Highly Quantum-Yield Enhancement of InP Quantum-Dot Emitting Red-Light Via Potassium Doping for Quantum-Dot Organic-Light-Emitting-Diode Display
Authors: Lee Seungjae , Lee Jieun , Lee Changjin , Park Jeagun 
Published: December 2020

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