Enhancing the Brightness of Cesium Lead Halide Perovskite Quantum Dots Based Green and Red Light-Emitting Devices through Fast Anion-Exchange

Abstract

Semiconductor quantum dots (QDs) have attracted an intensive attention in recent years as an excellent candidate material for the next-generation lighting and display, as well as optical communication technologies, because of their unique optical properties, such as size-dependent emission wavelength, narrow emission spectrum, and high luminescent efficiency.[1] Among the QD materials, in particular, metal halides perovskites, such as hybrid organic–inorganic CH3NH3PbI3, are newcomer optoelectronic material as an active layer in solar cells demonstrating the power conversion efficiency of 20%.[2] For light source or SUHD display applications, it is required a high quantum yield (QY) of higher than 80 % and a narrow full width of half maximum (FWHM) of less than 50 nm. Currently, InP/ZnS QDs have been utilized QD-LCD and QLED although their QY is less than 80 % and FWHM is around 45 nm. In this research, therefore, we synthesized perovskite quantum dots, CsPbBr3-xIX QDs, to overcome the drawback of the InP/ZnS QDs due to high QY, easily tuned emission colors, and high color purity, demonstrated optimized emitting lights of green (520~530nm) and red (630~640nm) by designing the concentration ratio of precursor of PbBr and PbI in CsPbX3(X=Br and I). In order to synthesize CsPbBr3-xIX QDs, ODE (5 mL) and PbI2 (0.087g), PbBr2 (0.069g) were loaded into 25- mL 3-neck flask and dried under vacuum for 1 h at 120 ºC. Dried oleylamine (0.5 mL) and OA (0.5 mL) were injected into the flask at 120 ºC under N2. After being solubilized a PbX2salt completely, the temperature in the flask was raised to 165 ºC for tuning the QD size and then Cs-oleate solution (0.4 mL, 0.125 M in ODE) was quickly injected. The reaction mixture was cooled down to the room temperature by the ice-water bath. The crude solution was separated by centrifuging. The synthesized QDs were centrifuged at 0 ºC. After centrifugation, the supernatant was discarded and the particles were redispersed in toluene. It was confirmed that the bright photoluminescence was tuned over the entire visible spectral region (500~700 nm), because all the anion exchange reactions led either to a blue shift, or to a red shift in the optical properties, resulted in the quantum yields of 10−80% and FWHM of 10−40 nm (from green to red), as shown in Fig. 1. In addition, it was confirmed that the synthesized QDs were square type in shape, well-crystallized, and the mean QD sizes varying with a composition ratio of Br and I precursor were from 10 to 12 nm, which probably led to the higher QY and narrow FWHM, as shown in Fig. 2 (a)-(e). In particular, interestingly, from the result of XRD shown in Fig. 2(f), it was confirmed that the anion-exchange reactions of the halide ions did not seem to affect the cationic sub-lattice, and that the cubic perovskite crystal structure was maintained although the temperature of the anion-exchange reaction was low. Finally, we have demonstrated that a CsPbX3 perovskite QDs was synthesized fast and simple anion exchange reaction. This result indicates that CsPbX3perovskite QDs are very promising core material for high resolution display application. Reference [1] H. B. Shen et al., Nano let. 2015, 15, 1211 [2] Y. Zhao et al., Chem. Soc. Rev., 2016, 45, 655-689. Acknowledgement * This work was financially supported by the IT R&D program of LG display [Cd-free green & red emitting core/shell QDs development through interface engineering design] and the Brain Korea 21 plus Project in 2014, Korea. Figure 1