Solid-State Anion Exchange Reactions for Color Tuning of CsPbX3

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Solid-state anion exchange reactions for color tuning of CsPbX3 perovskite nanocrystals Chris Guhrenz, Albrecht Benad, Christoph Ziegler, Danny Haubold, Nikolai Gaponik, and Alexander Eychmueller Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03980 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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Chemistry of Materials

Solid-state anion exchange reactions for color tuning of CsPbX3 perovskite nanocrystals Chris Guhrenz,†,‡ Albrecht Benad,†,‡ Christoph Ziegler,§ Danny Haubold,† Nikolai Gaponik,*,† and Alexander Eychmüller† †

Physical Chemistry, Technische Universität Dresden, Bergstr. 66b, 01062 Dresden, Germany

§

LUMINOUS! Center of Excellence for Semiconductor Lighting and Displays, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 639798, Singapore

ABSTRACT: Herein, we report on a room temperature anion exchange reaction of highly emitting, all-inorganic CsPbBr3 nanocrystals (NCs) taking place entirely in the solid state. A fast exchange from Br-to-I and Br-to-mixed Br/Cl without exertion of additional energy is observed within minutes to hours taking place by immobilizing the perovskite NCs on pure potassium halide salts (KCl, KBr, KI). By adjusting the halide ratios of the embedding salt matrix, the bright fluorescence of the CsPbX3 (X = Cl, Br, I) NCs can be tuned over a wide spectral range (400 – 700 nm) while maintaining the initial photoluminescence quantum yields of ~ 80 % and the narrow FWHM. We found that combinations of different initial CsPbX3 NCs and KX matrices result in different final halogen contents of the NCs. This is explained with a hostlattice limiting exchange mechanism. The anion exchange rate can be accelerated by pressing the soft, NC loaded salts at pressures of 2.2 GPa. Because of the “cold flow” behavior of the potassium salts during the pressing a complete embedding of the NCs into transparent salt pellets is achieved. This strategy allows for an easy adjustment of the NC loading as well as the form and the thickness of the resulting composite. An encapsulation of the NC-salt pellets with silicone yields robustness and stability of the embedded NCs under ambient conditions. The ease of handling and the superior stability makes the resulting perovskite composite materials attractive for various photonic and optoelectronic applications as demonstrated in a proof-of-concept color-converting layer for a light emitting diode (LED).

INTRODUCTION Perovskite materials with the structural composition ABX3 (where A is a cation of cesium (Cs+), methylammonium (MA+) or formamidinium (FA+); B is a cation of Sn2+ or Pb2+ and X is a halide of Cl-, Br- or I-) are presently in the focus of optoelectronic and photovoltaic applications due to their unique optical properties.1–10 The crystal structure and the photoconducting properties of bulk CsPbX3 all-inorganic perovskites were first reported by Møller in 1958.11 Unsuspectedly, the same perovskite structure is obtained even if the Cs+ is replaced by the small organic MA+ or FA+ cation, resulting in mixed organic-inorganic materials. By using different cation and anion combinations, the resulting hybrid perovskites can be tailored to tune their electronic, optical, magnetic, and mechanical properties.12 The combination of organic and inorganic cations (FA+ with Cs+) e.g. increases the efficiency and thermal stability of the perovskites in solar cell applications.13 In addition, bulk perovskite materials show cation14 and anion15 exchange processes in postsynthetic treatments. Along with the increasing developments in the field of bulk perovskite materials for optoelectronics, Protesescu et al. described the first synthesis of all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals

(NCs).16 This new class of materials is very attractive e.g. for the lighting and display technology because of their tunable photoluminescence (PL) over a broad range of the visible spectrum (400 – 700 nm) with high PL quantum yields (QYs) of up to 90 % and narrow full width at half maxima (FWHM).17–20 Color tuning can be achieved by using different lead halide ratios in the synthesis of the NCs. A control of their optical properties by means of a post-synthetic cation exchange was not achieved for these new materials (for neither Cs+ nor Pb2+).21 In contrast, anion exchange is well applicable for perovskites and has been already described for both MAcontaining bulk and NC solutions of hybrid perovskites treated by halide containing solutions or vapors.6,15,22,23 Moreover, Nedelcu et al.21, Akkermann et al.24 and Ramasamy et al.25 established methods for the postsynthetic tuning of the optical properties of CsPbX3 NCs in solution. In addition, hydrochloric and hydrobromic acid vapors can be used for anion exchange reactions of CsPbX3 NCs in thin films.26 Even though the emission of the resulting materials can be tuned over a broad visible range, both the stability of the NCs and the processability of the anion-exchanged solutions (with an excess of organic stabilizers or pure lead halide salts) still has to be improved. Taking this into account, looking for an

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alternative and easy to handle anion exchange approach is essential. Herein, we report on a method for the direct incorporation of pure CsPbX3 NCs into various ionic salt matrices using a typical material property of the potassium halide salts, namely “cold flow”. This procedure has been already described for Cd-containing NCs in one of our previous works where the ionic salts played a role of an inert matrix.27 In the present case however, solidstate anion exchange reactions between CsPbX3 NCs and the salt take place, turning the matrix into an active component of the composite. Using this concept, pure CsPbBr3 NCs have been incorporated into the robust salt matrix of KCl, KBr and KI salt under preservation of the initial high PL QY. The color tuning of the NCimpregnated salt is determined by the host halide used and can be speeded up within the pressing process resulting in highly-emitting pellets with tunable loading, size, shape and thickness. Starting with CsPbBr3 NCs, which are the most stable ones among the CsPbX3 (X = Cl, Br, I) NCs, this versatile technique enables the color tuning over a broad range of the spectral region by mixing different halide salts. Besides, this method can be expanded to other pure (CsPbCl3, CsPbI3) and mixed halide perovskite NCs. The final pellets combined with an additional, protective silicone coating makes these implemented NCs attractive for the use as color converters for light emitting diodes (LEDs) as demonstrated below.

EXPERIMENTAL SECTION Chemicals. All chemicals used were of analytical grade or of the highest purity available. 1-Octadecene (ODE, 90 %, Aldrich),oleic acid (OA, 90 %, Aldrich), oleylamine (OLA, 70 %, Aldrich), trioctylphosphine (TOP, 97 %, STREM), lead chloride (99.999 %, Aldrich), lead bromide (>98 %, Alfa Aeser), lead iodide (99 %, Aldrich), potassium chloride (KCl, >99.5 %, Aldrich), potassium bromide (KBr, >99 %, Aldrich), potassium iodide (KI, >99.5 %, Aldrich). Potassium halides (KX, X = Cl, Br, I) are milled to a fine powder before use and dried in a furnace at 120 °C for at least 24 hours. Synthesis of CsPbBr3 NCs. The synthesis of pure CsPbBr3 nanocrystals (NCs) was performed according to Protesescu et al. with slight variations.16 A suspension of 0.069 mg (0.188 mmol) PbBr2 in 5 mL ODE, 0.5 mL OA and 0.5 mL OLA was degassed under vacuum at 120 °C for 1 hour. The resulting solution was heated under argon to 150 °C and 0.4 mL of a Cs-oleate solution (0.125 M in ODE, prepared according to ref. 16) was injected rapidly. After 5 s the solution was quenched with an ice-water bath and cooled to room temperature. The resulting NC crude solution was centrifuged for 3 min at 6 krpm and the supernatant was discarded. Subsequently, the NCs were redispersed in 300 µL of n-hexane. After a second centrifugation step (3 min at 6 krpm) the precipitate was discarded and 600 µL n-hexane are added to the NC

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containing supernatant. The resulting solution was used for further studies. CsPbCl3 and CsPbI3 NCs were synthesized according to the strategy described above substituting PbBr2 by 0.052 g PbCl2 and 0.087 g PbI2, respectively. After the complete solubilization of the PbX2 (X = Cl, I) precursor (for CsPbCl3 NCs 1 mL of TOP was added before injection) an injection temperature of 170 °C for CsPbCl3 and 150 °C for CsPbI3 NCs was used. Preparation of NC-Salt Mixed Crystal Pellets. In a 10 mL pointed flask 500 mg of KX (X = Cl, Br, I) salt were mixed under stirring with 2 mL of n-hexane before a desired amount of the pure NC solution was added (standard loading: 4 mg NCs/g KX). The resulting suspension was mixed and the solvent was removed to dryness under reduced pressure using a rotary evaporator. Following, the NC-impregnated salt was dried under vacuum with Schlenk technique and transferred into a nitrogen filled glove box, where all of the following steps are performed. Before further use the NC-impregnated salt was milled to a fine powder. Normally, 100 mg of the NC-impregnated KX salt were pressed (at an Atlas 15T manual hydraulic press GS15011 from Specac) at 2 tons (equal to 2.2 GPa) for 5 min under vacuum. A 6 mm pressing die (model P0819 from msscientific GmbH) was used. The resulting pellets were stored under inert atmosphere for further use. For loadings between 2 and 20 mg NCs/g KBr salt, no significant influence on the resulting pellets has been seen for CsPbBr3 NC samples. However, higher loadings typically result in oily films on the pellet arising from residual organics present in the NC solution and from the NC surface. Emission Color Tuning. The color of the resulting pellets can be tuned by adjusting the ratios of KBr/KCl and KBr/KI, respectively. As will be discussed below, the efficiency of the anion exchange is increasing in a row Cl < Br < I. For red-emission color tuning, minimal amounts of KI (1.0 – 2.5 wt%) together with KBr were recrystallized from water, dried in a furnace at 120 °C and finally ball milled. The NCs were loaded on this recrystallized powder and dried under vacuum. The final color was obtained within the pressing step. Enhancement in Stability. The NC-impregnated powders of the pure halide salts are stable for several days under ambient conditions. Nevertheless, to assure a better reproducibility, we worked completely under inert atmosphere to preserve the high PL QYs. To prevent the final pellet from degradation through the influence of air and/or moisture, as prepared pellets were covered with a two-component silicone resin (ACC Silicones) resulting in an enhancement in stability and processability under ambient conditions. A template with holes of 8 mm is used for the silicone coating of the 6 mm pellets. First, 80 µL of the silicone resin are filled into the hole and polymerized at 60 °C for 3 hours. Next the NC-salt pellet is placed in the center of the hole and covered completely with 120 µL of fresh silicone, which polymerizes for 48 hours at room temperature.

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LED Preparation. For the preparation of a proof-ofconcept white LED, green- (CsPbBr3 NCs in KBr) and redemitting (anion-exchanged CsPbBr3 NCs in KI) pellets were joined in a nitrogen filled glove box with a twocomponent silicone resin (ACC Silicones) placed on top of a commercial blue-emitting InGaN LED (ASMT-MB00NDF00 model with a dominant wavelength at 460 nm at 350 mA from Avago Technologies). Absolute Measurement of Photoluminescence Quantum Yields (PL QYs) and UV/vis Absorption Measurements. Absolute PL QY measurements were performed using a FluoroLog-3 spectrofluorometer (Horiba Jobin Yvon) equipped with a Quanta-φ integrating sphere. The pellets were placed inside a Spectralon®-holder covered with a quartz-glass slip and mounted at the bottom of the integrating sphere. For green- and red-emitting samples an excitation wavelength of 450 nm and for blue-emitting samples of 350 nm was used. Pellets of the pure KX (X = Cl, Br, I) salt were used for blank measurements. UV/vis absorption spectra of the NC solutions were recorded using a Cary 50 spectrophotometer (Varian). The absorption spectra of the embedded CsPbBr3 NCs in KBr were acquired using a Cary 5000 spectrophotometer (Varian) with an integrating sphere setup. Transmission Electron Microscope (TEM). TEM images were recorded using a JEOL JEM-1400Plus microscope operated at 120 kV.

RESULTS AND DISCUSSION Using a hot-injection approach according to Protesescu et al., we synthesized highly emitting, pure CsPbBr3 NCs with cubic shape (~9 nm in edge length, Supporting Information, Figure S1).16 Impurities consisting of larger NCs and very small NCs were eliminated by two centrifugation steps. As prepared particles show characteristic narrow FWHM of ~ 23 nm with a PL maximum at 506 nm and a high PL QY of 77 % in n-hexane. Following, the pure NCs were embedded into KBr salt using a strategy from sample preparation e.g. for IR-spectroscopy, namely “cold flow”. Several soft salts, e.g. KCl, KBr, KI, CsI, NaCl, AgCl, etc., show this sintering behavior under pressure.28 The most prominent candidates are the potassium halide salts, which are soft and therefore show “cold flow” already at moderate pressures, resulting in transparent pellets. The incorporation of NCs into the ionic salt matrix has been already described for Cd-containing NCs in one of our previous works.27 Using this strategy, normally, 4 mg/g KBr of the pure CsPbBr3 NCs have been immobilized on the pure salt by removing the solvent under reduced pressure from a suspension of KBr salt in the NC solution. This strategy allows to load the NCs on the support material and to adjust the NC loading of the final pellet. The resulting NC-impregnated salt material shows a characteristic greenish color under ambient light and a pronounced green emission under UV light as can be seen from Figure 1. Pressing the NC-impregnated salt material

results in a green-emitting, transparent pellet with high PL QY (Figure 1). Starting from 77 % PL QY in solution this value is slightly increased after pressing and reached 83 %. The high pressure has no significant influence on the stability and emission color of the resulting pellets. Comparable results have been achieved for loadings between 2 – 20 mg/g KBr salt. The PL maximum of the green-emitting pellet is only slightly red-shifted from 506 nm in solution to 509 nm in the pellet (Figure 1). This moderate shift is due to the change in the dielectric constant of the surrounding media (hexane to KBr) and has been already observed in previous publications.27,29–33 The comparison of the absorption spectra of the CsPbBr3 NCs in solution and incorporated into a KBr pellet exhibits a preservation of the first excitonic transition and a small broadening of the peak (Supporting Information, Figure S2). Looking for a less hygroscopic matrix for CsPbBr3 NCs, we chose also the other soft potassium salts KCl and KI for the incorporation technique described above (Figure 2, top). Changing the halide in the embedding salt results in solid-state anion exchange reactions observable already after NC loading on the finely milled KX (X = Cl, I) powders. This exchange enables a compositional tuning of the NCs and hence a change of their optical properties. The powders and pellets resulted from the loading of the NCs on KCl and KI appeared to be highly emitting in the blue and red spectral region, respectively (Figure 2, bottom). The substitution of the halides starts immediately after complete vacuum evaporation of the solvent when the pure and dry CsPbBr3 NCs are in direct contact with the salt surface of KX (X = Cl, I).

Figure 1. PL spectra of the initial CsPbBr3 NC solution in n-hexane in comparison to NCs embedded in a KBr pellet. The inset shows a true color image of CsPbBr3 NCs loaded on KBr salt and the corresponding KBr pellet (loading: 4 mg/g KBr). The image was taken under illumination at 365 nm.

By changing the halide in the supporting salt, the emission of the NCs can be tuned to PL maxima of 464 nm for bare KCl and 669 nm for bare KI, respectively (Supporting Information, Figure S3). For both pellets with anion-exchanged NCs shown in Figure 2, the PL QYs are



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79 % and therefore they are as high as in the initial CsPbBr3 solution. In contrast, Akkerman et al. observed a decrease in the PL QYs during the solution processed anion exchange (PL QYs reduced to less than 40 %).24 The FWHM of the blue-emitting sample (~ 22 nm) is comparable with the initial bromide containing NCs while a typical small broadening (~ 39 nm) for the redemitting sample is observed (Supporting Information, Table S1). However, if expressed in energy scale, the FWHM of the CsPbBr3 NCs remained very narrow after the anion exchange (from 105 meV in the original solution to 127 meV in KCl and 108 meV in KI matrices, respectively).

Figure 2. Scheme of the embedding procedure of CsPbBr3 NCs into different KX (X = Cl, Br, I) salts (top). The anion exchange is not observable when the fine salt powder is dispersed in the NC solution, however, this process starts immediately after vacuum evaporation of the solvent, when the pure and dry CsPbBr3 NCs are in direct contact with the salt crystals. True color image of CsPbBr3 NCs loaded (loading: 4 mg/g salt) on KX (X = Cl, Br, I from left to right) salts and incorporated into KX pellets (bottom). The images are taken under illumination at 365 nm.


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According to the results from literature, pure CsPbI3 NCs are emitting, depending on the size of the NCs, in a spectral region between 650 – 700 nm (PL maximum), which is reached easily in our experiments via anion exchange.16 In contrast, pure CsPbCl3 NCs are emitting at ~ 400 nm, while the anion exchange from Br-to-Cl just allows us to reach 464 nm. Hence, we come to the conclusion that with the embedding of CsPbBr3 NCs into KCl only an incomplete exchange resulting in CsPb(Cl/Br)3 NCs is possible. The type of the halide anion has a significant influence on the time necessary for a complete anion exchange (Supporting Information, Table S2). The exchange with iodide is fast and already completed without pressing within 1 – 2 hours (Supporting Information, Video). In contrast, the anion exchange from Br-to-Cl is always incomplete, relatively slow and becomes clearly visible only after a few hours (Supporting Information, Figure S4). However, pressing the NCs-loaded KCl powder at “cold flow” conditions can accelerate this slow process. Under “cold flow” conditions the final perovskite composition in the NC-salt composite is reached within 5 minutes compared to several days without pressing. Summarizing our observations, the anion exchange reaction can be speeded up with the pressing process and faster rates are observed for iodide containing salt materials. The observed difference in the exchange rates matches the expectations based on the mobility of the halide ion vacancies in the perovskite lattice via the vacancy diffusion mechanism, as it was described for bulk CsPbX3 (X = Cl, Br).34 Therefore, the mobility at room temperature should increase from chloride via bromide to iodide. Comparing the activation energies for the migration of anion vacancies into the perovskite structure (0.29 eV for CsPbCl3 and 0.25 eV for CsPbBr3) further supports our findings. Overall the activation energies reported for these perovskite materials are relatively small in comparison to other ionic compounds, allowing for a relatively fast solid-state anion exchange at room temperature. Although the different migration rates explain the different anion exchange rates observed, the incomplete exchange of bromide to chloride cannot be resolved at this stage. An influence of the potassium ion diffusion can be excluded, because the alkali halides show a limited solubility in the perovskite structure.34 At this point, the lattice parameters of the potassium salts used have to be taken into account, since the exchange rate may also be limited by the diffusion of anions from the perovskite NCs to the host salt matrix. In this case, with an increase in the lattice parameter of the host salt crystal (Table 1), the diffusion of smaller anions into a broader lattice is preferred, while the diffusion of larger anions into a narrower lattice is restricted. This explains the complete anion exchange from Br-to-I resulting from an easy migration of the bromide ions into the broad lattice of KI. In contrast, the anion exchange from Br-to-Cl is limited by the diffusion of the larger bromide ions into the relatively narrow KCl matrix and therefore slow and incomplete.

Table 1. Overview about the lattice constants of pure KX 35,36 (X = Cl, Br, I) salts and the size of the halide anions.

Potassium salt

Lattice constant [Å]

Halide anion

Size [pm]

KCl

6.29

Cl-

167

6.60

-

Br

182

7.06

-

206

KBr KI

I

In support of the proposed exchange mechanism, we additionally studied the incorporation of pure CsPbCl3 and CsPbI3 NCs into various potassium halide salts (Figure 3 and Supporting Information, Table S1 and Figures S5–S7). If our assumptions are correct the anion exchange of pure CsPbCl3 NCs in KBr and KI should be complete, while the anion exchange of pure CsPbI3 NCs in KBr should be restricted and in KCl not or hardly observable.

Figure 3. Overview of the anion exchange process of pure CsPbX3 (X = Cl, Br, I) NCs embedded into various ionic matrices (KCl, KBr, KI). Multicolored lines symbolize the achievable spectral regions with particular perovskites in various salts. The spectral positions of the embedded perovskites in each pure salt are also shown. “N/A” indicates spectral regions which are not achievable by the corresponding perovskite NCs in any of the three salts. We have to note, that an additional slight color tuning is also possible via changing of the size of the initial NCs (quantum size effect). For example, it is expected, that for bigger perovskite NCs PL maxima up to 700 nm can be reached in the KI matrix.

Embedding pure CsPbCl3 NCs (PL maximum at 400 nm; 4 mg/g KX, X = Br, I) into KBr and KI salts results in pure cyan- (PL maximum at 490 nm) and red-emitting (at 673 nm) NCs, respectively (Supporting Information, Figure S6). The PL peak of the NCs in KBr starting with



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CsPbCl3 NCs is at higher energies compared to the direct incorporation of CsPbBr3 NCs into KBr. This is due to quantum confinement effects since the initial CsPbCl3 NCs are smaller than the initial CsPbBr3 NCs. Hence, the maxima values reached suggest complete anion exchanges, as expected from the host limiting theory discussed above. For the Cl-to-Br exchange however, the process is slow and it is necessary to press the salt matrix to achieve a complete PL shift. The moderate rate of the process has, however, its advantages as it allows to obtain mixed samples with intermediate PL colors. In comparison, the Cl-to-I exchange results directly in pure CsPbI3 NCs without showing any intermediate color. This direct and quick exchange was already observed by Nedelcu et al. for the Cl-to-I exchange reaction in solution and was attributed to the large difference in the ionic radii, leading to a higher stability of the singlehalide NCs.21 The embedding of pure CsPbI3 NCs (PL maximum at 678 nm; 10 mg/g KX, X = Cl, Br) into KCl (PL maximum at 657 nm) and KBr (at 573 nm) pellets further supports our proposed anion exchange mechanism. As seen from the reached PL maxima, the solid anion exchange from I-toBr was incomplete (Supporting Information, Figure S7). The process cannot be further forced even if a large excess of the bromide was used. This suggests the formation of mixed CsPb(Br/I)3 NCs and is in agreement with our proposed limitation of larger iodide ions migrating into the smaller KBr lattice. Furthermore, a direct anion exchange from I-to-Cl does not proceed at all, which originates from the restricted diffusion of the big iodide ions into the narrow KCl crystal lattice. An observed blueshift of the PL maximum of the resulting KCl pellet can be attributed to a partial anion exchange from I-to-Cl on the interface between the salt and perovskite NC, which reduces the NC size. An anion mixture of CsPb(Cl/I)3 is not described in the literature, but it was shown, that a high chloride doping level can stabilize the CsPbI3 phase.37 Using our embedding strategy, this was not observed during the NC incorporation. The stability of the embedded CsPbI3 NCs is limited to a few days, which supports the assumption that the chloride ions are etching the NC surface and are diminishing the NC size and stability. In general, directly synthesized CsPbI3 NCs, which are embedded into KI, show a lower stability than anion-exchanged, iodide-containing perovskites produced from CsPbCl3 and CsPbBr3 NCs, respectively. It is known from literature that colloidal CsPbI3 NCs show a low stability of less than two days.38 The PL efficiency is decreasing due to a phase transition of the perovskite NCs from the luminescent cubic to its non-luminescent orthorhombic phase.39,40 Indeed, the anion-exchanged CsPbI3 NCs on KI salt made from CsPbBr3 NCs show stability for several weeks, even when kept under ambient conditions (Supporting Information, Figure S8). The PL intensity is reducing over time, but clearly visible even after 14 days. This is an important step forward for making the iodide perovskite phase applicable. Taking into account the very high Br-to-I exchange rate and the different affinity of bromide and iodide for

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anion exchange reactions, we focused on the preparation of pellets with a peak emission at intermediate positions of the spectral region between 510 nm and 670 nm. For a color tuning in this region, CsPbBr3 NCs were chosen as starting material to be loaded on different KBr/KI salt mixtures. We observed that a simple addition of small amounts of KI salt to the NC loaded KBr salt resulted in pellets with local green- and red-emitting spots, indicating a complete anion exchange in this KI rich parts of the mixture. For a homogeneous color tuning of the entire pellet, the pure KBr and KI salts have to be intermixed properly before the immobilization of the NCs. For a better distribution of the halides within the crystallite, we recrystallized different ratios of KBr and KI from aqueous solution, dried the powder and milled it in a ball mill. The affinity of the perovskites to the iodide is so high, that already 2.5 wt% KI in the mixed salt result in a complete PL red-shift to ~ 670 nm in the final pellet. Additionally, moisture from air seems to have a crucial influence and supports the substitution of bromide to iodide within the lead halide perovskite crystal structure. Using a mixture of KBr/KI with 1.5 wt% KI, we produced a yellow-emitting pellet (Supporting Information, Figure S9) with a PL maximum at 587 nm and a broadened emission line widths of ~ 80 nm. This and a small shoulder at ~ 550 nm can be attributed to an insufficient homogenization of the recrystallized mixture resulting from the different solubility of KBr and KI in water. However, it shows the general possibility of tuning the emission color of the NCs over the entire region between 510 – 670 nm by using mixed crystals of KBr and KI with different ratios as the host material. To overcome the problem of inhomogeneous halide distribution within the mixed crystals a recrystallization of mixed salts from their melts can be suggested as a possible solution to reach the necessary homogeneity of the samples.

Figure 4. Silicone-encapsulated pellets of CsPbBr3 NCs in KBr (left) and anion-exchanged CsPbBr3 NCs in KI (right).

Typically, the pellets were coated with a protective silicone layer to avoid undesirable color changes and a decomposition of the NCs due to moisture (Figure 4). Thereby, the pellets can be handled under ambient conditions. The emission properties do not change during the encapsulation (Supporting Information, Figure S10). Pure CsPbBr3 NCs embedded in KBr and anion-exchanged CsPbBr3 NCs embedded in KI show a high stability of at being protected with the silicone layer. In comparison to the low PL QYs of less than 15 % for CsPbBr3 NCs embedded into PMMA as shown by Protesescu et al.,41 the high values of up to 80 % are still preserved while


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embedding CsPbBr3 NCs into KBr and are just slightly reduced to ~ 60 % during the additional silicone coating (Supporting Information, Figure S11). Our results show that the PL QY of the NC-salt pellet starts to decrease after 7 days being kept under ambient conditions. Due to the hygroscopy of the salt matrix the PL QY is reduced to half of its initial value after 14 days. In contrast, the silicone-encapsulated NC-salt pellet shows unaltered stability for at least 14 days. The initial reduction of the PL QY to ~60 % may be explained by the degradation of the NCs associated with the surface of the salt pellet when exposed to the aggressive silicone polymerization media. To demonstrate the applicability of our samples, we produced a proof-of-concept color conversion LED by using green- and red-emitting CsPbBr3 NCs embedded into KBr (2 mg CsPbBr3/500 mg KBr, 40 mg pellet) and KI (10 mg CsPbI3/500 mg KI, 30 mg pellet) and encapsulated in a protective silicone layer (Figure 5, top). Generally, the pellets can be milled to a powder, but a direct contact of different potassium halide salts has to be avoided to restrict an undesired anion exchange. Therefore, the pellets are simply stacked on top of a commercially available 1 W blue-emitting InGaN chip. In operation, the LED emits white light. In the corresponding PL spectrum the maxima of the CsPbBr3 and CsPbI3 NCs are clearly visible (Figure 5, bottom).

In summary, we have demonstrated a remarkable solid-state anion exchange reaction in CsPbX3 (X = Cl, Br, I) perovskite NCs while immobilized and pressed into soft potassium halide salts (KCl, KBr, KI) using “cold flow”. This first example of ion exchange involving NCs in the solid state greatly simplifies the post-synthetic tuning of the optical properties of the perovskite NCs. We propose a host matrix limiting anion exchange mechanism to explain the different mobilities of the halide ions for the different initial CsPbX3 (X = Cl, Br, I) NCs. After the “cold flow” the emission properties of the lead halide perovskites, such as high PL QY (up to ~ 80 %) and narrow FWHM, are preserved. By varying both the type of the pure halide perovskite NCs and the salt composition a spectral color tuning over a wide range of the visible spectral region can be accomplished (400 – 700 nm). Taking into account the quantum size effect of the pure NCs an additional fine-tuning of the resulting color by changing the size of the original NCs is possible. We also observed, that the described concept is applicable to other metal halide salts (e.g. cesium halide salts, Supporting Information, Figure S12) and results in similar solid-state anion exchange reactions. The embedding of the NCs into the inorganic ionic salt matrix provides a robust and processable surrounding. Furthermore, an additional silicone encapsulation of the perovskite loaded pellets combines advantages of both materials and ensures a high stability of the embedded NCs under ambient conditions. The excellent optical characteristics, adjustable NC loadings, form and thickness, and the easiness of handling make the resulting composite pellets attractive for applications in optoelectronics e.g. as color conversion materials for solid-state lighting, laser gain media and solar light concentrators. Additional studies should focus on the pellet purity of mixed crystals by using melted KCl/KBr or KBr/KI salts, respectively. Presumably, the solid-state anion exchange strategy can also be used for organic-inorganic perovskite materials like FAPbX3 and MAPbX3.

ASSOCIATED CONTENT

Figure 5. Proof-of-concept preparation of a white-light LED by stacking green- and red-emitting pellets on a commercially available blue-emitting InGaN LED under ambient conditions and in operation (top) with the corresponding PL spectra (bottom).

CONCLUSION

Supporting Information. TEM images of CsPbX3 (X = Cl, Br, I) NCs, absorption spectra of CsPbBr3 NCs in solution and embedded in KBr, true color images and PL spectra of CsPbX3 NCs embedded into various potassium halide salts, overview about the emission properties of the embedded CsPbX3 NCs in various salt matrices, PL evolution of the Brto-Cl exchange, PL spectra of anion-exchanged CsPbBr3 NCs on KI over time, true color image and PL spectra of loaded and NC-embedded mixed KBr/KI salt, stability tests of embedded CsPbBr3 NCs, PL spectra of CsPbBr3 NCs in cesium halide salts. Time laps video of the solid-state anion exchange of CsPbBr3 NCs on KI salt. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


Chemistry of Materials *E-mail: [email protected]

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡ These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the M-ERA.NET Network within the project ICENAP, GA 1289/3-1 as well as by the DFG Project EY 16/14-3. We also thank the Department of Inorganic Chemistry II of the TU Dresden for sharing their press facilities and Susanne Goldberg for TEM measurements.

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