Viability of Lead-Free Perovskites with Mixed Chalcogen and Halogen

Subscriber access provided by GAZI UNIV

Article

Viability of Lead-Free Perovskites with Mixed Chalcogen and Halogen Anions for Photovoltaic Applications Feng Hong, Bayrammurad Saparov, Weiwei Meng, Zewen Xiao, David B. Mitzi, and Yanfa Yan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00920 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Viability of Lead-free Perovskites with Mixed Chalcogen and Halogen Anions for Photovoltaic Applications Feng Hong,† Bayrammurad Saparov,|| Weiwei Meng,† Zewen Xiao,† David B. Mitzi,*,|| and Yanfa Yan*,† †

Department of Physics and Astronomy and Wright Center for Photovoltaics

Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA ||

Department of Mechanical Engineering and Materials Science, Duke University,

Durham, NC 27708, USA

Feng Hong,†,‡ Bayrammurad Saparov,||,§ Weiwei Meng,†,¶ Zewen Xiao,† David B. Mitzi,*,||,§ and Yanfa Yan*,† †

Department of Physics and Astronomy and Wright Center for Photovoltaics

Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA ‡

Department of Physics, Shanghai University, Shanghai 200444, China

||

Department of Mechanical Engineering and Materials Science, Duke University,

Durham, NC 27708, USA §

Department of Chemistry, Duke University, Durham, NC 27708, USA

¶

School of Physics and Technology, Center for Electron Microscopy, MOE Key

Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 22

ABSTRACT: We assess the viability for photovoltaic applications of proposed Pb-free perovskites with mixed chalcogen and halogen anions, AB(Ch,X)3 (A = Cs or Ba; B = Sb or Bi; Ch = chalcogen; X = halogen), by examining critical issues such as the structural, electronic/optical

properties

and

stability

through

the

combination

of

density-functional theory calculations and solid-state reactions. The calculations show that these quaternary Pb-free perovskites are thermodynamically unstable  they are prone to decompose into ternary and/or binary secondary phases or form phases with non-perovskite structures. Solid-state synthesis efforts confirm the theoretically predicted difficulty for preparing these compounds; all attempted reactions do not form the desired perovskite phases with mixed chalcogen and halogen anions under conditions examined. Instead, they form separate binary and ternary compounds. Despite earlier predictions of promising characteristics for these prospective perovskites for photovoltaics, our results suggest that, due to their instability, the Pb-free perovskites with mixed chalcogen and halogen anions may be challenging to form under equilibrium synthetic conditions.

2


Page 3 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Organic-inorganic lead halide (e.g., CH3NH3PbI3-based) perovskite thin-film solar cells have attracted substantial attention recently. Although the power conversion efficiency (PCE) has been rapidly improved to >20% in the past few years,1-7 the presence of the toxic heavy-metal (Pb) and instability under typical ambient humidity levels have raised concerns for the ultimate commercialization of lead halide perovskite

technologies.8-10

Theoretical

studies

have

revealed

that

the

three-dimensional (3-D) network of corner-sharing BX6 octahedra plays an important role for the superior photovoltaic properties of lead halide perovskites.11, 12 The results suggest that stable alternative perovskites made of nontoxic elements deserve exploration as prospective new photovoltaic materials. A few materials solutions to the toxicity of CH3NH3PbI3 have been proposed.13-16 Homovalent substitution of lead with tin in CH3NH3SnI3 addresses the issue of lead toxicity;14, 15 however, the stability of the Sn-analog is a concern since Sn normally prefers the tetravalent state, and the divalent Sn-based materials are therefore prone to oxidation. Perovskite-derivatives such as A3B2I9 (A = CH3NH3, Rb, Cs; B = Sb, Bi) have been proposed as candidates for Pb-free alternative PV absorbers.17-19 Recently, transition metal chalcogenide perovskites have also been found to exhibit band gaps suitable for efficient PV cell applications.20 In addition, Pb-free ternary non-perovskite chalcohalides such as Sb(S/Se)X (X=Br, I) have been proposed to be promising for ferroelectric photovoltaic applications.21 Nonetheless, so far, Pb-free absorbers have not demonstrated efficient solar cells. Therefore, although some progress has been made, the search for alternative Pb-free perovskite absorbers must continue in order to bring perovskite-based absorbers to the market. Previous theoretical studies have further shown that, aside from the three-dimensional (3-D) network of corner-sharing BX6 octahedra, the Pb lone-pair s orbitals also play a central role for the benign defect properties of Pb halide perovskites.11,12 To preserve the superior PV properties and eliminate the toxicity in alternative perovskites, Pb should be replaced by other nontoxic ions that possess lone-pair s orbitals, such as Sn, Sb, and Bi. If B is Sn, the perovskite is ASnI3 (A = 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 22

CH3NH3 or Cs) and, as mentioned above, the Sn-based materials are air-sensitive. If B is Sb or Bi, the perovskites must have mixed chalcogen and halogen anions in order to maintain charge neutrality within the stoichiometry AB(Ch,X)3, where A is an alkaline or alkaline-earth (or organic) cation, Ch is a chalcogen anion, and X is a halogen anion. One important potential benefit of such mixed chalcogenide-halide compositions is the fact that B-Ch interactions are likely to be more covalent, leading to overall more covalent bonding in the material and higher stability in ambient air. In addition, a recent theoretical study has shown that CH3NH3BiSeI2 and CH3NH3BiSI2 perovskites have band gaps suitable for PV applications.22 While the mixture of chalcogen and halogen anions provides more freedom for materials design and promises higher environmental stability, other potential issues such as anion ordering, phase separation and decomposition need to be studied, and if present, addressed. Here, we report our assessment on the viability for photovoltaic applications of Pb-free perovskites with mixed chalcogen and halogen anions, AB(Ch,X)3 (A = Cs or Ba; B=Sb or Bi), by examining critical issues such as the structural, optoelectronic properties and stability, using a combination of density-functional theory (DFT) calculations and solid-state reaction synthesis. We show: (1) anion ordering is energetically favorable in these prospective perovskites, which significantly affects the electronic properties; (2) the proposed perovskites exhibit weaker optical absorption coefficients than lead halide perovskites for a given band gap; and (3) these targeted quaternary perovskites are thermodynamically unstablethey are prone to decompose into ternary and/or binary secondary phases or form phases with non-perovskite structures. Our solid state reaction efforts confirm the difficulty for preparing these compounds; all tested reactions using mixed chalcogenide and halide do not form the targeted perovskite phases. Instead, they form separate binary and ternary compounds. Our results suggest that thermodynamic stability must be examined with care when new absorbers are theoretically conceived.

COMPUTATIONAL AND EXPERIMENTAL METHODS The DFT calculations were performed using the projector-augmented wave (PAW) 4


Page 5 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


method23,24 implemented in Vienna ab initio simulation package (VASP).25 The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE),26 and the screened hybrid Heyd-Scuseria-Ernzerhof (HSE)27, 28 functionals were used for exchange-correlation. The HSE functional consists of 25% exact Hartree-Fock exchange mixed with 75% PBE exchange. The cutoff energy for the plane-wave basis set was 550 eV. All atoms were relaxed until the Hellmann-Feynman forces on them were smaller than 0.01 eV/Å. A 8 × 8 × 8 k-point mesh can give a convergent result for one unit cell of the pseudo-cubic phase. A (√2 × √2 × 2) supercell （tetragonal phase）with a 6 × 6 × 4 k-point grid and (4× 4 ×4) supercell (pseudo-cubic) with Г-point calculations were adopted to study the disordering of anions. Solid state reactions at various temperatures (T = 140 – 1100 °C) were employed to target formation of each proposed structure. Given the challenges of making mixed-anion compositions, in most cases, reactants already featuring mixed-anion compositions, SbChX and BiChX (Ch = O, S, Se, Te; X = F, Cl, Br, I), were used as a starting material. These, in turn, were generally prepared by reacting Bi2Ch3 and Sb2Ch3 with BiX3 and SbX3, respectively, or in the cases of BiTeI and SbTeI, by reacting a stoichiometric mixture of elements.29-34 In one example, CsBr and BiSeBr were weighed in a nitrogen-filled glovebox in 1:1 molar ratio, targeting the 3D mixed-anion perovskite CsBiSeBr2. The mixture was carefully ground/homogenized inside the glovebox, and then cold-pressed into a pellet. The pellet was then placed into a quartz tube, evacuated down to ~7×10-7 Torr, and the quartz tube was flame-sealed. The reaction mixture was heated to 350 °C inside a box furnace, and kept at this temperature for 30 hours. Multiple homogenization/reheating steps, as well as a range of different cooling options (e.g., slow cooling vs quenching), were employed for most compositions. For CsBiSeBr2, the reaction mixture was also heated to 500 °C and 600 °C. During each annealing step, the masses of the pellets were monitored to ensure that volatile components were not lost during the reactions. Due to reactions with quartz, the oxyfluorides were not enclosed inside evacuated quartz tubes. For these compositions, the reactions were carried out inside a box 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 22

furnace under inert (N2) atmosphere.35,36 Finally, the product structures and phase compositions were checked using a PANalytical Empyrean powder X-ray diffractometer under ambient conditions and employing CuKα radiation.

RESULTS AND DISCUSSIONS DFT Calculations Anion ordering: The prospective atomic structures of quaternary perovskites, AB(Ch,X)3, are quite complex due to the likelihood of chemical ordering between Ch and X anions. The ordering significantly affects the stability of the phases. The situation is similar to the ordering of O and N seen in established oxynitride perovskites, such as SrTaO2N.37, 38 The O-N mixed sites result in different possible ordered and disordered structural configurations. The experimental measurements have shown that structures with Ta-N cis-chains (the angle of N-Ta-N is 90o) are more stable than other ordered structures. Using CsBiOF2 as a model system to explore AB(Ch,X)3-type structures, we examined the energetic stability of ordering between O and F. We have considered a variety of atomic structures both with and without Bi-O cis-chains, as shown in Figure 1, and used large supercells to study the structural disordering. Five of the supercells used have the pseudo-cubic structure. They contain 64 pseudo-cubic (1×1×1) unit cells (C1 in Fig. 1). These supercells were used to model five configurations of ordering (C2 to C6 in Fig. 1). Ten supercells with the tetragonal structure, containing 4 pseudo-cubic (1×1×1) unit cells, were used to model another ten configurations of anion ordering (T1 to T10 in Fig. 1). Among these configurations, random distributions of O and F were used only in the two pseudo-cubic structures (C2 and C3) to model the fully disordered structures. We have first calculated the total energy per formula unit (f.u.) to find the lowest energy ordered configuration. The results are listed in Table 1, in which ∆E is the total energy difference compared to that of the ordered C1 structure, in which O and F occupy apical and equatorial positions, respectively (no Bi-O cis-chains). It is seen that the C4, C5, C6, T3, T4, T5, T6, T8, and T9 configurations with Bi-O cis-chains 6


Page 7 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


have total energies about ~0.43-0.48 eV/f.u. lower than those without cis-chains, indicating that oxygen atoms prefer the sites that form the Bi-O cis-chains in CsBiOF2 perovskite. The atomic structure of the T5 configuration is shown in Figure 2, in which the cis-chain has been confined to a cis-cluster. It should be noted that anion ordering is not a unique property only for the prospective CsBiOF2 perovskite. For other proposed perovskites, the anion ordered/disordered structures have also been tested. We found that the configurations with cis-chains are also energetically more stable than those without cis-chains. These results highlight the importance of anion ordering in the structural and electronic characteristics of prospective AB(Ch,X)3-type structures.

Figure 1. Atomic structures of CsBiOF2 with various O/F ordered configurations. Bi-O bonds were marked as red lines to clearly show Bi-O cis-chains.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 22

Table 1. Total energy of various ordered structures referenced to the ordered C1 structure (see Fig. 1) for the prospective CsBiOF2 perovskite. (Unit: eV/f.u.) Pseudo-cubic phase

Tetragonal phase

Structure

Bi-O cis-chain

∆E

Structure

Bi-O cis-chain

∆E

C1

No

0

T1

No

0.033

C2

Mixed

-0.30

T2

No

0.002

C3

Mixed

-0.38

T3

Yes

-0.44

C4

Yes

-0.44

T4

Yes

-0.44

C5

Yes

-0.48

T5

Yes

-0.45

C6

Yes

-0.48

T6

Yes

-0.44

T7

Mixed

-0.19

T8

Yes

-0.43

T9

Yes

-0.45

T10

No

0.085

Figure 2. Top view (left panel) and side view (right panel) of atomic structures of the proposed CsBiOF2 perovskite with the T5 configuration. Bi-O bonds were marked to show the Bi-O cis-chains in the right panel.

Electronic properties: The energy positions of chalcogen anion p orbitals are different from those of halogen p orbitals. The perovskites with mixed chalcogen and halogen anions may therefore exhibit different electronic and optical properties than Pb perovskites with pure halogen anions. The difference can be revealed through the 8


Page 9 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


calculations of band structures, site-projected density of states (PDOS), and dielectric functions. As an example, the calculated GGA band structure and PDOS of CsBiSF2 with the T5 configuration (Fig. 2) are shown in Figure 3(a). PDOS shows clearly that the conduction band minimum (CBM) is derived from Bi 6p states, and the valence band maximum (VBM) is mainly composed of antibonding states from S 3p with a slight component of Bi 6s and 6p states. The band structure, therefore, is very similar to that of CH3NH3PbI3 perovskite,39 with both the top of the valence and bottom of the conduction bands being dispersive, which is favorable for charge transport. However, it also exhibits a distinctly different feature from CH3NH3PbI3. It can be seen from Fig. 3(a) that S 3p states are close to the VBM, whereas F 5p states are located in a lower energy range and do not make a significant contribution to the top of the valence band, in accordance with previous calculations on CH3NH3BiSI2.22 Such a difference can significantly affect the optical absorption, which can be evaluated by examination of the imaginary part of the dielectric function (ε2). We have also calculated the band structure of CsBiSF2 using HSE and HSE plus spin-orbital coupling (HSE+SOC). As shown in Fig. 3(b), including SOC in the calculation does not affect the valence band, but it splits the conduction band significantly, leading to a band gap of 0.86 eV that is smaller than the HSE or GGA calculated ones. Fig. 3(c) shows the ε2 of CsBiSF2, calculated using the PBE functional. For comparison the calculated ε2 of CH3NH3PbI3 is also given. A scissor operator was applied to CsBiSF2 by shifting the conduction bands down by ~ 0.2 eV to reproduce the band gaps obtained by HSE+SOC calculations (Fig. 3(b)). It is seen that the CsBiSF2 perovskite exhibits weaker optical absorption than CH3NH3PbI3 in the energy range near the absorption edge (see the insert in Fig. 3(c)). This is largely because, in the CsBiSF2 perovskite, the bottom of the conduction band (1.0 - 1.5 eV) does not contain degenerated Bi p states, as seen for Pb p states in CH3NH3PbI3, due to the lower symmetry caused by the mixed anions. The weaker optical absorption edge may exist more generally for mixed-anion perovskites due to the reduced 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 22

symmetries. It should be noted that the reduction of absorption coefficient near the band edge should depend on the energy difference between the Ch and X p orbitals. The reduction for a specific composition can be larger or smaller than that of CsBiSF2.

Figure 3. (a) The GGA calculated band structure and PDOS, and (b) HSE and HSE+SOC calculated band structures for CsBiSF2, and (c) GGA calculated imaginary parts of the dielectric functions of CsBiSF2 and CH3NH3PbI3 with an enlargement of the energy range of 0.9-1.8 eV shown in the inset in (c). For clarity, the 6s states of Bi are increased five times in the PDOS for CsBiSF2 (right panel in (a)).

Formation enthalpies: Although the proposed materials design strategy herein, and the reported optoelectronic properties from DFT are encouraging,22 an important parameter that must be further investigated is the stability of the proposed compositions. In order to examine whether or not the quaternary perovskites, AB(Ch,X)3, may theoretically form, we have calculated the formation enthalpies of these perovskites. Because all configurations with Bi-O cis-chain ordering have similar total energies, we chose the T5 configuration (Figure 1), which has less atoms than the pseudo-cubic supercells, to calculate the formation enthalpies of AB(Ch,X)3 perovskites with cis-chain ordering. The formation enthalpies (∆H) were calculated by (using CsBiOF2 as an example):

( ) ( ) Δ (CsBiOF ) Cs Bi (O ) (F ),

(1) 10


Page 11 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where Etot(CsBiOF2) is the total energy of a supercell with N CsBiOF2 formula units,

(Cs), (Bi), and (O ), and (F ) are the total energies of the

constituent elements in their solid or gas forms. Spin polarization was considered for the calculation of O2 and F2 gases. The calculated formation enthalpies are summarized in Table 2. It can be seen that all these compounds exhibit negative formation enthalpies, suggesting they should be stable theoretically and may potentially be synthesized experimentally from their constitute elements and gases.

Table 2. Calculated formation enthalpies ∆H (eV/f.u.) of AB(Ch,X)3 perovskites (A = Cs, B = Sb, Bi, Ch = chalcogen elements, and X = halogen elements). F

Cl

Br

-7.46 -6.59 -5.54 BaBiO2X

-11.35

-9.54

-8.96

-8.32

-9.08

-5.82 -5.04 -4.08 BaBiS2X

-8.30

-6.34

-6.04

-5.48

CsBiSeX2

-8.98

-5.74 -4.98 -4.05 BaBiSe2X

-8.14

-6.24

-5.89

-5.35

CsBiTeX2

-8.39

-5.25 -4.52 -3.64 BaBiTe2X

-7.19

-5.34

-4.89

-4.46

CsSbOX2

-10.82

-7.39 -6.56 -5.54 BaSbO2X

-11.56

-9.67

-9.16

-8.15

CsSbSX2

-8.83

-5.43 -4.67 -3.74 BaSbS2X

-8.37

-6.24

-6.10

-5.24

CsSbSeX2

-8.33

-5.33 -4.59 -3.69 BaSbSe2X

-7.89

-5.98

-5.60

-5.08

CsSbTeX2

-8.06

-4.82 -4.12 -3.27 BaSbTe2X

-6.96

-5.05

-4.61

-4.15

X:

F2

CsBiOX2

-10.81

CsBiSX2

Cl2

Br2

I2

X:

I

Decomposition energies: In the next step, we compared the formation energies of the target compositions with those of binary and ternary phases in the phase diagram. This is necessary to determine whether these compounds can be experimentally synthesized (i.e., will “win out” during the formation process over prospective secondary phases). The formation of secondary phases can also be described by the associated decomposition reactions, such as, for example, CsBiOF2→1/3(3CsF+Bi2O3+BiF3). The decomposition energy can be calculated by: 1/3[3Etot(CsF)+Etot(Bi2O3)+Etot(BiF3)]-Etot(CsBiOF2). If the decomposition energy is positive, the CsBiOF2 phase is stable against the formation of CsF, Bi2O3, and BiF3 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 22

binary phases. It should be noted that there are multiple decomposition pathways. If any of the decomposition paths gives a negative decomposition energy, then CsBiOF2 is considered unlikely to be synthesized experimentally under equilibrium conditions due to the formation of secondary phases. We have calculated the decomposition energies of all AB(Ch,X)3 perovskites listed in Table 2, with the consideration of numerous possible decomposition pathways, including the formation of binary and ternary phases. The results are summarized in Table 3. It is noted that once a negative decomposition energy was obtained, the search for decomposition pathways for a particular composition was terminated. Therefore, decomposition pathways with more negative energies could be possible and are not included in Table 3. The process of considering decomposition energies is complicated in prospective many-element systems, since the list of possible secondary phases is long and not always well established. Therefore, this process is greatly facilitated by the combination of theoretical and experimental approaches (as was done in the current study), wherein the experimental effort elucidates secondary phases for a given set of synthetic conditions and the computational approach can then be used to test whether the target composition might be stable over these secondary phases within some region of the equilibrium phase diagram. It is seen that all considered AB(Ch,X)3 perovskites show negative decomposition energies, indicating that these perovskites will be difficult to synthesize experimentally under thermal equilibrium conditions without the formation of secondary phases.

12


Page 13 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Calculated decomposition energies of all considered AB(Ch,X)3 perovskites (A=Cs, B=Sb, Bi, Ch = chalcogen elements, and X=halogen elements). (Unit: eV/f.u.) X:

F2

CsBiOX2

Cl2

Br2

I2

X:

F

Cl

Br

I

-0.09 -0.36 -0.12 -0.31 BaBiO2X -0.37 -0.03 -0.19 -0.29 BaBiS2X

-0.02

-0.01 -0.03

-0.02

-0.44

-0.57 -0.32

-0.24

-0.61

-0.68 -0.47

-0.37

CsBiTeX2

-0.51 -0.14 -0.27 -0.34 BaBiSe2X -0.32 -0.41 -0.51 -0.53 BaBiTe2X

-0.98

-1.00 -0.90

-0.68

CsSbOX2

-0.02 -0.12 -0.07 -0.28 BaSbO2X

-0.03

-0.09 -0.04

-0.06

CsSbSX2

-0.10 -0.12 -0.14 -0.25 BaSbS2X

-0.37

-0.39 -0.26

-0.19

CsSbSeX2

-0.60 -0.24 -0.22 -0.31 BaSbSe2X

-0.55

-0.63 -0.45

-0.33

CsSbTeX2

-0.65 -0.52 -0.47 -0.51 BaSbTe2X

-0.89

-0.96 -0.85

-0.66

CsBiSX2 CsBiSeX2

Competing phases: It is important to note that competing crystal structures may exist for the AB(Ch,X)3 compounds with the quarternary compositions. For example, according to previous experimental work, BaBiO2Cl and BaBiO2I exhibit the so-called Sillén X1-type structure,40,41 and not the perovskite structure. Our calculations show that the Sillén X1-type phases are ~0.66 and 0.94 eV/f.u. lower in energy than the perovskite structure for BaBiO2Cl and BaBiO2I, respectively. However, it should be noted that the Sillén X1-type phase may not always be energetically more stable than the perovskite phase for other compositions, as shown in Table 4. Table 4. Calculated formation enthalpy of selected AB(Ch,X)3 compounds with the Sillén X1-type and perovskite structures. (Unit: eV/f.u.) Phase

BaBiO2Cl

BaBiO2I

BaSbO2Cl

CsBiSF2

CsBiSeF2

Perovskite

-9.54

-8.32

-9.67

-9.08

-8.98

Sillén X1

-10.20

-9.26

-10.62

-8.90

-8.19

Experimental results Synthesis attempts. As shown above, our DFT calculations indicate that all 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 22

considered AB(Ch,X)3 perovskites may not be synthesized under thermal equilibrium conditions without the formation of secondary phases. As part of the process of reaching this conclusion (e.g., to elucidate secondary phases), we have attempted to synthesize some of the AB(Ch,X)3 perovskites via the solid-state-reaction approach. Because it is practically not possible to test all considered AB(Ch,X)3 perovskites, we selected some representative most promising compositions for our synthesis as listed in Table 5. Our selection of target compositions were mostly based on the radii and electronegativities of the chalcogenide and halide ions in the mixed-anion compositions. Significant difference in the sizes and electronegativities of chalcogenide and halide ions is more likely to result in a phase segregation. For example, CsBiOF2 with r(O2-) = 1.35 Å and r(F-) = 1.29 Å42 was selected as a promising composition, whereas CsBiOI2 (r(I-) = 2.2 Å) was not considered for experimental attempts. Beside Cs+, CH3NH3+ was also considered for the A ion. The results of our experimental work targeting the selected mixed-anion 3D perovskites are summarized in Table 5. Note that, in Table 5, products obtained are shown for a given set of experimental conditions. The product content changes depending on the reaction conditions. Generally, at lower temperatures, the compositions featuring monovalent A+ cations (A = Cs+, CH3NH3+) yielded perovskite or perovskite derivative products including the 2D-layered A3B2X9 (A = Cs+, CH3NH3+; B = Sb3+, Bi3+; X = Br-, I-). Upon heating at higher temperatures (< 450 °C), the high stability deficient perovskite impurities, A3B2X9, decompose into AX and BX3 salts. Further increase of reaction temperatures yields amorphous molten products with poor crystallinity. The targeted oxyfluorides CsBiOF2, BaBiO2F and BaSbO2F required high (< 500 °C) reaction temperatures as, at lower temperatures, no reaction was observed. Additionally, we attempted synthesis of BaBiO2F through fluorination of BaBiO3 with an excess of NH4F inside a sealed autoclave at 210 °C; however, these reactions resulted in Ba0.6Bi0.4F2.4.43 Although there are no known thermodynamic sinks in the Ba-containing target systems in analogy to the A3B2X9 phases, the resultant products generally showed a clear separation of halides and chalcogenides into separate compounds. Overall, the experimental results summarized 14


Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in Table 5 agree with the theoretically predicted difficulty of synthesizing these perovskites with mixed chalcogen and halogen anions (i.e., in no cases did evidence of the targeted compounds appear).

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 16 of 22

Table 5. Selected targeted 3-D perovskite compositions and their corresponding tolerance factor values (t) using the average anion sizes. The most typical products of reactions for a given composition and reaction temperature (T) are provided. Compound

t

Reactants

T

Products

CsBiOF2

0.96

CsF, BiOF

590 °C

CsxOy, Bi2O3, BiO0.14F2.72

BaBiO2F

0.88

BaO, BiOF

500 °C

BaBiO3-x, Bi2O3, BiOF, BaO2

BaSbO2F

0.99

BaO, SbOF

1090 °C

BaSb2O6, Ba4Sb2O9

CsBiSeBr2

0.91

CsBr, BiSeBr

350 °C

Bi2Se3, CsBr, Cs3Bi2Br9, impurities

BaSbSe2Br

0.93

Ba2Se3, Sb2Se3, SbBr3

500 °C

BaSb2Se4, Sb2Se3, BaBr2

CH3NH3BiTeI2

1.00

CH3NH3I, BiTeI

310 °C

CH3NH3I, BiTeI, Bi2Te3

16


Page 17 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CONCLUSION We have studied the chemical ordering, optoelectronic properties and stability of Pb-free AB(Ch,X)3 perovskites using DFT calculations and find that ordering between chalcogen and halogen anions is energetically highly favorable in these systems. We further computationally predict that none of the AB(Ch,X)3 perovskites may be synthesized experimentally under thermal equilibrium conditions without the formation of secondary phases. Solid-state-reaction efforts support the theoretical results; all tested reactions did not yield the desired perovskite phases with mixed chalcogen and halogen anions. Instead, they formed distinct chalcogenide and halide phases or non-perovskite mixed-anion phases. Our results point out that stability considerations should be considered with priority when new absorbers are proposed and tested using computational approaches. Since complex phase diagrams with many elements involve numerous possibilities for secondary phases and it is often not possible to a priori predict all likely phases, these results also highlight the usefulness of combining experimental (to understand possible secondary phases) and theoretical (to evaluate ultimate stability) approaches. Finally, it should be noted that the predicted decomposition of AB(Ch,X)3 mixed-anion perovskites into secondary phases assumes thermal equilibrium conditions and, therefore, even though selected compounds are predicted to phase segregate or form non-perovskite phases, this does not exclude the possibility that the targeted mixed-anion compounds may be formed as metastable phases under non-equilibrium conditions or by entropy stabilization.

AUTHOR INFORMATION * (D.B.M.) E-mail: [email protected]. Phone: +1-919-660-5356. (Y.Y.) E-mail: [email protected]. Phone: +1-419-530-3918.

ACKNOWLEDGEMENT The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

Award Number DE-EE0006712 and Ohio Research Scholar Program. This research used the resources of the Ohio Supercomputer Center and the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. One of the authors (BS) acknowledges support from a Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-AC05-06OR23100. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE. REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (3) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nature Photonics 2014, 8, 506-514. (4) Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630. (5) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (6) Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542-546. (7) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234-1237. (8) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized TiO2 with Meso-Superstructured Organometal Tri-Halide Perovskite Solar Cells. Nat. Commun. 2013, 4, 2885-2893. (9) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764-1769. 18


Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Christians, J. A.; Herrera, P. A. M.; Kamat, P. V. Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, 137, 1530-1538. (11) Yin, W.-J.; Shi, T.; Yan, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26, 4653-4658. (12) Yin, W.-J.; Shi, T.; Yan, Y. Superior Photovoltaic Properties of Lead Halide Perovskites: Insights from First-Principles Theory. J. Phy. Chem. C 2015, 119, 5253-5264. (13) Hao, F.; Stoumpos, C. C.; Chang, R. P. H.; Kanatzidis, M. G. Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites Enables Broadening of Absorption Spectrum in Solar Cells. J. Am. Chem. Soc. 2014, 136, 8094-8099. (14) Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A. A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; et al. Lead-Free Organic-Inorganic Tin Halide Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 3061-3068. (15) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-Free Solid-State Organic-Inorganic Halide Perovskite Solar Cells. Nature Photonics 2014, 8, 489-494. (16) Wang, K.; Liang, Z.; Wang, X.; Cui, X. Lead Replacement in CH3NH3PbI3 Perovskites. Adv. Electron. Mater. 2015, 1, 1500089. (17) Saparov, B.; Hong, F.; Sun, J. P.; Duan, H. S.; Meng, W. W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor. Chem. Mater. 2015, 27, 5622-5632. (18) Park, B. W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E. M. Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806–6813. (19) Lehner, A. J.; Fabini, D. H.; Evans, H. A.; Hébert, C.-A.; Smock, S. R.; Hu, J.; Wang, H.; Zwanziger, J. W.; Chabinyc, M. L.; Seshadri, R. Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A= K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics. Chem. Mater. 2015, 27, 7137-7148. (20) Sun, Y. Y.; Agiorgousis, M. L.; Zhang, P. H.; Zhang, S. B. Chalcogenide Perovskites for Photovoltaics. Nano Lett. 2015, 15, 581-585. (21) Butler, K. T.; Frost, J. M.; Walsh, A. Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited. Energy Environ. Sci. 2015, 8, 838-848. (22) Sun, Y. Y.; Shi, J.; Lian, J.; Gao, W.; Agiorgousis, M. L.; Zhang, P.; Zhang, S. B. Discovering Lead-Free Perovskite Solar Materials with a Split-Anion Approach. Nanoscale 2015, DOI: 10.1039/c5nr04310g. (23) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (24) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. (25) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

(26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (27) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207-8215. (28) Paier, J.; Hirschl, R.; Marsman, M.; Kresse, G. The Perdew-Burke-Ernzerhof Exchange-Correlation Functional Applied to the G2-1 Test Set Using a Plane-Wave Basis Set. J. Chem. Phys. 2005, 122, 234102. (29) Shevelkov, A. V.; Dikarev, E. V.; Shpanchenko, R. V.; Popovkin, B. A. Crystal Structures of Bismuth Tellurohalides BiTeX (X = Cl, Br, I) from X-Ray Powder Diffraction Data. J. Solid State Chem. 1995, 114, 379-384. (30) Dich, N. T.; Lošťák, P.; Horák, J. Preparation and Basic Physical Properties of BiTeI Single Crystals. Czech. J. Phys. 1978, 28, 1297-1303. (31) Petasch, U.; Göbel, H.; Oppermann, H. Untersuchungen zum Quasibinären System Bi2Se3/BiI3. Z. Anorg. Allg. Chem. 1998, 624, 1767-1770. (32) Poudeu Poudeu, P. F.; Söhnel, T.; Ruck, M. Homologous Silver Bismuth Chalcogenide Halides (N, x)P. I. Syntheses and Crystal Structures of the (0, 1)P Compound AgBi2S2Cl3 and of Three Members of the (1, x)P Solid Solution Series Ag2xBi4-2xS6-4xBr4x. Z. Anorg. Allg. Chem. 2004, 630, 1276-1285. (33) Schultz, P.; Keller, E. Strong Positive and Negative Deviations from Vegard's Rule: X-ray Powder Investigations of the Three Quasi-Binary Phase Systems BiSX1-xYx (X, Y = Cl, Br, I). Acta Crystallogr. B 2014, 70, 372-378. (34) Matar, S.; Reau, J.-M.; Rabardel, L.; Demazeau, G.; Hagenmuller, P. A High Temperature Variety of BiOF. Solid State Ionics 1983, 11, 77-81. (35) Choy, J. H.; Kim, J. Y.; Kim, S. J.; Sohn, J. S.; Han, O. H. New Dion-Jacobson-Type Layered Perovskite Oxyfluorides, ASrNb2O6F (A = Li, Na, and Rb). Chem. Mater. 2001, 13, 906-912. (36) Caruntu, G.; Spinu, L.; Wiley, J. B. New Rare-Earth Double-Layered-Perovskite Oxyfluorides, RbLnTiNbO6F (Ln = La, Pr, Nd). Mater. Res. Bull. 2002, 37, 133-140. (37) Yang, M. H.; Oró-Solé, J.; Rodgers, J. A.; Jorge, A. B.; Fuertes, A.; Attfield, J. P. Anion Order in Perovskite Oxynitrides. Nature Chem. 2011, 3, 47-52. (38) Clark, L.; Oró-Solé, J.; Knight, K. S.; Fuertes, A.; Attfield, J. P. Thermally Robust Anion-Chain Order in Oxynitride Perovskites. Chem. Mater. 2013, 25, 5004-5011. (39) Yin, W.-J.; Shi, T.; Yan, Y. Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903. (40) Kennard, M. A.; Darriet, J.; Grannec, J.; Tressaud, A. Cation Ordering in the Sillén X1-Type Oxychloride, BaBiO2Cl. J. Solid State Chem. 1995, 117, 201-205. (41) Charkin, D. O.; Berdonosov, P. S.; Dolgikh, V. A.; Lightfoot, P. A Reinvestigation of Quaternary Layered Bismuth Oxyhalides of the Sillén X1 Type. J. Solid State Chem. 2003, 175, 316-321. (42) Shannon, R. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. A 1976, 32, 751-767. 20


Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Soubeyroux, J. L.; Réau, J. M.; Wahbi, M.; Sénégas, J.; Soo, S. K. Neutron Diffraction Investigation of the Ba1−xBixF2+x Solid Solution. Solid State Commun. 1992, 82, 63-70.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 22

TOC GRAPHICS

22


Viability of Lead-Free Perovskites with Mixed Chalcogen and Halogen

Recommend Documents