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Polymer/Fullerene Nanocomposites: Design and Applications synopsizes state-of-the-art essentials and versatile inventions in polymers and fullerenes derived nanocomposites. As the design, fabrication and exploration of polymeric materials with fullerenes in advanced nanomaterials is progressing quickly because of their unique combination of properties, including optical, electronic, electrical, mechanical, thermal, photovoltaic, sensing, shape memory, capacitive, antimicrobial, and other applications, this book fills a void in literature compilation and assessment for a field still in its infancy. The introductory chapter of this manuscript provides a comprehensive update on the fundamentals and applications of fullerenes, with following chapters revealing the properties and essential aspects of polymeric nanocomposites. Reconnoiters state-of-the-art of fullerenes Focuses on fullerene nano-additives, developing covalent interactions, and physical dispersion with conjugated polymers and other polymeric matrices Emphasizes fullerene nanowhisker and nanoball nanofillers in nanocomposites Unfolds advanced applications of polymer/fullerene nanomaterials in stimuli-responsive systems, optoelectronic devices (photovoltaics, light emitting diodes and optical sensors), fuel cells, supercapacitors and biomedical fields
My thesis focuses on improving and understanding a relatively new type of solar cell materials system: polymer:fullerene bulk-heterojunction (BHJ) blends. These mixtures have drawn significant interest because they are made from low-cost organic molecules that can be cast from solution, which makes them a potential cheap alternative to traditional solar cell materials like silicon. The drawback, though, is that they are not as efficient at converting sunlight into electricity. My thesis focuses on this issue, and examines the loss processes holding back the efficiency in polymer:fullerene blends as well as investigates new processing methods for overcoming the efficiency limitations. The first chapter introduces the subject of solar cells, and polymer:fullerene solar cells in particular. The second chapter presents a case study on recombination in the high-performance PBDTTT polymer family, wherein we discovered that nongeminate recombination of an anti-Langevin origin was the dominant loss process that ultimately limited the cell efficiency. Electroluminescence measurements revealed that an electron back-transfer process was prevalent in active layers with insufficient PC$_{71}$BM content. This work ultimately made strong headway in understanding what factors limited the relatively unexplored but highly efficient PBDTTT family of polymers. In the next chapter, I further explore the recombination mechanisms in polymer:fullerene BHJs by examining the dark diode ideality factor as a function of temperature in several polymer:fullerene materials systems. By re-deriving the diode law for a polymer:fullerene device with Shockley-Read-Hall recombination, we were able to confirm that trap-assisted recombination through an exponential band-tail of localized states is the dominant recombination process in many polymer:fullerene active layers. In the third chapter, I present a generalized theoretical framework for understanding current transients in planar semiconductor devices, like those discussed above. My analysis reveals that the apparent free-carrier concentration obtained via the usual integral approach is altered by a non-trivial factor of two, sometimes leading to misinterpretations of the charge densities and overall device physics. This new perspective could have far-reaching effects on semiconductor research and technology. Finally, in the last two chapters, I discuss the device physics associated with a relatively novel method for fabricating nanoscale polymer:fullerene BHJs: solution sequential processing (SqP). In particular, I compare recombination in SqP vs. traditionally processed blend-cast devices, and demonstrate that SqP is a more scalable method for making BHJ solar cells. In the final chapter, I examine an unexpected discovery that occurred while working on the content in Chapter 5. Specifically, Chapter 6 examines electrode metal penetration in the SqP quasi-bilayer active layer architecture. Therein, we unexpectedly found that evaporated metal can readily penetrate into fullerene-rich layers, up to $\sim$70 nm or more. The details and consequences of this surprising occurrence are discussed in detail.
Since their discovery in 1977, the evolution of conducting polymers has revolutionized modern science and technology. These polymers enjoy a special status in the area of materials science yet they are not as popular among young readers or common people when compared to other materials like metals, paper, plastics, rubber, textiles, ceramics and composites like concrete. Most importantly, much of the available literature in the form of papers, specific review articles and books is targeted either at advanced readers (scientists / technologists / engineers / senior academicians) or for those who are already familiar with the topic (doctoral / postdoctoral scholars). For a beginner or even school / college students, such compilations are bit difficult to access / digest. In fact, they need proper introduction to the topic of conducting polymers including their discovery, preparation, properties, applications and societal impact, using suitable examples and already known principles/knowledge/phenomenon. Further, active participation of readers in terms of "question & answers", "fill-in-the-blanks", "numerical" along with suitable answer key is necessary to maintain the interest and to initiate the "thought process". The readers also need to know about the drawbacks and any hazards of such materials. Therefore, I believe that a comprehensive source on the science / technology of conducting polymers which maintains a link between grass root fundamentals and state-of-the-art R&D is still missing from the open literature.
Photovoltaic (PV) is attracting increasing interest as an important contribution to renewable energy supply. Organic photovoltaic (OPV) is a comparable young PV technology with a great potential towards low cost solar power. This is due to the intrinsic advantage of the incorporated organic semiconductors which are soluble. Solution processing allows high throughput coating and printing processes. Hence, energy intensive high temperature and vacuum steps can be avoided which reduces the fabrication costs and keeps energy payback times low. The performance of organic solar cells strongly depends on the structure of the solution cast photoactive layer which comprises a polymer-fullerene blend. The blend structure evolves during the film drying step which has been studied in this thesis. Starting point of this work was the hypothesis that drying process parameters are suitable for systematically tuning the structure formation during drying of solution cast polymer-fullerene films in order to generate optimized structures with improved photovoltaic performance. For the evaluation of this hypothesis the structure formation of the polymer-fullerene system Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl C61-butyric acid methyl ester (P3HT:PCBM) was investigated incorporating i) thin film drying kinetics, ii) phase behavior of polymer-fullerene solutions, iii) structure formation and iv) the drying process-structure-property relationship of solar cells. The generality of the obtained results has been studied in comparison with the behavior of Poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT). i) Within this thesis a dedicated coating and drying setup was developed which afforded precisely defined coating and drying process conditions as prerequisite for all obtained results. For the first time, the drying behavior of finally a few hundred nanometer thin films could be investigated at five measurement positions with laser reflectometry simultaneously. This allowed the elaboration of a spatially resolved numerical thin film drying model. ii) In conjunction with the measurement and simulation of the evolution of film composition it was required to determine important instants of phase transitions such as solubility limits. Therefore the binodal region of P3HT solutions has been determined in the temperature range of 0°C-60°C. Within the unstable region P3HT solutions phase separate into a sol and a gel phase. The fullerene PCBM exhibits only a single solubility limit. iii) In order to correlate the expected phase transitions according to the phase diagrams with the real structure formation, the above mentioned coating and drying setup was combined with synchrotron based in situ grazing incidence X-ray diffraction (GIXD) measurements. This gave unique insights into the mechanisms and dynamics of polymer-fullerene blend crystallization. After reaching P3HT solubility the crystallization proceeded with well-oriented interface-induced P3HT nucleation followed by P3HT crystal growth with increasing orientation distribution of the crystallites and PCBM aggregation in the final drying period. Furthermore strong polymer-fullerene interaction forces could be derived. By increasing the PCBM fraction it could be shown for the 1:2 P3HT:PCBM ratio that PCBM molecules brake the (020) π-π-stacking of P3HT lamellae which signifies a dramatic loss of hole mobility and consequently reduced device performance. It is further notable that increasing drying temperatures reduce the amount of (020) π-π-stacked P3HT molecules but lead to an increased amount of P3HT (100) crystallinity. Hence, drying temperature determines the preferred direction of crystal growth. iv) Besides a finer degree of phase separation, reduced drying temperatures also cause a higher amount of π-π-stacked polymers, longer effective polymer conjugation length, increased amount of vertical charge transport pathways and an increasingly rough topography due to larger polymer aggregates. Jointly this leads to improved power conversion efficiency at lower drying temperatures. Based on the elaborated knowledge a strategy for a 40% reduction of drying time with only small drawbacks in solar cell performance could be developed. Finally it was important to investigate the transferability of the obtained knowledge to other material systems. PSBTBT:PC71BM blends show similarities to that of P3HT:PCBM with partly interface induced polymer nucleation and subsequent fullerene aggregation in the final drying stage. The kinetics of molecular ordering however proceed fast enough such that the drying process under the investigated conditions cannot limit the structure formation. Hence, P3HT:PCBM is a suitable model system due to its sensitivity to many process parameters. According to the process influence on novel materials the results of this thesis can serve as a source for appropriate process strategies.
Written by an outstanding team of experts in the interdisciplinary areas of research, this book is based on a new classification of the different types of fullerene polymers according to their chemical structures. It covers all aspects, from different classes, to their synthesis and applications in material science. Of great interest to polymer and synthetic chemists, but also for material scientists and industrial chemists.
This in-depth experimental and theoretical account explores polymers and composites whose unusual properties (such as photophysical phenomena, electrical transport, phase transitions, and magnetic properties) stem from the incorporation of C60 in the material. Introductory chapters on the fundamental properties of fullerenes (C60, C70) and photophysical phenomena in fullerenes and polymers are also included.
This text covers a host of fullerene applications, including nanotubes, compounds of fullerenes with other elements and structures and polymerized fullerenes. It discusses properties of photoexcited states of fullerenes, neutral and charged states, nonlinear optical response (NLO) and electron-electron interactions.