Pure White‐Light Emission of Nanocrystal–Polymer Composites

Abstract

Incorporation of semiconductor nanocrystals (NCs) into functional polymers is a highly desirable approach to generate novel materials for use in optoelectronic devices, such as lightemitting diodes (LEDs) and photovoltaic cells. Basic requirement is a good miscibility of the NCs in such a polymer matrix. Much work has been done in developing suitable methods for preparation of the composites of NCs and polymers. One of the simplest methods is to mix NCs directly with polymers in organic solvents, but this often results in aggregation of the NCs in the polymer matrix due to a lack of covalent attachment. In situ synthesis of NCs in a polymer matrix tends to solve the aggregation problem. However, the photoluminescence (PL) of the resulting NC–polymer composites is dramatically quenched as a result of the absence of efficient passivation. The layer-by-layer self-assembly method is one of the best alternatives to solve these problems, although the amount of NCs incorporated into the composites is fairly low. The resulting assemblies are applicable to fabrication of ultrathin films. The use of tri-n-octylphosphine oxide (TOPO) to stabilize semiconductor NCs allows for facile ligand exchange and thus possible incorporation of NCs into a variety of functional polymers. To avoid surface oxidation of the NCs, size and size distribution changes and especially PL quenching, an alternative method is still much needed for incorporation of NCs into functional polymers. Preparation of NCs in aqueous medium offers a number of advantages such as low costs, ease of large-scale production and use of environment-friendly solvents. 7] Furthermore, these NCs can be easily modified with various functional groups during or after their preparation. Our recent work has shown that the charged NCs prepared in aqueous medium can be readily coated with polymers bearing the opposite charges to form NC–polymer composites that are soluble in organic solvents and solution processable for making films on substrates. This process overcomes the major problems associated with the miscibility of NCs with polymer matrix and the processability of resultant composites and also implies a potential of incorporating several different color-emitting NCs into a functional polymer. Herein we report the incorporation of CdTe NCs prepared in aqueous solution into carbazole-containing positively charged polymers (Scheme 1). The electrostatic interaction between CdTe NCs and charged polymers is expected to shorten the distance of CdTe and carbazole moieties, allowing for efficient electron transfer from carbazole to CdTe NCs. By incorporating