CsSnI₃: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

Abstract

CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.

Reentrant Structural and Optical Properties and Large Positive Thermal Expansion in Perovskite Formamidinium Lead Iodide

“…Fit over a temperature range of 200 K to 299 K, αV is greater than 200 × 10 -6 K -1 (R 2 = 99.8%). Thus, αV for the β-phase is larger than that for liquid mercury [21] and appears to be nearly the largest for any extended crystalline solid near ambient temperature. It was recently reported that αV = 220 × 10 -6 K -1 for the mixed A-site [CH3NH3]0.5[HC(NH2)2]0.5PbI3, though the precise temperature and fitting range are unspecified.…”

“…The increased values of the U┴ ADPs on cooling to the γ-phase suggest disorder that is not fully captured by this crystallographic model. [19,20] CsSnI3 γ (solid) 298 126 [21] Ag3[Co(CN)6] Solid 276 121 [22] Hg Liquid 293 182 [21] The unit cell volume per formula unit, V, is given in Figure 4(a). A comparison of the volumetric thermal expansion coefficient, αV, with those of other main-group halides and selected framework materials and liquids is given in Table 1.…”

Lead‐Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

“…Replacing the organic cations (MA + or FA + ) with the inorganic Cs + , CsSnI 3 perovskite shows a melting point of 435 °C,72, 141 suggesting superior intrinsic thermal stability. In contrast, hybrid perovskites MASnI 3 and FASnI 3 start to decompose at ≈200 °C.…”

“…Due to the 4s lone pairs of Ge is more active than the Pb 6s lone pair, Ge 2+ is easier to be oxidized leading to metallic conductivity in the hybrid materials and short‐circuit behavior in the photovoltaic devices similar to the case of Sn‐based perovskite 40, 135, 141. However, germanium has demonstrated much less toxicity compared to lead146 and is expected to be a promising candidate in the search for Pb‐free perovskite materials.…”

“…One of the major problems of Sn‐based PSCs is their poor stability originating from the facile formation of Sn vacancy associated with oxidation of Sn 2+ to Sn 4+ when exposed in air,46, 141, 150 which leads to poor reproducibility and deteriorates the devices rapidly 46, 51. For example, Xu et al151 studied the defect properties of CsSnI 3 perovskite and depicted the influence of defects and synthesis conditions on the photovoltaic performance.…”