During the past two decades, huge efforts have been devoted to various QDs as the light-emitting layer of QLEDs (Coe et al., 2002; Sun et al., 2007; Kwak et al., 2012; Seth and Samanta, 2016; Chen et al., 2017). Recently, perovskite QDs have attracted great attention due to their excellent optoelectronic performance, due to their tunable bandgap, high absorbance coefficient, and high photoluminescence quantum yield (PLQY) (Protesescu et al., 2015; Song et al., 2015; Tang et al., 2016; Heejae et al., 2017). However, the electroluminescence (EL) EQE of halide perovskite QLEDs is relatively lower to that of CdSe based QLEDs, which seriously limits further commercial application (Shirasaki et al., 2013; Dai et al., 2014; Wang et al., 2018). It is known that the photoluminescence (PL) performance of the QDs emitting layer plays a crucial role in QLED. Therefore, how to synthesize QDs with high quality is one critical step for preparing high performance QLEDs. It was recognized that the formation of core/shell structure between semiconductors is one effective approach to improve the performance of nanocrystals and QLEDs devices (Hines and Guyot-Sionnest, 1996; Li et al., 2011). Many research groups have reported that the non-radiative Auger recombination of CdSe QDs could be efficiently suppressed by being coated with larger bandgap materials such as CdS and ZnS, where better optical properties have been achieved (Chen et al., 2008; Bae et al., 2013; Efros and Nesbitt, 2016). However, few studies on the core/shell structure for perovskite materials have been reported. Therefore, it is still a challenge to investigate a perovskite-based core/shell structure and the corresponding QLEDs.

Herein, we synthesized novel colloidal perovskite core/shell QDs by covering CsPbBr₃ QDs with a CdS shell. Moreover, the inverted QLEDs (ITO/ZnO:Mg/QDs/CBP(4,4′-Bis(N-carbazolyl)-1,1′-biphenyl)/MoO₃/Al) based on CsPbBr₃/CdS core/shell QDs and pure CsPbBr₃ QDs were fabricated, respectively. The CsPbBr₃/CdS QDs based QLED exhibited the maximum luminance of 354 cd/m² with a CE of 0.3 cd/A and the best EQE of 0.4% was 5.4 times of the pure CsPbBr₃ QDs based QLEDs.

Based on the continuous injection method, a novel approach to overcome a non-radiative recombination of CsPbBr₃ QDs was developed by growing a thin CdS shell on the surface of the CsPbBr₃ core. Figure 1A shows the simulation of the CsPbBr₃/CdS core/shell structure. The X-ray diffraction (XRD) patterns of the CsPbBr₃/CdS core/shell QDs clearly show that two types of crystalline structures were formed (Figure 1B). Compared to the pure CsPbBr₃ QDs (Figure S2), the extra peaks of the CsPbBr₃/CdS XRD patterns are fairly consistent with the standard XRD patterns of a CdS-liked zinc blende structure. From the transmission electron microscope (TEM) images of CsPbBr₃ (Figure S1b) and CsPbBr₃/CdS QDs (Figure 1C), it can be seen that the point angle of the cubic shape become a circular arc. The high resolution TEM image of CsPbBr₃ (the inset of Figure S1b) and CsPbBr₃/CdS QDs (the inset of Figure 1C) clearly exhibit the crystal lattice (110) of the CsPbBr₃ nanoparticles and further verify the CsPbBr₃/CdS core/shell structure. In addition, the CsPbBr₃/CdS QDs exhibited a typical cubic shape with a larger average size (22.1 nm) than that of the CsPbBr₃ QDs (12.7 nm; Figure S1). To further demonstrate the successful formation of the CsPbBr₃ core and CdS shell, the mapping analysis and energy dispersive X-ray (EDX) spectroscopy of CsPbBr₃/CdS QDs was employed (Figures 1D,E) with the S and Cd elements uniformly distributed on the surface of the CsPbBr₃ core. Moreover, the PL spectra of CsPbBr₃/CdS QDs has a slightly blueshift (from 519 to 514 nm) compared with pure CsPbBr₃ QD (Figure 1F), which was probably induced by the CdS shell. A similar phenomenon was found and ascribed for the cationic inter-diffusion at the interface between the CsPbBr₃ core and CdS shell (Oladeji and Chow, 2005; Li et al., 2011; Kwak et al., 2015). On the other side, the CsPbBr₃/CdS showed a high photoluminescence quantum yield (PLQY) of 88% (Figure S3) and the full width at half-maximum (FWHM) was as narrow as 19 nm, which is similar to pure CsPbBr₃ QDs.

As shown in Figure 2A, the inverted QLEDs structure was fabricated with a sandwich structure of ITO (In₂O₃-SnO₂)/ZnO:Mg/QDs/CBP/MoO₃/Al, which was constituted by the transparent electrode ITO as a cathode, Mg doped ZnO as an electron transport layer, perovskite QDs as the light-emitting layer, CBP as a hole transport layer, Al as an anode, and MoO₃ as a hole injection layer. The flat-band energy levels of devices are shown in Figures 2B,C. After perovskite QDs were coated on the ZnO:Mg electron transport layer, the perovskite film was still soluble in organic solvents due to the presence of aliphatic ligands on the QDs. The structure above the light emitting layer was prepared by evaporation (Materials and Methods section).

As shown in Figures 2D,E, the peaks of the QLEDs EL spectra based on CsPbBr₃ QDs and CsPbBr₃/CdS QDs were consistent with the PL spectra, and the luminescence intensity increased with the voltage increase. The EL of pure CsPbBr₃ QDs based LEDs (Figure 2D), peaking at 519 nm with a FWHM of ~20 nm, was observed. And there is no emission peak from the hole/electron transport layer. The CsPbBr₃/CdS QDs based QLEDs also performed with a peak at 516 nm with a FWHM of ~20 nm. As depicted in Figure 2E, the emission from the hole/electron transport layer was hardly shown. The insets showed an operating device driven at 4 V, and the devices both exhibited a saturated and pure green color.

For the potential application in a light emission device, the characteristic of emitter layers is extremely important. For the inverted QLEDs with the CsPbBr₃ QDs emitting layer, the pure CsPbBr₃ QDs worked with poorer luminous characteristics, exhibiting a lower maximum luminance intensity of 65 cd/m² and the maximum CE of 0.14 cd/A (Figures 3A,B). It is worth mentioning that a maximum luminance of 354 cd/m² was obtained when the CsPbBr₃/CdS QDs were used as the emitting layer, and the maximum CE was boosted to 0.3 cd/A (Figures 3C,D). The overall performance of two kinds of devices were also described with the average EQE. The EQE of QLEDs with the CsPbBr₃ QDs emitting layer (0.4%) was 5.4 times larger than the pure CsPbBr₃ QDs (0.07%). The results are tabulated in Table 1 for comparison. Based on the same device structure with a different emitting layer, the turn-on voltage of CsPbBr₃/CdS based QLEDs was slightly lower than CsPbBr₃ based QLEDs. Moreover, the maximum CE of the CsPbBr₃/CdS based QLEDs was 2.1 times larger than the pure CsPbBr₃ based QLEDs, which could be ascribed to the valence bands (conduction bands) of the CdS shell which are lower (or higher) than that of CsPbBr₃ core, and a non-radiative Auger recombination of CsPbBr₃ was effectively suppressed (Park et al., 2014; Nasilowski et al., 2015; Niu et al., 2017). Additionally, the probable reason could be attributed to the fact that the extra carriers cannot escape from the surface of the QDs as the protecting layer of CdS, which leads to a higher radiative recombination rate, higher luminance and EQE.

In conclusion, we have developed novel all-inorganic core/shell perovskite QDs and applied it as an emitting layer in inverted QLEDs (ITO/ZnO:Mg/QDs/CBP/MoO₃/Al). The results show that the fabricated CsPbBr₃/CdS QDs based QLEDs exhibited enhanced performance compared with pure CsPbBr₃ QDs based QLEDs. The EQE of CsPbBr₃/CdS QDs was 5.4 times higher than that of pure CsPbBr₃ QDs, which demonstrates that the introduction of a CdS shell can increase the optoelectronic performance. The core/shell structure perovskite QDs present a new route of perovskite materials for light emission applications.

All datasets generated for this study are included in the manuscript and/or the Supplementary Files.

XT and JY contributed equally to this work. XT and JQ designed the experiment. JY and SL conducted the experiments and characterization. XT and JQ wrote and revised the paper. JY, WC, and ZH participated in the discussion. XT funded some of the subject experiments.