New Donor−Acceptor Random Copolymers with Pendent

ARTICLE pubs.acs.org/Macromolecules

New Donor
Acceptor Random Copolymers with Pendent Triphenylamine and 1,3,4-Oxadiazole for High-Performance Memory Device Applications Yi-Kai Fang,†,^ Cheng-Liang Liu,‡,^ Guei-Yu Yang,§ Po-Cheng Chen,§ and Wen-Chang Chen*,†,§ †

Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan 106 Department of Organic Device Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan § Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106 ‡

bS Supporting Information ABSTRACT: We report the synthesis and resistive-type switching memory characteristics based on new P(VTPAxBOXDy) and P(CNVTPAxBOXDy) random copolymers containing different donor/acceptor ratios (8/2, 5/5, and 2/8) of pendent electron-donating 4-vinyltriphenylamine (VTPA) or 4,40 -dicyano400 -vinyltriphenylamine (CNVTPA) and electron-withdrawing 2-(4-vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD). The effects of donor/acceptor ratios and cyano side group on the memory characteristics were explored and compared with properties of homopolymers, PVTPA, PCNVTPA, and POXD. The distinct electrical current
voltage (I
V) characteristics of the ITO/P(VTPAxBOXDy)/Al device changed from volatile memory to insulator depending on the relative donor/acceptor ratios. The ITO/P(VTPA8BOXD2) or PVTPA/Al device exhibited SRAM or DRAM behavior with an ON/OFF current ratio of 107
108. However, no switching phenomena were observed for a higher BOXD ratio. Moreover, the devices could endure 108 cycles under a voltage pulse and show a long retention time for at least 104 s under constant voltage stress. The low-lying HOMO energy level of BOXD was employed as the hole-blocking moiety as the charge transport occurred between the neighboring triphenylamine units. The charge trapping/spontaneously back-transferring of trapped carriers controlled the switching behavior. On the other hand, all the P(CNVTPAxBOXDy) memory devices exhibited nonvolatile nature with NDR behavior due to the preferred interaction of Al atoms with the cyano group. Here, the results demonstrated that the pendent polymers with specific donor
acceptor chromophores could tune the memory switching characteristics for advanced electronic device applications.

’ INTRODUCTION Donor
acceptor (D
A) polymer systems have widely investigated for organic electronics, such as light-emitting diodes,1,2 photovoltaic cells,3
16 field-effect transistors,11
18 and memory devices.19
21 They provide the advantages of flexibility, low cost, solution processability, and three-dimensional stacking capability as compared with inorganic counterparts.1
21 Because of the stable electron-donating nitrogen atom in the triphenylamine (TPA),22,23 the TPA-based D
A polymers were developed for various electronic applications.24
33 In addition, the energy level of TPA-based polymers can be chemically tuned through the different strength of electron acceptors to meet the requirement of original molecular design for specific organic electronics. For example, functional polyimides (PIs) containing electrondonating TPA moieties and electron-withdrawing phthalimide moieties were demonstrated for the device application of dynamic random access memory (DRAM), static random access memory (SRAM), write-once-read-many times (WORM), and flash memory via an external voltage bias or pulse.24
28 Soluble TPA-based polyazomethine grafted with graphene oxide acceptors was prepared r 2011 American Chemical Society

for bistable flash memory device.29 Besides, side-chain TPA
perylene block copolymers influenced the HOMO energy level and microphase-separated morphologies, which affected the solar cell performance significantly.30,31 We are particularly interested in the design and synthesis of D
A polymer systems for resistive-type memory device applications.27,34
37 The reported D
A materials system for the volatile and nonvolatile memory devices included small molecules,38,39 conjugated polymers,40
42 nonconjugated polymers with D/A chromophores (functional polyimide24
29,34 or pendent polymers36,43,44), and polymer nanocomposites (metal nanoparticle,45,46 fullerene,37,47 carbon nanotube,48 or graphene oxide29 embedded). The original switching mode of memory devices were operated through several proposed mechanisms such as the trapping/detrapping of charges, charge transfer effect, and filamentary conduction, as summarized by Kang and co-workers.19 Received: January 27, 2011 Revised: March 4, 2011 Published: March 25, 2011 2604

dx.doi.org/10.1021/ma200187e | Macromolecules 2011, 44, 2604–2612

Macromolecules

ARTICLE

Scheme 1. Synthesis of Random Copolymers P(VTPAxBOXDy) and P(CNVTPAxBOXDy)

The D
A copolymers comprising TPA and triazine or oxadiazole were only applied for light-emitting diode applications.22,23 However, the memory characteristics of such copolymers have not been fully explored yet. In this study, we report the synthesis of nonconjugated random copolymers with pendant donor and acceptor groups for memory device applications. The used electron-donating moiety was 4-vinyltriphenylamine (VTPA) or 4,40 -dicyano-400 vinyltriphenylamine (CNVTPA) while the electron-withdrawing moiety was the 2-(4-vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD) group. Here, various pendent random copolymers P(VTPAxBOXDy) or P(CNVTPAxBOXDy) with different D/A ratios were synthesized by the nitroxide-mediated free radical polymerization (NMRP). The thermal, optical, and electrochemical properties of the new synthesized copolymers were characterized and compared with those of the corresponding homopolymers, PVTPA, PCNTPA, and PBOXD. The memory behavior was measured by a simple sandwich device configuration consisted of spin-coated polymer films between ITO and Al electrodes. From the related energy levels of interface, the HOMO values and hole-injection ability were varied through the pendent TPA derivatives while the low-lying HOMO energy level of BOXD electron acceptor can block the transport of charge carriers. The experimental results suggested that the observed switching behavior was determined by D/A ratio through the ability of charge trapping/back-transferring of trapped charges and metal diffusion effect.

’ EXPERIMENTAL SECTION Materials. Benzoyl peroxide (BPO) and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) were obtained from Acros (Geel, Begium). Anhydrous toluene and dimethylformamide (DMF) were obtained from TEDIA (Fairfield, CT). All the chemicals were used as received and without further purification. Tetrabutylammonium perchlorate (TBAP, TCI) was recrystallized twice from ethyl acetate and then dried in vacuum prior to use. The monomers of 4-vinyltriphenylamine (VTPA), 4,40 -dicyano-400 -vinyltriphenylamine (CNVTPA), and 2-(4vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD) were prepared according to the literature.49,50 The detailed synthetic procedures of the

homopolymers, PVTPA, PCNTPA, and PBOXD, are reported in the Supporting Information.

General Polymerization Procedure of Random Copolymers. The procedure of random copolymers on based nitroxidemediated free radical polymerization (NMRP) is shown in Scheme 1. The reaction process comprises a mixture of VTPA (or CNVTPA) and BOXD monomers, free radical initiator (BPO), stable free radical (TEMPO), and DMF in glass tube with a magnetic stirring bar. The mixture was degassed and backfilled with N2 three times, sealed under N2 flow, and placed in 135 C oil bath for 4 h. After cooling, DMF was added to dissolve the polymers, and the solution was reprecipitated into methanol. The polymers were used for further purification by Soxhlet extraction in acetone. The copolymers were finally dried under vacuum at 60 C overnight. The weight-average molecular weights (Mw) of the copolymers, obtained by GPC using THF (for P(VTPAxBOXDy)) or DMF (for P(VTPAxBOXDy)) as the elute and polystyrene as standards, were 12 700
16 500 and 24 000
29 600 g/mol with the polydispersity index (PDI) of 1.24
1.37 and 1.17
1.31, respectively. The relative ratios of D/A segments in the random copolymers were estimated form elemental analysis. The reaction compositions and characterization are described as the following: P(VTPA8BOXD2): 474 mg of VTPA (1.74 mmol), 141 mg of BOXD (0.47 mmol), 6.6 mg of BPO (0.028 mmol), 5.0 mg of TEMPO (0.034 mmol), and DMF (1 mL) were used to afford 144 mg of gray solid (23.5%). 1H NMR (CDCl3), δ (ppm): 6.21
8.19 (br, 27H, Ar
H), 1.26
2.30 (br, 6H,
CH2
CH
). Anal. Calcd for [(C20H17N)0.8 þ (C22H16N2O)0.2]: C, 86.76; H, 6.09; N, 5.95. Found: C, 84.18; H, 5.99; N, 5.49. Mw and PDI estimated from GPC are 14 700 and 1.34, respectively. P(VTPA5BOXD5): 287 mg of VTPA (1.05 mmol), 343 mg of BOXD (1.14 mmol), 6.4 mg of BPO (0.026 mmol), 5.5 mg of TEMPO (0.033 mmol), and DMF (1 mL) were used to afford 165 mg of white solid (26.2%). 1H NMR (CDCl3) δ (ppm): 6.21
8.21 (br, 27H, Ar
H), 1.31
2.43 (6H,
CH2
CH
). Anal. Calcd for [(C20H17N)0.5 þ (C22H16N2O)0.5]: C, 84.38; H, 5.85; N, 7.03. Found: C, 82.68; H, 5.33; N, 6.75. Mw and PDI estimated from GPC are 15 000 and 1.37, respectively. P(VTPA2BOXD8): 112 mg of VTPA (0.41 mmol), 540 mg of BOXD (1.80 mmol), 6.30 mg of BPO (0.026 mmol), 5.2 mg of TEMPO (0.034 mmol), and DMF (1 mL) were used to afford 106 mg of white solid 2605

dx.doi.org/10.1021/ma200187e |Macromolecules 2011, 44, 2604–2612

Macromolecules (16.4%). 1H NMR (CDCl3) δ (ppm): 6.32
8.23 (br, 27H, Ar
H), 1.36
2.67 (6H,
CH2
CH
). Anal. Calcd for [(C20H17N)0.2 þ (C22H16N2O)0.8]: C, 82.24; H, 5.64; N, 7.99. Found: C, 81.29; H, 5.36; N, 7.97. Mw and PDI estimated from GPC are 12 700 and 1.24, respectively. P(CNVTPA8BOXD2): 503 mg of CNVTPA (1.57 mmol), 97 mg of BOXD (0.39 mmol), 6.67 mg of BPO (0.028 mmol), 5.0 mg of TEMPO (0.036 mmol), and DMF (1 mL) were used to afford 144 mg of gray solid (22.0%). 1H NMR (CDCl3) δ (ppm): 6.62
8.23 (br, 25H, Ar
H), 1.43
2.63 (br, 6H,
CH2
CH
). Anal. Calcd for [(C22H15N3)0.8 þ (C22H16N2O)0.2]: C, 81.37; H, 4.69; N, 12.78. Found: C, 80.10; H, 5.05; N, 12.52. Mw and PDI estimated from GPC are 25 000 and 1.17, respectively. P(CNVTPA5BOXD5): 338 mg of CNVTPA (1.06 mmol), 262 mg of BOXD (1.06 mmol), 7.10 mg of BPO (0.029 mmol), 5.95 mg of TEMPO (0.038 mmol), and DMF (1 mL) were used to afford 143 mg of gray solid (23.8%). 1H NMR (CDCl3) δ (ppm): 1.32
2.16 (br, 6H,
CH2
CH
), 6.41
8.18 (br, 25H, Ar
H). Anal. Calcd for [(C22H15N3)0.5 þ (C22H16N2O)0.5]: C, 80.04; H, 4.74; N, 12.29. Found: C, 78.5; H, 5.10; N, 11.93. Mw and PDI estimated from GPC are 25 640 and 1.19, respectively. P(CNVTPA2BOXD8): 147 mg of CNVTPA (0.457 mmol), 453 mg of BOXD (1.82 mmol), 6.92 mg of BPO (0.028 mmol), 5.80 mg of TEMPO (0.037 mmol), and DMF (1 mL) were used to afford 151 mg of gray solid (25.2%). 1H NMR (CDCl3) δ (ppm): 6.32
8.19 (br, 25H, Ar
H), 1.33
2.15 (br, 6H,
CH2
CH
). Anal. Calcd for [(C22H15N3)0.2 þ (C22H16N2O)0.8]: C, 78.51; H, 4.79; N, 12.72. Found: C, 77.38; H, 5.13; N, 11.39. Mw and PDI estimated from GPC are 28 000 and 1.31, respectively. Characterization. 1H NMR spectra were measured on a Bruker Avance 300 MHz FT-NMR spectrometer. Gel permeation chromatographic (GPC) analysis was performed on a Lab Alliance RI2000 instrument (one column, MIXED-D from Polymer Laboratories) connected with one refractive index detector from Schambeck SFD Gmbh. All GPC analyses were performed on polymer/THF (or DMF) solution at a flow rate of 1 mL/min at 40 C (70 C) and calibrated with polystyrene standards. Elemental analyses were performed with a Heraeus VarioEL-III-NCSH instrument. Thermogravimetric analysis (TGA) was conducted with a PerkinElmer Pyris 1 TGA at a heating rate of 20 C/min. Differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere at a heating rate of 20 C/min from
50 to 250 C using a TA Instruments DSC-Q100. Electrochemistry was performed with a CHI 611B electrochemical analyzer and a three-electrode cell in which ITO (polymer films area about 0.7  0.5 cm2) was used as a working electrode. A platinum wire was used as an auxiliary electrode. All cell potentials were taken with the use of a homemade Ag/AgCl, KCl(sat.) reference electrode. Absorption spectra were measured with a Hitachi U4100 UV
vis
NIR spectrophotometer. The thickness of the polymer film was measured with a Microfigure Measuring Instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.).

ARTICLE

Figure 1. 1H NMR spectra of (a) PVTPA, (b) PBOXD, and (c) P(VTPA8BOXD2) in CD2Cl2. was performed by a Keithley 4200-SCS semiconductor parameter analyzer equipped with a Keithley 4205-PG2 arbitrary waveform pulse generator. Al was used as the anode and ITO was set as the cathode (maintained as common) during the voltage sweep with a step of 0.1 V. The probe tip used 10 μm diameter tungsten wire attached to a tinned copper shaft with a point radius