Multicolored Mixed-Organic-Cation Perovskite Quantum Dots (FA

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Multicolored Mixed-Organic-Cation Perovskite Quantum Dots (FAxMA1-xPbX3, X = Br and I) for White Light-Emitting Diode Yawen Zhang, Guan Wu, Hui Dang, Kangzhe Ma, and Su Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02309 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Industrial & Engineering Chemistry Research

Multicolored Mixed-Organic-Cation Perovskite Quantum Dots (FAxMA1-xPbX3, X = Br and I) for White Light-Emitting Diode Ya-Wen Zhang,‡ Guan Wu,‡ Hui Dang, Kang-Zhe Ma and Su Chen*

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu Key Laboratory of Fine Chemicals and Functional Polymer Materials, Nanjing Tech University (former: Nanjing University of Technology), 5 Xin Mofan Road, Nanjing 210009, P. R. China.

ABSTRACT: Organometal halide perovskites (such as CH3NH3PbX3, X = Cl, Br, I) have received enormous interests due to their strikingly photoelectric properties. Here we develop a facile ligand-assisted reprecipitation method to synthesize NH2CH=NH2PbX3 (NH2CH=NH2+, FA; X = Br and I) perovskite quantum dots (QDs) at room temperature. The FAPbX3 perovskite QDs with uniform monodispersity (sized 4~7 nm) display relatively high photoluminescence quantum yields (PLQYs) of 60~75%. Through manipulating the mixed-organic-cation reactions, we achieve a series of multicolored perovskite QDs with continuously controllable emission wavelengths from 460 to 565 nm. Furthermore, we discuss the

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influence of ligands (oleic acid and n-octylamine) on PL properties and stabilities of perovskite QDs. Finally, we have successfully designed a white LED via compositing perovskite QDs and poly(methyl methacrylate) (PMMA), which presents a high color rendering index. Considering of those remarkable achievements, we believe our work will have great potential to meet various optoelectronic applications. INTRODUCTION In the last two decades, semiconductor quantum dots (QDs) have attracted enormous attentions in various optoelectronic applications, especially in the LEDs field1-5 due to their superior optical and chemical properties, such as broad excitation range, narrow emission spectra, high luminescent efficiency and chemical stability.6-9 It is well-known that the first reported LEDs based on CdSe QDs were achieved in twentieth century.10,11 After that, various QDs (e.g. CdTe, CdSe@ZnS, CdS, ZnCuInS, CuInS@ZnS, InP@ZnSeS, and CdSeS) have been fabricated as photoluminescent materials for wide applications in LEDs.12-19 However, the complicatedly rigorous synthetic process and relatively expensive raw materials seriously discourage their further development and practical applications. As a fast-growing demand for the new-generation of photoluminescent and photoelectric materials with facile fabrication, lower-cost and higher-performance, a new class of metal halide perovskites have been demonstrated as promising candidates in the applications of solar cells,20 lighting devices,21 and display backlights.22 In particular, a large progress of metal halide perovskites (such as 2 ACS Paragon Plus Environment

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CsPbX3, and CH3NH3PbX3 (CH3NH3+, MA; X = Cl, Br and I)) has been extensively utilized in LEDs due to their strong PL properties and stabilities. For instance, Kovalenko’s group reported the colloidal CsPbX3 perovskites at high temperature, which presented a high PL QYs of 90%, the entire visible spectral region of 410~700 nm and bright emission with wide color gamut.23 In order to further achieve the high quality perovskites at low temperature, Wong et al. reported a versatile anion exchange reaction to prepare the CH3NH3PbX3 perovskite nanorod arrays for the LEDs application, which offered a new strategy for optoelectronic device.24 Additionally, Zhang et al. developed a facile ligand-assisted reprecipitation (LARP) process to obtain brightly luminescent and colortunable CH3NH3PbX3 perovskites at room temperature. A wide-color gamut white LEDs was also demonstrated to enhance the color quality of display technique.25 Overall, these methods were all demonstrated to be facile, low-cost and controllable fabrication of perovskite QDs where the perovskite QDs exhibited brightly fluorescent emission in wide color gamut. Besides, those manipulations of emission spectra were mainly based on tuning the halide compositions21,25 or the anion exchange reactions.24,26,27 However, there are almost no reports of mix-organic-cation perovskite QDs with continuously tunable visible PL and not even applied in LEDs. Here, we report a simple LARP method to synthesize colloidal FAPbX3 (X = Br and I) perovskite QDs, and mix-organic-cation FAxMA1-xPbX3 perovskite QDs at room temperature. The monodisperse FAPbX3 perovskite QDs (sized 4~7 nm) exhibit 3 ACS Paragon Plus Environment


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relatively high photoluminescence quantum yields (PLQYs) of 60~75%. By adjusting the mixed organic MA+ and FA+, we obtain a range of multicolored FAxMA1-xPbX3 perovskite QDs with continuously controllable emission wavelength. Moreover, the PL properties and stabilities of perovskites are investigated by controlling the compositional ligands (oleic acid and n-octylamine). Furthermore, by virtue of compositing photoluminescent perovskite quantum dots and PMMA, the white LED is constructed. EXPERIMENTAL METHODS Materials Formamidine acetate, methylamine (CH3NH2, 33 wt % in absolute ethanol), n-octylamine (≥99%), PbBr2, PbI2, hydriodic acid (HI, 57 wt % in water), hydrobromic acid (HBr, 48 wt % in water), oleic acid (≥90%) and poly(methyl methacrylate) (PMMA, average MW ∼35000) were purchased from Aladdin Chemistry Co., Ltd. N,N-dimethylformamide (DMF), toluene, chloroform and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Synthesis of FAX and MAX (X = Br, I) NH2CH=NH2Br was prepared by mixing 3 g of formamidine acetate and 7.8mL of hydrobromic acid at the temperature of 0 oC for 2 h under continuous stirring. Then, rotary evaporation was applied to evaporate the solvent under the pressure of ~0.1 MPa at 45 oC, obtaining the precipitate. After that, the precipitate was washed three 4 ACS Paragon Plus Environment

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times with diethylether and dried under vacuum (65 oC) for 5 h. CH3NH3Br was synthesized by the reaction of methylamine with the corresponding acid. First, 24 mL methylamine was put into 100 mL absolute ethanol with continuous stirring at the temperature of 0 oC, forming a uniform mixed solution. Then, 8.6 mL hydrobromic acid was dropwise added into the mixed solution. The reaction solution was stirred for 2 h. Besides, rotary evaporation was applied to evaporate the solvent under the pressure of ~0.1 MPa at 45 oC, obtaining the precipitate. After that, the precipitate was washed three times with diethylether and dried under vacuum (65 oC) for 5 h. NH2CH=NH2I were synthesized by the similar method. Fabrication of FAPbX3 and hybrid FAxMA1-xPbX3 (X= Br, I) perovskite QDs The typical synthesis of FAPbBr3 perovskite QDs were as follows: a mixture of 0.2 mmol FABr, 0.2 mmol PbBr2, 40 µL n-octylamine and 0.75 mL oleic acid was dissolved in 5 mL of DMF, sonicating and forming a uniform precursor solution. Then, 50 µL precursor solution was gradually dropped into 5 mL toluene with vigorous stirring. After that, the preliminary colloidal FAPbBr3 QDs were obtained with strongly purple PL emission under UV light. After centrifugation at 5000 rpm for 10 min to discard the precipitates, a lyard colloidal solution with higher purity was obtained. FAPbI3 QDs were fabricated using the similar method. It should be noted that the fabrication of iodide-substituted samples need to operate in a glovebox so that to avoid fast degradation. The hybrid FAxMA1-xPbBr3 QDs were also prepared via the



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similar method by blending FABr and MABr with various proportions (the proportions of FABr and MABr were set as 1:1, 1:2, 1:4, and 1:7) in DMF solution. Construction of WLED The FA1/2MA1/2PbBr3 QDs (blue), FA1/5MA4/5PbBr3 QDs (green) and FAPbI3 QDs (red) were dissolved into chloroform with PMMA, respectively, forming three kinds of QDs/PMMA. Then, QDs/PMMA was further blended with thermocurable resin (silicone, Dow-Corning Co.) (QDs/PMMA:silicone was 2:3 (wt/wt)) and put into a vacuum chamber for some minutes to remove the bubbles and solvent. To construct white LED, a UV-LED chip with a peak wavelength centered at 400 nm was utilized as bottom component. After that, the three kinds of QDs/PMMA/silicone products were separately coated on the LED chip, and the WLED was achieved. All the optical performances were measured using a ZWL-600 instrument with integral sphere. Characterizations Transmission electron microscopy (TEM) was recorded by a JEOL JEM-2100 transmission electron microscope. Photoluminescence (PL) spectra measurements of the perovskite QDs were carried out on a Varian Cary Eclipse spectrophotometer. Ultraviolet−visible (UV−vis) absorption spectra were conducted using a Perkin-Elmer Lambda 850 UV−vis spectrometer. Fourier-transform infrared (FT-IR) spectra were recorded by a Nicolet 6700 FT-IR spectrometer with the KBr pellet technique ranging from 4000 to 500 cm-1 at a resolution of 4 cm-1. Quantum yield (QY) was measured


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according to an established procedure by using quinine sulfate in H2SO4 solution as a reference. The QY was calculated using the following equation (1): Q = (1) Where Q is QY, A is the measured integrated emission intensity, I is the optical density, and n is the refractive index. The subscript R refers to the quinine sulfate in 0.10 M H2SO4 standard solution (Φ = 0.54). The concentration of the samples for QY estimation should allow the first excitonic absorption peak below 0.05 to avoid any other reabsorption. Time-correlated single-photon counting (TCSPC) data were performed with an Edinburgh FL 900 photocounting system. The decay trace for perovskite QDs was fitted using biexponential functions Y(t) based on non-linear least squares analysis in equation (2): Y(t) = exp (− ⁄ ) + exp (− ⁄ )(2) Where α1 and α2 (α1+α2=1) are the fractional contributions of time-resolved decay lifetime of τ1 and τ2. The average lifetime could be concluded according to the following equation (3): τ=

+ (3) +

X-ray diffraction (XRD) was performed on a Rigaku Corporation D/max-rC rotating anode X-ray powder diffractometer using a copper target. The emission spectrum of the white LED was measured by a USB 4000 Miniature Fiber Optic Spectrometer of Ocean Optics. All the spectra were obtained at room temperature. RESULTS AND DISCUSSION 7 ACS Paragon Plus Environment


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The colloidal FAPbX3 and FAxMA1-xPbX3 perovskite QDs were synthesized via a facile LARP technique and the preparation process was shown in Scheme 1. To synthesize colloidal FAPbBr3 QDs as an example, a mixture of FABr and PbBr2, and ligands (oleic acid and n-octylamine) was dissolved in N,N-dimethylformamide (DMF), sonicating and forming a uniformly free-standing precursor solution25 (Scheme 1a). Then, the precursor solution was gradually dropped into 5 mL of toluene with vigorous stirring. After that, a brightly purple PL emission of FAPbBr3 QDs was observed under ultraviolet light (UV-light). The colloidal solution was further purified by centrifugation to discard the precipitates. The other perovskite QDs with various PL emissions were fabricated by the same method, and we obtained a serious of photoluminescent FAxMA1-xPbX3 (Scheme 1b). By utilizing three kinds of perovskite QDs with (red, green, blue) photoluminescence, the white LED (WLED) was constructed in Scheme 1c.


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Scheme 1. (a) Schematic illustration of the prepared process for FAPbBr3 perovskite QDs via LARP method. (b) A series of typical photoluminescent photographs of colloidal QDs (FAxMA1-xPbX3) under UV-light. (c) The application of WLED based on the perovskite QDs (Pe-WLED). The morphology and phase structure are characterized in Figure 1. Transmission electron microscopy (TEM) is utilized to describe the morphologies of FAPbBr3 QDs. It is clearly observed that FAPbBr3 QDs present a uniform monodispersity (Figure 1a), and their detailed size distributions are calculated in Figure 1b. Generally, the FAPbBr3 QDs have an average diameter of 5.78 nm with a size deviation of 0.61 nm. The phase structure of FAPbBr3 QDs are further characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) patterns. As illustrated in Figure 1c, the distinct lattice fringes of FAPbBr3 QDs represent the interplanar distances of 1.87 Å and 2.03 Å, corresponding to the (1 1 0) and (2 2 2) crystal faces, which are well in accordance with the XRD patterns (Figure 1d). The morphology, particle size distribution and XRD patterns of FAPbI3 QDs are also showed in Figure S1 and 2.



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Figure 1. (a) TEM image, (b) particle size histogram and (c) HRTEM image of colloidal FAPbBr3 QDs. (d) XRD patterns of FAPbBr3 QDs. FT-IR analyses are characterized to confirm the surface chemistry of the as-prepared FAPbBr3 and FAPbI3 perovskite QDs. As illustrated in Figure S3, the peaks in FAPbBr3 around 1400 cm-1 and 1714 cm-1 are ascribed to the bending vibration of O-H from carboxylic acid and C=O vibration, and similar peaks (1410cm-1, 1715 cm-1) are existed in FAPbI3, implying the existence of oleic acid ligand. The characteristic peaks (3445cm-1 and 1580 cm-1) in FAPbBr3 are identified as the -NH2 stretching vibration and bending vibration of -NH2 (similar peaks at 3405 cm-1 and 1630 cm-1 in FAPbI3), deriving from n-octylamine or formamidinium cation. The presence of these functional groups further demonstrate the existence of ligands on the surface of perovskite QDs, indicating the important role of ligands in the preparation process of FAPbX3.25,28 The molecular structure model and optical properties of FAPbBr3 perovskite QDs are further investigated. As seen the model structure of FAPbBr3 in Figure 2a, the FA+ 10 ACS Paragon Plus Environment

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and Pb2+ are in step with 12 and 6 Br-1, respectively, and the regular-octahedron of PbBr6 are corner-shared to form a 3D structure, which is similar to MAPbX3 structure.29,30 The molecular structure model of FAPbBr3 also is clarified as an ideal Br:Pb molar ratio close to 3:1, which is further demonstrated by Energy dispersive X-ray spectroscopy (EDX) in Figure 2b. It is illustrated that the molar proportion of Br:Pb is about 3.2:1, which is basically in accordance with the structural formula of FAPbBr3. The optical properties of colloidal FAPbBr3 perovskite QDs are studied utilizing UV-vis absorption spectrum and PL spectrum. As shown in Figure 2c, the FAPbBr3 QDs display the absorption characteristic peak (red line) at ~431 nm, Accordingly, a strong and symmetrical PL peak (black line) centered at 438 nm with a narrow full width at half maximum (FWHM) (≤20 nm) are represented, implying the high quality of as-prepared QDs. The brightly purple PL emission with a relatively high PLQY of 65% is also observed in the inset of Figure 2c. To gain more insight into the optical properties of FAPbBr3 QDs, time-resolved PL spectrum is measured. The typical fluorescence decay lifetime of FAPbBr3 QDs is investigated by using multidimensional time-correlated single photon counting (TCSPC) method. As shown in Figure 2d, FAPbBr3 QDs display the average decay lifetime of 9.47 ns. The UV-vis absorption, PL emission spectra and decay lifetime of FAPbI3 QDs are also illustrated in Figure S4. However, the FAPbI3 QDs show an asymmetric PL peak centered at 664 nm with larger FWHM (≥50nm) when is comparable to MA perovskite analogues of



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MAPbI3 (in Figure S5), which may be attributed to the relatively inhomogeneous distribution of particle sizes.

Figure 2. (a) Cubic perovskite crystal structure model of FAPbBr3, the little one for single unit cell of FAPbBr3 perovskite crystal. (b) EDX spectra of a typical sample of FAPbBr3 QDs. (c) UV-vis absorption (red line) and PL emission (black line) spectra (λex = 365 nm) of FAPbBr3 QDs in toluene. The inset was the fluorescent photographs under the ambient and UV light. (d) Time-resolved fluorescence decay curves of FAPbBr3 QDs. It is reported that the ligands have great influence on the PL properties and stabilities of perovskite QDs.25 Hence, we investigate the effects of oleic acid and n-octylamine ligands on FAPbBr3 perovskite QDs (Figure 3). Figure 3a, b, c and d, are the fluorescent photographs of FAPbBr3 QDs prepared under different combination of ligands under UV light and their corresponding PL emission spectra are shown in Figure 3e. Generally, the FAPbBr3 QDs prepared with mixed oleic acid and 12 ACS Paragon Plus Environment

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n-octylamine ligands display brightly purple PL with sharply narrow FWHM (≤20 nm) and high fluorescence intensity. When utilizing separated ligand, FAPbBr3 QDs represent little red shifts of PL peaks with the decrease of fluorescence intensities when compares with mixed ligands. However, FAPbBr3 QDs prepared without ligands exhibit a considerable red shift of PL peak with the widest FWHM (≥50 nm). The emission shift mechanism is because the ligands can control the kinetics of crystallization, which mainly contributes to the size control of perovskite quantum dots.25,31 Consequently, when ligands binding to the surface of QDs, the growth of QDs can be managed so that to control the particle sizes of QDs, which results the PL emission shifts. Figure S6 shows the influence of ligands on PL stabilities of FAPbBr3 QDs. Obviously, by utilizing the mixed ligands, FAPbBr3 QDs can maintain a good PL intensity after 12 hours. However, the fluorescence is decayed gradually after when utilizing the separated ligands (oleic acid or n-octylamine). Moreover, the PL intensity of FAPbBr3 QDs prepared without ligands decreases abruptly and will be completely quenched only after 2 hours. It is revealed that the decrease of ligands will increase the density of the defects, which results in the decrease of PL intensity of FAPbBr3 QDs,32 implying the paramount role of ligands in the PL stabilities of perovskite QDs.



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Figure 3. Fluorescent photographs of FAPbBr3 samples in toluene prepared under different combination of ligands. (a. with oleic acid and n-octylamine; b. with n-octylamine; c. with oleic acid; d. without oleic acid and n-octylamine.) (e) PL emission spectra of FAPbBr3 QDs prepared under different combination of ligands. We further comprehensively investigate the mixed-organic-cation colloidal perovskite QDs (FAxMA1-xPbX3). The LARP method is a versatile technique to fabricate mixed-organic-cation perovskite QDs via the solvent mixture of organic cation substitutions. By mixing FABr and MABr in precursor with different proportions (the proportions between FABr and MABr were 1:1, 1:2, 1:4, and 1:7), we fabricate a series of colloidal FAxMA1-xPbBr3 QDs with continuously tunable fluorescence, which are clearly observed in the optical photographs (Figure 4a). Figure 4b shows the detailed normalized PL emission spectra of the perovskite QDs in Figure 4a. When the mixing ratio is 1:1, we can observe the as-prepared QDs (FA1/2MA1/2PbBr3), emitting strong blue fluorescence with a PL peak centered at 460 nm. After increasing the proportion of MA+ (1:2, 1:4, 1:7), the PL emission wavelengths represent a gradual red shift (peaks centered at 502nm, 530nm, and 565 nm, respectively). Figure 4c describes the UV-vis absorption and PL emission spectra 14 ACS Paragon Plus Environment

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of the perovskite QDs in Figure 4a. From the integrated spectra, we can clearly see the

variation

of PL

emission and UV-vis

absorption peaks of hybrid

organic−inorganic perovskite QDs with different mixing ratios of organic cations.

Figure 4. (a) Optical images of typical FAPbX3 (X = Br, I) and FAxMA1-xPbBr3 QDs under ambient light (top) and a 365 nm UV lamp (down), respectively. (b) Normalized PL emission spectra of a series of perovskite QDs in Figure. 4a. (c) UV-Vis absorption (solid lines) and PL emission (dashed lines) spectra of the samples in Figure 4a. (From bottom to top: FAPbBr3, FA1/2MA1/2PbBr3, FA1/3MA2/3PbBr3, FA1/5MA4/5PbBr3, FA1/8MA7/8PbBr3, FAPbI3) By

taking

advantage

of

the

high-quality

FAPbX3

(X

=

Br,

I) and

mixed-organic-cation FAxMA1-xPbBr3 perovskite QDs with continuously tunable PL, we successfully construct a WLED via mixing three PL (red, green, blue) emission of perovskite QDs and PMMA polymer covered on a UV-LED chip. From the photograph of WLED in Figure 5a, we observe that the as-fabricated LED exhibits brightly white light. Figure 5b is the corresponding electroluminescent spectra of the device in Figure 5a, which further demonstrates the good combination of three kinds 15 ACS Paragon Plus Environment


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of perovskite QDs. The commission international de L'Eclarage (CIE) chromaticity diagram is described in Figure 5c. From the CIE chromaticity diagram, the WLED displays a superb color saturation, which may be because of the relative narrow emissions. It is revealed that the color coordinate of the sample is labeled as red circle (0.32, 0.30), which is close to the standard value of (0.33, 0.33). Besides, the as-prepared WLED device exhibits a relatively high color rendering index (CRI) of 85 and luminous efficiency of 40.2 lm W-1 under applied current of 350 mA. To further achieve high efficiency, two strategies can be conducted. One is to obtain high quality perovskite QDs with narrow FWHMs, high PLQYs and stability. The other is to develop the polymer with excellent luminousness, robust chemical resistance and good thermal stability.22 We will further improve the luminous efficiency of WLEDs in our future work. All the favorable performances of the LED imply the viability of fabricating WLED based on the novel fluorescent perovskite QDs.

Figure 5. (a) The as-fabricated Pe-WLED device tested under the applied voltage of 3 V and current of 148 mA. The corresponding electroluminescent spectra (b) and CIE 1931 chromaticity diagram (c) for the Pe-WLED device. The optical performances of 16 ACS Paragon Plus Environment

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the WLED measured using a ZWL-600 instrument with integral sphere under applied current of 350 mA.

CONCLUSIONS In summary, we report a simple and low-cost LARP method to synthesize brightly luminescent FAPbX3 and mixed-organic-cation FAxMA1-xPbBr3 perovskite QDs. The as-prepared perovskite QDs present strong luminescence with relatively high PLQYs of 60~75%. In addition, a series of continuously tunable PL wavelength from 460 nm to 565 nm is obtained by adjusting different proportional of mixed organic cations. Furthermore, we investigate the influence of oleic acid and n-octylamine ligands on the PL properties and stabilities of FAPbBr3 perovskite QDs. Through taking full consideration of the merits of perovskite QDs, we construct a WLED where the device displays a relatively high color rendering index of 85. We believe our results will have great potential to meet various optoelectronic applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org/ HRTEM image and particle size histogram of colloidal FAPbI3 QDs, XRD patterns of FAPbI3 QDs, FT-IR spectra of colloidal FAPbI3 and FAPbBr3 QDs, UV-vis absorption and PL emission spectra of FAPbI3 QDs, Time-resolved fluorescence



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decay curves of FAPbI3 QDs, PL emission spectra of FAPbI3 and MAPbI3 QDs, PL intensity variation of FAPbBr3 samples (PDF)

AUTHOR INFORMATION Corresponding Author *Correspondent author: Tel: 86-25-83172258, Fax: 86-25-83172258, E-mail: [email protected]

Author Contributions ‡

These authors have contributed equally to this work

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21474052), National Key Research and Development Program of China (2016YFB0401700), Natural Science Foundation of Jiangsu province (BK20170973) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Multicolored Mixed-Organic-Cation Perovskite Quantum Dots (FA

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