High Stable Perovskite-Quantum-Dot Using Ligand Engineering for Liquid-Crystals-Display Applications
Authors: Lee Ji-Eun , Lee Seung-Jae , Ko Yun-Hyuk , Prem Prabhakaran , Lee Kwang-Sup , Park Jea-Gun 
Published: February 2020

In recent years, semiconducting metal halide perovskite quantum dots have become attractive for use in many applications such as photovoltaics, photodetectors, light emitting diodes (LEDs), and lasers due to their outstanding electrical and optical properties.1 Particularly, metal halide perovskite materials are suitable for application in white LEDs (WLEDs) and displays as well as solar energy conversion materials due to their wide color tunability (400-800 nm), narrow photo-luminescence full width at half maximum (PL-FWHM) (~20 nm), and high photoluminescence quantum yields (PLQYs) (>80%), as well as their low temperature and cost-effective synthesis process, compared with inorganic luminescence materials such as phosphors.2,3

However, the instability of perovskite QDs has always been considered as the main hindrance significantly deteriorating the reliability and stability of their optoelectronic device and potential future applications.4 To overcome this issue of the instability of perovskite QDs, a few number of research work embedding QDs in organic polymer matrices, e.g., poly(methyl methacrylate) (PMMA), has been conducted to enable good processability and compatibility with typical packaging approaches.5 However, they exhibited limited stability improvement under harsh conditions like ultraviolet light excitation.6 Because of PMMA has a relatively high oxygen diffusion coefficient of 3.3 × 10–9 cm2 s−1 at 22 °C,7 it cannot completely prevent the photo-oxidation of QDs.
Here, we proposed an eﬃcient and simple method to improve the stability of perovskite QDs using effective ligand engineering. This proposed ligand Engineering was applied during the synthesis of perovskite QDs to modify and passivate their surface using organic ligand materials such as perfluorooctanoic acid (PFOA) as shown in Fig 1. PFOA belongs to fluorine functional group. The fluorine group is considered as promising ligand materials for perovskite QDs due to its outstanding optical properties and high stability against environmental and chemical factors.
In order to synthesize CsPbBr3-XIX QDs capping with PFOA ligand, 5 mL of octadecene (ODE) and 0.188 mmol of PbX2 (such as PbI2 and PbBr2) were loaded into 25 mL 3-neck flask and then dried under vacuum for 1h at 120 ºC. 0.5 mL of dried oleylamine (OAm) and different ratios of oleic acid (OA) to PFOA (OA:PFOA). The selected OA:PFOA ratios are 10:0, 6:4, 5:5, 4:6, and 3:7. The mixture of OA:PFOA was injected at 120 ºC under N2 gas ambient. After solutionization of a PbX2 salt in ODE, the temperature raised to 165 ºC then Cs-oleate solution (0.4 mL, 0.125 M in ODE) was quickly injected and maintained for 5 sec. later, the reaction mixture was cooled using an ice-water bath. The final crude solution was purified by centrifuging process. For smaller QDs, the synthesis process should be carried out at a reaction temperature of below 160 ºC. A centrifugation of below 0 ºC or/and an addition of tert-butanol to the crude solution (ODE:tBuOH=1:1 by volume) were found to be helpful to achieve a complete precipitation.of QDs. After centrifugation, the supernatant discarded and the precipitated particles were redispersed in toluene.
Both perovskite CsPbBr2I1 QDs capped and uncapped with PFOA ligand showed a cubical-like shape, high crystallinity, and well dispersity, as shown in HR-TEM images of Figs. 2(a) and (b). In addition, the diameters of perovskite CsPbBr2I1 QDs capped and uncapped with PFOA ligand were 10.2±2.39 and 11.5±2.61, respectively.
The PFOA capped perovskite CsPbBr2I1 QDs showed a higher absorption in visible region (400-800 nm) with increasing the mole fraction of PFOA in comparing to uncapped QDs, and PL peaks shifted to longer wavelengths (from 534 to 551 nm) due to the Ostwald ripening, as shown in Figs 3(a) and (b). The QDs had a broader PL-FWHM ranging from 24.4 to 35.1 nm and PL intensity ranging from 32,000 to 15,000 a.u. by increasing the mole fraction of PFOA as shown clearly in Figure 3 (c). However, It is worth to note that the PL intensity of perovskite CsPbBr2I1 QDs capped with PFOA ligand decreased 15 % after 7 days, while uncapped QDs decreased 57 % after the same period, as shown in Fig 3 (d). Finally, high stability of perovskite QDs capped with PFOA ligands can be a promising candidate material for QD-based display applications such as QD-LCD.

Figure 1