Study On The Preparation And Properties Of Nanocrystals/Polymer Optoelectronic Composite Materials

Semiconductor nanocrystals (NCs) directly synthesized in aqueous solution have attracted much attention due to their unique chemical and physical properties. Incorporation of these NCs into polymers is a highly desirable approach to generate novel materials for use in light-emitting diodes (LED) and photovoltaic (PV) cells. In this thesis, the composites of NCs and polymer were prepared by making use of the electrostatic interaction between them. The influence of interaction on the luminescent properties of the composite was systematically investigated. On this basis, the composites with different kinds of function were prepared by intentionally tuning the interaction between NCs and polymers.First, we designed a novel amphiphilic copolymer of poly (9-vinylcarbzol-co -octadecyl-4-vinylbenzyl-dimethyl-ammonium chloride) (CPVKOVDAC) with blue fluorescence. Octadecyl-4-vinylbenzyl-dimethyl-ammonium chloride (OVDAC) groups in the copolymer acted as intermedia to achieve the composites of the copolymers and functional NCs. The driving force of the composite formation was the electrostatic interaction between them. Three advantages can be envisaged in this approach: the introduction of carbazole moieties can improve the transport of holes in the composites; the high quality NCs are preformed, which avoids the photoluminescence (PL) quench in the ligand exchange or in situ method; the electrostatic interaction can make the NCs stable in the polymer matrix. The effect of electrostatic interaction on the PL properties of NC-polymer composites was systematically investigated through varying the ratios of NCs to polymers in the composites. Our experiment results indicated that the electrostatic interaction between CdTe NCs and polymers brought the carrier recombination process on NC surface while carbazole moieties had an important influence on the intrinsic recombination process of NC core states.By making use of this interaction, white-light PL and electroluminescence were obtained from the solution-processable NC-polymer composite. In comparison, white light-emitting NC-polymer composite was much easier to prepare than a polymer containing all blue, green and red chromospheres, and emitted purer and more stable white light over a range of applied voltages than dye-doped polymers and polymer blends. In order to further improve the quantum yield of the composites, we also directly grafted surfactant group on poly (vinylcarbazole) (PVK) to achieve PVK-based amphiphilic polymer. Different CdTe NCs prepared in aqueous medium were directly transferred to the organic phase by using the PVK-based polymers. Compared with the CPVKOVDAC copolymer, the quantum yield of NCs had been improved 50 % due to the shorter distant between carbazole groups and NCs. At the same time, CdTe NCs were prepared in the PVK matrix using in situ method. No ligand was capped on the surface of CdTe nanoparticles which made charges transport easily.The investigation of the NC-polymer composite was further extended into near infrared region. The CdHgTe NCs were prepared via a simple one-step synthesis by making use of a large difference in Ksp of CdTe and HgTe in aqueous medium. These NCs showed an excellent thermal stability and intense PL at 1100-1200 nm with the QY up to 45 %. ICP results indicated the formation process of CdHgTe NCs: since the solubility (Ksp) of HgTe in water is approximately 20 times lower than that CdTe, the speed of nucleation for HgTe should be much faster than that of CdTe when the concentrations of Hg2+ and Cd2+ are comparable. Therefore, the initial core should be rich in Hg due to its faster reaction rate for Hg. As the free Hg ion is being depleted from the reaction mixture, CdTe deposition becomes dominant in the subsequent crystal growth. Consequently, the shell that is in rich of Cd is formed on the core surface, without much mismatch owing to the similar crystallographic constants of CdTe and HgTe, yielding the alloy NCs with a gradient Hg concentration decreasing from the core to the surface. Because the outer layer is largely made of CdTe, it can act as an encapsulating shell for the HgTe-rich core. This unique structure ensured successful incorporation of CdHgTe NCs into a polymer matrix to achieve the transparent bulk composites with intense NIR emission at 1300 nm.The mechanism of interaction between NCs and polymers was further explored by investigating the behavior of pure CdTe NCs as well as NCs and polymers in solution. They could self-assemble into dendritic or leaf shapes, which depended on the interactions between NCs and polymers. The concentration and pH values of the solution, the ligands of NCs as well as the temperature and humidity had important influence on the morphology resulted. The study of composite based on electrostatic interaction can lay a basis for the better development of these optoelectronic materials.