Optical-Vibrational Properties of the Cs₂SnX₆ (X = Cl, Br, I) Defect Perovskites and Hole-Transport Efficiency in Dye-Sensitized Solar Cells

We report the vibrational and optical properties of the 'defect' perovskites Cs₂SnX₆ (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 TiO₂ substrates also strongly depends on the chemical composition of the materials. The Cs₂SnX₆ 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 Cs₂SnI₆ HTM and the Z907 dye performed best with a maximum power conversion efficiency of 4.23% at 1 sun illumination. The higher performance of Cs₂SnI₆ is attributed to efficient charge transport in the bulk material and hole extraction at the perovskite-Pt interface, as evidenced by electrochemical impedance spectroscopy.