We report the vibrational and optical properties of the 'defect' perovskites Cs 2 SnX 6 (X = Cl, Br, I) as well as their use as hole-transporting materials (HTMs) in solar cells. All three air-stable compounds were characterized using powder X-ray diffraction and Rietveld refinement. Far-IR reflectance, Raman, and UV−vis spectroscopy as well as electronic band structure calculations show that the compounds are direct band gap semiconductors with a pronounced effect of the halogen atom on the size of the energy gap and the vibrational frequencies. Scanning electron microscopy and atomic force microscopy confirmed that the morphology of the perovskite films deposited from N,N-dimethylformamide solutions on TiO 2 substrates also strongly depends on the chemical composition of the materials. The Cs 2 SnX 6 perovskites were introduced as hole-transporting materials in dye-sensitized solar cells, based on mesoporous titania electrodes sensitized with various organic and metal−organic dyes. The solar cells based on Cs 2 SnI 6 HTM and the Z907 dye performed best with a maximum power conversion efficiency of 4.23% at 1 sun illumination. The higher performance of Cs 2 SnI 6 is attributed to efficient charge transport in the bulk material and hole extraction at the perovskite-Pt interface, as evidenced by electrochemical impedance spectroscopy.

This study reports Raman and infrared spectra of hybrid organic-inorganic MAPbX3 perovskites (MA = CH3NH3, X = Cl, Br, I) and their mixed-halide derivatives. Raman spectra were recorded at three laser wavelengths (514, 785 and 1064 nm) under on-and offresonance conditions, as well as at room temperature and 100 K. Use of different excitation wavelengths allowed the unambiguous acquisition of 'true' Raman spectra from the perovskites, without degradation or photo-induced structural changes. Low frequency PbX vibrational modes were thoroughly identified by comparison of Raman and far-IR results. Red Raman frequency shifts for almost all MA vibrations from 200-3200 cm -1 , particularly intense for the torsional mode, were observed towards heavy halide derivatives, indicative of strengthening the interaction between halides and the organic cation inside the inorganic cage. Different MA-X bonding schemes are evidenced by torsional mode pairs emerging at the orthorhombic phase.MAPbBr3 was further characterized by variable temperature Raman measurements (100-295 K).Broadening of the MA rocking mode slightly above the tetragonal I to II phase transition is connected with disorder of the MA cation. Our results help understanding perovksite materials properties (ferroelectric domain formation, anomalous hysteresis) and their use as efficient light absorbers in solar cells. information is available free of charge via the internet at at

Abstract:The structure of the hybrid perovskite HC(NH2)2PbI3 (formamidinium lead iodide) reflects competing interactions associated with molecular motion, hydrogen bonding tendencies, thermally activated soft octahedral rotations, and the propensity for the Pb 2+ lone pair to express its stereochemistry. High-resolution synchrotron X-ray powder diffraction reveals a continuous transition from the cubic α-phase (Pm 3 m, #221) to a tetragonal β-phase (P4/mbm, #127) at around 285 K, followed by a first-order transition to a tetragonal γ-phase (retaining P4/mbm, #127) at 140 K. An unusual reentrant pseudosymmetry in the β-to-γ phase transition is seen that is also reflected in the photoluminescence. Around room temperature, the coefficient of volumetric thermal expansion is among the largest for any extended crystalline solid.Photovoltaic absorbers based on the hybrid perovskite HC(NH2)2PbI3 (formamidinium lead iodide) and its alloys exhibit impressive performance, [1] but the description of the crystal structure of this material is incomplete. In addition to this technological motivation, [2] dense hybrid materials with 3-D inorganic connectivity and isolated organic molecular ions combine features of traditional inorganic solids and open framework materials, and their composition-structure relations are of fundamental interest.The initial report of the preparation and characterization of HC(NH2)2PbI3 proposed perovskite structures of trigonal symmetry for the α-and β-phases on the basis of laboratory single crystal X-ray diffraction, [3] while a subsequent report assigned the structure of the cubic perovskite aristotype for the α-phase from neutron powder diffraction. [4] The structure of the γ-phase has not been reported. Figure 1. X-ray scattering intensity from HC(NH2)2PbI3 around selected lowangle Bragg peaks between 90 K and 490 K, normalized to maximum peak intensity. (a) The 211t tetragonal Bragg peak emerges upon cooling through a continuous phase transition around 285 K from the cubic α-phase to the tetragonal β-phase. (b) The 200c cubic Bragg peak splits continuously on cooling due to the emergent tetragonality. A first order transition to the pseudocubic γ-phase with tetragonal symmetry occurs at 140 K. [5] The 002t and 220t tetragonal peaks "fuse" across the β-γ transition while the 211t tetragonal peak remains.The disordered molecular cation, challenges associated with twinning in single crystals, and issues of pseudosymmetry led us to employ high-resolution synchrotron X-ray powder diffraction to follow the structure evolution of HC(NH2)2PbI3 between 90 K and 490 K. The temperature-dependent scattering intensity around instructive low-angle Bragg peaks is given in Figure 1. The continuous α-β phase transition around 285 K is evident in the emergence of the 211 tetragonal peak and the splitting of the 200 cubic peak into the 002 and 220 tetragonal peaks on cooling. The first-order β-γ transition can be seen in the abrupt change in intensities and peak positions at 140 K (transition temperature fro...

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The CsSnI perovskite and the corresponding SnF-containing material with nominal composition CsSnIF were synthesized by solid-state reactions and structurally characterized by powder X-ray diffraction. Both materials undergo rapid phase transformation upon exposure to air from the black orthorhombic phase (B-γ-CsSnI) to the yellow orthorhombic phase (Y-CsSnI), followed by irreversible oxidation into CsSnI within several hours. The phase transition occurs at a significantly lower rate in the SnF-containing material rather than in the pure perovskite. The high hole-carrier concentration of the materials prohibits the detection of Raman signals for B-γ-CsSnI and induces a very strong plasmonic reflectance in the far-IR. In contrast, far-IR phonon bands and a rich Raman spectrum are observed for the Y-CsSnI modification below 140 cm with weak frequency shift gradients versus temperatures between -95 and +170 °C. Above 170 °C, the signal is lost due to B-α-CsSnI re-formation. The photoluminescence spectra exhibit residual blue shifts and broadening as a sign of structural transformation initiation.