Ultrafast carrier and quasiparticle dynamics in strongly confined perovskite nanoplatelets

Over the past decade, lead halide perovskites (LHPs) have received considerable attention thanks to their impressive optoelectronic properties. Today, LHP-based devices are one of the most efficient single-junction solar cells, with power-conversion efficiencies reaching 25.7%. Likewise, nanostructures of the same material have emerged with tremendous potential for light-emitting and lasing applications. The perovskite complex and dynamic structures also make them fascinating for fundamental photophysical studies, not only to apply in the context of the device improvements but also for a much broader class of semiconductors.Typically, charge carrier dynamics on the ultrafast timescale, from a few nanoseconds down to tens of femtoseconds, are monitored by time-resolved spectroscopic techniques using ultrashort laser pulses. The aim of this work is to associate transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy for investigating early photophysical mechanisms in low-dimensional perovskite nanocrystals (PNCs). While most available TRPL experimental techniques are limited by a temporal resolution of a few picoseconds, broadband fluorescence upconversion spectroscopy (FLUPS) is employed to obtain two-dimensional spectro-temporal data with a sub-picosecond temporal resolution.Chapters 1 and 2 provide the scientific background relevant to the thesis. Chapter 1 establishes the fundamental photophysical concepts for bulk and confined semiconductors and provides a general overview of the PNC optoelectronic properties, reviewing the relevant literature. The second chapter focuses on TRPL spectroscopy and details the theoretical and experimental approaches for FLUPS, the primary technique employed in this work.Chapter 3 describes the synthetic procedure to obtain colloidal blue-emitting CsPbBr3 nanoplatelets (NPls). The product is then characterized by standard methods, including transmission electron microscopy, cyclic voltammetry and linear UV-Vis absorption and emission spectroscopy. The consequences of strong quantum and dielectric confinements are highlighted and induce unique optoelectronic properties. The importance of capping ligands engineering for passivating the perovskite surface states is also stressed. Capping ligands can also increase the NPl emission efficiency and long-term stability.Chapters 4 and 5 discuss the charge carrier dynamics under weak and strong excitation regimes. By combining FLUPS and TA data, complex photophysical behaviors within the first ten picoseconds are revealed. Under low excitation intensities, the NPl dynamics are dominated by an ensemble of independent excitons. Besides, at high pump fluences, enhanced carrier-carrier interactions induce surprisingly stable and emissive exciton-exciton pairs, also called biexcitons. The properties and recombination mechanisms of the biexcitons display typical quantum well signatures.Finally, chapter 6 investigates the charge separation and subsequent charge recombination mechanisms in CsPbBr3 NPls combining FLUPS, TA and nanosecond flash photolysis measurements. NPl-acceptor complexes are synthesized by attaching phenothiazine and p-benzoquinone to the NPl surface, acting as hole and electron acceptors, respectively. Excitation energy-dependent FLUPS measurements are carried out to unveil fast interfacial charge transfer from the perovskite band edge and ultrafast hot carrier transfer from higher energy levels.

Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites

Neha Arora, Ariadni Boziki, Mohammad Ibrahim Dar, Michael Graetzel, Gwénolé Jean Jacopin, Simone Meloni, Ursula Röthlisberger, Shaik Mohammed Zakeeruddin

Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2)(2)PbBr3, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (

2016

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

Andrés Burgos Caminal

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.

EPFL2020

Efficient ab initio semiclassical dynamics for linear and nonlinear electronic spectroscopy

Tomislav Begusic

Molecular spectroscopy is an essential experimental technique in both fundamental physical sciences and applied research. For example, electronic spectroscopy, in which light induces a change in the electronic state of a molecule, is a powerful tool for studying light-matter interactions appearing in solar energy conversion and light-emitting devices. To explain experimental findings and compare them with theoretical predictions, scientists often compute properties that can only be inferred indirectly, rather than simulating the observed spectroscopic signals. This is partly due to challenges associated with the simulation of electronic spectra, which requires both an accurate description of the electronic structure and the quantum-dynamical treatment of the motion of atomic nuclei. As the exact methods to solve the underlying quantum-mechanical problem scale exponentially with the number of atoms, this task must be performed approximately for all but the smallest molecules. In this thesis, we develop a set of methods to systematically study the effects of different approximations on the computed spectra. As our starting point, we consider a semiclassical method called the thawed Gaussian approximation, which has resurfaced in recent years as an efficient and robust alternative to simple but crude models or highly accurate but costly quantum dynamics methods. First, we present several practical improvements of the method, such as generalization to non-Condon effects or the efficient single-Hessian version, which enable a broader set of molecules to be studied. Second, we introduce a more fundamental modification of the methods - the inclusion of non-zero temperature effects. The underlying theory, which is inspired by the concept of so-called thermo-field dynamics, is exact and applies to any quantum dynamics method. Remarkably, within the thawed Gaussian approximation, the inclusion of non-zero temperature comes at nearly no additional computational cost. These methods, developed originally for steady-state linear spectra, are then employed to compute state-of-the-art, time-resolved spectra, which are typically measured with ultrashort laser pulses. More precisely, we study how the anharmonicity of the potential energy surfaces and mode-mode (Duschinsky) couplings affect pump-probe and two-dimensional electronic spectra. This approach, based on the thawed Gaussian wavepacket propagation, is the first method that can exactly evaluate the nonlinear electronic spectra of many-dimensional, shifted, distorted, and Duschinsky-rotated harmonic potentials. As in the case of linear absorption and emission spectra, we again use the concept of thermo-field dynamics to include non-zero temperature effects in the simulation of two-dimensional electronic spectra. The resulting wavepacket picture of two-dimensional spectroscopy at arbitrary temperature could strongly impact how we interpret and simulate multidimensional spectra in the future.