Facile synthesis strategy for cesium tin halide perovskite crystals toward light emitting devices and anti-counterfeiting flexible fiber

Abstract

All-inorganic metal halide perovskites are widely studied because of their excellent photoelectric properties. However, due to the toxicity of CsPbX₃ (X = Cl, Br, I) perovskites, it is difficult to apply them on a large scale. The lead-free nature and air stability make Cs₂SnX₆ (X = Cl, Br, I) perovskites possible candidates to replace CsPbX₃ perovskites. Herein, we report the perovskite crystals (PCs) based on Te(IV)-doped Cs₂SnCl₆: Cs₂Sn_1−xTe_xCl₆. Cs₂Sn_1−xTe_xCl₆ PCs showed yellow emission under a 365 nm ultraviolet lamp. The photoluminescence quantum yield (PLQY) of Cs₂Sn_0.94Te_0.06Cl₆ PCs was 57.09%, which was proposed to be from the triplet Te(IV) ion ³P₁ → ¹S₀ self-trapping excitons (STE) recombination. The perovskite crystals can be used to fabricate light-emitting diodes (LEDs). The fiber paper prepared from aramid chopped fibers (ACFs) and polyphenylene sulfide (PPS) fibers showed a bright yellow light under 365 nm ultraviolet light after being post-processed with Cs₂Sn_1−xTe_xCl₆ PCs solution. The ACFs/PPS compound fiber paper modified with Cs₂Sn_1−xTe_xCl₆ PCs maintained exceptional optical properties and could be stored in air for more than 4500 h. The fluorescence performance of the modified ACFs/PPS compound fiber paper could be applied to fluorescence anti-counterfeiting. The modification strategy and the applications in this work will provide a good choice for studying the optical performance of perovskites and broaden the application of ACFs/PPS compound fiber paper.

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Introduction

Lead halide CsPbX₃ (X = Cl, Br, I) perovskites are widely applied in optoelectronics because of their excellent optical performance.^1–3 For example, perovskites are widely used in photoelectric detection, solar cells, and other fields.^4–6 Perovskites are modified to improve their photoelectric conversion efficiency (PCE) and obtain the desired emission wavelength.^7–10 At the same time, the performance of perovskites can be improved by changing the morphologies of the perovskites.¹¹ The team of Miyasaka reported first metal halide perovskites solar cells with a PCE of 3.8%.¹² The PCE of perovskites solar cells is still rising.^13–15 However, the toxicity, and instability of lead-based perovskites make it difficult to use them in commercial applications.^16–18 The low toxicity and air stability of the element Sn(IV) make it possible for Cs₂SnX₆ (X = Cl, Br, I) to replace CsPbX₃.^19,20 However, although Cs₂SnCl₆ has a direct band gap, it hardly emits light under ultraviolet light. Therefore, synthesizing Cs₂SnCl₆ doped with other metal elements is of great significance to enhance its optical properties.

Doping refers to introducing impurities into intrinsic semiconductors to change their electrical properties.²¹ Existing studies have shown that doping has an important effect on improving the optical performance of perovskites.²² For instance, doping lanthanide into perovskite nanocrystals can greatly improve and expand the optical properties.²³ Sb-doped Cs₂SnCl₆ shows a photoluminescence peak at 630 nm with orange-red emission, and Bi-doped Cs₂SnCl₆ can yield blue self-trapping excitons (STE) emission. However, the unequal valence states between Bi³⁺ and Sn⁴⁺ make a large number of anion sublattices easily appear in the double-doped Cs₂SnCl₆, which may reduce the photoluminescence quantum yield (PLQY).²⁴ According to this view, tetravalent ion doping can significantly reduce the formation of extrinsic defects and promote optical properties. A lead-free perovskite Cs₂Sn_1−xTe_xCl₆ can show a bright yellow light under the irradiation of 365 nm ultraviolet light and has high anti-water stability.²⁵ A Te⁴⁺-doped Cs₂SnCl₆ vacancy-ordered perovskite variant shows green-yellow emission.²⁶ However, the previous synthesis methods required some special ligands or Teflon liners. Therefore, it is urgent to synthesize Cs₂Sn_1−xTe_xCl₆ using a more economical and convenient method. In this paper, Cs₂Sn_1−xTe_xCl₆ perovskite crystals (PCs) with different doping ratios were synthesized via a more convenient hydrothermal method. A light-emitting diode (LED) lamp was fabricated by combining a 385 nm chip and Cs₂Sn_0.94Te_0.06Cl₆ PCs. The LED lamp could emit yellow light driven by a 600 mA current. Furthermore, we have modified fiber paper prepared from aramid chopped fibers (ACFs) and polyphenylene sulfide (PPS) fiber by post-processing it with the Cs₂Sn_1−xTe_xCl₆ PCs solution. The modified ACFs/PPS compound fiber paper showed a bright yellow light under 365 nm ultraviolet light. The modified fiber paper could maintain exceptional optical properties even when soaked in a weak acid solution (pH = 5). The optical properties of the modified ACFs/PPS compound fiber paper can be applied in fluorescence anti-counterfeiting. The modification strategy and the application in this paper will provide a good choice for studying the optical properties of perovskites and broaden the application of ACFs/PPS compound fiber paper.

Results and discussion

Cs₂Sn_1−xTe_xCl₆ PCs with different doping ratios were synthesized via a more convenient hydrothermal method (shown in Fig. 1b). The ACFs/PPS compound fiber paper was prepared via a wet papermaking method (shown in Fig. 1c). The left part of Fig. 1a shows the crystal structure of Cs₂SnCl₆. Tetravalent tin(IV) ions usually occupy the central site and form the standalone [SnCl₆]²⁻ octahedral structure. The crystal structure of Cs₂Sn_1−xTe_xCl₆ is shown in the right part of Fig. 1a. The [TeCl₆]²⁻ replaced part of the [SnCl₆]²⁻ by doping with tetravalent Te(IV). The X-ray diffraction (XRD) patterns and photoluminescence (PL) spectra of Cs₂Sn_0.94Te_0.06Cl₆ with different reaction times are shown in Fig. S1 and S2.† The PL intensity of the Cs₂Sn_0.94Te_0.06Cl₆ PCs obtained after 60 minutes of reaction is the highest. The XRD pattern of un-doped Cs₂SnCl₆ matches well with the standard card (no.: 07-0197) (Fig. 2a). There were no impurity diffraction peaks observed after doping Te⁴⁺ into Cs₂SnCl₆ because of the similar coordination property to halogen of elements Te and Sn,²⁶ which proved that the synthesized Cs₂Sn_1−xTe_xCl₆ perovskite crystals were of high purity. At the same time, the optical properties (shown in Fig. 4(a)) of the doped perovskite crystals have been significantly improved. It can be reasonably inferred that part of [TeCl₆]²⁻ replaces [SnCl₆]²⁻ in the structure. The X-ray photoelectron spectroscopy XPS spectra (Fig. 2b) showed the specific elements’ content of Cs₂SnCl₆ and Cs₂Sn_0.94Te_0.06Cl₆. The detailed XPS spectra (Fig. 2c, d, and f) of Cs, Sn, and Cl of Cs₂Sn_0.94Te_0.06Cl₆ were practically consistent with Cs₂SnCl₆. In contrast, the XPS spectrum of Te of Cs₂Sn_0.94Te_0.06Cl₆ was different from that of Cs₂SnCl₆, which proved that Te had been doped into Cs₂SnCl₆ successfully. The atomic ratio of Cs, Sn, and Cl in un-doped Cs₂SnCl₆ was close to 2 : 1 : 6. The atomic ratio of Te in Cs₂Sn_1−xTe_xCl₆ increased with the doping concentration increasing (shown in Table S1†). The scanning electron microscope (SEM) image of un-doped Cs₂SnCl₆ is shown in Fig. 3a. The elemental mapping images of Cs, Sn, Te, and Cl in Cs₂Sn_0.94Te_0.06Cl₆ are shown in Fig. 3b–f. The SEM image of Cs₂Sn_0.94Te_0.06Cl₆ is shown in Fig. 3g. It can be observed that the crystal morphology of Cs₂Sn_1−xTe_xCl₆ after modification is almost unchanged.

Fig. 1 (a) The crystal structures of Cs₂SnCl₆ and Cs₂Sn_1−xTe_xCl₆ perovskite crystals. (b) and (c) Schematic diagrams of the synthesis of Cs₂Sn_1−xTe_xCl₆ perovskite crystals and ACFs/PPS compound fiber paper.

Fig. 2 (a) XRD patterns of Cs₂SnCl₆ and Cs₂Sn_1−xTe_xCl₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10) perovskite crystals. The bottom pattern is the standard card of Cs₂SnCl₆. (b) XPS spectra of Cs₂SnCl₆ and Cs₂Sn_0.94Te_0.06Cl₆. (c–f) XPS spectra of Cs₂SnCl₆ and Cs₂Sn_0.94Te_0.06Cl₆ clearly show the Cs, Sn, Te, and Cl elements.

Fig. 3 (a) SEM image of Cs₂SnCl₆. (b–f) Elemental mapping images of Cs, Sn, Te, Cl in Cs₂Sn_0.94Te_0.06Cl₆. (g) SEM image of Cs₂Sn_0.94Te_0.06Cl₆.

Fig. 4 (a) PL spectra of Cs₂SnCl₆ and Cs₂Sn_1−xTe_xCl₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10) PCs. (b) PLE spectra of Cs₂Sn_1−xTe_xCl₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10) PCs. (c) PL spectra of Cs₂Sn_0.94Te_0.06Cl₆ PCs with different excitation wavelengths. (d) PLE spectra of Cs₂Sn_0.94Te_0.06Cl₆ PCs under the different monitoring wavelengths. (e) and (f) 2D and 3D wavelength scanning spectra of Cs₂Sn_0.94Te_0.06Cl₆.

The PL and photoluminescence excitation (PLE) spectra of Cs₂Sn_1−xTe_xCl₆ PCs are shown in Fig. 4a and b. The un-doped Cs₂SnCl₆ was hardly luminous, while the PL intensities of Cs₂Sn_1−xTe_xCl₆ were significantly enhanced. It could be observed that the PL intensity of Cs₂Sn_1−xTe_xCl₆ was the highest when x = 0.06 and then decreased with the increase in Te⁴⁺ doping. According to previous reports, this reduction in PL was attributed to the concentration quenching process where the excitation energy migrated to the quenching sites through the lattice.²⁷ They calculated the critical distance (R) for [TeCl₆]²⁻–[TeCl₆]²⁻ energy transfer. According to the formula R⁶ = (0.6 × 10²⁸)(4.8 × 10⁻¹⁶f/E⁴)SO, the calculated R is almost three times the shortest Te–Te distance, which means that there is the possibility of energy transfer. f is the oscillator strength of the relevant ¹S₀→³P₁ transition and is taken to be 10⁻²; E, the energy of maximum spectral overlap, is 2.5 eV; SO, the spectral overlap of emission and absorption, amounts to 0.1 eV⁻¹. The peak position of the PL intensity showed a slight red shift with the increase in the amount of Te⁴⁺ doped. For halide perovskites with a soft lattice, the photogenerated electrons at the excited state levels tend to be coupled with lattice vibration, causing transient lattice distortion, changing nuclear coordinates, and producing local STE.²⁸ Tetravalent Te, as a typical ion with the outer electronic configuration of 5s², has five energy levels: ground state of ¹S₀ and singlet/triplet excited states of ¹P₁/³P_n (n = 0, 1, 2).²⁹ Self-trapping excitons widely exist in perovskites.³⁰ In some perovskite crystals, the electrons and holes generated by excitation will immediately be self-trapped because the self-trapped state is more stable. According to other reports, both a broad PL band and large Stokes shift are typical features of STE emission.^31,32 Take Cs₂Sn_0.94Te_0.06Cl₆ for example, the PL band starts at 450 nm and ends at 700 nm. The excitation wavelength is 385 nm, and the emission wavelength is 553 nm. Based on these data, it can be reasonably inferred that the fluorescence property of Te-doped Cs₂SnCl₆ is due to STE emission. Fig. 4c and d are the PL and PLE spectra of Cs₂Sn_0.94Te_0.06Cl₆ PCs. It can be observed that the PL and PLE spectra almost maintain the same wave profile no matter how the excitation and emission change, which proves that the emission band is the luminescence characteristic of Cs₂Sn_0.94Te_0.06Cl₆ PCs, not the surface defects or the lattice defects of the samples. The test results verify the STE and ion luminescence characteristics of Cs₂Sn_1−xTe_xCl₆. Fig. 4e and f are the two-dimensional (2D) and three-dimensional (3D) fluorescence spectra of Cs₂Sn_0.94Te_0.06Cl₆ PCs. It can be observed that the emission wavelength varies between 500 nm to 600 nm, while the excitation wavelength varies between 250 nm to 450 nm. The time-resolved (TRPL) decay curves (shown in Fig. 5a) of Cs₂Sn_1−xTe_xCl₆ PCs can be well fitted with the bi-exponential eqn (1):³³

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂). (1)

Fig. 5 (a) TRPL decay curves of Cs₂Sn_1−xTe_xCl₆ (x = 0.02, 0.06, 0.08) PCs. (b) Chromaticity coordinates (CIE 1931) of Cs₂Sn_1−xTe_xCl₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10) PCs.

The average fluorescence lifetime can be calculated using eqn (2):²¹

τ_avg = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂). (2)

It can be calculated that the average fluorescence lifetimes of Cs₂Sn_1−xTe_xCl₆ PCs are 2.06 μs (x = 0.02), 38.61 μs (x = 0.06) and 1.83 μs (x = 0.08). At the same time, combined with the PL spectra (Fig. 4(a)), the emission intensity of the sample is positively related to its lifetime. The chromaticity coordinate (CIE) values of un-doped Cs₂SnCl₆ PCs and Cs₂Sn_1−xTe_xCl₆ can be calculated to be (0.23, 0.21), (0.38, 0.52), (0.39, 0.54), (0.39, 0.55), (0.39, 0.54), (0.38, 0.53), which appeared in the yellow area of the CIE (Fig. 5b). The color purity of Cs₂Sn_1−xTe_xCl₆ PCs can be calculated from eqn (3):³⁴

(x, y), (x_i, y_i), and (x_d, y_d) conform to the CIE color coordinates of the Cs₂Sn_1−xTe_xCl₆ PCs, white light illumination, and dominant wavelength, respectively. For Cs₂Sn_0.94Te_0.06Cl₆ PCs, the concrete values of (x, y), (x_i, y_i), and (x_d, y_d) are (0.39, 0.55), (0.33, 0.33), (0.40, 0.59). It can be calculated that the color purity of Cs₂Sn_0.94Te_0.06Cl₆ PCs is 84.69%. The color purity of the sample indicated that it had the potential to manufacture LED lamps for general lighting. To further test and verify the application of Cs₂Sn_1−xTe_xCl₆ PCs for LEDs, a LED lamp was fabricated by combining the 385 nm n-UV chip and Cs₂Sn_0.94Te_0.06Cl₆ PCs. The electroluminescence (EL) spectrum of this LED lamp driven by the current is shown in Fig. 6e. The driving current is 600 mA and the corresponding output power is 3 W. The LED lamp produces CIE (shown in Fig. S3†) color coordinates of (0.35, 0.37) with a correlated color temperature (CCT) of 4879 K, which shows that Cs₂Sn_0.94Te_0.06Cl₆ perovskite crystals have potential in the manufacture of LED lamps for general lighting.

Fig. 6 (a) Fluorescent properties of ACFs/PPS compound fiber paper without modification and modified with Cs₂Sn_0.94Te_0.06Cl₆ PCs solution. (b) The stability of modified ACFs/PPS compound fiber paper in air. (c) The relative PL spectrum of modified ACFs/PPS compound fiber paper at different temperatures. (d) TGA of modified ACFs/PPS compound fiber paper. (e) Electroluminescence spectrum of light-emitting diode (LED) lamp fabricated via the combination of a near-ultraviolet 385 nm n-UV chip and Cs₂Sn_0.94Te_0.06Cl₆ perovskite crystals. (Inset) Digital images of the LED package with and without current input. (f) Fluorescent properties of modified ACFs/PPS compound fiber paper in weak acid solution.

Meanwhile, we modified the ACFs/PPS compound fiber paper by post-processing it with Cs₂Sn_0.94Te_0.06Cl₆ PCs solution. Fig. 6a shows that the modified ACFs/PPS compound fiber paper showed bright yellow light under UV light while the unmodified paper did not. Although stored in air for more than 4500 h, the modified fiber paper still showed a bright yellow light (Fig. 6b). Furthermore, the modified paper can still maintain luminescence characteristics when it was immersed in a weak acid solution (Fig. 6f). The PLQY of the modified fiber paper soaked in a weak acid solution was 17.63%. It could be observed that the fluorescence intensities of the modified fiber paper decreased with the increase in temperature (Fig. 6c). It was similar to the reduction in PL intensity due to the participation of phonons in nonradiative recombination at high temperatures. The thermal stability of the modified ACFs/PPS compound fiber paper can be verified by the thermogravimetric analysis (Fig. 6d). PL spectra of the ACFs/PPS composite fiber paper were measured at 50 °C and after falling back from 250 °C (shown in Fig. S4(a)†). The modified composite fiber paper could retain 82% of the original emission intensity when it fell from 250 to 50 °C. In addition, PL intensities of the ACFs/PPS composite fiber paper were measured at different temperatures. The intensity of PL decreased gradually with increasing temperature (shown in Fig. S4(b)†). However, after heating for 5 min, an enhancement in the PL intensity at some temperatures can be seen. Furthermore, the average intensity of PL can still maintain 98% of the original PL intensity when it falls from 100 to 40 °C. It can be observed that the sublimation temperature of the modified ACFs/PPS compound fiber paper is about 300 °C (Fig. 6c). Based on the above data, it can be concluded that the ACFs/PPS composite fiber paper has a certain degree of thermal stability. These test results have laid a good foundation for use of the modified ACFs/PPS compound fiber paper as a fluorescent anti-counterfeiting material.

Conclusions

To sum up, the Cs₂Sn_1−xTe_xCl₆ PCs were synthesized via a more convenient hydrothermal method. The synthesized perovskite crystals showed obvious fluorescence and the emission center was 553 nm. Cs₂Sn_0.94Te_0.06Cl₆ PCs showed the highest fluorescence intensity, where the PLQY could reach 57.09%. Through the PL and PLE spectra, it could be inferred that the fluorescence of Cs₂Sn_1−xTe_xCl₆ PCs was attributed to the doping of the Te element. The LED lamp fabricated by combining a 385 nm n-UV chip and Cs₂Sn_0.94Te_0.06Cl₆ PCs could emit yellow light driven by the current. Furthermore, ACFs/PPS compound fiber paper was post-processed with Cs₂Sn_1−xTe_xCl₆ PCs solution. The test results in this paper showed that the modified ACFs/PPS compound fiber paper can maintain fluorescence properties in air and a weak acid environment for a long time, which laid a good foundation for it as a fluorescent anti-counterfeiting material. This work provides a good choice to study the optical performance of perovskites crystals and broadens the application of the ACFs/PPS compound fiber paper.

Author contributions

Ziyao Hu: conceptualization, investigation, methodology, data curation, formal analysis, writing – original draft; Kun Nie: resources, conceptualization, supervision, project administration, funding acquisition, writing – review and editing; Xuyi Wang: project administration, funding acquisition; Xiuqiang Duan: investigation, methodology; Ranran Zhou: investigation, methodology; Mengyun Wu: investigation; Xiaoxue Ma: resources; Xiaodong Zhang: investigation; Luoxin Wang: project administration, funding acquisition; Lefu Mei: funding acquisition, project administration; Hua Wang: funding acquisition, project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Key Laboratory of Testing and Tracing of Rare Earth Products for State Market Regulation (Jiangxi University of Science and Technology) (TTREP2022YB04), the National Natural Science Foundation of China (51872269, 52274273), the Science and Technology Research Project of Hubei Provincial Department of Education (B2021091), Key Laboratory for New Textile Materials and Applications of Hubei Province (Wuhan Textile University) (FZXCL202107), the Open Project Program of High-Tech Organic Fibers Key Laboratory of Sichuan Province, and China and National Project Cultivation Plan of Wuhan Textile University. This work was supported by the Graduate Innovation Fund Project of Wuhan Textile University. The authors would like to thank Liu Nian and Liu Tianying from Shiyanjia Lab (https://www.shiyanjia.com) for the XPS and SEM characterizations.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr00301a

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