Técnicas espectroscópicas e de raios X para elucidação de propriedades fundamentais de perovskitas nanoestruturadas e bidimensionais

Author(s)

Raphael Fernando Moral

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Química
• Defense date: 2023-04-18

Examining board members

Ana Flávia Nogueira; René Alfonso Nome Silva; Jefferson Bettini; Lucimara Stolz Roman; Francisco das Chagas Marques

• Advisor: Ana Flávia Nogueira

Abstract

Organic and inorganic metal halide semiconductors, known as metal halide perovskites, are materials that have attracted the attention of many researchers around the world. The works transcribed and discussed here intend to demonstrate the enormous versatility of these materials, as well as their potential for applications in devices and their intriguing physical-chemical properties. Thus, to advance research in this field, this thesis seeks to demonstrate the potential of spectroscopic and X-ray techniques in the investigation of nanostructured perovskite materials. For this purpose, the text begins with an introduction covering the main aspects in physics of solids, in particular the concepts related to two-dimensional materials. Next, the main characteristics of 3D, quasi-2D, and 2D perovskites are presented in order to provide the reader with a theoretical foundation for the understanding of the works presented in the following chapters. In the second chapter, we present a work where we try to understand the origin of the emission profile of a 2D perovskite of composition (BA)2PbI4 (BA = butylammonium). Because it is a material that can be considered a quantum-well ensemble, it is expected in its emission spectrum only a narrow band related to the excitonic emission. However, what is observed is a dual-band emission profile, with a second broad band right next to the fundamental excitonic emission. The results of our experiments demonstrate that this second band comes from a strong exciton-phonon coupling that generates a replica of the excitonic ground state. This phonon is related to the vibrations of the polar heads of the butylammonium cations connected to the inorganic sublattice of [PbI6]4- octahedrons. Additionally, these vibrational modes suffer a variation in their fundamental frequency with the lowering of the temperature until the phase transition point. This behavior characterizes these vibrations as soft modes in the crystal lattice. In the third chapter, we explore the self-assembly properties of quasi-2D perovskites in their colloidal form (nanoplates). With wide- and small-angle X-ray scattering techniques, we demonstrate how nanoplates (with composition CsPbBr3 and CsPbI3) self-assemble in the solid state and in colloidal suspensions. While in hexane, which is a good solvent for these colloidal systems, the nanoplates self-assemble only at high concentrations (around 80 mg/mL); for decane, dodecane, and hexadecane the self-assembly is progressively facilitated due to the higher viscosity of these solvents. With the understanding gained from the experiments, we then discuss the implications of this self-assembly on energy transfer phenomena such as Förster resonant energy transfer (FRET). Through calculations based on experimental data, our estimates allowed us to conclude that the FRET efficiency for aggregated nanoplates (stacks) is greater than 50% for both studied compositions. In the fourth chapter, we explore the formation dynamics of 2D and quasi-2D perovskites on top of 3D perovskite films. This surface treatment is intended to increase the efficiency and stability of perovskite solar cells (PSCs). For this study, we treated 3D perovskites with two 2D-perovskite-forming cations in their iodide salts: phenylethylammonium iodide (PEAI) and 2-thiophenemethylammonium iodide (TMAI). In situ photoluminescence (PL) analysis reveal distinct mechanisms of layered perovskite formation for each cation. While for the PEAI salt the activation energy for the reactions is high and the material formed is predominantly its 2D perovskite with composition (PEA)2PbI4; for TMAI, the activation energy for the reactions appears to be lower than kBT, and the material formed is a mixture of its 2D and quasi-2D phases ((TMA)2PbI4 and TMA2[APbI3]PbI4 – A = a monovalent cation). This different composition of the films gives the solar cells different properties and efficiency, depending on the surface treatment conditions with these cations. Finally, in chapter five, we continue the in situ investigations, this time with phase-pure 2D and quasi-2D materials (not on top of 3D perovskites). Grazing-incidence wide-angle X-ray scattering (GIWAXS) and in situ PL experiments reveal a strong tendency for these materials to orient parallel to the substrate. This finding contrasts with some works that claim that these materials are oriented perpendicularly to the substrate and, therefore, present better electrical conductivity in photovoltaic devices, for example (AU)

• FAPESP's process: 19/25765-6 - In situ experiments with synchrotron based SAXS and WAXS to probe reactions and formation mechanisms of trending (nanostructured) lead halide materials
• Grantee: Raphael Fernando Moral
• Support Opportunities: Scholarships in Brazil - Doctorate