Furthermore, GaP QDs with green emission (520 nm) were applied as color conversion materials in the color conversion device with UV LED and blue LED chips. As a result, we obtained the green emission from the color conversion of GaP QD, and the average color conversion efficiency was calculated to be 10~15%. The performance of the devices was still lacking to use materials for single color conversion, but this was enough to confirm the possibility of GaP QDs as next-generation color conversion materials.
Introduction and Motivation
Colloid synthesis is also divided into two parts of methods, heating method and heat injection method (Figure 1.5). The Bawendi group pioneered10 the hot injection method and has developed the method to synthesize different types of semiconductor nanoparticles. In this thesis, an easy hot-injection method of new III-V semiconductor nanoparticles has been developed and the possibility of optical applications using these materials has been proposed.
Colloidal semiconductor nanoparticles (Quantum dots)
Core-shell structure
The three cases of core-shell structure can be defined and demonstrated: type I, type II, and conversely type I (the upper part of Figure 1.7). In type I, the band gap of shell materials is larger than that of core materials and both the electron and hole are confined in the core, so the type I shell is used to passivate the surface of the core for the purpose of improving the optical properties of QDs improve. In type II, the valence band or conduction band edge of shell material is in the band gap of the core.
Because Shell's growth leads to a smaller effective band gap and red shift of the QD emission, this is beneficial for solar cells. In the inverse type I, the band gap of shell materials is smaller than that of core materials, trapping electrons and holes in the shell depending on the thickness of the shell. To prevent nucleation of shell materials and fabricate 1-5 monolayers of shell materials on the surface of the core, the shell growth temperature during the shell growth step is generally lower than the core synthesis temperature.
Additionally, a syringe pump is used during shell growth step to slowly add shell precursors (Figure 1.8). Nowadays, we can synthesize core-shell structure without intermediate purification steps, and spray pump, then we can calculate the required amount of shell precursors to produce suitable shell thickness. Consequently, one of the developed synthesis methods for core-shell structure is SILAR (sequential ion layer . adsorption and reaction) method18 (Figure 1.8).
The research on core-shell structure has been developed in the direction of making more convenient synthesis method for core-shell structure, such as one-pot synthesis.
Cd-free quantum dots
Quantum dots color conversion devices
Discrete color mixing WLEDs
Conventional phosphors such as Y3Al4O12:Ce3+ are widely used in conventional light sources that produce yellowish-white lights as the limit of phosphors. CdSe QDs with a core–shell structure are widely used to fabricate discrete color mixing due to simple core tuning methods (eg: size, composition, etc.). In this application, there are only QD discrete blend WLEDs and organic and inorganic hybrid WLEDs. . In this study, a monolayer of CdSe QDs with an appropriate mixing ratio (red:green:blue = 1:2:10) is deposited in devices by simple spin coating.
Red QDs need a relatively small amount in the QD mixture because it has additional light emission mechanism, unlike blue QDs, which can be reabsorbed by the emission of green, blue, and red QDs. red emission occur. But the optical properties of the QD monolayer were difficult to maintain for a long period due to the high temperature from the LED operation. Some studies have suggested a solution to create QD hybrid layers by mixing organic materials (=charge transfer materials) and QDs of different colors29 (Figure 1.11 (b)).
The important point of the hybrid layers is to make a monodisperse mixture mixing QDs and organic materials that can induce an active energy transfer state. Furthermore, when simple spin coating is used to fabricate QD monolayers with large surface areas, the spin coating can cause phase separation and aggregation of QD30. When the microcontact printing method is used on the devices, the underlying organic layers of the LED can avoid contact with the non-polar solvent.
The key point of this method is to use barriers like SU8 between PDMS and QDs because PDMS has solubility in non-polar solvents such as hexane, chloroform.
Cadmium based QDs color conversion WLEDs
The multi-shell CdSe QDs improved the stability as soon as the bandgap is controlled with a thick shell, so the lifetime of the color converter was extended3 (Figure 1.13(b)). The multishell was synthesized by further heat injection of shell precursors and heating with washed core–shell structure CdSe (green = CdSe@ZnS QDs, red = CdSe@CdS@ZnS QDs) and shell precursors. This paper proved that CdSe QD color conversion WLED can be used as LCD backlight and which can express wider colors than HDTV.
This article was also published by the Samsung Advanced Institute of Technology, which is evidence closer to the commercialization of QD color conversion devices. For example, the researchers exchanged the ligand of CdSe QDs from oleic acid to thiol-containing organosilicate ligand and then mixed with surface-modified QDs and organosilicate materials for matrix34 (Figure 1.13(c)). Also, optical properties and chemical stabilities were improved by these ligand exchange steps in contrast to conventional ligand exchange methods.
The other method to improve the stability of QD color conversion devices is silica coating method. The typical silica coating method is inverse microemulsion method which has been widely used for surface coating of nanoparticles such as metal, metal oxide and QDs. However, microemulsion method usually gives adverse effect for silica coated QDs such as PL intensity decrease.
In order to solve the limitation of classical silica coating, researchers recently published a silicon coating method in a different way.
Cadmium free QDs color conversion WLEDs
A facile method to synthesize group III-V, Gallium phosphide Quantum
Experimental
- Materials
- Preparation of colloidal Gallium phosphide quantum dots
- Characterizations of gallium phosphide quantum dots
- Fabrication of InGaN/GaN blue LED chips
- Fabrication of Remote type GaP QDs color conversion LEDs
- Characterizations of Remote type GaP QDs color conversion LEDs
A typical synthesis of gallium phosphide quantum dots with a PL peak of ~520 nm (=green emission) was as follows. The resulting colloidal solution is cooled to room temperature under a gaseous stream of argon and precipitated with a mixture of ethanol, acetone and butanol. Gallium phosphide QDs with different band gaps can be prepared by controlling the precursor ratio.
Then, Cr and Au were deposited on both the exposed n-type GaN layer and the ITO layer for n- and p-type electrodes, respectively. The electrical and optical properties of both LED samples were measured using the optical spectrum analyzer and the precision. The synthesized GaP QDs in 50 フ hexane mixed with Secure 8110 ultraviolet curing resin (UV resin) by vortexing.
Homogeneous mixtures of GaP QDs and UV-resin were poured into a circular mold, and then the mixtures were cured with UV-light with 365 nm emission for 5 minutes. To record the EL spectrum, we made a customized zig system, as shown in Figure 2.1, and the entire measurement system, which includes an optical cable, was used from an ELT-1000 LED chip tester (Ecopia Corporation). In the characterization steps of the color converters, the LED chips were fully fixed to the board to prevent the chips from moving during the measurement.
After optimizing LED chip and optical cable positions, EL spectrum of color conversion LEDs was recorded while replacing the GaP QDs color conversion film.
Result and discussion
- The synthetic mechanism of GaP QDs
- XRD analysis for gallium phosphide quantum dots
- Optical properties & TEM images of GaP QDs
- Remote type GaP QDs color conversion devices
Furthermore, aggregation occurred easily in the GaP QD solution in a short time due to the lack of surfactant. We performed the synthesis of GaP QDs using different fatty acids ((lauric acid, palmitic acid, stearic acid, myristic acid). While GaP QDs show similar optical properties (absorbance, photoluminescence) in the reaction time from 0 to 60 min.
EL spectra of the remote type color conversion device fabricated with 1 wt% GaP QDs mixture on UV LED is shown in Figure 2.9(a). We can see green emission made by using optimized GaP QDs color conversion device with UV LED in figure 2.9(c). The color conversion efficiency (CCE) of various QDs concentration converters with UV-LED was calculated by reduced UV-LED emission to GaP QDs emission ratio.
To confirm the possibility of GaP QDs as next-generation color conversion WLEDS materials, we also measured the color conversion of GaP QDs at approx. Only green GaP QDs were used in these devices because large parts of the emission peaks overlapped. In this figure, the EL peak of the color conversion produced by the GaP QDs has a very similar shape compared to the PL of the green GaP QD solution.
The high luminous efficiency of the blue LED chip allows us to directly detect the detailed redshift of the EL spectrum created by GaP QDs (Figure 2.12(f)). Through the results of the upper color conversion device, we confirmed that color conversion of GaP QDs occurs in a wide wavelength range (from UV light to blue light) due to the wide absorption range of GaP QDs. Compared with CdSe, CIS QDs color conversion devices have made several examinations, our GaP QDs color conversion devices have some unsatisfactory points.
Integration of CuInS2-based nanocrystals for high efficiency and white light-emitting diodes. Large-scale synthesis of quasi-monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via sequential ionic layer adsorption and reaction. Synthesis of Cu-doped (d-dot) InP nanocrystals with ZnSe diffusion barrier as efficient and color-tunable NIR emitters.
Highly efficient cadmium-free quantum dot light-emitting diodes enabled by direct exciton formation within InP@ZnSeS quantum dots. Remote type white light emitting diode with high color gamut based on InP quantum dot color converters. Color conversion combinations of nanocrystal emitters for the generation of warm white light with high color rendering index.
Highly luminescent and photostable monolith with quantum dots and silicon dioxide and its application in light emitting diodes. Efficient white light emitting diodes constructed from highly fluorescent core/shell copper indium sulfide quantum dots. Stacked silicon dioxide film with embedded quantum dots on phosphor plate for superior white light emitting diode performance.
