Optical Properties Of Organic-Inorganic Hybrid Peroyskite

Novel light-harvesting materials for mesoscopic solar cells such as CH3NH3PbI3 (MAPbI3) have attracted much attention, following the pioneering work on organic-inorganic hybrid perovskites. While previous reports mostly focus on improving the device performance, equal emphasis should be given to the fundamental device physics. To date, some of the micro-mechanisms of the material properties of MAPbI3 are still ambiguous. Firstly, MAPbl3 exhibits an abnormal luminescent character of temperature dependence at around the phase transition temperature. However, the underlying physics is still controversial. Secondly, the stability of MAPbI3 has been becoming the bottleneck for the ccommercialization of MAPbl3 solar cells. Previous reports suggested that oxygen and the photocatalysis could induce an accelerated decomposition of MAPbl3 into PbI2.However, the author finds that oxygen with the assistance of the photocatalysis of the 532 nm laser results in a reversible change in the Raman sepectrum of MAPbl3. MAPbl3 of this special material character could be developed for oxygen sensors. Thirdly, intrinsic defects have import impacts on the transport property of the carriers. Particularly, it is recently reported that the passivation of the surface states can effectively eliminate the charge hysteresis in MAPbl3 based solar cells. The crystallization quality of MAPbI3 is highly dependent on the growth conditions which could introduce different types of intrinsic defects working as p or n-type doping. Thus to study the defect physics in MAPbl3 grown by different approaches is of great significance for improving the performance of MAPbI3 based solar cells.The main innovations of this article are as follows:1. TRPL and temperature-variable XRD were performed to study the luminescent and structure features of MAPbl3 at around phase-transition temperature. Our results clarify the origin of the two PL features of orthorhombic MAPbI3. As T is between 150 and 130 K, these two PL features are attributed to the phase coexistence of tetragonal and orthorhombic phases with different band-gap energies. As T falls below 130 K, the two PL features are dominated by the donor-acceptor pair (DAP) recombinations. We conclude that the tetragonal-orthorhombic phase transition for the solution-processed MAPbI3 is a gradual process which is related to the defects in the crystal.2. The measurements of Raman spectroscopy and X-ray photoelectron spectrum (XPS) were performed. It is found that photocatalysis on the surface of MAPbI3 can result in oxygen intercalation into the frameworks of MAPbI3. The oxygen intercalation induces the Raman specrum of MAPbI3 from PbI2-like to PbO-like. Lowering down the pressure in the experimental chamber can remove the intercalated oxygen in MAPbI3. The high sensitivity to the oxygen atmosphere observed in MAPbI3 perovskite grants it the potentials oxygen sensors.3. Sequential deposition method was modified to fabricate I-poor and I-rich MAPbI3 samples. Time-resolved photoluminescence (TRPL) and wavelength-dependent TRPL are used to study the PL dynamics in the samples, it is found that rich iodine in MAPbI3 is prone to deep trap centers in MAPbI3, while iodine vacancies introduce shallow defect states. Further study shows that PMMA can passivate the deep trap centers introduced by rich iodine on the surface of MAPbI3 to improve the PL property of MAPbI3. It is believed that the surface characteristics of MAPbI3 play an important role in its photoelectric properties.