Advanced Energy Materials

Published by Wiley-VCH Verlag
Online ISSN: 1614-6840
Publications
Article
Donor-acceptor (D-A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N-(1-octylnonyl)-2,7-carbazole] -alt-5,5-[4',7' -di(thien-2-yl)-2',1',3' -benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N-(1-octylnonyl)-2,7-bis-[(5-phenyl)thien-2-yl)carbazole (BPT-carbazole) (see Figure 1). The high-energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low-energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC70BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends.
 
Temperature of the TSC peak maximum of PC61BM for different values of TStop in the fractional TSC (triangles). b) Activation energies of the trap states in PC61BM according to the initial-rise method [Equation (2)] for the different values of TStop (squares) and the linear fit of the activation energies (line). For comparison the activation energies obtained by the Tmax method [Equation (3)] are also shown (circles).
DOOS distribution of PC61BM, as obtained by TSC TStart–TStop measurements.
TSC spectra of PC61BM, bisPC61BM, and PC71BM.
Article
The trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester), bisPC61BM (bis[6,6]-phenyl C61 butyric acid methyl ester) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), are investigated by thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes exhibits values in the order of 10^22 m^-3 with the highest trap density in bisPC61BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, being at 96 meV and 223 meV for PC71BM and 184 meV for bisPC61BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.
 
Article
Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime. A challenge is to develop 'ultrabarriers' that are impermeable, flexible, and preferably transparent. Another important requirement is that they must be compatible with organic electronics fabrication schemes (i.e. must be solution processable, deposited at room temperature and be chemically inert). Here, we report lifetime increase of 1,300 hours for poly(3-hexylthiophene) (P3HT) films encapsulated by uniform and continuous thin (~10 nm) films of reduced graphene oxide (rGO). This level of protection against oxygen and water vapor diffusion is substantially better than conventional polymeric barriers such as CytopTM, which degrades after only 350 hours despite being 400 nm thick. Analysis using atomic force microscopy, x-ray photoelectron spectroscopy and high resolution transmission electron microscopy suggest that the superior oxygen gas/moisture barrier property of rGO is due to the close interlayer distance packing and absence of pinholes within the impermeable sheets. These material properties can be correlated to the enhanced lag time of 500 hours. Our results provide new insight for the design of high performance and solution processable transparent 'ultrabarriers' for a wide range of encapsulation applications.
 
Article
Adding a small amount of single walled carbon nanotubes (SWCNTs) greatly improves graphene oxide (GO)'s performance as the anode modifying layer for polymer solar cells, putting it on par with the commonly used PEDOT:PSS. The high red/infrared transparency and water processability should make GO:SWCNTs an attractive interfacial layer material for high performance polymer solar cells with low bandgap polymers.
 
Layer structure of the single-junction GaAs solar cell.
Specular refl ection measured on the GaAs solar cells with bare, Si 3 N 4 , and syringelike NRAs/SiO 2 /Si 3 N 4 surfaces.
Device characteristics of the GaAs solar cells with different surface conditions.
Article
An effective antireflection coating of syringelike ZnO NRAs is demonstrated for GaAs solar cells. Energy conversion efficiency is significantly enhanced by up to 32%. GaAs is focussed on as a model system, but the hydrothermal growth of syringelike ZnO nanostructures is readily compatible with the existing processes in the photovoltaic industry, and highly applicable for various types of solar cells.
 
Article
Research on the luminescent solar concentrator (LSC) over the past thirty-odd years is reviewed. The LSC is a simple device at its heart, employing a polymeric or glass waveguide and luminescent molecules to generate electricity from sunlight when attached to a photovoltaic cell. The LSC has the potential to find extended use in an area traditionally difficult for effective use of regular photovoltaic panels: the built environment. The LSC is a device very flexible in its design, with a variety of possible shapes and colors. The primary challenge faced by the devices is increasing their photon-to-electron conversion efficiencies. A number of laboratories are working to improve the efficiency and lifetime of the LSC device, with the ultimate goal of commercializing the devices within a few years. The topics covered here relate to the efforts for reducing losses in these devices. These include studies of novel luminophores, including organic fluorescent dyes, inorganic phosphors, and quantum dots. Ways to limit the surface and internal losses are also discussed, including using organic and inorganic-based selective mirrors which allow sunlight in but reflect luminophore-emitted light, plasmonic structures to enhance emissions, novel photovoltaics, alignment of the luminophores to manipulate the path of the emitted light, and patterning of the dye layer to improve emission efficiency. Finally, some possible ‘glimpses of the future’ are offered, with additional research paths that could result in a device that makes solar energy a ubiquitous part of the urban setting, finding use as sound barriers, bus-stop roofs, awnings, windows, paving, or siding tiles.
 
Article
Chemical etching concurrently modifies pristine to layer-by-layer 3D-LiCoO 2 with a Co 3O 4 coating. Not only can the layered morphology affect the high electrochemical performance, but also the coating effect reduces the structure instability of the LiCoO 2 during cycling. This process can be applied to design other compounds to enhance their performance in Li-ion batteries.
 
Article
3DM-LiMnPO 4 balls and flakes are prepared via an impregnation method using a PMMA template. Both 3DM ball and flake structures demonstrated superior capacity retention to previously reported LiMnPO 4 prepared by various methods.
 
Article
CuS, CoS, and CuS/CoS onto fluorine-doped tin oxide glass substrates were deposited to function as counter electrodes for polysulfide redox reactions in CdS/CdSe quantum dot–sensitized solar cells (QDSSCs). Relative to a Pt electrode, the CuS, CoS, and CuS/CoS electrodes provide greater electrocatalytic activity, higher reflectivity, and lower charge-transfer resistance. Measurements of fill factor and short-current density reveal that the electrocatalytic activities, reflectivity, and internal resistance of counter electrodes play strong roles in determining the energy-conversion efficiency (η) of the QDSSCs. Because the CuS/CoS electrode has a smaller internal resistance and higher reflectivity relative to those of the CuS and CoS electrodes, it exhibits a higher fill factor and short-circuit current density. As a result, the QDSSC featuring a CuS/CoS electrode provides a higher value of η. Under illumination of one sun (100 mW cm−2), the QDSSCs incorporating Pt, CuS, CoS, and CuS/CoS counter electrodes provide values of η of 3.0 ± 0.1, 3.3 ± 0.3, 3.8 ± 0.2, and 4.1 ± 0.2%, respectively.
 
Article
An integrated computational and experimental study of FeS₂ pyrite reveals that phase coexistence is an important factor limiting performance as a thin-film solar absorber. This phase coexistence is suppressed with the ternary materials Fe₂SiS₄ and Fe₂GeS₄, which also exhibit higher band gaps than FeS₂. Thus, the ternaries provide a new entry point for development of thin-film absorbers and high-efficiency photovoltaics.
 
Article
A high electron mobility polymer, poly{[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene) (P(NDI2OD-T2)) is investigated for use as an electron acceptor in all-polymer blends. Despite the high bulk electron mobility, near-infrared absorption band and compatible energy levels, bulk heterojunction devices fabricated with poly(3-hexylthiophene) (P3HT) as the electron donor exhibit power conversion efficiencies of only 0.2%. In order to understand this disappointing photovoltaic performance, systematic investigations of the photophysics, device physics and morphology of this system are performed. Ultra-fast transient absorption spectroscopy reveals a two-stage decay process with an initial rapid loss of photoinduced polarons, followed by a second slower decay. This second slower decay is similar to what is observed for efficient P3HT:PCBM ([6,6]-phenyl C61-butyric acid methyl ester) blends, however the initial fast decay that is absent in P3HT:PCBM blends suggests rapid, geminate recombination of charge pairs shortly after charge transfer. X-ray microscopy reveals coarse phase separation of P3HT:P(NDI2OD-T2) blends with domains of size 0.2 to 1 micrometer. P3HT photoluminescence, however, is still found to be efficiently quenched indicating intermixing within these mesoscale domains. This hierarchy of phase separation is consistent with the transient absorption, whereby localized confinement of charges on isolated chains in the matrix of the other polymer hinders the separation of interfacial electron-hole pairs. These results indicate that local, interfacial processes are the key factor determining the overall efficiency of this system and highlight the need for improved morphological control in order for the potential benefit of high-mobility electron accepting polymers to be realized.
 
Article
Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3-n-hexylthiophene-2,5-diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.
 
Article
By tuning the frontier orbital energies through selective halogenation of the periphery of the organic framework, new light harvesting electron acceptors based on boron subphthalocyanine chloride (SubPc) have been made. Planar heterojunction organic photovoltaics made using a Cl 6-SubPc acceptor deliver an exceptionally high open-circuit voltage (∼1.3 V), good power conversion efficiency (∼2.7%) and improved operational stability in comparison to similar devices made using C 60 as the acceptor material.
 
Article
An advanced multifuelled solid oxide fuel cell (ASOFC) with a functional nanocomposite was developed and tested for use in a polygeneration system. Several different types of fuel, for example, gaseous (hydrogen and biogas) and liquid fuels (bio-ethanol and bio-methanol), were used in the experiments. Maximum power densities of 1000, 300, 600, 550 mW cm−2 were achieved using hydrogen, bio-gas, bio-methanol, and bio-ethanol, respectively, in the ASOFC. Electrical and total efficiencies of 54% and 80% were achieved using the single cell with hydrogen fuel. These results show that the use of a multi-fuelled system for polygeneration is a promising means of generating sustainable power.
 
Article
Thermoelectric materials can be optimized by tuning the carrier concentration with chemical doping. However, because the optimum dopant concentration typically increases with temperature, the optimum efficiency can not normally be achieved for a uniform material in a temperature gradient. Here, we show Ag-doped PbTe/Ag2Te composites exhibit high thermoelectric performance (∼50% greater than La-doped composites) because of a temperature induced gradient in the doping concentration caused by the temperature-dependent solubility of Ag in the PbTe matrix. This demonstrates a new mechanism to achieve a higher thermoelectric efficiency afforded by a given material system, and should be applicable to other thermoelectric materials.
 
Article
Some cations of ionic liquids (ILs) of interest for high-energy electrochemical storage devices, such as lithium batteries and supercapacitors, have a structure similar to that of surfactants. For such, it is very important to understand if these IL cations tend to aggregate like surfactants since this would affect the ion mobility and thus the ionic conductivity. The aggregation behaviour of ILs consisting of the bis(trifluoromethanesulfonyl)imide anion and different N-alkyl-N-methyl-pyrrolidinium cations, with the alkyl chain varied from C3H7 to C8H17, was extensively studied with NMR and Raman methods, also in the presence of Li+ cations. 2H NMR spin-lattice and spin-spin relaxation rates were analyzed by applying the “two step” model of surfactant dynamics. Here we show that, indeed, the cations in these ILs tend to form aggregates surrounded by the anions. The effect is even more pronounced in the presence of dissolved lithium cations.
 
Article
In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal-air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li-air and Zn-air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li-air and Zn-air batteries, with the aim of providing a better understanding of the new electrochemical systems.
 
Article
(VACNTs) are grown directly on a freestanding graphene paper (GP). The desirable carrier transport ability of the VACNTs, good conductivity and mechanical properties of the GP, and strong bonding between the VACNTs and the GP endow the hybrid structure with superior performance when utilized as the electrodes of lithium-ion batteries and dye-sensitized solar cells.
 
Article
A simple method was developed to prepare ultra-low Pt loading membrane electrode assembly (MEA) using vertically aligned carbon nanotubes (VACNTs) as highly ordered catalyst support for PEM fuel cells application. In the method, VACNTs were directly grown on the cheap household aluminum foil by plasma enhanced chemical vapor deposition (PECVD), using Fe/Co bimetallic catalyst. By depositing a Pt thin layer on VACNTs/Al and subsequent hot pressing, Pt/VACNTs can be 100% transferred from Al foil onto polymer electrolyte membrane for the fabrication of MEA. The whole transfer process does not need any chemical removal and destroy membrane. The PEM fuel cell with the MEA fabricated using this method showed an excellent performance with ultra-low Pt loading down to 35 μg cm−2 which was comparable to that of the commercial Pt catalyst on carbon powder with 400 μg cm−2. To the best of our knowledge, for the first time, we identified that it is possible to substantially reduce the Pt loading one order by application of order-structured electrode based on VACNTs as Pt catalysts support, compared with the traditional random electrode at a comparable performance through experimental and mathematical methods.
 
Article
The photovoltaic parameters, i.e., the short-circuit current, open-circuit voltage and device fill factor, of bulk heterojunction solar cells that use perylene diimide (PDI) derivatives as electron acceptors are often far below the theoretically expected values for reasons still not entirely understood. This article demonstrates that the photovoltaic characteristics of blend films of regioregular poly(3-hexylthiophene) (rr-P3HT) and PDI molecules are improved upon using a core-alkylated PDI derivative instead of the often used N-alkylated PDI molecules. A doubling of the power conversion efficiency of P3HT:PDI solar cells by using the core-alkylated PDI derivative is observed leading to an unprecedented power conversion efficiency of 0.5% for a P3HT:PDI solar cell under AM1.5 solar illumination. Furthermore, the optical properties of the novel PDI derivative are compared to two standard exclusively N-alkylated PDI derivatives by steady-state and time-resolved photoluminescence spectroscopy in solution and solid state. The experiments reveal that aggregation in the solid state determines the photophysics of all PDI derivatives. However, the emission energy and excited state lifetime of the aggregates are clearly influenced by the alkyl-substitution pattern through its effect on the packing of the PDI molecules. X-ray diffraction experiments before and after thermal annealing of PDI:polystyrene and PDI:P3HT blends reveal subtle differences in the packing characteristics of the different PDI derivatives and, problematically, that P3HT ordering is suppressed by all of the PDI derivatives.
 
Article
We demonstrate a new method for the direct conversion of heat to electricity using the recently discovered multiferroic alloy, Ni45Co5Mn40Sn101. This alloy undergoes a low hysteresis, reversible martensitic phase transformation from a nonmagnetic martensite phase to a strongly ferromagnetic austenite phase upon heating. When biased by a suitably placed permanent magnet, heating through the phase transformation causes a sudden increase of the magnetic moment to a large value. As a consequence of Faraday’s law of induction, this drives a current in a surrounding circuit. Theory predicts that under optimal conditions the performance compares favorably with the best thermoelectrics. Because of the low hysteresis of the alloy, a promising area of application of this concept appears to be energy conversion at small ΔT, suggesting a possible route to the conversion of the vast amounts of energy stored on earth at small temperature difference. We postulate other new methods for the direct conversion of heat to electricity suggested by the underlying theory.
 
Article
To develop a durable proton-exchange membrane (PEM) for fuel-cell applications, a series of sulfonated poly(benzoxazole thioether sulfone)s ( SPTESBOs) are designed and synthesized, with anticipated good dimensional stability (via acid–base cross linking), improved oxidative stability against free radicals (via incorporation of thioether groups), and enhanced inherent stability (via elimination of unstable end groups) of the backbone. The structures and the degree of sulfonation of the copolymers are characterized using Fourier-transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy (1H NMR and 19F NMR). The electrochemical stabilities of the monomers are examined using cyclic voltammetry in a typical three-electrode cell configuration. The physicochemical properties of the membranes vital to fuel-cell performance are also carefully evaluated under conditions relevant to fuel-cell operation, including chemical and thermal stability, proton conductivity, solubility in different solvents, water uptake, and swelling ratio. The new membranes exhibit low dimensional change at 25°C to 90°C and excellent thermal stability up to 250°C. Upon elimination of unstable end groups, the co-polymers display enhanced chemical resistance and oxidative stability in Fenton's test. Further, the SPTESBO-HFB-60 (HFB-60=hexafluorobenzene, 60 mol% sulfone) membrane displays comparable fuel-cell performance to that of an NRE 212 membrane at 80°C under fully humidified condition, suggesting that the new membranes have the potential to be more durable but less expensive for fuel-cell applications.
 
Article
This work introduces an effective, inexpensive, and large-scale production approach to the synthesis of a carbon coated, high grain boundary density, dual phase Li4Ti5O12-TiO2 nanocomposite anode material for use in rechargeable lithium-ion batteries. The microstructure and morphology of the Li4Ti5O12-TiO2-C product were characterized systematically. The Li4Ti5O12-TiO2-C nanocomposite electrode yielded good electrochemical performance in terms of high capacity (166 mAh g−1 at a current density of 0.5 C), good cycling stability, and excellent rate capability (110 mAh g−1 at a current density of 10 C up to 100 cycles). The likely contributing factors to the excellent electrochemical performance of the Li4Ti5O12-TiO2-C nanocomposite could be related to the improved morphology, including the presence of high grain boundary density among the nanoparticles, carbon layering on each nanocrystal, and grain boundary interface areas embedded in a carbon matrix, where electronic transport properties were tuned by interfacial design and by varying the spacing of interfaces down to the nanoscale regime, in which the grain boundary interface embedded carbon matrix can store electrolyte and allows more channels for the Li+ ion insertion/extraction reaction. This research suggests that carbon-coated dual phase Li4Ti5O12-TiO2 nanocomposites could be suitable for use as a high rate performance anode material for lithium-ion batteries.
 
Article
Single-walled carbon nanotubes (SWNTs) sorted by electronic type are employed as organic photovoltaic device anodes. Metal-enriched SWNT films yield device efficiencies that are fifty times greater than their semiconducting counterparts. Through sheet resistance, UV-vis-NIR optical absorbance, and X-ray photoelectron spectroscopy measurements, the OPV charge blocking layer PEDOT:PSS is found to reverse the original chemical doping of the SWNT films. The relative insensitivity of metallic SWNTs to chemical doping thus explains the improved performance of metal-enriched SWNT films as OPV anodes.
 
Article
Silicon exhibits the largest known capacity for Li insertion in anodes of Li-ion batteries. However, because of large volume expansion/phase changes upon alloying, Si becomes powder-like after a few charge-discharge cycles. Various approaches have been explored in the past to circumvent this problem, including the use of nanomaterials, particularly Si nanowires. However, even though nanowires resist cracking very well, anodes based on Si nanowires still see their original capacity fade away upon cycling, because of wire detachment from the substrate, due to the stress generated at their roots upon alloying with Li. Here, we present a silicon nanowire growth strategy yielding highly interconnected specimens, which prevents them from being individually detached from the substrate. We report a ∼100% charge retention after 40 cycles at C/2 rate, without charging voltage limitation. We also show that our anodes can be cycled at 8C rates without damage and we grow nanowires with a density of 1.2 mg/cm2, yielding anodes delivering a 4.2 mAh/cm2 charge density. Finally, we point out that a better understanding of the interactions of silicon with electrolytes is needed if the field is to progress in the future.
 
Article
The challenges of printing all layers in polymer solar cells from aqueous solution are met by design of inks for the electron-, hole-, active-, and metallic back electrode-layers. The conversion of each layer to an insoluble state after printing enables multilayer formation from the same solvent (water). The photograph here was taken just before screen printing of the aqueous silver ink.
 
Schematic of the fabrication of ZnOS nanowire photoanode. b) SEM and TEM images, and corresponding SAED of pristine ZnO nanowires. c) TEM image of ZnO nanowires with deposition of ZnS QDs. d) Elemental profile extract from STEM in direction of nanowires (indicated by arrow in TEM).
X-ray diffraction patterns of ZnO, ZnO nanowires sensitized with ZnS QDs, and ZnO-ZnS solid solution nanowires. b) bandgap plots of (F(R)hν)2 for ZnO and ZnO–ZnS solid solution nanowires.
Linear-sweep voltammograms from ZnO nanowires, ZnO nanowires with sensitization of ZnS QDs, and ZnO–ZnS solid solution nanowires. b) IPCE spectra of ZnO, ZnO nanowires with sensitization of ZnS QDs, and ZnO–ZnS solid solution nanowires at a potential of 0.5 V versus Ag/AgCl. c) H2 production upon illumination of each samples.
EXAFS spectra of Zn K-edge for ZnO and ZnO-ZnS solid solution nanowires. b) Wurtzite structure of ZnO and each scattering shell modes of central Zn atom. c) XANES spectra of Zn K-edge for ZnO nanowires, ZnO@ ZnS QDs, and ZnO–ZnS solid solution nanowires.
Sketch of nanostructure of ZnO–ZnS solid solution nanowires. The lower part displays pathways of electrons for generating hydrogen, and a relative energy diagram.
Article
A ZnO–ZnS solid solution nanowire array photoanode is developed based on an alternative sensitization of a ZnO–ZnS solid solution nanowire array for solar hydrogen generation with considerably enhanced photocurrent – more than 195% greater compared to pristine ZnO nanowires. This solid solution structure demonstrates a better photoactivity enhancement effect than traditional quantum dot sensitization, as well as allowing hydrogen generation.
 
Article
A novel ligand-assisted assembly approach is demonstrated for the synthesis of thermally stable and large-pore ordered mesoporous titanium dioxide with a highly crystalline framework by using diblock copolymer poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as a template and titanium isopropoxide (TIPO) as a precursor. Small-angle X-ray scattering, X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution scanning electron microscopy, and N2-sorption measurements indicate that the obtained TiO2 materials possess an ordered primary cubic mesostructure with large, uniform pore diameters of about 16.0 nm, and high Brunauer–Emmett–Teller surface areas of ∼112 m2 g−1, as well as high thermal stability (∼700 °C). High resolution TEM and wide-angle XRD measurements clearly illustrate the high crystallinity of the mesoporous titania with an anatase structure in the pore walls. It is worth mentioning that, in this process, in addition to tetrahydrofuran as a solvent, acetylacetone was employed as a coordination agent to avoid rapid hydrolysis of the titanium precursor. Additionally, stepped evaporation and heating processes were adopted to control the condensation rate and facilitate the assembly of the ordered mesostructure, and ensure the formation of fully polycrystalline anatase titania frameworks without collapse of the mesostructure. By employing the obtained mesoporous and crystallized TiO2 as the photoanode in a dye-sensitized solar cell, a high power-conversion efficiency (5.45%) can be achieved in combination with the N719 dye, which shows that this mesoprous titania is a great potential candidate as a catalyst support for photonic-conversion applications.
 
Article
Differential resistance analysis has been shown to be effective at studying the charge carrier losses in bulk heterojunc-tion solar cells. The variations of differential resistance with light intensity and Voc provide strong evidence for the identification of the recombination kinetics, revealing bimolecular as the dominating recombination mechanism in PCDTBT:PC 60BM system and showing an evolution to trap-assisted recombination in presence of PC 84BM trap states.
 
Article
In this study, the effect of plasmonic core-shell structures, consisting of dielectric cores and metallic nanoshells, on energy conversion in dye-sensitized solar cells (DSSCs) is investigated. The structure of the core-shell particles is controlled to couple with visible light so that the visible component of the solar spectrum is amplified near the core-shell particles. In core-shell particle – TiO2 nanoparticle films, the local field intensity and light pathways are increased due to the surface plasmons and light scattering. This, in turn, enlarges the optical cross-section of dye sensitizers coated onto the mixed films. When 22 vol% of core-shell particles are added to a 5 μm thick TiO2 film, the energy conversion efficiency of DSSCs increases from 2.7% to 4.0%, in spite of a more than 20% decrease in the amount of dyes adsorbed on the composite films. The correlation between core-shell particle content and energy conversion efficiency in DSSCs is explained by the balance among near-field effects, light scattering efficiency, and surface area in the composite films.
 
Article
This image presents a scanning electron microscopy image of solid state dye-sensitized solar cell with a plasmonic back reflector, overlaid with simulated field intensity plots when monochromatic light is incident on the device. Plasmonic back reflectors, which consist of 2D arrays of silver nanodomes, can enhance absorption through excitation of plasmonic modes and increased light scattering, as reported by Michael D. McGehee, Yi Cui, and co-workers.
 
Article
A mature approach: Allowing solutions of poly[(4,4-didodecyldithieno[3,2-b: 2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P1) to stand in a solvent of marginal quality leads to interchain aggregation. These supramolecular structures lead to thin films with higher charge carrier mobilities and internal order, as determined by the fabrication of thin film transistors and grazing-incidence wide angle X-ray scattering (GIWAXS) measurements, respectively. Aging of solutions is therefore a very straightforward method to modify the optoelectronic properties of solution-processable organic semiconductors.
 
Article
Understanding the morphology of polymer-based bulk heterojunction (BHJ) solar cells is key to improving device efficiencies. Blends of a low-bandgap silole-containing conjugated polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b;2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (PSBTBT) with phenyl-C61-butyric acid methyl ester (PCBM) were investigated using different processing conditions. Scanning force microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry and neutron reflectivity studies showed that thermal annealing did not induce obvious changes in the structure of the active layer. Grazing-incidence X-ray diffraction and small-angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and a brief thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains with 10-nm in size, leading to an improvement in photovoltaic performance.
 
Article
Layered Na 0.85Li 0.17Ni 0.21Mn 0.64O 2 is synthesized and evaluated as a cathode in Na batteries. The resulting P2 crystal structure allows for single-phase reversible Na intercalation reaction demonstrating 100 mAhg -1. The cathode can be discharged with high rate (65 mAhg -1 at 25 C rate). The presence of Li and the electronic ordering of Ni(II)/Mn(IV) in the transition metal layer is responsible for material stability.
 
Article
Nanostructured V2O5 thin films have been prepared by means of cathodic deposition from an aqueous solution made from V2O5 and H2O2 directly on fluorine-doped tin oxide coated (FTO) glasses followed by annealing at 500°C in air, and studied as film electrodes for lithium ion batteries. XPS results show that the as-deposited films contained 15% V4+, however after annealing all the vanadium is oxidized to V5+. The crystallinity, surface morphology, and microstructures of the films have been investigated by means of XRD, SEM, and AFM. The V2O5 thin film electrodes show excellent electrochemical properties as cathodes for lithium ion intercalation: a high initial discharge capacity of 402 mA h g−1 and 240 mA h g−1 is retained after over 200 cycles with a discharging rate of 200 mA g−1 (1.3 C). The specific energy density is calculated as 900 W h kg−1 for the 1st cycle and 723 W h kg−1 for the 180th cycle when the films are tested at 200 mA g−1 (1.3 C). When discharge/charge is carried out at a high current density of 10.5 A g−1 (70 C), the thin film electrodes retain a good discharge capacity of 120 mA h g−1, and the specific power density is over 28 kW kg−1.
 
Article
Bulk-type all-solid-state inorganic Li-ion batteries can be prepared in one step in few minutes using spark plasma sintering. The self-supported cells display thick composite electrodes of up to 800 μm, which offer high surface capacities of up to 10 mAh.cm-2. Such technology is safer than classical Li-ion batteries and offers good electrochemical properties at temperatures above 100 °C.
 
Article
Low cost, high performance redox flow batteries are highly demanded for up to multi-megawatt levels of renewable and grid energy storage. Here, we report a new vanadium redox flow battery with a significant improvement over the current technologies. This new battery utilizes a sulfate-chloride mixed solution, which is capable of dissolving more than 2.5 M vanadium or about a 70% increase in the energy storage capacity over the current vanadium sulfate system. More importantly, the new electrolyte remains stable over a wide temperature range of -5 to 60oC, potentially eliminating the need of active heat management. Its high energy density, broad operational temperature window, and excellent electrochemical performance would lead to a significant reduction in the cost of energy storage, thus accelerating its market penetration.
 
Article
A new kind of flow battery is fueled by semi-solid suspensions of high-energy-density lithium storage compounds that are electrically 'wired' by dilute percolating networks of nanoscale conductor particles. Energy densities are an order of magnitude greater than previous flow batteries; new applications in transportation and grid-scale storage may result.
 
Article
The effect of Bi (semimetal) nanoinclusions in nanostructured Bi2Te3 matrices is investigated. Bismuth nanoparticles synthesized by a low temperature solvothermal method are incorporated into Bi2Te3 matrix phases, synthesized by planetary ball milling. High density pellets of the Bi nanoparticle/Bi2Te3 nanocomposites are created by hot pressing the powders at 200 °C and 100 MPa. The effect of different volume fractions (0–7%) of Bi semimetal nanoparticles on the Seebeck coefficient, electrical conductivity, thermal conductivity and carrier concentration is reported. Our results show that the incorporation of semimetal nanoparticles results in a reduction in the lattice thermal conductivity in all the samples. A significant enhancement in power factor is observed for Bi nanoparticle volume fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal conductivity and increase the power factor resulting in an increase in figure of merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coefficient and electrical conductivity as a function of carrier concentration data are consistent with the electron filtering effect, where low-energy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.
 
Article
Vanadium pentoxide (V2O5) layered nanostructures are known to have very stable crystal structures and high faradaic activity. The low electronic conductivity of V2O5 greatly limits the application of vanadium oxide as electrode materials and requires combining with conducting materials using binders. It is well known that the organic binders can degrade the overall performance of electrode materials and need carefully controlled compositions. In this study, we develop a simple method for preparing freestanding carbon nanotube (CNT)-V2O5 nanowire (VNW) composite paper electrodes without using binders. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode (Spectracarb 2225) as the cathode. The supercapacitor with CNT-VNW composite paper electrode exhibits a power density of 5.26 kW Kg−1 and an energy density of 46.3 Wh Kg−1. (Li)VNWs and CNT composite paper electrodes can be fabricated in similar manner and show improved overall performance with a power density of 8.32 kW Kg−1 and an energy density of 65.9 Wh Kg−1. The power and energy density values suggest that such flexible hybrid nanocomposite paper electrodes may be useful for high performance electrochemical supercapacitors.
 
Article
Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In the most extensively characterized system, the poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase-pure domains form in BHJs due to immiscibility of these two components.
 
Article
Polymer:fullerene blends were screened in a combinatorial approach using inkjet printing thin film libraries for photovoltaic devices. The application of inkjet printing enabled a fast and simple experimental workflow from film preparation to the study of structure-property-relationships with a very high material efficiency. Inkjet printing requires less material for the preparation of thin film libraries in comparison to other dispensing techniques, like spin-coating. Two polymers (PCPDTBT, PSBTBT) and two fullerene derivatives (mono-PCBM, bis-PCBM) were investigated in various blend ratios, concentrations, solvent ratios, and film thicknesses. Morphological and optical properties of the inkjet printed films were investigated and compared with spin-coated films. This study shows the principle of an experimental setup from solution preparation to film characterization for the combinatorial investigation of large polymer:fullerene libraries.
 
Article
There is an urgent need for alternative energy resources due to the rapid rise in the price of fossil fuels and the great danger of the increasing greenhouse effect caused by carbon dioxide emission. Sunlight provides by far the largest of all carbon-neutral energy sources. Therefore, the current solar- or photovoltaic-cell-based technologies, which can utilize solar energy, are of extreme importance. Dye-sensitized solar cells (DSSCs) are of particular interest because they can offer a number of advantages when compared to existing photovoltaic technologies. In this review, recent advances in carbon-related nanomaterials and their application as materials for DSSCs are discussed. Carbon nanomaterials such as carbon nanotubes and graphene display remarkable electrical, thermal, and mechanical properties that enable several exciting applications in DSSCs. The progress on the utilisation of carbon nanotubes, graphene, and their nanocomposites is reviewed as highly prospective materials to replace transparent conductive oxide (TCO) layers and counter electrodes in DSSCs. Moreover, carbon nanomaterials enable improvement of the performance of absorbing layers in working photoanodes by enhancing the light absorption and electron transport across the semiconducting nanostructured film. The application of carbon nanotubes, graphite particles, and graphene as additives towards the improved efficiency of the electrolyte in these solar cells is also discussed. Finally, a brief outlook is provided on the future development of carbon nanomaterial composites as prospective materials for DSSCs, particularly as components for printable solar cells, which are expected to play an important role in the future solar-cell market.
 
Article
A multi-component catalyst Ni-VOx/AC (VOx is comprised of V2O5 and VO2, x = 2.18) was synthesized by a wet impregnation method. The synthesized Ni-VOx/AC shows a superior catalytic effect on de/hydrogenation of Mg. The MgH2+Ni-VOx/AC composites can absorb 6.2wt.-% hydrogen within only 1 min at 150 degrees C under a hydrogen pressure of 2 MPa and desorb 6.5 wt.-% hydrogen within 10 min at 300 degrees C under an initial hydrogen pressure of 1 KPa, which overcomes a critical barrier for practical use of Mg as a hydrogen storage material. A significant decrease of activation energy (E-a) indicates that Ni-VOx/AC catalyst is highly efficient for Mg de/hydrogenation, which may be ascribed to the synergistic effect of bimetals (metal oxides) and nanocarbon.
 
Article
Graphitic carbons with ordered mesostructure and high surface areas (of great interest in applications such as energy storage) have been synthesized by a direct triblock-copolymer-templating method. Pluronic F127 is used as a structure-directing agent, with a low-molecular-weight phenolic resol as a carbon source, ferric oxide as a catalyst, and silica as an additive. Inorganic oxides can be completely eliminated from the carbon. Small-angle XRD and N2 sorption analysis show that the resultant carbon materials possess an ordered 2D hexagonal mesostructure, uniform bimodal mesopores (about 1.5 and 6 nm), high surface area (∼1300 m2/g), and large pore volumes (∼1.50 cm3/g) after low-temperature pyrolysis (900 °C). All surface areas come from mesopores. Wide-angle XRD patterns demonstrate that the presence of the ferric oxide catalyst and the silica additive lead to a marked enhancement of graphitic ordering in the framework. Raman spectra provide evidence of the increased content of graphitic sp2 carbon structures. Transmission electron microscopy images confirm that numerous domains in the ordered mesostructures are composed of characteristic graphitic carbon nanostructures. The evolution of the graphitic structure is dependent on the temperature and the concentrations of the silica additive, and ferric oxide catalyst. Electrochemical measurements performed on this graphitic mesoporous carbon when used as an electrode material for an electrochemical double layer capacitor shows rectangular-shaped cyclic voltammetry curves over a wide range of scan rates, even up to 200 mV/s, with a large capacitance of 155 F/g in KOH electrolyte. This method can be widely applied to the synthesis of graphitized carbon nanostructures.
 
Article
Identifying the important factors governing the oxygen reduction kinetics at solid oxide fuel cell cathodes is critical for enhanced performance, particularly at reduced temperatures. In this work, a model mixed conducting perovskite materials system, SrTi1–xFexO3–δ, is selected, offering the ability to systematically control both the levels of ionic and electronic conductivity as well as the energy band structure. This, in combination with considerably simplified electrode geometry, serves to demonstrate that the rate of oxygen exchange at the surface of SrTi1–xFexO3–δ is only weakly correlated with either high electronic or ionic conductivity, in apparent contradiction with common expectations. Based on the correlation found between the position of the Fermi energy relative to the conduction band edge and the activation energy exhibited by the exchange rate constant, it is possible to confirm experimentally, for the first time, the key role that the minority electronic species play in determining the overall reaction kinetics. These observations lead to a new conceptual model describing cathode kinetics and provide guidelines for identifying cathodes with improved performance.
 
Article
Solution-processed zinc oxide nanocrystals (ZnO NCs) hybridized with insulating poly(ethylene glycol) (PEG) are introduced as a cathode interlayer in bulk heterojunction organic photovoltaic cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methyl ester (PC61BM) blends. The performance of devices with ZnO-PEG interlayers exhibit an excellent maximum power conversion efficiency (PCE) of 4.4% with a fill factor (FF) of 0.69 under optimized conditions. This enhanced device performance is attributed to decreased series resistance from the hole blocking properties of ZnO, as well as the facilitated electron transport due to the reduced area of ZnO domain boundaries upon addition of PEG. The addition of PEG also lowers the electron affinity of ZnO, which leads to a nearly Ohmic contact at the polymer/metal interface. Moreover, the ZnO-PEG interlayer serves as an optical spacer that enhances light absorption and thereby increases the photocurrent. The addition of PEG permits control over layer thickness and refractive indices. Improved photon energy absorption is supported by optical simulations. Devices with highly stable metals such as Ag and Au also show dramatically enhanced performance comparable to conventional devices with Al cathode. Due to its simplicity and excellent characteristics, this multifunctional interlayer is suitable for high performance printed photovoltaic cells.
 
Top-cited authors
Jaephil Cho
  • Ulsan National Institute of Science and Technology
Oki Gunawan
  • IBM Research
Meilin Liu
  • Georgia Institute of Technology
Gerbrand Ceder
  • University of California, Berkeley
Teodor Todorov