Highly stable metal halides Cs2ZnX4 (X = Cl, Br) with Sn2+ as dopants for efficient deep-red photoluminescence

Yan Zhang Lei Zhou Lei Zhang Wei Luo Wei Shen Ming Li Rongxing He

Citation:  Yan Zhang, Lei Zhou, Lei Zhang, Wei Luo, Wei Shen, Ming Li, Rongxing He. Highly stable metal halides Cs2ZnX4 (X = Cl, Br) with Sn2+ as dopants for efficient deep-red photoluminescence[J]. Chinese Chemical Letters, 2023, 34(2): 107556. doi: 10.1016/j.cclet.2022.05.070 shu

Highly stable metal halides Cs2ZnX4 (X = Cl, Br) with Sn2+ as dopants for efficient deep-red photoluminescence


  • In the past few years, metal halide perovskites as luminescent materials have been widely studied because of their intriguing optoelectronic properties, including spectral tenability and high photoluminescence quantum yields (PLQYs) [1-4]. As the key to the development of next-generation displays, luminescent materials used as light-emitting diode (LED) are required not only to meet the color purity standards [5-7], but also to be low-cost and environmental friendliness [8-10]. Although great progresses have been achieved, the stability issue of perovskite is still a bottleneck [11-14]. Furthermore, lead-free metal halide perovskites with intense emission in deep-red spectral region (> 660 nm) are very rare and remain great developing challenge [15, 16], although they are more desirable for display, photoelectric detection, biological tissue imaging, and solid-state lighting [17-20]. Therefore, it is of great significance to develop environmental-friendly, stable metal halides with highly efficient deep-red luminescence.

    Recently, the zero-dimensional (0D) metal halide Cs2ZnCl4 with wide bandgap of 4.2 eV and negligible luminescence [21] are once again becoming excellent candidates as active materials, because the luminescence properties of the materials used Cs2ZnCl4 as host can be greatly improved by doping strategy. For example, Cheng et al. [22] reported the Cu+-doped blue emitting Cs2ZnX4 with PLQY of 65.3%. Su et al. [23] developed a near-infrared emission single crystal by incorporating Sb3+ into the Cs2ZnCl4 matrix with high PLQY of 69.9%. Although the PLQYs of these materials have been greatly enhanced compared with that of the pristine Cs2ZnCl4, their properties are still unsatisfactory, especially their structural stability towards humidity and thermal. Generally, there are many factors that determine the color tunability and PL efficiency of emitting materials, and the structure of luminescent center is a key one of them [24-28]. Because Cs2ZnCl4 has an orthorhombic crystal structure with disconnected [ZnCl4]2− tetrahedrons, when the doped metal ion Mx+ (x = 1, 2) is introduced into Cs2ZnCl4, a part of [ZnCl4]2− tetrahedrons should be replaced by [MCl4]y (y = 2, 3) tetrahedrons, which is the origin of strong emission for doped Cs2ZnCl4 [22, 29]. Previous reports [30, 31] have demonstrated that the four coordinated Sn2+ ions are prone to induce red emission. When Sn2+ is used as a dopant, more luminescent centers associated with Sn2+ ions are introduced into the lattice of Cs2ZnCl4, which induces the generation of self-trapped excitons (STEs), ultimately leading to the formation of STE states with a deeper self-trapping depth [32-34]. Therefore, it is anticipated that the band gap of Cs2ZnCl4 would be reduced further and the deep-red emission can thus be realized by introducing Sn2+ into the lattice. The Sn2+-doped Cs2ZnCl4 microcrystal has recently been developed by Wu and co-workers [35]. While its PL efficiency is unknown and the behind PL mechanism is poorly understood. For Cs2ZnCl4: Sn, the synthesis of its single crystals with lower defect densities is challenging. Thus, efficient deep-red emission is highly expected in Cs2ZnCl4: Sn single crystals, and the corresponding photophysical properties remain to be unveiled further.

    Herein, we report a Sn2+-doped 0D metal halide Cs2ZnCl4 (Cs2ZnCl4: Sn) with a broadband deep-red emission peaked at about 700 nm. By introducing Sn2+ into Cs2ZnCl4, an unprecedented improvement of PLQY (~99.4%) and outstanding stability (the PLQY of Cs2ZnCl4: Sn only decreases 4% when it is exposed to the air with relative humidity of 80% for 370 days) were realized. To the best of our knowledge, this is the best performance reported to date for any lead-free metal halides with deep-red emission. Detailed spectral characterizations and density functional theory (DFT) calculations revealed that the bright deep-red emission in Cs2ZnCl4: Sn originates from the STEs induced by the doped Sn2+ ion. For comparison, the Sn2+-doped Cs2ZnBr4 (Cs2ZnBr4: Sn) emitting deep-red light (710 nm) was also synthesized using the same method, and its PLQY (~33.9%) and stability are also very prominent.

    Cs2ZnCl4 and Sn2+-doped Cs2ZnCl4 (i.e., Cs2ZnCl4: Sn) single crystals were obtained through solvothermal reaction. The detailed synthesis and characterization are presented in Supporting information. It is reported that the crystal of Cs2ZnCl4 is orthorhombic space group Pnam [21]. The [ZnCl4]2− tetrahedrons are isolated from each other by Cs+ cations and thus form a 0D structure (Fig. 1a). Powder X-ray diffraction (PXRD) patterns of Cs2ZnCl4 and Cs2ZnCl4: Sn were measured and the main diffraction peaks were found to be unique to Cs2ZnCl4 (Fig. 1b). Compared with Cs2ZnCl4, the diffraction peak of Cs2ZnCl4: Sn at 20.5° (2θ) moves to a smaller angle, indicating that Sn2+ is incorporated into the Cs2ZnCl4 successfully. Moreover, Cs2ZnBr4 and Cs2ZnBr4: Sn were also synthesized and verified by PXRD (Fig. S1 in Supporting information). The oxidation state of Sn2+ was confirmed to be positively divalent by X-ray photoelectron spectroscopy (XPS) (Fig. S2 in Supporting information). As shown in Fig. S2d, the main bands located at 487 and 496 eV are derived from the Sn2+ 3d5 state. Inductively coupled plasma optical emission spectroscopy (ICP-OES) reveals the actual content of Sn2+ in various Cs2ZnCl4: Sn and Cs2ZnBr4: Sn samples. The results shown in Table S1 (Supporting information) unveil the actual Sn2+ content of the maximum feeding ratio samples for Cs2ZnCl4: Sn and Cs2ZnBr4: Sn is only 1.42% and 1.12%, respectively, implying that the feeding Sn2+ precursor was only partially embedded into the Cs2ZnX4 lattice. Moreover, the slight shift of the XRD peaks towards a small angle further demonstrates the incorporation of trace amounts of Sn2+ in Cs2ZnX4. Nevertheless, the photophysical properties of Cs2ZnX4 have been modulated greatly after Sn2+ doping, which will be discussed later. In this article, unless otherwise specified, the samples with the highest actual doping rate (i.e., Cs2ZnCl4: 1.42%Sn and Cs2ZnBr4: 1.12%Sn) were chosen for all tests, and abbreviated as Cs2ZnCl4: Sn and Cs2ZnBr4: Sn for simplicity. To shed new light on the structural changes of doped samples, Raman spectra of Cs2ZnCl4 and Cs2ZnCl4: Sn were measured. As shown in Figs. 1c and d and Table S2 (Supporting information), the Raman peaks of Cs2ZnCl4 and Cs2ZnCl4: Sn are very similar, indicating that the introduction of Sn2+ does not destroy the lattice of Cs2ZnCl4. Here, the relative intensity and the positions of Zn-Cl stretching mode (ν1-ν4) and Cl-Zn-Cl bending mode (ν5-ν9) are changed slightly for Cs2ZnCl4: Sn due to the distortion of [ZnCl4]2− tetrahedron after Sn2+ doping. The above analysis confirmed that Sn2+ was successfully doped into the lattice of Cs2ZnCl4 by substituting a small part of Zn2+.

    Figure 1

    Figure 1.  (a) Crystal structure of Cs2ZnCl4. (b) PXRD diffractograms of Cs2ZnCl4 and Cs2ZnCl4: Sn. Raman spectrum of Cs2ZnCl4 (c) and Cs2ZnCl4: Sn (d).

    The optical properties of these materials were characterized by UV–vis absorption spectroscopy and PL spectroscopy. The solid-state UV–vis spectrum of Cs2ZnCl4 in Fig. 2a exhibits absorption below 400 nm, with an intense absorption peak at ~270 nm, indicating the negligible absorption in visible-light region. While a shoulder peak between 300 and 400 nm emerges after Sn2+ doping, which can be attributed to the absorption of [SnCl4]2− tetrahedron, suggesting the successful embedding of Sn2+ in Cs2ZnCl4 [36]. As shown in Fig. 2b, an additional narrow-band peak (~350 nm) appears in the PL excitation (PLE) spectrum, which is the main feature of the red emission observed for the Sn2+-doped materials [37, 38]. Figs. 2b and c and Table S3 (Supporting information) provide the room-temperature PL properties of Cs2ZnCl4 with different Sn2+ content, revealing that the PLQY gradually increases with the increase of Sn2+ doping concentration. This is because that more luminescent centers associated with Sn2+ ions are introduced into the lattice of Cs2ZnCl4 and the optimum luminescent performance is obtained when the actual doping amount reaches nSn: nZn = 1.42%. After that, impurity such as CsSnCl3 was detected. For Cs2ZnCl4: 1.42%Sn, a deep-red emission centered at ~700 nm is observed under 280 nm excitation with PLQY of 99.4% and the corresponding Commission Internationale de L'Eclairage (CIE) coordinates is (0.58, 0.39). It is worth adding that, to the best of our knowledge, such PLQY is the highest value of deep-red light among the reported metal-halide single crystals (Table S4 in Supporting information). The PL decay curves of Cs2ZnCl4: Sn with different Sn2+ concentrations were shown in Fig. 2d, and the corresponding data were presented in Table S5 (Supporting information). The average PL lifetime in Fig. 2d indicates a slight increase in PL lifetime with the increase of doping concentration, and the lifetime of Cs2ZnCl4: 1.42%Sn is up to 18.3 µs. Such long lifetime, large Stokes shift and broad PL line suggest the deep-red emission might be from the radiative recombination of triplet STEs in [SnCl4]2− tetrahedron, by inhibiting the non-radiative recombination in Cs2ZnCl4.

    Figure 2

    Figure 2.  (a) UV–vis absorption spectra of Cs2ZnCl4 and Cs2ZnCl4: Sn. (b) Room-temperature PL and PLE spectra of Cs2ZnCl4: Sn. (c) CIE chromaticity diagram of Cs2ZnCl4: Sn. (d) Room-temperature PL decay curves of Cs2ZnCl4: Sn.

    To gain more insight into the effect of Sn2+ on the photophysical properties of Cs2ZnCl4, first principle calculations were carried out. As displayed in Fig. 3a, the calculated energy bandgap of Cs2ZnCl4 is 4.62 eV, while that of Sn2+-doped Cs2ZnCl4 is reduced to 3.42 eV (Fig. 3b), which is in agreement with the experimentally reduced trend of bandgap (4.15 eV for Cs2ZnCl4 and 3.58 eV for Cs2ZnCl4: Sn). It is worth noting that the calculated band structure of Sn-doped Cs2ZnCl4 is more flatter, compared to that of the undoped Cs2ZnCl4, suggesting the excited carriers are more localized after the incorporation of Sn2+. Upon examining the projected density of states (DOS) (Fig. S3 in Supporting information), the resultant valence band maximum (VBM) of Cs2ZnCl4 is mainly composed of Cs-p, Zn-d and Cl-p orbitals, and the conduction band minimum (CBM) consists of Cs-d, Zn-s and Cl-p orbitals. Conversely, for Cs2ZnCl4: Sn, its VBM are mainly contributed by Sn-s and Cl-p orbitals, and CBM are mainly composed of Sn-p and Cl-p orbitals. This verifies that the embedded Sn2+ ions change the position of VBM and CBM of Cs2ZnCl4 and reduce the band gap, resulting in the deep-red emission in Sn2+-doped Cs2ZnCl4. In addition, the calculated charge density (Figs. 3c and d) demonstrates that the carriers are delocalized in both VBM and CBM of Cs2ZnCl4, whereas those are localized in [SnCl4]2− tetrahedrons in Sn2+-doped Cs2ZnCl4, agreeing perfectly with the DOS results. The highly localized charge density induces a statically expressed distortion of the [SnCl4]2− tetrahedrons due to the presence of stereoactive 5s2 lone pair in Sn2+ [33, 39], which is conducive to the formation of STEs, and thereby leading to the broadband deep-red emission [40].

    Figure 3

    Figure 3.  Calculated band structure (a) and charge densities (VBM (left) and CBM (right)) (b) of Cs2ZnCl4. Calculated band structure (c) and charge densities (VBM (left) and CBM (right)) (d) of Cs2ZnCl4: Sn. The red circle denotes the doped Sn2+ ion.

    The radiative recombination in 0D metal halides can originate from many aspects, such as permanent defects, free excitons (FE) and STEs. To probe into the origin of the deep-red emission, PLE and PL spectra of Cs2ZnCl4: Sn monitored at different wavelengths were carried out, and the consistency of the peaks shown in Fig. S4 (Supporting information) indicates the deep-red emission origins from an identical excited states. Further, upon the excitation power dependent PL measurements, the PL intensity of deep-red emission for Cs2ZnCl4: Sn is linear with the excitation power (Fig. 4a), eliminating the possibility that the deep-red emission is resulted from permanent defects. In contrast to FE emission that typically features with narrow PL band, small Stokes shifts and short lifetime is at nanosecond level, the PL spectrum of Sn2+-doped Cs2ZnCl4 exhibits an ultra-broad full width at half maximum (FWHM) over 158 nm, a large Stokes shifts of 420 nm and a long PL lifetime up to 18.3 µs at room temperature, ruling out the possibility of FE emission. Combined with the calculated results, it can be reasonably speculated that the strong deep-red light of Cs2ZnCl4: Sn is originated from the triplet STE emission induced by Sn2+ dopant. To prove such conjecture, temperature-dependent PL spectra of Cs2ZnCl4: Sn was measured and the corresponding results were depicted in Fig. 4b and Fig. S5 (Supporting information). With the decrease of temperature, the maximum peak position gradually redshifts from 700 nm to 740 nm, as a result of the intense electron-phonon interaction [41]. Meanwhile, the PL intensity enhances with the decrease of temperature, suggesting the suppressed of nonradiative recombination. When the temperature is lower than 150 K, an obvious emission centered at 470 nm appears. Furthermore, as shown in Fig. S6 (Supporting information), the exciton binding energy (Eb) can be calculated with 190 meV, which is much higher than the thermal energy at room temperature (26 meV), providing a high probability of radiative combination in Cs2ZnCl4: Sn [42]. Meanwhile, the relationship between FWHM and temperature can be fitted, with a calculated Huang-Rhys factor (S) value of 62.6. Such a large S indicates the presence of a strong electron-phonon coupling effect in Cs2ZnCl4: Sn, which enables the formation of STEs and the broadening of PL band. To further probe into such emission, PL decay curves of Cs2ZnCl4: Sn were measured at 80 K (Fig. 4c and Table S6 in Supporting information). For comparison, the PL and PL decay lifetimes of Cs2ZnCl4 were also measured at the 80 K (Fig. S7 in Supporting information and Table S6). As shown in Table S6, the lifetime of the emission peak at 450 nm in Cs2ZnCl4 is 20 ns, much shorter than that of the emission peak at 470 nm in Cs2ZnCl4: Sn (10.33 µs), indicating that the emission at 470 nm also comes from the triplet STEs. For Sn2+ ions with 5s2 electronic configuration, its excited states can be composed of one singlet state (1P1) and three triplet states (3P0, 3P1 and 3P2), where the triplet states 3P0, 3P1 and 3P2 can be further split due to spin-orbit interactions [43]. Undoubtedly, for Cs2ZnCl4: Sn, the emission peak at 740 nm in 80 K is attributed to the 3P11S0 transition of Sn2+ ion [33], while the emission peak at 470 nm in 80 K may be assigned to the transition of 3P21S0, as a result of the break of its forbidden nature at low temperature. In this situation, the crystal symmetry might be modulated or broken to convert the forbidden transition of 3P21S0 to an allowed transition, thus further enabling the radiative emission, which has also been seen in bismuth-based materials [44].

    Figure 4

    Figure 4.  (a) Dependence of the emission intensity at 700 nm of Cs2ZnCl4: Sn on the excitation power at 300 K (λex = 405 nm). (b) Temperature-dependent PL spectra of Cs2ZnCl4: Sn. (c) PL decay curves of Cs2ZnCl4: Sn monitoring at 470 and 740 nm at 80 K (λex = 280 nm). (d) Possible schematic diagram of the energy level of Cs2ZnCl4: Sn.

    Following the above results, the possible photophysical process of Cs2ZnCl4: Sn is illustrated in Fig. 4d. Briefly, upon the photoexcitation, electrons in the ground state are promoted to the excited states of [SnCl4]2−. And simultaneously, the highly localized electrons in excited states combining with the stereoactive 5s2 lone pair in Sn2+ induce structural distortion of [SnCl4]2− polyhedron that further leads to the formation of STEs [45]. Then, the excitons will undergo an intersystem crossing (ISC) process to triplet STE state, where some of them are quickly self-trapped to relatively high energy states, enabling the blue light (~470 nm) emission, while the rest will transfer to low energy STE state, finally resulting in the highly efficient deep-red emission with a large Stokes shift and long lifetime.

    To expand the scope of materials, Cs2ZnBr4: Sn was also synthesized and studied to uncover the effect of halogen on the PL properties, details of which can be found in Supporting information. Furthermore, as an excellent luminescent material, except for the outstanding luminescent efficiency, stability is also important for practical applications. As illustrated in Fig. S10a (Supporting information), the thermal stability of Cs2ZnCl4: Sn and Cs2ZnBr4: Sn was evaluated by thermogravimetry analysis (TGA), and the results reveal that both of the two materials are thermally stable up to 610 ℃, indicating their excellent thermal stability that is extremely important for optoelectronic applications. Moreover, their humidity stability was also investigated. As shown in Fig. S10b (Supporting information), the XRD profiles of the two materials after exposing to the atmosphere (relative humidity > 80%) for one year exhibit almost no significant change compared with that of their freshly prepared samples, manifesting that both Cs2ZnCl4: Sn and Cs2ZnBr4: Sn are resistant to the humidity. Surprisingly, after exposing to atmosphere for one year, the PLQYs of Cs2ZnCl4: Sn decreased by only 4% (~30% for Cs2ZnBr4: Sn) compared with its initial values (Fig. S10c in Supporting information), further confirming its excellent PL stability towards humidity. Generally, Sn2+ is unstable and can be easily oxidized to Sn4+ in air. In these two materials, the actual doping concentration of Sn2+ is extremely low (1.42% for Cs2ZnCl4: Sn and 1.12% for Cs2ZnBr4: Sn), thus Sn2+ is hardly oxidized in atmosphere with high humidity. Moreover, it is noteworthy that the Sn-X (X = Cl, Br) bond lengths here (2.28 Å for Cs2ZnCl4: Sn and 2.43 Å for Cs2ZnBr4: Sn) are significantly shorter than that in the previous reports (both Sn-Cl and Sn-Br are > 2.5 Å) (Table S7 in Supporting information), which allows for stronger interactions between the Sn2+ ion and the halogen, resulting in the excellent stability of the studied materials.

    In summary, we have successfully prepared the Sn2+-doped 0D free-lead metal halides Cs2ZnX4: Sn (X = Cl, Br) with bright deep-red emission. The Cs2ZnCl4: Sn exhibits deep-red emission (700 nm) with ultra-high PLQY (99.4%) owing to the introduction of Sn2+ ions which can reduce the band gap of the native and induce triplet STE emission. Such a superior PL performance is attributed to the 5s2 lone pair electron expression of the Sn2+ ion. Most importantly, Cs2ZnCl4: Sn possess outstanding PL and structural stability, maintaining the PLQY under ambient air with relative humidity over 80% for more than 370 days. The present work not only sheds insight into the luminescent mechanism of Cs2ZnX4: Sn (X = Cl, Br), but also provides an efficient, stable and environment-friendly deep-red emitting material for promising optoelectronic devices and biological imaging.

    The authors report no declarations of interest.

    We acknowledge the financial supports from National Natural Science Foundation of China (Nos. 91741105, 22109130), Chongqing Municipal Natural Science Foundation (Nos. cstc2018jcyjAX0625, cstc2021jcyj-msxmX1180), and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (No. CXTDX201601011).

    Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.070.

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  • Figure 1  (a) Crystal structure of Cs2ZnCl4. (b) PXRD diffractograms of Cs2ZnCl4 and Cs2ZnCl4: Sn. Raman spectrum of Cs2ZnCl4 (c) and Cs2ZnCl4: Sn (d).

    Figure 2  (a) UV–vis absorption spectra of Cs2ZnCl4 and Cs2ZnCl4: Sn. (b) Room-temperature PL and PLE spectra of Cs2ZnCl4: Sn. (c) CIE chromaticity diagram of Cs2ZnCl4: Sn. (d) Room-temperature PL decay curves of Cs2ZnCl4: Sn.

    Figure 3  Calculated band structure (a) and charge densities (VBM (left) and CBM (right)) (b) of Cs2ZnCl4. Calculated band structure (c) and charge densities (VBM (left) and CBM (right)) (d) of Cs2ZnCl4: Sn. The red circle denotes the doped Sn2+ ion.

    Figure 4  (a) Dependence of the emission intensity at 700 nm of Cs2ZnCl4: Sn on the excitation power at 300 K (λex = 405 nm). (b) Temperature-dependent PL spectra of Cs2ZnCl4: Sn. (c) PL decay curves of Cs2ZnCl4: Sn monitoring at 470 and 740 nm at 80 K (λex = 280 nm). (d) Possible schematic diagram of the energy level of Cs2ZnCl4: Sn.

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  • 发布日期:  2023-02-15
  • 收稿日期:  2022-02-07
  • 接受日期:  2022-05-21
  • 修回日期:  2022-04-26
  • 网络出版日期:  2022-05-26
通讯作者: 陈斌, bchen63@163.com
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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