor copolymer semiconductors with moderate to high molecular weight. Incorporation of seven different thiophene moieties of varying electron-donating strength into the NDI-copolymers facilitates the tuning of the electronic structure (HOMO/LUMO energy levels), solid-state morphology, the nature of charge transport, and the carrier mobilities in organic field-effect transistors (OFETs). Given the similarity of the LUMO levels (−5.4 eV). It is also interesting to note the variation of device geometry from top-contact to bottom-contact leads to the appearance or enhancement of hole transport in some polymers, which could be due to decreased hole injection barrier induced by the work function changes of electrodes during substrate preparation. The easily tunable electronic energy levels, solid-state morphology, and optoelectronic properties make the NDI-based copolymer semiconductors promising for n-channel and ambipolar components in organic optoelectronic devices and complementary electronic circuits.

Figure 8. (a) Electronic energy levels of NDI copolymer semiconductors. (b) Average electron mobility of NDI copolymer semiconductors in both top-contact (TC; black square) and bottomcontact (BC; red circle) OFET architectures. (c) Average hole mobility of NDI copolymer semiconductors in TC device (green upward triangle) and BC device (blue downward triangle). Error bar represents one standard deviation.

tends to decrease with the hole mobility increases when bottom-contact geometry is used (Table 4), resulting in a lower μe/μh ratio. Furthermore, the bottom-contact OFETs of P2, P4, and P6 exhibited ambipolar charge transport, whereas the top-contact devices showed unipolar electron transport. The appearance/enhancement of hole transport cannot be explained by reduction of contact resistance caused by a change in device geometry, because bottom-gate/top-contact devices generally have lower contact-resistance than bottom-gate/bottom-contact devices. The difference in charge carrier mobilities between top-contact and bottom-contact devices might in part come from the surface modification and work-function changes of the electrodes during the substrate preparation.51 Plasma treatment on gold electrodes has been shown to increase the workfunction to higher than 5.1 eV.51,52 The energy barriers between gold electrodes and LUMO energy levels of the polymer semiconductors become larger after plasma treatment, whereas the barriers with rather low-lying HOMO energy levels get smaller. This is likely the origin of the observed enhanced hole injection and thus ambipolar charge transport in some of the NDI-based copolymer transistors. Although the devices in multiple batches were carefully fabricated, small unintentional variation in device processing conditions cannot be ruled out as a possible factor in large standard deviations in carrier mobility in the bottom-contact devices of polymer semiconductors with lower-lying HOMO energy levels (