Progress of Lead‐Free Halide Double Perovskites

Abstract

Although impressive performance has been obtained, PSCs are still far from commercial or real-life availability due to serious issues such as toxicity [15,16] and poor stability to heat, [17] oxygen, [18] moisture, [19,20] electric field, [21] and light. [18,22] The toxic nature of hybrid organic-inorganic lead halide perovskites has been traced to the presence of Pb in its chemical composition. [15,16,[23][24][25] Pb 2+ readily dissolves in water (e.g., rain water) to form a toxic solution capable of causing serious environmental pollution, harmful to human beings and the ecosystem. Besides their sensitivity to moisture, oxygen, light, electric field, or thermal stress, an existing self-degradation pathway [26,27] is also a big issue in hybrid organic-inorganic lead halide perovskites. Mixed-halide and mixed-cation perovskites have been investigated to address these issues. [28][29][30][31][32] The group IV elements, tin (Sn) [23,[33][34][35] and germanium (Ge), [34,36] have been employed as the replacements for Pb. However, the device performance through this approach has fallen short of the Pb-based ones. For example, the PCEs reported for Sn-based perovskite solar cells are usually less than 10%. [23,[33][34][35][37][38][39][40] In addition, the easy oxidation of Sn and Ge from the +2 state to the +4 state due to their high energy 5s and 4s orbitals makes them less promising for application in stable and long-term PSCs. [41] High throughput calculations also demonstrate that these substitutions are likely to compromise the ideal optoelectronic properties of MAPbI 3 . [42,43] Furthermore, low dimensional (e.g., 2D, 1D, and 0D) perovskites have also been used to address the stability issues in PSCs. [44][45][46][47][48] Recently, a stabilized PCE of 21.7% resulting from a 2D/3D bilayer PSC was reported. [49] However, the highest certified PCE in a 2D-only planar PSC is 15.3%, [50] which is far below that of their 3D perovskite-based counterparts. It thus stimulates the interest to develop new classes of materials which can solve the issues of toxicity and stability while still maintaining the fascinating properties of lead-based perovskite materials.Recent theoretical calculations demonstrate that a halide double perovskite structure, A 2 B′B″X 6 , which could be formed through a replacement of two toxic Pb 2+ in the crystal lattice with a pair of nontoxic heterovalent (i.e., monovalent and trivalent) metal cations, is a promising alternative to realize high-performance, lead-free, and stable PSCs. [51,52] Although, spectroscopic limited maximum efficiency (SLME) calculations revealed an efficiency limit less than 8% for the most prominent member www.advancedsciencenews.com double perovskites with a vacancy ordered structure. [88] In particular, the unit cell axis of Cs 2 AgBiBr 6 is given to be ≈11.25 Å, [25] which is two times larger than that of MAPbBr 3 (≈5.92 Å). [89] Both Ag + and Bi 3+ occupy the B-site of the crystal lattice with slightly varied metal-halide bond lengths. The dissimilar bond lengths st...

Structurally Stabilizing and Environment Friendly Triggers: Double‐Metallic Lead‐Free Perovskites

“…It also includes a transition from ns 2 states of Bi 3+ in A 2 AgBiX 6 to ns 0 states of In 3+ in A 2 CuInX 6 . Such optimized electronic band structure can be contributed to the close d states of Cu + in A 2 CuInX 6 perovskite . Moreover, the compound Cs 2 NaSbCl 6 has the cubic elpasolite structure, and Cs 2 NaSbBr 6 also possesses this type of structure …”

Post‐Treatment Engineering of Vacuum‐Deposited Cs2NaBiI6 Double Perovskite Film for Enhanced Photovoltaic Performance

“…The research of a thin‐film photovoltaic device based on a hybrid organic–inorganic lead (Pb) halide perovskite material has been significantly progressed during the last decade, and the world record power conversion efficiency (PCE) has recently reached 24.2% . The promising performance of perovskite solar cells (PSCs) is already competitive with CdTe and polycrystalline silicon‐based solar cells . Despite the rapid development of PSCs, the instability and the toxicity of a classical Pb‐based perovskite material remain the potential risks for its large‐scale commercial application .…”

“…[1][2][3][4][5][6] The promising performance of perovskite solar cells (PSCs) is already competitive with CdTe and polycrystalline silicon-based solar cells. [7] Despite the rapid development of PSCs, the instability and the toxicity of a classical Pb-based perovskite material remain the potential risks for its large-scale commercial application. [8][9][10] As a result, much effort has been made on exploring lead-free perovskite materials with high stability and, meanwhile, low toxicity.…”

Ag/In lead‐free double perovskites

“…Eventually, what a single cation cannot achieve, can be done, perhaps, by the cooperation of two: this is the idea which pushed several groups to explore the coexistence of two different B‐site cations in the same compound, thus extending the chemical template of the search from that of single perovskites to the double perovskites of the form A 2 BB'X 6 ; in fact, if B and B′ are not too different and size‐compatible with A and X, these materials can grow structurally ordered and highly symmetric, with B and B′ alternating in regular patterns; clearly, the chemical equivalence with Pb 2+ requires a B 1+ and B′ 3+ combination.…”

“…References , , and provide useful overviews of the literature concerning both actually synthesized and theoretically simulated halide double perovskites; it is remarkable that many of them adopt the cubic Fm3m symmetry, as a consequence of the regular alternation of B and B′ along the three Cartesian directions (so called “G‐type” order); this signals an improved thermodynamic stability with respect to the Pb‐based prototypes, also favored by the presence of an atomic cation, in place of the molecule, at the A‐site; the great potential of these systems is also apparent by the wide variety of band gaps allowed by the presence of two B‐site cations, spanning all the useful range for photovoltaic and optical applications. However, having the correct band gap is only a necessary precondition: good amount of the examined double perovskites are found having indirect band gaps and/or absorption spectra smothered by symmetry‐forbidden inter‐band transitions .…”