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J. Phys. Chem. B 2010, 114, 10294–10301
Electrical Memory Characteristics of a Nondoped π-Conjugated Polymer Bearing Carbazole Moieties Samdae Park,† Taek Joon Lee,† Dong Min Kim, Jin Chul Kim, Kyungtae Kim, Wonsang Kwon, Yong-Gi Ko, Heungyeal Choi, Taihyun Chang, and Moonhor Ree* Department of Chemistry, Center for Electro-Photo BehaViors in AdVanced Molecular Systems, DiVision of AdVanced Materials Science, National Research Lab for Polymer Physical Chemistry, Polymer Research Institute, and BK School of Molecular Science, Pohang UniVersity of Science & Technology, Pohang 790-784, Republic of Korea ReceiVed: February 4, 2010; ReVised Manuscript ReceiVed: July 9, 2010
Poly[bis(9H-carbazole-9-ethyl)dipropargylmalonate] (PCzDPM) is a novel π-conjugated polymer bearing carbazole moieties that has been synthesized by polymerization of bis(9H-carbazole-9-ethyl)dipropargylmalonate with the aid of molybdenum chloride solution as the catalyst. This polymer is thermally stable up to 255 °C under a nitrogen atmosphere and 230 °C in air ambient; its glass-transition temperature is 147 or 128 °C, depending on the polymer chain conformation (helical or planar structure). The charge-transport characteristics of PCzDPM in nanometer-scaled thin films were studied as a function of temperature and film thickness. PCzDPM films with a thickness of 15-30 nm were found to exhibit very stable dynamic random access memory (DRAM) characteristics without polarity. Furthermore, the polymer films retain DRAM characteristics up to 180 °C. The ON-state current is dominated by Ohmic conduction, and the OFF-state current appears to undergo a transition from Ohmic to space-charge-limited conduction with a shallow-trap distribution. The ON/OFF switching of the devices is mainly governed by filament formation. The filament formation mechanism for the switching process is supported by the metallic properties of the PCzDPM film, which result in the temperature dependence of the ON-state current. In addition, the structure of this π-conjugated polymer was found to vary with its thermal history; this change in structure can affect filament formation in the polymer film. 1. Introduction Electrically resistive switching organic molecules and polymeric materials have recently gained much attention from academia and industry because they exhibit significant advantages over inorganic silicon- and metal-oxide-based memory materials in the fabrication of volatile and nonvolatile memory devices: their dimensions can easily be miniaturized, and their properties can easily be tailored through chemical synthesis.1-20 In particular, polymeric materials exhibit easy processability, flexibility, high mechanical strength, and good scalability. In general, they require only very simple solution processes such as spin coating, dip coating, spray coating, and inkjet printing, which can be carried out at low cost, and further are easily fabricated in the multistack layer structures required for highdensity memory devices.4-20 Thus, significant research effort is currently devoted to the development of polymer switching materials with properties and processability that meet the requirements of memory devices. As a result, some candidate polymeric materials with memory effects and their applications have been reported.4-30 One interesting group of polymers is the fully π-conjugated polymers.11,12,18 Poly(o-anthranilic acid) and its copolymers with aniline have been reported to exhibit electrically bistable ON-OFF switching characteristics that are suitable for fabrication of nonvolatile memory devices.11,12 Poly(diethyl dipropargylmalonate) has been found to exhibit dynamic random access memory (DRAM) characteristics or * To whom correspondence should be addressed. Phone: +82-54-2792120. Fax: +82-54-279-3399. E-mail:
[email protected]. † S. Park and T. J. Lee contributed equally to this work.
write-once-read-many-times (WORM) memory behavior without polarity, depending on the film thickness.18 Films with a thickness of 30 nm have been found to exhibit very stable WORM memory characteristics, whereas films with a thickness of 62-120 nm exhibit DRAM characteristics. Another interesting group of polymers is nonconjugated polymers containing carbazole moieties in the main chain or side group: poly(Nvinylcarbazole) (PVK),21,22 poly[N-vinylcarbazole-co-europium(vinylbenzoate)(2-thienoyltrifluoroacetone)2phenanthroline],23 poly(2-(9H-carbazol-9-yl)ethyl methacrylate-co-4-(5-(4tert-butylphenyl-1,3,4-oxadiazol-2-yl)phenyl methacrylate-coeuropium-methacrylate-thenoyltrifluoroacetone-phenanthroline) (PCzOxEu),24 poly(N-vinylcarbazole-co-N-fullerenylvinylcarbazole) (PVK-PVK_C60),25 poly(2-(9H-carbazol-9-yl)ethyl methacrylate),26 6-O-[4-(9H-carbazol-9-yl)butyl]-2,3-di-O-methyl cellulose,28 poly(N-vinylcarbazole-co-N-vinylphenylfluorenylcarbazole) (PVK-PVK_PF),29 and poly(3,3′-bis(N-ethylenyloxycarbazole)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-CBZ PI).30 These polymers exhibit ON-OFF switching and WORM memory or DRAM behaviors, depending on the polymer backbone’s chemical structure.21-30 In general, the memory characteristics of a polymer have been found to depend on its chemical structure, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), conductivity, thickness, and temperature of the polymer layer as well as the employed electrodes’ work function. Their memory characteristics can also be influenced by the characteristics of their interfaces with the electrodes. Further, the exact switching mechanism of polymer memory devices is
10.1021/jp101062a 2010 American Chemical Society Published on Web 07/23/2010
Nondoped π-Conjugated Polymer-Bearing Carbazole Moieties
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Figure 1. (a) Chemical structure of PCzDPM, and (b) schematic diagram of a memory device fabricated with a nanometer-scale thin film of the PCzDPM polymer and aluminum (Al) top and bottom electrodes.
still a subject of debate. Thus, development of polymer-based memory devices remains in the exploration stage. In this study we attempted to chemically mold fully π-conjugated polymer and carbazole-containing nonconjugated polymer as a single polymer system in order to improve electrical memory performance by utilizing advantageous digital memory characteristics of both polymer systems. We succeeded to synthesize poly(bis(9H-carbazole-9-ethyl)dipropargylmalonate) (PCzDPM) (Figure 1a), which is a new π-conjugated polymer containing carbazole moieties. This polymer is highly soluble in organic solvents and is thus easily processed as nanoscale thin films through conventional solution spin, roll, or dip coating and subsequent drying. The PCzDPM polymer exhibits excellent DRAM behavior with a high ON/OFF ratio (up to 107) for film thicknesses
