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.

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.

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We study the hydrostatic pressure dependence of the Y123 lattice by synchrotron angle-dispersive powder diffraction up to 12.7 GPa in order to detect any lattice instabilities or phase separation observed by Raman measurements. In the pressure range (3.7 GPa < p < 10 GPa) mainly the c-axis (and to a smaller extend the a-, b-axis) undergoes a clear deviation from the expected equation of state. Upon the pressure release the data follow the anticipated dependence showing a strong hysteresis. At the pressure of ∼3.7 GPa new peaks appear in the diffraction patterns, which can be attributed to another apparently coherent phase that exhibits enhanced disorder and texture effects. The intensity of the new peaks decreases with increasing pressure and upon pressure release they disappear for p < 3.9 GPa. The in-plane Cu-O pl bonds, the Cu2-Cu1 distance and the fractional coordinate of Ba atom along the c-axis of the Y123 phase show modifications at characteristic pressures in complete agreement with the Raman measurements under pressure, strongly indicating a pressure-induced lattice instability and phase separation.

High resolution synchrotron x-ray powder diffraction combined with micro-Raman spectroscopy are used to investigate the effect of Pr substitution for Y in optimally doped or overdoped polycrystalline YBa2Cu3Oy (Y123) compounds. The spectral analysis of the Raman-active B1g-symmetry mode indicates a phase separation into coexisting nanoscopic environments consisting of pure Y123 and Pr123 and a mixed (Y–Pr)123 phase of almost the nominal amount of Pr. The Y123 phase disappears at x≈0.6 where superconductivity is suppressed, while the formation of the pure Pr123 phase is correlated with the increase of local lattice distortions at the Cu and Ba sites, the presence of crystal defects and the increase of microstrains, as obtained by analyzing the anisotropic XRD peak broadening. The comparison of the Ba Ag-symmetry phonon shift for the Y1−xPrxBa2Cu3Oy and YBa2−zPrzCu3Oy compounds as well as lattice dynamic calculations proves that, when Pr substitutes for Y, it also occupies an amount of Ba sites. The data from Pr, La or Ca substitution for Y indicate that loss of superconductivity is correlated with the substitution of Pr, La for Ba and Ca for Y, though the underlying effects may not be the hole filling by these occupancies.