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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*,†

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Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States ‡ Department of Physics, Shanghai University, Shanghai 200444, China § Department of Mechanical Engineering and Materials Science and ∥Department of Chemistry, Duke University, Durham, North Carolina 27708, United States ⊥ 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 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 solidstate reactions. The calculations show that these quaternary Pbfree perovskites are thermodynamically unstablethey are prone to decompose into ternary and/or binary secondary phases or form phases with nonperovskite 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.

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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 analogue 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 Pbfree alternative PV absorbers.17−19 Recently, transition metal © 2016 American Chemical Society

chalcogenide perovskites have also been found to exhibit band gaps suitable for efficient PV cell applications.20 In addition, Pbfree ternary nonperovskite 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 = CH3NH3 or Cs) and, as mentioned above, the Sn-based materials are air-sensitive. If B is Sb or Bi, the perovskites may Received: January 27, 2016 Revised: March 8, 2016 Published: March 9, 2016 6435

DOI: 10.1021/acs.jpcc.6b00920 J. Phys. Chem. C 2016, 120, 6435−6441

Article

The Journal of Physical Chemistry C

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 cischains.

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COMPUTATIONAL AND EXPERIMENTAL METHODS The DFT calculations were performed using the projectoraugmented wave (PAW) method23,24 implemented in the 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 pseudocubic phase. A (√2 × √2 × 2) supercell (tetragonal phase) with a 6 × 6 × 4 k-point grid and (4 × 4 × 4) supercell (pseudocubic) with Γ-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 nitrogenfilled 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 and 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 h. Multiple homogenization/reheating steps, as well as a range of different cooling options (e.g., slow cooling vs quenching), were

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 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 CH3 NH 3 BiSeI 2 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 nonperovskite structures. Our solidstate 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. 6436


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chains 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-

employed for most compositions. For CsBiSeBr2, the reaction mixture was also heated to 500 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 furnace under an 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 Cu Kα radiation.

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RESULTS AND DISCUSSION 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 90°) 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 cischains, as shown in Figure 1, and used large supercells to study the structural disordering. Five of the supercells used have the pseudocubic structure. They contain 64 pseudocubic (1 × 1 × 1) unit cells (C1 in Figure 1). These supercells were used to model five configurations of ordering (C2−C6 in Figure 1). Ten supercells with the tetragonal structure, containing 4 pseudocubic (1 × 1 × 1) unit cells, were used to model another ten configurations of anion ordering (T1 to T10 in Figure 1). Among these configurations, random distributions of O and F were used only in the two pseudocubic 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 ΔE 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-

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 are marked to show the Bi−O cis-chains in the right panel.

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. 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 calculations of band structures, siteprojected density of states (PDOS), and dielectric functions. As an example, the calculated GGA band structure and PDOS of CsBiSF2 with the T5 configuration (Figure 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 Figure 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 Figure 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. Figure 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 (Figure 3(b)). It is seen that the CsBiSF2 perovskite exhibits

Table 1. Total Energy of Various Ordered Structures Referenced to the Ordered C1 Structure (See Figure 1) for the Prospective CsBiOF2 Perovskite (Units: eV/f.u.) pseudocubic phase

tetragonal phase

structure

Bi−O cis-chain

ΔE

structure

Bi−O cis-chain

ΔE

C1 C2 C3 C4 C5 C6

no mixed mixed yes yes yes

0 −0.30 −0.38 −0.44 −0.48 −0.48

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10

no no yes yes yes yes mixed yes yes no

0.033 0.002 −0.44 −0.44 −0.45 −0.44 −0.19 −0.43 −0.45 0.085 6437


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Figure 3. (a) 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)).

Table 2. Calculated Formation Enthalpies ΔH (eV/f.u.) of AB(Ch,X)3 Perovskites (A = Cs, B = Sb, Bi, Ch = Chalcogen Elements, and X = Halogen Elements) X:

F2

Cl2

Br2

I2

X:

F

Cl

Br

I

CsBiOX2 CsBiSX2 CsBiSeX2 CsBiTeX2 CsSbOX2 CsSbSX2 CsSbSeX2 CsSbTeX2

−10.81 −9.08 −8.98 −8.39 −10.82 −8.83 −8.33 −8.06

−7.46 −5.82 −5.74 −5.25 −7.39 −5.43 −5.33 −4.82

−6.59 −5.04 −4.98 −4.52 −6.56 −4.67 −4.59 −4.12

−5.54 −4.08 −4.05 −3.64 −5.54 −3.74 −3.69 −3.27

BaBiO2X BaBiS2X BaBiSe2X BaBiTe2X BaSbO2X BaSbS2X BaSbSe2X BaSbTe2X

−11.35 −8.30 −8.14 −7.19 −11.56 −8.37 −7.89 −6.96

−9.54 −6.34 −6.24 −5.34 −9.67 −6.24 −5.98 −5.05

−8.96 −6.04 −5.89 −4.89 −9.16 −6.10 −5.60 −4.61

−8.32 −5.48 −5.35 −4.46 −8.15 −5.24 −5.08 −4.15

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) (Units: eV/f.u.) X:

F2

Cl2

Br2

I2

X:

F

Cl

Br

I

CsBiOX2 CsBiSX2 CsBiSeX2 CsBiTeX2 CsSbOX2 CsSbSX2 CsSbSeX2 CsSbTeX2

−0.09 −0.37 −0.51 −0.32 −0.02 −0.10 −0.60 −0.65

−0.36 −0.03 −0.14 −0.41 −0.12 −0.12 −0.24 −0.52

−0.12 −0.19 −0.27 −0.51 −0.07 −0.14 −0.22 −0.47

−0.31 −0.29 −0.34 −0.53 −0.28 −0.25 −0.31 −0.51

BaBiO2X BaBiS2X BaBiSe2X BaBiTe2X BaSbO2X BaSbS2X BaSbSe2X BaSbTe2X

−0.02 −0.44 −0.61 −0.98 −0.03 −0.37 −0.55 −0.89

−0.01 −0.57 −0.68 −1.00 −0.09 −0.39 −0.63 −0.96

−0.03 −0.32 −0.47 −0.90 −0.04 −0.26 −0.45 −0.85

−0.02 −0.24 −0.37 −0.68 −0.06 −0.19 −0.33 −0.66

skites. 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 pseudocubic supercells, to calculate the formation enthalpies of AB(Ch,X)3 perovskites with cis-chain ordering. The formation enthalpies (ΔH) were calculated by (using CsBiOF2 as an example)

weaker optical absorption than CH3NH3PbI3 in the energy range near the absorption edge (see the inset in Figure 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 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. 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 perov-

ΔH =

1 solid solid (Cs) − Etot (Bi) Etot(CsBiOF2) − Etot N 1 gas gas (O2 ) − Etot (F2) − Etot 2

(1)

where Etot(CsBiOF2) is the total energy of a supercell with N solid gas CsBiOF2 formula units, and Esolid tot (Cs), Etot (Bi), Etot (O2) and gas Etot (F2) 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 6438


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The Journal of Physical Chemistry C potentially be synthesized experimentally from their constituent elements and gases. 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 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. 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 structure40,41 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 (Units: eV/f.u.) phase

BaBiO2Cl

BaBiO2I

BaSbO2Cl

CsBiSF2

CsBiSeF2

perovskite Sillén X1

−9.54 −10.20

−8.32 −9.26

−9.67 −10.62

−9.08 −8.90

−8.98 −8.19

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. Table 5. Selected Targeted 3-D Perovskite Compositions and Their Corresponding Tolerance Factor Values (t) Using the Average Anion Sizesa compound

t

reactants

T

products CsxOy, Bi2O3, BiO0.14F2.72 BaBiO3−x, Bi2O3, BiOF, BaO2 BaSb2O6, Ba4Sb2O9 Bi2Se3, CsBr, Cs3Bi2Br9, impurities BaSb2Se4, Sb2Se3, BaBr2

CsBiOF2

0.96

CsF, BiOF

590 °C

BaBiO2F

0.88

BaO, BiOF

500 °C

BaSbO2F CsBiSeBr2

0.99 0.91

BaO, SbOF CsBr, BiSeBr

1090 °C 350 °C

BaSbSe2Br

0.93

500 °C

CH3NH3BiTeI2

1.00

Ba2Se3, Sb2Se3, SbBr3 CH3NH3I, BiTeI

310 °C

CH3NH3I, BiTeI, Bi2Te3

a

The most typical products of reactions for a given composition and reaction temperature (T) are provided.

Our selection of target compositions was 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 (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

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EXPERIMENTAL RESULTS Synthesis Attempts. As shown above, our DFT calculations indicate that all 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 6439


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expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

compounds. Overall, the experimental results summarized 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).

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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 nonperovskite 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 nonperovskite phases, this does not exclude the possibility that the targeted mixed-anion compounds may be formed as metastable phases under nonequilibrium conditions or by entropy stabilization.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(D.B.M.) E-mail: [email protected]. Phone: +1-919-6605356. *(Y.Y.) E-mail: [email protected]. Phone: +1-419-5303918. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 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-AC0205CH11231. 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 6440


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