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Organic solar cells have emerged as new promising photovoltaic devices due to their potential applications in large area, printable and flexible solar panels. Organic Solar Cells: Materials and Device Physics offers an updated review on the topics covering the synthesis, properties and applications of new materials for various critical roles in devices from electrodes, interface and carrier transport materials, to the active layer composed of donors and acceptors. Addressing the important device physics issues of carrier and exciton dynamics and interface stability and novel light trapping structures, the potential for hybrid organic solar cells to provide high efficiency solar cells is examined and discussed in detail. Specific chapters covers key areas including: Latest research and designs for highly effective polymer donors/acceptors and interface materials Synthesis and application of highly transparent and conductive graphene Exciton and charge dynamics for in-depth understanding of the mechanism underlying organic solar cells. New potentials and emerging functionalities of plasmonic effects in OSCs Interface Degradation Mechanisms in organic photovoltaics improving the entire device lifetime Device architecture and operation mechanism of organic/ inorganic hybrid solar cells for next generation of high performance photovoltaics This reference can be practically and theoretically applied by senior undergraduates, postgraduates, engineers, scientists, researchers, and project managers with some fundamental knowledge in organic and inorganic semiconductor materials or devices.
"We employ the experimental technique THz Time Domain spectroscopy (THz-TDS) to study the optoelectronic properties of potential photovoltaic materials. This all-optical method is useful for probing photoconductivities in a range of materials on ultrafast timescales without the application of physical contacts. Using this technique we study the process of carrier multiplication (CM) - the excitation of multiple charge carriers by a single photon - in indium nitride (InN). InN possesses a number of properties favorable for efficient CM. However, we find that CM in InN is rather inefficient, contributing only to a modest efficiency increase in a potential InN based solar cell. Additionally, we study the dynamics of photoexcited carriers in 2-dimensional graphene with emphasis on the process of multiple hot carrier generation, which is related to carrier multiplication. A very efficient energy transfer from an optically excited charge carrier into multiple hot carriers is shown. We also perform a study of the photoconductivity of two types of 1-dimensional graphene-based semiconductors, flat graphene nanoribbons and carbon nanotubes. Free charge carriers are observed immediately after excitation. The mobility of these carriers is found to vary significantly for the different types of 1-D conductors. The applicability of these graphene based conductors in organic solar cell architectures is briefly discussed. Finally, we explore the carrier transport properties of colloidal TiO2 films commonly used in dye- and quantum dot sensitized solar cells. We find that the photoresponse is dominated by long percolation pathways of connected particles, responsible for the materials long range conductivity."--Samenvatting auteur.
Organic solar cells have emerged as new promising photovoltaic devices due to their potential applications in large area, printable and flexible solar panels. Organic Solar Cells: Materials and Device Physics offers an updated review on the topics covering the synthesis, properties and applications of new materials for various critical roles in devices from electrodes, interface and carrier transport materials, to the active layer composed of donors and acceptors. Addressing the important device physics issues of carrier and exciton dynamics and interface stability and novel light trapping structures, the potential for hybrid organic solar cells to provide high efficiency solar cells is examined and discussed in detail. Specific chapters covers key areas including: Latest research and designs for highly effective polymer donors/acceptors and interface materials Synthesis and application of highly transparent and conductive graphene Exciton and charge dynamics for in-depth understanding of the mechanism underlying organic solar cells. New potentials and emerging functionalities of plasmonic effects in OSCs Interface Degradation Mechanisms in organic photovoltaics improving the entire device lifetime Device architecture and operation mechanism of organic/ inorganic hybrid solar cells for next generation of high performance photovoltaics This reference can be practically and theoretically applied by senior undergraduates, postgraduates, engineers, scientists, researchers, and project managers with some fundamental knowledge in organic and inorganic semiconductor materials or devices.
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.
Real insight from leading experts in the field into the causes of the unique photovoltaic performance of perovskite solar cells, describing the fundamentals of perovskite materials and device architectures. The authors cover materials research and development, device fabrication and engineering methodologies, as well as current knowledge extending beyond perovskite photovoltaics, such as the novel spin physics and multiferroic properties of this family of materials. Aimed at a better and clearer understanding of the latest developments in the hybrid perovskite field, this is a must-have for material scientists, chemists, physicists and engineers entering or already working in this booming field.
Exfoliation of graphene in 2004 initiated intensive attention on layered two-dimensional (2D) materials. So far, except for graphene, a large family of 2D materials has been reported, such as transition metal dichalcogenides (TMDCs), hexagonal boron nitride, black phosphorene, metal nitrides/carbides and their van der Waals heterostructures, etc. The large number of species, unique electrical and optical properties and versatile functionalities render 2D materials as promising materials for electronic, optoelectronic and photovoltaic devices. This thesis investigates ultrafast charge dynamics in 2D material system based on real-time time-dependent density functional theory method within Ehrenfest framework. This work is divided into three major parts. In first part, we investigate the ultrafast interlayer charge transfer process in the graphene/WS2 heterostructure. Our results demonstrate that photo-induced holes transfer from WS2 to graphene more efficient than electrons. The ultrafast charge dynamics arises from the coupling to nuclear vibrations and its amplitude and polarity show a strong dependence on the external electric fields. Further analysis reveals that carrier dynamics in the heterostructure is the result of competition between interlayer and intralayer relaxation process, which is governed by the couplings between carriers and their acceptor states. This work establishes a firm correlation between the charge dynamics and couplings between states in 2D heterostructures, and provide practical methodology to manipulate carrier dynamics at heterointerfaces. In second part, we study the carrier multiplication (CM) phenomenon in six monolayer TMDCs MX2 (M = Mo, W; X = S, Se, Te). Our results present that CM is observed in all six TMDCs. The threshold energy of CM can be substantially reduced to 1.75 bandgap (Eg) via couplings to phonon modes. Since electron-phonon couplings can result in significant changes in electronic structures, even trigger semiconductor-metal transition, and eventually assist CM beyond threshold limit. Chalcogen vacancies can further decrease the threshold due to sub-gap defect states. In particular for WS2, CM occurs with excitation energy of only 1.51Eg. Our results identify TMDCs as attractive candidate materials for efficient photovoltaic devices with the advantages of high photo-conductivity and phonon-assisted CM characteristic. In third part, we report the effect of doping levels on CM in graphene. Our calculation results indicate that doping level can introduce remarkable differences in CM conversion efficiency in graphene. Specifically, the CM quantum yield can be promoted from 1.41 to 1.89 when the Fermi level rising from 0.40 eV to 0.78 eV via n-doping. Consistently, time- and angle-resolved photoemission spectroscopy measurements on n-doped graphene present the same correlation between doping levels and CM conversion efficiency. Our results provide a practical strategy to promote the performance of graphene as a photovoltaic material by tuning doping levels.
Semiconductors Probed by Ultrafast Laser Spectroscopy, Volume II discusses the use of ultrafast laser spectroscopy in studying fast physics in semiconductors. It reviews progress on the experimental and theoretical understanding of ultrafast events that occur on a picosecond and nanosecond time scale. This volume discusses electronic relaxation in amorphous semiconductors and the physical mechanisms during and after the interaction of an intense laser pulse with a semiconductor. It also covers the relaxation of carriers in semiconductors; transient optical pulse propagation; and methods of tim ...
There is increasing evidence that the initially generated excited state species in bulk heterojunction solar cell photoactive layers are critical to device performance. At present however, an understanding of the nature and dynamics of such excited states still remains limited. This thesis presents a study of the ultrafast exciton dynamics in bulk heterojunction organic and hybrid organic-inorganic solar cells. Fluorescence upconversion is used to elucidate the dynamics of such transient species allowing internal properties of the blend systems to be probed including changes in film morphology and ultrafast energy loss mechanisms. An understanding of such processes is an important step forward in the evolution of molecular semiconductor based solar cells. The first chapter focuses on the main experimental technique, fluorescence upconversion, and how this can be employed to study excited states. In particular, this section addresses one of the main unanswered questions in the field and attempts to correlate the exciton dynamics with the structure of the common photoactive polymer poly(3-hexylthiophene) (P3HT). Three structural variations of P3HT are studied and their exciton dynamics associated with differing internal processes occurring within the polymers. These include self localisation, and different types of long-range energy transfer mechanisms. The following two chapters build upon the knowledge of exciton dynamics obtained from the first chapter. First, a study is made of amorphous polymers with different acceptors, all based on phenyl-C61-butyric acid methyl ester (PCBM). The distinct interactions of the PCBM-type molecules with the polymer results in different electron transfer dynamics, from which the exciton diffusion length of the polymer in real bulk heterojunction blends is extracted using a simple model. Second, the ultrafast excited state dynamics of a crystalline polymer with the same PCBM-type acceptors is studied. Correlation of these dynamics with thermal analysis of the blend films allows the morphology of the films to be extracted and allows two different mechanisms of microstructure development to be identified. In the final chapter, the effect of acceptor aggregation on exciton dynamics and charge generation yields in hybrid organic-inorganic blend films has been studied. Such aggregation has been shown to be essential for efficient charge generation in all-organic solar cells but has often been assumed to be less important in such inorganic hybrids. More aggregated acceptor nanoparticles are shown to not only result in greater than expected exciton quenching but are also shown to result in a greater yield of long-lived charges. This study is extended to show that in-situ grown inorganic nanoparticles exhibit superior performance to standard pre-synthesised inorganics.