Design Optimization of Lead-Free Perovskite Cs2AgInCl6:Bi

Apr 9, 2019 - The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University ...
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Design Optimization of Lead-Free Perovskite Cs2AgInCl6:Bi Nanocrystals with 11.4% Photoluminescence Quantum Yield Ying Liu, Yuyu Jing, Jing Zhao, Quanlin Liu, and Zhiguo Xia Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00410 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Chemistry of Materials

Design Optimization of Lead-Free Perovskite Cs2AgInCl6:Bi Nanocrystals with 11.4% Photoluminescence Quantum Yield Ying Liu⊥, †, Yuyu Jing⊥, †, Jing Zhao†, Quanlin Liu†, and Zhiguo Xia*,†,‡ The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China ‡ State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou, 510641, China †

ABSTRACT: Lead-free halide double perovskites continue to draw increasing attention in view of their nontoxicity and stability compared to lead-based perovskites. Herein, we optimized the colloidal synthesis of undoped and Bi-doped Cs2AgInCl6 nanocrystals (NCs) via the facile hot-injection process using nontoxic and available precursors. The synthesis conditions including temperature, ligands and the hydrochloric acid are investigated to boost the photoluminescence quantum yield (PLQY) of Cs2AgInCl6:Bi NCs with the trace amount of Bi doping. And the broad-band orange emission peaking at 580 nm, is observed with the PLQY as high as ~11.4%, and the related luminescence mechanism has been discussed. This work provides a reliable avenue toward the optimization of optical properties for emerging lead-free halide perovskite NCs with enhanced PLQY.

■ INTRODUCTION Lead-based halide perovskites (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) are now established as one of the most appealing optical and optoelectronic materials.1-4 They possess high absorption coefficients, rapid carrier recombination rates and near-unity photoluminescence quantum yields (PLQY), and so on.5 However, despite all these attractiveness, lead halide perovskites face two main issues, the toxicity of Pb and the lack of enough stability in common working conditions with humidity and light, which are considered to be bottlenecks for their commercial applications.6-8 Therefore, it is of great significance to explore nontoxic and stable perovskite alternatives. To obtain Pb-free metal halide perovskites, one simple method is to replace Pb2+ cations with the less toxic group IV metal ion, Sn2+. But Sn2+-based perovskites suffer from the chemical instability of the elements, hindering their commercial applications under ambient conditions.9 Therefore, the double perovskites with a general formula A2M+M3+X6 (A = Cs+ or CH3NH3+; M+ = Cu+, Ag+, Na+; M3+ = Bi3+, Sb3+, In3+; X = Cl, Br, I), emerge as a promising new alternative to lead-based halide perovskites.10-13 And such a three-dimensional double perovskite structure is constructed via the replacement of two toxic Pb2+ ions with one M+ cation and one M3+ cation compared with Pb-based 3D perovskites. There exist two types of semiconductors among the double perovskites, viz., indirect-bandgap and direct-bandgap type. For indirect-bandgap materials, such as Cs2AgBiBr6 and Cs2AgBiCl6, not only photons but also phonons are involved in the absorption and recombination resulting in relatively low absorption coefficients and low PLQY.14-17 In contrast, the double perovskites with direct-bandgap show high absorption coefficients and increased PLQY and rapid carrier recombination rates resulting from only photons involved absorption and recombination process. Cs2AgInCl6 crystals as one of the direct-bandgap double perovskites,

have been under the spotlight recently since the pioneering work by Volonakis et al.10 and the independent work simultaneously in our group11. Considering the emerging advanced optical applications of double perovskites, there are increasing demand for high-quality NCs dispersible in solution or films.16, 18, 19 To the best of our knowledge, there are only three reports on the synthesis of Cs2AgInCl6 double perovskites NCs till now, including the hot-injection route and anti-solvent recrystallization synthesis,20-22 however, the previous methods either contain harmful reagents, such as benzoyl chloride and chlorotrimethylsilane, or produce NCs with a very low PLQY. Therefore, optimized green synthetic approaches that are capable to produce highquality NCs are highly desired. Besides, the bulk Cs2AgInCl6 crystals/powders are also characterized by a weak PL emission and doping approaches have been utilized to improve the optical properties. Mn-doped Cs2AgInCl6 NCs exhibited an orange emission at 620 nm but originating from Mn2+ luminescence with the PLQY of ~16±4%.20 For the powders, Cs2Ag0.6Na0.4InCl6 doped with 0.04% Bi3+ leads to white emission with an excellent quantum efficiency, (86±5) %.23 And a PLQY of 36.6% for the violet-emitting Cs2AgIn0.9Bi0.1Cl6 NCs capped with OA is observed, but the reported PLQY of orange emission at 570 nm is only 2%.22 Here, we demonstrate a facile hot-injection method using oleic acid, oleylamine and octadecene as ligands and the solvent to prepare Cs2AgInCl6 and Bi-doped Cs2AgInCl6 NCs. We further discuss the design optimization and the optical properties of as-prepared NCs depending on synthesis temperatures, the amount of ligands and the hydrochloric acid and optimize the synthesis condition to obtain the NCs those possess the high crystallinity and excellent optical property. The undoped Cs2AgInCl6 NCs exhibit a blue emission at 470 nm and the Bi-doped Cs2AgInCl6 NCs exhibit a 580 nm broad orange emission with the PLQY of 11.4%. We also explore the contribution of Bi doping in the valence band maximum (VBM) and conduction band minimum

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(CBM) to understand the nature of band edges, and thus the subsequent effect on optical transition probabilities across these states has been analyzed. ■ EXPERIMENTAL SECTION Materials. Cs2CO3 (99.9%, Aldrich), AgNO3 (analytical pure, Beijing Chemical Works, China), InCl3 (99.99%, Aldrich), BiCl3 (analytical pure, Sinopharm Chemical Reagent Co., Ltd, China), 1-octadecene (ODE, >90%, Aldrich), oleic acid (OA, analytical pure, Aldrich), oleylamine (OLA, 80-90%, Aldrich), hexane (≥ 98%, Aldrich), hydrochloric acid (HCl, analytical pure, Beijing Chemical Works, China), ethyl acetate (analytical pure, Beijing Chemical Works, China). All chemicals were used as received without further purification. Synthesis of Cs-Oleate. Cs2CO3 (0.814 g), ODE (10 mL) and OA (2.5 ml) were loaded into a 50 mL three-neck flask, dried and degassed under vacuum at 120 oC for 1 h. And then to obtain a clear solution, the mixture was heated under N2 to 150 oC. The Cs-oleate solution was stored under N and pre-heated 2 before injection. Synthesis of Cs2AgInCl6 and Cs2AgInCl6:1%Bi NCs. In a typical reaction, AgNO3 (61 mg), InCl3 (80 mg), BiCl3 (1.1 mg), ODE (14 ml), OA (1-2 ml), OLA (1-2 ml) and HCl (0-0.98 ml) were loaded into a 50 ml three-necked flask and heated to 120 oC for 1 h, and then the temperature was raised to 180-280 oC under N2 atmosphere and 0.8 ml hot Cs-oleate solution was injected quickly under vigorous stirring. Five seconds after the injection, the mixture was cooled down by a water bath. A centrifugation process (4 min at 8000 rpm) was performed to separate the NCs from the crude solution. After that, the crude NCs were redispersed in hexane and centrifuged again for 4 min at 5000 rpm. Then, the precipitates were discarded and the supernatant was poured into another centrifuge tube with ethyl acetate added, following a centrifugation process for 5 min at 10000 rpm. Finally, the NCs were obtained by discarding the supernatant. Characterization. The X-ray diffraction (XRD) measurements were conducted on a PANalytical X’Pert3 Powder diffractometer (Cu Kα radiation, operating at 40 kV and 40 mA) under ambient conditions. All samples were prepared by dissolving nanocrystals into hexane and dropping the concentrated solutions onto glass sample holders for XRD measurements. Energy dispersive X-ray (EDX) spectroscopy was performed on a probe aberration corrected microscope, JEOL JEM-ARM200CF, at 200 kV. Transmission electron microscopy (TEM) imaging was performed using a 120 kV JEM2010. Samples for TEM measurements were prepared by dropping dilute NC solutions on ultrathin carbon film coated 300 mesh copper grids. UV-Vis absorption spectra of the colloidal solutions were collected at room temperature using the Hitachi UH4150 US–vis–near-infrared spectrophotometer. The steady-state photoluminescence (PL) spectra and photoluminescence excitation (PLE) were collected with FLSP9200 Edinburgh Instruments equipped with the Xe900 lamp and the PMT detector. The room-temperature luminescence decay curves were also obtained using the same FLSP9200 Instrument with an nF920 flash lamp as the excitation source. Samples for these PL measurements were prepared by loading 3 mL dilute NC hexane solutions in the quartz cuvettes. PLQY was measured by an Absolute Photoluminescence Quantum Yield Measurement System (PELM 3011OperationManual from Enli Technology) with an integrating sphere. The ICP elemental analysis was conducted

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by inductively coupled plasma mass spectroscopy (ICP-MS). The samples were dissolved in the nitric acid (HNO3 2% (v/v)) and heated up to 180 oC on a hotplate. Cs+ and In3+ ions were dissolved in the solution. But AgCl precipitated due to its low solubility, thus separated from the acid solution via centrifugation. Finally, the supernatant was diluted by adding Milli-Q water and analysed directly without any further operations.

■ RESULTS AND DISCUSSION The structural model of Bi-doped Cs2AgInCl6 double perovskite is shown in Figure 1a, which crystallizes in a cubic unit cell with the space group Fm 3 m, in which [AgCl6] and [(In/Bi)Cl6] octahedra form a 3D framework. We conducted a colloidal hot-injection method to synthesize Cs2AgInCl6 NCs as depicted in Figure 1b. As described in the experimental section, the related precursors (AgNO3, InCl3 and BiCl3) are dissolved in octadecene together with ligands (OA and OLA) and hydrochloric acid (HCl) and then the Csoleate solution is injected at the optimized injection temperature 280 oC. Briefly, the precursor solution changed from turbid to clear and gradually turned to dark color with the increasing temperature during the vacuum operation process. The corresponding X-ray diffraction (XRD) patterns for the NCs synthesized at 180-280 oC are shown in Figure 1c. All the XRD patterns of Cs2AgInCl6:Bi NCs mostly match that of the standard file of the reported Cs2AgInCl6 bulk materials with a small amount of Bi doping (1%). For the NCs synthesized at 180-230 oC, the careful checking of the XRD patterns revealed that there are some peaks of impurity phases between 30-35 o besides the major phase while the XRD patterns of NCs synthesized at 260 and 280 oC are in good agreement with that of reported bulk Cs2AgInCl6, indicating that the full crystallization can be obtained at temperature higher than 260 oC. Relatively, the TEM micrographs of Cs2AgInCl6:Bi synthesized at 180 and 230 oC both show impurities apart from cubic-shaped NCs (Figure 1d, e). Additionally, at the synthesis temperature 280 oC, as shown in Figure 1f, g, the TEM and highresolution TEM (HRTEM) images exhibited pure and uniform cubic-shaped nanocrystals with high crystallinity and 3.8 Å lattice spacing compatible with the (022) surface of the Cs2AgInCl6 structure.

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Chemistry of Materials choose 280 oC as the optimal synthesis temperature for purpose of high-quality NCs.

Figure 1. (a) Structural model and (b) synthesis illustration of Cs2AgInCl6:Bi NCs. (c) XRD patterns of Bi-doped Cs2AgInCl6 NCs prepared at different synthesis temperature (180-280 oC) with the corresponding reflections of bulk Cs2AgInCl6. TEM images of as-prepared NCs with the synthesis temperature at (d) 180 oC, (e) 230 oC, (f) 280 oC; (g) HRTEM images of NCs with synthesis temperature at 280 oC. (h) UV-vis absorption (dash line), photoluminescence (excitation at 368 nm) and photoluminescence excitation (emission at 580 nm) spectra of Bi-doped Cs2AgInCl6 NCs dispersion in hexane obtained at different synthesis temperature (180-280 oC), and the insets shows the samples irradiated under 365 nm UV lamp. (i) Variations of the photoluminescence quantum yields depending on the synthesis temperature (180-280 oC).

Next, we investigated the optical properties of Cs2AgInCl6:Bi NCs synthesized at different temperature. As can be seen from Figure 1h, all the Cs2AgInCl6:Bi NCs possess a typical absorption spectrum featuring a sharp absorption peak positioned at about 368 nm with a absorption tail starting at longer wavelengths (The details will be discussed below together with Cs2AgInCl6 NCs.), which are consistent with the reported Cs2AgInxBi1-xCl6 NCs.22 All the PL spectra excited at 368 nm exhibit broad emissions peaked at around 580 nm and the photoluminescence excitation (PLE) spectra (gray lines) obtained at the 580 nm emission coincide with the absorption peak at 368 nm. The inset images of Figure 1h present the Cs2AgInCl6:Bi NCs hexane solutions in the quartz cuvettes under UV light and we can obviously see the improvement of the PL intensity with increasing synthesis temperature. Accordingly, we then measured the PLQYs of the Cs2AgInCl6:Bi NCs synthesized at 180-280 oC (Figure 1i). With the increase in temperature, the PLQY curve rises rapidly from 180-260 oC; While since 260 oC, the curve rises slowly up to the highest PLQY of around 11.4% with a stable platform. With further rising temperature above 280 oC, the PLQY may increase but would not increase much, and the synthesis condition will not be safe. Therefore, considering the XRD and TEM data and the corresponding photoluminescence analysis, we

Figure 2. TEM and HRTEM images of Cs2AgInCl6:Bi NCs with synthesis temperature at (a) 180 oC, (b) 200 oC, (c) 230 oC, (d) 280 oC demonstrating the transformation diagrams of NCs with the increase of temperature. (e) Schematic representation on the phase formation of Cs2AgInCl6:Bi NCs at different synthesis temperatures (180-280 oC) in three steps. (I) Complexing of optimized amount of precursors including 1 mL OA, 1 mL OLA and 0.28 mL HCl. (II) AgNO3 and InCl3 reacting with OA to form Ag-oleate and In-oleate and oleylammonium chlorine (oleylamine binds HCl). Ag0 particles form by the reduction of Ag+ in the presence of amine ligands. (III) Formation of Cs2AgInCl6 NCs at different synthesis temperatures followed by the injection of CsOA.

To figure out the reason on the high synthesis temperature around 280 oC is the optimal, we examined the TEM and HRTEM images of Cs2AgInCl6:Bi NCs synthesized at 180-280 oC. For the NCs synthesized at 180 oC, there are many dark spherical nanoparticles and precursor impurities with the lower contrast (Figure 1d and Figure 2a). The precursor impurities have good crystallinity with around 2.86 and 4.07 Å lattice parameters according to the corresponding FFT (fast Fourier transform) image and we assign to the Cs3In2Cl9 composition as confirmed by XRD and TEM analysis (Figure S1).24, 25 In addition, from the previous work,15, 26 OLA can reduce silver ions to silver nanoparticles as it can undergo metal-ion-induced oxidation to nitriles, so we think that the spherical dark nanoparticles should be Ag nanoparticles. And we also conducted the XPS experiment on the Cs2AgInCl6:Bi NCs. Ag 3d spectra is presented in Figure S2, suggesting that there are two components after deconvolution, attributed to Ag2O (367.7 eV) and Ag0 (368.2 eV), respectively, which confirms that the Ag nanoparticles may come from the synthesis process. With the synthesis temperature increasing to 200 oC, the TEM images of the asprepared NCs show the decrease in the precursor impurities and the dark nanoparticles mostly attached to the cubic-shaped NCs (Figure 2b). When the temperature increases to 230 oC, precursor impurities with the lower contrast grow up to irregular-shaped NCs and dark nanoparticles become smaller (Figure 2c). For the reaction at 280 oC, only the pure and uniform cubic-shaped NCs form

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with a few dark nanoparticles can be observed (Figure 2d). On the basis of the results discussed above, we proposed the reaction scheme as presented in Figure 2e. Initially, the InCl3 and AgNO3 salt dissolved in a mixture of octadecene, OLA and OA at 120 oC (Figure 2e, step 1) and then increase the temperature to form complex (Figure 2e, step 2). AgNO3 and InCl3 react with OA to form Ag-oleate, In-oleate and oleylammonium chlorine (oleylamine binds HCl).27, 28 But in the reducing environment provided by amine ligands, Ag+ can be reduced to Ag nanoparticles so that there are Ag nanoparticles in the precursors (the black dots in Figure 2e, step 2). The TEM image of precursors at 120 oC (Figure S3) also demonstrated the dark nanoparticles, which suggested the presence of Ag nanoparticles in the reaction precursors. To figure out whether the reaction time of the precursors affect the prevalence of Ag(0) nanoparticles, the experiment with longer reaction time of the precursors was conducted. Figure S4 showed the TEM images of Bi-doped Cs2AgInCl6 NCs with the reaction time of 30 min and 60 min of precursors. It is found that there are more or larger Ag(0) nanocrystals for the longer reaction time, about 60 min (Figure S4b), compared with the normal one (Figure S4a), which indicates that longer reaction time may promote the prevalence of Ag(0) particles. In step 3 (Figure 2e), Cs2AgInCl6 NCs can be formed with impurities, such as Cs3In2Cl9 and Ag nanoparticles followed by the injection of Cs-oleate (CsOA) at different temperatures. Here, to see if the reaction time (after Cs(OA) injection) affect the presence of Ag(0) particles, the experiment of longer reaction time (30 min after Cs(OA) injection) were conducted and checked from the related TEM analysis (Figure S5). The nanocrystals treated with 30 min (Figure S5b, d) clearly grow, and it seems that there are no obvious change on the amounts of Ag(0) nanoparticles compared with those obtained with 5 s reaction time (Figure S5a, c). The results show that longer growth time leads to small increase in size as also found in Cs2AgBiBr615 and the reaction time plays almost no role in the presence of Ag(0) particles. Thus the injection temperature up to 280 oC, normal reaction time of precursors and 5s reaction time after Cs(OA) injection result in decrease in the impurity phases and the particle size of Ag nanoparticles.29

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Figure 3. XRD patterns of impurities with (a) no HCl added, (b) 0.14 mL HCl, (c) 0.28 and 0.42 mL HCl added showing the impurities depending on different added amounts of HCl and ligands during the synthesis. TEM images of Bi-doped Cs2AgInCl6 NCs with (d) 0.28 mL HCl, (e) 0.42 mL HCl and (f) 0.98 mL HCl added during the synthesis. (g) XRD patterns of impurities with OA/OLA = 2. (h) Photographs of impurities with OA/OLA = 2 under the ambient light and 365 nm UV lamp. (i) TEM image of Bi-doped Cs2AgInCl6 NCs with OA/OLA = 1:2. Additionally, unlike the hot-injection synthesis of CsPbX3 (X = Cl, Br and I), it is worth mentioning that an appropriate amount of HCl additive is indispensable during the synthesis. Therefore we examined the effect of different amounts of hydrochloric acid (0, 0.14, 0.28 and 0.42 mL) adding while keeping the same ratio of ligands and the synthesis temperature at 280 oC as shown in Figure 3a-f. When no HCl was added, the precursors could not be dissolved completely as seen from the Figure S6 and the final nanocrystals are obtained in a very low yield. So we checked the XRD patterns of the sediment (impurities and bulk target product) after the second centrifugation (centrifugation for 3 min at 5000 rpm). The sediment with no HCl added includes Ag, AgCl and bulk Cs2AgInCl6 as shown in Figure 3a, while the sediments with HCl added do not contain any Ag and AgCl (Figure 3b, c). The results indicate that the HCl additive can promote the full ionization of Ag and prevent the formation of AgCl impurity, because the carboxylate groups in metal-oleate complexes can be protonated by the addition of HCl, facilitating the formation of reactive metal monomer.30 Besides, HCl can also help the dissolution of AgCl through the (at least transient) formation of dissolvable AgCl2- ions. Once 0.14 mL HCl was added, it also results in a very low yield of the final product and the XRD patterns of the sediment show the formation of the Cs3In2Cl9 impurity originating from the lack of Cl- ions in the precursor (Figure 3b). That is ascribed to the fact that high concentration of Cl- can promote the coordination of Ag+ and Cl-, in the form of AgCl2- ions and so promote the full ionization of Ag. Although the sediments with both 0.28 mL and 0.42 mL HCl added almost contain no other impurities, when checking the TEM of final NCs (Figure 3d-f), the sample with 0.42 mL HCl and 0.98 mL HCl added contains larger dark nanoparticles, corresponding to a low PLQY of 6.7% and 3.8%, respectively. It seems that excessive HCl additive will lead to the growth of Ag nanoparticles and furthermore do harm to the PLQY.31 Therefore, the optimum volume of HCl (under the present optimized reaction condition) is 0.28 mL. Furthermore, the ligands also play a crucial role in the formation of the nanocrystals according to previous studies32. We varied the volume ratio of oleic acid (OA) to oleylamine (OLA) (OA/OLA = 2, 1 and 1/2) to optimize the synthesis while keeping the added volume of HCl fixed at 0.28 mL and the synthesis temperature at 280 oC. The variations of the XRD patterns with different volume ratios of OA to OLA are shown in Figure 3g. When OA is added more than OLA, almost no Cs2AgInCl6 NCs are obtained and the XRD patterns of the sediment after the second

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Chemistry of Materials centrifugation show that there exist InCl3, AgCl and Cs2AgInCl6:Bi microcrystals included in the impurities (Figure 3g). The impurities with OA/OLA = 2 give violet light under the daylight and exhibit orange emission under the UV light (Figure 3h), which proves that the existence of AgCl and Cs2AgInCl6:Bi microcrystals. Furthermore, if OA is added less than OLA, the as-prepared Cs2AgInCl6 NCs contains more Ag nanoparticles as shown in Figure 3i, also resulting in a lower PLQY (6.5%). Besides, with more OLA added, the Ag/In ratio becomes smaller from the EDS data (Table S1), possibly resulting from more reduction of Ag+ in the presence of more amine ligands. These results mentioned above suggest that more OA can result in the formation of AgCl impurity, insolubilization of InCl3 and very low yield of NCs.33 Indeed, when the OLA is insufficient, the deprotonation of OA would be suppressed and OA presents mainly in the form of dimers due to the hydrogenbonding interaction rather than in the form of carboxylate anions. Furthermore, less OA ligands would affect the insolubilization of InCl3 because OA ligands could function as a complexing agent for In3+ ions. In contrast, more OLA will lead to the reduction of Ag+ ions because of the reducing environment created by amine ligands.30, 33, 34

Figure 4. (a) XRD patterns of Cs2AgInCl6 and Bi-doped Cs2AgInCl6 NCs. TEM and HRTEM images of (b, c) Bi-doped Cs2AgInCl6 NCs and (d, e) undoped Cs2AgInCl6. (f) UV-vis absorption (dash lines), PL (excitation at 368 nm, gray lines) and PLE (emission at 580 nm for Bi-doped NCs and 470 nm for undoped NCs) spectra of Bi-doped Cs2AgInCl6 NCs and undoped Cs2AgInCl6 NCs dispersion in hexane, and the insets shows the respective samples under 365 nm UV lamp. Time resolved PL decay curves of (g) Bi-doped NCs and (h) undoped

NCs. (i) Fluorescent mechanism of Cs2AgInCl6 NCs and Bidoped Cs2AgInCl6 NCs.

The comparison on the structure and optical properties of undoped and Bi-doped Cs2AgInCl6 NCs is show in Figure 4 to investigate the photoluminescence mechanism, especially the effect of the trace amount of Bi addition. The XRD characterizations of the undoped and 1% Bi-doped samples presented in Figure 4a reveal that they both crystallize in the double perovskite Cs2AgInCl6 structure without any other undesired secondary phases, suggesting that crystal structures are not distorted by the slight Bi doping. In addition, TEM-EDS and ICP analyses results performed on Bi-doped and undoped Cs2AgInCl6 confirmed the expected Cs2AgInCl6 stoichiometry and detected the presence of Bi addition (Table S2). Figure 4b-e shows TEM and HRTEM micrographs of Cs2AgInCl6 and Cs2AgInCl6:1%Bi NCs. Both samples consist of the cubicshaped NCs with a mean size of 9.79 nm (undoped sample) and 10.95 nm (1% Bi-doped sample), as further evidenced by the detailed TEM and particle size analysis (Figure S7). The HRTEM image of Bi-doped Cs2AgInCl6 NCs presents high crystallinity with lattice spacing of 0.37 nm, corresponding to the (022) diffraction peaks, which is the same as what observed for the undoped sample. Figure 4f comparatively shows the PL, PLE and the absorption spectra of Cs2AgInCl6 and Cs2AgInCl6:1%Bi NCs. The insets give the images of the NC in the hexane solutions. The absorption of Cs2AgInCl6 NCs starts at around 350 nm peaked at ~270 nm, with the optical bandgap of 4.25 eV calculated from the corresponding Tauc plot (Figure S8a). Different from the undoped Cs2AgInCl6 NCs, the absorption onset of Bi-doped samples is observed at ~390 nm with an evident excitonic absorption peak around 368 nm and the optical bandgap obtained from the corresponding Tauc plot is about 3.28 eV (Figure S8b). The PL spectra are both recorded by the 368 nm excitation. The undoped sample exhibits a weak blue emission at 470 nm and Bi-doped sample shows the broad orange emission with the peak at 580 nm. The PL decay curves of undoped and doped samples are also totally different, which indicates that there are two origins for the PL emission. For the Bi-doped Cs2AgInCl6 NCs, the PL decay curve at 580 nm is considerably slower with a lifetime (~1633 ns) of greater than that (