Room-temperature synthesis of excellent-performance CsPb1-xSnxBr3 perovskite quantum dots and application in light emitting diodes

Materials & Design 185 (2020) 108246

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Room-temperature synthesis of excellent-performance CsPb1-xSnxBr3 perovskite quantum dots and application in light emitting diodes Jidong Deng a, Haoran Wang b, Jiao Xun a, Jingxi Wang a, Xuyong Yang b, Wei Shen a, Ming Li a, Rongxing He a, * a

Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai, 200072, China

b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The Sn2þ doped CsPb1-xSnxBr3 perovskite quantum dots were synthesized at room temperature rather than traditional hot injection.
The CsPb0.9Sn0.1Br3 perovskite quantum dots obtain high PLQY and excellent stability.
The CsPb0.9Sn0.1Br3 quantum dots synthesized at room temperature are potential light emitting diode materials.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 July 2019 Received in revised form 27 September 2019 Accepted 30 September 2019 Available online 10 October 2019

The traditional hot injection (HI) process needs high temperature, inert gas protection, and localized injection operation, which severely hinder their large-scale industrialization. Moreover, the CsPb1xSnxBr3 HI-QDs exhibit poor stability. Herein, we report the room-temperature (RT) synthesis of CsPb1xSnxBr3 perovskite QDs by modified ligand-assisted reprecipitation (LARP) approach. Compared with the CsPb1-xSnxBr3 HI-QDs reported in literatures, the CsPb1-xSnxBr3 RT-QDs show higher photoluminescence quantum yield (PLQY) and better stability: the CsPb0.9Sn0.1Br3 RT-QDs obtain the highest PLQY of more than 91%, and the stability of the film made with this QDs still maintain more than 80% of its original fluorescence strength after 120 days in air environment. Because of the superior PLQY, light-emitting diodes (LEDs) based on the RT-QDs is constructed, and it exhibits an external quantum efficiency (EQE) of 1.8%, a luminance of 1600 cdm-2, a current efficiency of 4.89 cdA-1, a power efficiency of 6.41 lmw1, and a low on-voltage of 3.6 V. The present work provides a feasible method for large-scale industrial synthesis of perovskite QDs at room temperature and shows that the CsPb1-xSnxBr3 RT-QDs are promising for highly efficient LEDs. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Keywords: Room-temperature synthesis CsPb1-xSnxBr3 perovskite quantum dots High quantum yield Outstanding stability Light-emitting diodes

1. Introduction

* Corresponding author. E-mail address: [email protected] (R. He).

In recent years, all inorganic perovskite quantum dots (QDs) (CsPbX3, X ¼ Cl, Br, I) have attracted significant attention [1e3] due to their superior performance in optoelectronic devices such as light-emitting diodes (LEDs) [4], lasers [5e7], photodetectors [8,9]

https://doi.org/10.1016/j.matdes.2019.108246 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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solar cells [10e15] and Soft-X-Ray detectors [16]. In particular, all inorganic CsPbBr3 perovskite QDs synthesized by Kovalenko and co-workers [17], and Li and co-workers [18] were extensively studied because they exhibit ultrahigh photoluminescence quantum yield (PLQY) and low threshold lasing, making them potential emitters for electroluminescent displays. However, the toxicity of heavy metal lead in CsPbBr3 perovskite raises great concerns about the environmental pollution. Thus, Jellicoe et al. developed CsSnBr3 perovskite QDs using the hot injection (HI) method [19]. Unfortunately, the existence of divalent tin makes CsSnBr3 perovskite QDs extremely unstable due to Sn2þ ions are easily oxidized to Sn4þ ones, which results in low PLQY. On the other hand, the stable Cs2SnI6 perovskite QDs prepared by Wang et al. also exhibit a low PLQY owing to the poor ion conductivity of Sn [20,21]. In fact, in the halide perovskite family, divalent lead plays an important role in stabilizing perovskite and providing matching energy levels, thus they display the outstanding optoelectronic performance. In this sense, the partial substitution of lead with other ions is a good strategy to obtain stable and high PLQY perovskite QDs. In 2016, Zhang and his co-workers synthesized CsPb1-xSnxBr3 perovskite QDs by partial replacement of Pb2þ with Sn2þ using the HI method [22]. They considered that the partial lead substitution not only reduces the toxicity of material, but also improves the device performance of LEDs. Note that with the increase of Sn content x, the PLQY of the QDs decreases rapidly, which is mainly ascribed to the unstable Sn oxidation in air. Recently, Wang and coworkers fabricated CsPb1-xSnxBr3 perovskite QDs by partially replacing Pb2þ with highly unstable Sn2þ [23]. For the Sn2þ substituted QDs with the best ratio of x ¼ 0.33 (CsPb0.67Sn0.33Br3 QDs), the absolute PLQY is as high as 83% because that a small number of Sn2þ doping effectively suppresses the formation of trions. However, the above mentioned CsPb1-xSnxBr3 perovskite QDs were prepared using the HI method, which inevitably needs high temperature and inert atmosphere, and thus leads to high cost and limited output. Therefore, it is necessary to develop milder methods of synthesizing perovskite QDs. Although CsPbX3 (X ¼ ClxBryI1xy, 0  x, y  1) perovskite QDs have been synthesized at room temperature [24], the Sn2þ doped CsPb1-xSnxBr3 QDs fabricated by means of the RT method are still not reported. The most probable reason should be that Sn2þ is easily oxidized to Sn4þ at room temperature in air. In this work, we present a facile RT method to synthesize CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.0e0.9) with Sn2þ replacement based on mixed metal cation. Our experiments demonstrate that the RT-QDs of CsPb12þ substitution exhibit significantly improved PLQY xSnxBr3 with Sn compared with the HI analogues. When the relative amount of Sn is about 10% (x ¼ 0.1), the CsPb0.9Sn0.1Br3 RT-QDs achieve the highest PLQY of more than 91%, which is higher than that of CsPb1-xSnxBr3 HI-QDs respectively replaced with Sn2þ and Sn4þ ions. Moreover, the film based on this QDs shows extremely high stability because it keeps more than 80% of its original fluorescence strength after 120 days of exposure to atmosphere. The RT-QDs of CsPb1-xSnxBr3 are employed as light emitter in the LEDs. The device based CsPb0.9Sn0.1Br3 RT-QDs exhibits encouraging performance. Partial lead replacement at room temperature reduces manufacturing costs and ultimately improves device performance, which provides a potential method and opens up a world of new optoelectronic materials for low-cost and low-toxic perovskite QDs LEDs. 2. Experimental section 2.1. Chemicals and reagents All materials were used as received and used without purification, unless otherwise noted. Cesium bromide (CsBr, Aladdin,

99.99%), lead (II) bromide (PbBr2, Alfa Aesar, 99.999%), Tin(II) Bromide (SnBr2, Alfa, 99.2%), oleic acid (OA, Alfa) Aesar, tech, 90%), oleylamine (OAm, Acros, approximate C18 content 80e90%), toluene (anhydrous 99.8%), hexane (analytical reagent 97%). PEDOT: PSS (Clevious PVP AI 4083) was purchased from Heraeus, dimethylsulfoxide (DMSO, 99.8%) and N, N-dimethylformamide (DMF, 99.9%) were purchased from Sigma-Aldrich. PVK, TPBI, and LiF were purchased from Luminescence Technology, respectively. 2.2. Synthesis of CsPb1-xSnxBr3 QDs The schematic diagram of synthesis processes are provided in the Supporting Information (Fig. S1). The synthesis of CsPbBr3 perovskite QDs was carried out by modified ligand-assisted reprecipitation (LARP) approach [25], through injection of 0.2 mL of precursor mixture (which we denote by a generic term “precursor” further on) into a “bad solvent” toluene (10 mL) under vigorous stirring (Fig. S1). The precursor was prepared by mixing 0.2 mmol CsBr, 0.2 mmol PbBr2, 0.05 mL OAm, and 0.1 mL OA in “good solvent” DMF (1 mL) and DMSO (1 mL), respectively (see Fig. S1). The perovskite QDs solution was centrifuged (8000 rpm, 5 min), then the precipitate was washed by toluene/ethylacetate solution (1:3 in volume ratio). After that, the solution was centrifuged at 5000 rpm for 5 min. Finally, the QDs were dried for 12 h in vacuum drying box, and then the obtained powders were used to characterize its physical properties by the XRD, XPS, etc., such as the structure and morphology. The optical properties (PL, PLQY) were characterized by re-dissolving the powders into n-hexane to form colloids for further characterization. Varied Sn content was controlled by changing the concentration of SnBr2 in the precursor to obtain the CsPb1-xSnxBr3 QDs (x ¼ 0.1, 0.2, 0.3, 0.5, 0.7 0.9). In order to ensure good reproducibility, the CsPb1-xSnxBr3 QDs were synthesized repeatedly. All samples were synthesized at room temperature with a range of 20e25  C (outdoor temperatures range from ~35  C in summer to ~10  C in winter, but the air-conditioning was used in laboratory). 3. Results and discussion The as-prepared inorganic perovskite QDs with mixed-metal cations were investigated by the powder XRD and transmission electronic microscopy (TEM) and the results were shown in Fig. 1. As shown in Fig. 1g, the XRD patterns show that the CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.0e0.1) have cubic structure, which is in good agreement with that of cubic phase PDF#54-0752 reported by the previous works [26,27]. However, the shape of the QDs (x ¼ 0.3e0.9) become irregular with increasing Sn2þ concentration. Further, it can be found that the CsPb1-xSnxBr3 QDs (x ¼ 0.1e0.9) have the main XRD characteristic peaks of CsPbBr3 perovskite QDs, indicating that Sn doping can basically maintain the framework of CsPbBr3. However, many impurity p- eaks appear with the increase of Sn content (x > 0.1), which is due to the formation of Cs2SnBr6 phase (its peaks were assigned by red triangle in Fig. 1g). Especially, when x ¼ 0.9, a large number of Cs2SnBr6 phase is formed, which seriously destroyed the original Pb-based perovskite skeleton and thus reduced the stability. Therefore, the corresponding characteristic peaks in XRD patterns are weakened (its peaks were assigned by blue pentacle in Fig. 1g). Recently, Liu synthesized the Sn-doped CsPbBr3 QDs by the traditional HI method and obtained similar results [23]. Besides, the synthesis of pure perovskite QDs CsSnBr3 at room temperature has not been reported. Even though the CsSnBr3 QDs have been synthesized by the traditional HI, they are very unstable because Sn2þ is easily oxidized, resulting in a serious decline in PL efficiency to almost undetectable value (<0.01%) [19,28]. Therefore, the range of Sn content x in this work is

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Fig. 1. TEM images of CsPb1-xSnxBr3 perovskite QDs with (a) x ¼ 0.0, (b) x ¼ 0.1, (c) x ¼ 0.3, (d) x ¼ 0.5, (e) x ¼ 0.7, and (f) x ¼ 0.9. (g) The XRD patterns of CsPb1-xSnxBr3 perovskite powders as a function of Sn content x.

0.0e0.9. For better understanding this change, TEM images of the CsPb1-xSnxBr3 QDs are shown in Fig. 1aef for comparison. As given in Fig. S2, the x value of Sn element actually incorporated in the CsPb1-xSnxBr3 perovskite QDs were determined by the energy dispersive X-ray spectroscopy (EDX) in the scanning electron microscope (SEM). Further, the molar ratio of Pb to Sn was determined more accurately by the inductively coupled plasma-optical emission spectrometer (ICP-OES) (see Table S1). Our results shown that the proportion of elements given by EDX is consistent with that by ICP-OES, indicating the determined element ratio in the asprepared perovskite QDs is reliable. The intensities of PL spectra of CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.0e0.9) with different amounts of Sn are shown in Fig. 2b. When the relative amount of Sn is more than 10%, the PL intensity begins to decrease (that is, the luminescence intensity of the CsPb0.9Sn0.1Br3 QDs is the highest under the same conditions). This

indicates that a small amount of Sn doping can promote exciton recombination through radiative pathway, thereby improving optical performance. However, the PL strength decreases rapidly with the increase of Sn content. This is because that the introduction of excessive amount of Sn generates a large number of defect states with oxidation. These defect states lead to more non-radiative recombination centers, which will provide additional nonradiative decay paths for Sn2þ doped QDs. The absorption and emission spectra of CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.0e0.9) are shown in Fig. 2c. There is no significant change in the absorption peak positions, which indicates that their band gaps are almost the same due to the fact that the band gap is defined by the Pb 6s-Br 4p hybridized orbitals and the Pb 6p orbitals in the perovskite QDs [29,30]. For the emission spectra shown in Fig. 2c, with the increase of the amount of doped Sn (the value of x from 0.0 to 0.9), the emission peaks of CsPb1-xSnxBr3 QDs shift slightly towards the blue

Fig. 2. (a) Photographs of CsPb1-xSnxBr3 perovskite QDs in hexane solution under 365 nm UV lamp illumination. (b) Intensities of the PL spectra of CsPb1-xSnxBr3 perovskite QDs. (c) The steady-state absorption and PL spectra of CsPb1-xSnxBr3 perovskite QDs.

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band, which is consistent with the previous experimental results reported by Liu et al. [23]. To understand the kinetics of excitons and free carriers in mixed metal cation CsPb1-xSnxBr3 perovskite QDs, we performed the timecorrelated single-photon counting (TCSPC) measurements. Timeresolved PL decay curves of the CsPb1-xSnxBr3 perovskite QDs synthesized with different amounts of Sn (x ¼ 0.0e0.9) are given in Fig. 3, and the fitting average lifetimes (tave) are provided in Table S2. As a whole, the time-resolved PL decays of the CsPb1xSnxBr3 QDs (x ¼ 0.0e0.9) show average lifetime in the range of 0.62e14.00 ns with the change of Sn content. To be specific, the exciton lifetimes of CsPbBr3 QDs and CsPb0.9Sn0.1Br3 QDs are 8.89 ns and 14.00 ns, respectively. The increase in lifetime implies that small amount of Sn2þ substitution reduces the density of nonradiative recombination centers. Ultimately, the quality of the sample was improved. Furthermore, from Fig. 3 and Table S2, we found that the PL lifetimes of the CsPb1-xSnxBr3 QDs (x ¼ 0.3e0.9) decrease rapidly with the increase of doping Sn2þ, which can be explained as the formation of much more defect states of nonradiative recombination centers produced by excessive amount of Sn2þ. These results are in good agreement with the analyses of the PL strength and XRD patterns (see Fig. 2). In order to determine whether the Sn doping amount of x ¼ 0.1 is optimal, we measured the PL lifetime of CsPb1-xSnxBr3 QDs with x ¼ 0.2 (see Fig. S3). The results shown that the doping amount of 0.1 is the best. The absolute PLQY of the CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.0e0.9) was also measured using a calibrated integrating sphere and the results are listed in Table S3. It is found that the PLQY changing trend of CsPb1-xSnxBr3 QDs are consistent with the variation of the average lifetime and radiative one. Among these CsPb1-xSnxBr3 QDs, the absolute PLQY of CsPb0.9Sn0.1Br3 perovskite QDs is up to 91.9%, which is much higher than that of CsPb1-xSnxBr3 HI-QDs [23]. This may be attributed to the fact that a small amount of Sn doping into the CsPbX3 perovskite QDs lattice can improve the order of local coordination environment of lead and enhance the short-range order of the lattice without introducing a new recombination channel, which is conducive to radiation recombination. However, for other CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.3e0.9), the lifetime and absolute PLQY decreased with the increase of Sn doping content. The decreased lifetime implies that the excessive Sn cation could increase emission decay of free exciton caused by

Fig. 3. PL lifetime of mixed-metal cation based CsPb1-xSnxBr3 perovskite QDs (x ¼ 0.0e0.9).

nonradiative energy transfer to the trap states. The excessive Sn2þ ions could act as additional nonradiative relaxation channels, leading to a reduction of the radiative decay. Further, as reported by the previous literatures, the fast-decaying luminescence of Snbased perovskite originates from band-edge states and shallow states induced by the intrinsic defects sites [31,32]. In addition, the easy oxidation of Sn2þ to Sn4þ results in the much higher defect state densities (~1017 cm3) than that of the perovskites containing only lead [33,34]. The chemical composition and valence states of each element in CsPb0.9Sn0.1Br3 perovskite QDs was verified by the X-ray photoelectron spectroscopy (XPS) (Fig. S4). The XPS results show that the elements in the CsPb0.9Sn0.1Br3 perovskite QDs remain their original valence states, and Sn should be þ2 valence (see Fig. S4). In order to study the excited state dynamics of CsPb0.9Sn0.1Br3 perovskite QDs, the ultrafast femtosecond transient absorption (fsTA) spectroscopy, which can provide detailed radiative and nonradiative processes of excited states, was performed. Fig. 4a shows the decay associated spectra (DAS) and retrieved decay times according to the global fitting. Obviously, TA dynamics were decomposed into three components, the ultrafast 2 ps component, middle 228 ps component and ultra-long-lifetime 1 ns component. This indicates that there are three main decay processes for the excited carriers of materials [35]. Usually, Auger recombination process can be presented as a ultrafast decay component under high pump intensity [36]. Therefore, the observed ultrafast component (2 ps) should be assigned as a combination of Auger recombination and charge transfer from the excited state to the trapping state (>20 ps). For the 228 ps component, it is considered to be the lifetime of typical electron-hole pair intrinsic radiative decay in semiconductor materials [37e40]. As mentioned above, the PL lifetime of CsPb0.9Sn0.1Br3 perovskite QDs is the longest, which is attributed to the fact that a suitable amount of tin doping greatly improves the lattice and reduces the density of defect states. The ultra-longlifetime component (>1 ns) features a negative tail below the band gap energy toward 650 nm, which indicates the existence of sub-band gap states transition proved by Wu and Zheng [41,42]. That is to say, the ultra-long-lifetime component can be attributed to excitonic trapping states associated decay pathways. Combining the above results we can formulate the excited state dynamics model of CsPb0.9Sn0.1Br3 perovskite QDs (Fig. 4b), which includes Auger recombination and excited state charge transfer (2e30 ps), intrinsic radiative (228 ps), and the decay of the long-lived trapping states (>1 ns). The high PLQY of the CsPb0.9Sn0.1Br3 RT-QDs can be attributed to the fact that the Sn2þ doping improves the QDs lattice and partially passivates the trap states. The stability of perovskite QDs is critical for their practical applications. The room temperature fluorescence intensity (lem ¼ 519 nm) of CsPb0.9Sn0.1Br3 perovskite QDs as a function of time are shown in Fig. 5. As can be seen from Fig. 5a, the PL position and shape of CsPb0.9Sn0.1Br3 perovskite QDs in solution did not change significantly after 120 days exposure to air at room temperature, indicating that these QDs have outstanding stability under ambient conditions. From the quantitative variation of fluorescence intensity (Fig. 5b), one can see that the PL intensity can be maintained above 90% of its original value after 120 days. This is far superior to other reported CsPb1-xSnxBr3 HI-QDs [22,23]. Further, we tested the moisture stability of film made with this RTQDs. The result shows that the film can maintain more than 80% of its initial FL intensity after 120 days exposure to air (see Fig. S4). Obviously, the RT synthesized CsPb0.9Sn0.1Br3 perovskite QDs exhibit outstanding stability in solution and film in ambient air. In view of the excellent optical properties and stability of the perovskite RT-QD CsPb0.9Sn0.1Br3, we explored its application in

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Fig. 4. (a) DAS and decay times retrieved from the global analysis of all the fs-TA spectra under 320 nm pulsed laser excitation. (b) Excited dynamics model of CsPb0.9Sn0.1Br3 perovskite QDs.

Fig. 5. (a) Time-dependent PL spectrum of CsPb0.9Sn0.1Br3 perovskite QDs in solution. (b) Linear graph of the corresponding PL intensity as a function of time.

LEDs (note that according to the above discussion, the CsPb1(x ¼ 0.3e0.9) RT-QDs were not used for LEDs because they are unstable). The structure of LEDs based on this QDs is displayed in Fig. 6. The device based on the CsPb0.9Sn0.1Br3 RT-QDs consisting of the spin-coated layers of poly (3, 4-ethylenedioxyth-iophene) poly (styrenesulfonate) (PEDOT:PSS), poly (4-butylphenyldiphenyl-amine) (poly-TPD), perovskite layer, and evaporated

xSnxBr3

layers of 1,3,5-tris (1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi) and LiF/Al, was manufactured and the fabrication details is presented in the Supporting Information. For comparison, the LEDs device based on the CsPbBr3 QDs was also fabricated (see Fig. S5). The energy levels of the different components in the LEDs are given in Fig. 6b. The performance of device with CsPb0.9Sn0.1Br3 QDs is shown in Fig. 7. At a certain voltage, the best device based on

Fig. 6. (a) Schematic illustration of the perovskite QDs LEDs device structure. (b) Energy band diagram of the different components in the perovskite QDs LEDs.

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Fig. 7. (a) The current density-voltage of CsPb0.9Sn0.1Br3 perovskite QDs LED as a function of the applied bias. (b) The current efficiency as a function of the current density. (c) The EQE. The inset shows the photograph of the operating device with an emitting area of 2 mm  2 mm. (d) The PL (red) and EL (green) spectra of CsPb0.9Sn0.1Br3 QDs LED device.

CsPb0.9Sn0.1Br3 QDs exhibits a low on-voltage of 3.6 V and a maximum brightness of 1600 cdm-2. The small turn-on voltage indicates that upon partial Sn2þ substitution, the injection of charge carriers is easier. The maximum current efficiency (CE), external quantum efficiency (EQE), power efficiency (PE) are 4.89 cdA-1, 1.8% and 6.41 lmw1, respectively (Fig. 7). Note that two prepared separately 10% Sn2þ doped QDs were used to fabricate the LEDs devices and their performances were tested under the same conditions. The results show that they have similar EQE of about 1.8% and other properties, indicating good reproducibility. Fig. 7d shows the electroluminescence spectrum (EL) and the corresponding PL spectrum of the CsPb0.9Sn0.1Br3 QDs LED operating at 12 V. The central emission wavelength of EL (green) spectrum is about 523 nm. Compared with the PL (red) spectrum, the red shift is only 4 nm, which is caused by aggregation of the QDs during spincoating the film. In addition, the profile of peak is well symmetrical and no other impurity peaks were observed (Fig. 7d). This means that at higher current density, the resulting material retains its original electronic structure [43,44]. As shown in Fig. S6b, the 1931 Commission Internationale del’Eclairage (CIE) color coordinates of CsPb0.9Sn0.1Br3 perovskite QDs LED is (0.19, 0.71), which are almost located at the edge of the CIE graph, indicating the high color purity of this device with the CsPb0.9Sn0.1Br3 RT-QDs. According to the above experimental results, one can find that the CsPb1-xSnxBr3 perovskite QDs synthesized at room temperature have broad application prospects in high efficiency LEDs. 4. Conclusions In summary, we synthesized the Sn2þ doped perovskite QDs CsPb1-xSnxBr3 (x ¼ 0.0e0.9) at room temperature. TCSPC and fs-TA measurements were used to study the excited state dynamics of the as-synthesized CsPb1-xSnxBr3 (x ¼ 0.1) RT-QDs. Three decay processes of the excited state were assigned. The high PLQY (91.9%)

of the CsPb0.9Sn0.1Br3 perovskite QDs is attributed to the improved lattice of the RT-QDs and the partial passivation of trap states by appropriate Sn2þ doping. The LED device based the CsPb0.9Sn0.1Br3 perovskite QDs displays a luminance of 1600 cdm-2, a CE of 4.89 cdA-1, an EQE of 1.8%, a PE of 6.41 lmw1, and a low turn-on voltage of 3.6 V. The present work demonstrates that mixed metal cation illuminate materials with high PLQY and excellent stability can be obtained by room temperature synthesis, which is a considerable progress in realizing practical low-cost light sources, promoting existing applications and proposing new potential. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by Natural Science Foundation of China (91741105, 21173169), Chongqing Municipal Natural Science Foundation (cstc2018jcyjAX0625). And program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011). We thank Feifan Yang for PLQY measurement and Ruiling Zhang for fs-TA measurement. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matdes.2019.108246. References [1] C. Bi, S. Wang, Q. Li, S.V. Kershaw, J. Tian, A.L. Rogach, Thermally stable copper(II)-Doped cesium lead halide perovskite quantum dots with strong blue emission, J. Phys. Chem. Lett. 10 (2019) 943e952. [2] H. Zhao, Y. Zhou, D. Benetti, D. Ma, F. Rosei, Perovskite quantum dots

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