LEAD-FREE PEROVSKITE SINGLE CRYSTAL FOR OPTOELECTRONIC APPLICATIONS

Abstract

Hybrid halide perovskite materials have attracted strong attention as potential semiconductor materials for optoelectronic device applications. Especially, single-crystalline halide perovskites with no grain boundaries and low trap densities are likely to show superior optoelectronic performances in comparison to their polycrystalline film counterparts. However, most of the research has been done on lead-based perovskite materials and their toxicity as well as modest operational stability, have been major limiting factors impeding their commercial application. As a suitable alternative, lead-free perovskite single crystals have garnered significant attention recently and many reports have shown their successful use in a variety of detection applications. This thesis aims to provide a comprehensive description of lead-free bismuth halide single perovskite and double perovskite single crystals including growth and fundamental properties investigation, structure-property relationship with cation and anion transmutation, carrier dynamics and excited state photo-physics study using optical and electrical spectroscopy techniques, photodetection and X-ray detection performance. In the beginning, the potential of perovskite materials in optoelectronic devices and the need for lead-free perovskites are discussed in chapter 1. The single crystal growth methods and thermodynamics are discussed in chapter 2 followed by the instrumental description and the corresponding parameters. Further, the growth, fundamental properties, and photodetection application of all inorganic cesium bismuth halide Cs3Bi2X9 (X = Cl, Br, and I) perovskite single crystals are presented in chapter 3. It was observed that Cs3Bi2X9 single crystals exhibit higher Urbach energy than lead-halide crystals, which reveals a high degree of local structural disorderness and a short range of crystallinity. The structural disorder affects not only the optical properties but also the magnetic and spin relaxation properties. Further, the light-induced negative photoconductivity with slow recovery was found in the Cs3Bi2Cl9 and Cs3Bi2Br9 crystals which is attributed to the formation of meta-stable trap states and charged excitons. The negative photoconductivity-based photodetectors show high sensitivity and a large on-off ratio. Overcoming the large thickness of single crystals, perovskite single-crystalline thin film is fabricated with controlled thickness for photodetection applications. The Cs3Bi2I9 perovskite single crystal thin film exhibits photodetection performance comparable to other perovskites-based materials. Further in chapter 4, the charge carrier dynamics with cation and anion transmutation were discussed. The role of A-site cation size and its nature on the ion transport dynamics is studied in the A3Bi2I9 [A= methylamine (MA) and formamidinium (FA)] perovskite single crystals using the temperature-dependent electrochemical impedance spectroscopy. Additionally, the less acidic and less polar character of the Formamidinium (FA) cation and strong hydrogen bond plays an important role to reduce the defect states. These bismuth halide single crystals exhibit crystalline-liquid duality and demonstrate strong deviation from the classical Dulong-Petit law at room temperature. This is attributed to the dynamic disorder and strong anharmonicity in their soft crystal lattice. The fundamental limitation and photophysics of Cs3Bi2I9 crystal revealed by the ultrafast transient absorption spectroscopy is discussed in chapter 5. It is observed that the self-trapping and localization of charge carriers as small polarons limit carrier transport in Cs3Bi2I9 single crystalline film. The Fröhlich and deformation potential are synergistically coupled to the electronic dynamics, leading to the self-trapping of charge carriers and the subsequent formation of localized polarons within a few picoseconds in the Cs3Bi2I9 perovskite lattice. Moreover, it is found that the hot carriers in Cs3Bi2I9 single crystal are coupled with crystal lattice that controls the hot carrier dynamics. An unusual slow hot carrier cooling is observed which is associated with longitudinal optical (LO) and coherent longitudinal acoustic phonons (CLAPs) emission during the deexcitation of the hot carriers. Although bismuth halide perovskites with A3Bi2X9 structure exhibit structural diversity and interesting optoelectronic characteristics, the low dimensionality can result in weaker performance. Therefore, while finding the alternative of the lead-halide perovskite, electronic dimensionality should be considered as the main factor. In this direction, if two Pb2+ cations are replaced with heterovalent (i.e., monovalent (I) and trivalent (III)) cations, this cation transmutation approach leads to the formation of lead-free halide double perovskites A2B(I)B(III)X6 structure keeping the three-dimensional connectivity of lattices and it permits the adaptability for different compositional changes. We have studied the fundamental properties and detection application of double perovskite Cs2AgBiX6 (X= Cl, Br) single crystals. Growth and fundamental characteristics are presented in chapter 4. Double perovskites exhibit three-dimensional lattice connectivity and extended absorption than their single perovskite Cs3Bi2X9 (X= Cl, Br) analogues with excellent optoelectronic and thermal stability. With the photophysics and excited state dynamics in bismuth halide single crystals, revealed by the optical spectroscopy techniques, we also focused on the carrier dynamics, dielectric relaxation, and conduction mechanism using the using temperature-dependent electrochemical impedance spectroscopy (td-EIS) in correlation with the modulus spectroscopy. It is found that the polarons are the main conduction carriers in the Cs2AgBiBr6 semiconductor. The temperature-activated polaron relaxation from non-Debye type to Debye type is observed in Cs2AgBiBr6 crystals. Further, the X-ray detection and imaging of the lead-free single and double perovskite single crystals are demonstrated in chapter 6. The X-ray detection and imaging application of Cs2AgBiCl6, Cs2AgBiBr6 and Cu-Cs3Bi2Br9 single crystals are demonstrated. Findings in this chapter reveal that the Bi-halide perovskite single crystals show great potential as practical X-ray detectors and imaging for medical radiography and dose monitoring. In the last part of the thesis, the seventh chapter will have the conclusion of the thesis and the future outlook. This chapter presents the current challenges and unresolved issues in the performance of Pb-free perovskite single crystal-based devices and prospective future investigations in this field. The efforts made in this thesis direct to the development of high-quality perovskite semiconductors crystals using solution-processed techniques. Further, investigation of fundamental photophysics and fabrication of highly sensitive photodetector based on negative photo-conductivity is explored. Efforts are made for developing large-area single-crystalline thin films for photodetection and studying their underlying photophysics. The fundamental limitation of lattice and unknown photophysics are understood in detail in these lead-free perovskites. Consequently, this thesis shines light on that how lead-free semiconductors are comparable to lead-halide perovskite and other conventional semiconductors. Finally, the possibility of the practical application of lead-free semiconductors in photodetectors and X-ray detectors is explored. The comprehensive understanding of the structure-property relationship and applications of these new materials is believed to advance the research in materials science. Keywords: lead-free perovskite, bismuth halide perovskites, perovskite single crystals, structural disorder, double perovskites, doped perovskites, ultrafast dynamics of lead-free perovskites, negative photoconductivity, photodetector, X-ray detector, X-ray imaging, dielectric relaxation, conductivity relaxation, single-crystalline thin film, crystal-liquid duality, specific heat, ion migration, spin dynamics, polaron formation, hot carrier cooling, electron-phonon coupling, polaron hopping, modulus spectroscopy, impedance spectroscopy of perovskite single crystals, Debye type relaxation