Mechanistic Insights Into The Effect Of Light Soaking On Carriers Recombination Dynamics In CH₃NH₃PbI₃ Film

In this thesis, the effect of light soaking on carriers recombination dynamics in CH3NH3 Pb I3 film have been studied by use of steady-state fluorescence spectroscopy, microwave absorption dielectric spectroscopy, time correlated single photon counting(TCSPC) technology and X-ray diffraction etc. The light soakings on CH3NH3 Pb I3 film are mainly carried out by a He-Ne laser(632.8 nm) and a semiconductor laser(532nm continuous wave). The specific research contents were divided into two parts: 1.The effect of light soaking on fluorescence quantum yields of CH3NH3 Pb I3 film. 2. The polarization formation in CH3NH3 Pb I3 film under light soaking.1. Focusing on fluorescence quantum of CH3NH3 Pb I3 film, the thesis has studied its evolution with light soaking. Under light soaking the fluorescence quantum fields increases first, then deceases. Firstly, the evolution is characterized in different conditions, including film thickness, exciting light intensity, atmospheric environment and the laser spot size on the film. The experimental results shown that the whole evolution, namely the increase and decrease of fluorescence quantum fields, would be finished in shorter time with the thinner CH3NH3 Pb I3 film, the stronger intensity of the exciting light. Moreover, the increase part of the whole evolution is significantly enhanced in the O2 environment. Secondly, the reversibility of the evolution of fluorescence quantum yields was analyzed. The results confirm that the evolution of fluorescence quantum yields of CH3NH3 Pb I3 film is a reversibly process. With more dark time, the CH3NH3 Pb I3 film of the sample is closer to the initial state.Thirdly, by use of microwave absorption dielectric spectroscopy and TCSPC technology, we investigated the underlying mechanism for the evolution of fluorescence quantum yields of CH3NH3 Pb I3 film. Our experimental results indicate that the fluoresce lifetime synchronously varies with fluorescence quantum fields. Basing on related references and our experimental results, we propose that photoinduced thermal effect and photoinduced chemical reaction between CH3NH3 Pb I3 film and oxygen gas occurs in the film under light soaking, which would eliminate some trap sates and enhance the fluorescence quantum fields. On the other hand, light soaking also enhance the rotation movement of CH3NH3+, and induce the distortion of inorganic scaffold. The distortion of inorganic scaffold results in a decrease of CH3NH3 Pb I3 film.2. Under the light soaking a polarization process in CH3NH3 Pb I3 film has been studied in the second part of the thesis. When the film is process in the sequence of light-dark-light, the fluorescence intensity will be higher than that in the upper “light” stage at first, then decrease slowly to a comparable level with that in the upper “light” stage. Such change in fluorescence intensity makes the fluorescence intensity trace form an obvious step. We supposed that a new polarization occurs in the CH3NH3 Pb I3 film, which suppresses the recombination radiation between electron and hole. At the present time, references points to three possible mechanisms for the fluorescence intensity step of CH3NH3 Pb I3 film under light soaking. They are ion migration, ferroelectricity and charge trapping, respectively. According to tour experiments, the relationship between fluorescence intensity step and ion migration can be exclude. However, with ferroelectricity and charge trapping, we can not specify the exact one being responsible for the polarization process in the CH3NH3 Pb I3 film.