Metal halide perovskites have revolutionized the field of solution-processable photovoltaics. Within just a few years, the power conversion efficiencies of perovskite-based solar cells have been improved significantly to over 20%, which makes them now already comparably efficient to silicon-based photovoltaics. This breakthrough in solution-based photovoltaics, however, has the drawback that these high efficiencies can only be obtained with lead-based perovskites and this will arguably be a substantial hurdle for various applications of perovskite-based photovoltaics and their acceptance in society, even though the amounts of lead in the solar cells are low. This fact opened up a new research field on lead-free metal halide perovskites, which is currently remarkably vivid. We took this as incentive to review this emerging research field and discuss possible alternative elements to replace lead in metal halide perovskites and the properties of the corresponding perovskite materials based on recent theoretical and experimental studies. Up to now, tin-based perovskites turned out to be most promising in terms of power conversion efficiency; however, also the toxicity of these tin-based perovskites is argued. In the focus of the research community are other elements as well including germanium, copper, antimony, or bismuth, and the corresponding perovskite compounds are already showing promising properties.

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Progress on lead-free metal halide perovskites for photovoltaic applications: a review

Chalcogenide perovskites are proposed for photovoltaic applications. The predicted band gaps of CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with the distorted perovskite structure are within the optimal range for making single-junction solar cells. The predicted optical absorption properties of these materials are superior compared with other high-efficiency solar-cell materials. Possible replacement of the alkaline-earth cations by molecular cations, e.g., (NH3NH3)(2+), as in the organic-inorganic halide perovskites (e.g., CH3NH3PbI3), are also proposed and found to be stable. The chalcogenide perovskites provide promising candidates for addressing the challenging issues regarding halide perovskites such as instability in the presence of moisture and containing the toxic element Pb.

Chalcogenide perovskites for photovoltaics.

Chalcogenide perovskites – an emerging class of ionic semiconductors

Through density functional theory calculations, we show that the alloy perovskite system BaZr1–xTixS3 (x 0) may not be stable under thermal equilibrium growth conditions. Calculations of decomposition energies suggest that introducing compressive strain may be a plausible approach to stabilize BaZr1–xTixS3 thin films.

Alloying and Defect Control within Chalcogenide Perovskites for Optimized Photovoltaic Application

Transition metal perovskite chalcogenides are a new class of versatile semiconductors with high absorption coefficient and luminescence efficiency. Polycrystalline materials synthesized by an iodine-catalyzed solid-state reaction show distinctive optical colors and tunable bandgaps across the visible range in photoluminescence, with one of the materials' external efficiency approaching the level of single-crystal InP and CdSe.

Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenides

Synthesis, structure, and electronic and physical properties of the two SrZrS3 modifications

The herewith-introduced antimonides Zr 1 1 Sb 1 8 and Zr 1 0 . 4 V 0 . 6 Sb 1 8 were prepared by high-temperature techniques; both arc-melting and solid-state reactions at 1200 °C starting from α-ZrSb 2 and the metals Zr and V in powder form are possible methods. These isostructural compounds represent an unprecedented metal:antimony ratio of 11:18 and form a new structure type. Zr 1 1 Sb 1 8 crystallizes in the tetragonal space group I42d, with the lattice dimensions a = 676.94(4) pm and c = 6007.3(5) pm, while the V-containing phase forms a slightly smaller unit cell with a = 676.48(8) pm and c = 6005.6(9) pm (Z= 4). Their structures are comprised of an Sb atom substructure with several intermediate Sb-Sb bonds starting at 311 pm, which is reminiscent of that found in the series (Ti,M) 5 Sb 8 (M = Zr, Hf, Nb, Mo) published last year. Interwoven with this network is the Zr atom network, which forms a diamond-like metal atom substructure with long Zr-Zr contacts of ca. 360 pm. Band structure calculations based on the linear muffin tin orbital approach reveal these antimonides to be mainly stabilized by strong M-Sb and intermediate Sb-Sb bonds, and additionally-to the smallest extent-by M-M bonds (M = Zr, V). In agreement with the electronic structure calculations, Zr 1 1 Sb 1 8 is metallic with a small positive Seebeck coefficient.

Zr11Sb18: A new binary antimonide exhibiting an unusual Sb atom network with nonclassical Sb-Sb bonding

We identified a new ternary molybdenum antimonide-sulfide, Mo2SbS2. Black, needle-shaped crystals of Mo2SbS2 can be synthesized by reacting stoichiometric amounts of the elements at 850 °C for 7 da...

Synthesis, structure, and physical properties of mixed valent Mo2SbS2, the first superconducting antimonide-sulfide

Recently, a novel structure map was presented for metal-rich pnictides and chalcogenides M2Q (M = valence-electron poor transition metal, Q = pnicogen or chalcogen). Herewith, we introduce ZrTiAs, which crystallizesin perfect agreement with our structure mapin the La2Sb type. While Zr2As apparently forms the Zr2P type, the (to date) hypothetical “Ti2As” could not have been prepared yet. ZrTiAs was obtained by arc-melting Zr, Ti, and TiAs2. This arsenide crystallizes in the tetragonal space group I4/mmm, with lattice dimensions of a = 379.28(6) pm, c = 1480.2(3) pm, and V = 212.94(7) 106 pm3 (Z = 4). In contrast to (Zr,Ti)Sb, ZrTiAs comprises complete ordering of the Zr and Ti atoms. However, the Zr atom in ZrTiAs may in part be replaced by Ti, i.e., a phase range of Zr1-δTi1+δAs exists with (at least) 0 ≤ δ ≤ 0.42(1). The ZrTiAs structure exhibits strong Zr−As, Ti−As, and Ti−Ti bonds, and to a minor extent Zr−Zr and Zr−Ti bonding interactions.

Chi-Shen Lee

Papers

Synthesis, structure, and electronic and physical properties of the two SrZrS3 modifications

Zr11Sb18: A new binary antimonide exhibiting an unusual Sb atom network with nonclassical Sb-Sb bonding

Synthesis, structure, and physical properties of mixed valent Mo2SbS2, the first superconducting antimonide-sulfide

Structure Prediction Using Our Semiempirical Structure Map: The Crystal Structure of the New Arsenide ZrTiAs

Ti4MoAs3 and Ti3.7Mo1.3As3: the predicted structure changes in Ti5As3 by a partial substitution of Mo for Ti