its volatility and reversible/irreversible decomposition reaction even at low temperature, MA is gradually avoided in the material designs. [9] At the same time, FA is more thermally stable than MA due to its stronger hydrogen bonding with PbX 6 octahedra and benign reversible decomposition reaction below 85 °C, and it is the primary cation in practically all current high-performance PSCs. [10,11] Also, FA has no irreversible (nonselective) back reaction. [12] At the same time, in order to approach the bandgap of 1.34 eV according to the Schottky-Queisser (S-Q) limit, from initially MAPbI 3 to double cation (MAFA or FACs), [10,13,14] triple cation (CsMAFA) based perovskites, [5] eventually to quadruple (CsRbMAFA) based perovskites, [4,[15][16][17] and recently FAPbI 3 are dominated ones. [18][19][20] FAPbI 3 has an ideal, narrow bandgap since FA remains the largest organic cation that fits into a 3D perovskite crystal structure. [21] The implicit or explicit goal of perovskite research is to obtain a black FA-based (stable phase) perovskite at room temperature. Avoiding yellow phase impurities encouraged the advancement of processing techniques and elaborate multication, multi-halide mixtures.It has been proved that the invasion of Br anion at the X site is effective for stabilizing the black phase FA-based and other perovskites. But it leads to an adverse blue-shift of the bandgap disproportionately. For example, there is a spanning of 700 meV persisting in MAPbI x Br 1-x from 2.28 eV (MAPbBr 3 ) to 1.58 eV (MAPbI 3 ). [22] Furthermore, from a stability perspective, introducing Br anion in iodide-based perovskites is unfavorable since Br/I mixtures will undergo severe anion segregation Lead halide-based perovskite solar cells (PSCs) are intriguing candidates for photovoltaic technology because of their high efficiency, low cost, and simple process advantages. Owing to lead toxicity, PSCs based on partially/fully substituted Pb with tin have attracted tremendous attention, which would enable the ideal bandgap to approach the Shockley-Queisser (S-Q) limit. Especially, methylammonium (MA), bromide-free, tin-based perovskites are striking, because of the intrinsic poor stability of MA and blue shift caused by the incorporation of Br − . The first section of this review emphasizes the motivation for studying single-junction MA, Br-free, and Sn-based perovskites. The film quality improvement strategies of Sn-based perovskites, including additive, composition, dimensional, and interface engineering toward high-efficiency devices are comprehensively overviewed. Moreover, strategies to improve stability, where shelf, thermal and operational stabilities of the devices are summarized. Finally, this review concludes with a discussion of actual limitations and future prospects for Sn-based PSCs.

The combination of topological phase and intrinsic beyond-room-temperature ferromagnetism is expected to realize the quantum anomalous Hall effect at a high temperature. However, no beyond-room-temperature intrinsic ferromagnetism has been reported in either topological insulator or topological crystalline insulator (TCI) so far. Here, we report Cr-doping in TCI-phase SnTe crystals which possess highly tunable beyond-room-temperature intrinsic ferromagnetism including T c, magnetic moment, and coercivity by varying Cr contents and crystal thickness. With the increase of the Cr content, the T c increases by 159 K from 221 to 380 K and the saturation magnetic moments increase by ∼23.6 times from 0.018 to 0.421 μB/f.u. This intrinsic beyond-room-temperature ferromagnetism is fully demonstrated by the anomalous Hall effect and magneto-optical Kerr effect in a single Cr x Sn1–x Te nanosheet. Moreover, the room-temperature tunneling magnetoresistance effect has been realized by using a Cr x Sn1–x Te flake, a Fe thin film, and a commercially compatible ultrathin AlO x tunneling barrier. This work indicates a great potential of Cr x Sn1–x Te crystals in room-temperature magnetoelectronic and spintronic devices.

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