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Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals

Lead-based halide perovskites are emerging as the most promising class of materials for next generation optoelectronics. However, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double-perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs2BiAgCl6, we perform structural refinement using single-crystal X-ray diffraction, and we characterize its optical properties via optical absorption and photoluminescence measurements. This new perovskite belongs to the Fm-3m space group, and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-Vis and PL measurements we obtain an indirect gap of 2.2 eV. The new compound is very stable under ambient conditions.

Perovskites are among the most fascinating crystals, and play important roles in a variety of applications, including ferroelectricity, piezzoelectricity, high-T c superconductivity, ferromagnetism, giant magnetoresistance, photocatalysis and photovoltaics. [1][2][3][4][5][6][7][8] The majority of perovskites are oxides and are very stable under ambient temperature and pressure conditions. 4,9 However, this stability is usually accompanied by very large band gaps, therefore most oxide perovskites are not suitable candidates for optoelectronic applications. The most noteworthy exceptions are the ferroelectric perovskite oxides related to LiNbO 3 , BaTiO 3 , Pb(Zr, Ti)O 3 and BiFeO 3 , which are being actively investigated for photovoltaic applications, reaching power conversion efficiencies of up to 8%. 9 The past five years witnessed a revolution in optoelectronic research with the discovery of the organic-inorganic lead-halide perovskite family. These solution-processable perovskites are fast becoming the most promising materials for the next generation of solar cells, achieving efficiencies above 20%. [10][11][12][13] Despite this breakthrough, hybrid lead-halide perovskites are known to degrade due to moisture and heat, 14 upon prolonged exposure to light, 15 and are prone to ion or halide vacancy migration, leading to unstable operation of photovoltaic devices. 16,17 At the same time the presence of lead raises concerns about the potential environmental impact of these materials. 18, 19 Given these limitations, identifying a stable, non-toxic halide perovskite optoelectronic material is one of the key challenges to be addressed in the area of perovskite optoelectronics.
The starting point of our search for a lead-free halide-perovskite is the prototypical inorganic compound of the family CsPbI 3 . CsPbI 3 is an ABX 3 perovskite where the heavy metal cations Pb 2+ and the halide anions I − occupy the B and X sites, respectively, while Cs + occupies the A site. The most obvious route to replacing Pb in this compound is via substitution of other group-14 elements such as Sn and Ge. However both elements tend to undergo oxidation, for example from Sn 2+ to Sn 4+ , leading to a rapid degradation of the corresponding halide perovskites. [20][21][22][23] More generally, it should also be possible to substitute lead by other divalent cations outside of group-14 elements. However, our previous high-throughput computational screening of potential candidates showed that the homovalent substitution of lead in halide perovskites impacts negatively the optoelectronic properties by increasing band gaps and effective masses. 24 Another possible avenue is to consider heterovalent substitution, that is the formation of a double perovskite structure with a basic formula unit A 2 BB X 6 . 4 This type of compounds are abundant in the case of oxides and are well known for their ferroelectric, ferromagnetic and multiferroic properties. 4 Additionally, double perovskites have been explored in order to tune the band gap of oxide perovskites. 25,26 On the other hand, halide double perovskites remain a much less explored class of materials. To date, the best known halide double perovskites are based on alkali and rare-earth metals, and are investigated for applications as scintillators in radiation detectors. 27 In order to replace the divalent Pb cations and maintain the total charge neutrality, the B and B sites have to be occupied by one monovalent and one trivalent cation. We search for our B 3+ metallic cations among the pnictogens, and consider Bi and Sb as the most suitable choices.
Arsenic is less desirable owing to its toxicity. We investigate the electronic properties of these hypothetical compounds from first principles using density functional theory (DFT) in the local density approximation (LDA). We construct 'rock-salt' double perovskites, whereby B and B alternate in every direction (shown in Figure 1a).
The rock-salt ordering is known to be the ground state for most oxide double perovskites, 4 therefore it can be expected to hold also in the present case. For each model structure we perform full structural optimization using DFT-LDA and calculate the electronic band gaps using the hybrid PBE0 functional as described in the Supporting Information.
In Figure 1b-c we show a comparative view of the band gaps calculated for the entire Cs 2 B B X 6 family. We find that all band gaps are below 2.7 eV, spanning the visible and near infrared optical spectrum. The band gaps are indirect and increase as we move up the halogen or the pnictogen column in the periodic table, but do not follow a monotonic trend with respect to the size of the noble metal cation. This behaviour can be explained by the character of the electronic states at the band edges. Indeed, a shown in Figure S1 of the Supporting Information, the conduction band bottom and valence band top in each case are predominantly of pnictogen-p and halogen-p character, respectively. As we move up in the periodic table the energy of the halogen-p states decreases, thus lowering the energy of the valence band top. Similarly, the energy of the pnictogen-p states decreases when moving up in the periodic table, thus lowering the energy of the conduction band bottom. The electron and hole effective masses calculated at the band edges exhibit an anisotropic behaviour in most cases (see Table S1). Throughout the entire family of compounds the electron masses are more isotropic than the hole masses. For clarity, in Figure 1 we report the transport effective masses, 29 as defined in the Supporting Information. We note that all compounds exhibit small carrier effective masses between 0.1 and 0.4 m e , very close to those calculated for CH 3 NH 3 PbI 3 within the same level of theory. 30 The electronic band structures of these halide double perovskites (shown in Figure S2  for Cs 2 BiAgCl 6 , Cs 2 BiCuCl 6 and Cs 2 BiCuBr 6 , while for the other compounds the CBM is at the L (π/a, π/a, π/a) point. The FCC cubic crystals of the former three compounds are indirect band gap semiconductors, however a small disturbance of the conventional unit cell symmetry could render the direct optical transition allowed. This is shown in Figure S4, where the band structure of Cs 2 BiAgCl 6 is calculated in the conventional unit cell (which corresponds to two primitive cells). Here, as a result of Brillouin zone folding in the conventional FCC unit cell, the band gap of Cs 2 BiAgCl 6 becomes direct at the Γ point. In practice, this effect could be realized by incorporating an organic cation, like methylammonium (CH 3 NH + 3 ) or formamidinium (CHN 2 H + 4 ) into the cuboctahedral cavity. We illustrate this possibility by calculating the band structure of the hypothetical orthorhombic CH 3 NH 3 BiAgCl 6 (constructed as described in the Supporting Information); as expected we obtain a direct band gap, as shown in Figure S5.
Having established that the family of A 2 BB X 3 halide double perovskites, based on B = Sb, Bi and B = Cu, Ag, Au exhibits promising optoelectronic properties, we move to the synthesis and optical characterization of a representative member of this group of compounds. We adapt the synthesis process of Cs 2 BiNaCl 6 , reported in Ref., 31 to allow for the incorporation of a noble metal. Of the three noble metals under consideration, Ag has an ionic radius which is closest to that of Na (1.02 Å vs 1.15 Å). For this reason we proceed to synthesize Cs 2 BiAgCl 6 by conventional solid-state reaction as described in detail in the Supporting Information. In Figure 2a we show the X-ray Diffraction Pattern for a single crystal (∼30µm diameter). We observe sharp reflections for the crystallographic 0kl, h0l and hk0 planes. These reflections show characteristics of m3m symmetry that reveal systematic absences for (hkl; h + k, k + l, h + l = 2n) corresponding to the face-centered space groups F432, F43m and Fm3m. The latter was selected for structure refinement after confirmation that Cs 2 BiAgCl 6 crystallizes in an FCC lattice. We find that there is no significant distortion of octahedral symmetry about the Bi 3+ . The atomic positions from the structural refinement are listed in Table S2    a X-ray diffraction pattern for a Cs 2 BiAgCl 6 single crystal at 293 K. hkl shown for three different planes, i.e. 0kl, h0l and hk0. All wave vectors are labeled in reciprocal lattice units (rlu) and a * , b * and c * denote reciprocal lattice vectors of the cubic cell of the Fm3m structure. b UV-Vis optical absorption spectrum of Cs 2 BiAgCl 6 . The inset shows the Tauc plot, corresponding to an indirect allowed transition [assuming the expression: (αhν) 1/2 = C(hν − E g ), where α is the absorption coefficient, hν is the energy of the incoming photon E g is the optical band gap and C is a constant]. The straight lines are fitted to the linear regions of the absorption spectrum and Tauc plot, and the intercepts at 2.32 eV and 2.54 eV marked on the plot are calculated from the fit. c Steady-state photoluminescence (PL) spectrum of Cs 2 BiAgCl 6 , deposited on glass. d Time resolved photoluminescence decay of Cs 2 BiAgCl 6 , deposited on glass. The data is fitted using a biexponential decay function. The decay lifetimes of 15 ns (fast) and 100 ns (slow) is estimated from the fit. The Band structure of Cs 2 BiAgCl 6 calculated along the high symmetry path L(π/a, π/a, π/a) -Γ (0,0,0) -X (0,0,2π/a) without (left) and with (right) spin-orbit coupling. The black points on the fully relativistic band structure marked 'i-iv' mark the conduction band bottom at L and Γ and the valence band top at Γ and X, respectively. For each of the states we show the electronic wavefunctions. The conduction band bottom is primarily of Bi-p and Cl-p character, while the valence band top consists of Ag-d and Cl-p character. The shape of all four wavefunctions is consistent with metal-halide σ -bonds.