Abstract

Quantum dots (QD) have been researched intensively among several luminescent materials because they can simply adjust wavelength, high efficiency, high stability, and high color purity properties. In particular, eco-friendly InP based core-shell QDs without environmental regulations (i.e., restriction of hazardous substances in Europe) has been a great of attention.1 However, it is difficult to synthesize InP-based core-shell QDs, which requires a high temperature, a long reaction time, and a high reactivity precursor since the InP-based core-shell QDs need a strong covalent bonds, degrading quantum yield(QY)2. For this reason, many defects in QDs are generated during a InP core growth. Among the blue(B)-, green(G)-, and red(R)-light emitting QDs, especially R-light emitting InP based core-shell QDs contain numerous crystalline defects since the core size should be increased to reduce the energy band gap. In our study, we enhanced highly QY of R-light emitting InP based core-shell QDs(R-QDs) by doping potassium ions via injecting potassium ion precursor such as potassium iodide during a InP core growth. We characterized the mechanism why potassium ion doping highly enhanced QY by observing the crystallinity of core-shell QD and electron paramagnetic resonance(EPR) analysis.In general, the R-QDs contain vacancy defect in the InP core-QDs which emit the red-light with ~625-nm in wavelength, degrading QY by lattice scattering, as shown in Fig. 1(a). These vacancy defects induced lattice-scattering and cause a decrease of QY. Here, the vacancy defects in the InP core-QDs would be passivated by doping potassium ions via using potassium iodide as a dopant, as shown in Fig. 1(b). In the InP core-QDs, the mole fraction of potassium ions to indium ions linearly increased with the potassium iodide concentration, measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), as shown in Fig. 1(c). Then, the InP core/ZnSe inter-shell/ZnSeS inter-shell/ZnS oueter-shell QDs (i.e.; InP core-shell QDs) were synthesized by using a precipitation method. The QY of the InP core-shell QDs peaked at a specific potassium iodide concentration; i.e., 91% at the potassium iodide concentration of 3%, as shown in Fig. 1(d). Otherwise, both full-width at half maximum (FWHM) and the wavelength emitting red-light gradually increased with the potassium iodide concentration.To understand the dependency of QY on the potassium iodide concentration, the crystallinity of InP core-shell QDs were investigated as a function of the potassium iodide concentration during growing the InP core-QDs, where in four different crystalline plane peaks were found; (unknown), (111), (220), and (311), as shown in Fig. 1(e). Particularly, the crystalline peak intensity of (unknown) rapidly increased with increasing QY, indicating the doping of potassium ions during growing the InP core-QDs passivated vacancy defects so that it suppressed an un-proper growth along unknown crystalline direction, as shown in Fig. 1(e). In addition, the exciton lifetime exponentially increased with QY, measured by time-resolved photoluminescence (TRPL), meaning that the passivation of vacancy defects with potassium ions enhanced the exciton lifetime of the InP core-shell QDs. It was found that there was a good correlation between QY and the crystalline peak intensity of (unknown) or the exciton lifetime, implying that the passivation of vacancy defects in the InP core-QDs by doping potassium iodide ions would enhance the QY of the InP core-shell QDs, as shown in Fig.1(f). Finally, the international commission on illumination (CIE) 1931 color spaces of a QD OLED display using R-, G-, and B-QD functional color filters, as shown in Fig.1(g), achieved 121.7% (NTSC) and 91.1% (Rec. 2020), as shown in Fig. 1(h) inset figure. Reference [1] Tamang, S.; Lincheneau, C.; Hermans, Y.; Jeong, S.; Reiss, P.; (2016). Chemistry of InP Nanocrystal Syntheses. Chem. Mater. 28, 2491−2506[2] Jang, E.; Kim, Y.; Won, Y.; Jang, H.; Choi, S.; (2020). Environmentally Friendly InP-Based Quantum Dots for Efficient Wide Color Gamut Displays. ACS Energy Lett. 5, 1316−1327 Figure 1

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