Alcohol-soluble Quantum Dots: Synthesis And Light Emitting Applications

Quantum dots(QDs)are emerging as new generation luminescent materials for many lighting technologies due to their unique optical and electrical properties.In particular,quan-tum dots liquid crystal display(QLCD)technologies have already been successfully applied in commercial TVs toward enhanced color performance.In another direction,quantum dots based light-emitting diodes(QLED)become to emerging star for new generation of low-power display technology to supplement organic light-emitting diodes(OLED).To push them forward,there has been of great interests to develop low cost alternative materials.I-III-VI Cu In S₂and Cu In Se₂based QDs are potential low cost candidates for white lighting applica-tions due to their color tunable and broad emission spectra.Although quantum dots with tunable emissions and high photoluminescence quantum yields(PLQYs)have been well developed,the application of QLCD and QLED require the incorporation QDs into polymer matrix or sandwiched devices.Currently,the compatibility hindered the exploration of lighting and display applications.For example,(1)The obtained QDs from colloidal synthesis can be dissolved in nonpolar organic solvent which is harmful to the human body and the environment.Also,selection of orthog-onal solvent for organic transport layers had been a critical problem in all-solution processed QLED.(2)In a multilayered electroluminescent device,the long alkyl chain ligands of quantum dots are not favorable to the charge injection from adjacent layers to quantum dots,which seriously limited the performance of QLED.Meanwhile,the serious compatibility issues be-tween quantum dots and polymer matrix lead to quantum dot agglomeration and severely reduces the optical conversion efficiency of the quantum dots films.To solve the above problems,ligand modification on surface has been considered as one of the most essential means on tuning optoelectronic properties and processability of quan-tum dots.Compared with oil-and water-soluble quantum dots,alcohol-soluble quantum dots,with unique solubility and hydroxyl-terminated ligands,exhibit“green”processability,or-ganic solvent resistance,tunable electrical band gap and multiple reaction sites on surface.Thus,alcohol-soluble quantum dots possess notable performance and processing superiorities in high performance optoelectronic device.This dissertation focus on synthesis and charac-terization of alcohol-soluble quantum dots and their applications in electroluminescence and photoluminescence devices.Herein after,we developed two simple and accessible ligand exchange strategies for fabricating Cu In S₂and Cd Se based alcohol-soluble quantum dots with PLQYs up to 70%and 80%,respectively.The use of alcohol-soluble quantum dots for light-emitting diodes were demonstrated.The main achievements are highlighted as follow.(1)The phase transfer of Cu In S₂based QDs from nonpolar solvents into alcohols was developed by combining in-situ ligand exchange strategy and heat-assisted ligand exchange strategy with 6-mercaptohexanol as ligands.The resulting alcohol-soluble quantum dots show high PLQYs(>70%)and excellent solubility in alcoholic solvents.(2)Electroluminescent devices based on alcohol-soluble Cu In S₂quantum dots have a maximum luminance of 8,735 cd·m^-2and an external quantum efficiency of 3.22%.The current efficiency and power efficiency of the optimized devices are 9.43 cd·A^-1and 6.09lm·W^-1,respectively.Moreover,alcohol-soluble QDs bring the solvent resistance for the fabrication of multiple layered QLED devices.(3)We demonstrated the fabrication of composite materials by embedding alcohol-soluble quantum dots into alcohol-soluble polymers or siloxane glass.These highly transparent com-posites can load high concentration of quantum dots up to 40 wt%.These results opened up a new research field on alcohol-soluble quantum dots composite and may inspire important work toward practical applications.