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.