Antimony doped Cs₂SnCl₆ with bright and stable emission

Abstract

Lead halide perovskites, with high photoluminescence efficiency and narrow-band emission, are promising materials for display and lighting. However, the lead toxicity and environmental sensitivity hinder their potential applications. Herein, a new antimony-doped lead-free inorganic perovskites variant Cs₂SnCl₆:xSb is designed and synthesized. The perovskite variant Cs₂SnCl₆:xSb exhibits a broadband orange-red emission, with a photoluminescence quantum yield (PLQY) of 37%. The photoluminescence of Cs₂SnCl₆:xSb is caused by the ionoluminescence of Sb³⁺ within Cs₂SnCl₆ matrix, which is verified by temperature dependent photoluminescence (PL) and PL decay measurements. In addition, the all inorganic structure renders Cs₂SnCl₆:xSb with excellent thermal and water stability. Finally, a white light-emitting diode (white-LED) is fabricated by assembling Cs₂SnCl₆:0.59%Sb, Cs₂SnCl₆:2.75%Bi and Ba₂Sr₂SiO₄:Eu²⁺ onto the commercial UV LED chips, and the color rendering index (CRI) reaches 81.

Key words： perovskite; lead-free; antimony doping; orange-red emission

Introduction

Lead halide perovskites have attracted tremendous research interests due to their excellent performance in photovoltaic, electroluminescence, bio imaging, display etc [¹–⁷]. In terms of light emitting applications, the performances of lead halide perovskite light-emitting diodes (PeLEDs) have been improved dramatically to over 20% of external quantum efficiency in a short period [⁸–¹⁰]. Lead halide perovskites have also exhibited many other favorable advantages such as high photoluminescence quantum yield (PLQY) (>95%), narrow full width at half maximum (FWHM<20 nm), low-cost solution synthesis [¹¹,¹²]. Besides, by adjusting the type and proportion of halogens, the emission spectrum of lead halide perovskites can cover the entire visible spectrum and the color gamut could even reach 140% of National Television Standards Committee (NTSC) color standards [¹³,¹⁴]. However, lead halide perovskites contain poisonous Pb element, which not only damages humans’ nervous system but also ruins local ecosystems [¹⁵]. Moreover, the poor stability of lead halide perovskites is another annoying disadvantage, such as the high susceptibility toward moisture and thermal treatment etc [¹⁶–¹⁸]. Hence, developing perovskites or perovskite variants, with non-toxic compositions and good stability is a key step toward practical applications.

Several lead-free halide perovskites have been reported, such as CsSnX₃ (X= Cl, Br, I), MA₃Bi₂X₉ and Cs₃Sb₂X₉ etc [¹⁹–²²]. These materials have barely exhibited satisfactory photoluminescence and stability. By tuning the crystal structure and reducing the electronic dimensionality, Ma and coworkers reported a series of luminescent lead-free perovskite variants via the use of long chain amines for dimensionality reduction, such as (1-butyl-1-methylpyrrolidinium (C₉NH₂₀))₂SbCl₅ and (Tetraphenylphosphonium (Ph₄P))₂SbCl₅ [²³,²⁴]. The stability against water and heat however needs to be further improved due to the presence of organic amines. Recent studies have demonstrated the use of doping as another effective method to tune the photoluminescence properties, like Mn²⁺ or rare earth ion doped lead halide perovskites [²⁵–²⁷]. The strong exciton confinement in ion luminescent center, similar with dimensionality reduction in essence, is also beneficial for photoluminescence. Dopant emission is relatively less explored, particularly in an inorganic perovskite structure which enjoys the good stability toward external conditions. It is thus scientifically interesting to explore more doping ions in lead-free perovskites and produce multi-colored emissions for potential lighting and display applications.

Herein we introduced antimony cations (Sb³⁺) into Cs₂SnCl₆, the previously established matrix material [²⁸], to obtain a new phosphor with a broadband orange-red emission (peak position at 602 nm; FWHM ~101 nm). By introducing Sb³⁺ into the vacancy-ordered double perovskite Cs₂SnCl₆, a significant boost of PLQY (~37%) was observed compared to the non-luminescent Cs₂SnCl₆. The stability against water and heat of Sb³⁺ doped Cs₂SnCl₆ is superior among halide perovskites due to its all-inorganic structure. At last, we demonstrated a white light-emitting diode (white-LED) by assembling Cs₂SnCl₆:0.59%Sb, Cs₂SnCl₆:2.75%Bi and Ba₂Sr₂SiO₄:Eu²⁺ onto the commercial UV LED chips, and the color rendering index (CRI) reaches 81. The high stability and CRI endow this all-inorganic lead-free material with highly promising prospects in lighting application.

Experimental section

Materials and Chemicals: Cesium chloride (CsCl, 99.99%), tin chloride (SnCl₂, 99.999%) and antimony(III) oxide (Sb₂O₃, 99.9%) were purchased from Alfa Aesar. Hydrochloric acid (HCl, 37wt% in water) was purchased from Sinopharm Chemical Reagent Co., Ltd, China. All materials and chemicals were used without further purification, unless otherwise noted.

Growth of Cs₂SnCl₆:xSb crystals: Cs₂SnCl₆:xSb crystals were grown from HCl solution by hydrothermal method. 336.72 mg (2 mmol) CsCl, 189.60 mg (1 mmol) SnCl₂, and 1.458−43.73 mg (0.01−0.3 mmol) Sb₂O₃ and 4 mL HCl were loaded into a PTFE container successively. The container was placed in a hydrothermal autoclave and heated to 180°C for dissolving the raw materials completely. Crystals were obtained by slowly cooling the solution down to room temperature over the course of 20 h. The obtained crystals of Cs₂SnCl₆:xSb³⁺ were rinsed with methanol three times to remove the surface impurity, dried naturally and then collected for further analysis.

Fabrication of white-LED devices: White-LED was fabricated by using a UV LED chips (EPILEDS, 380 nm) to excite the phosphor-silicone mixture. The mixture was obtained after gelling in air naturally by mixing Cs₂SnCl₆:xSb³⁺, Cs₂SnCl₆:xBi³⁺, commercial green phosphors (GaAlSiN₃:Eu²⁺) and tetraethyl orthosilicate (TEOS) with the addition of proper amount of water and HBr to promote the hydrolysis of TEOS. The UV LEDs were driven by a Keithley 2400 source meter and emission spectra were recorded on a PR655 Portable spectrophotometer.

Structural characterization: The X-ray diffraction (XRD) experiments were performed by powder X-ray diffraction (PXRD) (D8 ADVANCE, Bruker, using a Cu Ka rotating anode). The surface of Cs₂SnCl₆:xSb crystals were analyzed using XPS with Al Ka excitation (Genesis, EDAX Inc., 300 W). Thermogravimetric analysis (TGA) results were obtained using a PerkinElmer Diamond TG/DTA6300, while the Cs₂SnCl₆:0.59%Sb crystals were heated from room temperature (around 20°C) to 700°C at a rate of 10°C min^-1 in N₂ flow within an alumina crucible.

Optical characterization: UV-Vis absorption spectra were measured with powder samples. PLQY, steady-state and time-resolved photoluminescence (TRPL) spectra were recorded on an Edinburgh Instruments FLS980 spectrometer with a red-sensitive photomultiplier tube (R928), equipped with a xenon lamp and a TCSPC module (diode laser excitation at l = 375 nm) and an integrating sphere. The temperature-dependent photoluminescence (PL) spectra were measured with a temperature controller system. The spectra were corrected for the monochromator wavelength dependence and photomultiplier response functions provided by the manufacturer. The measurements were performed using dried, powdered polycrystalline samples. No filters were used during the TRPL measurement.

Results and discussion

Fig.1 (a) XRD patterns of Cs₂SnCl₆:xSb powders with representative Sb content. The inset is the crystal structure of vacancy ordered double perovskite Cs₂SnCl₆. Dark purple spheres: Cl; tawny spheres: Cs; gray spheres: Sn. (b) XPS survey spectrum for Cs₂SnCl₆:0.59%Sb. (c) Calculated polyhedron of the chemical potential region where Cs₂SnCl₆ is stable against possible competitive phases. (d) Calculated formation enthalpies (DH) of neutral Sb_i and Sb_Sn as a function of the chemical potentials (Dm_Cs, Dm_Sn), where (Dm_Cs, Dm_Sn) moves along the F-E-D-C-B-A-G-F lines in (c)

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Sb³⁺-doped Cs₂SnCl₆ crystals were synthesized through the conventional hydrothermal method. In detail, cesium chloride (CsCl), tin chloride (SnCl₂) and antimony oxide (Sb₂O₃) were added into HCl aqueous solution in a PTFE container of hydrothermal reactors. Then the solution was kept at 180°C for 10 h to ensure the complete dissolution of all precursors. After a slow cooling down process from 180°C to room temperature for 20 h, tiny crystals could be formed in the solution. To ensure the total removal of surface-adsorbed ions, the tiny crystals were rinsed by methanol for three times. The Sb/(Sb+ Sn) molar ratios in the solution were set at 0, 0.99%, 4.76%, 9.09%, 16.66% and 23.08%. The whole procedure was conducted in air atmosphere. Here we utilized Sb₂O₃ instead of SbCl₃ as Sb³⁺ precursors due to the high hygroscopicity of SbCl₃ and hence the difficulty in accurate weighing. The reason why we chose Sn (II) as the tin source will be discussed in the later part of the calculation for chemical potential (Dm) and formation enthalpies (DH). X-ray diffraction (XRD) measurement was applied to determine the structure of the samples and the result is shown in Fig. 1(a). Without any impurity phase, all the diffraction peaks of products matched well with Cs₂SnCl₆ crystal structure (ISCD #9023) with a space group of Fm\bar{\mathrm{3}}m. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to precisely determine the chemical compositions of products. As shown in Table S1, the ICP-OES-determined Sb/(Sb+ Sn) concentrations are 0.20%, 0.41%, 0.59%, 0.89% and 0.98%, while the feeding concentrations are 0.99%, 4.76%, 9.09%, 16.66% and 23.08%, respectively. To simplify later discussion, we label the samples as Cs₂SnCl₆:xSb, where x is the real concentration provided by ICP-OES. It should be noted that the Sb concentrations are fairly low compared to the Bi concentrations reported in our previous work [²⁸]. X-ray photoelectron spectroscopy (XPS) measurement was utilized to further verify the composition and obtain the valence state of the elements within the products. The XPS survey spectrum (Fig. 1(b)) shows the characteristic peaks for Cs, Sn, Cl and Sb. As shown in Fig. S2(a), the peaks locating at 496.3 and 487.8 eV correspond to Sn⁴⁺ 3d3/2 and 3d5/2, respectively, proving that the Sn²⁺ was completely oxidized to Sn⁴⁺ after the reaction process in the hydrothermal reactor. The peaks locating at 540.0 and 532.9 eV correspond to Sb³⁺ 3d3/2 and 3d5/2, respectively (Fig. S2(b)). There is an additional peak at 531.7eV in Fig. S2(b), which was tentatively attributed to the presence of Sb-O or Sn-O on the crystal surface due to the hydrolysis of antimony chloride and tin chloride [²⁹–³²]. From the XRD, ICP-OES and XPS measurements, we speculate that the Sb³⁺ was incorporated into the Cs₂SnCl₆ matrix successfully. Moreover, the chemical potential window and formation enthalpies for Sb interstitial (Sb_i) and Sb-Sn substitution (Sb_Sn) were calculated (Figs. 1(c) and 1(d)). The DH>0 means that it is not conducive to forming such products. Hence the results reveal that Sb_i can hardly form because of the too high DH values in the whole chemical potential regions, and Sb_Sn can also hardly form under Sn⁴⁺-rich/Sn²⁺-poor conditions (i.e., in the high Dm_sn regions C-F in Fig. 1(c)). Instead, the use of Sn⁴⁺-poor/Sn²⁺-rich condition (i.e., in the low Dm_sn regions A, B, and G) is favorable for the formation of Sb_Sn, where the DH values are low or even negative. Thereby we chose Sn (II) as the tin source for the synthesis of antimony ion doped Cs₂SnCl₆, in analogy with our previous synthesis of Cs₂SnCl₆:xBi. The refined lattice parameters of Cs₂SnCl₆:xSb samples increased as the Sb concentration increased (see Fig. S1 and Table S2 in the Supporting Information), which is consistent with ion radius of Sn⁴⁺ (0.69 Å) and Sb³⁺ (0.76 Å) for Sb-Sn substitution.

Fig.2 (a) Optical absorption spectrum of Cs₂SnCl₆:0.59%Sb, the insets show the images of Cs₂SnCl₆:0.59%Sb under the natural light (left) and UV irradiation (right). (b) Excitation and photoluminescence spectra of Cs₂SnCl₆:0.59%Sb

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Tab.1 Photophysical properties of Cs₂SnCl₆:xSb at room temperature (x is the content of antimony; l_ex is the wavelength at the excitation maximum; l_em is the wavelength at the emission maximum)

x	lex/nm	lem/nm	FWHM/nm	Stokes shift/nm	PLQY
0	N/A	N/A	N/A	N/A	N/A
0.20%	365	601	101	236	25.9%
0.41%	364	601	102	237	28.3%
0.59%	365	602	101	237	37.0%
0.89%	365	604	102	239	32.0%
0.98%	366	602	100	236	21.9%

Then the PL properties of Cs₂SnCl₆:xSb were studied by steady-state and TRPL spectroscopy. Sb-doped Cs₂SnCl₆ samples exhibited a broadband orange photoluminescence under 365 nm UV excitation (inset of Fig. 2(a)), while Cs₂SnCl₆ exhibited no photoluminescence. The results were summarized in Table 1, and Cs₂SnCl₆:xSb with various doping concentrations exhibited similar emission and excitation spectra. PLQY was directly measured by FLS980 spectrometer with the excitation light at 365 nm. The PLQY reached the highest value of 37.0% when doping concentration x was 0.59%. Further increase of the dopants caused the self-quenching effect, which is a typical character of ionoluminescence [³³]. The ionoluminescence nature of this material will be discussed in the later part.

The Cs₂SnCl₆:0.59%Sb sample, with the highest PLQY of 37.0%, was selected as the representative sample for further exploration. Figure 2(a) shows the optical absorption spectra of Cs₂SnCl₆:0.59%Sb crystals which match reasonably well with the excitation spectra (Fig. 2(b)). In the optical absorption spectrum (Fig. S3) for series of Cs₂SnCl₆:xSb crystals, we could clearly observe that the absorbance intensity increased with the increase of Sb content within the region of 300−400 nm. Thereby we attributed the absorption in this region to the Sb³⁺ induced ion absorption. As shown in Fig. 2(b), Cs₂SnCl₆:0.59%Sb exhibited a broad emission peak (FWHM>100 nm) at 602 nm with a large Stokes shift of 237 nm. We suspected that the broad emission with large Stokes shift was due to that outermost S-P electron orbital transition of Sb³⁺ was highly sensitive to the distortion of Sb-Cl polyhedron and crystal field [²⁴,³⁴–³⁶].

Fig.3 (a) Temperature-dependent photoluminescence spectra of Cs₂SnCl₆:0.59%Sb. (b) Schematic diagram of luminescence process in Cs₂SnCl₆:xSb. (c) Schematic of the potential energy curves of Cs₂SnCl₆:xSb in a configuration space. (d) PL decay curve of Cs₂SnCl₆:0.59%Sb bulk crystals (l_ex = 365 nm, l_em = 602 nm). The red curve is a fit to the experimental data with a double exponential decay function. (e) Excitation spectra of PL monitored at different emission wavelengths. (f) Emission spectra of PL monitored at different excitation wavelengths

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To figure out the mechanism of luminescence, herein we conducted the temperature dependent photoluminescence (TDPL) measurement for Cs₂SnCl₆:0.59%Sb (Fig. 3(a)) and clearly observed two emission peaks with the decreasing temperature. This emission character is a strong indicator of ionoluminescence.

As studied in many previous papers, for ions with the outer electronic configuration as s² (here is 5s² for Sb³⁺), the ground state of s² ion is ¹S₀, whereas the excited state (sp) splits into four energy levels, namely ¹P₁, ³P₀, ³P₁ and ³P₂ (Fig. 3(b)). According to the transition rule, ¹S₀-¹P₁ transition is allowed and ¹S₀-³P₁ transition is partially allowed due to a spin-orbit coupling for heavy atoms, while ¹S₀-³P₂ and ¹S₀-³P₀ transitions are totally forbidden at the electric dipole transition level [³⁵,³⁷,³⁸]. The photoluminescence within the visible light region is possibly from the transition of ³P₁-¹S₀ and also the double emission peaks at low temperature conforms to the asymmetric doublet characteristics for ³P₁-¹S₀ transition [³⁸]. Several previous papers have documented that the transition of ³P₁-¹S₀ could be split into two transitions due to the Jahn-Teller effect of ³P₁ [³⁵,³⁸,³⁹], and here the two derived transition bands are termed as H transition (2.48 eV) and L transition (2.07 eV) (i.e., the blue and orange lines in Fig. 3(c)). The intensity of these two transitions could be changed as we altered the measurement temperature. As shown in Fig. S4, the PL intensity based from H transition was much higher than L transition in the low-temperature region (83−143 K). Upon the temperature further increased, the PL intensity based from L transition became stronger than H transition due to the thermal population of the carrier from H transition band to L transition band. The same phenomenon has been observed in many ionoluminescence with ns² ions as the luminescent centers, such as Sb³⁺ doped phosphate/borax/germanate, In⁺ doped KCl [³⁶].

Unlike our previous work about Cs₂SnCl₆:Bi whose photoluminescence derived from defect band induced by Bi-doping [²⁷], here Cs₂SnCl₆:Sb exhibited discrete ionoluminescence properties. We suspected the reason for this difference was that there was no continuous defect band formed since the excited state was strongly localized with the tiny content of antimony.

In addition, the photoluminescence decay behavior of the activator in host materials is significant to gain more insight into the luminescence mechanism. Figure 3(d) shows the time-resolved PL decay curve of Cs₂SnCl₆:0.59% Sb measured at 600 nm excited by 365 nm light at room temperature. The curve can be fitted well by the following biexponential fitting (Eq. (1)):

(1)A(t)={{\displaystyle A}}_{\mathrm{1}}\exp (-t/{{\displaystyle \tau }}_{\mathrm{1}})+{{\displaystyle A}}_{\mathrm{2}}\exp (-t/{{\displaystyle \tau }}_{\mathrm{2}}),

where A(t) is the time variation of PL intensity at time t, A₁ and A₂ are the initial PL intensity of two component, and {{\displaystyle T}}_i(i=\mathrm{1},\mathrm{2}) represents the decay time of i component.

Accordingly, we obtained a short-lived PL lifetime of 153.8 ns with a percentage of 39.9% and a long-lived PL lifetime of 821.2 ns with a percentage of 60.1%. According to previous study [³⁶], the excited state of ³P₁ for Sb³⁺ has two recombination route, i.e., ³P₁-³P₀ transition within the excited state with a fast decay rate and ³P₁-¹S₀ radiative recombination with a slow decay rate. Thereby we could attribute the observed two decay lifetimes of Cs₂SnCl₆:0.59% to ³P₁-³P₀ and ³P₁-¹S₀ transitions.

For the representative Cs₂SnCl₆:0.59%Sb sample, when the monitored emission wavelength was varied from 550 to 600 nm, the corresponding excitation spectra (Fig. 3(e)) show negligible peak shift or shape change. Similarly, when the excited wavelength varied from 350 to 400 nm, the corresponding emission spectra (Fig. 3(f)) show negligible changes as well. These negligible changes conformed to the characteristics of ionoluminescence, and verified that the photoluminescence was not derived from Raman scattering or defects.

Fig.4 (a) TGA and DSC of Cs₂SnCl₆:0.59%Sb. (b) PL stability of Cs₂SnCl₆:0.59%Sb by illuminating with UV light (365 nm). The measurements were conducted in air without any encapsulation. (c) PL stability of Cs₂SnCl₆:0.59%Sb after immersed into deionized water for different durations

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It is also notable that the synthesized Cs₂SnCl₆:xSb samples exhibit good stability. In Fig. 4(a), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) shows that the Cs₂SnCl₆:0.59%Sb sample melt around 396°C and decomposed at 606°C. This decomposition temperature is much higher than organic-inorganic hybrid perovskite ABX₃ (A= CH₃NH₃, HC(NH₂)₂; B= Pb, Sn; X= Cl, Br, I) [²³,⁴⁰]. We further utilized 365 nm UV light (UVLS-26, 1200 uW at a distance of three inches) to irradiate Cs₂SnCl₆:0.59%Sb sample for 24 h and observed little PL decay (Fig. 4(b)). Such observation indicated that Cs₂SnCl₆:Sb had excellent photo-stability due to its all-inorganic structure. Unlike conventional lead halide perovskite CsPbBr₃ and antimony-based metal halide hybrids (like Cs₃Sb₂Br₉, (C₉NH₂₀)₂SbCl₅ and (Ph₄P)₂SbCl₅), Cs₂SnCl₆:0.59%Sb retained its luminescence after 120 min soaking in water (Fig. S5). In addition, Cs₂SnCl₆:0.59%Sb exhibited no degradation within storage in air for two months, demonstrating its good air stability (Fig. S6). Figure 4(c) shows that Cs₂SnCl₆:0.59%Sb retained 92% of the original PL intensity after immersing in deionized water for 24 h. We suspected that this good water-stability possibly derived from the presence of SbOCl on the surface (see Figs. S2 and S7 in the Supporting Information) and the inherently low solublility of Cs₂SnCl₆ in water, similar to our previous Cs₂SnCl₆:xBi phosphor.

Fig.5 (a) Luminescence spectra from Cs₂SnCl₆:0.59%Sb-based LEDs with cold white emission and (inset) photo of an operating LED. (b) CIE coordinates and CCTs corresponding to white-LED device (white star) and Cs₂SnCl₆:0.59%Sb crystals (red triangle). (c) Emission spectra of white-LED device at different driving currents

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The high stability and large Stokes shift endow the all-inorganic lead-free Cs₂SnCl₆:xSb material with highly promising prospects in a variety of applications like lighting, fluorescence imaging technique and so on. To demonstrate the application of lighting, we fabricated a commercial UV chip (380 nm) pumped cold white-LED by mixing Cs₂SnCl₆:0.59%Sb with our home-made blue phosphor Cs₂SnCl₆:2.75%Bi and green phosphor Ba₂Sr₂SiO₄:Eu²⁺ into the silicone. The ratio between each phosphor are optimized to obtain the best white emission. Figure 5(a) shows the emission spectra and the image of the working white-LED. Figure 5(b) shows the Commission Internationale de L'Eclairage (CIE) coordinates of Cs₂SnCl₆:0.59%Sb phosphor and our white-LED. For the orange emission of Cs₂SnCl₆:0.59%Sb, it reveals a CIE color coordinate of (0.55, 0.45) and correlated color temperature (CCT) of 2087 K. With a blue/green/orange ratio of 1:1:1, a cold white-LED with CIE color coordinate of (0.30, 0.37), CCT of 6815 K and the CRI of 81, was obtained. The white-LED exhibited excellent color stability at different operating currents (Fig. 5(c)), as well as under various operating times (Fig. S8), which was ascribed to the negligible energy transfer among different phosphors and the great stability respectively. These excellent performances enable the Cs₂SnCl₆:xSb to be a promising candidate for lighting phosphor.

Conclusion

In summary, we have designed and synthesized a Cs₂SnCl₆:xSb phosphor with orange-red emission and photoluminescence quantum efficiency of ~37%, as well as good stability against both water and heat. This material has a photoluminescence emission at 602 nm and a large FWHM of 101 nm, which precisely matches our previous blue perovskite for lighting application. The temperature dependent PL and PL decay verify the ionoluminescence of Sb³⁺ within Cs₂SnCl₆ matrix. Finally, a cold white-LED with a high CRI of 81 was fabricated by mixing this material with our previously synthesized blue phosphor Cs₂SnCl₆:Bi and commercial green phosphor.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51761145048, 61725401 and 51702107), the National Key R&D Program of China (No. 2016YFB0700702) and the China Postdoctoral Science Foundation (No. 2018M632843). The authors thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, WNLO. The work at Tokyo Institute of Technology was conducted under the Tokodai Institute for Element Strategy (TIES) funded by the MEXT Elements Strategy Initiative to Form Core Research Center.

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