Ultra-broadband time-resolved THz spectroscopy of perovskites and nanomaterials

The fundamental study of charge carrier dynamics has been shown to be of great interest for optoelectronics, paving the way forward for future developments. Often, some of the key processes that determine the photophysics of a material occur on very short timescales. In order to study and characterize them, we can use time-resolved spectroscopic techniques on the ultrafast timescale. These generally rely on pulsed lasers with ultra-short pulse lengths, in the order of tens to hundreds of fs. One such technique is time-resolved THz spectroscopy (TRTS), especially useful in the study of semiconductors due to its sensitivity to charge carrier concentration and mobility. Nonetheless, it can also be used to study low energy vibrations and other photoexcited species such as excitons. In this work, we focus on recent developments based on gas photonics that easily enables the extension of the probed frequency range towards 20 THz and beyond, giving it the name of ultra-broadband TRTS. This dissertation revolves around the technique and its application to study two of the hottest topics in optoelectronics: nanomaterials and lead halide perovskites (LHPs). This is often done in combination with other spectroscopic techniques. LHPs have become one of the most studied materials in recent years due to their outstanding performance in photovoltaics and their multitude of other potential applications. Nonetheless, many properties still need to be fully understood. In addition, lower dimensionality variations of LHPs are gaining increasing attention due to interesting qualities such as stability, emission quantum yield and confinement effects. Chapters 1 and 2 are introductory chapters. The former introduces both the theory and the recent literature that is relevant to the following chapters. Meanwhile, the latter gives a detailed explanation of ultra-broadband TRTS, including data analysis, setup description and substrate suitability. It also gives a summarized description of other time-resolved techniques that were employed. The following chapters are ordered in increasing complexity and realization of the potential of ultra-broadband TRTS. Each one focuses on a different material, exploring different photophysics that can be studied with TRTS. Chapter 3 is a time-resolved photophysical study of Se nanowires. These materials have generated great interest for their potential applications, from catalysis to optoelectronics. Here, we study thin-film samples obtained from a novel and simple solution-based synthesis process. The charge carrier dynamics and mobility are studied for the first time employing ultrafast techniques. In chapter 4, bulk LHPs are studied with an interest towards the early evolution of charge carriers, including hot carrier cooling and polaron formation. The study focuses on the rise in photoconductivity that can be observed in LHPs of diverse composition over the first ps after excitation. It proposes a complex model to unify previously reported mechanisms based on the fluence and wavelength dependencies. Finally, chapter 5 turns towards the study of 2D LHPs where the dielectric and quantum confinement effects completely alter the photophysics, resulting in a strong excitonic character. While the previous two chapters mainly focus on the use of frequency-averaged TRTS dynamics, this one also makes use of frequency-resolved spectra as well as broadband fluorescence up-conversion spectroscopy to study the carrier-exciton dynamics.

Spin-valley optoelectronics based on two-dimensional materials

Dmitrii Unuchek

Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

Ultrafast Laser Direct-Writing of Self-Organized Microstructures in Ge-Sb-S Chalcogenide Glass

Yves Bellouard, Gözden Torun

The structuring of mid-IR materials, such as chalcogenide glass (ChG), at the micro and nano scales, is of high interest for the fabrication of photonic devices in general, and for spectroscopy applications in particular. One efficient method for producing regular patterns with a sub-micron to micron length scale is through self-organization processes occurring during femtosecond laser exposure. These processes occur in a broad set of materials, where such self-organized patterns can be found not only on the surface but also within the material volume. This study specifically investigates the case of chalcogenide glass (Ge23Sb7S70) exposed to femtosecond laser pulses, inducing pulse-to-pulse nanostructure formation that is correlated to the glass network structural evolution using Raman spectroscopy as well as morphological and elemental microscopy analysis.

2022

A Time-Resolved Photophysical Study of Hybrid Organic-Inorganic Perovskite Photovoltaic Materials

Arun Aby Paraecattil

The search for new photovoltaic materials has been driven by the combined need to exploit sources of energy that are clean and sustainable, while simultaneously doing it in a cost effective manner. In this light, hybrid organic-inorganic perovskites have recently emerged as an extremely promising photonic material, and their application as a functional photovoltaic layer has resulted in device efficiencies that rival long established silicon based technologies. Rapid progress in device efficiencies have occurred over the last years (22.1% being the current record). However, the simultaneous growth in basic studies of the material have not resulted in a conclusive understanding of the fundamental process that occur subsequent to photoexcitation. Hence, there is a pressing need to identify the pathways that currently limit device performance and provide direction for future work in materials and device engineering. Towards this goal, we investigate two perovskite compositions (CH3NH3PbI3 and (FAPbI3)0.85(MAPbBr3)0.15) using time-resolved (THz and electroabsorption) spectroscopic techniques. Chapter 1 and 2 provide a general introduction into the investigated system and the experimental techniques that have been used. In chapter 3, we detail time-resolved THz measurements and report on experimental evidence for carrier recombination through an indirect transition, as well as a direct recombination pathway that is present at higher carrier densities. We calculate temperature dependent carrier mobilities (at THz frequencies) and bimolecular recombination constants. Through which we identify phonon scattering as the primary limiting mechanism for carrier transport, and temperature dependent bimolecular recombination that is mediated by the relative mobility of the charge carriers. Analysis of the complex photoconductivity spectra using the Drude-Smith model revealed a large difference in carrier scattering between the two perovskite films that could be attributed to the significantly different morphologies. In chapter 4 we apply time-resolved electroabsorption spectroscopy (TREAS) to insulated CH3NH3PbI3 layers and investigate the macroscopic carrier transport dynamics under applied electric fields. Transport within the 40nm perovskite grain was discovered to be diminished by a factor ¿ 2 relative to the high frequency mobility obtained through THz spectroscopy. The averaged carrier mobility across the 280nm film length was reduced by a factor ¿ 4, due to the presence of grain boundaries and defects. Preliminary investigations also identified spectral signatures associated with carrier accumulation at the perovskite interface and delayed extraction at the contacts. Chapter 5 deals with complete solar cells formed using (FAPbI3)0.85(MAPbBr3)0.15 as the active layer and we report the first application of the TREAS technique to complete perovskite devices. Our results reveal that the improved morphology of the film results in film averaged mobilities that are near the intrinsic values obtained using THz spectroscopy. Analysis of transient absorption spectra revealed an electroabsorption signature, its dynamics can be correlated with the disassociation of a transient excitonic species to form free charge carriers.