Charge Transportations and Digital Memory Characteristics of Electron-Donating and Accepting Polymers

Abstract

Studying charge transportations of electronics are very crucial because it is related with the device characteristics and performances. So far, the various methods are proposed to measure the charge transport properties. Nowadays, the electronic devices are getting smaller and the materials in used of it is become finer and thinner. Thus, it makes difficulty and unreliability of previous estimation methods of charge transport properties. Compared to silicon-based materials, the functional polymers in electrical memory devices have a lot of attention due to its good scalability, low-cost potential, low-power consumption, three-dimensional stacking capability and large capacity for data storage. In addition, polymers can be modified by chemical synthesis to have various electroactive moieties and self-assembly characteristics. The electrical memory characteristics have permanent and temporary behavior by thickness, nanostructure of thin films and chemical structures. Moreover, compared to other organic electronics, there are no study on the relationship between memory performances and charge transport properties. The charge transport properties of memory devices can be affected by film conditions and polymer itself, but the relationship between memory characteristics and charge transport properties has not been identified. Therefore, the correlation among electrical memory characteristics, chemical structure of functional polymers, nanostructure of thin films and charge transport properties should be examined. In Chapter II, as the dimension of the electronic functional materials become finer, the intact information of nanoscale thin film are overwhelmed by interfering side effects coming from heterojunction for measurement. In particular, the carrier-resolved characteristics of organic thin films have not been experimentally explored yet due to its low conductivity and the fragile molecular coupling to external stimuli, despite the strong demand for information about the phenomena inside thin film for the breakthrough of nanoscale device technology. Here, in contrast to general beliefs, we show that the portion of the primary electron or photon beam that effectively interacts with the target material is sensitively dependent on the transport properties of the sample and demonstrate that the estimated effective primary current allows in situ measurement of the evolution of the charge-resolved mobility of nanoscale thin film without the need for heterojunction formation and forcing field. This novel method opens new opportunities for in situ analysis of the charge-resolved transport properties of nanomaterials in development across the electrical critical point. In Chapter III, soluble aromatic polyimides and polyvinyls were prepared with incorporating pyridine moiety and its derivatives in the backbone and the side groups respectively: 6F-Py-i polymers based on the polyimide backbone (6F-Py-1 to 6F-Py-7) and PVPy-i polymers based on the polyvinyl backbone (PVPy-1 to PVPy-4). All polymers were found to be amorphous. The 6F-Py-i polymers were thermally stable up to 511545 C; the PVPy-i polymers were stable up to 362376 C. Their glass transitions, thin film densities, molecular orbitals, and band gaps were determined. The electrical devices fabricated with the polymers in an electrode/polymer/electrode structure revealed p-type unipolar write-once-read-many times (namely, permanent) or dynamic random access memory or dielectric behavior, depending on the substituents of pyridine unit and the film thicknesses. In particular, such digital memory characteristics were found to originate from the pyridine moieties possessing a high charge affinity in the polymers. However, the pyridine moieties were found to still need at least two or more aromatic substituents to get an enough power to stabilize charges via utilizing the resonance effects provided by the substituents. Overall, this study demonstrated that the pyridine unit conjugated with two or more aromatic substituents is a very useful component to design and synthesize digital memory materials based on thermally stable polyimides and other high performance polymers. The 6F-Py-i polymers have potential for the low-cost mass production of high-performance programmable unipolar permanent memory devices with very low power consumption. In Chapter IV, three different series of vinyl copolymers bearing electron donating and accepting moieties in various compositions and their homopolymers were synthesized by reversible addition-fragmentation chain transfer polymerizations. They all were soluble in conventional organic solvents and gave good quality nanoscale films via conventional coating and drying processes. They were thermally stable up to 242 C or higher temperatures. Their optical and electrochemical properties as well as electron densities and mass densities were measured. The nanoscale film morphologies were further examined by synchrotron grazing incidence X-ray scattering analysis; they were confirmed as amorphous or structurally-featureless films. All polymers exhibited various electrical properties depending on the polymers and film thicknesses. In particular, only p-type digital memory characteristics were observed within certain film thickness windows, regardless of electron donating polymers, electron accepting polymers, and their copolymers. Moreover, all polymers revealed high memory performances with low switching-ON voltages, high ON/OFF current ratios and high reliabilities even in air ambient conditions. The memory behaviors followed Ohmic conduction and trap-limited space charge limited conduction in the OFF-state and Ohmic conduction in the ON-state. However, the film thickness window showing digital memory characteristics was significantly dependent upon the compositions of electron donating and accepting moieties. Higher fraction of electron donating moieties provided wider film thickness window for digital memory. For this aspect, the electron accepting polymers could gain great benefits, whereas the electron donating polymers could attain only negative impacts. Overall, all polymers of this study are suitable for the low-cost mass production of high-performance programmable memory devices. In Chapter V, the first digital nonvolatile memory devices fabricated with deoxyribonucleic acids (DNAs) and their mimicking brush polymers were reported. Salmon testes and calf thymus DNAs (StDNA and CtDNA) in natural form, sodium salt, and surfactant complex were chosen and a series of brush polyacrylates containing nucleobase moieties at the bristle ends were newly synthesized as DNA-mimicking polymers by reversible addition-fragmentation chain transfer polymerization. StDNA and CtDNA thin films in devices revealed p-type unipolar write-once-read-many-times memory (WORM) behaviors with low switching-on voltage and high ON/OFF current ratio. Similar memory behaviors were observed for the DNA-mimicking brush polymers based on polyacrylate dielectrics. For all observed memory behaviors, switching mechanisms were investigated. The memory behaviors were found to have a film thickness window because of the charge trapping and hopping nature of nucleobase moieties. In case of two mimicking polymers containing adenine and guanine moieties, the memory behaviors were further varied somewhat with the degree of ordering and orientation. The results collectively confirmed that the digital nonvolatile memory characteristics of the DNAs and mimicking brush polymers are originated from the nucleobase moieties which can play as charge-trapping sites and stepping-stones in hopping process to transport charges. Overall, this study demonstrates that DNA and DNA-mimicking polymers are good candidate materials for the production of p-type permanent memory devices with high performance, high stability and low power consumption. In Chapter VI, the electrical memory devices composed of P3HT are investigated. The film thickness and annealing temperature are varied, and the effects of these changes on electronic characteristics are quantified by conducting GIWAXS and XR experiments. Film conditions affected the film orientation, long-range order, edge and face-on ratio, surface roughness, and also changed the electrical memory behaviors. Electrical memory behaviors were governed by the combination of Ohmic and trap-limited space charge limited conduction by a hopping process that involves thiophene units as charge traps and stepping-stones. The electrical memory behaviors were influenced by the film conditions, which are higher film thicknesses and fractions of edge-on structure revealed narrow film thickness window of electrical memory operation. Overall, the P3HT polymer can be used to create high-performance memory devices that have very low power consumption.