Jun
Zhou‡
a,
Ximing
Rong‡
bc,
Maxim S.
Molokeev
def,
Xiuwen
Zhang
*b and
Zhiguo
Xia
*a
aThe Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail: xiazg@ustb.edu.cn; Fax: +86-10-82377955; Tel: +86-10-82377955
bCollege of Electronic Science and Technology, Shenzhen University, Guangdong 518060, China. E-mail: xiuwenzhang@szu.edu.cn
cKey Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
dLaboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia
eSiberian Federal University, Krasnoyarsk, 660041, Russia
fDepartment of Physics, Far Eastern State Transport University, Khabarovsk, 680021, Russia
First published on 28th December 2017
The electronic and optical properties of perovskites are related to the local structures of the compounds and define their functional applications. Herein we have prepared a double perovskite Cs2AgSbCl6, which crystallized in the cubic structure with the space group Fm-3m and the material is found to have a varied band gap associated with different body colors. The anti-site defect model was established to investigate transposition influence on the optical and electronic properties of the double-perovskite Cs2AgSbCl6, and the proposed model clearly explained the as-observed variable body color. Cs2AgSbCl6 perovskite has a high decomposition temperature and is stable upon prolonged exposure to air and moisture, which emphasize its potential in the field of photovoltaic absorbers and optoelectronic applications.
There are different structural design principles adopted in the search for potential Pb-replacements. One straightforward idea to solve the Pb toxicity issue involves replacing Pb2+ by Sn2+ from the same periodic group, but the chemical instability of Sn2+ and poor performance give rise to limitations for their further use.8,9 Another promising approach to replace Pb is substitution by more stable cations, Bi3+ or Sb3+, which is isoelectronic with Pb2+. However, the change of divalent to trivalent metal oxidation state implies a need for vacancies on the metal site and therefore lower dimensional structures and wider band gaps for the resulting A3M2VX9 (A = K+, Cs+, Rb+, [NH4]+ and [CH3NH3]+; V = vacancy; M = Bi and Sb; X = Cl, Br and I).10,11
Nevertheless, the fact is that there are only a few bivalent or trivalent cations alternative to Pb2+ that form stable and non-toxic perovskites.12 Therefore, another viable route is combination with a monovalent and a trivalent cation together, which leads to the formation of a double perovskite structure with the basic formula A2B′B′′X6 (A = CH3NH3+ or Cs+; B′ = Na+, Cu+ or Ag+; B′′ = Bi3+, Sb3+ or In3+; X = Cl−, Br−, or I−).13–15 The double perovskites were first investigated in the 1970s, initially in the context of ferroelectrics.16 Recently, in the light of the intense interest in halide perovskites, our group reported Cs2AgInCl6 with a band gap of 3.23 eV and it can be stable for several weeks in air.17 Later, Tran et al. successfully controlled the direct vs. indirect nature of the band gap in halide perovskites in Cs2AgSb1−xInxCl6 solid solution.18 Herein, we adopted a hydrothermal method to grow pure Cs2AgSbCl6 crystals with a varied band gap, and we established an anti-site defect model by density functional theory (DFT) to investigate transposition influence on the electronic and optical properties of the double-perovskite Cs2AgSbCl6. This site-equal transposition, which easily occurred during the experimental process, did not change the atomic ratio and had a big influence on the band gap. Moreover, the Cs2AgSbCl6 crystal was found to be stable in air for several weeks and has a high decomposition temperature. This material shows great potential for optoelectronics application via further band gap engineering.
Compound | Cs2AgSbCl6 |
Sp.Gr. | Fm-3m |
a, Å | 10.70093 (6) |
V, Å3 | 1225.36 (2) |
Z | 4 |
2θ-interval, ° | 5–120 |
R wp, % | 11.57 |
R p, % | 8.39 |
R exp, % | 9.40 |
χ 2 | 1.23 |
R B, % | 2.01 |
A visualization of the double perovskite structure is presented in Fig. 1b, and the fractional atomic coordinates are also given in Table 2. Cs2AgSbCl6 exhibits a 3-dimensional framework of corner-sharing alternating octahedra of [AgCl6] and [SbCl6], with Cs+ cations located in the cavities formed between the octahedra, resulting in a double cell, i.e., elpasolite K2NaAlF6 structure, as also observed in Cs2AgInCl6.17 The Sb–Cl bond length (2.63(1) Å) is slightly shorter than that of Ag–Cl (2.72(1) Å), as expected.
x | y | z | B iso | |
---|---|---|---|---|
Cs | 1/4 | 1/4 | 1/4 | 2.71 (9) |
Ag | 1/2 | 1/2 | 1/2 | 1.6 (1) |
Sb | 0 | 0 | 0 | 1.1 (1) |
Cl | 0.2456 (11) | 0 | 0 | 3.1 (1) |
Fig. 2a shows the typical SEM images of the as-prepared Cs2AgSbCl6 microcrystals, which are comprised of numerous uniform rhombic dodecahedron particles with a size of 15–60 μm. A closer observation of the magnified images for one microcrystal (Fig. 2b) demonstrates that the rhombic dodecahedron shows the typical fcc-crystal features, a tetradecahedral crystallization habit forming a truncated octahedral. Besides, the exhibited facets of Cs2AgSbCl6 were determined to be (100) and (111). Additionally, the well faceted microcrystal shapes generally indicate the high structural quality of the powder product, as obtained for several other compounds.27–29
Fig. 2 (a) Typical SEM images of Cs2AgSbCl6 crystals, (b) one representative microcrystal showing different facets. |
It is also worth mentioning that the body colors of the as-prepared powders can be continuously changed from yellow to dark green and eventually to near black with increasing volumes of HCl from 0.5 mL to 1.5 mL (Fig. 3a). Moreover, all of these samples are pure phase, and this phenomenon can be controlled by the added amount of HCl in the reaction solution. To determine the optical bandgaps for these different colored samples, the UV-vis diffuse reflection spectra of Cs2AgSbCl6 have been measured. As shown in Fig. 3b, as the color of the samples deepens, the reflection curve becomes lower and lower. The band gap of Cs2AgSbCl6 can be estimated according to eqn (1)30
[F(R∞)hv]n = A(hv − Eg) | (1) |
F(R∞) = (1 − R)2/2R = K/S | (2) |
Fig. 4 Electronic band structure of the double-perovskite Cs2AgSbCl6 by HSE (a) and PBE (b) approaches. |
Since Ag and Sb have rather similar local environments in the present cubic double-perovskite Cs2AgSbCl6 phase (e.g. Ag[Cl6Cs8Sb6Ag12] and Sb[Cl6Cs8Ag6Sb12] that differ only in the third shell, and similar atomic radii (0.16 nm for Ag and 0.145 nm for Sb32)), it is worthwhile to study the anti-site defect between them. Furthermore, because we considered the anti-site defect with swapped Ag and Sb atoms, the ratio of each element will not be changed, making the defect structures possibly allomorphic. Fig. 5a shows a diagrammatic sketch of two anti-site defect models of Cs2AgSbCl6 structures based on a primitive (with a rhombic structure) enlarged 2 × 2 × 2 super cell (Cs16Ag8Sb8Cl48). In these two anti-site models, an Ag atom makes atomic transposition with its nearest neighbor (NN) Sb atom and second-nearest neighbor (2NN) Sb atom. Before selecting the size of the super cell, we tested the size effect of the defect energy, as shown in Fig. 5b. For NN and 2NN structures, Edefect has been converged in accordance with the results of 2 × 2 × 2 and 3 × 3 × 3 super cells, meaning that the 2 × 2 × 2 super cell is large enough to avoid the boundary effect caused by DFT calculations. And the structural parameters of primitive Cs2AgSbCl6 and 3 Cs16Ag8Sb8Cl48 (balanced, NN and 2NN) structures after full relaxation are shown in Table 3. Compared with the balanced structures, the lattices of NN and 2NN defect structures expanded and the total energy was increased by 0.625 eV (NN) and 0.975 eV (2NN) due to the influence of the internal defect on the lattice. For the NN anti-site model, the structure is no longer cubic, this is mainly because the transposition occurred within the diagonal of the face rather than the body diagonal, causing the lattice to expand uncoordinated with a distortion. For the 2NN anti-site model, the lattice also slightly expands (by 0.39%). Due to the relatively low formation energy of anti-site defects (e.g. 0.625 eV for the NN anti-site), a significant amount of Ag–Sb anti-site defects would form. The Ag and Sb anti-site defects would be connected to their NN, 2NN, 3NN,… neighboring anti-sites in the crystal.
Structure | E tot (eV) | ΔE (eV) | ΔE/atom (meV) | V (Å3) | L a (Å) | L b (Å) | L c (Å) | α (o) | β (o) | γ (o) |
---|---|---|---|---|---|---|---|---|---|---|
Primitive | −32.703 | — | — | 319 | 7.672 | 7.672 | 7.672 | 60 | 60 | 60 |
Balanced | −261.625 | 0 | 0 | 2554 | 15.343 | 15.343 | 15.343 | 60 | 60 | 60 |
NN | −261.000 | 0.625 | 7.81 | 2586 | 15.397 | 15.424 | 15.397 | 59.94 | 60.12 | 59.94 |
2NN | −260.650 | 0.975 | 12.19 | 2584 | 15.403 | 15.403 | 15.403 | 60 | 60 | 60 |
Fig. 6 shows the electronic band structures of NN (Fig. 6a) and 2NN (Fig. 6b) Cs16Ag8Sb8Cl48 anti-site structures from the PBE calculations, respectively. Since a super cell is required in the process of calculating defects, the band folding will occur and result in the gap fold of the band structure to the gamma point, so direct band gap is chosen to be discussed. The direct band gaps of the anti-site structures decrease from 2.36 eV (pristine) to 1.36 eV (NN) and 1.13 eV (2NN). The decrease of the direct gap from NN to 2NN anti-sites reflects the effect of the local electric field between the oppositely charged Ag and Sb anti-site defects. Details of band gaps are presented in Table 4. As NN and 2NN are allotropes of double-perovskite Cs2AgSbCl6 structures, the obvious gap differences indicate that this defect model could stand a good chance in the experiments.
Structure | Calculation method | E directg/eV | E indirectg/eV | E overallg/eV |
---|---|---|---|---|
Primitive | HSE | 3.33 | 2.35 | 2.35 |
Primitive | PBE | 2.36 | 1.40 | 1.40 |
NN | PBE | 1.36 | — | 1.36 |
2NN | PBE | 1.13 | 0.90 | 0.90 |
Fig. 6c shows the imaginary optical absorption coefficient of the dielectric functions of 3 Cs16Ag8Sb8Cl48 structures. The absorption edges coincided with the direct band gaps (see Table 4) and showed the order Ebalanced > ENN > E2NN, which is consistent with the band gap variation in these structures. As we discussed before, since a significant amount of anti-sites with NN, 2NN,… nearest neighbor configurations would form at the experimental temperature, Cs2AgSbCl6 would have strong optical absorption for photons with energy much lower than its pristine optical band gap. It would be hard to define strictly the actual optical gap of Cs2AgSbCl6 containing defects. However, the optical absorbance of Cs2AgSbCl6 containing defects towards visible light (below its optical band gap of ∼3.33 eV according to HSE calculations) can be continuously varied due to the formation of anti-site defects. Indeed, in the experiments, we observed almost continuously constant optical absorbance for most of the visible light in Cs2AgSbCl6 below its pristine optical band gap (see Fig. 3b) indicated by a clear optical absorption shoulder that doesn't change much for different samples (the variation of optical gap from 2.61 to 2.24 eV could not explain the significant variation of sample colors). The main difference between the samples with different colors is the magnitude of reflectance for visible light (see the right side of Fig. 3b), i.e. the darker sample absorbs more visible light, in other words it reflects less light. A possible reason is that the darker sample contains more Ag–Sb anti-site defects. Since 100% Ag–Sb anti-sites in Cs2AgSbCl6 lead to the same structure as the pristine crystal and zero defect formation energy, in local regions of Cs2AgSbCl6, clusters of Ag–Sb anti-sites could form, leading to high anti-site density. The formation of anti-site clusters is hard to control precisely in the current synthesis of bulk compounds. The above optical absorption feature is rather different from the widely studied tunability of sample color by controlling the sizes of nano-crystals (e.g. in ref. 33), where the optical absorption shoulder changes significantly for different samples with different nano-crystal sizes. In our experiments, we also observed a slight variation of optical absorption shoulders, which may reflect the change of nano-crystal sizes. Furthermore, although there are contributions from phonon assisted absorption in Fig. 3b, the phonon effect could not explain the sharp sample color variation since phonon assisted absorption could not go below the fundamental gap of pristine Cs2AgSbCl6 (2.35 eV according to HSE calculations). The variety of visible light absorbance in Cs2AgSbCl6 is related to the as-observed colors of the materials, which also have some implications for optical properties.
Fig. 7 PXRD patterns (a) and UV-vis diffuse reflectance spectra (b) of Cs2AgSbCl6 after 0 days, 7 days, 14 days, 21 days, 28 days and 35 days of exposure to light and moisture conditions. |
Halide perovskites are known for low decomposition and formation energies. Since thermal stability is also an important parameter for halide perovskites in the prospective applications, we carried out thermogravimetric and differential scanning calorimetry (TG-DSC) analysis of a Cs2AgSbCl6 powder sample (Fig. 8). From the recorded TG-DSC result during the heat-treatment of Cs2AgSbCl6 in an argon flow, it is apparent that there are two major stages of rapid weight loss in the TGA curve at 356 and 756 °C, accompanying their corresponding exothermic peaks, which indicates that Cs2AgSbCl6 may decompose by two steps. The first weight loss of about 28.55 wt%, observed at 250–500 °C, is ascribed to the evaporation of SbCl3. Notably, SbCl3 constitutes 32.21% of the total weight of Cs2AgSbCl6, and therefore, the decomposition reaction equation might be described as follows: Cs2AgSbCl6 → SbCl3 + Cs2AgCl3. Finally, the third serious weight loss of 42.64 wt%, observed at 500–1000 °C and centered at around 756 °C, is in good agreement with the theoretical value of CsCl (47.55%) evaporation, and the decomposition process could proceed according to the scheme below: Cs2AgCl3 → AgCl + 2CsCl.34 The above results showed that Cs2AgSbCl6 is relatively stable to mass loss up to 250 °C. After that, some obvious decomposition reaction occurs enabling the materials to lose the functionality completely.
Footnotes |
† Electronic supplementary information (ESI) available: The crystallographic information (CIF) of Cs2AgSbCl6. See DOI: 10.1039/c7ta10062k |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2018 |