the perovskite material onto transparent substrates covered with a compact TiO 2 layer and an optional mesoporous TiO 2 (or Al 2 O 3 ) scaffold layer. [34] The p-i-n structure, which involves depositing the perovskite material onto transparent substrates which are covered with an HTL, such as the poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS). [35] So far, PSCs based on both mesoporous and planar structure exhibit high performance and stability, however, the comparison of the advantages of two different strucutres in stability is still under debate. [36] Mesoporous Perovskite Solar CellsRecently, a new generation of photovoltaic converters, mesoporous solar cell [37] has attracted more consideration due to their low material cost, simple fabrication process, high energy conversion efficiencies, [38] like dye-sensitized solar cell, [38c] and mesoporous perovskite solar cell. [5a,21,39] PCE up to 9.7% has been achieved by using CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite nanocrystals as the light absorber to fabricate a solid-state MPSC. [21] After that the research focuses on solid-state MSCs began to transfer from DSSCs to MPSCs. [32a,40] In an MPSC, a compact layer is usually deposited on fluorine doped tin oxide (FTO) layer, which usually extracts electrons and block holes. Three strategies are broadly used for depositing the TiO 2 layer: 1) Spin-coating the colloidal dispersion of TiO 2 nanoparticles followed by a thermal treatment (titanium source: TiCl 4 , [41] titanium isopropoxide, [42] tetra-n-butyl-titanate; [43] 2) spin-coating titanium precursor solutions followed by a thermal treatment (titanium source: TiCl 4 , [44] titanium isopropoxide, [23] titanium diisopropoxide bis(acetylacetonate) 12 ); 3) spray pyrolysis deposition (titanium source: titanium diisopropoxide bis(acetylaceto nate) 18 ). [45] Low-temperature sintering approaches to prepare an

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...

Daily temperature variations induce phase transitions and lattice strains in halide perovskites, challenging their stability in solar cells. We stabilized the perovskite black phase and improved solar cell performance using the ordered dipolar structure of β-poly(1,1-difluoroethylene) to control perovskite film crystallization and energy alignment. We demonstrated p-i-n perovskite solar cells with a record power conversion efficiency of 24.6% over 18 square millimeters and 23.1% over 1 square centimeter, which retained 96 and 88% of the efficiency after 1000 hours of 1-sun maximum power point tracking at 25° and 75°C, respectively. Devices under rapid thermal cycling between −60° and +80°C showed no sign of fatigue, demonstrating the impact of the ordered dipolar structure on the operational stability of perovskite solar cells.

The chemical composition engineering of lead halide perovskites via a partial or complete replacement of toxic Pb with tin has been widely reported as a feasible process due to the suitable ionic radius of Sn and its possibility of existing in the +2 state. Interestingly, a complete replacement narrows the bandgap while a partial replacement gives an anomalous phenomenon involving a further narrowing of bandgap relative to the pure Pb and Sn halide perovskite compounds. Unfortunately, the merits of this anomalous behavior have not been properly harnessed. Although promising progress has been made to advance the properties and performance of Sn‐based perovskite systems, their photovoltaic (PV) parameters are still significantly inferior to those of the Pb‐based analogs. This review summarizes the current progress and challenges in the preparation, morphological and photophysical properties of Sn‐based halide perovskites, and how these affect their PV performance. Although it can be argued that the Pb halide perovskite systems may remain the most sought after technology in the field of thin film perovskite PV, prospective research directions are suggested to advance the properties of Sn halide perovskite materials for improved device performance.