Visible Light Response, Electrical Transport, And Amorphization In Compressed Organolead Iodine Perovskite

Recent scientific advances on the organic-inorganic hybrid perovskites are mainly focused on the improvement of power conversion efficiency. So far, how compression tunes their electronic and structural properties remain less understood. This paper the CH₃NH₃PbI₃（MAPbI₃） nanorods have been examined as an example. By combining in situ photocurrent, impedance spectroscopy, and X-ray diffraction（XRD） measurements, we have studied the electrical transport and structural properties of compressed MAPbI₃ nanorods with the diamond anvil cell（DAC）. At the same time, we employed transmission electron microscopy to characterize morphology of MAPbI₃ nanorods under pressure. We found that:the pressure has a significant effect on the photocurrent. And MAPbI₃ nanorods went a phase transition under high pressure. With the phase transition, the electrical transport parameters vary abnormally. According to our results, we have discussed the physical mechanism of the phase transition and electrical transport properties. In addition, we have studied the connection between the phase transitions and the transport properties. The details are listed below:1. By in situ high-pressure visible light response measurement, we have studied the effect of pressure on the photocurrent of MAPbI3 nanorods. The results indicated that the photocurrent increased sharply as the light turned on and then dropped rapidly to the original value when the light was turned off, indicating a good optoelectronic character. The visible light response of MAPbI₃ remains robust below 3 GPa while it is suppressed when the pressure increased to 4 GPa. Continue to increase pressure, we can not observe the obvious photoelectric response signal.2. The electrical transport properties of MAPbI₃ nanorods were systematically studied by means of high pressure in situ AC impedance spectroscopy measurement. Only one semicircle describing the bulk contribution can be observed in the impedance spectroscopy results of MAPbI₃, which is why the bulk conduction process dominates the electrical transport behaviour of MAPbI₃, which is extremely desirable for MAPbI₃-based electronic devices. In addition, the electrical transport parameters of MAPbI₃ including bulk and grain boundary resistances, relaxation frequencies, and relative permittivity were obtained, and the electrical transport parameters have discontinous changes at 0.6 GPa and 4.4 GPa.3. In situ high pressure XRD experiments were conducted to monitor the structural modification of MAPbI3 under compression. The tetragonal–orthorhombic structural transition of MAPbI₃ occurred at-0.5GPa and completed at-0.7GPa. the phase transition could be attributed to the tilting and distortion of Pb I6 octahedra. When the applied pressure is higher than 4.2 GPa, the sample starts to be partly amorphous. when the sample is pressurized up to 11.6 GPa and then quenched to ambient pressure, showing the reversible character when the sample is pressurized up to 20.9 GPa and then quenched to ambient pressure, showing the irreversible character.4. HRTEM observations on the decompressed samples demonstrate that at 0.3 GPa and 0.6 GPa, the MAPbI₃ samples still retain their nanorod character. Upon increasing the pressure up to 0.9 GPa, the nanorods can’t be observed anymore, instead the samples were compressed into a compact one. In the HRTEM image of the decompressed sample some nanodomains which exhibit a short range order were observed. The average sample size changed from 0.5–0.6 μm in length and 80–100 nm in diameter to arbitrary shapes with the size of ～10 nm at 0.9 GPa. which means that the MAPbI₃ samples belong to the class of soft materials. The deviatoric stress in the MAPbI₃ sample chamber makes a nonhydrostatic condition before 0.9 GPa because of the nanorod character. Before the nanorods crush into nanopieces, the deviatoric stress is still high enough to lead to the appearance of the mixed phase of tetragonal and orthorhombic at 0.5 GPa...