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Lead-free, stable mixed halide double perovskites Cs2AgBiBr6 and Cs2AgBiBr6−xClx – A detailed theoretical and experimental study
Journal Pre-proofs Lead-Free, Stable Mixed Halide Double Perovskites Cs2AgBiBr6 and Cs2AgBiBr6-xClx- A Detailed Theoretical and Experimental Study Mrinmoy Kumar Chini, Sriram Goverapet Srinivasan, Naveen Tailor, Yukta, Dennis Salahub, Soumitra Satapathi PII: DOI: Reference:
S0301-0104(19)30933-4 https://doi.org/10.1016/j.chemphys.2019.110547 CHEMPH 110547
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Chemical Physics
Received Date: Revised Date: Accepted Date:
4 August 2019 2 October 2019 4 October 2019
Please cite this article as: M. Kumar Chini, S. Goverapet Srinivasan, N. Tailor, Yukta, D. Salahub, S. Satapathi, Lead-Free, Stable Mixed Halide Double Perovskites Cs2AgBiBr6 and Cs2AgBiBr6-xClx- A Detailed Theoretical and Experimental Study, Chemical Physics (2019), doi: https://doi.org/10.1016/j.chemphys.2019.110547
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Lead-Free, Stable Mixed Halide Double Perovskites Cs2AgBiBr6 and Cs2AgBiBr6-xClx- A Detailed Theoretical and Experimental Study Mrinmoy Kumar Chini1, Sriram Goverapet Srinivasan2, Naveen Tailor1, Yukta1, Dennis Salahub3,4,5, and Soumitra Satapathi*,1 1Department 2Tata
of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, 247667, India
Research Development and Design Center, TCS Research, 54B Hadapsar Industrial Estate, Pune, Maharashtra 411013, India
3Department
of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4 4Centre for Molecular Simulation, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4 5Institute for Quantum Science and Technology, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4
*Corresponding Author: Soumitra Satapathi, Email ID: [email protected]
ABSTRACT Recently, lead-free halide-based double perovskites (DPs) have emerged as a promising candidate for photovoltaic and optoelectronic applications. Here we report the synthesis, characterization and electronic structure calculations on lead-free Bi-based mixed-halide double perovskites of Cs2AgBiClxBr6-x stoichiometry. The introduction of Cl- dopant converts these DPs from cubic to orthorhombic or tetragonal crystal structures. Electronic structure calculations indicate that both the bandgap and the nature of the band edges of these materials are intimately related to the kind of halides constituting the BiX6 octahedra. When the BiX6 octahedra contains both Cl- and Br- ions, the band gap increases linearly with increase in chloride content. On the other hand, when the BiX6 octahedra comprise of only one halide (Br- or Cl-) ion, the band gap remains nearly equal to that of Cs2AgBiBr6 until the Cl- dopant concentration reaches 50%. Experimental characterization of the synthesized mixed-halide DPs show that the band gap value of 1.98 eV and 1.97 eV, obtained for Cs2AgBiBr6 and Cs2AgBiClxBr6-x, respectively, matches with our theoretical calculations. The PXRD spectrum indicates polycrystallinity in these DPs and changes slightly after one month, 1
showing reasonably good stability. The elemental analysis showed incorporation of a small amount of Cl- (7 weight%) in the perovskite composition. In summary, we have performed detailed computational studies and synthesized and characterized mixed halide-based DPs with high stability for solar cells application. KEYWORDS: Double perovskite, band gap, density functional theory, mixed halide, non toxicity 1. INTRODUCTION The current focus on the introduction and development of stable, non toxic and environmentally friendly photovoltaic materials with suitable band gaps in the visible region attracts a remarkable interest for the synthesis and computational study of lead-free halide-based double perovskites (DPs). Previously, lead-based halide perovskites have achieved power conversion efficiencies as high as ~24.2% (1), due to their excellent physical and optical properties such as solution processibility (2), efficient light absorption (3), excellent charge carrier mobility along with long diffusion length (4). However, the presence of lead in organometallic perovskites has raised concerns regarding toxicity (5-7) and stability issues (8-11) which prevent the widespread deployment of this technology for commercial applications (12-19). Consequently, a tremendous urge for the development of lead-free perovskite materials, has led researchers to the synthesis of several lead-free perovskite materials. Among them, the substitution of Pb2+ by heterovalent M3+ cations such as non-toxic Bi3+, which is isoelectronic with Pb2+ has been accomplished. But incorporation of a highly charged Bi3+ ion into the three-dimensional A1+M2+X3 structure resulted in undesirable optoelectronic properties compared to the lead-based counterparts (20-23). This was overcome by introducing Bi3+ ions into the Elpasolite structure which is commonly known as the double perovskite structure having general formula A2M1+M3+X6 where, M1+ and M3+ represent monovalent and trivalent cations, respectively (24). Recently, several reports on Bi3+-based double perovskites with Ag+ as the monovalent cation with favourable band gap, excellent photoluminescence lifetime, high stability, long carrier recombination lifetime and comparable 2
charge carrier effective masses have been reported, rendering them as a very promising material for photovoltaic applications (24-29). McClure et al. have synthesized Cs2AgBiBr6 and Cs2AgBiBCl6 and have demonstrated favourable band gaps and superior moisture stability for these double perovskites compared to CH3NH3PbX3 (24). Slavney et al. have shown excellent photo-physical properties of Cs2AgBiBr6 compounds (25). Karmakar et al. have successfully prepared low band gap DP material including Cu(II)-Doped Cs2AgSbBr6 (30). Recently, Gao et al. have demonstrated high-quality Cs2AgBiBr6 DP film for inverted planar heterojunction solar cells with moderate Efficiency (2.2%) (31). Filip et al. have indicated that mixed cations-based Cs2BiAg1−xCuxCl6 can be synthesized with a reduced band gap (~1.6−1.9 eV) (32). Li et al. have synthesized a remarkably stable hybrid lead-free (CH3NH3)2AgSbI6 DP which has turned out to be a promising light absorber with a suitable optical band gap (1.93 eV) for photo-voltaic applications (33). Therefore, lead-free DPs, especially Cs2AgBiBr6, have enormous potential as an alternative to the lead-based counterpart with low carrier effective masses and low band gaps. The tuning of electronic properties and enhancement in stability can be obtained by compositional mapping with different composition in halide-based double perovskites (26-28). These reported double-halide perovskites possess the advantages of absorption spectra in the visible range of the solar spectrum and significant stability under ambient conditions (29,30) along with nontoxicity which favours them as excellent candidates for photovoltaics applications. However, the long term stability of these materials continues to remain a concern. Doping the hybrid organic-inorganic perovskite MAPbI3 with Cl- ions (in order to form mixed-halide perovskites of MAPbI3-xClx stoichiometry) was shown to nearly retain the bandgap of MAPbI3 but result in an improved charge transport within the active perovskite layer as well as a better solar cell performance. However, to the best of our knowledge, no reported studies deal with the variation in halide composition of the DPs and its effect on the stability and optoelectronic properties.
3
In this work, we have systematically carried out the theoretical investigation of incorporation of Cl in the Cs2AgBiBr6 structure by Density Functional Theory (DFT) calculations in order to understand the bulk and electronic structures of the mixed halide-based double perovskites Cs2AgBiClxBr6-x (x = 0 to 6). We have also synthesized and characterized the mixed halide double perovskites, namely Cs2BiAgBr6 (CBAB) and Cs2BiAgBr6-xClx (CBABC) for two different compositions to verify our computational study. We show that the bandgap of the mixed halide perovskites is strongly dependent on the nature of the halides constituting the BiX6 octahedra. If the BiX6 octahedra contain both Br- and Cl- ions, the band gap is seen to increase linearly with the chloride content, whereas the bandgap remains nearly a constant (equal to the band gap of Cs2AgBiBr6) until 50% Cl- doping, if only one kind of halide ion constitutes a BiX6 octahedra. In the former case, the contribution to the valence band edge arises from both Cl and Br ions while in the latter, only Br- ions contribute to the valence band edge as long as they remain the dominant anion. Furthermore, we have also synthesized and characterized these proposed mixed halidesbased lead-free double perovskites and compared their photophysical properties. Our results show that Cl-doped mixed halide double perovskites are promising photovoltaic materials as they depict a favorable band gap and enhanced ambient stability simultaneously. 2. COMPUTATIONAL METHODS AND DETAILS Density Functional Theory (DFT) calculations were carried out to understand the bulk and electronic structures of the mixed halide double perovskites Cs2AgBiClxBr6-x (x = 0 to 6). All calculations were carried out using the VASP planewave software.[34-37] The valence electronic states were expanded in a basis of planewaves up to a planewave kinetic energy of 600 eV. The Projector Augmented Wave (PAW) approach[38,39] was used to describe the core–valence interaction, while the
PBE GGA functional[40] was used to describe the electron exchange-
correlation interactions. Geometry optimizations were deemed to have converged when the forces on the atoms dropped below 10-3 eV/A, while the SCF convergence threshold was set to 10-7 eV. 4
Complete cell optimizations followed by re-relaxations were carried out for all the systems in order to avoid any artifacts due to Pulay stress. During the SCF procedure, a Pulay mixing scheme was used for charge density mixing (41). The Brillouin zone was sampled using an (8x8x8) Gammacentred k-point mesh. Spin Orbit Coupling (SOC) was included in the computation of the electronic band structure and band gap. The SOC electronic structure was computed using the geometry optimized with the PBE GGA functional. For all structures, ALFOW-ONLINE (42) was used to obtain the coordinates of the high symmetry points in the Brillouin zone for band structure calculations. Molecular graphics were generated using the VESTA software (43). 3. EXPERIMENTAL METHODS Cesium bromide (CsBr) (99%), Silver bromide (AgBr), Bismuth (III) bromide (BiBr3) and Bismuth (III) chloride (BiCl3) (98%) were purchased from Avra Synthesis Pvt. Ltd. and used without any further purification. Deionised water and hydrochloric acid/ hydrobromic acid were used as solvents. FESEM images were taken by FESEM QUANTA 200 FEG under an electron excitation energy of 20 keV using an ETD detector at a working distance of 9.5 mm. Differential thermal analysis (DTA) and thermogravimetric analysis (TG) analysis were performed using an EXSTAR TG/DTA 6300 instrument. In these experiments, the sample (10.51 mg) was placed with reference alumina powder (10.5 mg) under nitrogen gas flow (200 ml/min) and heated at a rate of 10oC/min in the temperature range of 34 °C to 900 °C. Ultraviolet-visible absorption spectra of the polycrystalline materials were collected using a Microprocessor UV-Vis Double Beam spectrophotometer LI-2800. Photoluminescence measurements were performed with an RF-6000 Spectro-Fluorophotometer. Synthesis of Double Perovskites Polycrystalline materials of halide, Cs2BiAgBr6Cl0 (CBAB) and mixed halide double perovskites, Cs2BiAgBr6-xClx (CBABC) were synthesized from a concentrated HBr/ HCl solution
5
containing stoichiometric CsBr (2mM), AgBr (1mM), and BiBr3/ BiCl3 (1mM), respectively, at a temperature of 125 °C for 1h. The detailed synthetic method is described below (25). Synthesis of Cs2BiAgBr6Cl0 (CBAB) The initial solution was prepared by dissolving 1 mM AgBr and 1 mM BiBr3 in acidic solution of concentrated HBr (12 mL, 8.84 M), followed by stirring at 90 °C for 10 minutes. Then, 2 mM CsBr in solution of HBr was added slowly to the previous reaction mixture, with continuous stirring for 1 h at 125 °C to ensure the completion of the reaction. Finally the solution was gradually cooled down to 70 °C at a cooling rate of 20 °C/ h. Upon cooling down to room temperature, we obtained orange-coloured polycrystalline material of Cs2BiAgBr6Cl0 composition. The obtained material was washed with ethanol and dried under vacuum. Synthesis of Cs2BiAgBr6-xClx (CBABC) The procedure for the synthesis of Cs2BiAgBr6-xClx was similar to that of Cs2BiAgBr6Cl0. Here, we had taken BiCl3 instead of BiBr3 in acidic solution of concentrated HCl. In this case also, we have obtained the orange-coloured polycrystalline material of Cs2BiAgBr6-xClx.
.
BiBr3
AgBr
BiCl3
AgBr
2CsBr HBr, 125 oC 2CsBr HCl/ HBr, 125 oC
Cs2BiAgBr6-0Cl0
Cs2BiAgBr6-xClx
Scheme 1 Reaction scheme for Cs2BiAgBr6Cl0 and Cs2BiAgBr6-xClx
Double perovskite film preparation The synthesized polycrystalline double perovskite compounds were dissolved (60mg/ 1 mL) in anhydrous DMSO solvent (99.9%, Sigma-Aldrich) by sonication for 10 minutes followed by heating at 75 °C. Following complete dissolution of the DPs, the studied DPs (100 μL) were spincoated on top of the preheated substrate (Cleaned ITO) at 75 °C at 1000 rpm for 30 s. The 6
substrates were then initially annealed at 75 °C for 15 min in a controlled-humidity chamber. Again, a 2nd set of spin-coated substrates were subsequently annealed at 250 °C for 5 min in the humidity chamber to have the desired DP phase. The treated substrates were then subjected to UV-Vis, photo-luminescence (PL), Thermogravimetric analysis, Raman and thin film XRD measurements. 4. RESULTS AND DISCUSSIONS 4.1 Density Functional Theory Calculations DFT calculations were carried out to understand the changes in the structural and electronic properties upon the formation of mixed-halide double perovskites. The halide double perovskites A2B’B”X6 (A = Cs, B’ = Ag, B” = Bi, X =Cl/Br) crystallize in the cubic elpasolite structure (Fm3m space group), containing connected corner-sharing B’X6 and B”X6 octahedra. The B’X6 and B”X6 octahedra are arranged in an alternating fashion such that each of the B’ and B” cations are surrounded by six B”X6 and B’X6 octahedra, respectively. The A cation resides at the centre of the cuboctahedral cavity formed by eight octahedra (i.e., four pairs of B’- and B”-centered octahedra). The lattice parameters and unit cell volumes for the chloride and bromide double perovskites are reported in Table 1, showing that our DFT based values are in very good agreement with experiments and earlier reported theoretical values. Table 1 DFT optimized and experimental lattice parameters and unit cell volumes for two halide double perovskites. DFT calculations utilized a planewave kinetic energy cut off of 600 eV and PBE GGA exchange correlational functional. The lattice parameters correspond to a conventional unit cell containing 40 atoms (4 formula units).
Composition
Lattice Parameter (Å)
Unit Cell Volume (Å3)
DFT
Experiment11
DFT
Experiment11
Cs2AgBiCl6
10.965
10.777
1318.41
1251.82
Cs2AgBiBr6
11.496
11.271
1519.30
1431.86
7
Two different kinds of system were considered to model the mixed halide perovskites. In the first kind, the bromide ions of the BiBr6 octahedra were sequentially replaced by chloride ions resulting in five intermediate compositions, with ‘x’ varying from 1 to 5. These systems were modelled using their primitive cell consisting of 10 atoms (1 formula unit). Fig. 4 shows representative BiClxBr6-x octahedra for ‘x’ varying from 0 to 6. We name these structures Bi-MHO-xi, MHO being the acronym for ‘Mixed Halide Octahedra’, x corresponding to the number of bromide ions replaced by chloride ions, and ‘i’ corresponding to different configurations of the chloride ions (if any) at a given value of ‘x’.
Fig. 1 Representative structures of the BiClxBr6-x octahedra at various values of ‘x’ ranging from 0 to 6. Bi atoms are purple, Br atoms are brown and Cl atoms are green in colour. For instance, at x = 2, two different configurations (Bi-MHO-2a and Bi-MHO-2b, Fig. 1c and 1d respectively) of the chloride ions were considered. In Bi-MHO-2a configuration (Fig. 1c), one axial and one equatorial bromide ion of the BiBr6 octahedra was replaced by a chloride ion, while in BiMHO-2b, two diametrically opposite bromide ions were replaced by chloride ions. Similarly, in BiMHO-3a (Fig. 1e), two equatorial and one axial bromide ions were replaced by chloride ions while in Bi-MHO-3b (Fig. 1f), three equatorial bromide ions were replaced by chloride ions. Finally, in 8
Bi-MHO-4a (Fig. 1g), three equatorial and one axial bromide ions were replaced by chloride ions while in Bi-MHO-4b (Fig. 1h), all the four equatorial bromide ions were replaced by chloride ions. In the second kind of system, one, two and three of the BiBr6 octahedra were replaced by BiCl6 octahedra to obtain three compositions, Cs2AgBiCl1.5Br4.5, Cs2AgBiCl3Br3 and Cs2AgBiCl4.5Br1.5 respectively. The primitive cell for CsAgBiCl1.5Br4.5 and CsAgBiCl4.5Br1.5 contained 40 atoms each while that for Cs2AgBiCl3Br3 contained 20 atoms. We call these structures Bi-WO-x, where WO stands for ‘Whole Octahedra’ and x corresponds to the proportion of Cl in the double perovskite. Table 2 lists the optimized lattice parameters, unit cell volumes and the space group of the mixed halide perovskites. Table 2 Optimized lattice parameters, unit cell volumes and space groups of the various mixed halide double perovskites. All calculations were done on the primitive unit cells.
System
a (Å)
b (Å)
c (Å)
α (deg.) β (deg.) γ (deg.)
Volume (Å3)
Space Group
Bi-MHO-1
8.116
8.031
8.031
60.701
59.650
59.650
370.113
I4mm
Bi-WO-1.51 11.380 11.380 11.380
90.00
90.00
90.00
1473.80
Pm-3m
Bi-MHO-2a 8.025
7.938
8.025
60.357
59.363
60.357
361.609
Imm2
Bi-MHO-2b 8.112
7.950
7.950
61.359
59.321
59.321
362.208
I4/mmm
Bi-MHO-3a 7.941
7.932
7.941
59.965
60.00
59.965
353.483
R3m
Bi-MHO-3b 7.858
7.934
8.027
58.982
59.931
61.087
353.700
Fmm2
Bi-WO-3.02 7.971
7.971 11.234
90.00
90.00
90.00
713.791
P4/mmm
Bi-MHO-4a 7.935
7.848
7.848
60.668
59.633
59.633
345.342
Imm2
Bi-MHO-4b 7.933
7.785
7.933
60.617
58.767
60.617
346.228
I4/mmm
Bi-WO-4.51 11.118 11.118 11.118
90.00
90.00
90.00
1374.305
Pm-3m
Bi-MHO-5
60.310
59.379
60.310
337.335
I4mm
7.839
7.765
7.839
1Primitive
unit cell contains 40 atoms (4 formula units)
2Primitive
unit cell contains 20 atoms (2 formula units)
From Table 2, we notice that while the pure halide double perovskites were cubic in nature, the structures of all the mixed halide perovskites are either tetragonal or orthorhombic in nature. Furthermore, as the ‘x’ varies from 0 to 6, the unit cell volumes (and the densities) of the mixed
9
halide perovskites decrease monotonically due to an increase in the proportion of the smaller sized chloride ions. In all the double perovskites studied here, the Ag and Bi cations are octahedrally coordinated by the halide ions. However, not all of the Ag or Bi cations are equivalent to each other in all the mixed halide double perovskites. For instance, in the WO-1.5 systems, one of the Ag cations is coordinated to 6 bromide ions while the other three Ag cations are equivalent to each other and coordinated to 4 bromide and 2 chloride ions. Similarly, in the WO-3 systems, two of the Ag cations are coordinated to 4 bromide and 2 chloride ions, while the other two Ag cations are coordinated to 2 bromide and 4 chloride ions. On the other hand, in the MHO systems, all the Ag (or Bi) cations are equivalent to each other, each being coordinated to x chloride and (6-x) bromide ions. Consequently, the Ag (Bi)-X bond distances are dependent on both the system kind (i.e., MHO vs WO) and the proportion of the 2nd halide in the structure. Table 3 lists the Ag-Cl, Ag-Br, Bi-Cl and Bi-Br bond distances in all the structures studied here. Table 3 Ag-X and Bi-X bond distances (in Å) in various mixed halide double perovskites.
System
Bi-Cl
Bi-Br
Ag-Cl
Ag-Br
Cs2AgBiBr6
-
2.884
-
2.860
Bi-MHO-1
2.746
2.884, 2.866
2.741
2.857, 2.883
Bi-WO-1.5
2.749
2.889, 2.861
2.941
2.801, 2.829
Bi-MHO-2a
2.747
2.884, 2.867
2.739
2.852, 2.884
Bi-MHO-2b
2.727
2.886
2.776
2.850
Bi-MHO-3a
2.749
2.866
2.734
2.883
Bi-MHO-3b
2.750, 2.730
2.884, 2.866
2.764, 2.737
2.890, 2.847
Bi-WO-3
2.756, 2.728
2.890, 2.859
2.889, 2.880
2.778, 2.727
Bi-MHO-4a
2.749, 2.730
2.869
2.764, 2.733
2.871
Bi-MHO-4b
2.730
2.885
2.774
2.828
Bi-WO-4.5
2.764, 2.730
2.853
2.829, 2.795
2.706
Bi-MHO-5
2.748, 2.730
2.867
2.762, 2.724
2.850
Cs2AgBiCl6
2.731
-
2.749
-
10
From Table 3, we see that while the Bi-X bond distances vary little across various structures, the Ag-X bond distances are more strongly dependent on whether the mixed halide perovskites are of the MHO or WO kind. Specifically, in the WO structures, we see that the Ag-Cl and Ag-Br bond distances are longer and shorter respectively, compared to their values in the MHO structures. The influence of these structural features is profoundly reflected in the band gaps of these systems. Fig. 2 shows the variation in the band gap (with SOC) of the Cs2AgBiClxBr6-x double perovskites as a function of ‘x’ for both MHO and WO systems while Fig. S1 to S13 in the supporting information show the band structure (with and without SOC) of these double perovskites. Table S1 in the supporting information provides the band gaps computed with and without SOC included.
Fig. 2 Variation in the band gaps of the Cs2AgBiClxBr6-x mixed halide double perovskites with increase in ‘x’. The dotted line is a linear fit to the band gaps of the MHO type mixed halide double perovskites. All the perovskites show an indirect band gap. From Fig. 2, S1 to S13 and Table S1, we notice that, generally, the band gap of the mixed halide perovskites increase as the amount of chlorine content increases since the chlorine ‘p’ orbitals are both less diffuse and lie lower in energy than the bromine ‘p’ orbitals. Furthermore, the band gaps of all the mixed halide double perovskites are indirect in nature. However, the specific features of 11
the band structure in terms of the atoms and orbitals contributing to the band edges and the band gap of the mixed halide perovskites are strongly dependent on the kind of its structure (i.e., MHO vs WHO). For the pure halide double perovskites, Cs2AgBiBr6 (Fig. S1) and Cs2AgBiCl6 (Fig. S10), the top of the valence band (VBM) and the bottom of the conduction band (CBM) lie at the X (π/a, 0,π/a) and L (π/a,π/a,π/a) points in the Brillouin zone, respectively. The VBM consists of the Bi ‘s’, Ag ‘d’ and halide ‘p’ orbitals while the CBM is predominantly comprised of the Bi ‘p’ and Ag ‘s’ orbitals. Inclusion of SOC results in the formation of an isolated conduction band in the middle of the GGA band gap, which comprises the Bi ‘p’ and Ag ‘s’ states, together with a minor contribution from the halide ‘p’ states. The reduction in the band gap upon inclusion of SOC effects is larger for Cs2AgBiCl6 (0.26 eV) compared to Cs2AgBiBr6 (0.17 eV) (see Table S1). These findings are in line with earlier reports on the electronic structure of these pure halide double perovskites (24,26). With increase in the chloride content, the electronic features of the mixed double halide perovskites in the MHO structure continue to retain the same qualitative features. Since all the Ag (and Bi) cations are equivalent to each other in these structures, the magnitude of the band gap depends only on the amount of chloride ions in the material and increases linearly with increase in the chloride content. Similarly, the reduction in the band gap upon inclusion of SOC effects also increases linearly with the chloride content. However, the degeneracy of the ‘split off’ conduction band state due to SOC depends on the local coordination environment of the Bi (and consequently Ag) cation. In the pure halide double perovskites; this state is twofold degenerate throughout the Brillouin zone. However, in all MHO double perovskites other than Bi-MHO-2b and Bi-MHO-4b, the Bi (and consequently Ag) cations are ‘off centred’ in the BiClxBr6-x octahedra, due to two different Bi-Br and/or Bi-Cl distances. As a result, the split-off conduction band state does not remain two-fold degenerate throughout the Brillouin zone. This degeneracy is recovered for Bi-MHO-2b (Fig. S9) and BiMHO-4b (Fig. S13) structures wherein the distances of all the Cl (or Br) ions to the Bi cations are the same, resulting in no off-centring of the BiClxBr6-x octahedra. 12
In contrast to the MHO structures, the WO structures show band-structure contributions arising from only a subset of the cations and anions depending on their local coordination environments. For instance, in the WO-1.5 structures, there are three Bi-Br6 octahedra and one Bi-Cl6 octahedra, resulting in the VBM predominantly consisting of ‘s’ orbitals of the Bi cation from the Bi-Br6 octahedra, ‘d’ orbitals of the Ag cation and the ‘p’ orbitals of the Br- ion. The CBM predominantly consists of the ‘p’ orbitals of all the Bi cations in the Bi-Br6 octahedra and the ‘s’ orbitals of all the Ag cations. No contribution is made by the ‘p’ orbitals of the Bi cation in the Bi-Cl6 octahedra and the Cl- ions to the CBM. Similarly, in the WO-3.0 structures that contain one Bi-Cl6 and one Bi-Br6 octahedra, the VBM again consists of the ‘s’ orbitals of the Bi cation from the Bi-Br6 octahedra, ‘d’ orbitals of the Ag cation that is coordinated to 4 Br- ions and 2 Cl- ions, and the ‘p’ orbitals of the Br- ions. The CBM consists of the ‘p’ orbitals of the Bi cation in the Bi-Br6 octahedra and the ‘s’ orbitals of the Ag cation that is coordinated to 4 Br- ions. Once again, no contribution is made by the ‘p’ orbitals of the Bi cation in the Bi-Cl6 octahedra, the ‘s’ orbitals of the Ag cation coordinated to 4 Cl- and 2 Br- ions, and the Cl- ions to the CBM. Finally, in the WO-4.5 structures that contain three Bi-Cl6 octahedra and one Bi-Br6 octahedra, the VBM is comprised of the ‘s’ orbitals of the Bi cation belonging to the Bi-Br6 octahedra, ‘d’ orbitals of the three Ag cations that are coordinated by 4 Cl- and 2 Br- ions, and the ‘p’ orbitals of the halides (both Br- and Cl-) coordinated to the same Ag cations. The CBM has contributions from the ‘p’ and‘s’ orbitals of the aforementioned Bi and Ag cations, respectively. No contributions are made to the CBM or VBM by one Ag cation and the six Cl- ions that are coordinated to it. Thus we find that only the bromide ions contribute to the band edges as long as they remain the dominant halide ion in the double perovskite. Consequently, the band gap of these perovskites is smaller than the value of their MHO counterparts at the same halide compositions. Furthermore, these band gaps hardly change with increase in the chloride content, as long as the bromide ions remain the majority halide ions. For example, the band gap remains the same as the chloride content doubles from Bi-WO-1.5 (1.332 eV) to Bi-WO-3.0 structures (1.343 eV), as mentioned in Table S3. In fact, the inclusion of SOC effects causes a slight 13
reduction in the band gap as the chloride content increases from Bi-WO-1.5 (1.060 eV) to Bi-WO3.0 (1.034 eV). More importantly, these band gaps are even smaller than the band gaps of the pure bromide perovskites Cs2AgBiBr6. At 50% chloride composition (i.e., Bi-WO-3.0), the band gap, including SOC effects, is 0.342 eV smaller than its’ MHO counterpart. Thus, by exercising control over synthesis conditions and procedures so as to prepare mixed halide double perovskites in the WO structure, one can retain the more favourable bandgap of pure bromide double perovskite and yet have as much as 50% of the anions as chlorides in order to obtain enhanced ambient stability. Motivated by these results, we have synthesised pure Br based DP and Cl- incorporated mixed halide DPs, Cs2BiAgBr6-xClx by varying the stoichiometry of the precursors and various amount of solvents in order to enhance environmental stability and improve the performance of the DP materials. The synthesized materials were characterized by UV-Vis, PL, Raman spectroscopy, PXRD, TGA, and FESEM techniques.
4.2 EXPERIMENTAL STUDIES Halide double perovskites, namely, Cs2BiAgBr6Cl0 (CBAB) and Cs2BiAgBr6-xClx (CBABC), were synthesized from their precursor solutions by reacting at temperature 125 °C for 1h (Scheme 1). The synthesized perovskites CBAB and CBABC crystallized as reddish and yellowish orange compounds, respectively. These materials were hardly soluble in most of the common solvents except anhydrous DMSO. This can be attributed to stronger polarity of DMSO which is assumed to form stable adduct with DPs by donating its’ lone pair (44). Therefore, we have chosen anhydrous DMSO as the solvent for our experimental studies. Our chosen cations such as Ag+ and Bi3+ ions occupy the B’ and B” sites in the ordered double-perovskite lattice with different metalbromide/chloride bond lengths to some extent. Therefore, in order to examine the applicability of the synthesized mixed halide double perovskites, we have incorporated Bi3+ as a BIII site cation and Cs+ as A-site cation in the A2BIBIIIX6-xYx (X, Y = halide) perovskite framework.
14
To measure the absorption spectra, we have dissolved as-prepared DPs solution in DMSO and fabricated thin films of DPs on hot ITO substrate by spin coating. This preheating step has been performed in order to have good quality DPs film in terms of improved surface coverage which would in turn improve optical absorption. The onset of absorption in case of CBAB and CBABC DPs solution was observed at ~386 nm (λmax = 334 nm) and ~391 nm (λmax = 342 nm) (Fig. S14 and S15). On the other hand, the observed onset value for the thin film of CBAB and CBABC was found at ~625 nm (λmax = 513 nm) and ~630 nm (λmax = 513 nm), respectively (Fig. 3a). The observed red shift in the thin film UV-Vis spectra of CBAB and CBABC compared to their solution phase can be attributed to the strong exciton absorption peak, which is due to the confinement of excitons in the crystal structure of CBAB and CBABC planar film. From the UV-Visible experiment, slightest amount of changes in onset and corresponding band gap values has been observed for CBAB and CBABC in both solution and thin film. The band gap of the thin film from UV-vis study is estimated around 1.98 eV and 1.97 eV, respectively from the onset values. It is to be noted that the measured band gap values can vary to some extent for the same compound depending upon the various sample preparation methods and measurement techniques. The
Fig. 3 (a) UV-Vis spectra and (b) XRD pattern of thin films of CBAB and CBABC.
15
compounds with observed absorption onset are expected to be coloured compounds and this is in agreement with our synthesized yellow and reddish yellow coloured materials. From the UV-visible spectrum, we can hardly rationalize the differences found in the electronic structure of the synthesized mixed halide based double perovskites with the trivalent cations, Bi+3 (Fig. 3a). We have observed the corresponding photoluminescence peaks around 373 nm for both of the compounds in solution. From the PL spectra, we observed that slight changes induced upon Cl doping or incorporation in pure Br based DPs (Fig. S14 and S15). This finding is in correlation with experimentally observed UV-Vis spectra in solution phase. But, in our case, we are not able to observe PL peaks for films even after annealing at 250 °C though reports are available on achieving low intensity PL peaks for pure CBAB (25). This can be attributed to the weak radiative recombination. The green-coloured fluorescent PL images obtained by treating the unannealed thin film samples under an optical microscope correspond to the expected emission peak of double perovskite films (Fig. S16). On the basis of these observations, we propose to assign the photoluminescence peak to a phonon-assisted recombination across the indirect band gap and the leading edge of the optical absorption onset to the direct band gap (28). The preparation of good quality thin films of the double perovskites is challenging, especially for the bromide/chloride based DPs as they usually form non-uniform and discrete grain sized morphologies. Therefore, we have focused on preparing the thin film of DPs to carry out XRD measurements. The thin film X-ray diffraction (XRD) spectrum of the freshly prepared thin films of CBAB and CBABC have shown sharp peaks for both of the polycrystalline DPs (Fig.3b). We have obtained few other peaks other than peaks for our desired structure (cubic elpasolite) of the materials which originate from the unreacted precursor molecules and side phases such as AgBr and Cs3Bi2Br9. The diffraction peaks at 12.8°, 30.9° and 44.2° correspond to the side phases (45). Furthermore, it has been reported that annealing above 250 °C can possibly remove the unwanted side phases like Cs3Bi2Br9 and AgBr to produce pure DPs phase. It is to be noted that, owing to the presence of these unreacted precursors and side phases, PL signals are suppressed. The XRD peaks 16
40 20 0
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Fig. 4 (a) TGA plot showing comparable thermal stability for CBAB and CBABC (a) and Raman spectrum for CBAB (b) and CBABC (c), respectively.
change slightly after 30 days of air-exposing which indicates reasonable stability of the synthesized DPs (CBABC) (Fig. S17). The powder samples were also subjected to thermo gravimetric analysis to evaluate the thermal stability. The decomposition temperature was found to be 410 °C (Fig. 4a) in N2 gas flow. The flow rate is 10 °C/ minute. Thus, the studied materials have both exhibited excellent thermal stability. This can be attributed to the employment of Cs and enrichment of CsBr components which induce higher lattice strain resulting in moderate grain sized crystals and better stability than the organic counterpart (28). In addition, it is evident that incorporation of Cl does not impact the thermal stability of the DPs. The apparently observed minimal changes might be attributed to the low level of Cl doping and the meagre distortion of cubic elpasolite structure of CBAB to CBABC DP structure. Raman analysis has also been carried out for further structural analysis. The peaks obtained for phase pure CBAB were at 72.19 cm-1 and 174.29 cm-1 (Fig. 4b) while these peaks were at 83.19 cm-1 and 183.79 cm-1 for chlorine doped CBABC (Fig. 4c). Here, we have observed considerable difference in Raman shift in case of the CBABC than CBAB. The observed changes in the Raman spectrum can be attributed to changed lattice vectors, electron–phonon coupling alongwith carrier scattering in the perovskite crystals. 17
The powder samples exhibited a crystalline morphology, as shown in the top view FESEM images (Fig. 5, S18, S19), and further elemental composition was analyzed by EDS to investigate the stoichiometry of elements namely Cs, Bi, Ag, Br and Cl. EDS analysis showed a small amount of Cl incorporation (~7%, weight percentage) in the case of CBABC perovskites composition (Table S2) and stoichiometry i.e. molar proportion of elements is somewhat balanced in the final structure. By analysing FESEM and EDS data, we can assume the formation of two products such as bromide-based phase pure CBAB (Cs2BiAgBr6-0Cl0) (Table S3) and chloride doped CBABC (Cs2BiAgBr6-xClx). The observed difference in the stoichiometry is attributed to the error in the EDS measurement and the presence of unreacted compounds. In this regard, Colella, S. et al have also found out that Cl incorporation in a perovskite structure is at a very low concentration (below 3−4%) irrespective of the components ratio in the precursor solution (46). It is reported that the DP composition is not possible to be tuned extensively through the variation of Cl/ Br molar ratio in the subjected precursor solutions. This can be attributed to the strong difference in the halogen ionic
Fig. 5 FESEM images of powder samples of CBABC and elemental images indicating the presence and uniform distribution of Cs, Bi, Ag, Br and Cl in polycrystalline DPs. 18
radii interferes with the formation of a continuous solid solution. However, the Cl incorporation in the structure would remarkably improve the charge transport within the perovskite layer. Therefore, our results show that both enhanced stability and favourable band gap can be realized using mixed halide double perovskites. In particular, by exercising control over synthesis conditions and procedures so as to prepare mixed halide double perovskites in the WO structure, one can retain the more favourable bandgap of pure bromide double perovskite and yet have as much as 50% of the anions as chlorides to obtain enhanced ambient stability. 5. CONCLUSION Mixed halide double perovskites (DPs) based on Silver, Bismuth, Chlorine and Bromine have been synthesized. Density functional theory calculations together with detailed experimental calculations show that these materials are promising photovoltaic absorbers owing to their ideal bandgap and high stability in ambient conditions. The theoretical results further show that the band gap and the nature of the band edges are intimately related to the nature of the halide ions constituting the BiX6 octahedra. When the BiX6 octahedra contains both chlorine and bromine ions, the bandgap increases linearly with increase in the chloride content and both the halide ions contribute to the valence band edge. On the other hand, when the octahedra contains only one kind of halide ion (Br or Cl), the bandgap remains nearly equal to that of the bromide DP upto 50% Cl doping. In this case, only the bromide ions contribute to the valence band edge, with no contribution arising from the chloride ions. ACKNOWLEDGMENT MKC acknowledge IIT Roorkee for Post Doctoral Fellowship. SS acknowledge DST Solar Energy Research Grant [Grant No: DST SERI- S147].
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Highlights: Fighting climate change demands a boost in the development of renewable energies. Solar energy based on novel photovoltaics cells is a potential renewable energies technology. Halide perovskites are one of the most promising materials for delivery of the next generation of solar cells. A currently intensely debated topic in perovskite solar cells concerns the use of lead, a constituent of most of the halide perovskites so far demonstrated as effective photovoltaic materials. Here, we discuss the synthesis, characterization and electronic structure calculations on lead-free Bi-based mixed-halide double perovskites of Cs2AgBiClxBr6-x stoichiometry. Some of the novelty and highlights of this paper are the following: In this paper, the tuning of electronic properties and enhancement in stability is obtained by compositional mapping with different composition in halide-based double perovskites. We have systematically carried out the theoretical investigation of incorporation of Cl in the Cs2AgBiBr6 structure by Density Functional Theory (DFT) calculations in order to understand the bulk and electronic structures of the mixed halide-based double perovskites Cs2AgBiClxBr6-x (x = 0 to 6). We have also synthesized and characterized the mixed halide double perovskites, namely Cs2BiAgBr6 (CBAB) and Cs2BiAgBr6-xClx (CBABC) for two different compositions to verify our computational study. 21
We have shown that both the bandgap and the nature of the band edges of these materials are intimately related to the kind of halides constituting the BiX6 octahedra. This kind of observation of structure-property is very important to understand the stability in these types of devices. The detailed computational studies and synthesis and characterization of mixed halide-based double perovskites with high stability as discussed here will be an important step for the development of next generation lead free double perovskite solar cells.
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S0301-0104(19)30933-4 https://doi.org/10.1016/j.chemphys.2019.110547 CHEMPH 110547
To appear in:
Chemical Physics
Received Date: Revised Date: Accepted Date:
4 August 2019 2 October 2019 4 October 2019
Please cite this article as: M. Kumar Chini, S. Goverapet Srinivasan, N. Tailor, Yukta, D. Salahub, S. Satapathi, Lead-Free, Stable Mixed Halide Double Perovskites Cs2AgBiBr6 and Cs2AgBiBr6-xClx- A Detailed Theoretical and Experimental Study, Chemical Physics (2019), doi: https://doi.org/10.1016/j.chemphys.2019.110547
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Lead-Free, Stable Mixed Halide Double Perovskites Cs2AgBiBr6 and Cs2AgBiBr6-xClx- A Detailed Theoretical and Experimental Study Mrinmoy Kumar Chini1, Sriram Goverapet Srinivasan2, Naveen Tailor1, Yukta1, Dennis Salahub3,4,5, and Soumitra Satapathi*,1 1Department 2Tata
of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, 247667, India
Research Development and Design Center, TCS Research, 54B Hadapsar Industrial Estate, Pune, Maharashtra 411013, India
3Department
of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4 4Centre for Molecular Simulation, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4 5Institute for Quantum Science and Technology, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4
*Corresponding Author: Soumitra Satapathi, Email ID: [email protected]
ABSTRACT Recently, lead-free halide-based double perovskites (DPs) have emerged as a promising candidate for photovoltaic and optoelectronic applications. Here we report the synthesis, characterization and electronic structure calculations on lead-free Bi-based mixed-halide double perovskites of Cs2AgBiClxBr6-x stoichiometry. The introduction of Cl- dopant converts these DPs from cubic to orthorhombic or tetragonal crystal structures. Electronic structure calculations indicate that both the bandgap and the nature of the band edges of these materials are intimately related to the kind of halides constituting the BiX6 octahedra. When the BiX6 octahedra contains both Cl- and Br- ions, the band gap increases linearly with increase in chloride content. On the other hand, when the BiX6 octahedra comprise of only one halide (Br- or Cl-) ion, the band gap remains nearly equal to that of Cs2AgBiBr6 until the Cl- dopant concentration reaches 50%. Experimental characterization of the synthesized mixed-halide DPs show that the band gap value of 1.98 eV and 1.97 eV, obtained for Cs2AgBiBr6 and Cs2AgBiClxBr6-x, respectively, matches with our theoretical calculations. The PXRD spectrum indicates polycrystallinity in these DPs and changes slightly after one month, 1
showing reasonably good stability. The elemental analysis showed incorporation of a small amount of Cl- (7 weight%) in the perovskite composition. In summary, we have performed detailed computational studies and synthesized and characterized mixed halide-based DPs with high stability for solar cells application. KEYWORDS: Double perovskite, band gap, density functional theory, mixed halide, non toxicity 1. INTRODUCTION The current focus on the introduction and development of stable, non toxic and environmentally friendly photovoltaic materials with suitable band gaps in the visible region attracts a remarkable interest for the synthesis and computational study of lead-free halide-based double perovskites (DPs). Previously, lead-based halide perovskites have achieved power conversion efficiencies as high as ~24.2% (1), due to their excellent physical and optical properties such as solution processibility (2), efficient light absorption (3), excellent charge carrier mobility along with long diffusion length (4). However, the presence of lead in organometallic perovskites has raised concerns regarding toxicity (5-7) and stability issues (8-11) which prevent the widespread deployment of this technology for commercial applications (12-19). Consequently, a tremendous urge for the development of lead-free perovskite materials, has led researchers to the synthesis of several lead-free perovskite materials. Among them, the substitution of Pb2+ by heterovalent M3+ cations such as non-toxic Bi3+, which is isoelectronic with Pb2+ has been accomplished. But incorporation of a highly charged Bi3+ ion into the three-dimensional A1+M2+X3 structure resulted in undesirable optoelectronic properties compared to the lead-based counterparts (20-23). This was overcome by introducing Bi3+ ions into the Elpasolite structure which is commonly known as the double perovskite structure having general formula A2M1+M3+X6 where, M1+ and M3+ represent monovalent and trivalent cations, respectively (24). Recently, several reports on Bi3+-based double perovskites with Ag+ as the monovalent cation with favourable band gap, excellent photoluminescence lifetime, high stability, long carrier recombination lifetime and comparable 2
charge carrier effective masses have been reported, rendering them as a very promising material for photovoltaic applications (24-29). McClure et al. have synthesized Cs2AgBiBr6 and Cs2AgBiBCl6 and have demonstrated favourable band gaps and superior moisture stability for these double perovskites compared to CH3NH3PbX3 (24). Slavney et al. have shown excellent photo-physical properties of Cs2AgBiBr6 compounds (25). Karmakar et al. have successfully prepared low band gap DP material including Cu(II)-Doped Cs2AgSbBr6 (30). Recently, Gao et al. have demonstrated high-quality Cs2AgBiBr6 DP film for inverted planar heterojunction solar cells with moderate Efficiency (2.2%) (31). Filip et al. have indicated that mixed cations-based Cs2BiAg1−xCuxCl6 can be synthesized with a reduced band gap (~1.6−1.9 eV) (32). Li et al. have synthesized a remarkably stable hybrid lead-free (CH3NH3)2AgSbI6 DP which has turned out to be a promising light absorber with a suitable optical band gap (1.93 eV) for photo-voltaic applications (33). Therefore, lead-free DPs, especially Cs2AgBiBr6, have enormous potential as an alternative to the lead-based counterpart with low carrier effective masses and low band gaps. The tuning of electronic properties and enhancement in stability can be obtained by compositional mapping with different composition in halide-based double perovskites (26-28). These reported double-halide perovskites possess the advantages of absorption spectra in the visible range of the solar spectrum and significant stability under ambient conditions (29,30) along with nontoxicity which favours them as excellent candidates for photovoltaics applications. However, the long term stability of these materials continues to remain a concern. Doping the hybrid organic-inorganic perovskite MAPbI3 with Cl- ions (in order to form mixed-halide perovskites of MAPbI3-xClx stoichiometry) was shown to nearly retain the bandgap of MAPbI3 but result in an improved charge transport within the active perovskite layer as well as a better solar cell performance. However, to the best of our knowledge, no reported studies deal with the variation in halide composition of the DPs and its effect on the stability and optoelectronic properties.
3
In this work, we have systematically carried out the theoretical investigation of incorporation of Cl in the Cs2AgBiBr6 structure by Density Functional Theory (DFT) calculations in order to understand the bulk and electronic structures of the mixed halide-based double perovskites Cs2AgBiClxBr6-x (x = 0 to 6). We have also synthesized and characterized the mixed halide double perovskites, namely Cs2BiAgBr6 (CBAB) and Cs2BiAgBr6-xClx (CBABC) for two different compositions to verify our computational study. We show that the bandgap of the mixed halide perovskites is strongly dependent on the nature of the halides constituting the BiX6 octahedra. If the BiX6 octahedra contain both Br- and Cl- ions, the band gap is seen to increase linearly with the chloride content, whereas the bandgap remains nearly a constant (equal to the band gap of Cs2AgBiBr6) until 50% Cl- doping, if only one kind of halide ion constitutes a BiX6 octahedra. In the former case, the contribution to the valence band edge arises from both Cl and Br ions while in the latter, only Br- ions contribute to the valence band edge as long as they remain the dominant anion. Furthermore, we have also synthesized and characterized these proposed mixed halidesbased lead-free double perovskites and compared their photophysical properties. Our results show that Cl-doped mixed halide double perovskites are promising photovoltaic materials as they depict a favorable band gap and enhanced ambient stability simultaneously. 2. COMPUTATIONAL METHODS AND DETAILS Density Functional Theory (DFT) calculations were carried out to understand the bulk and electronic structures of the mixed halide double perovskites Cs2AgBiClxBr6-x (x = 0 to 6). All calculations were carried out using the VASP planewave software.[34-37] The valence electronic states were expanded in a basis of planewaves up to a planewave kinetic energy of 600 eV. The Projector Augmented Wave (PAW) approach[38,39] was used to describe the core–valence interaction, while the
PBE GGA functional[40] was used to describe the electron exchange-
correlation interactions. Geometry optimizations were deemed to have converged when the forces on the atoms dropped below 10-3 eV/A, while the SCF convergence threshold was set to 10-7 eV. 4
Complete cell optimizations followed by re-relaxations were carried out for all the systems in order to avoid any artifacts due to Pulay stress. During the SCF procedure, a Pulay mixing scheme was used for charge density mixing (41). The Brillouin zone was sampled using an (8x8x8) Gammacentred k-point mesh. Spin Orbit Coupling (SOC) was included in the computation of the electronic band structure and band gap. The SOC electronic structure was computed using the geometry optimized with the PBE GGA functional. For all structures, ALFOW-ONLINE (42) was used to obtain the coordinates of the high symmetry points in the Brillouin zone for band structure calculations. Molecular graphics were generated using the VESTA software (43). 3. EXPERIMENTAL METHODS Cesium bromide (CsBr) (99%), Silver bromide (AgBr), Bismuth (III) bromide (BiBr3) and Bismuth (III) chloride (BiCl3) (98%) were purchased from Avra Synthesis Pvt. Ltd. and used without any further purification. Deionised water and hydrochloric acid/ hydrobromic acid were used as solvents. FESEM images were taken by FESEM QUANTA 200 FEG under an electron excitation energy of 20 keV using an ETD detector at a working distance of 9.5 mm. Differential thermal analysis (DTA) and thermogravimetric analysis (TG) analysis were performed using an EXSTAR TG/DTA 6300 instrument. In these experiments, the sample (10.51 mg) was placed with reference alumina powder (10.5 mg) under nitrogen gas flow (200 ml/min) and heated at a rate of 10oC/min in the temperature range of 34 °C to 900 °C. Ultraviolet-visible absorption spectra of the polycrystalline materials were collected using a Microprocessor UV-Vis Double Beam spectrophotometer LI-2800. Photoluminescence measurements were performed with an RF-6000 Spectro-Fluorophotometer. Synthesis of Double Perovskites Polycrystalline materials of halide, Cs2BiAgBr6Cl0 (CBAB) and mixed halide double perovskites, Cs2BiAgBr6-xClx (CBABC) were synthesized from a concentrated HBr/ HCl solution
5
containing stoichiometric CsBr (2mM), AgBr (1mM), and BiBr3/ BiCl3 (1mM), respectively, at a temperature of 125 °C for 1h. The detailed synthetic method is described below (25). Synthesis of Cs2BiAgBr6Cl0 (CBAB) The initial solution was prepared by dissolving 1 mM AgBr and 1 mM BiBr3 in acidic solution of concentrated HBr (12 mL, 8.84 M), followed by stirring at 90 °C for 10 minutes. Then, 2 mM CsBr in solution of HBr was added slowly to the previous reaction mixture, with continuous stirring for 1 h at 125 °C to ensure the completion of the reaction. Finally the solution was gradually cooled down to 70 °C at a cooling rate of 20 °C/ h. Upon cooling down to room temperature, we obtained orange-coloured polycrystalline material of Cs2BiAgBr6Cl0 composition. The obtained material was washed with ethanol and dried under vacuum. Synthesis of Cs2BiAgBr6-xClx (CBABC) The procedure for the synthesis of Cs2BiAgBr6-xClx was similar to that of Cs2BiAgBr6Cl0. Here, we had taken BiCl3 instead of BiBr3 in acidic solution of concentrated HCl. In this case also, we have obtained the orange-coloured polycrystalline material of Cs2BiAgBr6-xClx.
.
BiBr3
AgBr
BiCl3
AgBr
2CsBr HBr, 125 oC 2CsBr HCl/ HBr, 125 oC
Cs2BiAgBr6-0Cl0
Cs2BiAgBr6-xClx
Scheme 1 Reaction scheme for Cs2BiAgBr6Cl0 and Cs2BiAgBr6-xClx
Double perovskite film preparation The synthesized polycrystalline double perovskite compounds were dissolved (60mg/ 1 mL) in anhydrous DMSO solvent (99.9%, Sigma-Aldrich) by sonication for 10 minutes followed by heating at 75 °C. Following complete dissolution of the DPs, the studied DPs (100 μL) were spincoated on top of the preheated substrate (Cleaned ITO) at 75 °C at 1000 rpm for 30 s. The 6
substrates were then initially annealed at 75 °C for 15 min in a controlled-humidity chamber. Again, a 2nd set of spin-coated substrates were subsequently annealed at 250 °C for 5 min in the humidity chamber to have the desired DP phase. The treated substrates were then subjected to UV-Vis, photo-luminescence (PL), Thermogravimetric analysis, Raman and thin film XRD measurements. 4. RESULTS AND DISCUSSIONS 4.1 Density Functional Theory Calculations DFT calculations were carried out to understand the changes in the structural and electronic properties upon the formation of mixed-halide double perovskites. The halide double perovskites A2B’B”X6 (A = Cs, B’ = Ag, B” = Bi, X =Cl/Br) crystallize in the cubic elpasolite structure (Fm3m space group), containing connected corner-sharing B’X6 and B”X6 octahedra. The B’X6 and B”X6 octahedra are arranged in an alternating fashion such that each of the B’ and B” cations are surrounded by six B”X6 and B’X6 octahedra, respectively. The A cation resides at the centre of the cuboctahedral cavity formed by eight octahedra (i.e., four pairs of B’- and B”-centered octahedra). The lattice parameters and unit cell volumes for the chloride and bromide double perovskites are reported in Table 1, showing that our DFT based values are in very good agreement with experiments and earlier reported theoretical values. Table 1 DFT optimized and experimental lattice parameters and unit cell volumes for two halide double perovskites. DFT calculations utilized a planewave kinetic energy cut off of 600 eV and PBE GGA exchange correlational functional. The lattice parameters correspond to a conventional unit cell containing 40 atoms (4 formula units).
Composition
Lattice Parameter (Å)
Unit Cell Volume (Å3)
DFT
Experiment11
DFT
Experiment11
Cs2AgBiCl6
10.965
10.777
1318.41
1251.82
Cs2AgBiBr6
11.496
11.271
1519.30
1431.86
7
Two different kinds of system were considered to model the mixed halide perovskites. In the first kind, the bromide ions of the BiBr6 octahedra were sequentially replaced by chloride ions resulting in five intermediate compositions, with ‘x’ varying from 1 to 5. These systems were modelled using their primitive cell consisting of 10 atoms (1 formula unit). Fig. 4 shows representative BiClxBr6-x octahedra for ‘x’ varying from 0 to 6. We name these structures Bi-MHO-xi, MHO being the acronym for ‘Mixed Halide Octahedra’, x corresponding to the number of bromide ions replaced by chloride ions, and ‘i’ corresponding to different configurations of the chloride ions (if any) at a given value of ‘x’.
Fig. 1 Representative structures of the BiClxBr6-x octahedra at various values of ‘x’ ranging from 0 to 6. Bi atoms are purple, Br atoms are brown and Cl atoms are green in colour. For instance, at x = 2, two different configurations (Bi-MHO-2a and Bi-MHO-2b, Fig. 1c and 1d respectively) of the chloride ions were considered. In Bi-MHO-2a configuration (Fig. 1c), one axial and one equatorial bromide ion of the BiBr6 octahedra was replaced by a chloride ion, while in BiMHO-2b, two diametrically opposite bromide ions were replaced by chloride ions. Similarly, in BiMHO-3a (Fig. 1e), two equatorial and one axial bromide ions were replaced by chloride ions while in Bi-MHO-3b (Fig. 1f), three equatorial bromide ions were replaced by chloride ions. Finally, in 8
Bi-MHO-4a (Fig. 1g), three equatorial and one axial bromide ions were replaced by chloride ions while in Bi-MHO-4b (Fig. 1h), all the four equatorial bromide ions were replaced by chloride ions. In the second kind of system, one, two and three of the BiBr6 octahedra were replaced by BiCl6 octahedra to obtain three compositions, Cs2AgBiCl1.5Br4.5, Cs2AgBiCl3Br3 and Cs2AgBiCl4.5Br1.5 respectively. The primitive cell for CsAgBiCl1.5Br4.5 and CsAgBiCl4.5Br1.5 contained 40 atoms each while that for Cs2AgBiCl3Br3 contained 20 atoms. We call these structures Bi-WO-x, where WO stands for ‘Whole Octahedra’ and x corresponds to the proportion of Cl in the double perovskite. Table 2 lists the optimized lattice parameters, unit cell volumes and the space group of the mixed halide perovskites. Table 2 Optimized lattice parameters, unit cell volumes and space groups of the various mixed halide double perovskites. All calculations were done on the primitive unit cells.
System
a (Å)
b (Å)
c (Å)
α (deg.) β (deg.) γ (deg.)
Volume (Å3)
Space Group
Bi-MHO-1
8.116
8.031
8.031
60.701
59.650
59.650
370.113
I4mm
Bi-WO-1.51 11.380 11.380 11.380
90.00
90.00
90.00
1473.80
Pm-3m
Bi-MHO-2a 8.025
7.938
8.025
60.357
59.363
60.357
361.609
Imm2
Bi-MHO-2b 8.112
7.950
7.950
61.359
59.321
59.321
362.208
I4/mmm
Bi-MHO-3a 7.941
7.932
7.941
59.965
60.00
59.965
353.483
R3m
Bi-MHO-3b 7.858
7.934
8.027
58.982
59.931
61.087
353.700
Fmm2
Bi-WO-3.02 7.971
7.971 11.234
90.00
90.00
90.00
713.791
P4/mmm
Bi-MHO-4a 7.935
7.848
7.848
60.668
59.633
59.633
345.342
Imm2
Bi-MHO-4b 7.933
7.785
7.933
60.617
58.767
60.617
346.228
I4/mmm
Bi-WO-4.51 11.118 11.118 11.118
90.00
90.00
90.00
1374.305
Pm-3m
Bi-MHO-5
60.310
59.379
60.310
337.335
I4mm
7.839
7.765
7.839
1Primitive
unit cell contains 40 atoms (4 formula units)
2Primitive
unit cell contains 20 atoms (2 formula units)
From Table 2, we notice that while the pure halide double perovskites were cubic in nature, the structures of all the mixed halide perovskites are either tetragonal or orthorhombic in nature. Furthermore, as the ‘x’ varies from 0 to 6, the unit cell volumes (and the densities) of the mixed
9
halide perovskites decrease monotonically due to an increase in the proportion of the smaller sized chloride ions. In all the double perovskites studied here, the Ag and Bi cations are octahedrally coordinated by the halide ions. However, not all of the Ag or Bi cations are equivalent to each other in all the mixed halide double perovskites. For instance, in the WO-1.5 systems, one of the Ag cations is coordinated to 6 bromide ions while the other three Ag cations are equivalent to each other and coordinated to 4 bromide and 2 chloride ions. Similarly, in the WO-3 systems, two of the Ag cations are coordinated to 4 bromide and 2 chloride ions, while the other two Ag cations are coordinated to 2 bromide and 4 chloride ions. On the other hand, in the MHO systems, all the Ag (or Bi) cations are equivalent to each other, each being coordinated to x chloride and (6-x) bromide ions. Consequently, the Ag (Bi)-X bond distances are dependent on both the system kind (i.e., MHO vs WO) and the proportion of the 2nd halide in the structure. Table 3 lists the Ag-Cl, Ag-Br, Bi-Cl and Bi-Br bond distances in all the structures studied here. Table 3 Ag-X and Bi-X bond distances (in Å) in various mixed halide double perovskites.
System
Bi-Cl
Bi-Br
Ag-Cl
Ag-Br
Cs2AgBiBr6
-
2.884
-
2.860
Bi-MHO-1
2.746
2.884, 2.866
2.741
2.857, 2.883
Bi-WO-1.5
2.749
2.889, 2.861
2.941
2.801, 2.829
Bi-MHO-2a
2.747
2.884, 2.867
2.739
2.852, 2.884
Bi-MHO-2b
2.727
2.886
2.776
2.850
Bi-MHO-3a
2.749
2.866
2.734
2.883
Bi-MHO-3b
2.750, 2.730
2.884, 2.866
2.764, 2.737
2.890, 2.847
Bi-WO-3
2.756, 2.728
2.890, 2.859
2.889, 2.880
2.778, 2.727
Bi-MHO-4a
2.749, 2.730
2.869
2.764, 2.733
2.871
Bi-MHO-4b
2.730
2.885
2.774
2.828
Bi-WO-4.5
2.764, 2.730
2.853
2.829, 2.795
2.706
Bi-MHO-5
2.748, 2.730
2.867
2.762, 2.724
2.850
Cs2AgBiCl6
2.731
-
2.749
-
10
From Table 3, we see that while the Bi-X bond distances vary little across various structures, the Ag-X bond distances are more strongly dependent on whether the mixed halide perovskites are of the MHO or WO kind. Specifically, in the WO structures, we see that the Ag-Cl and Ag-Br bond distances are longer and shorter respectively, compared to their values in the MHO structures. The influence of these structural features is profoundly reflected in the band gaps of these systems. Fig. 2 shows the variation in the band gap (with SOC) of the Cs2AgBiClxBr6-x double perovskites as a function of ‘x’ for both MHO and WO systems while Fig. S1 to S13 in the supporting information show the band structure (with and without SOC) of these double perovskites. Table S1 in the supporting information provides the band gaps computed with and without SOC included.
Fig. 2 Variation in the band gaps of the Cs2AgBiClxBr6-x mixed halide double perovskites with increase in ‘x’. The dotted line is a linear fit to the band gaps of the MHO type mixed halide double perovskites. All the perovskites show an indirect band gap. From Fig. 2, S1 to S13 and Table S1, we notice that, generally, the band gap of the mixed halide perovskites increase as the amount of chlorine content increases since the chlorine ‘p’ orbitals are both less diffuse and lie lower in energy than the bromine ‘p’ orbitals. Furthermore, the band gaps of all the mixed halide double perovskites are indirect in nature. However, the specific features of 11
the band structure in terms of the atoms and orbitals contributing to the band edges and the band gap of the mixed halide perovskites are strongly dependent on the kind of its structure (i.e., MHO vs WHO). For the pure halide double perovskites, Cs2AgBiBr6 (Fig. S1) and Cs2AgBiCl6 (Fig. S10), the top of the valence band (VBM) and the bottom of the conduction band (CBM) lie at the X (π/a, 0,π/a) and L (π/a,π/a,π/a) points in the Brillouin zone, respectively. The VBM consists of the Bi ‘s’, Ag ‘d’ and halide ‘p’ orbitals while the CBM is predominantly comprised of the Bi ‘p’ and Ag ‘s’ orbitals. Inclusion of SOC results in the formation of an isolated conduction band in the middle of the GGA band gap, which comprises the Bi ‘p’ and Ag ‘s’ states, together with a minor contribution from the halide ‘p’ states. The reduction in the band gap upon inclusion of SOC effects is larger for Cs2AgBiCl6 (0.26 eV) compared to Cs2AgBiBr6 (0.17 eV) (see Table S1). These findings are in line with earlier reports on the electronic structure of these pure halide double perovskites (24,26). With increase in the chloride content, the electronic features of the mixed double halide perovskites in the MHO structure continue to retain the same qualitative features. Since all the Ag (and Bi) cations are equivalent to each other in these structures, the magnitude of the band gap depends only on the amount of chloride ions in the material and increases linearly with increase in the chloride content. Similarly, the reduction in the band gap upon inclusion of SOC effects also increases linearly with the chloride content. However, the degeneracy of the ‘split off’ conduction band state due to SOC depends on the local coordination environment of the Bi (and consequently Ag) cation. In the pure halide double perovskites; this state is twofold degenerate throughout the Brillouin zone. However, in all MHO double perovskites other than Bi-MHO-2b and Bi-MHO-4b, the Bi (and consequently Ag) cations are ‘off centred’ in the BiClxBr6-x octahedra, due to two different Bi-Br and/or Bi-Cl distances. As a result, the split-off conduction band state does not remain two-fold degenerate throughout the Brillouin zone. This degeneracy is recovered for Bi-MHO-2b (Fig. S9) and BiMHO-4b (Fig. S13) structures wherein the distances of all the Cl (or Br) ions to the Bi cations are the same, resulting in no off-centring of the BiClxBr6-x octahedra. 12
In contrast to the MHO structures, the WO structures show band-structure contributions arising from only a subset of the cations and anions depending on their local coordination environments. For instance, in the WO-1.5 structures, there are three Bi-Br6 octahedra and one Bi-Cl6 octahedra, resulting in the VBM predominantly consisting of ‘s’ orbitals of the Bi cation from the Bi-Br6 octahedra, ‘d’ orbitals of the Ag cation and the ‘p’ orbitals of the Br- ion. The CBM predominantly consists of the ‘p’ orbitals of all the Bi cations in the Bi-Br6 octahedra and the ‘s’ orbitals of all the Ag cations. No contribution is made by the ‘p’ orbitals of the Bi cation in the Bi-Cl6 octahedra and the Cl- ions to the CBM. Similarly, in the WO-3.0 structures that contain one Bi-Cl6 and one Bi-Br6 octahedra, the VBM again consists of the ‘s’ orbitals of the Bi cation from the Bi-Br6 octahedra, ‘d’ orbitals of the Ag cation that is coordinated to 4 Br- ions and 2 Cl- ions, and the ‘p’ orbitals of the Br- ions. The CBM consists of the ‘p’ orbitals of the Bi cation in the Bi-Br6 octahedra and the ‘s’ orbitals of the Ag cation that is coordinated to 4 Br- ions. Once again, no contribution is made by the ‘p’ orbitals of the Bi cation in the Bi-Cl6 octahedra, the ‘s’ orbitals of the Ag cation coordinated to 4 Cl- and 2 Br- ions, and the Cl- ions to the CBM. Finally, in the WO-4.5 structures that contain three Bi-Cl6 octahedra and one Bi-Br6 octahedra, the VBM is comprised of the ‘s’ orbitals of the Bi cation belonging to the Bi-Br6 octahedra, ‘d’ orbitals of the three Ag cations that are coordinated by 4 Cl- and 2 Br- ions, and the ‘p’ orbitals of the halides (both Br- and Cl-) coordinated to the same Ag cations. The CBM has contributions from the ‘p’ and‘s’ orbitals of the aforementioned Bi and Ag cations, respectively. No contributions are made to the CBM or VBM by one Ag cation and the six Cl- ions that are coordinated to it. Thus we find that only the bromide ions contribute to the band edges as long as they remain the dominant halide ion in the double perovskite. Consequently, the band gap of these perovskites is smaller than the value of their MHO counterparts at the same halide compositions. Furthermore, these band gaps hardly change with increase in the chloride content, as long as the bromide ions remain the majority halide ions. For example, the band gap remains the same as the chloride content doubles from Bi-WO-1.5 (1.332 eV) to Bi-WO-3.0 structures (1.343 eV), as mentioned in Table S3. In fact, the inclusion of SOC effects causes a slight 13
reduction in the band gap as the chloride content increases from Bi-WO-1.5 (1.060 eV) to Bi-WO3.0 (1.034 eV). More importantly, these band gaps are even smaller than the band gaps of the pure bromide perovskites Cs2AgBiBr6. At 50% chloride composition (i.e., Bi-WO-3.0), the band gap, including SOC effects, is 0.342 eV smaller than its’ MHO counterpart. Thus, by exercising control over synthesis conditions and procedures so as to prepare mixed halide double perovskites in the WO structure, one can retain the more favourable bandgap of pure bromide double perovskite and yet have as much as 50% of the anions as chlorides in order to obtain enhanced ambient stability. Motivated by these results, we have synthesised pure Br based DP and Cl- incorporated mixed halide DPs, Cs2BiAgBr6-xClx by varying the stoichiometry of the precursors and various amount of solvents in order to enhance environmental stability and improve the performance of the DP materials. The synthesized materials were characterized by UV-Vis, PL, Raman spectroscopy, PXRD, TGA, and FESEM techniques.
4.2 EXPERIMENTAL STUDIES Halide double perovskites, namely, Cs2BiAgBr6Cl0 (CBAB) and Cs2BiAgBr6-xClx (CBABC), were synthesized from their precursor solutions by reacting at temperature 125 °C for 1h (Scheme 1). The synthesized perovskites CBAB and CBABC crystallized as reddish and yellowish orange compounds, respectively. These materials were hardly soluble in most of the common solvents except anhydrous DMSO. This can be attributed to stronger polarity of DMSO which is assumed to form stable adduct with DPs by donating its’ lone pair (44). Therefore, we have chosen anhydrous DMSO as the solvent for our experimental studies. Our chosen cations such as Ag+ and Bi3+ ions occupy the B’ and B” sites in the ordered double-perovskite lattice with different metalbromide/chloride bond lengths to some extent. Therefore, in order to examine the applicability of the synthesized mixed halide double perovskites, we have incorporated Bi3+ as a BIII site cation and Cs+ as A-site cation in the A2BIBIIIX6-xYx (X, Y = halide) perovskite framework.
14
To measure the absorption spectra, we have dissolved as-prepared DPs solution in DMSO and fabricated thin films of DPs on hot ITO substrate by spin coating. This preheating step has been performed in order to have good quality DPs film in terms of improved surface coverage which would in turn improve optical absorption. The onset of absorption in case of CBAB and CBABC DPs solution was observed at ~386 nm (λmax = 334 nm) and ~391 nm (λmax = 342 nm) (Fig. S14 and S15). On the other hand, the observed onset value for the thin film of CBAB and CBABC was found at ~625 nm (λmax = 513 nm) and ~630 nm (λmax = 513 nm), respectively (Fig. 3a). The observed red shift in the thin film UV-Vis spectra of CBAB and CBABC compared to their solution phase can be attributed to the strong exciton absorption peak, which is due to the confinement of excitons in the crystal structure of CBAB and CBABC planar film. From the UV-Visible experiment, slightest amount of changes in onset and corresponding band gap values has been observed for CBAB and CBABC in both solution and thin film. The band gap of the thin film from UV-vis study is estimated around 1.98 eV and 1.97 eV, respectively from the onset values. It is to be noted that the measured band gap values can vary to some extent for the same compound depending upon the various sample preparation methods and measurement techniques. The
Fig. 3 (a) UV-Vis spectra and (b) XRD pattern of thin films of CBAB and CBABC.
15
compounds with observed absorption onset are expected to be coloured compounds and this is in agreement with our synthesized yellow and reddish yellow coloured materials. From the UV-visible spectrum, we can hardly rationalize the differences found in the electronic structure of the synthesized mixed halide based double perovskites with the trivalent cations, Bi+3 (Fig. 3a). We have observed the corresponding photoluminescence peaks around 373 nm for both of the compounds in solution. From the PL spectra, we observed that slight changes induced upon Cl doping or incorporation in pure Br based DPs (Fig. S14 and S15). This finding is in correlation with experimentally observed UV-Vis spectra in solution phase. But, in our case, we are not able to observe PL peaks for films even after annealing at 250 °C though reports are available on achieving low intensity PL peaks for pure CBAB (25). This can be attributed to the weak radiative recombination. The green-coloured fluorescent PL images obtained by treating the unannealed thin film samples under an optical microscope correspond to the expected emission peak of double perovskite films (Fig. S16). On the basis of these observations, we propose to assign the photoluminescence peak to a phonon-assisted recombination across the indirect band gap and the leading edge of the optical absorption onset to the direct band gap (28). The preparation of good quality thin films of the double perovskites is challenging, especially for the bromide/chloride based DPs as they usually form non-uniform and discrete grain sized morphologies. Therefore, we have focused on preparing the thin film of DPs to carry out XRD measurements. The thin film X-ray diffraction (XRD) spectrum of the freshly prepared thin films of CBAB and CBABC have shown sharp peaks for both of the polycrystalline DPs (Fig.3b). We have obtained few other peaks other than peaks for our desired structure (cubic elpasolite) of the materials which originate from the unreacted precursor molecules and side phases such as AgBr and Cs3Bi2Br9. The diffraction peaks at 12.8°, 30.9° and 44.2° correspond to the side phases (45). Furthermore, it has been reported that annealing above 250 °C can possibly remove the unwanted side phases like Cs3Bi2Br9 and AgBr to produce pure DPs phase. It is to be noted that, owing to the presence of these unreacted precursors and side phases, PL signals are suppressed. The XRD peaks 16
40 20 0
200
400
600
800
Temperature (C)
CBAB
Intensity (Counts)
60
Intensity (Counts)
Weight (%)
CBABC CBAB
80
0
850
5000
100
4000 3000 2000 1000 0
0
100
200
300
400 -1
Raman Shift (cm )
500
CBABC
800 750 700 650 600
0
100
200
300
400
500
-1
Raman Shift (cm )
Fig. 4 (a) TGA plot showing comparable thermal stability for CBAB and CBABC (a) and Raman spectrum for CBAB (b) and CBABC (c), respectively.
change slightly after 30 days of air-exposing which indicates reasonable stability of the synthesized DPs (CBABC) (Fig. S17). The powder samples were also subjected to thermo gravimetric analysis to evaluate the thermal stability. The decomposition temperature was found to be 410 °C (Fig. 4a) in N2 gas flow. The flow rate is 10 °C/ minute. Thus, the studied materials have both exhibited excellent thermal stability. This can be attributed to the employment of Cs and enrichment of CsBr components which induce higher lattice strain resulting in moderate grain sized crystals and better stability than the organic counterpart (28). In addition, it is evident that incorporation of Cl does not impact the thermal stability of the DPs. The apparently observed minimal changes might be attributed to the low level of Cl doping and the meagre distortion of cubic elpasolite structure of CBAB to CBABC DP structure. Raman analysis has also been carried out for further structural analysis. The peaks obtained for phase pure CBAB were at 72.19 cm-1 and 174.29 cm-1 (Fig. 4b) while these peaks were at 83.19 cm-1 and 183.79 cm-1 for chlorine doped CBABC (Fig. 4c). Here, we have observed considerable difference in Raman shift in case of the CBABC than CBAB. The observed changes in the Raman spectrum can be attributed to changed lattice vectors, electron–phonon coupling alongwith carrier scattering in the perovskite crystals. 17
The powder samples exhibited a crystalline morphology, as shown in the top view FESEM images (Fig. 5, S18, S19), and further elemental composition was analyzed by EDS to investigate the stoichiometry of elements namely Cs, Bi, Ag, Br and Cl. EDS analysis showed a small amount of Cl incorporation (~7%, weight percentage) in the case of CBABC perovskites composition (Table S2) and stoichiometry i.e. molar proportion of elements is somewhat balanced in the final structure. By analysing FESEM and EDS data, we can assume the formation of two products such as bromide-based phase pure CBAB (Cs2BiAgBr6-0Cl0) (Table S3) and chloride doped CBABC (Cs2BiAgBr6-xClx). The observed difference in the stoichiometry is attributed to the error in the EDS measurement and the presence of unreacted compounds. In this regard, Colella, S. et al have also found out that Cl incorporation in a perovskite structure is at a very low concentration (below 3−4%) irrespective of the components ratio in the precursor solution (46). It is reported that the DP composition is not possible to be tuned extensively through the variation of Cl/ Br molar ratio in the subjected precursor solutions. This can be attributed to the strong difference in the halogen ionic
Fig. 5 FESEM images of powder samples of CBABC and elemental images indicating the presence and uniform distribution of Cs, Bi, Ag, Br and Cl in polycrystalline DPs. 18
radii interferes with the formation of a continuous solid solution. However, the Cl incorporation in the structure would remarkably improve the charge transport within the perovskite layer. Therefore, our results show that both enhanced stability and favourable band gap can be realized using mixed halide double perovskites. In particular, by exercising control over synthesis conditions and procedures so as to prepare mixed halide double perovskites in the WO structure, one can retain the more favourable bandgap of pure bromide double perovskite and yet have as much as 50% of the anions as chlorides to obtain enhanced ambient stability. 5. CONCLUSION Mixed halide double perovskites (DPs) based on Silver, Bismuth, Chlorine and Bromine have been synthesized. Density functional theory calculations together with detailed experimental calculations show that these materials are promising photovoltaic absorbers owing to their ideal bandgap and high stability in ambient conditions. The theoretical results further show that the band gap and the nature of the band edges are intimately related to the nature of the halide ions constituting the BiX6 octahedra. When the BiX6 octahedra contains both chlorine and bromine ions, the bandgap increases linearly with increase in the chloride content and both the halide ions contribute to the valence band edge. On the other hand, when the octahedra contains only one kind of halide ion (Br or Cl), the bandgap remains nearly equal to that of the bromide DP upto 50% Cl doping. In this case, only the bromide ions contribute to the valence band edge, with no contribution arising from the chloride ions. ACKNOWLEDGMENT MKC acknowledge IIT Roorkee for Post Doctoral Fellowship. SS acknowledge DST Solar Energy Research Grant [Grant No: DST SERI- S147].
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Highlights: Fighting climate change demands a boost in the development of renewable energies. Solar energy based on novel photovoltaics cells is a potential renewable energies technology. Halide perovskites are one of the most promising materials for delivery of the next generation of solar cells. A currently intensely debated topic in perovskite solar cells concerns the use of lead, a constituent of most of the halide perovskites so far demonstrated as effective photovoltaic materials. Here, we discuss the synthesis, characterization and electronic structure calculations on lead-free Bi-based mixed-halide double perovskites of Cs2AgBiClxBr6-x stoichiometry. Some of the novelty and highlights of this paper are the following: In this paper, the tuning of electronic properties and enhancement in stability is obtained by compositional mapping with different composition in halide-based double perovskites. We have systematically carried out the theoretical investigation of incorporation of Cl in the Cs2AgBiBr6 structure by Density Functional Theory (DFT) calculations in order to understand the bulk and electronic structures of the mixed halide-based double perovskites Cs2AgBiClxBr6-x (x = 0 to 6). We have also synthesized and characterized the mixed halide double perovskites, namely Cs2BiAgBr6 (CBAB) and Cs2BiAgBr6-xClx (CBABC) for two different compositions to verify our computational study. 21
We have shown that both the bandgap and the nature of the band edges of these materials are intimately related to the kind of halides constituting the BiX6 octahedra. This kind of observation of structure-property is very important to understand the stability in these types of devices. The detailed computational studies and synthesis and characterization of mixed halide-based double perovskites with high stability as discussed here will be an important step for the development of next generation lead free double perovskite solar cells.
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