Showing papers in "Advanced Materials in 2008"

Hollow Micro-/Nanostructures: Synthesis and Applications**

Abstract: Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices

Abstract: One of the greatest challenges for our society is providing powerful electrochemical energy conversion and storage devices. Rechargeable lithium-ion batteries and fuel cells are amongst the most promising candidates in terms of energy densities and power densities. Nanostructured materials are currently of interest for such devices because of their high surface area, novel size effects, significantly enhanced kinetics, and so on. This Progress Report describes some recent developments in nanostructured anode and cathode materials for lithium-ion batteries, addressing the benefits of nanometer-size effects, the disadvantages of 'nano', and strategies to solve these issues such as nano/micro hierarchical structures and surface coatings, as well as developments in the discovery of nanostructured Pt-based electrocatalysts for direct methanol fuel cells (DMFCs). Approaches to lowering the cost of Pt catalysts include the use of i) novel nanostructures of Pt, ii) new cost-effective synthesis routes, iii) binary or multiple catalysts, and iv) new catalyst supports.

Mechanically Strong, Electrically Conductive, and Biocompatible Graphene Paper

Abstract: A study was conducted to demonstrate that highly ordered graphene paper can be prepared by directional flow-induced assembly of graphene sheets that are well dispersed in solution. Moderate thermal annealing can enhance its mechanical stiffness and strength, and also electrical conductivity. Scanning electron microscopy (SEM) analysis reveals that the surface of the graphene paper is quite smooth and the fracture edges of the papers exhibit a layered structure through the entire cross-section. The study has also shown the results of cell culture experiments, which indicate that graphene paper may be biocompatible and therefore suitable for biomedical applications. The combination of the exceptional mechanical strength, thermal stability, high electrical conductivity, and biocompatibility makes graphene paper a promising material for many technological applications, such as inclusion in heart valves.

Deoxygenation of Exfoliated Graphite Oxide under Alkaline Conditions: A Green Route to Graphene Preparation

Abstract: Graphene – a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice – is the basal building block in all graphitic materials. Since it was first reported in 2004, graphene has attracted great interest because of the unique electronic, thermal, and mechanical properties arising from its strictly 2D structure, and to its potential technical applications. However, producing graphene on a large scale using existing mechanical methods is still unfeasible. Searching for alternative chemical approaches is an urgent matter. However, the hydrophobic nature of graphene and its strong tendency to agglomerate in solvents present a great challenge to the development of fabrication methods, and severely restrict its promising applications. Although the mechanism involved remains unproven, the chemical reduction of readily available exfoliated graphite oxide (GO) with reducing agents such as hydrazine and dimethylhydrazine is a promising strategy in the large-scale production of graphene. Unfortunately, the reducing agents involved are very hazardous, and the graphene obtained presents irreversibly agglomerated features in solvents that do not contain polymer surfactants. Here, we report a new green route for the synthesis of processable graphene on a large scale. We observed that a stable graphene suspension could be quickly prepared by simply heating an exfoliated-GO suspension under strongly alkaline conditions at moderate temperatures (50–90 8C) (Figure 1a). Our initial purpose was to introduce functional groups to exfoliated GO by free-radical addition. Surprisingly, the addition of NaOH to the GO suspension – to improve the solubility of the alkyl free-radical initiator, which is carboxyl-terminated – was accompanied by a fast, unexpected color change (from yellow-brown to homogeneous black). Careful experiments revealed that exfoliated GO can undergo fast deoxygenation in strongly alkaline solutions, resulting in stable aqueous graphene suspensions

Applications of Nanoparticles in Biology

Abstract: The wide variety of core materials available, coupled with tunable surface properties, make nanoparticles an excellent platform for a broad range of biological and biomedical applications. This Review provides an introduction to nanoparticle–biomolecular interactions as well as recent applications of nanoparticles in biological sensing, delivery, and imaging of live cells and tissues.

Developments in Nanostructured Cathode Materials for High‐Performance Lithium‐Ion Batteries

Abstract: Nanostructured materials lie at the heart of fundamental advances in efficient energy storage and/or conversion, in which surface processes and transport kinetics play determining roles. This Review describes some recent developments in the synthesis and characterization of nanostructured cathode materials, including lithium transition metal oxides, vanadium oxides, manganese oxides, lithium phosphates, and various nanostructured composites. The major goal of this Review is to highlight some new progress in using these nanostructured materials as cathodes to develop lithium batteries with high energy density, high rate capability, and excellent cycling stability resulting from their huge surface area, short distance for mass and charge transport, and freedom for volume change in nanostructured materials.

Tin-Nanoparticles Encapsulated in Elastic Hollow Carbon Spheres for High-Performance Anode Material in Lithium-Ion Batteries†

Abstract: Lithium batteries, as a main power source or back-up power source for mobile communication devices, portable electronic devices and the like, have attracted much attention in the scientific and industrial fields due to their high electromotive force andhigh energy density. Tomeet the demand for batteries having higher energy density and improved cycle characteristics, in recent years, a great deal of attempt has been made to develop new electrode materials or design new structures of electrode materials. For anode materials, among them, some elementary substances such as silicon (Si), germanium (Ge), or tin (Sn) provide promising alternative to conventional carbonaceous anode active materials, because they are capable of alloying with more lithium and thus leading to the extreme high initial capacity density. For example, metallic tin has recently been widely concerned as one of the promising anode materials for lithium batteries due to the following reasons. Firstly, its theoretical specific capacity (Li4.4Sn, 992mAhg ) ismuchhigher than that of conventional graphite (LiC6, 372 mA h g ). Secondly, the tin anode has higher operating voltage than graphite, so it is less reactive and the safety of batteries during rapid charge/discharge cycle could be improved. Furthermore, a significant advantage of metallic tin over graphite is that it does not encounter solvent intercalationwhich causes irreversible charge losses at all. Unfortunately, the biggest challenge for employing metallic tin as applicable active anode materials is that it is suffering from huge volume variation during Liþ insertion/extraction cycle, which leads to pulverization of the electrode and very rapid capacity decay. Without appropriate structure design, the tin electrode typically fails after only a few discharge/charge cycles. It is therefore very desirable to design a new tinbased materials mainly composed of metallic tin with high specific capacity as well as good cycle performance. Some metal/oxides and carbon nanocomposites have been reported with high capacity and capacity retention when used as anodematerials, because the carbon shell has itself good electronic conductivity and prevents the aggregation of active materials, and especially thin carbon shell has good elasticity to effectively accommodate the strain of volume change during Liþ insertion/extraction. Very recently, tin-encapsulated spherical hollow carbon was synthesized by the pyrolysis of tin-containing organic precursors have exhibited higher capacity and better cycle performance than unencapsulated mixture materials, in which the content of tin active substance was only 24 wt%. Nanostructured tin dispersed in a carbonmatrix and carbon-encapsulated hollow tin nanopartides were also reported as superior anode materials. These studies showed that both coating tin nanomaterials with carbon layer and dispersing tin nanoparticles in carbon matrix are effective to improve their electrochemical properties in lithium ion batteries. It is obvious that thehigher content of and smaller size of tin, as well as the thinner carbon coating will greatly contribute to the further enhancement of material performance since the lithium storage density in tin ismuch higher than that in carbon. Meanwhile, this tin-based anode material has to be designed to own enough void volume to compensate the volume expansion during Liþ insertion, which is important to improve its cycle performance. In the presentwork,we therefore designed anewapproach to synthesize tin nanoparticles encapsulated elastic hollow carbon spheres (TNHCs) with uniform size, in which multiple tin nanoparticles with a diameter of less than 100 nm were encapsulated inone thin hollow carbon spherewith a thickness of only about 20 nm, thus leading to both the content of Sn up to over 70% by weight and the void volume in carbon shell as high as about 70–80%by volume. This void volume and the elasticity of thin carbon spherical shell efficiently accommodate the volume change of tin nanoparticles due to theLi-Sn alloying-dealloying reactions, and thus prevent the pulverization of electrode. As a result, this type of tin-based nanocomposites have very high specific capacity of >800 mA h g 1 in the initial 10 cycles, and >550mAh g 1 after the 100th cycle, as well as excellent cycling [*] Prof. L.-J. Wan, W.-M. Zhang, Dr. J.-S. Hu, Prof. Y.-G. Guo, S.-F. Zheng, L.-S. Zhong, Prof. W.-G. Song Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing 100080 (P.R. China) E-mail: wanlijun@iccas.ac.cn

Self-Supported Formation of Needlelike Co3O4 Nanotubes and Their Application as Lithium-Ion Battery Electrodes†

Abstract: Lithium ion batteries (LIBs) are currently the dominant power source for portable electronic devices. The ever-growing need for high capacity and/or high power, especially for emerging large-scale applications (e.g., electric cars), has prompted numerous research efforts towards developing new high-performance electrode materials for next-generation LIBs. As a new class of negative electrode materials for LIBs discovered in 2000, transition metal oxides (Co3O4 in particular) can in principle deliver as high as three times the capacity of currently used graphite (< 372 mAh g). However, they usually suffer from poor capacity retention upon cycling and/or poor rate capability, which remain major challenges for use in practical cells. These problems have long been partly attributed to the large volume changes during repeated lithium uptake and removal reactions, which cause local stress and eventually lead to electrode failure, and the formation of a polymer/gel-like layer and solid electrolyte interface (SEI) by catalyzed degradation of electrolyte. One generally accepted strategy to alleviate these problems is to prepare nanometer-sized materials with designed structures. Indeed, there is increasing evidence showing that tailored nanostructured materials can significantly improve electrochemical properties compared to their bulk counterpart. 19] For example, electrochemically prepared nano-architectured Fe3O4-Cu and Ni-Sn electrodes are shown to exhibit high rate capabilities, and virus-templated Co3O4 nanowires have recently been demonstrated as LIB electrodes with improved properties. Although nanotubes of a wide range of materials in various types have been prepared by different strategies, it remains a challenge to synthesize nanotubes of metal oxides with isotropic crystal structure. Recently, Yang and others have prepared single-crystalline nanotubes of semiconductors (e.g., GaN and Si) and metal oxides (e.g., Fe3O4) by using a novel “epitaxial casting” against single-crystalline nanowires. More recently, single-crystalline spinel ZnAl2O4 nanotubes have been fabricated based on the Kirkendall effect, normally applied for preparation of hollow nanoparticles, in which amorphous Al2O3 was coated on ZnO nanowires by atomic layer deposition followed by annealing at high temperature. Herein, we report a one-step self-supported topotactic transformation approach for synthesis of needlelike nanotubes made of electrochemically active Co3O4. As-prepared Co3O4 nanotubes are shown to manifest ultrahigh capacity with improved cycle life and high rate capability. Figure 1 schematically illustrates the self-supported topotactic transformation process for synthesis of needlelike Co3O4 nanotubes. At the early stage of the synthesis, needlelike b-Co(OH)2 nanorods with growth direction along [001] are formed due to the highly anisotropic hexagonal crystal structure (Fig. 1a). With the constant mediation of air, C O M M U N IC A TI O N

Bio-Inspired, Smart, Multiscale Interfacial Materials

Abstract: In this review a strategy for the design of bioinspired, smart, multiscale interfacial (BSMI) materials is presented and put into context with recent progress in the field of BSMI materials spanning natural to artificial to reversibly stimuli-sensitive interfaces. BSMI materials that respond to single/dual/multiple external stimuli, e.g., light, pH, electrical fields, and so on, can switch reversibly between two entirely opposite properties. This article utilizes hydrophobicity and hydrophilicity as an example to demonstrate the feasibility of the design strategy, which may also be extended to other properties, for example, conductor/insulator, p-type/n-type semiconductor, or ferromagnetism/anti-ferromagnetism, for the design of other BSMI materials in the future.

A Nanoreactor Framework of a Au@SiO2 Yolk/Shell Structure for Catalytic Reduction of p‐Nitrophenol

Abstract: Accordingly,the interactions between metal nanoparticles and supportsshould be precisely manipulated in order to maximize theirsynergetic effects. Metal particles should be dispersed evenlyon the supports in order to prevent aggregation betweenparticles in close proximity, and to maintain metal–supportcontact areas. Numerous processes have been developed foreffective dispersion of metal nanoparticles in bifunctionalcatalysts.

Efficient and Flexible ITO‐Free Organic Solar Cells Using Highly Conductive Polymer Anodes

Abstract: Despite the relatively low efficiencyin comparison with conventional inorganic solar cells, thepotential of roll-to-roll processing and large-area processa-bility on flexible low-cost substrates renders conjugatedpolymer-based organic solar cells (OSCs) very attractive asa cost-effective solution to the problem of energy shortage.Among the available BHJ systems, poly(3-hexylthiophene)(P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C

Exciton Diffusion Measurements in Poly(3‐hexylthiophene)

Abstract: The problem of making reliable measurements of exciton diffusion lengths in organic semiconductors is addressed. The exciton diffusion length is an extremely important quantity in the operation of organic solar cells. We focus on the polymer P3HT because of its widespread use in solar cells and are able to fit the exciton diffusion in a range of films with a single diffusion constant, showing that our approach is particularly robust.

Fmoc-diphenylalanine self assembles to a hydrogel via a novel architecture based on π–π interlocked β-sheets

Abstract: The self assembly of peptide hydrogelators that carry aromatic substituents can be modeled by a novel nanocylindrical architecture. The proposed model suggests that the nanocylinders are formed by anti-parallel β-sheets interlocked by the π-stacking interactions of fluorenyl groups and phenyl rings. This explanation is consistent with the structures observed in TEM and the data obtained by a variety of spectroscopic techniques.

Narrow-bandgap diketo-pyrrolo-pyrrole polymer solar cells : the effect of processing on the performance

Abstract: A study has reported on the application of a novel narrow bandgap polymer, poly[3,6-bis-(4'-dodecyl-[2,2']bithiophenyl-5-yl)-2,5-bis-(2-ethyl-hexyl)-2, 5-dihydropyrrolo[3,4-]pyrrole-1,4-dione] (pBBTDPP2) in photovoltaic cells. The conjugated polymer is based on alternating quaterthiophene (BBT) and diketopyrrolo-pyrrole (DPP) units with a bandgap of1.4eV, which is close to the optimum for single, bandgap polymer solar cells. After careful optimization of the processing parameters and morphology, the donor polymer in combination with C60 and C70 PCBM derivatives affords external quantum efficiencies of ~0.4 over a broad spectral range and power conversion efficiencies of 3.2 and 4.0%, respectively under simulated AM1.5G solar light conditions. Significant molecular weights of Mn=20000 and M w=67000g/mol (DPI)=3.35) also were obtained, as determined by gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene as eluent

Controlled Preparation of MnO2 Hierarchical Hollow Nanostructures and Their Application in Water Treatment

Abstract: Manganese oxides are of considerable importance in technological applications, including ion-exchange, molecular adsorption, catalysis, and electrochemical supercapacitors owing to their structural flexibility combined with novel chemical and physical properties. Up to now, various nanostructures of MnO2, such as nanoparticles, [6] nanorods/-belts/-wires/-tubes/fibers, nanosheets, mesoporous/molecular sieves and branched structures, urchins/orchids, and other hierarchical structures have been synthesized by different methods. Over the past years, fabrication of hierarchical hollow nanostructures has attracted significant interest because of their widespread potential applications in catalysis, drug delivery, acoustic insulation, photonic crystals, and other areas. Until now, the general approach for preparation of hollow structures has involved the use of various removable or sacrificial templates, referred to as “hard”, such as monodispersed silica, polymer latex spheres and reducing metal nanoparticles, as well as “soft” ones, for example, emulsion droplets/ micelles and gas bubbles. Furthermore, lots of one-pot template-free methods for generating hollow inorganic microand nanostructures have been developed employing novel mechanisms, including the nanoscale corrosion-based insideout evacuation and Kirkendall effect. Recently, rhombododecahedral silver cages have been prepared by self-assembly coupled with the precursor crystal-templating approach. By treating the external morphologies of hollow structures, unique properties can be obtained. Thus, it is desirable to develop easy methods to control the morphologies of assembled systems with well-defined hierarchical structures. Herein, we report a simple controlled preparation of hierarchical hollow microspheres and microcubes of MnO2 nanosheets through self-assembly with an intermediate crystaltemplating process. As shown in Figure 1, the synthesis is performed by a three-step process. Particularly, discrete spherical and cubic hollow MnO2 nanostructures with controlled morphologies can be prepared by changing the morphologies of MnCO3 precursors, which can be simply obtained by adding the (NH4)2SO4 solution in the reaction system, and the

Conjugated Polymer Photovoltaic Materials with Broad Absorption Band and High Charge Carrier Mobility

Abstract: Polymer solar cells (PSCs) have attracted great attention in recent years because of their advantages of easy fabrication, low cost, light weight, and potential for flexible devices. However, the power conversion efficiency (PCE) of the PSCs needs to be improved for future commercial applications. Factors limiting the PCE of the PSCs include the low exploitation of sunlight due to the narrow absorption band of conjugated polymers, and the low charge-transport efficiency in the devices due to the lower charge-carrier mobility of the polymer photovoltaic materials. In this Research News article, recent progress in new conjugated polymer photovoltaic materials fabricated by our group and others is reviewed, including polythiophene (PT) and poly(thienylene vinylene) derivatives with conjugated side chains for a broad absorption band, crosslinked PT derivatives with conjugated bridges for higher hole mobility, and low-bandgap donor–acceptor copolymers for broad, red-shifted absorption to match the solar spectrum.

High‐Speed Roll‐to‐Roll Nanoimprint Lithography on Flexible Plastic Substrates

Abstract: The ability of microto nanometer-scale patterning on flexible substrates can enable many new applications in the area of photonics and organic electronics. A major roadblock has remained for many practical applications of patterned nanostructures, which is the throughput of nanopattern replication and the associated cost issues. Among the emerging techniques, nanoimprint lithography (NIL) clearly stands out as a promising technology for high-throughput and highresolution nanometer-scale patterning, which can achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in other traditional techniques. Developments in this area have enjoyed great momentum in the past decade and numerous applications, such as in Si electronics, organic electronics and photonics, magnetics, and biology have been exploited by many researchers. On the other hand, the current process and throughput in NIL (on the order of a few minutes per wafer) is still far from meeting the demands of many practical applications, especially in photonics, biotechnology, and organic optoelectronics. The concept of roller imprinting has been pursued by previous investigators as a means to improve speed. However, the procedure was to imprint a small piece of Si mold onto a Si substrate, which is not too different from that of conventional NIL except that a rod is used to apply pressure rather than a flat plate. The reverse nanoimprinting or nanotransfer printing methods produce positive-tone polymer or metal patterns, which in principle can also be applied to roll-to-roll printing processes. In addition, Lee et al. proposed a bilayer transfer process from a ‘‘rigiflex’’ mold to a Si wafer, and pointed out that the process can potentially be extended to a roller bilayer transfer process. However, these are yet to be demonstrated.

Stretchable electronics: Materials strategies and devices

Abstract: New electronic materials have the potential to enable wearable computers, personal health monitors, wall-scale displays and other systems that are not easily achieved with established wafer based technologies. A traditional focus of this field is on the development of materials for circuits that can be formed on bendable substrates, such as sheets of plastic or steel foil. More recent efforts seek to achieve similar systems on fully elastic substrates for electronics that can be stretched, compressed, twisted and deformed in ways that are much more extreme than simple bending. This article highlights some recent progress in this area, with an emphasis on materials approaches and demonstrated devices.

Versatile Approach for Integrative and Functionalized Tubes by Strain Engineering of Nanomembranes on Polymers

Abstract: Flexible electronics, extremely sensitive sensors, strained-silicon technology, andmacromolecule separation, are only a few examples stimulating increasing interest in free-standing nanomembranes, which can be fabricated out of thin solid films, particle nanocomposites, organic layers, organic/inorganic networks, and even graphene sheets. Strain engineering offers an advanced strategy to deterministically rearrange such nanomembranes into threedimensional micro-/nanostructures including tubes, helices, rings, wrinkles and other advanced microarchitectures, all of which serve for applications in electronics, mechanics, fluidics, and photonics. The fabrication often requires a selective underetching procedure to release the nanomembranes from their substrate, which heavily constraints the number of desirable materials for exciting applications in, e.g., metamaterials or biomedical research. The material choice is limited, because the selective underetching not only removes the underlying sacrificial layer but also in many cases dissolves the nanomembrane material itself. We circumvent this problem for a broad range of materials and material combinations by a new approach outlined in Figure 1a. A pre-stressed inorganic nanomembrane deposited at low temperatures onto a polymer sacrificial layer (here: photoresist) is released from the substrate surface by removing

Fullerene bisadducts for enhanced open-circuit voltages and efficiencies in polymer solar cells

Abstract: A fullerene bisadduct can enhance the efficiency of polymer:fullerene bulk heterojunction solar cells. The bisadduct has a LUMO that is 100 meV higher compared to that of [6,6]-phenyl C-61 butyric acid methyl ester (PCBM). This increases the open-circuit voltage of polymer: fullerene bulk heterojunction solar cells based on poly(3-hexylthiophene) and bisadduct PCBM to 0.73 V, while maintaining high fill factors and currents.

Synthesis of Hierarchically Structured Metal Oxides and their Application in Heavy Metal Ion Removal

Abstract: Hierarchically structured metal oxides have two or more levels of structure. Their nanometer-sized building blocks provide a high surface area, a high surface-to-bulk ratio, and surface functional groups that can interact with, e.g., heavy metal ions. Their overall micrometer-sized structure provides desirable mechanical properties, such as robustness, facile species transportation, easy recovery, and regeneration. In combination these features are suitable for the removal of heavy metal ions from water. Several general synthesis routes for the fabrication of metal oxides with various morphologies and hierarchical structures are discussed including soft and hard template-assisted routes. These routes are general, reliable, and environmentally friendly methods to prepare transition and rare earth metal oxides, including cobalt oxide, iron oxide, and ceria. As-prepared hierarchically structured metal oxides show excellent adsorption capacities for AsV and CrVI ions in water.

Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes

Abstract: Cost effective and highly efficient renewable energy is becoming ever more important in our age of rising energy prices and global climate change. Solar energy is a nonexhaustible and green energy. Organic solar cells (OSC) have the merits of low cost and simplistic fabrication in addition to compatibility with flexible plastic substrates over large areas. They have therefore been considered a promising energy conversion platform for clean and carbon-neutral energy production. In recent years, the power conversion efficiency of OSCs based on conjugated polymers has steadily increased through improved energy harvesting, enhanced exciton separation in improved device structures, and optimization of processing parameters, e.g., solvent evaporation time, and annealing conditions. Most OSCs are built on indium tin oxide (ITO) coated substrates because ITO offers transparency in the visible range of the electromagnetic spectrum as well as good electrical conductivity. However, ITO is not the optimum electrode for solar cell applications as it has been reported that the band structure of ITO hinders efficient photocurrent generation. Moreover, the poor mechanical stability of ITO can cause device failure when an ITO-coated flexible substrate is bent. In addition, the limited supply of indium and the increasing demand from the rapidly expanding display market have increased the cost of ITO drastically, which potentially prevents the realization of low cost and large scale OSC fabrication. Therefore, there is a strong need to find alternative materials that can replace ITO as high transparency electrode. Some examples that have been investigated recently are nanotube networks, and Ag wire grids. In this communication, we report on high transparency metal wire grid electrodes for organic solar cell applications. The high transparency metal electrodes are fabricated by nanoimprint lithography (NIL), and have several advan-

Controlling Morphology in Polymer–Fullerene Mixtures

Abstract: In the past several years, polymer–fullerene mixtures have been intensely studied for use in organic solar cells because they can be deposited from solution, are compatible with lowcost roll-to-roll fabrication technology, and have shown high power conversion efficiency (g), up to 4–5%. The best devices consist of a single bulk-heterojunction active layer, in which the polymer (donor) and fullerene (acceptor) are deposited from a common solvent. As the solvent dries the donor and acceptor components separate into domains. The final efficiency of the solar cell has been shown to be extremely sensitive to the size, composition, and crystallinity of the formed domains. Improvement of the morphology in devices fabricated with a mixture of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and regioregular poly(3-hexylthiophene) (P3HT) has been achieved by using heat-treatment techniques and long-time solvent curing, with resulting record efficiencies. More recently, a method for increasing the crystallinity of the P3HT component has been introduced which involves filtering preformed nanofibers of P3HT out of solution and mixing the prepared nanofiber dispersion with PCBM to increase the efficiency of as-cast devices. Interestingly, the best device performance was achieved by mixing lower-molecular-weight (MW) amorphous P3HT back into the solution to reduce the crystalline content of the active layer and, thereby, to increase connection between crystalline domains. Studies of the MW impact on P3HT/PCBM solar cells have indicated that a large polydispersity and number-average molecular weight (Mn) over 19000 g mol -1 leads to improved efficiency. Morphology studies of organic field-effect transistor (OFET) devices indicate that the increased MW leads to better network formation between crystalline domains. The morphology of these improved devices has been studied using transmission electron microscopy (TEM), grazing-angle X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), NMR, and a variety of other optical and electrical techniques. The morphology studies give a picture of a device in which the P3HT forms aligned/crystalline domains, between which are amorphous segments of P3HT and PCBM. These domains form with larger size and crystallinity for higher heat-treatment temperatures and longer solvent soaking times. Depending on the fabrication and measurement techniques, the aligned domains of P3HT are depicted as fibers or as shapeless masses. The majority of these studies do not, however, allow quantification of the percentage of the P3HT that is agglomerated/ crystalline in the final device. Only by making use of the nanofiber filtration technique have the authors been given the ability to control the crystalline content of the P3HT in solution and in the final device. The disadvantages of this technique are the necessity of more complicated preparation, and filtered P3HT is restricted to a fibrous form that requires the addition of amorphous P3HT to provide connections between crystalline domains. We present here a simple method to determine the agglomerated–amorphous ratio of the P3HT and to control the degree of agglomeration/crystallinity of the P3HT in the final device by using a solvent mixing method and no further heat-treatment or prepreparation of the polymer. The most obvious change that heat-treatment and solvent soaking yield on a P3HT:PCBM layers is the change in color. It has been widely reported that the P3HT absorption red-shifts and a series of vibronic peaks become visible at k ∼ 600 nm, 550 nm, and 510 nm. This red-shift has been assigned to increased planarization and stabilization of the P3HT chains that accompanies self-stacking of the polymer. The crystal structure for these self-stacking domains has been solved by using XRD and TEM, and shows a herringbone interconnection of the alkyl chains and an a-dimension stacking distance of 1.6 nm. The p–p chain stacking of the P3HT chains in crystallites has been measured to be 0.38 nm. The herringbone structure and planarization of the P3HT with heating has been confirmed using heteronuclear solid-state NMR measurements. The red-shift in the UV-vis spectrum occurs proportionally to the degree of agglomeration of the P3HT. The pure amorphous electronic spectrum of P3HT or a mixture of P3HT and PCBM is simple to measure. A solution UV-vis spectrum can also be measured in the liquid state. If C O M M U N IC A TI O N