The electrical transport properties of CHNHPbBr (MAPbBr) polycrystals were in situ investigated by alternating-current impedance spectroscopy under high pressures up to 5.6 GPa. It is confirmed that ionic and electronic conductions coexist in MAPbBr. As pressure below 3.3 GPa ions migration is the predominant process, while above 3.3 GPa electronic conduction becomes the main process. An obvious ionic-electronic transition can be observed. The pressure dependent photo responsiveness of MAPbBr was also studied by in situ photocurrent measurements up to 3.8 GPa. The mixed conduction can be clearly seen in photocurrent measurement. Additionally, the photocurrents remain robust below 2.4 GPa, while they are suppressed with pressure-induced partial amorphization. Interestingly, the photoelectric response of MAPbBr can be enhanced by high pressure, and the strongest photocurrent value appears in the high-pressure phase II at 0.7 GPa, which is similar to previous results in both MAPbI and MASnI.

Pressure effects on the ionic transport and optoelectrical properties of lead halide perovskites are still largely terra incognita. Herein, we have conducted in situ alternating current (AC) impedance spectroscopy on both CsPbBr3 powders and single crystals with random planes at pressures of up to 9.2 GPa and 6.8 GPa, respectively. Through the selection of different simulation equivalent circuit models of AC impedance spectroscopy, we have obtained the pressure-dependent electrical parameters of CsPbBr3. The current results indicate that all the CsPbBr3 samples show mixed ionic-electronic conduction from ambient pressure to 2.3 GPa and pure electronic conduction at pressures above 2.3 GPa. We have also conducted in situ photocurrent measurements on CsPbBr3 powders at pressures up to 2.9 GPa. The emergence of extremely sharp and needle-like peaks at every moment of light irradiation at pressures below 2.3 GPa is attributed to the mixed conduction within CsPbBr3, and the photocurrent of CsPbBr3 could hardly be detected at pressures above 2.9 GPa. Additionally, the photoelectric response of CsPbBr3 can be enhanced by compression, and the strongest photocurrent value appears in the high-pressure phase at 1.4 GPa.

Further efficiency boost of organic-inorganic perovskite solar cells is hampered by limited knowledge on ion migration, inductive loops, and the relationship between structures and properties in organometal halide perovskites. In this work, in situ alternating current impedance spectroscopy measurements on CH(NH2)2PbBr3 (FAPbBr3) have been carried out under high pressure up to 4.8 GPa. The inductive loop has been discovered at low frequencies and can be tuned dramatically by applying pressure, which is attributed to large FA ion migration in FAPbBr3. Two discontinuous changes are observed in both ionic and electronic resistances around phase transition pressure. The pressure dependent photoresponse of FAPbBr3 has also been studied by in situ photocurrent measurements under high pressure up to 3.8 GPa. It indicates that the photocurrent of FAPbBr3 can be enhanced remarkably at 1.3 GPa and the largest photocurrent value in FAPbBr3 is nearly 10 times larger than that in CH3NH3PbBr3 and about 3 times larger than that in CH3NH3PbI3.

In situ impedance measurements were employed to investigate the electrical transport properties of BaMoO under pressures of up to 20.0 GPa. Two anomalous changes in the electrical parameters were found, related to the pressure-induced structural phase transitions. The dielectric performance of BaMoO was improved by pressure. The dispersion in the real part of dielectric constant versus frequency weakens with increasing pressure. Based on the first-principles calculations, the increases of resistance with increasing pressure in the tetragonal and monoclinic phases were mainly caused by the increasing defect levels. The decrease of the relative permittivity in the tetragonal phase was attributed to pressure-induced strengthening in electronic localization around Mo atoms, which hindered the polarization of Mo-O electric dipoles.

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