Insight into the nanoscale carrier transport in the rapidly developing class of solutionprocessed semiconductors known as metal halide perovskites is the focal point for these studies. Further advancement in fundamentally understanding photophysical processes associated with charge carrier transport is needed to realize the true potential of perovskites for photovoltaic applications. In this work, we study photogenerated carrier transport to understand the underlying transport behavior of the material on the 10s to 100s nanometer lengthscales. To study these processes, we employ a temporally-resolved and spatially-resolved technique, known as transient absorption microscopy, to elucidate the charge carrier dynamics and propagation associated with metal halide perovskites. This technique provides a simultaneous high temporal resolution (200 fs) and spatial resolution (50 nm) to allow for direct visualization of charge carrier migration on the nanometer length scale. There are many obstacles these carriers encounter between photogeneration and charge collection such as morphological effects (grain boundaries) and carrier interactions (scattering processes). We investigate carrier transport on the nanoscale to understand how morphological effects influence the materials transport behavior. Morphological defects such as voids and grain boundaries are inherently small and traditionally difficult to study directly. Further, because carrier cooling takes place on an ultrafast time scale (fs to ps), the combined spatial and temporal resolution is necessary for direct probing of hot (non-equilibrium) carrier transport. Here we investigate a variety of ways to enhance carrier transport lengthscales by studying how non-equilibrium carriers propagate throughout the material, as well as, carrier cooling mechanisms to extend the non-equilibrium regime. For optoelectronic devices based on polycrystalline semiconducting thin films, grain boundaries are important to consider since solution-based processing results in the formation of well-defined grains. In Chapter 3, we investigate equilibrium carrier transport in metal halide perovskite thin films that are created via the highly desired solution processing method. Carrier transport across grain boundaries is an important process in defining efficiency due to the literary discrepancies on whether the grains limit carrier transport or not. In this work, we employ transient absorption microscopy to directly measure carrier transport within and across the boundaries. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation. The results of this work provide insight into why this defect tolerant material performs so well. Photovoltaic performance (power conversion efficiency) is governed by the ShockleyQueisser limit which can be overcame if hot carriers can be harvested before they thermalize. To convert sunlight to usable electricity, the photogenerated charge carriers need to migrate long distances and or live long enough to be collected. It is unclear whether these hot carriers can migrate a long enough distance for efficient collection. In Chapter 4, we report direct visualization of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy. This work demonstrates three distinct transport regimes. (i) Quasiballistic transport, (ii) nonequilibrium transport, and (iii) diffusive transport. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers of transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites to ultimately overcome the Shockley-Queisser limit. In the next work, we investigated a way to extend non-equilibrium carrier lifetime, which ultimately corresponds to an accelerated carrier transport. From the knowledge of the hot carrier transport work, we showed a proof of concept that the excess kinetic energy corresponds to long range carrier transport. To further develop the idea of harvesting hot carriers, one must investigate a way to make the carriers stay hot for a longer period (i.e. cool down slower). In Chapter 5, we slow down the cooling of hot carriers via a phonon bottleneck, which points toward the potential to overcome the Shockley-Queisser limit. Open questions remain on whether the high optical phonon density from the bottleneck impedes the transport of these hot carriers. We show a direct visualization of hot carrier transport in the phonon bottleneck regime in both single crystalline and polycrystalline lead halide perovskites, more specifically, a relatively new class of alkali metal doped perovskites (RbCsMAFA), which has one of the highest power conversion efficiencies. Remarkably, hot carrier diffusion is enhanced by the presence of a phonon bottleneck, the exact opposite from what is observed in conventional semiconductors such as GaAs. These results showcase the unique aspects of hot carrier transport in hybrid perovskites and suggest even larger potential for hot carrier devices than previously envisioned by the initial results presented in Chapter 4. The final chapter will be divided into two sections, as we summarize and highlight our collaborative efforts towards homogenization of carrier dynamics via doping perovskites with alkali metals and our work on two-dimensional hybrid quantum well perovskites. Further studies on the champion solar cell (RbCsMAFA) were performed to elucidate the role inorganic cations play in this material. By employing transient absorption microscopy, we show that alkali metals Rb+ and Cs+ are responsible for inducing a more homogenous halide (Iand Br- ) distribution, despite the partial incorporation into the perovskite lattice. This translates into improved electronic dynamics, including fluorescence lifetimes above 3 µs and homogenous carrier dynamics, which was visualized by ultrafast microscopy. Additionally, there is an improvement in photovoltaic device performance. We find that while Cs cations tend to distribute homogenously across the perovskite grain, Rb and K cations tend to phase segregate at precursor concentrations as low as 1%. These precipitates have a counter-productive effect on the solar cell, acting as recombination centers in the device, as argued from electron beam-induced current measurements. Remarkably, the high concentration of Rb and Cs agglomerations do not affect the open-circuit voltage, average lifetimes, and photoluminescence distribution, further indicating the perovskite’s notorious defect tolerance. A new class of high-quality two dimensional organic-inorganic hybrid perovskite quantum wells with tunable structures and band alignments was studied. By tuning the functionality of the material, the strong self-aggregation of the conjugated organic molecules can be suppressed, and 2D organic-halide perovskite superlattice crystals and thin films can be easily obtained via onestep solution-processing. We observe energy transfer and charge transfer between adjacent organic and inorganic layers, which is extremely fast and efficient (as revealed by ultrafast spectroscopy characterizations). Remarkably, these 2D hybrid perovskite superlattices are stable, due to the protection of the bulky hydrophobic organic groups. This is a huge step towards the practicality of using perovskites for optoelectronics, since stability is always a huge concern with water-sensitive materials. The molecularly engineered 2D semiconductors are on par with III-V quantum wells and are promising for next-generation electronics, optoelectronics, and photonics.

<div>Recently, atomically thin two-dimensional (2D) layered materials such as graphene and transition metal dichalcogenides (TMDCs) have emerged as a new class of materials due to their unique electronic structures and optical properties at the nanoscale limit. 2D materials also hold great promises as building blocks for creating new heterostructures for optoelectronic applications such as atomically thin photovoltaics, light emitting diodes, and photodetectors. Understanding the fundamental photo-physics process in 2D semiconductors and heterostructures is critical for above-mentioned applications. </div><div>In Chapter 1, we briefly describe photo-generated charge carriers in two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors and heterostructures. Due to the reduced dielectric screening in the single-layer or few-layer of TMDCs semiconductors, Columbo interaction between electron and hole in the exciton is greatly enhanced that leads to extraordinary large exciton binding energy compared with bulk semiconductors. The environmental robust 2D excitons provide an ideal platform to study exciton properties in TMDCs semiconductors. Since layers in 2D materials are holding by weak van de Waals interaction, different 2D layers could be assembled together to make 2D heterostructures. The successful preparation of 2D heterostructures paves a new path to explore intriguing optoelectronic properties.</div><div>In Chapter 2, we introduce various optical microscopy techniques used in our work for the optical characterization of 2D semiconductors and heterostructures. These optical imaging tools with high spatial and temporal resolution allow us to directly track charge and energy flow at 2D interfaces.</div><div>Exciton recombination is a critical factor in determining the efficiency for optoelectronic applications such as semiconductor lasers and light-emitting diodes. Although exciton dynamics have been investigated in different 2D semiconductor, large variations in sample qualities due to different preparation methods have prevented obtaining intrinsic exciton lifetimes from being conclusively established. In Chapter 3, we study exciton dynamics in 2D TMDCs semiconductors using ultrafast PL and transient absorption microscopy. Here we employ 2D WS2 semiconductor as a model system to study exciton dynamics due to the low defect density and high quantum yield of WS2. We mainly focus on how the exciton population affects exciton dynamics. At low exciton density regime, we demonstrate how the interlayer between the bright and dark exciton populations influence exciton recombination. At high exciton density regime, we exhibit significant exciton-exciton annihilation in monolayer WS2. When comparing with the bilayer and trilayer WS2, the exciton-exciton annihilation rate in monolayer WS2 increases by two orders of magnitude due to enhanced many-body interactions at single layer limit. </div><div>Long-range transport of 2D excitons is desirable for optoelectronic applications based on TMDCs semiconductors. However, there still lacks a comprehensive understanding of the intrinsic limit for exciton transport in the TMDCs materials currently. In Chapter 4, we employ ultrafast transient absorption microscopy that is capable of imaging excitons transport with ~ 200 fs temporal resolution and ~ 50 nm spatial precision to track exciton motion in 2D WS2 with different thickness. Our results demonstrate that exciton mobility in single layer WS2 is largely limited by extrinsic factors such as charge impurities and surface phonons of the substrate. The intrinsic phonon-limited exciton transport is achieved in WS2 layers with a thickness greater than 20 layers.</div><div>Efficient photocarrier generation and separation at 2D interfaces remain a central challenge for many optoelectronic applications based on 2D heterostructures. The structural tunability of 2D nanostructures along with atomically thin and sharp 2D interfaces provides new opportunities for controlling charge transfer (CT) interactions at 2D interfaces. A largely unexplored question is how interlayer CT interactions contribute to interfacial photo-carrier generation and separation in 2D heterostructures. In Chapter 5, we present a joint experimental and theoretical study to address carrier generation from interlayer CT transitions in WS2-graphene heterostructures. We use spatially resolved ultrafast transient absorption microscopy to elucidate the role of interlayer coupling on charge transfer and photo-carrier generation in WS2-graphene heterostructures. These results demonstrate efficient broadband photo-carrier generation in WS2-graphene heterostructures which is highly desirable for atomically thin photovoltaic and photodetector applications based on graphene and 2D semiconductors.</div><div>CT exciton transport at heterointerfaces plays a critical role in light to electricity conversion using 2D heterostructures. One of the challenges is that direct measurements of CT exciton transport require quantitative information in both spatial and temporal domains. In order to address this challenge, we employ transient absorption microscopy (TAM) with high temporal and spatial resolution to image both bright and dark CT excitons in WS2-tetrance and CVD WS2-WSe2 heterostructure. In Chapter 6, we study the formation and transport of interlayer CT excitons in 2D WS2-Tetracene vdW heterostructures. TAM measurements of CT exciton transport at these 2D interfaces reveal coexistence of delocalized and localized CT excitons. The highly mobile delocalized CT excitons could be the key factor to overcome large CT exciton binding energy in achieving efficient charge separation. In Chapter 7, we study stacking orientational dependent interlayer exciton recombination and transport in CVD WS2-WSe2 heterostructures. Temperature-dependent interlayer exciton dynamics measurements suggest the existence of moiré potential that localizes interlayer excitons. TAM measurements of interlayer excitons transport reveal that CT excitons at WS2-WSe2 heterointerface are much more mobile than intralayer excitons of WS2. We attributed this to the dipole-dipole repulsion from bipolar interlayer excitons that efficiently screen the moiré potential fluctuations and facilitate interlayer exciton transport. Our results provide fundamental insights in understanding the influence of moiré potential on interlayer exciton dynamics and transport in CVD WS2-WSe2 heterostructures which has important implications in optoelectronic applications such as atomically thin photovoltaics and light harvesting devices. </div><div><br></div>

<p> Excitons represent the electronic excited state of organic semiconductor and many low-dimensional inorganic semiconductors. In solar energy conversion systems, exciton transport affects how fast the charges reach the electrodes thus governs the performance of photovoltaic cells. In optoelectronic applications such as semiconductor lasers and light-emitting diodes, exciton radiative rate determines the efficiency of luminescence in competition to various nonradiative processes. Therefore, understanding how exciton migrates over space as well as its decay dynamics are vital for the design of highly efficient optoelectronic devices. To interrogate these photophysical processes requires experimental tools with simultaneous high temporal and spatial resolution. In this thesis, I introduce two transient imaging systems (photoluminescence imaging with 300 ps time resolution, and transient absorption microscopy with 200 fs time) that are innovative tools to directly probe excited state dynamics and transport in sub-μm domains. The techniques were applied to a type of promising semiconductor, perovskites, including surface-passivated hybrid perovskite and 2D layered perovskites to explore the fundamental mechanisms that affect exciton transport. The fundamental understanding of excitons shed light on the underlying physics such as exciton delocalization, exciton-exciton interaction, and how these properties affected by the static and dynamic disorders of the material. We further demonstrated a novel twisted superlattice using ultrathin perovskites that confines excitons due to increased density of state from the moiré flat bands. In addition, excitons can be accelerated by strongly interacts photons, forming polariton quasiparticles that possess small effective mass. This is demonstrated by coupling 2D layered perovskites to a plasmonic array. We further showcase the formation of bulk polaritons without an external optical cavity in a self-assembled organic aggregate. Experimental investigation into these intriguing phenomena provide an approach to study fundamental processes such as many-body interaction and quantum coherence. </p>