“Breathing” organic cation to stabilize multiple structures in low-dimensional Ge-, Sn-, and Pb-based hybrid iodide perovskites

Abstract

Low-dimensional hybrid inorganic–organic perovskites are excellent candidates for stable optoelectronic devices. The dimensionality of these perovskites depends largely on the organic and inorganic compositions, as well as the synthetic conditions. Here, we report five new hybrid iodides, (ETU)₄Ge₅I₁₈, (ETU)GeI₄, (ETU)SnI₄, (ETU)PbI₄, and (ETU)₃Pb₂I₁₀ using only one type of organic cation, namely, S-(2-aminoethyl)isothiouronium (ETU). (ETU)GeI₄ and (ETU)SnI₄ belong to the (110)-oriented structure-type with “3 × 3” sawtooth corrugated layers and crystallize in a structure with the orthorhombic space group Pbca. (ETU)₄Ge₅I₁₈ crystallizes in a structure with the triclinic space group P1̄, featuring a 2D layered structure with combinations of corner, edge, and face-sharing [GeI₆] octahedra. For the Pb-based series, (ETU)PbI₄ has the conventional (100) – oriented 2D type whereas (ETU)₃Pb₂I₁₀ has a unique 0D structure. Remarkably, the unstable 2D orange-phase (ETU)PbI₄ transforms to a stable 0D yellow phase (ETU)₃Pb₂I₁₀, accompanied by the reduction of the C–S–C angle of the organic cation ETU. The optical band gaps are largely regulated by the diverse types of structure and are in the range of 1.8 eV to 2.8 eV. (ETU)SnI₄ is the only material showing notable photoluminescence at room-temperature. Our work showcases the flexibility of the organic cation in determining the structural dimensionality and provides a new strategy in generating new hybrid materials.

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Introduction

Hybrid halide perovskite materials with small exciton binding energy, high carrier lifetime, high absorption coefficient and other favorable properties have been extensively studied for optoelectronic applications.^1–6 Three-dimensional (3D) halide perovskites have a general formula of AMX₃ (A = Cs⁺, CH₃NH₃⁺, HC(NH₂)₂⁺; M = Pb, Sn or Ge; X = halide),^4,7,8 for a limited set of A-site cations. In contrast, low-dimensional perovskites enjoy tremendous chemical flexibility as well as structural diversity, offering the opportunity to use a large variety of functional organic molecules as cations.^9–12 2D perovskites can be obtained via chemically sectioning the 3D perovskite lattice along different planes, resulting in (100)-, (110)- and (111)-oriented perovskites, which have layered or corrugated sheets that are sandwiched between large organic cations.^9,13,14 Other derivatives with lower dimensionality such as 1D or 0D structures often consist of edge-shared or face-shared octahedra.^15,16 The reduction of dimensionality in these materials has an enormous influence on their physical and optoelectronic properties, leading to other specific applications.^17,18

A promising strategy for novel functional hybrid materials is to combine the properties of the inorganic frameworks with the functionality and flexibility of the organic component. Especially for 2D perovskites, the selection of cations has a great influence on the formation of different types of layers such as (110)-oriented layered types. These “sawtooth” (110)-oriented 2D perovskites are named “n × n”, where n represents the number of octahedra that form half of the sawtooth, thus these structures can be regarded as “2 × 2”, “3 × 3”, “4 × 4”, etc.¹⁹ Previously, the “2 × 2” structural type has been extensively studied.^20–22 In the case of the “3 × 3” type, α-(DMEN)PbBr₄ (DMEN = 2-(dimethylamino)ethylamine) was the first example, and was constituted of alternating rows of cis and trans octahedra.¹⁹ Recently, Tremblay et al. have reported a new “3 × 3” corrugated 2D hybrid perovskite, (4NPEA)₂PbI₄ (4NPEA = 4-nitrophenylethylammonium), demonstrating that the optical property is similar to other 2D perovskites with distorted [PbI₆] octahedra.²³ Guo et al. have further reported that [ImH][TzH]PbBr₄, ([ImH⁺] = imidazolium, [TzH⁺] = 1,2,4-triazolium), adopting a “3 × 3” structural type, is the first example of a bromide perovskite incorporating two types of ordered organic cations.²⁴ Other similar “3 × 3” corrugated 2D Pb-based perovskites have also been reported and employed in solar cell devices.²⁵ In contrast, the Sn-based “3 × 3” corrugated 2D perovskites are much rarer, with the only example being (ImEA)[SnI₄] (ImEA²⁺ = 1-(2-ammonioethyl)-1H-imidazol-3-ium).²⁶ For the “4 × 4” corrugated structures, α-[NH₃(CH₂)₅NH₃]SnI₄ ²⁷ and α-(HA)SnI₄ ²⁸ are the only two examples reported. An additional consideration in our work is that most reports on hybrid halide perovskites have mainly focused on Pb- and Sn-based phases. Within the same group, Ge²⁺ has a similar electron configuration of Sn²⁺ and Pb²⁺, and significant effect of its lone pair.²⁹ However, Ge-based phases are much less explored.^30,31

Here, by using S-(2-aminoethyl)isothiouronium (ETU) as the organic templating cation, five new hybrid iodides, namely (ETU)₄Ge₅I₁₈, (ETU)GeI₄, (ETU)SnI₄, (ETU)PbI₄ and (ETU)₃Pb₂I₁₀, are reported. (ETU)GeI₄ and (ETU)SnI₄ are structurally analogous, featuring interesting corrugated “3 × 3” layered structures which belong to the (110)-oriented 2D perovskite family. (ETU)GeI₄ represents the first Ge-based example of the “3 × 3” type of corrugated sawtooth 2D perovskite. A different structure, (ETU)₄Ge₅I₁₈, is also obtained by controlling the reaction ratio of the precursor organic cation and the inorganic precursors. Interestingly, an orange phase 2D perovskite (ETU)PbI₄ can be further converted to the yellow phase 0D structure (ETU)₃Pb₂I₁₀, with the reduction of the C–S–C angle of the sulfur-containing ammonium cation ETU. The above compounds have optical band gaps in the range of 1.8–2.8 eV, which can be tuned with varying structural types and octahedral connectivity. At room-temperature, only (ETU)SnI₄ exhibits photoluminescence (PL), which could be due to the relatively stable configuration since it is the only material with just one structure (Ge and Pb each have two). This work demonstrates the utility of flexible organic cations in dictating a range of hybrid structures that are significant in the development of materials for advanced optoelectronic applications.

Experimental

Synthesis

All chemicals were purchased from Sigma-Aldrich and used without further purification or modification.

(ETU)₄Ge₅I₁₈ and (ETU)GeI₄. GeO₂ (0.1 g, 1 mmol) powder was dissolved in 47% hydriodic acid (8.0 mL) and 50% aqueous H₃PO₂ (1.5 mL) by heating under stirring for 30 min at 127 °C until the solution clarified. Then, S-(2-aminoethyl)isothiouronium bromide hydrobromide (0.168 g, 0.8 mmol) was added to the above solution under heating and stirring for an additional 5 min. Yellow needle crystals of (ETU)₄Ge₅I₁₈ precipitated during slow cooling. Orange plate crystals of (ETU)GeI₄ were formed in the same fashion except the ratio of S-(2-aminoethyl)isothiouronium bromide hydrobromide (0.281 g, 1 mmol) to GeO₂ (0.1 g, 1 mmol) was enlarged to 1 : 1.

(ETU)SnI₄. SnCl₂·2H₂O (0.225 g, 1 mmol) powder was dissolved in 47% hydriodic acid (8.0 mL) and 50% aqueous H₃PO₂ (1.5 mL) by heating under stirring for 10 min at 127 °C until the solution turned light yellow. Then S-(2-aminoethyl)isothiouronium bromide hydrobromide (0.281 g, 1 mmol) was added to the above solution, and the resultant mixture was stirred and heated for 5 min. Dark red plate-like crystals precipitated during slow cooling.

(ETU)PbI₄ and (ETU)₃Pb₂I₁₀. PbO (0.223 g, 1 mmol) powder was dissolved in 47% hydriodic acid (4.0 mL) and 50% aqueous H₃PO₂ (1.0 mL) by heating under stirring for 5 min at 127 °C until the solution turned bright yellow. Then S-(2-aminoethyl)isothiouronium bromide hydrobromide (0.281 g, 1 mmol) was added to the above solution, and the resultant mixture was evaporated to approximately at half its original volume by stirring and heating. The solution was then left to stand overnight and allowed to crystallize into orange block crystals (ETU)PbI₄ at room temperature. However, the orange phase (ETU)PbI₄ is unstable. After removing a part of the crystals in the mother liquid, all of the orange block crystals in contact with the mother liquid quickly changed to yellow block crystals. Similarly, orange block crystals placed in the air also turned to yellow block crystals about one hour. Similar synthetic approaches were used to directly synthesize the yellow block crystals (ETU)₃Pb₂I₁₀ except S-(2-aminoethyl)isothiouronium bromide hydrobromide (0.562 g, 2 mmol) to PbO (0.223 g, 1 mmol) ratio was doubled to 2 : 1.

Single crystal X-ray diffraction

The single-crystal diffraction data was collected on a Bruker APEX-II CCD diffractometer for (ETU)SnI₄ and (ETU)₃Pb₂I₁₀ at 298 K, as well as on a Bruker D8 Venture diffractometer for (ETU)GeI₄, (ETU)₄Ge₅I₁₈ at 298 K and (ETU)PbI₄ at 100 K using Mo-Kα (λ = 0.71073 Å) radiation. The data collection and reduction were carried out using the Bruker APEX3 program for all compounds.³² The crystal structures were solved by direct method and refined using the OLEX2 program package based on F² with refinements of full-matrix least-squares.^33,34 All the non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms attached to C and N atoms of organic molecules were positioned geometrically and refined isotropically. The data has been deposited in the CCDC database under 2169490–2169494.†

Powder X-ray diffraction (PXRD)

The powder X-ray diffraction (PXRD) data were collected on SmartLab X-ray diffractometer (Cu-Kα, λ = 1.54056 Å) at a voltage of 45 kV and a current of 200 mA in the range of 5–50° with the step size of 10° min⁻¹ to confirm the purity of each sample at room temperature. By using the crystallographic file from the single-crystal X-ray diffraction experiment, the simulated powder pattern was calculated via Mercury software.

Optical absorption spectroscopy

The solid UV-Vis absorbance spectra were recorded at room temperature on a Shimadzu UV-3600i Plus spectrophotometer with BaSO₄ as the reference substance with the wavelength range at 200 nm to 900 nm. The reflectance spectra were converted to absorption spectra according to the Kubelka–Munk function: α/S = F(R) = (1 − R)²/2R, where S is the scattering coefficient, α is the absorption coefficient, and R is the reflectance.³⁵

Steady-state photoluminescence (PL)

Steady-state PL spectra were conducted on LabRAM HR evolution laser confocal micro-Raman spectroscopy at room temperature with a 532 nm laser.

Computational details

Density functional theory (DFT) calculations were performed using the Vienna Ab initio simulation package^36–38 (VASP), version 5.4.4. The Perdew–Burke–Ernzerhof³⁹ (PBE) functional and projector-augmented wave^40,41 (PAW) pseudopotentials were used with a plane-wave cut-off energy of 700 eV and energy convergence of 10⁻⁵ eV or better. PAW potentials were chosen following the version 5.2 guidelines. Automatic k-mesh generation⁴² was used with a length parameter (R_k) of 40 or better. Spin–orbit coupling corrections were included for the Pb-based structures. For band-structure calculations, an appropriate k-path was generated using SeeK-path.^43,44 Density of states were plotted and analysed using Sumo.^45,46

Results and discussion

Unlike the relatively common perovskite-like organic–inorganic hybrids formed by rigid cations, a flexible organic cation can lead to richer and more diverse structures. The flexible cation ETU used here forms two different compounds with Ge and Pb, respectively, and one layered compound with Sn. The crystal structures of all five compounds are shown in Fig. 1 and 2, and crystallographic data are tabulated in Tables S1 and S2.† (ETU)₄Ge₅I₁₈ crystallizes in the triclinic space group P1̄, and the inorganic sheets consist of mixed corner-sharing, edge-sharing and face-sharing [GeI₆] octahedra (Fig. 1a and S2†). (ETU)GeI₄ crystallizes in a structure with the orthorhombic space group Pbca. It adopts a layered structure templated by the divalent organic cation, where each layer is formed with corner-sharing of [GeI₆] octahedra resulting in a “3 × 3” type of sawtooth corrugated structure (Fig. 1b). This is the first example of “3 × 3” corrugated 2D Ge-based perovskite. (ETU)SnI₄ is structurally analogous to (ETU)GeI₄ (Fig. 1c). We further quantified the individual distortion of the octahedra for [GeI₆] and [SnI₆] by determining the bond length distortion (Δd) and octahedral angle variance (σ²) listed in Table S3† (calculated using VESTA software). We note that the average Δd (3.18 × 10⁻²) and average σ² (12.76) for (ETU)SnI₄ is less than the value of (ETU)GeI₄, where the larger radius of Sn compared to that of Ge accounts for the smaller distortion in the Sn analogue. The octahedral distortion index Δd for all three individual [PbI₆] in (ETU)₃Pb₂I₁₀ is either zero or close to zero, which indicates the stable nature of the compound and the higher tolerance for the 0D structure.

Fig. 1 Crystal structures of (a) (ETU)₄Ge₅I₁₈, (b) (ETU)GeI₄, and (c) (ETU)SnI₄ and structure of the organic cation used here. Elements indicated by: dark blue (I), yellow (S), blue (N), dark gray (C). All hydrogen atoms are omitted for clarity.

Fig. 2 Crystal structures of (a) (ETU)PbI₄ and (b) (ETU)₃Pb₂I₁₀. Elements indicated by: dark blue (I), yellow (S), blue (N), dark grey (C). Shown also are the metal halide units including distorted octahedra (light blue) and trimer octahedra (dark green) for (ETU)₃Pb₂I₁₀. All H atoms are omitted for clarity.

For the Pb-based materials, (ETU)PbI₄ crystallizes in a structure with the monoclinic space group P2₁/c, featuring the conventional (100)-oriented 2D structure with corner-sharing [PbI₆] octahedra (Fig. 2a). The Br analogue (ETU)PbBr₄ has been previously reported by Kundu et al. and belongs to the (110)-oriented type.⁴⁷ (ETU)₃Pb₂I₁₀ crystallizes in the trigonal space group R3̄, where the isolated [PbI₆] octahedra and [Pb₃I₁₂]⁶⁻ trimer are surrounded by ETU cations and iodide anions, giving rise to an 0D structure (Fig. 2b and S3†). Interestingly, the 2D orange phase (ETU)PbI₄ can be transformed to the stable 0D yellow phase (ETU)₃Pb₂I₁₀ when the concentration of the mother liquor is low or exposed in air. This conversion is irreversible.

More interestingly, the degree of cations torsion angle (C–S–C angle) also varies in different structures. One can easily see from Fig. 3a and b that degree of cation distortion is changed in the two distinct 2D Ge-based structures. The torsion angles of the C–S–C in (ETU)₄Ge₅I₁₈ structure are 100° and 102.5° (Fig. 3b), respectively, whereas the torsion angles of the C–S–C are 106.4° and 103.4° in the (ETU)GeI₄ structure (Fig. 3a). In Pb-based series, there are two different conformational cations with the C–S–C torsion angles of 105.5° and 103.1° within the 2D orange phase (ETU)PbI₄ (Fig. 3c). Owing to the inherent flexibility of the cations in the crystal structure, the cation has only one configuration in the 0D phase, with the C–S–C torsion angle of 103.9° (Fig. 3d). Generally speaking, the yellow phases (ETU)₄Ge₅I₁₈ and (ETU)₃Pb₂I₁₀ with smaller C–S–C angles are more stable than the orange phases (ETU)GeI₄ and (ETU)PbI₄. The dynamic movement of ETU cations reaches a final state, where the yellow phases are stabilized.

Fig. 3 The layout of cationic entities in the structures of (a) (ETU)GeI₄, (b) (ETU)₄Ge₅I₁₈, (c) (ETU)PbI₄, and (d) (ETU)₃Pb₂I₁₀. Elements indicated by: orange (Ge), pink (Pb), dark blue (I), yellow (S), blue (N), dark gray (C), and all H atoms are omitted here for clarity. Inserted: optical microscope picture of corresponding single crystals.

Differences in the structure remarkably affect the photophysical properties of these materials. The optical band gaps of the five compounds reported here were determined by diffuse reflectance UV–vis spectroscopy measurements (Fig. 4a). These materials all show clear excitonic features close to the band edges. (ETU)₄Ge₅I₁₈ has a band gap of 2.30 eV, with an average Ge–I–Ge angle of 111.2°, while (ETU)GeI₄ has a slightly smaller band gap (2.14 eV), due to a larger averaged angle of 162.3°. These hybrid germanium iodides, reasonably have narrower band gaps compared with reported 2D Ge–Br perovskites.³¹ (ETU)SnI₄ has the narrowest band gap of the series (1.87 eV), which is consistent with band gap value of α-(HA)SnI₄ with “4 × 4” corrugated structure.²⁸ Within the Pb series, (ETU)PbI₄ with the 2D structure has the lower band gap of 2.17 eV while (ETU)₃Pb₂I₁₀ with the 0D structure has higher band gap (2.71 eV) because the later has lower dimensionality. These two phases are easily distinguishable by color, which are consistent with previously reported.^16,48

Fig. 4 (a) UV–vis absorption spectra of (ETU)₄Ge₅I₁₈, (ETU)GeI₄, (ETU)SnI₄, (ETU)PbI₄ and (ETU)₃Pb₂I₁₀ at room temperature. (b) Steady-state photoluminescence (PL) spectra (excited at 532 nm) for (ETU)SnI₄ at room temperature.

To further evaluate the optical properties of these materials, we performed photoluminescence (PL) measurements on single crystals at room temperature. Unfortunately, no significant emission peaks were observed in Ge-and Pb-based materials, which may be related to structural parameter variations as reported previously.^49,50 Under other stimuli such as low-temperature or with extra pressure, PL may be able to be observed. The PL spectrum for (ETU)SnI₄ is presented in Fig. 4b and has a significantly emission peak at 620 nm under excitation at 532 nm with a wide full width at half maximum (FWHM) of 74 nm, as well as an Stokes shift of 88 nm. This PL band is closely overlapped with the absorption profile and appears to be due to band edge recombination process, in which the photogenerated free excitons (FE) relax back to the valence band. The PL emission energy is generally consistent with the previously reported band edge emission of many 2D tin–iodine perovskites.²⁸ It is intriguing that (ETU)SnI₄ is the only compound here found to have PL at room temperature, and also the only compound that is composed of ETU with metal iodide with just one composition (Ge and Pb each have two). This may be due to that the dynamics of the organic cations suppress radiative recombination processes in the Ge- and Pb-based phases, whereas the Sn analogue has higher rigidity to maintain the FE emission.

Density functional theory (DFT) calculations were performed in order to gain a better understanding of how the various structure-types presented here affect the electronic structures of these compounds. Fig. 5 displays the calculated density of states (DOS) for (ETU)₃Pb₂I₁₀, (ETU)PbI₄, (ETU)₄Ge₅I₁₈, and (ETU)GeI₄, which allow us to estimate the band gaps of these materials and understand the contribution of different orbitals to the valence and conduction bands. First, considering the Pb series of compounds, we find that the trend in the band gaps is qualitatively similar to what was observed experimentally, with (ETU)PbI₄ having a significantly smaller bandgap than (ETU)₃Pb₂I₁₀ (1.22 eV versus 2.22 eV, respectively). Both band gaps are underestimated compared to experiment, which is typical of DFT calculations using the PBE functional. However, the band gap for (ETU)PbI₄ is underestimated by almost 1 eV, compared with approximately 0.5 eV for (ETU)₃Pb₂I₁₀. This discrepancy may be due to the fact that (ETU)PbI₄ is inherently unstable, and the crystal structure used for the calculations needed to be collected at 100 K, versus 298 K for the (ETU)₃Pb₂I₁₀ structure. Examining the orbital contributions to the DOS, the valence band edges for both compounds are dominated by the I 5p orbitals with a small contribution from the Pb 6s orbitals. In the case of the conduction band, both compounds show a contribution from the Pb 6p states at the band edges, with a contribution from the ETU orbitals (N 2p, C 2p, and S 3p) slightly higher in energy. The dimensionality of these two compounds manifests itself very clearly in the DOS, where the 0D (ETU)₃Pb₂I₁₀ shows a very “spiky” DOS due to the molecular-like nature of its structure, while the 2D (ETU)PbI₄ shows a more continuous curve. In the Ge-based compounds, the band gap trends are again reproduced, where (ETU)₄Ge₅I₁₈ has a calculated gap of 1.94 eV and (ETU)GeI₄ has a gap of 1.71 eV. In this case, both band gaps are underestimated by approximately 0.4 eV. The orbital-projected DOS for these compounds shows that, similarly to the lead compounds, the valence band edge is predominately made up of the I 5p orbitals with a small contribution from the Ge 4s orbitals. Interestingly, for (ETU)₄Ge₅I₁₈, the edge of the conduction band is dominated by states originating from the ETU cation. These states may be lower in energy compared to those in the other compounds due to the lower C–S–C angle in the ETU cation, which, as previously noted, appears to enhance stability. The DOS for (ETU)GeI₄ shows similar features, but the conduction band edge appears to be primarily comprised of the Ge 4p states and the contributions from the ETU cation are shifted to slightly higher energies. In summary, the DOS for both (ETU)PbI₄ and (ETU)GeI₄ is similar to that of other 2D halide perovskite materials. Additionally, the DOS for (ETU)₄Ge₅I₁₈ illustrates how conformational changes in the organic cation can potentially influence the makeup of the band edges in these materials.

Fig. 5 DFT-calculated DOS for (a) (ETU)₃Pb₂I₁₀, (b) (ETU)PbI₄, (c) (ETU)₄Ge₅I₁₈, and (d) (ETU)GeI₄.

Conclusions

In summary, we have synthesized and characterized the structure, optical and electronic properties of five new hybrid iodides by selecting flexible organic species as the stabilizing cations. Among them, two intriguing new “3 × 3” corrugated (110)-oriented type 2D perovskites (ETU)GeI₄ and (ETU)SnI₄ are reported, where (ETU)GeI₄ is the first Ge-based example of the “3 × 3” type 2D structure. Due to the flexibility of the bond angle of the organic cation, (ETU)PbI₄, featuring conventional (100)-oriented type 2D structure, can be readily transformed into a new yellow-phase 0D structure (ETU)₃Pb₂I₁₀. The materials with larger band gaps and lower dimensionality show smaller torsion angles of the organic cations, hinting the movement of the cations are stabilized. Our work highlights the importance of properly manipulate the interactions between the organic and inorganic counterparts, suggesting new design principles for inorganic–organic hybrid halides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, under grant SC0012541. The work at SUSTech was supported by SUSTech startup grant (Y01216150). The authors are grateful to the assistance of SUSTech Core Research Facilities.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2169490–2169494. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01247b

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