Synthesis of fluorescent organic nano-dots and their application as efficient color conversion layers

Color conversion layers (CCLs) are essential components of numerous contemporary display and lighting technologies. Organic nanomaterials are capable of very competitive performance in this context. We present a new approach to develop CCLs in light-emitting diodes using organic "nano-dots".

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Fluorescent organic materials at the nano or submicron scale often exhibit novel physicochemical properties, with great potential for use in a range of technological applications. Their unique properties have spurred intense research efforts and tremendous breakthroughs have been made in the field of biomedicine, creating numerous applications like cell imaging, bio-sensing, optical sensing, etc., which were inconceivable in past decades. Despite impressive achievements, sub-micron-sized fluorescent organic particles have yet to make their mark in the optoelectronic field, where bulk organic fluorophores and inorganic quantum dots have seen notable success as efficient emitters in light-emitting diodes and displays.

Herein, we show the utility of surfactants in the synthesis and processing of sub-micron, organic fluorophore dispersions by thoroughly characterizing the effect of ionic and non-ionic surfactants on the properties of these particles. Using this information, we identify surfactant processing conditions that result in nearly 100 % conversion of organic fluorophores into sub-micrometer particles, or nano-dots, with outstanding performance as color conversion layers. Such water dispersions are environmentally benign and efficiently convert light. They can be used for a range of fluorophores covering a full spectral gamut, with excellent color purity, including full-width at half-maximum values as low as 21 nm. Compared to inorganic references, the organic nano-dot-based color conversion layers show superior color conversion efficiency and substantially improved long-term stability.

Nano-dot dispersions of different fluorescent organic light emitters were prepared as shown in Figure 1.

Figure 1. Schematic diagrams. (a) Synthesis process of Ttrz-DI nano-dot dispersion using surfactants. Chemical structures of (b) Ttrz-DI, (c) CzDABNA, (d) 4tBuMB (e) TNAP fluorescent materials. In (b) – (e), the background color corresponds to the emission color of each fluorophore.

Typically, a fluorescent organic emitter such as Ttrz-DI was dissolved in an organic solvent like tetrahydrofuran to make a stock solution. Then a surfactant (such as Triton X-100) was dissolved separately in the same solvent. After that, the fluorophore solution was mixed with variable amounts of the surfactant solution. Then, deionized water was injected rapidly at room temperature into the fluorophore-surfactant solution to make a fine dispersion of nano-dots. The concentration of the surfactant played a vital role in the synthesis and poor results (low yields, large particles) were observed when the concentration of surfactant was lower than its critical micelle concentration. The dispersions were filtered through polytetrafluoroethylene (PTFE) syringe filters with different pore sizes and subjected to dialysis using cellulose acetate tubes for 12 hours to remove the excess surfactant from the dispersions, which were then concentrated under vacuum. After evaporation of the excess solvent, the concentrated nano-dot dispersions could then be used or diluted with additional deionized water as necessary to obtain desired concentrations for optical studies and film processing.

After synthesizing the nano-dots, we investigated the influence of surfactant type and quantity on the morphology of the nan-dots. Photoluminescence spectroscopy and dynamic light scattering techniques revealed the presence of submicron-level particles in the dispersion. This led us to investigate the size of the particles using various analytical techniques like optical microscopy, scanning electron microscopy, and atomic force microscopy. Interestingly, the size of the nano-dots was primarily dependent on the amount of surfactant used in the synthesis, rather than the structure of the fluorophore or the type of surfactant. In the case of Ttrz-DI, the nano-dots reached a minimum average size of 120 nm when the concentration of surfactant was 6 mM.

We also investigated the yield of the nano-dots using UV-Vis spectroscopy. The nano-dot yield was affected by the surfactant concentration, solvents, anti-solvents, and anti-solvent injection temperatures. From this work, we found that the optimum condition for the nano-dots synthesis was at 6 mM surfactant concentration using tetrahydrofuran as a solvent and water as an anti-solvent at room temperature. Under optimized conditions, the nano-dot yields were almost 100%.

Figure 2. Color conversion characteristics. (a) Color conversion efficiency (CCE) of Ttrz-DI nano-dot films used as color conversion layers. (b) Color conversion efficiency comparison of 4CzIPN nano-dot film, Ttrz-DI nano-dot film, 4tBuMB nano-dot film and QD film. The color of each bar corresponds to the emission color of each fluorophore. (c) Stability test of nano-dot films under constant UV exposure. (d) Color conversion performance stability of new and aged films.

Finally, we explored the applicability of these nano-dot dispersions in the field of optoelectronics. To produce white light from blue light-emitting diodes, color conversion layers are necessary that can convert the blue light to green and red respectively (in displays), or other colors which when added to blue, generate white light. We fabricated such red and green color conversion films with nano-dots, which were investigated in color conversion applications. The nano-dots were dispersed in 10 % polyvinyl alcohol solution and drop-cast to prepare the films. The color conversion properties of these films were tested using a 400 nm excitation light source, mimicking the back panel blue radiation of light-emitting diodes. A portion of this radiation excited the nano-dot films resulting in the emission of light of a specific wavelength corresponding to the nano-dots emission spectra (Fig. 2a & 2b). We have compared these films with state-of-the-art inorganic quantum dot color conversion layers and found that the organic nano-dots show impressive results in both efficiency and stability (Fig 2c & 2d). With these superior characteristics and eco-friendly replacements for conventional color converting materials, the organic nano-dots have tremendous potential to influence the future of display and white light-emitting devices.

For further information, please read our published article by Khan, Walker, Kwon and coworkers in Nature Communications, 13, Article number: 1801 (2022).

Bright Walker

Associate Professor, Kyung Hee University