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Synthesis, Properties and Modelling of Cs1-xRbxSnBr3 Solid Solution: a New Mixed-cation Lead-free All-Inorganic Perovskite System Andrea Bernasconi, Aurora Rizzo, Andrea Listorti, Arup Mahata, Edoardo Mosconi, Filippo De Angelis, and Lorenzo Malavasi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00837 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Chemistry of Materials

Synthesis, Properties and Modelling of Cs1-xRbxSnBr3 Solid Solution: a New Mixed-cation Lead-free AllInorganic Perovskite System

Andrea Bernasconi,a Aurora Rizzo,b Andrea Listorti,b,c* Arup Mahata,d Edoardo Mosconi,d Filippo De Angelis,d,* and Lorenzo Malavasia,* aDepartment

bCNR

of Chemistry and INSTM, Viale Taramelli 16, 27100, Pavia, Italy

NANOTEC, Institute of Nanotechnology, c/o Campus Ecotecne, via Monteroni, 73100

Lecce, Italy cDipartimento

di Matematica e Fisica “E. De Giorgi”, Università del Salento, Via per Arnesano,

73100 Lecce, Italy dComputational

Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, via Elce di

Sotto 8, I-06123 Perugia, Italy

AUTHOR INFORMATION Corresponding Authors *Andrea Listorti: [email protected]; *Filippo de Angelis: [email protected]; *Lorenzo Malavasi: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

In the present work, the substitution of Cesium (Cs+) with Rubidium (Rb+) in fully-inorganic tin bromide perovskites Cs1-xRbxSnBr3, has been experimentally demonstrated by synthesizing pure single-phase samples in the CsSnBr3-Cs0.70Rb0.30SnBr3 compositional range. The substitution of Cs with Rb is responsible of structural modification from cubic to orthorhombic symmetry, that have been correlated with optical properties, as the band gap varies from 1.719 to 1.817 eV from CsSnBr3 to Cs0.70Rb0.30SnBr3 sample. Noticeably all the rubidium embedding alloys present good air-stability. All these results are very straightforward and open the possibility to exploit the electrical and optical capabilities of this very promising family of lead-free materials.

TOC GRAPHICS


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INTRODUCTION

Organic-inorganic hybrid halide perovskite A+B2+X3 have been largely investigated in the last decade due to their optimal optical properties, combined with a progressively increased efficiency that is actually well above the value of 20% with the latest certified value of 23.7%.1-5 In this family of materials, there is a large variety of possible compositions that have been well characterized. For example, multiple organic molecules have been incorporated in the A-site of the material, with methylammonium (MA+) and formamidinium (FA+) as the most common ones, giving the possibility to properly tune the optical properties as a function of stoichiometry.6,7 The drawback related to the presence of these organic cations is given by their low thermal and humidity stability that limits their utilization.8,9 This issue can be surmounted by replacing the organic molecule with inorganic monovalent cations, like for example Cs.10 Regarding the B2+ cation, lead has been intensely used up to now but one of the challenges in this research’s field is to partially, or totally, replace it in order to overcome its toxicity. Tin has already proven to be a good candidate for this substitution, but careful evaluation of the problem related to its oxidation in air environment from Sn2+ to Sn4+ must be taken into account.11-13 The feasibility of this substitution is given by the very similar ionic radius of Sn2+ (e.g. 1.10 Å) compared to that of Pb2+ (e.g. 1.19 Å), which implies a similar Goldschmidt tolerance factor t, a quantity that correspond to: 𝑡=

𝑟𝐴 + 𝑟𝐵 2(𝑟𝐵 + 𝑟𝑋)

(1)

where rA, rB and rX are the ionic radius of the monovalent and bivalent cations, and of the halide, respectively.14 The huge interest towards all-inorganic Pb-free materials is witnessed by the intense


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work in the design of solar cells based on such phases as well as on different cations.15-18 Very recently, the synthesis of fully inorganic CsSnI3 and CsSnBr3 perovskites has been demonstrated, together with a full characterization of their structural features.19,20 In particular, CsSnBr3 exhibits cubic symmetry (Pm-3m space group, -phase) at room temperature with a unit cell parameter of 5.8043(3) Å and it undergoes a cubic to tetragonal phase transition at about 286 K (space group P4/mbm, β-phase); it finally becomes orthorhombic at about 250 K (space group Pnma, γ-phase). Noticeably an established strategy towards the stabilization of lead based halide perovksite has foreseen the inclusion of multiple cations in the A+ site, this strategy allowed stability towards ions migration and humidity driven degradation, moreover it lead to an improved processabilty for these alloys.21,22 It is therefore of fundamental importance to extend the library of fully inorganic Sn-based perovskites for optoelectronic applications, in order to modulate their physical properties, get a further insight into their structural features and to evaluate their air-stability. In this work, we explored the possible formation of mixed compositions within the CsSnBr3-RbSnBr3 joint. Thus a complete investigation of the synthesis, structural and optical properties on the Cs1xRbxSnBr3

system, through a combined experimental and computational approach, is presented

here. Noticeably, the substitution of Cs+ with Rb+ within the CsSnBr3 phase was not reported in the literature while only the synthesis of RbSnBr3 single crystals has been reported together with a Raman spectroscopy characterization.23


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RESULTS AND DISCUSSION

Figure 1 reports the x-ray powder diffraction (XRD) patterns of the samples considered in the present work, i.e. Cs1-xRbxSnBr3 with x=0, 0.05. 0.10, 0.15, 0.20, and 0.30. The measurements have been carried out at a wavelength of 0.206589 Å (see the Experimental for details). Details on the samples synthesis are reported in the Experimental Methods (SI). Indexed patterns for each of the crystal structures found (i.e. cubic, tetragonal and orthorhombic) are reported in the SI.

Figure 1 – Room temperature XRD patterns of the samples of the Cs1-xRbxSnBr3 system (x=0, 0.05, 0.10, 0.15, 0.20, and 0.30), λ=0.206589 Å.

A Rb content of 0.3 has been found to represent the boundary between single and multiple phase compound (samples with x>0.30, shown in Figure 1a of the SI). A closer inspection of the


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diffraction patterns reported in Figure 1 indicates that the crystal structure progressively changes from cubic (x=0) to tetragonal (0.05≤x≤0.20) to orthorhombic for the highest Rb content (x = 0.30). Clear evidence of this phase sequence is provided by the (220) reflection proper of α phase in CsSnBr3 sample splitting into the (222) and (400) reflections proper of β phase in Cs0.95Rb0.05SnBr3 sample, and into the (242), (400) and (004) reflections proper of γ phase in Cs0.70Rb0.30SnBr3.

Figure 2. Left panel: comparison of XRD patterns form Cs1-xRbxSnBr3 samples with x=0, 0.05 and 0.30.

Figure 2 (right panel) shows the trend of the pseudo-cubic lattice volume determined from the Rietveld refinement of the diffraction patterns (results of the refinement reported in the SI) showing a quasi-linear reduction by increasing the Rb-content, thus obeying the Vegard’s law of solid solutions. It is of interest to highlight that CsSnBr3 is cubic at room temperature even displaying a tolerance factor of 0.873 being at the limit for the existence of a cubic perovskite (cubic: t=0.9-1). On the other hand, the introduction of the smaller Rb ion (Rb+=1.72 Å, Cs+=1.88 Å, for 12-fold coordination) progressively reduces the t value to 0.862 in the Cs0.70Rb0.30SnBr3 sample, and even this small variation can induce a significant distortion in the material average


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structure leading to an orthorhombic structure. All the refined lattice parameters values are displayed in Table 1 while all the refined parameters are available in the Supporting Information.

Table 1: Optical bandgap (from total absorption spectra) and lattice parameters for the single-phase samples of the Cs1-xRbxSnBr3 system. The Tauc plots were used to extract optical band gap of the perovskite alloys. Rb content Bandgap (eV) a (Å) b (Å) c (Å) Volume (Å3) (x) Z=1 0

1.719

5.80498(3)

5.80498(3)

5.80498(3)

195.615(3)

0.05

1.721

8.18740(7)

8.18740(7)

5.82457(6)

195.22(1)

0.1

1.738

8.17822(2)

8.17822(2)

5.82686(4)

194.86(1)

0.15

1.760

8.17171(7)

8.17171(7)

5.82784(6)

194.58(1)

0.2

1.762

8.16709(8)

8.16709(8)

5.82640(7)

194.31(1)

0.3

1.817

8.1731(1)

11.6290(2)

8.1349(1)

193.29(1)

To assess the impact of Rb substitution into the CsSnBr3 lattice, absorbance and photoluminescence measurements have been collected on the single-phase samples of the Cs1xRbxSnBr3 solid

solution (0 ≤ x ≤ 0.30). The total absorption spectra are displayed in Figure 3.


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Figure 3 – Total absorption spectra of Cs1-xRbxSnBr3 samples with 0≤x≤0.3.

As it can be noticed, the absorption edge shifts towards shorter wavelengths as Rb+ is progressively introduced in the structure, with a bandgap (reported in Table 1) that increases accordingly, moving towards higher energies by increasing x in Cs1-xRbxSnBr3.


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Figure 4 – Steady state (main panel) and time resolved (inset) photoluminescence spectra of Cs1-xRbxSnBr3 samples with 0≤x≤0.3. the legend are common in the main panel and in the inset, where only CsSnBr3(black line); Cs0.95Rb0.05SnBr3 (red line); Cs0.85Rb0.15SnBr3(magenta line); Cs0.75Rb0.25SnBr3(navy line) decays are reported. In grey the instrumental response function. For the details of the measurements please see SI.

The emission spectra reported in Figure 4 show, in line with the absorption, a monotonic shift towards high energies when Rb is added to CsSnBr3. These shifts well match the shifts observed with absorption measurements, however some peculiarities can be noticed. The shifts in emission band energetics appear to be smaller among the Rb containing samples in comparison to the related absorption spectra and only the first step from the reference compound (CsSnBr3) to the first alloy of the series (Cs0.95Rb0.05SnBr3), is associated to a sizeable energy shift of about 0.03 eV. Light emission in halide perovskite materials is a complex phenomenon associated to the


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carrier recombination and influenced by a multiplicity of factors such as trap distribution and diverse direct and indirect band edge transitions. These influences would justify the differences in the energy shifts in the cases of absorption and emission measurements. Excited state lifetimes (Figure 4 inset and Table S1) are also perturbed and in particularly increased by the introduction of the Rb cation. Beside the main emission peak discussed above, a low-intensity feature around 760 nm has been always recorded in the all the samples without any shift and without any clear correlation in its intensity with the Rb-content. The record of the emission in an integrative sphere, aiming at resolving scattered components, or by masking the samples to evaluate self-absorption artifacts did not lead to the disappearance of the 760 nm peaking band.24 Such low-intensity feature may be related to very small amounts of secondary phases existing in the prepared samples. As a matter of fact, in the high-resolution synchrotron diffraction data reported in the present work, small amounts, in the order of 1-2%, of Rb/CsSn2Br5 phase has been detected and, in addition, we may not exclude the presence of poorly crystalline additional phases, which may be responsible for this weak peak in the PL spectra. These observations well match the structural modification and band-gap perturbation, determined by the pseudo-cubic lattice volume trend showing a linear reduction with the increase of Rbcontent, (Figure 2) thus obeying the Vegard’s law of solid solutions. Therefore upon addition of small amounts of Rb the absorption and photoluminescence (PL) bands (figure 3 and 4) are blueshifted. The influence of the smaller Rb on Cs perovskite could lower the effective Cs/Rb cation radius in the new perovskite compound contributing to shift the tolerance factor towards a tetragonal lattice structure.25 Furthermore, it is noteworthy an increase in charge carrier lifetime induced by the Rb inclusion in CsSnBr3.26


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The tendency of alloy formation, stability and the electronic properties have been studied by the density functional theory (DFT) calculations. We have studied the following composition: CsSnBr3,

Cs0.94Rb0.06SnBr3,

Cs0.87Rb0.13SnBr3,

Cs0.81Rb0.19SnBr3,

Cs0.75Rb0.25SnBr3

and

Cs0.69Rb0.31SnBr3 as has been shown in Figure 5. The different positions of the Rb atoms have been considered by varying the Rb-Rb distance and the octahedral position for Cs0.87Rb0.13SnBr3 (Figure S2), Cs0.81Rb0.19SnBr3 (Figure S3), Cs0.75Rb0.25SnBr3 (Figure S4), and Cs0.69Rb0.31SnBr3 (Figure S5) systems and the energetically most stable geometries are shown in Figure 5. However, relative energies for each stoichiometry varies in the range of 8, 12, 38 and 44 meV for Cs0.87Rb0.13SnBr3, Cs0.81Rb0.19SnI3, Cs0.75Rb0.25SnBr3, and Cs0.69Rb0.31SnBr3, respectively, thus a slight geometrical preference for certain sites (Rb atoms at nearest octahedra of the equatorial plane and nearest axial octahedra to some extent) during alloy formation is retrieved by increasing the Rb content (see Table S1-7 for details). Cohesive energies per formula unit are almost constant (within 40 meV) for the explored compositions with x=0, 0.06, 0.13, 0.19, 0.25 and 0.31, suggesting that mixing entropy may play a significant role in stabilizing alloys against pure phase. The calculated HSE06SOC band gap of the same compounds are 1.72, 1.77, 1.78, 1.79, 1.82 and 1.85 eV respectively which matches very nicely with the experimental band gap values calculated from the absorption spectra, see Figure 3 and Table 1).


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Figure 5: Optimized structures of Cs1-xRbxSnBr3 compositions with a) x=0, b) x=0.06, c) x=0.13, d) x=0.19, e) x=0.25 and f) x=0.31.

The inorganic cation in CsSnBr3 has spherical symmetry and there is no built-in dipole and hence holds cubic structure with a direct band gap. However, due to the incorporation of smaller Rb cation, there is structural distortion in the octahedra and the distortion increases with increasing Rb concentration. As a result, Rashba splitting has been observed and it increases with increasing the Rb concentration (Table 2). Although the Rashba parameters are quite small compared to MAPbI3, however can justify the slower recombination rate upon Rb incorporation as observed in experiment. In fact, the Rb-doped system possesses an indirect band gap (Table S8).27 The presence of accessible indirect transitions could be also responsible, along the previously discussed effects, of the excited state lifetime increase determined via TRPL measurements of Rb containing alloys.


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Table 2: Shift of valence and conduction band (Δk), band splitting energy (ε≠) and Rashba interaction coefficient (α) for the pristine and Rb-doped systems. See Figure S2-S6 (in Supporting Information) for the corresponding optimized geometries.

System

Δk (×10−3 Å−1)

ε≠ (meV)

α (eV Å)

VB

CB

VB

CB

VB

CB

CsSnBr3

0.02

0.04

0.00(2)

0.00(7)

0.05

0.07

Cs0.94Rb0.06SnBr3

0.08

0.19

0.04

0.13

0.22

0.34

Cs0.87Rb0.13SnBr3 –I

0.12

0.31

0.08

0.36

0.32

0.57

Cs0.81Rb0.19SnBr3–III

0.15

0.41

0.12

0.62

0.38

0.75

Cs0.75Rb0.25SnBr3 –III

0.21

0.55

0.22

1.03

0.51

0.93

Cs0.69Rb0.31SnBr3–VIII

0.24

0.67

0.26

1.45

0.55

1.08

The indication of an increased local structural distortion in the Cs1-xRbxSnBr3 solid solution by increasing the Rb content obtained from the DFT calculations has been experimentally evaluated by means of X-ray total scattering measurements coupled to Pair Distribution Function (PDF) analysis on two representative compositions: x=0 (CsSnBr3) and x=0.3, i.e. the limit of the solid solution existence. As determined by powder x-ray diffraction, the average structure is cubic for x=0 and orthorhombic for x=0.3 (see Figure 1 and Table 1). The fit of the short-range (1-10 Å) PDF for CsSnBr3 with the Pm-3m space group (-phase), does not provide a satisfactory description of the data (see Figure S6). In the analogous hybrid organic-inorganic perovskites of the MAPbX3 family, the local structure has been found to be always orthorhombically distorted also for high-symmetry average structures.28-30 This result appears to be also present in fully inorganic perovskites since a better description of the local structure of CsSnBr3 can be achieved


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through the use of an orthorhombic structure (Pnma space group), as shown in the following Figure (6a). The fit results are reported in the SI (Table S9).

Figure 6: a) Refinement of 300 K x-ray PDF of CsSnBr3 against the orthorhombic (Pnma) model; b) Refinement of 300 K x-ray PDF of Cs0.7Rb0.3SnBr3 against the orthorhombic (Pnma) model. Blue open circles: experimental data; red line: calculated; green line: difference (shifted by -0.5 for ease of visualization).

By increasing the Rb content to 0.3, the shape of the PDF changes, in particular in the region around 4 Å (marked with a red circle in Figure 6b), which is dominated by Cs/Rb-Br and Br-Br bond pairs, where a clear splitting of that peak can be seen from Figure 6b. The fit of the 1-10 Å PDF data for Cs0.7Rb0.3SnBr3 shown in Figure 6b has been carried out, as for CsSnBr3, with the Pnma orthorhombic structure (which corresponds also to the average structure for this composition). However, in order to catch the peculiar features of the PDF data for this high Rbcontent sample, in particular the peak splitting highlighted in Figure 6b, the fit of the data required the use of anisotropic thermal displacement parameters for Cs/Rb and Br ions. The fit results are reported in the SI. An increase of the corresponding uiso passing from x=0 to x=0.3 has been


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observed for all the atoms, in particular for Cs/Rb and Br2 sites. Clearly, the local positional disorder induced by the presence of mixed cations on the A-site is compensated in the fit by an increase of the displacement parameters. The increased distortion passing from x=0 to x=0.3 can be observed in the spread of the three Sn-Br octahedral distances which move from 2.917(5), 2.922(7) and 2.956(7) Å at x=0 to 2.801(8), 2.920(7) and 2.982(7) Å at x=0.3. Such increase of the distortion of perovskite lattice by replacing Cs with Rb, in agreement with the above reported computational data, has been also theoretically predicted for the CsSnI3 system.12 Finally, it is worth mentioning that a key point of the current research on Sn-based fully inorganic perovskites focuses on their chemical stability. We started a preliminary test of the relative stability to the environment by exposing selected samples i.e., CsSnBr3 and Cs0.7Rb0.3SnBr3, to moist laboratory air (relative humidity about 60%) for about 10 days at room temperature. Figure 7 below shows the diffraction patterns after synthesis and storage in glovebox (T0) and after air-exposure (T1) for CsSnBr3 (left pane, cubic structure) and Cs0.7Rb0.3SnBr3 (right panel, orthorhombic structure).


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Cs0.70Rb0.30SnBr3

CsSnBr3

Figure 7 – On the left side, X-ray diffraction pattern comparison between CsSnBr3 after synthesis (T0) and after one week of exposure in air conditions (T1). On the right side, X-ray diffraction pattern comparison between Cs0.7Rb0.3SnBr3 after synthesis (T0) and after one week of exposure in air conditions (T1). Black vertical bar represented the position of the different hkl reflection for α (left side) and γ (right side) phases.

A close inspection to the patterns of Figure 7 clearly demonstrates that in both cases there are not appreciably differences between T0 and T1 measurements and no signals of the formation of secondary phases due to oxidation and/or degradation of the samples are found. While this preliminary results are limited to a relatively short time-window, they suggests that full inorganic perovskites of the Cs1-xRbxSnBr3 system have an improved stability with respect to their hybrid counterparts.


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CONCLUSION In this paper we synthesized for the first time the Cs1-xRbxSnBr3 solid solution by means of solidstate reaction and assessed its structure and optical properties by a combined experimental and theoretical approach. The solubility limit to achieve single-phase materials has been found to be around x=0.3. By increasing the Rb-content in CsSnBr3, a linear contraction of the unit cell together with a transition from a cubic to an orthorhombic crystal structure has been observed. This couples to a progressive blue-shift of the band-gap from 1.719 eV for CsSnBr3 to 1.817 eV for Cs0.70Rb0.30SnBr3. An analogous trend for the emission has been confirmed by PL measurements where the increase of Rb-content leads to an increase of the carrier lifetime. The optical properties results have been confirmed by DFT calculations, which pointed out that the Rb incorporation into the cubic CsSnBr3 perovskite structure is accompanied by an increase of the octahedral distortion at the local scale. Such evidence has been further corroborated by x-ray PDF data showing an enhanced local distortion in mixed samples. Finally, preliminary stability tests confirmed that samples of the Cs1-xRbxSnBr3 system are very stable in laboratory moist air. This work reports a new series of all-inorganic lead-free perovskites with improved stability, which are appealing materials for several applications in the photovoltaic filed and in tandem cell architectures.


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ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Experimental and Computational details. AUTHOR INFORMATION The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge the project PERSEO-“PERrovskite-based Solar cells: towards high Efficiency and lOng-term stability” (Bando PRIN 2015-Italian Ministry of University and Scientific Research (MIUR) Decreto Direttoriale 4 novembre 2015 n. 2488, project number 20155LECAJ) for funding.


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Inorganic Perovskite Syst

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