Morphology and Memory Device Applications of Self-assembly Amphiphilic Block Copolymer:PCBM Composites
Date Issued
2011
Date
2011
Author(s)
Chen, Jhao-Cheng
Abstract
As an emerging area in organic electronics, polymer memories have become an active research topic in recent years since they are likely to be an alternative or supplementary technology to the conventional memory fabrication. Nanocomposite containing self-assemble block copolymer and functional nanoparticle is one of potential candidates for memory device application owing to benefits of the self-assembly of block copolymer and the electronic characteristic of nanoparticle. Thus, this thesis mainly focuses on the fabrications, characterizations and morphologies of nanocomposites combined with their applications on memory device.
In the first part of this thesis (Chapter 2), the morphology, optical and electrochemical properties of poly (styrene-block-4-vinylpyridine) (PS-b-P4VP): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) composite thin films are reported. Two PS-b-P4VP copolymers with different volume fraction (f PS =0.70 (L1) and 0.26 (L2)) are employed to blend with PCBM. The optical absorption results indicate that the formation of weak supramolecular interaction between the P4VP block and the PCBM makes the free PCBM molecule preferentially accumulate within the P4VP domains. Thus, the utilization of a P4VP block demonstrates a way to form electronic domains containing PCBM within block copolymer self-assembled nanostructures. Furthermore, morphologies based on PS-b-P4VP:PCBM composite thin films are identified via AFM topography, TEM micrograph and Small Angle X-ray Scattering (SAXS), which demonstrate that two specifically self-organized nanostructures based on different volume fraction of PS-b-P4VP, perpendicularly cylindrical array (L1, where PCBM molecules disperse in the cylindrical cores normally) and horizontal cylindrical array (L2, in which PCBM molecules disperse in the cylindrical corona horizontally), were successfully fabricated via solvent annealing.
In the second part of this thesis (Chapter 3), nanocomposite thin films based on PS-b-P4VP:PCBM with and without solvent annealing are used to evaluate the electronic memory device applications. All the memory devices are fabricated in the same configuration: ITO/electroactive layer/Al. No matter what the morphology is, the device is merely an insulator without bistable characteristic if the electroactive layer is a pure polymer film. For L1 composite system, electroactive layer containing 5wt% PCBM with perpendicularly cylindrical array via solvent annealing shows nonvolatile switching current-voltage (I-V) characteristics, a write-once read-many times memory (WORM), whose yield is about 90% and switch-on threshold voltage is located around -1.7 V with a narrow distribution. However, electroactive layer containing 5wt% PCBM but without solvent annealing reveals a low yield (<50%) and wide distribution of threshold voltage. The electroactive layers containing 10wt% PCBM with and without solvent annealing exhibit the characteristic of semiconductor. For L2 composite system, electroactive layer containing 5wt% PCBM with horizontally cylindrical array via solvent annealing displays a volatile static random access memory (SRAM) behavior, whose yield is about 80% and threshold voltage is located around -2.7 V with a narrow distribution. Nevertheless, electroactive layer containing 5wt% PCBM but without solvent annealing only reveals a low yield (< 25%) and lager variation of threshold voltage. The electroactive layers containing 10wt% PCBM with and without solvent annealing also exhibit as semiconductors. Both the memory devices containing 5wt% PCBM with specific morphologies, perpendicularly cylindrical array and horizontally cylindrical array, exhibit high on/off current ratios of 106 ~ 107 and long retention time of 104 second. The mechanism of the switching behaviors is based on electron-trapping with space charge limited current (SCLC) theory. The present study suggests the self-assembly nanocomposites with specific nanostructures have potential application on memory devices. The relationship between the morphologies, electronic properties and device performance were also established in this study.
Subjects
Self-assembly
block copolymer
PCBM
Morphology
Memory
Type
thesis
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