Solution-processed hybrid perovskite of CH3NH3PbI3 (MAPbI3) exhibits an abnormal luminescence behavior at around the tetragonal-orthorhombic phase transition temperature. The combination of time resolved photoluminescence (PL), variable excitation power PL, and variable-temperature X-ray diffraction (XRD) allows us to clearly interpret the abnormal luminescence features in the phase transition region of MAPbI3. Both PL and XRD results unambiguously prove the coexistence of the tetragonal and orthorhombic phases of MAPbI3 in the temperature range of 150 to 130 K. The two luminescence features observed in the orthorhombic phase at T < 130 K originate from free excitons and donor-acceptor-pair (DAP) transitions, respectively. The comprehensive understanding of optical properties upon phase transition in MAPbI3 will benefit the development of new optoelectronic devices.
drive the ultraviolet/optical variations. However, the medium energy X-ray NVA is 2-4 times that in the ultraviolet, and the single-epoch, absorption-corrected X-ray/γ-ray luminosity is only about 1/3 that of the ultraviolet/optical/infrared, suggesting that at most ∼1/3 of the total low-energy flux could be reprocessed high-energy emission.The strong wavelength dependence of the ultraviolet NVAs is consistent with an origin in an accretion disk, with the variable emission coming from the hotter inner regions and non-variable emission from the cooler outer regions. These data, when combined with the results of disk fits, indicate a boundary between these regions near a radius of order R ≈ 0.07 lt-day. No interband lag would be expected as reprocessing (and thus propagation between regions) need not occur, and the orbital time scale of ∼1 day is consistent with the observed variability time scale. However, such a model does not immediately explain the good correlation between ultraviolet and X-ray variations.
Methylammonium lead iodide (MAPbI 3 ) perovskite has emerged as a dazzling nova in the solar cell realm. However, the robustness or stability of the material exposed to different ambiences is a key issue. In this paper, resonance Raman spectroscopy is combined with surface and bulk crystal characterizations to interpret the oxygen intercalation phenomenon on the surface of MAPbI 3 . We observe that oxygen can intercalate into the frameworks of MAPbI 3 with the assistance of laser excitation. By lowering down the pressure in the experimental chamber, the intercalated oxygen can be readily removed. Xray photoelectron spectroscopy and X-ray diffraction characterizations suggest that Pb−O bonds are mainly formed on the surface of MAPbI 3 but are constrained to avoid the formation of PbO compound. The quantum chemical calculation based on density functional theory supports the above conclusions. The understanding of oxygen intercalation in MAPbI 3 shall benefit the improvement of stability of the important solar cell materials.
Methylammonium lead-iodide (CH3NH3PbI3, hereafter referred to as MAPbI3) perovskite has emerged as a dazzling nova in the solar cell realm. To date, the surface physics of these materials is still puzzling, but in this work, we demonstrate that the optical dynamics in MAPbI3 is primarily determined by the surface states. Pb dangling bonds on the surface of MAPbI3 introduce shallow electronic states. The carrier localization effect for these electronic states is rather weak as the lifetimes of the carriers on the iodine-poor surface are comparative to those in the interior region of MAPbI3. In contrast, rich-iodine on the surface of MAPbI3 induces deep trap centers for the carriers, which are detrimental to long carrier diffusion lengths. It is further proved that the surface passivation, which surprisingly prolongs the carrier diffusion lengths, mainly works on the rich-iodine on the surface rather than the Pb dangling bonds. This better understanding of the surface physics could provide essential information for improving the performance of photoelectronic devices based on MAPbI3 perovskites.
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