Inorganic Semiconductor Nanocrystal / Conjugated Polymer Composites, Characterization, And The Photoelectric Effect Between The Composite Components

Conjugated polymer/semiconductor nanocrystals composites not only have the surface effects and quantum effect of nano-materials,but also combine the rigidity,thermal stability of inorganic nanocrystals with the flexibility,easy solution processiblility of conjugated polymers.These make the composites produce many special excellent photoelectrical properties for the extensive research and application in optoelectronic devices,such as light-emitting diodes,photovoltaic cells,and so on.By combining conjugated polymers with semiconductor nanocrystals,the mobility of charge carriers in the former can be increased and an ultrafast charge transfer between the two takes place at the high surface interface.These behaviors make the composites particularly popular in the research of polymer-based photovoltaic devices.However,there are some difficulties in the preparation of the composites.Moreover,the research of exciton interaction between the components needs long-term research both in theory and application. Previous studies have revealed that nanocrystals will aggregate in the polymer matrix during the preparation of composites,which will severely affect the device performances. The knowledge of transport of charge carriers and the mobility of electron in the composites is an important reference for the choice of materials in photovoltaic devices. However,little is understood about it.This dissertation is focused on modifying the preparation of the composites,and studying the mechanism of the interaction between the conjugated polymer and semiconductor nanocrystals.The contents are as follows:1.An electron-rich unit—bithiophene was introduced into the structure of poly(p-phenyleneethynylene)(PPE),thus an electron-donating conjugated polymer PPE-BTH was synthesized.By combining PPE-BTH with TOPO capped CdSe nanocrystals(TOPO-CdSe) and pyridine covered CdSe nanocrystals(Py-CdSe), electroactive interaction were studied.Fluorescence resonance energy transfer was dominant between PPE-BTH and TOPO-CdSe,whose efficiency was increased with the nanocrystals concentration.Photoinduced charge transfer occurred between PPE-BTH and Py-CdSe.The quenching process involves a static quenching and a dynamic quenching mechanism.2.By easily introducing the binding sites—amino groups into the chain structure of a conjugated polymer based on fluorene units,an amine-containing multidentate ligands polymer PDMAEMA-PF-PDMAEMA was synthesized through atom transfer radical polymerization(ATRP).The chemical structure was determined by ¹H NMR and ¹³C NMR.The multidentate ligand replaced TOPO form the surface of CdSe nanocrystals by ligand exchange process,and conjugated to CdSe forming tight conjugated polymer/nanocrystals composite.The ligand exchange process was proved by ³¹P NMR.The morphology of the composite was studied by HRTEM,showing an excellent dispersion of CdSe in the polymer matrix.The PL spectra shows photoinduced charge transfer has been faciliated by the complex formation.3.An amphiphilic conjugated-ionic triblock copolymer PFDMA-Q with positive charges located on the non-conjugated chains was prepared by quaternization of PDMAEMA-PF-PDMAEMA.We have demonstrated by steady-state photoluminescence spectra,photoluminescence excitation spectra and fluorescence lifetime measurements that efficient fluorescence resonance energy transfer has occurred from the polymer as the energy donor to CdTe quantum dots served as the acceptor.Using CdTe quantum dots with emission maxima at 550,580,610 nm as the energy donor,we have observed that more effective FRET process occurred in the larger sized QDs and polymer blended systems.And these also demonstrated that even relatively long insulating chains do not severely interrupt the energy transfer from the polymer to QDs.4.A cationic conjugated polymer PFE-NEt₂MeI with the positive charges located on the main chain of the polymer was synthesized.A significant PL quenching of the polymer was observed in the composite of polymer and CdTe quantum dots.We have demonstrated by steady-state photoluminescence spectra,photoluminescence excitation spectra and fluorescence lifetime measurements that photoinduced charge transfer was dominant in the composite system.5.A facile way to construct the composite of CdTe nanocrystals and a derivative polymer of polystyrene containing light-emitting conjugated moieties.These emissive flexible random copolymers were synthesized via radical polymerization,which were composed of two kinds of functional side chain groups providing the optoelectronic properties and the electrostatic attachment with the CdTe nanocrystals,respectively. Furthermore,the chemical structures of these random flexible copolymers were clearly confirmed by ¹H and ¹³C NMR.On the other side,the structure and morphology of these nanocomposites were characterized by FT-IR and TEM, respectively,showing that these nanocrystals were well dispersed in the polymeric matrix.In addition,the UV-Vis and PL emission spectra revealed that fluorescence resonance energy transfer process from the emissive random copolymer to the CdTe nanocrystals took place,with low transfer efficiency.We can conclude that these semiconductor nanocomposites were not proper materials applied in photovoltaic cell, but could be the potential light-emitting materials for LEDs application.