film interface with halide anion

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Emissions at perovskite quantum dot/ film interface with halide anion exchange Yang Liu, Fushan Li, Qianqian Li, Kaiyu Yang, Tailiang Guo, Xiaoming Li, and Haibo Zeng ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00966 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Photonics

Emissions at perovskite quantum dot/film interface with halide anion exchange

Yang Liua, Fushan Lia,*, Qianqian Lia, Kaiyu Yanga, Tailiang Guoa, Xiaoming Lib, Haibo Zengb,*

aInstitute bMIIT

of Optoelectronic Displays, Fuzhou University, Fuzhou 350002, China

Key Laboratory of Advanced Display Materials and Devices, Institute of

Optoelectronics & Nanomaterials, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Corresponding emails: [email protected] (F. Li); [email protected] (H. Zeng)

Abstract Solution processable hybrid perovskite film and quantum dots (QDs) have shown promising application in optoelectronic devices owing to their inherent crystal structures, ionics property and electronic nature. The manipulation of halide anions, for both perovskite bulks and quantum dots, is definitely critical for rendering such materials with peculiar optoelectronic characteristics. Herein, the tunable emissions at the solid interface between CH3NH3PbI3 film and CsPbBr3 quantum dots via halide anion exchange are reported. By in-situ photoluminescence and X-ray diffraction measurements, the gradual substitution of Br ions in the CsPbBr3 QDs lattice by I ions in CH3NH3PbI3 film and the reversed ions exchange process is elaborately monitored, respectively. In addition, the 1

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morphology of CsPbBr3 QDs on CH3NH3PbI3 film and micronet is preserved according to atomic force microscope and high-magnification scanning electron microscope observations. Furthermore, the time course of the anion exchange can be tuned by tailoring the density of ligands on CsPbBr3 QDs. By exquisite manipulation of halide anions exchange, capability is achieved to tailor the emission color continuously at the micro-scale. This work expands the understanding of perovskite anion exchange to new dimensions, and implies their potential applications in optoelectronic devices owing to the gradient energy level stem from the interfacial transition. Keywords: CsPbBr3 quantum dots; Perovskites; Anion exchanges; Interfaces; Multicolored film.

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The promising embodiment of low-cost solution-processed inorganic-organic hybrid perovskite materials in optoelectronic or electronic devices including solar cell,

1-2

light-emitting diodes,3 photodetector,4 and the likes,5-6 has been attracting extensive attention owing to its excellent photoelectric properties including high absorption coefficient, extended diffusion length of excitation and high external quantum efficiency (EQE) in the visible wavelength range.7-10 However, CH3NH3PbI3 tends to deteriorate into PbI2 and CH3NH3I at environmental atmosphere since it is volatile to humidity, oxygen and UV light.11-12 Inorganic cesium has been introduced to replace organic element to enhance the stability of inorganic-organic hybrid perovskite, while the poor solubility of precursors, coupled with the inferior orthorhombic phase (a larger bandgap of 2.82 eV for CsPbI3), limits its prosperity in optoelectronic and electronic devices. During the past two years, inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I) quantum dots (QDs), overcoming the poor solubility and inferior crystal lattice of CsPbX3 bulks, have been emerging as novel star materials with a combination of narrow size distributions, narrow emission line widths, and high photoluminescence quantum yield (PLQY) up to 90%.13-15 For both perovskite bulks and QDs, the bandgap can be simply tailored via the manipulation of halide components (Cl, Br, and I) or their proportions. In addition, the ionic property of halide perovskite plays a key role in explaining a range of important observations, including the hysteresis of current-voltage 3


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characteristics of perovskite-based solar cells or the self-compensating mechanism for charged point defects.16-17 It should be noted that, for perovskite-structured materials, a preferred promising avenue to a boom of optoelectronic materials is cation and anion exchange reactions. However, various attempts of cation-exchange reactions for CsPbX3 QDs have been proved unsuccessful owing to their single ionic charge and the rigid nature of the cationic sublattice.18 Recently, the ability of halide ions diffusing into perovskite lattice has been demonstrated in hybrid halide perovskites thin film with CH3NH3- halides or halogen gases as sources of the exchanging halide ions.19-20 Peidong Yang et al. introduced an anion exchange technique to convert the CH3NH3PbBr3 nanorod array into a CH3NH3PbI3 nanorod array by annealing at 140 to 150 °C with CH3NH3I vapor as halide source. And the fast anion-exchange reactions of inorganic cesium lead halide perovskites QDs by adding different halide source or mixing QDs with diverse halide ions in the QDs dispersion has been reported.18,

21-22

Although the works

above-mentioned have validated the possibility of anion exchange, they are confined to the post processing of either perovskite film or QDs respectively. That is, the anion exchange between the solid interface between perovskites bulks and QDs is still unexplored. At the same time, the composite interface between perovskite film or QDs is very important for understanding the unique performance of solid state perovskite-based optoelectronic devices. Herein, we create a composite interface of CH3NH3PbI3 film and CsPbBr3 QDs, and 4


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demonstrate the halide anions migration between them for the first time. Given that the nonpolar property of CsPbBr3 QDs solvent (octane), halide anions migration is assumed to happen at the solid-phase interface between CH3NH3PbI3 film and CsPbBr3 QDs film. This exchange was found to last hundreds to thousands of seconds when CsPbBr3 QDs were deposited on CH3NH3PbI3 film by spin coating. By observing the in-situ photoluminescence (PL) emission of the samples, we follow the gradual substitution of Br ions in the CsPbBr3 QDs lattice by I ions in CH3NH3PbI3 film. The gradual shift of the PL emission from green to red was in agreement with the continuous formation of homogeneous CsPbBrxI3−x QDs. In addition, we show that anion exchange process between CsPbBr3 QDs and CH3NH3PbI3 film could be tuned by the ligand density of oleylamine and oleic acid ligands. And we demonstrated the capability to tailor the emission color continuously, especially at the micro-scale area via this simple process. This work is of great importance in post processing of perovskite QDs and perovskite films, the exquisite tailoring of energy level at the interface between them, and expanding the understanding of perovskites anion exchange to new dimensions. Results and discussion. The monodispersed CsPbBr3 QDs used here were synthesized by injecting Cs-oleate into PbBr2 solution under the atmosphere of nitrogen, the nucleation and growth of which takes only several seconds owing to the ionic nature of the chemical bonding in perovskites. According to TEM images in Figure 1a, the CsPbBr3 QDs have an average 5


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diameter of 11 nm with a cubic shape determined by the perovskite crystal structure, which is clearly revealed by XRD patterns presented in Figure 1b. The as-synthesized CsPbBr3 QDs dispersed in octane exhibit an absorption peak at 487 nm, a strong PL centered at 516 nm and a narrow full wideness half maximum (FWHM) of 21 nm (Figure 1c). The QDs solution exhibits green color under natural light (Inset of Figure 1c left), and emits bright green light when irradiated by 365 nm UV light source (Inset of Figure 1c right). The short radiative lifetimes of 8.6 ns，as depicted in Figure 1d, is in agreement with the previous report.23 The CH3NH3PbI3 film used here was deposited on an indium-tin-oxide (ITO)-coated glass substrate by a modified two-step method24. In order to demonstrate the film being perovskites, XRD patterns for CH3NH3PbI3 film on ITO-coated glass is shown in Figure S1. The main diffraction peak at 14.1º (110) and 28.2º (220) were identified as the characteristic peaks of CH3NH3PbI3, similar to the works reported before.25-26 To fabricated a composite interface of CH3NH3PbI3 film and CsPbBr3 QDs, the synthesized CsPbBr3 QDs dispersed in octane was deposited by spin coating. The non-polar solvent of octane was chosen owing to the solvent resistance of CH3NH3PbI3 layers to octane solvent (Figure S2). According to the morphologies of the CH3NH3PbI3 film and CsPbBr3 QDs/CH3NH3PbI3 film hybrid characterized by AFM in Figure 1e,f, , the CsPbBr3 QDs were scattered on CH3NH3PbI3 film with a large part of them accumulating at concave field, indicating that the morphology of CH3NH3PbI3 crystals was maintainedand a composite interface of CH3NH3PbI3 film and CsPbBr3 QDs was formed. 6


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Amazingly, halide anions exchange happens at the solid interface between CH3NH3PbI3 film and CsPbBr3 quantum dots (QDs) as is illustrated in Figure 2a. After oleylamine and oleic acid ligands capped CsPbBr3 QDs were deposited on CH3NH3PbI3 film, the green CsPbBr3 QDs turn to red CsPbBrxI3−x QDs owing to the partially replacement of Br ion by I ions from CH3NH3PbI3 film. In contrast to the direct anion exchange by rinsing CH3NH3PbI3 in halide source dissolved in polar solvent, which can decompose and reconstruct the crystal structure, the non-polar solvent of octane would not function like this. Figure 2b showed the PL images of the sample under the irradiation of UV light (365 nm) at 60 s, 360 s, 600 s and 1200 s after spin coating CsPbBr3 QDs on perovskite film, respectively. It is interesting that the PL emission of the sample shifts from green to red after the spin coating of CsPbBr3 QDs. The evolution of PL emission from green to red is proposed to be related to the gradual anion exchange between CH3NH3PbI3 film and CsPbBr3 QDs. An in-situ PL study of the anion exchange between CsPbBr3 QDs and CH3NH3PbI3 crystals is presented in Figure 2c. Most of the anion exchange occurred rapidly within the initial period and proceeded for hundreds of seconds. The gradual change of the PL emission from green to red accords with the continuous formation of homogeneous CsPbBrxI3−x QDs at the surface.18, 21 Importantly, the integrated intensities of the PL spectra of the exchanged sample remain comparable to that of the original one, indicating that high PL quantum yields were sustained throughout the exchange process. In comparison with anion exchange of perovskite QDs occurring in solution18 and organic-inorganic hybrid perovskite film dipped in solution,20 the anion 7


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exchange of CsPbBr3 QDs and CH3NH3PbI3 film in our work is much more dilatory. The migration of halide anions leads to the CsPbBr3 QDs transforming to CsPbBrxI3-x QDs even though the QDs are capped with oleylamine and oleic acid ligands. The impact of ligand density on anion exchange will be discussed later. The gradually changing halide ion stoichiometries throughout the anion exchange process are further confirmed by time-dependent XRD pattern. The scanning angle ranging from 10º to 25º was chosen to make sure that one single smooth XRD curve can be completed as soon as possible (in 45 s). During the process, the XRD curves of CH3NH3PbI3 film feature a slightly shift of peak positions to larger angle direction (Figure 3a), indicating shrinkage of the lattice due to larger I ions in CH3NH3PbI3 film being partially replaced by smaller Br ion,20,

27

which is in accordant with our PL

observations mentioned above. From the enlarged XRD pattern in Figure 3b, a small yet broadening diffraction peak at 15.2º, corresponding the (100) peak of CsPbBrxI3−x QDs, appeared at the initial period, then moved to smaller angle direction and was finally covered up by the much more intense (100) peak of CH3NH3PbBryI3-y film. This further proved the reversed ions exchange process where smaller Br ions in CsPbBr3 QDs were partially substituted by I ions in CH3NH3PbI3 film. The (200), (211) and (202) peaks of CH3NH3PbI3 film showed the same shift tendency as illustrated in Figure 3c and 3d. Moreover, the black triangle in Figure 3c, corresponding to the XRD peak of the ITO-coated glass substrate (Figure S1), stands still throughout the anion exchange, conforming the accuracy of our XRD results. 8


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Here, in order to further confirm the presence of anion exchange between CsPbBr3 QDs and CH3NH3PbI3 film, in-situ PL of CsPbBr3 QDs on a polymethylmethacrylate (PMMA)-layer-coated CH3NH3PbI3 film was taken as shown in Figure 3e. It is indicated that PL shift did not take place during the measurement, implying the failure of anion exchange. The inserted thin PMMA barrier prevented the anion migration between CsPbBr3 QDs and CH3NH3PbI3 film. Moreover, the overlapped PL intensity though out the measurement demonstrated superior stability of CsPbBr3 QDs in ambient condition.28-29 Up to now, there have, for different scenario, been varied explanations including Schottky defects, Frenkel defects, lattice distortions (due to accumulated charges, dissolved impurities and nonuniform strain), soften lattice (caused by the light illumination induced bond weakening), and open space and wrong bonds at grain boundaries. However, the fundamental mechanism is not so clear, or pioneer works are not consistent with each other very well; and more theoretical work with identical model are needed to understand ion migration in perovskite for varied scenario fundamentally. Owing to the absence of applied electric field and the elimination of light stimulation (the anion exchange happened in dark environment), coupled with nano structure of perovskite QDs, we attribute the ion migration pathway here to open space and wrong bonds at grain boundaries, or defects in internal and external of the QDs. In order to disclose the driving force for the I-Br halogen exchange at the CH3NH3PbI3 substrate/CsPbBr3 QDs interface, theoretical calculations based on density 9


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functional theory method were conducted. For the stable adsorption of QD on film surface, halogen bonding (XB) plays an important role, which is fascinating because of its effectiveness in driving the formation of supramolecular architectures or interface interaction.30 The Br-I distances being shorter than their sums of the van der Waal’s radii (Br-I 3.81 Ǻ) can be treated as strong XB interaction.31 In this work, XB contributes to the achievement of stable adsorption. Afterwards, the Br-I exchange is driven by the requirement of surface/cluster stabilization. According to Figure 3f, when the I-Br distance is 3.0 Ǻ, the most stable configuration can be obtained. This distance is greatly shorter than the van der Waal’s radii sum of Br–I (3.81 Ǻ), suggesting a strong XB interaction. And the adsorption energy is calculated as -65.24 kcal/mol, indicating a strong chemical adsorption. Furthermore, after I-Br exchange, a stabilization energy of 7.61 kcal/mol can be observed. These data suggest that the adsorption and I-Br exchange are both thermodynamically favorable. The stabilization energy of 7.61 kcal/mol led by I-Br exchange can be explained by the stabilization of surface and cluster. In this calculation models, capping ligands of perovskite QDs are neglected to simplify the calculation process, which, however can suppress the anion exchange by passivating surface dangling bonds or defects to some degree (which will be discussed in detailed in the following parts). In addition, there are interspace (between capping ligands) on the surface of QDs, where the atoms are bared with dangling bonds; these bared parts are more likely to be attracted by the surface of CH3NH3PbI3 substrate, where the interval is smaller than 1 nm, offering possible path for anion exchange. Furthermore, the surface 10


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atom-surfactant interaction is unstable, which can be confirmed by that ligands are easily washed away from the surface and perovskite QDs are liable to grow in ambient environment. Conclusion could be drawn that at the CH3NH3PbI3 substrate/CsPbBr3 QDs inter-surface, the I-Br exchange is driven by strong XB interaction and requirement of surface/cluster stabilization. The anion exchange process between CsPbBr3 QDs and CH3NH3PbI3 film could be further tuned by the density of oleylamine and oleic acid ligands. The density of ligands passivating CsPbBr3 QDs, tuned by various purification cycles, has an substantial impact on charge injection in quantum dot-based optoelectronic devices.32 Photographs of the CsPbBr3 QDs solution sample with different purifying cycles under the UV light were given in Figure S3a and 2b. The surface ligand variation on QDs was also investigated by Fourier transform infrared (FTIR) spectra as shown in Figure S3c. The FTIR peaks, located at 2923 and 2854 cm-1, correspond to C-C and C-H stretching modes of the -CH2group, correlating negatively with treating cycles. The FTIR results reveal that the density of surface ligand capping CsPbBr3 QDs indeed decreased with the purification cycles. In-situ PL indicating the anion exchange using CsPbBr3 QDs treated with one and three purification cycles were provided in Figure 4a and 4b. For CsPbBr3 QDs purified via one and three cycles, the tendency of the PL color change from green to red sustained, indicating continuous transformation of CsPbBr3 QDs to homogeneous CsPbBrxI3−x QDs. However, the anion-exchange time decreased with the diminishment of ligand density. When emission wavelength reached 620 nm during anion exchange, it takes about 6000 s, 11


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600 s and 350 s for CsPbBr3 QDs with one, two, and three purification circles, respectively. Two reasons can account for the manipulation of anion-exchange time by ligand density, as depicted in Figure 4c and 4d. On one hand, passivating ligands would create a thin barrier at the interface between CsPbBr3 QDs and CH3NH3PbI3 film, which will suppress halide anion migration and then prolong the anion exchange time. In addition, since halide ion is responsible for ionic conductivity via the vacancy diffusion mechanism in perovskite18, defects for CsPbBr3 QDs

33

and purification process brings in surface degradation or

32,

the increasing defects in CsPbBr3 QDs themselves with

purification times could form new routines for halide anion migration. For in-depth understanding of the anion exchange, we carried out a well-designed experiment by depositing CsPbBr3 QDs on CH3NH3PbI3 micronet. The anisotropic CH3NH3PbI3 micronet, prepared following a modified one-step method, can be attributed to the polar aprotic solvent N, N-Dimethyl formamide (DMF) and the slow evaporation rate.25, 34 Figure 5a illustrate the schematic of the CsPbBr3 QDs deposited on perovskite micronet and the gaps. The XRD pattern in Figure 5b confirmed the fully transformation from precursors to perovskite crystals. The coexistence of the XRD peaks of the CH3NH3PbI3 micronet and CsPbBr3 QDs capped in gaps (Figure 5b) further supports our conclusion. Figure S4 depict the CH3NH3PbI3 micronet morphology characterized with PL microscopy. The CH3NH3PbI3 micronet fails to exhibit visible images under the irradiation of blue and green light (Figure S4b and S4c) owing to its smaller bandgap. After the deposition of CsPbBr3 QDs on CH3NH3PbI3 micronet, the existence of 12


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ACS Photonics

CsPbBr3 QDs on CH3NH3PbI3 micronet and the gaps is conformed according to the HRSEM images in Figure 5c-5e. The PL microscopy figures further manifest CsPbBr3 QDs-coated CH3NH3PbI3 morphology with micronet and the gaps between them (Figure 5f-5h). Under the irradiation of blue light source, the sample emitted red light at the region of CH3NH3PbI3 micronet, while showed green color in the gap where only CsPbBr3 QDs exists, as highlighted by yellow dotted circle (Figure 5f-5h). When illuminated by green excitation light source, CH3NH3PbI3 micronet parts emit red light, while the gap turn black. The results imply that only at the areas where CsPbBr3 QDs adhere to CH3NH3PbI3 micronet did anion exchange occur, while the CsPbBr3 QDs capped into the gap between the micronet keep its composition, consistent to that of CsPbBr3 QDs on CH3NH3PbI3 film. In an elegant study of the anion-exchange process, we obtained multicolored areas (in micrometer scale) in a single PL microscopic image by designing different CsPbBr3 QDs thickness via drop casting instead of spin coating (Figure S5). The small fluctuation of CsPbBr3 QDs thickness leads to different PL shift periods during anion-exchange process. Via this method, we obtained the green, yellow and red color in a single PL image. The multicolored ring stripe, indicating varied thickness of CsPbBr3 QDs, is due to multi-pinned coffee ring effect35 during the evaporation of solvent (Figure S6). For green area, the CsPbBr3 QDs at the surface fail to get through anion exchange, while CsPbBr3 QDs has transformed to CsPbBrxI3-x for red area due to their smaller thickness. The uneven morphology and varied color in micro or nano scale implies potential application in unrepeatable and colorful anti-counterfeit 13


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labels.

Conclusions. To conclude, we observe the halide migration between CH3NH3PbI3 film and CsPbBr3 QDs, which last hundreds to thousands of seconds when CsPbBr3 QDs were deposited on CH3NH3PbI3 film. During the exchange reactions, the morphology of perovskite film and CsPbBr3 QDs is well preserved. In addition, it is indicated that the ligand density of oleylamine and oleic acid ligands can readily tune the anion exchange process between CsPbBr3 QDs and CH3NH3PbI3 film. PL microscopic images of CsPbBr3 QDs on perovskite micronet under the irradiation of blue light and green light were given by a well-design method. By exquisite manipulation of halide anions exchange, we have achieved the capability to tailor the emission color continuously, especially at the micro-scale area, indicating the promising applications in optoelectronic devices and anti-counterfeit labels. Experimental Section Chemicals ： PbBr2, PbI2, Cs2CO3 (from Aladdin-reagent), oleylamine (OAm, 80-90% pure, from Aladdin-reagent), oleic acid (OA, 90% pure, from Sigma-Aldrich), 1-octadecene (ODE, 90% pure, from Sigma-Aldrich), hexane (from Aladdin-reagent), ethylacetate (99.5% pure, from Aladdin-reagent), poly methyl methacrylate (PMMA, average Mw ~996000 by GPC, from Sigma-Aldrich) and CH3NH3I (from Xi'an Polymer 14


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Light Technology Corp.) were received and used without further purification. CsPbBr3 QDs Synthesis and Purification: 0.407 g Cs2CO3 was loaded into 100 mL 3-neck flask along with 20 ml ODE and 1.25 ml OA, dried under vacuum for 10 min and then heated under N2 at 120 ºC until all Cs2CO3 reacted with OA. Cs-oleate in ODE was pre-heated to 100 ºC before injection. 5 mL ODE and 0.69 g PbBr2 were loaded into 25 mL 3-neck flask and dried under vacuum for 10 min and heated under N2 flow at 120 ºC. 5 ml OA and 5 ml OAm were injected at 120 ºC under N2. After complete solubilization of PbBr2, the temperature was raised to 155 ºC and 4 ml pre-heated Cs-oleate solution was quickly injected and, 5 s later, the reaction mixture was cooled in ice-water bath. The crude solution was split into three parts for the purification process of QDs. First, ethyl acetate was added into the crude solution with a volume ratio of 3:1, and then the precipitate was collected separately after centrifugation and dispersed in 2 mL hexane. 6 mL ethyl acetate was added into the 2 mL hexane dispersion with a volume ratio of 3:1, the precipitate was collected and redispersed in hexane. The process was repeated according to our varied purification circles, and the final QDs was dispersed in octane. Film Formation: For two-step method, PbI2 and CH3NH3I were dissolved into N, N-dimethylformamide (DMF) and isopropanol with concentrations of 461 mg ml-1 for PbI2 and 30 mg ml -1 for CH3NH3I, respectively, and both solutions were heated at 70 °C for being fully dissolved before spin coating. For perovskite CH3NH3PbI3 micronet prepared by one step method, the hybrid perovskite CH3NH3PbI3 precursor solution was prepared by mixing PbI2 and CH3NH3I at 1:1 equimolar ratio dissolved in DMF solvent, 15


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heating at 70 °C for 12 h inside a nitrogen-filled glovebox with oxygen and moisture levels less than 1 ppm. The as-synthesized CsPbBr3 QDs in octane was deposited on substrate by spin coating at a rotate speed of 2000 r.p.m. For PMMA layer, PMMA solution (10mg/mg in chlorobenzene) was spin coated on CH3NH3PbI3 films at the rotate speed of 2000 r.p.m and baked at 120 °C for 30 min, and then followed by CsPbBr3 QDs with the same processing parameters above. Characterization: The UV−Vis absorption spectra were tested with a UV/Vis/NIR spectrophotometer (Shimadzu ，

UV-3600). X-ray diffraction (XRD) spectra was

collected with an X'Pert PRO diffractometer (PANalytical).

The transmission electronic

microscopy (TEM) image of CsPbBr3 QDs was recorded using JEOL JEM-2100F microscope. The surface morphologies of the film were characterized by atomic force microscopy (AFM, Bruker Multimode 8). The HRSEM images were taken via Gmini300. The steady-state PL spectra were collected with a Hitachi F-4600 fluorescence spectrophotometer, by exciting the samples using a Xe lamp coupled to a monochromator. Time-resolved PL measurement was collected by fluorescence lifetime measurement system (HORIBA scientific). The photoluminescence (PL) microscopic images of the samples were characterized by fluorescent microscope (Olympus BX51M). This purposefully restricted excitation equalizes the emission intensities. In addition, the filter wavelength tolerance is below ±2nm, which is less than half of the conventional figure, and makes it possible to maximize fluorescence light transmission from the samples. The images under blue irradiation were carried out by choosing an ultraviolet excitation 16


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(wideband) mirror unit with a excitation filter (460-490 nm) and emission filter (520 nm), while, for those images under green light irradiation, an ultraviolet excitation (wideband) mirror unit with a excitation filter (510-550 nm) and emission filter (590 nm) was utilized. Computational Methods: In the theoretical calculation using density functional theory (DFT) methods, the Vienna ab initio simulation package (VASP) program was adopted.36

The

exchange-correlation

functions

were

treated

by

the

Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA),37 and the plane-wave basis set with a cutoff energy of 400 eV within the projector augmented wave (PAW)38 method was used. The Brillouin-zone integration was performed on a grid of 1×1×1 Monkhorst-Pack special K-points. A vacuum layer thickness of 20 Å was applied perpendicular to the slab to avoid artificial interactions between the slab and its periodic images. The accuracy in the energy convergence is set as 10-5 Ry. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: …… X-ray diffraction patterns for ITO-coated glass, CsPbI3 QDs and CH3NH3PbI3 film on ITO-coated glass; Photograph of CsPbBr3 QDs solution sample with different purifying cycles; FTIR spectra of the CsPbBr3 QDs with different purifying cycles; PL microscopic images of perovskite micronet and CsPbBr3 QDs droping on perovskite 17


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micronet under the irradiation of white light, blue light, green light; Absorption spectra of CH3NH3PbI3 layers without rinsing and washed by octane respectively. AUTHOR INFORMATION *E-mail: [email protected] (F. Li); [email protected] (H. Zeng) Notes The authors declare no conﬂict of interest.

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Reference (1) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376-1379. (2) Tu, Y. G.; Wu, J. H.; He, X.; Guo, P. F.; Wu, T. Y.; Luo, H.; Liu, Q. Z.; Wang, K.; Lin, J. M.; Huang, M. L.; Huang, Y. F.; Lan, Z.; Li, S. Z. Solvent engineering for forming stonehenge-like PbI2 nano-structures towards efficient perovskite solar cells. J. Mater. Chem. A 2017, 5, 4376-4383. (3) Xiao, Z. G.; Kerner, R. A.; Zhao, L. F.; Tran, N. L.; Lee, K. M.; Koh, T. W.; Scholes, G. D.; Rand, B. P. Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nat. Photonics 2017, 11, 108-115. (4) Li, F.; Wang, H.; Kufer, D.; Liang, L. L.; Yu, W. L.; Alarousu, E.; Ma, C.; Li, Y. Y.; Liu, Z. X.; Liu, C. X.; Wei, N. N.; Wang, F.; Chen, L.; Mohammed, O. F.; Fratalocchi, A.; Liu, X. G.; Konstantatos, G.; Wu, T. Ultrahigh Carrier Mobility Achieved in Photoresponsive Hybrid Perovskite Films via Coupling with Single-Walled Carbon Nanotubes. Adv. Mater. 2017, 29, 1602432. (5) de Arquer, F. P. G.; Gong, X. W.; Sabatini, R. P.; Liu, M.; Kim, G. H.; Sutherland, B. R.; Voznyy, O.; Xu, J. X.; Pang, Y. J.; Hoogland, S.; Sinton, D.; Sargent, E. Field-emission from quantum-dot-in-perovskite solids. Nat. Commun. 2017, 8,14757. (6) Gu, C.; Lee, J. S. Flexible Hybrid Organic-Inorganic Perovskite Memory. ACS nano 19


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(13)Shi, Z. F.; Li, Y.; Zhang, Y. T.; Chen, Y. S.; Li, X. J.; Wu, D.; Xu, T. T.; Shan, C. X.; Du, G. T. High-Efficiency and Air-Stable Perovskite Quantum Dots Light-Emitting Diodes with an All-Inorganic Heterostructure. Nano Lett. 2017, 17, 313-321. (14)Lignos, I.; Stavrakis, S.; Nedelcu, G.; Protesescu, L.; Demello, A. J.; Kovalenko, M. V. Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping. Nano Lett. 2016, 16, 1869-1877. (15)Shan, Q. S.; Li, J. H.; Song, J. Z.; Zou, Y. S.; Xu, L. M.; Xue, J.; Dong, Y. H.; Huo, C. X.; Chen, J. W.; Han, B. N.; Zeng, H. B. All-inorganic quantum-dot light-emitting diodes based on perovskite emitters with low turn-on voltage and high humidity stability. J. Mater. Chem. C 2017, 5, 4565-4570. (16)Xiao, Z. G.; Yuan, Y. B.; Shao, Y. C.; Wang, Q.; Dong, Q. F.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. S. Giant switchable photovoltaic effect in organometal trihalide perovskite devices. Nat. Mater. 2015, 14,193. (17)Walsh, A.; Scanlon, D. O.; Chen, S. Y.; Gong, X. G.; Wei, S. H., Self-Regulation Mechanism for Charged Point Defects in Hybrid Halide Perovskites. Angew. Chem., Int. Ed. 2015, 54, 1791-1794. (18)Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M. J.; Kovalenko, M. V. Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). Nano Lett. 2015, 15, 5635-5640. (19)Solis-Ibarra, D.; Smith, I. C.; Karunadasa, H. I. Post-synthetic halide conversion and selective halogen capture in hybrid perovskites. Chem. Sci. 2015, 6, 4054-4059. 21


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Figure captions: Figure 1 (a)TEM images of CsPbBr3 QDs. The scale bar is 20 nm. (b) XRD pattern of CsPbBr3 QDs film after different purified cycles on glass. (c) Absorption and PL spectra of the CsPbBr3 QDs in octane. Inset: photograph of the CsPbBr3 QDs solution sample (left) and under the UV light (right). (d) Time-resolved PL decays for CsPbBr3 QDs. AFM images of (e) CH3NH3PbI3 film on ITO-coated glass substrate and (f) spin-coated CsPbBr3 QDs on CH3NH3PbI3 film. Figure 2 (a) Schematic representation of the CsPbBr3 QDs deposited on perovskite film. (b) PL images of CsPbBr3 QDs on perovskite film under the irradiation of UV light (365 nm) at 60 s, 360 s, 600 s and 1200 s after spin coating. (c) In-situ PL study of the anion exchange between CsPbBr3 QDs and CH3NH3PbI3 film. Figure 3 (a) In-situ X-ray diffraction (XRD) and (b, c, d) enlarged XRD study of the anion exchange between CsPbBr3 QDs and CH3NH3PbI3 film. The black triangle corresponds to the XRD peak of the substrate. (e) In-situ PL of CsPbBr3 QDs on a PMMA-layer-coated CH3NH3PbI3 films. The inset is a PL image of the sample. (f) Energy profile with PbBr6 octahedron approaches to CsPbI3 (001) surface. Figure 4 In-situ PL study of the anion exchange between CsPbBr3 QDs treated with (a) one and (b) three purification cycles. Schematic representation of the anion exchange process when CsPbBr3 QDs were purified after (c) one cycle and (d) three cycles repectively. Figure 5 (a) Schematic representation of the CsPbBr3 QDs deposited on perovskite 25


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micronet and the gaps. (b) X-ray diffraction patterns for ITO-coated PET substrate, CsPbI3 QDs film and CsPbBr3 QDs/CH3NH3PbI3 micronet on ITO-coated PET substrate. The black triangles correspond to the XRD peaks of the CH3NH3PbI3 micronet, while green diamonds stand for that of CsPbBr3 QDs capped in gaps. (c) Low-magnification SEM image of the perovskite micronet after being coated by CsPbBr3 QDs on the surface. (d, e) High-magnification SEM images taken from the yellow rectangular area. PL microscopic images of CsPbBr3 QDs on perovskite micronet at varied amplification factors under the irradiation of (f1-f3) white light, (g1-g3) blue light, (h1-h3) green light. x1, x2 and x3 correspond to scale bars of 200 μm, 50 μm and 5 μm respectively (x=f, g, h).

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Figure 1

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Figure 2

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Figure 3

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Figure 5

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For Table of Contents Use Only

Composite interface of perovskite quantum dot/film via halide anion exchange

Yang Liua, Fushan Lia,*, Qianqian Lia, Kaiyu Yanga, Tailiang Guoa, Xiaoming Lib, Haibo Zengb,*

Solution processable hybrid perovskite film and quantum dots (QDs) have shown promising application in optoelectronic devices owing to their inherent crystal structures, ionics property and electronic nature. The manipulation of halide anions, for both perovskite bulks and QDs, is definitely critical for rendering such materials with peculiar optoelectronic characteristics. Herein, the tunable emissions at the solid interface between CH3NH3PbI3 film and CsPbBr3 QDs via halide anion exchange are reported. This work expands the understanding of perovskite anion exchange to new dimensions, and implies their potential applications in optoelectronic devices owing to the gradient energy level stem from the interfacial transition.

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Keywords: CsPbBr3 quantum dots; Perovskites; Anion exchanges; Interfaces; Multicolored film.

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film interface with halide anion

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