Download or read book Charge Carrier Dynamics in Colloidal Quantum Dots written by and published by . This book was released on 2015 with total page 186 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Investigation of Charge Carrier Dynamics in Organo halide Perovskite and Colloidal Quantum Dot Semiconductors written by Robert Stewart and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Crystallite surfaces often dominate the optical and electronic properties of nanocrystalline semiconductors because the fraction of atoms at the surface experience different crystal field environments than bulk atoms, and as a consequence, surfaces influence the energetic landscape of the entire crystallite. Contributing electronic states that are either isoenergetic or below the semiconductor band edges, surface states mediate charge conduction and recombination - two critical processes in optoelectronic device performance. Utilizing a combination of inorganic synthesis, surface characterization, and time-resolved optical spectroscopy, the research presented herein begins to identify the link between charge carrier dynamics and the underlying surface chemistry in two emerging, yet promising, nanocrystal semiconductor systems: organo-halide perovskites and colloidal quantum dots (CQD).Notably, my research provided one of the first reports that charge recombination centers in lead halide perovskite films are localized almost exclusively on the surface of crystallites. Passivation of these nanocrystal surfaces with small molecules that contain strongly coordinating functional groups caused charge-carrier lifetimes in perovskite thin-films to approach the bulk radiative limit reported for single crystal analogues. Likewise, my research contributed to an understanding that surfaces in lead sulfide (PbS) CQDs produce electronic energy levels that are sufficiently delocalized to provide charge conduction pathways in CQD thin-film arrays. Given the strong coupling to the QD surface, charge carrier diffusion lengths were shown to be highly sensitive to the character of surface-bound ligands. My PhD research highlights the importance of understanding the interplay between surface chemistry and nanocrystal semiconductor photophysics as wellivas the importance of selecting surface treatment strategies capable of passivating diversesurfaces to eliminate energetic inhomogeneity while simultaneously allowing strongelectronic coupling across interfaces.

Download or read book Carrier Dynamics and Lasing Applications of Colloidal Quantum Dots written by Golam Bappi and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Colloidal quantum dots have seen progress over the last three decades as an active material for solution processed optoelectronics. Quantum dots offer a tunable optical bandgap from the UV to the mid-IR via control over size and chemical composition. Their optical and electronic properties can be further manipulated through surface engineering and heterostructuring. These materials are processed from solution, enabling low-cost fabrication; and are compatible with a wide range of substrates.In this thesis, I investigate properties of colloidal quantum dots for lasing applications. My findings illuminate fundamental processes that determine their performance in lasing; and point to strategies to overcome present-day limitations. First, I investigate the effects of temperatures reached during continuous-wave excitation on the charge carrier dynamics in CdSe/CdS core/shell QDs, and their effect on the lasing threshold. Modelling and experimental characterization reveal a temperature-activated sub- picosecond electron trapping process that depletes the population of excited QDs. Accordingly, a small decrease in the athermal lasing threshold can yield a large decrease in the continuous- wave lasing threshold due to reduced heat generation. In CdSe/CdS QDs, built-in biaxial strain reduces the valence band-edge degeneracy, lowering the athermal and CW lasing threshold by 30% and 70% respectively. Next I investigate graded CdSe/CdS shells on infrared InAs QDs to suppress non-radiative biexciton Auger recombination. Infrared InAs QDs are promising materials for infrared light emitting devices, but their Auger lifetime is much shorter than those found in more widely explored cadmium and lead chalcogenide materials. The graded CdSe/CdS shells on InAs which I develop herein result in a 2x increase in the Auger lifetime relative to the best value reported in prior InAs QD literature. Finally, I propose a method to achieve nanosecond deep-blue lasing using CsPbCl3 QDs. These perovskite quantum dots suffer from fast biexciton Auger lifetimes, and are consequently able to sustain lasing only under femtosecond pulsed photoexcitation. Forming a superlattice of QDs with aligned dipoles, and coupling them to a high Q-factor distributed feedback grating, is a step toward quasi-CW lasing in this materials system. I design the grating for single mode operation within the gain spectrum of the CsPbCl3 QDs.

Download or read book Charge Carrier Dynamics in Lead Sulfide Quantum Dot Solids written by Rachel Hoffman Gilmore and published by . This book was released on 2017 with total page 117 pages. Available in PDF, EPUB and Kindle. Book excerpt: Quantum dots, also called semiconductor nanocrystals, are an interesting class of materials because their band gap is a function of the quantum dot size. Their optical properties are not determined solely by the atomic composition, but may be engineered. Advances in quantum dot synthesis have enabled control of the ensemble size dispersity and the creation of monodisperse quantum dot ensembles with size variations of less than one atomic layer. Quantum dots have been used in a variety of applications including solar cells, light-emitting diodes, photodetectors, and thermoelectrics. In many of these applications, understanding charge transport in quantum dot solids is crucial to optimizing efficient devices. We examine charge transport in monodisperse, coupled quantum dot solids using spectroscopic techniques explained by hopping transport models that provide a complementary picture to device measurements. In our monodisperse quantum dot solids, the site-to-site energetic disorder that comes from size dispersity and the size-dependent band gap is very small and spatial disorder in the quantum dot superlattice often has a greater impact on charge transport. In Chapter 2, we show that improved structural order from self-assembly in monodisperse quantum dots reduces the interparticle spacing and has a greater impact than reduced energetic disorder on increasing charge carrier hopping rates. In Chapter 3, we present temperature-dependent transport measurements that demonstrate again that when energetic disorder is very low, structural changes will dominate the dynamics. We find increasing mobility with decreasing temperature that can be explained by a 1-2 Å contraction in the edge-to-edge nearest neighbor quantum dot spacing. In Chapter 4, we study optical states that are 100-200 meV lower in energy than the band gap. Because we work with monodisperse quantum dots, we are able to resolve this trap state separately from the band edge state and study its optical properties. We identify the trap state as dimers that form during synthesis and ligand exchange when two bare quantum dot surfaces fuse. The findings of this thesis point to the importance of minimizing the structural disorder of the coupled quantum dot solid in addition to the energetic disorder to optimize charge carrier transport.

Download or read book Charge Carrier Dynamics in Nontoxic Semiconductor Quantum Dots written by Alexander Florian Richter and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Surface Effects on Charge Carrier Dynamics in Semiconductor Quantum Dots written by Pooja Tyagi and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "Due to the large surface-to-volume ratio of quantum dots, their surface conditions play a significant role in determining their electronic and optical properties. In this thesis, we show that the presence of surface states modifies the optical selection rules in quantum dots and enhances the rate of surface charge trapping. These surface-induced effects have profound impact on the measurement of multiexciton recombination and carrier multiplication processes. Specifically, in transient absorption studies, surface states result in additional decay timescales which may be misattributed to multiexciton recombination processes. Additionally, they lead to large "apparent" carrier multiplication yields even under conditions where it is forbidden by energy conservation. The surface-dependent transient absorption studies presented in this work suggest ways to identify and minimize the undesirable surface-induced signals. Interestingly, surface-induced processes also result in significant electrostatic effects. We show that due to the piezoelectric nature of wurtzite CdSe quantum dots, the strong electric field created by surface charge trapping can drive coherent acoustic phonons in these systems. We further show that the amplitude of this piezoelectric response can be controlled by altering the surface conditions of the quantum dot. Finally, we theoretically investigate the effect of multiple surface layers on carrier localization in nanostructures. We find that in a core/barrier/shell configuration, layered nanostructures offer independent control over electron and hole wave functions. These results suggest design principles for wave function engineering in potential quantum dot applications in light emitting devices, photovoltaics and optical amplification." --

Download or read book Charge Carrier Dynamics and the Development of Optical Gain in Semiconductor Quantum Dots written by Ryan R. Cooney and published by . This book was released on 2009 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Colloidal Quantum Dot Optoelectronics and Photovoltaics written by Gerasimos Konstantatos and published by Cambridge University Press. This book was released on 2013-11-07 with total page 329 pages. Available in PDF, EPUB and Kindle. Book excerpt: Captures the most up-to-date research in the field, written in an accessible style by the world's leading experts.

Download or read book Nanocrystal Quantum Dots written by Victor I. Klimov and published by CRC Press. This book was released on 2017-12-19 with total page 485 pages. Available in PDF, EPUB and Kindle. Book excerpt: A review of recent advancements in colloidal nanocrystals and quantum-confined nanostructures, Nanocrystal Quantum Dots is the second edition of Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, originally published in 2003. This new title reflects the book’s altered focus on semiconductor nanocrystals. Gathering contributions from leading researchers, this book contains new chapters on carrier multiplication (generation of multiexcitons by single photons), doping of semiconductor nanocrystals, and applications of nanocrystals in biology. Other updates include: New insights regarding the underlying mechanisms supporting colloidal nanocrystal growth A revised general overview of multiexciton phenomena, including spectral and dynamical signatures of multiexcitons in transient absorption and photoluminescence Analysis of nanocrystal-specific features of multiexciton recombination A review of the status of new field of carrier multiplication Expanded coverage of theory, covering the regime of high-charge densities New results on quantum dots of lead chalcogenides, with a focus studies of carrier multiplication and the latest results regarding Schottky junction solar cells Presents useful examples to illustrate applications of nanocrystals in biological labeling, imaging, and diagnostics The book also includes a review of recent progress made in biological applications of colloidal nanocrystals, as well as a comparative analysis of the advantages and limitations of techniques for preparing biocompatible quantum dots. The authors summarize the latest developments in the synthesis and understanding of magnetically doped semiconductor nanocrystals, and they present a detailed discussion of issues related to the synthesis, magneto-optics, and photoluminescence of doped colloidal nanocrystals as well. A valuable addition to the pantheon of literature in the field of nanoscience, this book presents pioneering research from experts whose work has led to the numerous advances of the past several years.

Download or read book Effects of Surface and Chemical Composition on the Charge Carriers of Alloyed Quantum Dots written by Joseph Daniel Keene and published by . This book was released on 2015 with total page 102 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Semiconducting Nanocrystals Synthesis Colloidal Stability and Charge Carrier Dynamics written by Shuai Chen and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Colloidal nanocrystals (NCs) with controllable morphologies and sizes are at the core of nanomaterials research. Among different NCs (zero-dimensional quantum dots, one-dimensional nanorods, etc.), two-dimensional cadmium selenide (CdSe) nanoplatelets (NPLs) with lateral extension of over 100 nm2 and atomically precise thickness are one of the most promising semiconductors for optoelectronic applications. However, several issues remain for implementing CdSe NPLs for applications, including: (1) containing toxic and heavy elements, and (2) lacking a fundamental understanding of the colloidal stability and the process of charge carrier generation in solids. This thesis aims to synthesize environmentally friendly alternatives, such as indium phosphide (InP) NCs with varied shapes, and shed light on colloidal stability and charge carrier generation and dynamics using conventional CdSe NPLs as the model system. The introductory Chapter 1 briefly presents an overview of nanomaterials and nanotechnology, including quantum confinement in NCs, quantum dots, InP NCs, the synthesis status of CdSe NPLs, colloidal NCs stabilization in dispersion, photophysics of CdSe NPLs, ligand exchange method, and the thermal annealing method. In Chapter 2, we introduce terahertz (THz) spectroscopy basics, including the generation and detection mechanism, experimental setups, and the conductivity models for data analysis to infer charge transport properties in materials of interest. Chapters 3-5 present the key results of this thesis. In Chapter 3, we synthesize InP NCs with different shapes and sizes by utilizing triphenyl phosphite as the phosphorus source. We show that InP NCs can be synthesized and formed from In2O3 NCs. Our findings open new synthetic possibilities by utilizing a cost-effective, non-pyrophoric, and non-toxic phosphorus precursor. In Chapter 4, we combine THz photoconductivity measurements with simulations to study the colloidal stability and aggregation process in CdSe NPL dispersion. We demonstrate that increasing the facet area of NPLs and the solvent length can reduce the critical concentration at which NPLs start aggregating. Our simulation attributes the effect to the solvation force-dominated colloidal stability for the NPL system; the solvation force is enhanced by increasing the NPL lateral area and the length of solvent molecules. In Chapter 5, we report that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. We observe a direct correlation between the temperature-dependent ligand decomposition and the NPL-NPL distance shortening (by TEM). This leads to a strong red-shift in the absorption band edge (by UV-vis studies) and enhanced electrical transport Abstract in NPL films. Our results illustrate a straightforward manner to control the interfacial electronic coupling strength for developing functional optoelectronics through thermal treatments.

Download or read book Quantum Dot Molecules written by Jiang Wu and published by Springer Science & Business Media. This book was released on 2013-10-28 with total page 383 pages. Available in PDF, EPUB and Kindle. Book excerpt: A quantum dot molecule (QDM) is composed of two or more closely spaced quantum dots or “artificial atoms.” In recent years, QDMs have received much attention as an emerging new artificial quantum system. The interesting and unique coupling and energy transfer processes between the “artificial atoms” could substantially extend the range of possible applications of quantum nanostructures. This book reviews recent advances in the exciting and rapidly growing field of QDMs via contributions from some of the most prominent researchers in this scientific community. The book explores many interesting topics such as the epitaxial growth of QDMs, spectroscopic characterization, and QDM transistors, and bridges between the fundamental physics of novel materials and device applications for future information technology. Both theoretical and experimental approaches are considered. Quantum Dot Molecules can be recommended for electrical engineering and materials science department courses on the science and design of advanced and future electronic and optoelectronic devices.

Download or read book Design and Optimization of Colloidal Quantum Dot Solids for Enhanced Charge Transport and Photovoltaics written by Sangjin Lee (Ph. D.) and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Colloidal quantum dots (CQDs) have attracted much attention due to their distinctive optical properties such as wide spectral responses and tunable absorption spectra with simple size control. These properties, together with the advantages of solution processing and superior robustness to organic materials, have motivated the recent investigation of CQD-based solar cells, which have seen rapid growth in power conversion efficiency in just the last 10 years, to a current record of over 10%. However, in order to continue to push the efficiencies higher, a better understanding of the charge transport phenomena in CQD films is needed. While the carrier transport mechanisms between isolated molecules have been explored theoretically and the device-scale mobility of CQD layers has been characterized using experimental measurements such as time-of-flight analysis and field-effect-transistor measurements, a systematic study of the connection between these two distinct scales is required in order to provide crucial information regarding how CQD layers with higher charge carrier mobility can be achieved. While a few strategies such as ligand exchanges, band-like transport, and trap-state-mediated transport have been suggested to enhance the charge carrier mobility, inhomogeneity in CQD solids has been considered a source of the mobility degradation because the electronic properties in individual CQDs may have dispersions introduced in the synthesis and/or in the deposition process, leading to the deviations of the localized energy states from the regular positions or the average energy levels. Here, we suggest that control over such design factors in CQD solids can provide important pathways for improvements in device efficiencies as well as the charge carrier mobility. In particular, we have focused on the polydispersity in CQDs, which normally lies in the range of 5-15%. The effect of size-dispersion in CQD solids on the charge carrier mobility was computed using charge hopping transport models. The experimental film deposition processes were replicated using a molecular dynamics simulation where the equilibrium positions of CQDs with a given radii distribution were determined under a granular potential. The radii and positions of the CQDs were then used in the charge hopping transport simulator where the carrier mobility was estimated. We observed large decreases (up to 70%) in electron mobility for typical experimental polydispersity (about 10%) in CQD films. These large degradations in hopping charge transport were investigated using transport vector analysis with which we suggested that the site energy differences raised the portion of the off-axis rate of charge transport to the electric field direction. Furthermore, we have shown that controlling the size distribution remarkably impacts the charge carrier mobility and we suggested that tailored and potentially experimentally achievable re-arrangement of the CQD size ensemble can mediate the mobility drops even in highly dispersive cases, and presents an avenue towards improved charge transport. We then studied the degradation in CQD solar cells with respect to the polydispersity and how these enhanced charge transport from re-design of CQD solids can boost the photovoltaic performances. In addition, we estimated the potential in the binary CQD solids in terms of their improved charge transport and efficient light absorption. Combined with the accurate size-dependent optical absorption model for CQDs, our hopping model confirmed that the inclusion of smaller CQDs could enhance both the charge transport and the solar light absorption, leading to the enhanced average charge generation rates and solar cell performance.

Download or read book Investigating Carrier Dynamics in Photovoltaic Materials written by Mohammad Mehdi Taheri and published by . This book was released on 2021 with total page 209 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding photoexcited carrier dynamics is crucial for designing high-performance optoelectronic devices. Carrier cooling in semiconductors, charge transfer across interfaces, and recombination mechanisms are critical processes in photophysical systems that typically occur on the time scale of less than a picosecond to several nanoseconds. Ultrafast techniques, including ultraviolet-visible-infrared transient absorption (TA), time-resolved terahertz spectroscopy (TRTS), and time-resolved photoluminescence (TRPL), are ideal tools for studying charge carrier dynamics at such timescales. This thesis will focus on the application of complementary spectroscopy techniques and modeling to investigate carrier dynamics within CdSe/CdS core/shell colloidal quantum dots (QDs) and Cu3AsS4 and CdTe thin films.CdTe solar technology has attracted the photovoltaic (PV) community for the past three decades owing to its low production cost and record efficiency of 22.1%. However, some challenges must be overcome to further improve its efficiency to the 25% range. Cu3AsS4 thin film is a promising emerging candidate as a PV absorber material due to its earth-abundant and nontoxic constituent elements, but its optoelectronic properties are not well known. Carrier dynamics reveal important details about the recombination processes that limit PV performance. Improvements in the PV device efficiency require a full understanding of the routes for carrier recombination processes.TRPL, which measures emission, has conventionally been used to evaluate recombination mechanisms in thin film PVs, but carrier redistribution often dominates the response at short times. Here we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both TRTS, which measures photoconductivity, and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We were able to distinguish and quantify bulk and surface recombination in CdTe and Cu3AsS4 thin films, which is critical for the development of thin film PVs with higher efficiency.We also investigated the carrier dynamics in functionalized CdSe/CdS core/shell QDs using complementary ultrafast TA and TRPL spectroscopies and kinetic modeling. Cd-chalcogenide QDs have been widely studied because of their excellent optical properties and their facile tunability. The Cd-chalcogenide QDs have been studied for more than 20 years, but the ambiguities in the interpretation of the TA spectra are still under debate. For one thing, the photoexcited TA signal in Cd-chalcogenide QDs has been fully attributed to conduction band electrons, neglecting any contributions from valence band holes. In this work, we present a comprehensive picture of the electronic processes in photoexcited CdSe/CdS core/shell QDs. We have demonstrated through complementary spectroscopic experiments and kinetic modeling that holes affect the TA results and can contribute ~ 30% to the visible range and ~ 72% to the mid-IR range. The comprehensive picture of photophysical processes provided by the complementary ultrafast techniques and kinetic modeling in this work can accelerate both the fundamental science and application development of nanostructured and molecular systems.This thesis will focus on the application of spectroscopy techniques and modeling to investigate carrier dynamics in optoelectronic systems including thin film PVs and colloidal CdSe based QDs. The methodologies presented in this thesis can serve as a guideline for the accurate interpretation of spectroscopic measurements not only for the cases studied here but also for other optoelectronic systems.

Download or read book Colloidal Quantum Dot Optoelectronics and Photovoltaics written by Gerasimos Konstantatos and published by Cambridge University Press. This book was released on 2013-11-07 with total page 478 pages. Available in PDF, EPUB and Kindle. Book excerpt: Capturing the most up-to-date research in colloidal quantum dot (CQD) devices, this book is written in an accessible style by the world's leading experts. The application of CQDs in solar cells, photodetectors and light-emitting diodes (LEDs) has developed rapidly over recent years, promising to transform the future of clean energy, communications, and displays. This complete guide to the field provides researchers, students and practitioners alike with everything they need to understand these developments and begin contributing to future applications. Introductory chapters summarise the fundamental physics and chemistry, whilst later chapters review the developments that have propelled the field forwards, systematically working through key device advances. The science of CQD films is explained through the latest physical models of semiconductor transport, trapping and recombination, whilst the engineering of organic and inorganic multilayered materials is shown to have enabled major advances in the brightness and efficiency of CQD LEDs.

Download or read book Carrier Dynamics in Semiconductor Quantum Dots written by Jörg Siegert and published by . This book was released on 2006 with total page 99 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Optical Properties of Semiconductor Nanocrystals written by S. V. Gaponenko and published by Cambridge University Press. This book was released on 1998-10-28 with total page 263 pages. Available in PDF, EPUB and Kindle. Book excerpt: Examines the optical properties of low-dimensional semiconductor structures, a hot research area - for graduate students and researchers.