Physical Properties Of Bismuth Based Perovskite Solar Cell Material Rb₃Bi₂I₉ Under High Pressure

In recent years,organic-inorganic lead halide perovskite materials have attracted much attention due to their excellent optical properties,and their photoelectric conversion efficiency has been growing rapidly.Although the photoelectric conversion efficiency of the solar cell has been steadily improved,the main disadvantage of this kind of perovskite is its low toxicity and stability.The structure of perovskite is destroyed by the oxidation of antimony in the air.Antimony perovskite is not the best photoelectric material.Therefore,bismuth based perovskite compounds are expected to be the first choice materials to solve the lead pollution problem of perovskite solar cells.In this paper,the author selected Rb₃Bi₂I₉ material as the research object.By means of high-pressure in-situ X-ray diffraction（XRD）,Raman,absorption spectrum and electrical measurement based on diamond anvil cell（DAC）,the physical properties of Rb₃Bi₂I₉ were characterized.The specific research results are as follows:Firstly,According to the XRD results of Rb₃Bi₂I₉,the spatial group of Rb₃Bi₂I₉ is a monoclinic structure of P2₁/n,which has a low non central structure symmetry.With the increase of pressure,Rb₃Bi₂I₉ x-ray diffraction spectrum did not detect the disappearance of the original peak and the emergence of new peak.All Bragg diffraction peaks showed a monotonous trend of broadening,and finally reached the pressure-induced amorphization.At 14.4 GPa,Rb₃Bi₂I₉ single crystal was completely opaque black,but the conversion of the color was completely reversible.The absorption spectrum of Rb₃Bi₂I₉ changes with the pressure,and the absorption edge continuously shifts red,which is accompanied by its shift from the visible region to the near infrared region.At high pressure,Rb₃Bi₂I₉ shows an indirect gap condition of 2.04 eV at ambient temperature.At high pressures,the band gaps of Rb₃Bi₂I₉ undergo a continuous reduction process.The results of Raman spectra are consistent with those of XRD,indicating that the amorphization process of the material is reversible and returns to the original state after pressure relief.Secondly,the electrical transport properties of Rb₃Bi₂I₉ are systematically studied by means of high pressure in-situ AC impedance spectroscopy.The results show that the resistance of Rb₃Bi₂I₉ decreases with the increase of pressure.At atmospheric pressure,the large resistance of Rb₃Bi₂I₉ is related to its wide band gap.With the increase of pressure,the change of resistance value is not obvious with the pressure reach up to about 9.5 GPa,then the resistance value continues to decrease rapidly.At about 15 GPa,the resistance value decreases the most.Considering the resistance of the measurement system itself,Rb₃Bi₂I₉ may be metallized during compression.By analyzing the Nyquist diagram obtained from the impedance spectrum data,we found that the electric transport process of Rb₃Bi₂I₉ sample is only electron conduction,which shows that pure electron conduction dominates the electric transport process of Rb₃Bi₂I₉ sample.Lastly,the phenomenon of pressure induced metallization of Rb₃Bi₂I₉ is confirmed by the measurement of its high pressure in-situ variable temperature resistivity.The experimental results show that the resistivity of Rb₃Bi₂I₉ decreases with the increase of temperature before 15.2 GPa,which shows obvious semiconductor characteristics.When the pressure was 15.8 GPa,the resistance of Rb₃Bi₂I₉ increased with the increase of temperature,showing metal characteristics,which confirmed the semiconductor metal transition.The change rule of resistivity under pressure is related to the depth of impurity energy level.With the closing of piezoinduced energy gap,the phase transition from semiconductivity to metallicity of Rb₃Bi₂I₉ takes place at 15.8 GPa.