Author/EditorSankaramangalam Balachandran, Bhagya Lakshmi
Alternative Titleハロゲン化鉛ペロブスカイトにおける光および電気励起発光の顕微分光学的研究
Note (General)Lead halide perovskites are gaining rapid attention in solar cells, LEDs, lasers, etc. on account of their attractive properties like easy synthesis, tunable bandgap, and excellent charge carrier dynamics. However, their commercialization is still hindered by certain factors, mainly being the stability concern. In this thesis, I carry out a microspectroscopic investigation of photoluminescence (PL) and electroluminescence (EL) in lead halide perovskites microcrystals for understanding the factors that decrease photoluminescence and electroluminescence efficiencies. This thesis has five chapters. In chapter 1, an introduction of lead halide perovskite by giving focus to its chemical structure, optoelectronic properties, various synthesis methods, and their applications is provided. Perovskite blinking is one of the main issues leading to the loss of photoluminescence and electroluminescence efficiency in devices, which is discussed in detail. In chapter 2, various methods like the pressure-induced solid-state method, antisolvent vapor-assisted crystallization, and inverse temperature crystallization to synthesize perovskites crystals are summarized. Various characterization methods like absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, single-particle microspectroscopy, X-ray diffraction technique, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) for understanding the properties are also discussed in detail. In chapter 3, photoluminescence studies on perovskite pellets synthesized by a solid-state, pressure-induced method are carried out. The synthesized perovskite pellet shows very good optical properties, which match those synthesized by wet chemical routes involving solvents like N, N-dimethylformamide, and dichloromethane. Due to the large thickness of the pellet samples, electroluminescence studies are not feasible. Photon recycling is a phenomenon seen in thick samples, in which emissions from the sample get reabsorbed and emitted multiple times. This phenomenon is known to improve solar cell efficiencies. The perovskite pellets synthesized by the pressure-induced method showed a distribution of energy states, confirmed by photoluminescence spectral and lifetime studies. From the photoluminescence lifetime studies, the rates of various charge carrier recombination processes are extracted. These rates reveal that energy transfer outweighs other radiative and nonradiative processes occurring in the pellet. This fact is clarified by preparing mixed halide perovskites which confirm nonradiative energy transfer in the photon recycling mechanism. In chapter 4, electroluminescence and photoluminescence studies are conducted on perovskite microcrystals using single-particle microspectroscopy and fluorescence spectroscopy. An interesting phenomenon of EL blinking is observed from these microcrystals which are characterized by high-intensity and low-intensity EL bursts in the EL trajectories. The nature of EL blinking in perovskites is clarified with the help of statistical analysis. In a microcrystal showing multiple-emitting sites, truncated power-law behaviors for ON- and OFF-time probability densities are observed. Such a truncated behavior is commonly characterized as type-A blinking arising due to the charging-discharging processes. However, in an ensemble of crystals, a linear power-law behavior is seen for the ON- and OFF-time probability distributions. This is the signature of type-B blinking due to the charge carrier trapping-de-trapping processes. From these studies, MAPbBr3 microcrystals show both the type of blinking. The main factor responsible for EL blinking is the migrating halide vacancies in the microcrystals, which act as EL quenchers. These vacancies randomly shift their locations. Such a mechanism dominates the whole microcrystal. The above studies made it possible to understand the origin and mechanism of EL blinking in perovskites. In chapter 5, the role of halide vacancies in EL blinking is studied. For this purpose, perovskite microcrystals of varying bromide compositions are prepared by varying the molar ratio of precursors. An under-stoichiometric sample, which is expected to carry many halide vacancies, shows a linear power-law behavior in the ON- and OFF-time probability densities. This behavior, as stated above, is typical of type-B blinking, which is due to the trapping and de-trapping of charge carriers. The trapping of charge carriers occurs due to the excess halide vacancies. However, a stoichiometric and an over-stoichiometric perovskite sample show type-A blinking as suggested by the truncated power-law behavior of the ON- and OFF-time probabilities. This is due to the charging-discharging mechanism resulting from the formation of trions and associated Auger-type nonradiative recombination. The role of halide vacancies in EL blinking is verified by analyzing MAPbBr3 microcrystals post-treated with MABr solutions to fill the vacancies. An enhancement in the photo- or electroluminescence intensities of the MABr treated MAPbBr3 microcrystals is verified by correlating single-particle microspectroscopic studies with SEM-EDS studies. The PL and EL experiments on lead halide perovskites help understand the low efficiencies of perovskite-based electroluminescent devices. The studies in this thesis help improve the efficiencies of perovskite-based optoelectronic, photovoltaic, and electroluminescent devices.
(主査) 教授 Biju Vasudevan Pillai, 教授 八木 一三, 准教授 髙野 勇太
環境科学院（環境物質科学専攻）
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2022-01-10T16:22:37+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション