Cs2NaBiCl6:Mn2+ â A new orange-red halide double perovskite

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CsNaBiCl:Mn – A new orange-red halide double perovskite phosphor Jackson D Majher, Matthew B. Gray, T. Amanda Strom, and Patrick M Woodward Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05280 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Chemistry of Materials

Cs2NaBiCl6:Mn2+ – A new orange-red halide double perovskite phosphor Jackson D. Majher†, Matthew B. Gray†, T. Amanda Strom#, and Patrick M. Woodward†* †Department

of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210,

United States #Department of Material Science, UC Santa Barbara, 2066C Materials Research Lab, Santa Barbara, CA 93106, United States ABSTRACT: In this work, we report on the promising photoluminescent behavior of the cubic double perovskite Cs2NaBiCl6 doped with Mn2+ ions. Localized excitations centered on Bi3+ ions in the host lattice strongly absorb near-UV light. In the undoped host compound only very weak photoluminescence is observed, but in manganese-doped samples, energy transfer from Bi3+ to Mn2+ leads to intense orange-red photoluminescence. A broad emission peak centered at 590 nm is assigned to the 4T1 → 6A1 transition of octahedrally coordinated Mn2+. The excitation spectrum contains peaks at 294 nm and 354 nm that arise from 6s2 → 6s16p1 excitations of Bi3+ ions. If the chloride ions are partially replaced by bromide ions, the strongest excitation peak red-shifts to 375 nm. The lack of expensive reagents and toxic elements, and the ability to tune the excitation and emission spectra through chemical substitution make Cs2NaBiCl6−xBrx:Mn2+ a promising phosphor system.

Introduction Halide perovskites with formula ABX3 (A = CH3NH3+; Cs+; B = Pb2+, Sn2+, Ge2+; X = Cl−, Br−, I−) have been widely studied for use in various optoelectronic applications, most notably as the absorbing layer in photovoltaic cells.1-5 The efficiency of perovskite solar cells has rapidly climbed over the last decade with power conversion efficiency (PCE) improving from 3.8% to a reported 20.35% for planar, single-junction cells.3,5 The photoluminescent properties of halide perovskite quantum dots, particularly CsPbX3, have also attracted interest due to their near unity quantum efficiencies and tunable emission spectra.6-8 Inorganic halide double perovskites with formula A2BIBIIIX6 (A = Cs+, Rb+; BI = Ag+, K+, Na+, Li+; BIII = Bi3+, Sb3+, In3+; X = Cl−, Br−, I−) are site-ordered variants of the perovskite structure that exhibit an alternating (rock salt) pattern of BI and BIII ions on the octahedral sites. These double perovskites are attractive because they do not contain the toxic Pb2+ ion and offer enhanced chemical and thermal stability. While many A2BIBIIICl6 double perovskites can be prepared, very few examples of semiconducting bromide and iodide analogs have been reported, Cs2AgBiBr6 being the most notable exception.9 The larger band gaps of the double perovskites along with an often-reduced electron dimensionality presents many challenges for the use of these materials as solar absorbers in photovoltaic cells.10,11 However, halide double perovskites show promise for other applications including scintillators,12,13 solid state lasers,14 photocatalysts,15 and down-conversion phosphors.16 The use of halide double perovskites in phosphor converted light emitting diodes (pc-LEDs) may be one of their most promising potential applications. Pc-LEDs can produce white light with efficiencies 10 times higher than incandescent lights and double that of fluorescent lights. Since lighting accounts for roughly one-fifth of global

electricity consumption17, improvements in the efficiency, stability and/or cost of pc-LEDs can have tremendous societal impact.18 Most pc-LEDs pair a blue-emitting Ga1−xInxN LED with a yellow-emitting phosphor, usually Y3Al5O12:Ce3+. A limitation of this approach is the slight deficiency of orange and red components in the emitted light, which is perceived as a “cool white.” For applications that demand warmer white light, a combination of green and red phosphors is used in conjunction with the blue LED.19 Cs2AgInCl6 is a cubic double perovskite (a ≈ 10.47 Å) with a direct bandgap of 3.3–3.5 eV.20,21 Recently several research groups have focused on the photoluminescence (PL) of this double perovskite, which in the pure form exhibits very weak red photoluminescence.22-24 The inefficient nature of PL in Cs2AgInCl6 has been attributed to the parity-forbidden nature of the radiative relaxation of free and self-trapped excitons.16 To improve efficiency, Nanda and Nag doped Cs2AgInCl6 with the activator Mn2+, which leads to red PL (λmax = 619 nm). However, the reported quantum efficiencies are still relatively low, 3 – 5%.22 Luo et al. pursued a different strategy by alloying Cs2AgInCl6 and Cs2NaInCl6 to change the local symmetry and relax the parity-forbidden nature of the exciton recombination.16 This approach yielded bright PL with a broad emission peak that spanned much of the visible spectrum, peaking near 550 nm in Cs2(Ag0.60Na0.40)InCl6. The quantum yield was reported to be an impressive 86 ± 5%; a figure that makes halide double perovskites of considerable interest for use in pc-LEDs. Cs2AgInCl6 is but one member of a large family of double perovskites. In this paper, we report the absorption and emission characteristics of another halide double perovskite, Cs2NaBiCl6, doped with Mn2+. The undoped Cs2NaBiCl6 host has a cubic structure at room temperature that distorts to tetragonal below 90 K.25 Incorporation of the highly electropositive Na+ cations on alternating octahedral 1

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when opening the hydrothermal autoclaves as unreacted phosphonic acid can ignite upon exposure to air. MnCl2∙4H2O was added so that the Mn:(Na+Bi) molar ratio was 0.20. The vessel was sealed and heated in a furnace to 150 °C for 6 hours before cooling at 1 °C/hour to 25 °C. Single crystals of the desired product were filtered and washed with diethyl ether (Fisher Scientific). The crystals were ground into a fine powder for subsequent characterization steps. Powder X-ray diffraction (PXRD) data was collected on a Bruker D8 Advance powder diffractometer (40 kV, 40 mA, sealed Cu X-ray tube) equipped with a Lynxeye XE-T position sensitive detector. Data was collected with an incident beam monochromator (Johansson type SiO2crystal). Rietveld refinements of laboratory PXRD data were carried out using the TOPAS-Academic (Version 6) software package to determine the crystal structure.29 UV-visible diffuse reflectance spectroscopy (DRS) data was collected from 180–890 nm with an Ocean Optics USB4000 spectrometer equipped with a Toshiba TCD1304AP (3648-element linear silicon CCD array). The spectrometer was used with an Ocean Optics DH-2000-BAL deuterium and halogen UV−vis−NIR light source and a 400 μm R400-7-ANGLE-VIS reflectance probe. The detector was calibrated using a Spectralon Diffuse Reflectance Standard. Electron paramagnetic resonance (EPR) spectra were collected on a Bruker EMX-Plus Electron Spin Resonance spectrometer at room temperature. Spectra were collected at 60 dB with a scan range between 3000 and 3700 G. The modulation amplitude used was 10 Gauss. Luminescent data was obtained using a Horiba Fluoromax-4 (Xenon source, 1 mm excitation and emission slit widths, 1 nm step size) equipped with a solid state sample holder. Luminescent data was analyzed using the FluorEssence (v3.5) software powered by Origin. All photo-luminescent quantum yield (PLQY) measurements were performed in a Jovin Horiba FluoroMax4 equipped with a Quanta-φ integrating sphere (15 cm) and a PTFE sample cup. Non-luminescing barium sulfate (Wako, #022-00425, Lot SDF1291) was used as the blank reference sample. All samples were excited at 355 nm (λmax for most samples) and the luminescence signal integrated from 500 to 775 nm. Radiometric, sphere, and dark count corrections were applied during data acquisition while corrections for filters and integration time differences were applied in the FluorEssence™ analysis package for Quantum Yield (FluorEssence v3.8.0.60, Origin v8.6001). Additional experimental details are available in the Supporting Information. Thermogravimetric analysis (TGA), was collected on a Thermogravimetric Analyzer TGA Q50. Samples were heated under a nitrogen stream of 50 mL/min with a heating rate of 10 °C/min between 25 °C and 425 °C. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was performed using a PerkinElmer Optima 4300DV Inductively Coupled Plasma Optical Emission Spectrometer. Samples were dissolved using

sites results in an electronic structure that is effectively zero-dimensional, and can be understood by analyzing localized excitations of [BiCl6]3− octahedra.26 This view is further supported by band structure calculations that reveal narrow bands, consistent with localized electronic states that are hypothesized to be favorable for self-trapped and dopant-bound excitons.27 The results presented here show that when Mn2+ is incorporated into Cs2NaBiCl6 it acts as an activator leading to a bright orange-red phosphor. Further chemical substitution, replacing Cl− with Br− ions, leads to a red-shift of the excitation spectrum, and an enhancement of the absorption of blue light. Experimental The following reagents were purchased and used without modification: HCl (Fisher Scientific, 37%), HBr (Sigma Aldrich, 48%), Bi2O3 (J.T. Baker, 99%), CsCl (Alfa Aesar, 99.9%), NaCl (GFS Chemicals, 99%), In2O3 (Alfa Aesar, 99.994%), and MnCl2∙4H2O (Mallinckrodt, 99%). AgCl was synthesized on a gram scale via a precipitation reaction between KCl (Fisher Scientific, 99.8%) and AgNO3 (Alfa Aesar, 99.9%). The resulting white product was washed three times with deionized H2O and dried overnight via vacuum filtration. While drying, the filter flask was covered with foil to prevent photoreduction of Ag(I) into Ag(0). To prepare Mn2+-doped polycrystalline Cs2NaBiCl6 samples, 25 mL of concentrated HCl was added to a roundbottom flask and heated to 80 °C in an oil bath. For a synthesis that yields ~2.0 g of product, Bi2O3 (1.41 mmol), NaCl (2.82 mmol), and varying amounts of MnCl2∙4H2O (0.03–7.03 mmol) were added to the flask and stirred. Upon full solvation of these precursors, CsCl (5.63 mmol) was added, leading to the immediate formation of a white precipitate. The solution was left to stir for an additional 20 minutes before removing it from the oil bath and allowing the product to cool to room temperature to ensure complete reaction of precursors. The precipitate was then filtered using a porous fritted funnel, washed several times with neat ethanol (Decon Labs Inc., 200 proof), and dried overnight via vacuum filtration. The resulting powder was ground for 30 minutes and heated in an oven at 210 °C for 10 hours to ensure homogenous Mn2+ incorporation. For comparison, a modified procedure for the synthesis of Mn2+-doped Cs2AgInCl6 samples reported by Nandha et al. was carried out.22 In2O3 (1.43 mmol), AgCl (2.86 mmol), and MnCl2∙4H2O (0.57 mmol) were dissolved in concentrated HCl and CsCl (5.70 mmol) was added to trigger precipitation. Due to the limited solubility of AgCl in solution, the CsCl was added before the AgCl precursor was completely solvated. The solution stirred for 3 hours at 80 °C to ensure the complete formation of the product. The resulting powder was filtered and washed with ethanol. To prepare Mn2+ doped Cs2NaBiCl6–xBrx samples, stoichiometric amounts of CsCl, NaCl, and Bi2O3 were added to a Teflon-lined Parr reactor containing varying mixtures of HCl/HBr along with phosphonic acid (H3PO2, SigmaAldrich, 50 wt. % in H2O), which can act as a stabilizing agent.28 The ratio of hydrohalic to phosphonic acid was kept at a 9:1 volume/volume ratio. Caution should be taken 2


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Chemistry of Materials concentrated nitric acid, and the resulting solution was diluted with deionized water. Results PXRD patterns of the pure and doped products indicated phase pure samples of Cs2NaBiCl6 (Figure 1). The lack of peak splitting is consistent with cubic Fm3m space group symmetry and the refined lattice parameter of 10.84229(7) Å for the undoped sample is in good agreement with the value reported in the literature, 10.839(1) Å.30 The Na+ and Bi3+ ions are fully ordered, as evidenced by the strong intensity of the (111) peak. Rietveld refinements give Cs–Cl, Na–Cl, and Bi–Cl bond distances of 12×3.83363(4) Å, 6×2.736(5) Å, and 6×2.686(5) Å, respectively, all of which are similar to bond distances reported in related chloride double perovskites (see SI for more details).

S1), Cs2Na1−xBi1−xMn2xCl6 samples with x ranging from 0.0003 to 0.036, as determined by ICP-OES, were synthesized. PXRD analysis shows a very small but systematic decrease in lattice parameter as the Mn2+ incorporation increases (Table S1 and Figure S2). A full description of the calculations used to estimate the composition is provided in the Supporting Information. To confirm the oxidation state of the incorporated manganese in the host material, EPR measurements were taken on several doped samples (Figure 2). EPR spectra presents a six-fold hyperfine coupling pattern characteristic of isolated Mn2+ ions, with minor broadening at higher doping levels, most likely due to exchange interactions between proximal Mn2+ ions. Observed hyperfine splitting of 8.7–8.9 mT and g-factor of 2.012 are in good agreement to those reported in other Mn2+ doped perovskite systems.22,24,31

Figure 1. Rietveld refinement of the PXRD scan of Cs2NaBiCl6, performed using TOPAS-6 Academic. The observed data, calculated fit, and difference curve are shown with black dots, a red line, and a blue line, respectively. The cubic double perovskite structure is shown in the inset (Na- and Bi-centered octahedra in red and gray respectively, Cs+ ions as blue spheres).

Figure 2. Room temperature X-band EPR scan of manganesedoped samples showing 6-fold hyperfine coupling pattern characteristic of high-spin Mn2+.

All samples were analyzed for thermal stability via thermogravimetric analysis, which indicated that the undoped Cs2NaBiCl6 host is stable up to 450 °C, as shown in Figure S4. The presence of Mn2+ slightly decreases the temperature at which decomposition begins, but the decrease appears to be independent of Mn2+ content. In addition, no apparent loss of luminescence is observed after heating the samples at 210 °C for 10 hours. UV-visible diffuse reflectance spectra of selected Cs2NaBiCl6:Mn2+ samples are presented in the SI (Figure S5). The reflectance data was transformed to pseudoabsorbance (Figure 3) using the Kubelka-Munk (KM) function,  = (1−R)2/2R, where α is the optical absorption coefficient and R is the reflectance.32 Four absorption bands, all associated with 6s2 → 6s1p1 transitions of the [BiCl6]3− octahedra can be identified: The spin forbidden 1S0 → 3P0,1,2 transition that is split into three peaks by spin-orbit coupling, and the spin allowed 1S0 → 1P1 transition. 33 The

To prepare Mn2+-doped samples, MnCl2∙4H2O was dissolved in concentrated HCl solution, along with stoichiometric amounts of Bi2O3 and NaCl. As with the undoped sample, CsCl was added to the solution to trigger precipitation. We have assumed that Mn2+ substitutes equally for Na+ and Bi3+, Cs2Na1−xBi1−xMn2xCl6, thereby maintaining charge balance, but we cannot rule out the possibility that Mn2+ substitutes preferentially on one of the two sites accompanied by charge compensating defects. The starting Mn:(Na+Bi) ratios were varied from 0.01 to 2.5 but MnCl2∙4H2O impurities were observed in the PXRD patterns of the product once this ratio approached 3.0. ICP-OES analysis of single-phase samples (Mn:(Na+Bi) < 3.0) shows that Mn2+ incorporation increases as it’s concentration in solution increases, but only a few percent (1–3%) of the Mn2+ in solution is incorporated into the perovskite structure. As shown in the supporting information (Table 3



broad peak at ~270 nm is assigned to the 1S0 → 3P2 transition, while the peaks at 318 nm and 345 nm are assigned to the 1S0 → 3P1 transition. The splitting of the 1S0 → 3P1 transition is attributed to a dynamic Jahn-Teller effect, induced by the coupling of lattice vibrations with empty T1u orbitals with antibonding Bi 6p – Cl 3p character. Given the cubic to tetragonal transition that occurs upon cooling below 90 K, dynamic coupling with lattice vibrations that distort the Oh symmetry of the Bi3+ site is not unreasonable. The shoulder at 378 nm is assigned to the 1S0 → 3P0 transition. The weak intensity of this peak is due to the spinand parity-forbidden nature of the transition. The assignments are tentative, but consistent with those made in a previous temperature dependent study of the optical properties of Cs2NaBiCl6 single crystals.26 In the previous work, the fully allowed 1S0 → 1P1 transition was observed near 200 nm; a region that falls outside of the range of detection of our instruments.

excitation spectrum coincide reasonably well with onset of absorption associated with the 1S0 → 3P2 (294 nm) and 1S0 → 3P1 (354 nm) transitions. It is worth noting that the wavelength of the strongest excitation peak at 354 nm, is quite similar to the strongest peak in the excitation spectrum of Cs2SnCl6 doped with Bi3+ (358−364 nm), which also contains localized [BiCl6]3− octahedra.35

Figure 4. Excitation (blue) and emission (red) scans for

Cs2Na1−xBi1−xMn2xCl6 (x = 0.004). Wavelengths used for

excitation and emission measurements are given in top middle.

Unlike the diffuse reflection measurements, the 354 nm 3P excitation peak does not show signs of splitting, for 1 reasons that are not obvious. It is possible that strong absorbance near the 320 nm absorbance maxima leads to luminescence quenching by nonradiative decay paths involving defect states at the surface.36 Further study on samples where the Bi3+ ion concentration is diluted by a non-absorbing species like Y3+ are needed to confirm or refute this hypothesis. The origin of the peak at 428 nm is not understood. It may be associated with absorbing centers near point defects, such as Cl− vacancies. Regardless of its origin, the presence of an excitation peak in the blue region of the spectrum could be attractive for use in pc-LED applications.18,37 For use as a phosphor in white lighting applications, it would be beneficial to further red-shift the excitation wavelength to better overlap with the emission spectrum of Ga1−xInxN LED sources. To achieve this red-shift, it is necessary to decrease the crystal field splitting of the Bi 6s and 6p orbitals. This logic led us to explore the optical properties of manganese-doped Cs2NaBiCl6−yBry solid solutions. Unfortunately, the solution synthesis procedure in mixed hydrohalic acid solutions does not lead to phase pure products, due to the formation of an undesired Cs3Bi2Br9 phase. Similar problems were encountered in solid state reactions, although it was apparent from an increased lattice parameter that some degree of bromide incorporation into the perovskite phase was taking place. Fortunately, phase pure samples of Cs2NaBiCl6−yBry (y ≤ 0.6) could be isolated from the growth of single crystals via 1S

Figure 3. Kubelka-Munk transformation of diffuse reflectance data for Cs2Na1−xBi1−xMn2xCl6 samples. The inset shows a magnified view of the shoulder at ~378 nm.

Samples doped with Mn2+ exhibit orange-red photoluminescence with a moderately broad emission band centered at 590 nm (Figure 4). The emission can confidently be assigned as originating from a 4T1 → 6A1 transition of octahedrally coordinated Mn2+.33 When no dopant is added, the material exhibits very weak luminescence centered at 730 nm; this has been attributed to the emission of Bi3+ cations located near defect sites.34 Photo-luminescent quantum yield (PLQY) measurements indicate a maximum PLQY of 15% in the sample with highest Mn2+ concentration. PLQY steadily decreases as the Mn2+ concentration decreases (Table S5). This result suggests that developing strategies to increase the concentration of Mn2+ incorporation in the lattice may lead to samples with improved PLQY. The excitation spectrum (Figure 4) shows peaks centered at 294 nm, 354 nm, and 428 nm, and a shoulder on the most intense peak at 310 nm. The dominant two peaks in the

0→

4


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Chemistry of Materials hydrothermal methods. Refinement of the PXRD scans (Figure S6) indicate a lattice parameter increase from 10.8405(2) Å to 10.9060(2) Å with a maximum bromide incorporation of y = ~0.6 (Br/(Cl+Br) = 10%). Attempts to further increase the bromide content resulted in a mixture of Cs3Bi2Br9−yCly and NaCl1−yBry impurity phases.

The absorption spectra of Cs2NaBiCl6−yBry:Mn2+ samples are similar to the pure chloride end-member, but the peaks are broadened and shifted to longer wavelengths (see Figure 5). Accordingly, the excitation spectra shift to longer wavelengths as the bromide content increases, with the most intense excitation occurring at 375 nm, a red-shift of 21 nm. Fortuitously, bromide incorporation also results in a significant increase in the intensity of the ~430 nm excitation peak, along with a slight red-shift of its maximum from 428 nm to 435 nm. The increased intensity may be due to the decreased symmetry of Bi3+ sites in the mixed halide samples. The maximum level of substitution, x = 0.6, suggests a mixture of [BiCl5Br]3− and [BiCl6]3− centers. That does not necessarily preclude the existence of Bi-centered octahedra with anywhere from 2 to 6 bromide ligands, such centers may be responsible for the ~430 nm excitation peak. The incorporation of bromide ions will also break the local inversion symmetry at the Bi3+ site, which will relax the selection rules. Incorporation of bromide has a negligible effect on the emission profile (Figure 5), which suggests that the dilute Mn2+ dopants are not coordinated by bromide ions. Discussion It is instructive to compare the PL of Cs2NaBiCl6:Mn2+ and Cs2NaBiCl6−yBry:Mn2+ with other halide double perovskite phosphors. The most direct comparison is with Cs2AgInCl6:Mn2+.22 The biggest difference between the two is the nature of the absorption. The covalency of the Ag−Cl bonds makes Cs2AgInCl6 a wide bandgap semiconductor with a three-dimensional electronic structure. When Cs2AgInCl6 absorbs photons, it creates electron-hole pairs that can presumably migrate through the lattice until they are trapped and undergo recombination at the Mn2+ sites. In contrast, Cs2NaBiCl6 absorbs light through localized excitations of [BiCl6]3− octahedra, and upon returning to the ground state it transfers energy to neighboring Mn2+ sites that act as activators. Given the spin-forbidden nature of the electronic transitions on Mn2+ the transfer is likely due to Dexter energy transfer. Practically speaking, the redemission of Cs2AgInCl6:Mn2+ (λmax = 632 nm) may be advantageous for some applications, such as tri-color pcLEDs employing red and green phosphors, but the low quantum yield and lack of absorbance at wavelengths longer than 350 nm is not ideal. Figure 6 compares the intensity and position of the emission peak for Cs2NaBiCl6 and Cs2AgInCl6 hosts doped with similar levels of Mn2+. The much weaker emission of the latter is consistent with the rather low PLQY of 2% measured on Cs2AgInCl6:Mn2+ samples prepared in our lab. From an absorption point of view, Cs2NaBiCl6:Mn2+ shares many similarities with the vacancy ordered perovskite, Cs2SnCl6 doped with Bi3+. This phosphor emits in the blue (λmax = ~460 nm) with reported quantum yield values that range from 38 to 78%.35 In Cs2SnCl6:Bi3+, isolated [BiCl6]3− octahedra act as sensitizers, with an excitation maximum near 360 nm that is quite similar to the strongest excitation peak in Cs2NaBiCl6:Mn2+. The absorption processes in Cs2NaBiCl6 and Cs2SnCl6:Bi3+ appear to be very similar, but

Figure 5. Normalized Kubelka-Munk spectra (top), excitation (middle), and emission (bottom) spectra for Cs2Na1−xBi1−xMn2xCl6−yBry (y = 0, 0.3 (5%) and 0.6 (10%)) with approximate Mn2+ concentration x = 0.004. 5



in the former compound energy transfer to Mn2+ leads to bright luminescence in the orange-red region of the spectrum rather than the blue luminescence of the latter.

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improved while tailoring the emission and excitation spectra to be favorable for lighting applications. Cs2NaBiCl6−yBry:Mn2+ may also be an intriguing candidate for perovskite nanocrystals as the tunable excitation may be coupled with a higher concentration of Mn2+ incorporation. Halide substitution often results in tunable emission properties in nanocrystals as well and this may provide a route to red shift the emission observed in this material by incorporating higher levels of bromide ions. As seen for Cs2AgInCl6:Mn2+ quantum yields of nanocrystals are often higher than that those obtained in the bulk samples (Table 1). Differences in processing conditions compared to Cs2AgInCl6 should also be noted. Solution-based film deposition and/or nanocrystal synthesis of halide double perovskites containing Ag+ can be challenging due to the low solubilities of AgCl and AgBr in aqueous solutions. Sodium salts are much easier to dissolve, and thus more amenable to solution deposition processes. This is an attractive feature for applications that rely on solution processing. Finally, it’s worth noting that replacing In3+ with Bi3+ is highly advantageous from a cost perspective.

Figure 6. A comparison of the emission spectra of Cs2NaBiCl6:Mn2+ (ex = 354 nm) and Cs2AgInCl6:Mn2+ (ex = 310 nm). Both samples made using identical Mn2+ loadings (Mn:(B+B’) = 0.2).

Table 1. List of the PL characteristics of Mn2+ doped halide perovskite materials in bulk and nanocrystal (NC) form.

The quantum yield of Cs2NaBiCl6:Mn2+ is lower than the values reported for Cs2(Ag1−xNax)InCl6 and Cs2SnCl6:Bi3+.16,35 A likely explanation is the mismatch in the concentration of Bi3+ sensitizers and Mn2+ activators. The spin-forbidden nature of the electronic transitions on Mn2+ renders long range Forster energy transfer highly inefficient, thus the energy transfer between Bi3+ and Mn2+ must rely on the Dexter mechanism, which is only operative over short distances. It is not clear if the energy transfer is limited to Mn2+ ions that have substituted for Na+ and share a common chloride ion with Bi3+, or if Mn2+ ions that substitute for Bi3+ and are thus surrounded by Na+-centered octahedra can also participate. In either case, at the relatively low doping levels of 2−3% that can be achieved, most of the Bi3+ absorbing centers are not adjacent to a Mn2+ activator. Given that every Mn2+ ion that substitutes for Na+ will have six Bi3+ neighbors, crude calculations suggest that at the highest doping levels ~10% of the Bi3+ ions in the host lattice have a Mn2+ nearest neighbor. Thus, PLQY values of the same order of magnitude are not surprising. Diluting the Bi3+ absorbing centers with an isovalent replacement like Y3+ or In3+ that does not absorb in the 350–450 nm range could lead to significant gains in quantum efficiency, provided the Bi3+ and Mn2+ ions can be kept in close proximity to each other. Substitutions that lead to blue luminescence via recombination of excitons, as seen in Cs2SnCl6:Bi3+ while maintaining red or orange-red luminescence via Mn2+ activators is another attractive possibility. This could lead a white emitting phosphor not entirely different from the classic haloapatite phosphors doped with Mn2+ and Sb3+. Given the size of the halide double perovskite family the possibilities are extensive, and there is good reason to expect that the efficiency can be

Material

Bulk QY

NC’s QY

λem max (nm)

Ref.

Cs2AgInCl6:Mn2+

3-5%

16%

632, 620

22,25

Cs2NaBiCl6:Mn2+

15%

--

590

This work

CsPbCl3:Mn2+

--

54%

580

37

Conclusion The results reported here show that the perovskite Cs2Na1−xBi1−xMn2xCl6 is a promising new orange-red phosphor. Bi3+ ions in the host lattice absorb near UV light and transfer this energy to Mn2+ activators that emit light from 525 to 700 nm via the spin-forbidden 4T1 → 6A1 transition. The optical absorption and excitation spectra can be red-shifted through isovalent substitution of bromide ions for chloride ions. The ease of synthesis, use of inexpensive reagents, and stability of the host lattice are attractive for potential applications. The modest quantum efficiencies of ~15% can likely be improved, perhaps significantly, if routes can be found to better match the concentrations of sensitizers and activators.

ASSOCIATED CONTENT Supporting Information. (Table S1, Figure S2) Comparisons of Mn2+ incorporation with lattice parameter, (Table S1, Figure S1) Comparison of amount of Mn2+ used in synthesis vs. amount incorporated into product, (Figure S3) PXRD of Mn2+doped Cs2NaBiCl6, (Table S2) Structural refinement of Cs2NaBiCl6, (Table S3) Bond length analysis, (Figure S4) TGA of Mn2+-doped Cs2NaBiCl6, (Figure S5) Diffuse Reflectance Spectra of Mn2+-doped Cs2NaBiCl6, (Figure S6) PXRD of Mn2+6


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Chemistry of Materials doped Cs2NaBiCl6−xBrx, (Table S4) Lattice parameters of Mn2+doped Cs2NaBiCl6−xBrx, (Figure S7, Figure S8) Digital photos of powders and crystals of Mn2+-doped Cs2NaBiCl6, (Figure S9, Table S5) Quantum yield of Mn2+-doped Cs2NaBiCl6, (Table S6) ICP-OES analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

5. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible- light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050–6051. 6. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 2015, 15, 3692–3696. 7. Li, X.; Wu, Y.; Zhang, S.; Cai, B.; Gu, Y.; Song, J.; Zeng, H. CsPbX3 Quantum dots for lighting and displays: Roomtemperature synthesis, photoluminescence superiorities, underlying origins and white lightemitting diodes. Adv. Funct. Mater. 2016, 26, 2435–2445. 8. Wei, S.; Yang, Y.; Kang, X.; Wang, L.; Huang, L.; Pan, D. Room-temperature and gram-scale synthesis of CsPbX3 (X = Cl, Br, I) perovskite nanocrystals with 50-85% photoluminescence quantum yields. Chem. Commun. 2016, 52, 7265–7268. 9. McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M. Cs2AgBiX6 (X = Br, Cl): New visible light absorbing, leadfree halide perovskite semiconductors. Chem. Mater. 2016, 28, 1348–1354. 10. Xiao, Z., Meng, W., Wang, J., Mitzi, D., Yan, Y.; Searching for promising new perovskite-based photovoltaic absorbers: the importance of electronic dimensionality. Mater. Horiz., 2017, 4, 206–216. 11. Zhao, X., Yang, D., Ren, J., Sun, Y., Xiao, Z., Zhang, L.; Rational design of halide double perovskites for optoelectronic applications, Joule, 2018, 2, 1662–1673. 12. Luo, J.; Li, S.; Wu, H.; Zhou, Y.; Li, Y.; Liu, J.; Li, J.; Li, K.; Yi, F.; Niu, G.; Tang, J.; Cs2AgInCl6 double perovskite single crystals: Parity forbidden transitions and their application for sensitive and fast UV photodetectors. ACS Photonics 2018, 5, 398–405. 13. Pan, W.; Wu, H.; Luo, J.; Deng, Z.; Ge, C.; Chen, C.; Jiang, X.; Yin, W. J.; Niu, G.; Zhu, L.; Yin, L.; Zhou, Y.; Xie, Q.; Ke, X.; Sui, M.; Tang. Cs2AgBiBr6 single-crystal X-ray detectors with a low detection limit. Nat. Photonics 2017, 11, 726–732. 14. Aull, B. F.; Jenssen, H. P. Impact of ion-host interactions on the 5d-to-4f spectra of lanthanide rareearth-metal ions. II. The Ce-doped elpasolites. Phys. Rev. B 1986, 34, 6647–6655. 15. Zhou, L.; Xu, Y. F.; Chen, B. X.; Kuang, D. B.; Su, C. Y. Synthesis and Photocatalytic Application of stable leadfree Cs2AgBiBr6 perovskite nanocrystals. Small 2018, 1703762, 1–7. 16. Luo, J.; Wang, X.; Li, S.; Liu, J.; Guo, Y.; Niu, G.; Yao, L.; Fu, Y.; Gao, L.; Dong, Q.; Zhao, C.; Leng, L.; Ma, F.; Liang, W.; Wang, L.; Jin, S.; Han, J.; Zhang, L.; Etheridge, J.; Wang, J.; Yan, Y.; Sargent, E.; Tang, J. Efficient and stable

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Jackson D. Majher: 0000-0002-4160-4201 Matthew B. Gray: 0000-0002-9526-4732 Patrick M. Woodward: 0000-0002-3441-2148

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Funding was provided by the National Science Foundation under award number DMR-1610631. PLQY measurements in the MRL Shared Experimental Facilities by A.S. are supported by the MRSEC Program of the NSF under Award No. DMR 1720256; a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org). Special thanks to Eric McClure for stimulating discussions on synthetic methods. Further thanks to Dr. Nicole Karn and Anthony Lutton for assistance with TGA and ICP-OES, respectively.

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Cs2NaBiCl6:Mn2+ â A new orange-red halide double perovskite

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Cs2NaBiCl6:Mn2+ â A new orange-red halide double perovskite