Synthesis and optical study of lead halide perovskites for opto-electronic applications

• Main Author: Ha, Son Tung
• Other Authors: Xiong Qihua
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Physics
• Online Access: http://hdl.handle.net/10356/73518
• Institution: Nanyang Technological University

Organic-inorganic metal halide perovskite is an emerging class of semiconductors. In the past few years, we have witnessed many breakthrough applications of this material, especially in the field of thin film solar cells, light emitting diodes, and lasers. The perovskite (ABX3 or A2BX4) is a rich family of material with thousands of possible compositions by changing either: organic component (A), metal (B), or halogen (X). These perovskite compounds are famous for their numbers of excellent optical and electrical properties such as: high absorption coefficient, long carrier diffusion length, low defect density, good carrier mobility, and so on. In addition, the bandgap of the material can be tuned from ultraviolet to near infrared by simply changing the chemical composition. With the ease of low-temperature solution process-ability and abundance of the sourced materials, the perovskite is rapidly becoming a strong candidate for many practical applications in optoelectronics. In the early day of perovskite research, the most conventional method to prepare sample is via either spin-coating or thermal evaporation method. These methods usually result in polycrystalline film with many pin-holes and defects. As a physicist, my first aim is to find a way to prepare highest quality, single crystal samples for studying their intrinsic and fundamental properties. I have utilized Chemical Vapor Deposition (CVD) method to grow single crystal perovskites. This method has advantages over other single crystal growth methods such as: gas-phase reaction without solvent which can cause residuals or byproducts, inert growth environment, good reproducibility, and highly controllable growth parameters. Indeed, I have developed a facile two-step method to prepare nanoplatelets and nanowires of the perovskite which will be presented in Chapter 3. I also used solution growth method to prepare two dimensional perovskite crystals and mechanical exfoliation to prepare the perovskite flake for optical measurements. In Chapter 4, I will present the fundamental study on optical properties of the perovskite such as: absorption and photoluminescence properties, measurement of carrier diffusion length, phase transition, and phonon-assisted upconversion photoluminescence. Variety of optical instruments were employed to study in this thesis such as: Micro-Raman spectroscopy (Horiba T64000), micro-UV-VIS transmission spectroscopy (CRAIC20), time-resolved photoluminescence spectroscopy, femto-second laser system (Spectra-Physics), etc. The fundamental study of the material at the beginning has led to some important applications in optically pumped laser and laser cooling. The excellent optical properties of the perovskite crystals compared to conventional film sample also confirmed the high quality of samples grown by my method. Chapter 5 is devoted for optically pumped lasing application in the perovskite single crystals. In this chapter, the lasing behavior of several materials will be discussed in detail such as: layered PbI2 (i.e., precursor of perovskite), 3D perovskite nanoplatelets, and 2D perovskite flake. In most cases, the lasing mechanism was attributed to whispering gallery mode (WGM) cavity which was naturally formed in the crystal. Indeed, my work on room temperature, near infrared lasing of CH3NH3PbX3 nanoplatelets published in Nano Letters is the very first reports on lasing of this material. The observation of lasing in 2D perovskite flake was not achieved before. Finally, I will present the experimental demonstration of laser cooling in perovskite. The material has been known to have strong electron/exciton-phonon interaction in the literature. In the study of the material optical properties, I have discovered that both 2D and 3D perovskites have super strong phonon-assisted upconversion photoluminescence, a critical condition to achieve laser cooling in semiconductor. By carefully design experiment and sample fabrication, I were able to achieve net laser cooling in the material and substantially extend the toolbox for future application of the field. In conclusion, my thesis presented a complete study on high quality single crystal lead halide perovskites. In the synthesis part, I have introduced a facile method to obtain high crystalline perovskite nanoplatelets and nanowires using a home-build CVD setup. Using these crystals and optical spectroscopy, important fundamental properties of lead halide perovskite were studied such as: carrier diffusion length, temperature-dependent band gap shift, phonon-assisted upconversion photoluminescence behavior, etc. These crystals were then used as WGM optical cavity to achieve lasing at room temperature. Laser cooling in perovskite was also demonstrated for the first time.