Open AccessCCS ChemistryCOMMUNICATION1 Oct 2021Multiple Hydrogen Bond-Induced Structural Distortion for Broadband White-Light Emission in Two-Dimensional Perovskites Xiaoting Liu†, Ziqi Yang†, Chengda Ge, Hanming Li, Mingwei Hao, Chenlian Wan, Yilong Song, Bao Li and Qingfeng Dong Xiaoting Liu† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 †X. Liu and Z. Yang contributed equally to this work.Google Scholar More articles by this author , Ziqi Yang† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 †X. Liu and Z. Yang contributed equally to this work.Google Scholar More articles by this author , Chengda Ge State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Hanming Li State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Mingwei Hao State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Chenlian Wan State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Yilong Song State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Bao Li State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Qingfeng Dong *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000484 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Low-dimensional hybrid lead-halide perovskites with broadband white-light emission upon near-UV excitation have attracted immense scientific interest due to their potential application for the next generation of solid-state lighting as well as scintillators for radiation detection. Recently, broadband emission material is mostly reported in structural distorted perovskites. However, it is still unclear how to generate structural distortion in low-dimensional perovskites. Herein, we find strong structural distortion could be realized by introducing multiple hydrogen bonds for stronger supramolecular interactions between the organic cations and metal-halide inorganic frameworks, which is favorable to exciton self-trapping for broadband white-light emission in hybrid perovskites. The multifunctionalized metformin-configured two-dimensional (2D)-layered perovskites, MFPbClxBr4-x (x = 0–4), presents a broadband white-light emission, which is mainly attributed to electron–phonon coupling induced self-trapped excitons (STEs) in the highly distorted structure. Meanwhile, the emission of the novel MFPbClxBr4-x can be simply tuned from “cold” to “warm” white light by tailoring the anionic component of mixing-halide perovskites. It is of high value to explore the possibility of biguanide derivatives as a new type of template of cations for structural diversity of perovskite material systems as well as to understand structure−property relationships in structural design for realizing desired properties for photovoltaic applications. Download figure Download PowerPoint Introduction Organic−inorganic hybrid-halide perovskites (OIHPs) have attracted a great deal of attention for their excellent optical-electric properties, such as a high absorption coefficient, a high charge-carrier mobility, and a long carrier recombination lifetime, making OIHPs excellent candidates in many important areas, including solar cells,1–4 light-emitting diodes,5–8 lasers,9 solid-state lighting,10 and scintillators11–13 for radiation detection. Lately, organic–inorganic lead-halide perovskites have been found to be a promising class of scintillator materials for X-ray imaging, with the ability to convert high-energy X-ray photons into low-energy visible photons. Compared with conventional scintillators, the emerging hybrid perovskites for X-ray detectors exhibit attractive virtues of facile synthesis, fast response, and easily tunable radioluminescence across the visible spectrum simply by adjusting composition of mixed-halide perovskites. However, the strong self-absorption of this mixture restricts their practical applications. There are two methods that could overcome the effect of self-absorption. One is element-doping, which can change the photoluminescence (PL) emission of the resulting materials, and the other is to construct broadband extrinsic self-trapped exciton (STE) emission materials in low-dimensional perovskites. Unfortunately, doping leads to the long decay time of the materials, which is not suitable for X-ray detection. By contrast, the STE broadband emission in hybrid perovskites can realize large Stokes shifts, avoid self-absorption, and maintain the short decay time. Accordingly, this type of single-component broadband emission perovskite materials is a potential candidate for high-efficiency X-ray scintillation, and provides a way to further explore perovskite scintillator materials for radiation detection. Recently, a series of (100)-oriented, (110)-oriented, and (111)-oriented two-dimensional (2D)-layered perovskites with large Stokes shifts and broadband emissions attributed to STEs or charge carriers trapped on defects have been reported.14–19 These perovskites showed potential applications for single-component white-light emitters as well as scintillator materials for radiation detection due to the advantages in structural diversity, high tunability, and color stability. It has been reported that the broadband white-light emission on low-dimensional perovskites originated from excitons self-trapping under UV excitation,13,20–24 which was strongly related to the dimensionality of the structure. Extensive research has proven the feasibility of forming self-trapped states when the dimensionality of hybrid perovskites is reduced from 3D to 2D, 1D or 0D. In particular, the stronger electron–phonon coupling contributed to structural distortion and the formation of self-trapped states within low-dimensional perovskites.20 However, the limited variety of amine-functionalized interlayer cations greatly limited the development of solid-state lighting and scintillator materials, and only a few broadband emission perovskite materials were reported. Apart from traditional A-site organic cations, methylammonium (MA) and formamidinium (FA), guanidinium cation (GA) is an excellent donor of hydrogen bonds, which can interact with the inorganic lead-halide frameworks. Moreover, guanidinium derivatives have been extensively studied in many fields, especially in medical applications,25 biological systems,26 and functional materials27 due to the presence of several potential donor sites for supramolecular host–guest interaction. Here, we choose a biguanide derivative, metformin (MF) as the organic moiety in perovskites and find that the multiamine groups functionalized biguanidinium shows strong supramolecular interaction with the metal-halide inorganic framework in perovskites. This forms a type of novel single-layered 2D organic–inorganic hybrid perovskite. The inorganic sublattice of MFPbClxBr4-x (x = 0–4) perovskites exhibits structural distortion in each PbX6 octahedra. Multihydrogen-bonding interaction enables MF cations bonded with both bridging halide and terminal halide within inorganic layers at the same time. Meanwhile, an obviously imperfect mismatched arrangement was presented in the layered perovskites for steric effect of the larger and asymmetric biguanidine interlayer cation. All these together contributed to the highly disordered lattice structures, which further influenced the optical properties of the MFPbClxBr4-x perovskites. It was found that the greatly distorted 2D structure of MFPbClxBr4-x resulted in large Stokes shifts and broadband white-light emissions. The large Stokes shift in emission avoided the heat effect of self-absorption, which provided a new pathway for developing the solid-state lighting materials and scintillator materials for radiation detection. Single crystals of MFPbClxBr4-x were obtained by a solution process, and crystal structures as well as photophysical properties were studied. And the emission of the layered perovskites can be simply tuned by halogen substitution. By gradually increasing chloride content, the PL spectrum can be tuned from “cold’’ to ‘‘warm’’ white light. The photophysical studies of MFPbClxBr4-x revealed that the broadband emission primarily originated from electron–phonon coupling induced self-trapped states in the greatly distorted supramolecular low-dimensional perovskites.28,29 Results and Discussion The single crystalline of the novel-layered perovskites, MFPbBr4, was prepared by slow cooling of the acid solution of precursor Lead(II) oxide and MF hydrochloride (Synthesis Details can be seen in the Supporting Information). Figure 1a exhibits the elongated plate-shaped images of the crystals. The structure of MFPbBr4 was determined using single-crystal X-ray diffraction (SCXRD), which crystallizes in the noncentrosymmetric monoclinic P21 space group with a = 6.2198 (9) Å, b = 11.7999 (18) Å, and c = 19.7260 (3) Å at room temperature. Details of crystal data and structure refinements of MFPbBr4 are listed in Supporting Information Table S1. The 2D MFPbBr4 perovskites consisted of inorganic layers of corner-sharing PbX6 metal-halide octahedra extending along the ab crystallographic plane and organic interlayer cations situated in the interlayer region of the inorganic framework ( Supporting Information Figure S1a). Supporting Information Figure S1b shows that the inorganic layers stack in an imperfect arrangement seen from the top-down direction. The mismatched displacement was caused by the steric effect arising from the size and shape of the interlayer biguanide cation as well as supramolecular interaction between the organic interlayer cations and inorganic layers. Powder X-ray diffraction (PXRD) patterns confirmed the phase purity and crystallinity of MFPbBr4 ( Supporting Information Figure S3). Figure 1 | (a) Image of MFPbBr4 bulk single crystals under ambient light. (b) Structure of the organic biguanide cation. (c) Hydrogen-bonding interaction between the organic cations and the inorganic layers (highlighted by a green dashed line). Terminal bromine (labeled with an orange circle) and bridging bromine (labeled with a purple circle) in the inorganic layers. (d) The distorted inorganic layers of the MFPbBr4. (e) The distance of the inorganic layers (labeled with orange, green, and purple dashed lines, respectively). Yellow, black, red, cyan, and gray spheres represent Pb, Br, N, C and H atoms, respectively. Download figure Download PowerPoint The inorganic sublattice of 2D perovskites showed local structural distortion in the metal coordination sphere, which deviated from ideal octahedral symmetry. In MFPbBr4, we found that each PbBr6 octahedra was highly distorted within the inorganic structure, displaying the Pb–Br bond lengths varying from 2.81 to 3.19 Å, while the Br−Pb−Br bond angles ranged from 81.92° to 97.05°. The degree of PbX6 octahedral structural distortion was quantitatively evaluated by using the octahedral elongation (λoct), bond angle variance (σoct2), and octahedra bond-length distortions (Δoct) according to the following three equations. 30,31 λ oct = 1 6 ∑ i = 1 6 ( d i / d 0 ) 2 (1) σ oct 2 = 1 11 ∑ i = 1 12 ( θ i − 90 ) 2 (2) Δ oct = 1 6 ∑ i = 1 6 [ ( d i − d av ) / d av ] 2 (3)where di is the individual Pb−X bond lengths, d0 is the center-to-vertex distance of a regular polyhedron of the same volume, and dav is the average Pb−X bond length in eqs 1 and 3, θ i is the individual X−Pb−X angles in eq 2, respectively. The value of the λ oct is calculated to be 1.0015, σ oct 2 is 43.61, and Δ oct is 13.70 × 10−4, indicating that the PbBr6 octahedra are highly distorted within the layered MFPbBr4 perovskites. Importantly, on account of the conjugated effect of the MF2+ interlayer cation, MF cation was able to form multiple hydrogen bonds with the bridging Br− as well as terminal Br− in the single-layered MFPbBr4. The structure of conjugated interlayer organic molecule is depicted in Figure 1b. The MF cation proved to be an excellent donor of hydrogen bonds with the inorganic framework. The formation of hydrogen-bonding networks between the hydrogen of amino groups in MF organic cation and bromine ions in lead-halide inorganic framework is presented in Figure 1c. As illustrated in Figure 1c, in MFPbBr4, the MF cation can form hydrogen bonds with the bridging Br− as well as terminal Br−, which are highlighted by a green dashed line. Bonding with the terminal Br− is responsible for the in-plane distortion to some extent. Meanwhile, the H bonds are formed with the bridging Br− anions deeper inside the inorganic layers, which further expand the in-plane distortion. The distortion of the inorganic sublattice in the novel 2D-layered perovskites arose from interoctahedral tilting due to the hydrogen-bonding interaction, which deviated from the metal—(μ halide)—metal angle of 180°. The metal—(μ halide)—metal angle (θtilt) has two types, including in-plane (θin) and out-of-plane (θout). Accordingly, the distortion Dtilt (180° − θtilt) has in-plane distortion Din (180° − θin) and out-of-plane distortion Dout (180° − θout). Figure 1d presents the in-plane and out-of-plane views of the inorganic layers in MFPbBr4, measuring θtilt of the Pb–(μ Br)–Pb to be 168.38°, θout is 177.50°. And the inorganic layers of MFPbBr4 displayed an in-plane distortion with Dout calculated to be 2.48° and a larger in-plane distortion with a Din value of 11.62°, induced by the hydrogen-bonding interaction between the MF cation and inorganic layers. Supporting Information Table S2 lists several 2D-layered perovskites with distorted structure, showing that the novel 2D MFPbBr4 has larger structural distortion owing to the multiple hydrogen-bonding interactions. Notably, the multiple hydrogen bonds also had an impact on the interlayer distance simultaneously, which greatly shortened the Br⋯Br distance. The mismatched inorganic layers have a larger distance with a value of ∼7.1974 Å, because of the larger size and the stereochemical configuration of MF cation. While the distance between the mismatched inorganic layers was greatly reduced to ∼6.7403 Å, even to ∼4.9525 Å, on account of the fact that the multiple hydrogen bonds can attract the inorganic layers to get closer, as shown in Figure 1e. Besides, hydrogen bonding appeared to be a key factor in stabilizing the packing of the distorted crystal structure. In addition, the multiple hydrogen-bonding interaction had an impact on the crystal structure and resulted in the distorted inorganic metal-halide layers, which further influenced the optical and electrical properties of material for the structure−property relationship. Therefore, it is of interest to further study the optical properties of the novel 2D-layered material. As presented in Figure 2a, the MFPbBr4 crystals showed a bluish white-light PL broadband emission under 365 nm light irradiation with Commission Internationale de l’Eclairage (CIE) color coordinates of (0.2492, 0.2858), a color-rendering index (CRI) of 85, and the correlated color temperature (CCT) of 14872 K, comparable with “cold” white light. Absorption spectra of MFPbBr4 single crystals are shown in Supporting Information Figure S2. The PL quantum yield (PLQY) of the single crystals was roughly 4%. In addition, the PL emission of MFPbBr4 was facilely tuned from “cold” white light to “warm” white light simply by halide substitution without changing organic cations. By gradually increasing chloride content in the precursor solution, we got MFPbClxBr4-x (x = 0–4) perovskites with broadband white-light PL emission (Figures 2a and 2b) and see a red-shift due to the smaller electronegativity of Cl compared with Br, which is consistent with previous studies. Figure 2a shows that broadband emission can be tuned across nearly the whole visible spectrum with the PL peak shifted toward 580 nm by increasing the amount of Cl. The detailed synthesis method of MFPbClxBr4-x with broadband emission is presented in the Supporting Information. Emission spectra measured at different excitation wavelengths indicate that the emissions derive from the same excited states ( Supporting Information Figures S5 and S6). Excitation laser power-dependent steady-state PL spectra of the MFPbClxBr4-x (x = 3.6) are shown in Supporting Information Figure S7. At 293 K, with the increase of the excitation laser power, the PL intensity increased, indicating the intrinsic, excitonic nature of the broadband emission. Moreover, the 2D perovskites showed good thermal stability ( Supporting Information Figure S4). The time-resolved PL (TRPL) spectroscopy of MFPbClxBr4-x was carried out at room temperature. It is clear from Figure 2c that the lifetime was longer as the ratio of Cl in 2D perovskites MFPbClxBr4-x was increased The calculated average lifetime of MFPbBr4 (x = 0) is 1.6290 ns fitted by a biexponential function (Figure 2c). The CIE coordinates of the broadband emission of MFPbClxBr4-x (x = 3.6) were (0.3862, 0.4217), accompanied by a CRI of 85 and a CCT of 4145 K, producing “warm” white-light emission that meets the requirements for indoor lighting devices. In addition, the average lifetime of MFPbClxBr4-x (x = 3.6) was calculated to be 8.603 ns, which is longer than that of MFPbBr4, because of the highly distorted structure and populated STEs increasing the chlorine in the mixed-halide perovskites, as shown in Figure 2c. The degree of distortion is larger for mixed-halide perovskites than that in single-halide ones, which is favorable to the free excitons (FEs) trapping to self-trapped states.21 Figure 2 | (a) Normalized PL spectra of 2D MFPbClxBr4-x (x = 0–4) perovskites and (b) CIE chromaticity coordinates. (c) TRPL decay curves. Download figure Download PowerPoint Temperature-dependent steady-state PL measurement was carried out to further investigate the mechanism of broadband emission in MFPbClxBr4-x (x = 3.6), which showed that broadband emission was contributed from electron–phonon coupling generated STEs in the highly distorted structure.32,33 PL intensity increased as temperature decreased from 300 to 100 K, as shown in Figure 3a. Thermal activation (kBT) was low at a lower temperature, while the activation energy of detrapping (Ea,detrap) was higher than thermal activation (kBT). When kBT < Ea,detrap, it can effectively prevent STEs from detrapping back to the FEs state, which can increase the concentration of STEs as well as increase the possibility of radiation recombination from the self-trapped state to the ground state (GS).20 Consequently, the PL intensity of the broadband emission greatly increased as temperature decreased. Simultaneously, the PL emission was a broad peak ranging from 400 to 800 nm, centered at 580 nm at 300 K, and the broad PL peak tended to become narrower as temperature lowered from 300 to 100 K. Overall, the bandwidth showed the notable upward trend in the new-layered perovskites while the PL intensity decreased dramatically as temperature increased from 100 to 300 K (Figure 3b). Previous studies have proved that broadband emission becomes narrower as temperature decreases, arising from the easily enhanced electron–phonon coupling.34 And the temperature-dependent full width at half maximum (FWHM) (Γ) can be estimate according to the following model35: Γ ( T ) = Γ 0 + Γ phonon [ e ( E LO / k B T ) − 1 ] − 1 + Γ inhomo e − E b / k B T (4)where Γ0 represents the FWHM at 0 K, Γphonon represents the electron−phonon coupling constant, Γinhomo represents the inhomogeneous broadening coefficient due to trapped states, ELO is energy of the longitudinal-optical phonon energy, and Eb is the binding energy of the trapped states, respectively. The fitting result is presented in Supporting Information Figure S9, illustrating the strong electron−phonon coupling with ELO = 132 cm−1. This result is quite consistent with the Raman stretching vibration of the Pb-halide bonds within the inorganic framework ( Supporting Information Figure S8). The study suggests that the broadband emission of MFPbClxBr4-x mainly stems from electron–phonon coupling-generated STEs in the highly distorted structure33, presented in Figure 3d. Figure 3 | (a) Temperature-dependent steady-state PL spectra of 2D MFPbClxBr4-x (x = 3.6). (b) Temperature dependence of the bandwidth and PL intensity. (c) Temperature-dependent TRPL decay curves of the MFPbClxBr4-x (x = 3.6). (d) Configuration coordinate diagram for FEs and STEs. The paths refer to exciton self-trapping (red arrow) in MFPbClxBr4-x perovskites. Download figure Download PowerPoint Furthermore, we performed temperature-dependent TRPL spectroscopy measurements to better comprehend the origin of the broadband white-light emission of the 2D materials. As shown in Figure 3c, the lifetime of the broad STE emission at ∼580 nm became increasingly longer as the temperature decreased from 300 to 100 K. The lengthening of the broadband emission lifetime can be ascribed to the thermal equilibrium between the FEs and STEs that govern the thermally activated trapping of FEs in the STE state.12,36 Conclusion We fabricate the novel 2D perovskites, MFPbClxBr4-x, with corner-sharing distorted 2D-layered structure, which exhibit broadband white-light emission that covers nearly the whole spectrum of visible region simply by adjusting the ratio of halides in composition of MFPbClxBr4-x. We find that it is easier to realize broadband emission by judiciously selecting the organic interlayer cations for stronger supramolecular interaction with the inorganic lattice in the low-dimensional perovskites. Our study highlights the potential of guanidine derivatives as a promising new type of templating cations, which can interact with the inorganic framework for larger structural distortion in perovskites. This work provides a new pathway to further extend the range of low-dimensional perovskites with intriguing white-light emission for applications in the field of solid-state optical and scintillator materials. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no conflict of interest. Notes The crystal structures have been deposited at the Cambridge Crystallographic Data Centre (deposition numbers: CCDC 1963853), and can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/structures; by emailing [email protected]cam.ac.uk; or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 21875089). Q.D. conceived the idea, supervised the project, and conducted the initial experiment; X.L. conducted most of the experiments and characterizations; Z.Y., C.G., and H.L. contributed to materials design and synthesis; M.H., C.W., and Y.S. contributed to the test of TRPL; B.L. contributed to the single-crystal data and structure refinements; Q.D. and X.L wrote the paper. All authors have given approval to the final version of the manuscript.
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