Graphene mate rials have been researched as viable alternatives of transparent conductors electrodes (TCEs) in this thesis. Current study focuses on few layer graphene (FLG), reduced graphene oxide (rGO) and their hybrids with carbon nanotubes (CNTs) for TCE applications inorganic solar cells (OSCs). FLGs and rGOs have been prepared by mechanical and microwave-assisted exfoliation methods. This films of these materials have been produced by hot-spray method. Results of charge transport characterizations by four-point probes, transparency (UV-Vis), measurements, along with morphological (SEM, TEM) and topgraphic (AFM) studies of films have been presented. UPS studies were performed to determine for a work-function. XPS,Raman and Photoluminescence studies have been employed to obtain the information about the structural quality of the samples.
This book provides a systematic presentation of the principles and practices behind the synthesis and functionalization of graphene and grapheme oxide (GO), as well as the fabrication techniques for transparent conductors from these materials. Transparent conductors are used in a wide variety of photoelectronic and photovoltaic devices, such as liquid crystal displays (LCDs), solar cells, optical communication devices, and solid-state lighting. Thin films made from indium tin oxide (ITO) have thus far been the dominant source of transparent conductors, and now account for 50% of indium consumption. However, the price of Indium has increased 1000% in the last 10 years. Graphene, a two-dimensional monolayer of sp2-bonded carbon atoms, has attracted significant interest because of its unique transport properties. Because of their high optical transmittance and electrical conductivity, thin film electrodes made from graphene nanosheets have been considered an ideal candidate to replace expensive ITO films. Graphene for Transparent Conductors offers a systematic presentation of the principles, theories and technical practices behind the structure–property relationship of the thin films, which are the key to the successful development of high-performance transparent conductors. At the same time, the unique perspectives provided in the applications of graphene and GO as transparent conductors will serve as a general guide to the design and fabrication of thin film materials for specific applications.
The isolation of free-standing graphene in 2004 was the spark for a new scientific revolution in the field of optoelectronics. Due to its extraordinary optoelectronic and mechanical properties, graphene is the next wonder material that could act as an ideal low-cost alternative material for the effective replacement of the expensive conventional materials used in organic optoelectronic applications. Indeed, the enhanced electrical conductivity of graphene combined with its high transparency in visible and near-infrared spectra, enabled graphene to be an ideal low-cost indium tin oxide (ITO) alternative in organic solar cells (OSCs). The prospects and future research trend in graphene-based TCE are also discussed. On the other hand, solution-processed graphene combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates making it compatible with printing and coating technologies, such as roll-to-roll, inkjet, gravure, and flexographic printing manufacturing methods. This chapter provides an overview of the most recent research progress in the application of solution-processed graphene-based films as transparent conductive electrodes (TCEs) in OSCs. (a) Chemically converted graphene (CCG), (b) thermally and photochemically reduced graphene oxide, (c) composite reduced graphene oxide-carbon nanotubes, and (d) reduced graphene oxide mesh films have demonstrated their applicability in OSCs as transparent, conductive electrodes.
This dissertation, "Transparent Electrode Design and Interface Engineering for High Performance Organic Solar Cells" by Di, Zhang, 张笛, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: With the growing needs for energy, photovoltaic solar cells have attracted increasing research interests owing to its potentially renewable, feasible and efficient applications. Compared to its inorganic counterparts, organic solar cell (OSC) is highly desirable due to the low-cost processing, light weight, and the capability of flexible applications. While rapid progress has been made with the conversion efficiency approaching 10%, challenges towards high performance OSCs remain, including further improving device efficiency, fully realizing flexible applications, achieving more feasible large-area solution process and extending the stability of organic device. Having understood the key technical issues of designing high performance OSCs, we focus our work on (1) introducing flexible graphene transparent electrodes into OSCs as effective anode and cathode; (2) interface engineering of metal oxide carrier transport layers (CTLs) in OSCs through incorporating plasmonic metal nanomaterials;(3)proposing novel film formation approach for solution-processed CTLs in OSCs in order to improve the film quality and thus device performance. The detailed work is listed below: 1. Design of transparent graphene electrodes for flexible OSCs Flexible graphene films are introduced into OSCs as transparent electrodes, which complement the flexibility of organic materials. We demonstrate graphene can function effectively as both the anode and cathode in OSCs: a) Graphene anode: we propose an interface modification for graphene to function as anode as an alternative to using aconventional polymer CTL. Using the proposed interfacial modification, graphene OSCs show enhanced performance. Further analysis shows that our approach provides favorable energy alignment and improved interfacial contact. b) Graphene cathode: efficient OSCs using graphene cathode are demonstrated, using a new composite CTL of aluminum-titanium oxide (Al-TiO2).We show that the role of Al is two-fold: improving the wettability as well as reducing the work function of graphene. To facilitate electron extraction, self-assembledTiO2is employed on the Al-covered graphene, which exhibits uniform morphology. 2. Incorporation of plasmonic nanomaterialsinto the metal oxide CTLinOSCs By incorporating metallic nanoparticles (NPs) into the TiO2CTLin OSCs, we demonstrate the interesting plasmonic-electrical effect which leads to optically induced charge extraction enhancement. While OSCs using TiO2CTL can only operate by ultraviolet (UV)activation, NP-incorporated TiO2enables OSCs to perform efficiently at a plasmonic wavelength far longer than the UV light. In addition, the effciency of OSCs incorporated with NPs is notably enhanced. We attribute the improvement to the charge injection of plasmonically excited electrons from NPs into TiO2. 3. Formation of uniform TiO2CTLfor large area applications using a self-assembly approach A solution-processed self-assembly method is proposed for forming large-area high-quality CTL films. Owing to the careful control of solvent evaporation, uniform film is formed, leading to enhanced OSC performance. Meanwhile, our method is capable of forming large-area films. This approach can contribute to future low-cost, large-area applications. DOI: 10.5353/th_b5295530 Subjects: Electrodes - Design and construction Solar cells - Mater
The isolation of free-standing graphene in 2004 was the spark for a new scientific revolution in the field of optoelectronics. Due to its extraordinary optoelectronic and mechanical properties, graphene is the next wonder material that could act as an ideal low-cost alternative material for the effective replacement of the expensive conventional materials used in organic optoelectronic applications. Indeed, the enhanced electrical conductivity of graphene combined with its high transparency in visible and near-infrared spectra, enabled graphene to be an ideal low-cost indium tin oxide (ITO) alternative in organic solar cells (OSCs). The prospects and future research trend in graphene-based TCE are also discussed. On the other hand, solution-processed graphene combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates making it compatible with printing and coating technologies, such as roll-to-roll, inkjet, gravure, and flexographic printing manufacturing methods. This chapter provides an overview of the most recent research progress in the application of solution-processed graphene-based films as transparent conductive electrodes (TCEs) in OSCs. (a) Chemically converted graphene (CCG), (b) thermally and photochemically reduced graphene oxide, (c) composite reduced graphene oxide-carbon nanotubes, and (d) reduced graphene oxide mesh films have demonstrated their applicability in OSCs as transparent, conductive electrodes.
The Organic photovoltaic cell (OPV) is a promising technology because of its potential for low-cost high-throughput roll-to-roll manufacturing. Significant improvements have been achieved in power conversion efficiency (PCE) of OPV cells during last two decades. While recent progress in raising the PCE has been encouraging, the PCE of organic solar cells is still limited and needs to be improved to meet the requirement for commercial applications. Further improvements in both material properties and device architectures are necessary. Photocurrent generation in an OPV cell is fundamentally different from the process that takes place in their inorganic counterparts. A detailed understanding of the operation mechanisms of OPV cells and optimization of the fundamental electronic properties of the system (or material) are critical. In this work, I will first discuss major factors that limit the efficiency of bilayer OPV cells, such as exciton binding energy, exciton diffusion length, charge separation and open-circuit voltage. The exciton binding energy is one of the key parameters that govern the operation of OPV cells, and determines the required energy band offset between donor and acceptor, and thus the achievable open-circuit voltage of the donor-acceptor combination. Exciton diffusion is a main bottleneck limiting photocurrent of a bi-layer OPV cell, which depends on material properties and film morphology. The energy loss between optical excitation and extracted electrical power is mainly due to the energy band offset between donor and acceptor in OPV cells. The PCE limit for single junction OPV cell can be estimated based on the findings. In the second part of this work, I will focus on transparent conductors, which are essential components of thin-film optoelectronic devices. Sputtered Indium-Tin-Oxide (ITO) is currently the most commonly used transparent electrode material, but it has a number of shortcomings. There is a clear need for alternative transparent electrodes whose optical and electrical performance is similar to that of ITO but without its drawbacks. The next generation transparent conductor should also be lightweight, flexible, cheap, environmental attractive, and compatible with large-scale manufacturing methods. I will discuss the possibility of using graphene thin films as a replacement for ITO. Theoretical estimates indicate that graphene thin films are promising transparent electrodes for thin-film optoelectronic devices, with an unmatched combination of sheet resistance and transparency. For the first time, we demonstrated that solution-processed graphene thin films can serve as transparent conductive anodes for both OPV cells and organic light-emitting diodes (OLEDs). The graphene electrodes were deposited on quartz substrates by spin-coating of an aqueous dispersion of functionalized graphene, followed by a vacuum anneal step to reduce the sheet resistance. Small molecular weight organic materials and a metal cathode were directly deposited on the graphene anodes, resulting in devices with a performance comparable to control devices on ITO transparent anodes. Device modeling has been explored to compare the performance between graphene-based device and ITO-based control device. Transfer of graphene films to a foreign flexible substrate was also demonstrated which opens up new opportunities for low-cost flexible organic opto-electronics.
The aim of this thesis is to develop an understanding of the science and engineering in applying chemical vapor deposition (CVD) graphene as the transparent conductor in photovoltaic devices. Transparent conducting oxides currently dominate the transparent conductor market but suffer drawbacks that make them unsuitable certain applications. Graphene is mechanically robust, chemically inert, and has work function that can be tuned by chemical doping, making it a versatile substitute that is compatible many types of devices. We start by demonstrating a scalable method for directly transferring graphene onto a variety of substrates and exploring a doping method that vastly enhances the conductivity of graphene films. These developments improve the attractiveness of CVD graphene for transparent electrode applications. Next, we apply graphene to various types of devices to assess key advantages and challenges. We develop an understanding of the importance of the interface in graphene/silicon Schottky barrier solar cells and apply our understanding to achieve record efficiency in these devices. We also explore graphene/SrTiO3 Schottky junctions, where the graphene itself is responsible for absorbing visible light and show that these devices can be used as tunable photodetectors. We demonstrate highly-transparent organic solar cells with all-graphene electrode as well as inkjet-printed perovskite solar cells with graphene electrodes. Finally, we use graphene/perovskite Schottky barrier solar cells to gain a better understanding of carrier dynamics in perovskite films.
Firstly, this thesis details the background and challenges we are confronted with and then puts forward the research objectives and significance. Secondly, a comprehensive literature review is presented including the preparation approaches developed till now to produce 2D nanomaterials, 2D nanomaterialsbased composites and their applications in PV aspects. Then the general methodologies involved are introduced. In chapter 4, a salt-assisted direct exfoliation method is first developed and as-produced graphene nanosheets are characterized. Conductive and transparent graphene thin films with various thicknesses are fabricated by vacuum filtration method. Chapter 5 introduces the graphene hybrid thin films with enhanced conductivity by the combination with 1D silver nanowires. This hybrid films based on as-produced graphene nanosheets can be employed as the electrode in perovskite solar cells. Chapter 6 extends the salt-assisted exfoliation method to prepare 2D graphene-analogue transition metal dichalcogenide nanomaterials. Single-and few-layered 2D nanosheets are obtained and can readily disperse in aqueous solutions. And MoS2́2 nanosheets are solution processed into thin films and integrated into organic solar cells as a hole transport layer. Chapter 7 develops a CVD method to synthesize high-quality monolayer graphene and investigates the electrical properties of graphene FET devices with functionalization of polymer brushes and subsequent immobilization of biomolecules. Finally, chapter 8 concludes this thesis and provides future perspectives.
This title features 11 new chapters unique to this edition, including chapters on grain boundaries in graphene, 2D metal carbides and carbonitrides, mechanics of carbon nanotubes and nanomaterials, biomedical applications, oxidation and purification of carbon nanostructures, sintering of nanoceramics, hydrothermal processing, nanofibers, and nanomaterials safety. It offers a comprehensive approach with a focus on inorganic and carbon-based nanomaterials, including fundamentals, applications, synthesis, and characterization. This book also provides a unique angle from the nanomaterial point of view on application, synthesis, and characterization not found in any other nanomaterials book on the market.
Edited by well-known pioneers in the field, this handbook and ready reference provides a comprehensive overview of transparent conductive materials with a strong application focus. Following an introduction to the materials and recent developments, subsequent chapters discuss the synthesis and characterization as well as the deposition techniques that are commonly used for energy harvesting and light emitting applications. Finally, the book concludes with a look at future technological advances. All-encompassing and up-to-date, this interdisciplinary text runs the gamut from chemistry and materials science to engineering, from academia to industry, and from fundamental challenges to readily available applications.
