Measuring charge transport from transient photovoltage rise times. A new tool to investigate electron transport in nanoparticle films

Charge transport rate at open-circuit potential (V(oc)) is proposed as a new characterization method for dye-sensitized (DS) and other nanostructured solar cells. At V(oc), charge density is flat and measurable, which simplifies quantitative comparison of transport and charge density. Transport measured at V(oc) also allows meaningful comparison of charge transport rates between different treatments, temperatures, and types of cells. However, in typical DS cells, charge transport rates at V(oc) often cannot be measured by photocurrent transients or modulation techniques due to RC limitations and/or recombination losses. To circumvent this limitation, we show that charge transport at V(oc) can be determined directly from the transient photovoltage rise time using a simple, zero-free-parameter model. This method is not sensitive to RC limitation or recombination losses. In trap limited devices, such as DS cells, the comparison of transport rates between different devices or conditions is only valid when the Fermi level in the limiting conductor is at the same distance from the band edge. We show how to perform such comparisons, correcting for conduction band shifts using the density of states (DOS) distribution determined from the same photovoltage transients. Last we show that the relationship between measured transport rate and measured charge density is consistent with the trap limited transport model.     

Orbit-Homogeneity in Permutation Groups

This paper introduces the concept of orbit-homogeneity of permutationgroups: a group G is orbit-t-homogeneous if two sets of cardinalityt lie in the same orbit of G whenever their intersections witheach G-orbit have the same cardinality. For transitive groups,this coincides with the usual notion of t-homogeneity. Thisconcept is also compatible with the idea of partition transitivityintroduced by Martin and Sagan. Further, this paper shows that any group generated by orbit-t-homogeneoussubgroups is orbit-t-homogeneous, and that the condition becomesstronger as t increases up to [n/2], where n is the degree.So any group G has a unique maximal orbit-t-homogeneous subgroup_t(G), and _tG _t–1(G). Some structural results for orbit-t-homogeneousgroups, and a number of examples, are also given. 2000 MathematicsSubject Classification 20B10.     

Interface engineering for solid-state dye-sensitised nanocrystalline solar cells: the use of an organic redox cascade

We demonstrate the formation of a charge transfer cascade at a nanostructured TiO2/dye/polymer/molecular hole transport multilayer interface. Charge recombination dynamics at this interface are shown to be retarded when the ionisation potential of the polymer layer exceeds that of the molecular hole transport layer.     

Estimation of electronic coupling for intermolecular electron transfer from cross-reaction data

Sixty-five electron-transfer reactions including 27 new 0, +1 couples have been added to our data set of cross-reactions between 0 and +1 couples, bringing it to 206 reactions involving 72 couples that have been studied by stopped-flow kinetics in acetonitrile containing supporting electrolyte at 25 degrees C, formal potentials determined by cyclic voltammetry, and analyzed using Marcus cross-rate theory. Perhaps surprisingly, a least-squares analysis demonstrates that intrinsic rate constants exist that predict the cross-rate constants to within a factor of 2 of the observed ones for 93% of the reactions studied, and only three of the reactions have a cross-rate constant that lies outside of the factor of 3, that corresponds to a factor of 10 uncertainty in the rate constant for an unknown couple. Many triarylamines, which have very high intrinsic reactivity, are included among the newly studied couples. The enthalpy contribution to the Marcus reorganization energy, lambda'v, has been calculated for 46 of the couples studied, at the (U)B3LYP/6-31+G (or for the larger and lower barrier compounds, at the less time-consuming (U)B3LYP/6-31G) level. In combination with a modified Levich and Dogodnadze treatment that assumes that the rate constant is proportional to (KeHab2/lambda1/2) exp[-DeltaG/RT], this allows estimation of the electronic coupling (Hab) at the transition state for intermolecular electron transfer, (more properly H'ab, the product of the square root of the encounter complex formation constant times Hab) for these couples. Although the principal factor affecting intermolecular electron-transfer rate constants is clearly lambda, H'ab effects are easily detectable, and the dynamic range in our estimates of them is over a factor of 600.     

Coating single-walled carbon nanotubes with phospholipids

Single-walled carbon nanotubes (SWNTs), being hydrophobic by nature, aggregate in water to form large bundles. However, isolated SWNTs possess unique physical and chemical properties that are desirable for sensing and biological applications. Conventionally isolated SWNTs can be obtained by wrapping the tubes with biopolymers or surfactants. The binding modes proposed for these solubilization schemes, however, are less than comprehensive. Here we characterize the efficacies of solubilizing SWNTs through various types of phospholipids and other amphiphilic surfactants. Specifically, we demonstrate that lysophospholipids, or single-chained phospholipids offer unprecedented solubility for SWNTs, while double-chained phospholipids are ineffective in rendering SWNTs soluble. Using transmission electron microscopy (TEM) we show that lysophospholipids wrap SWNTs as striations whose size and regularity are affected by the polarity of the lysophospholipids. We further show that wrapping is only observed when SWNTs are in the lipid phase and not the vacuum phase, suggesting that the environment has a pertinent role in the binding process. Our findings shed light on the debate over the binding mechanism of amphiphilic polymers and cylindrical nanostructures and have implications on the design of novel supramolecular complexes and nanodevices.     

Measuring inter-DNA potentials in solution

Interactions between short strands of DNA can be tuned from repulsive to attractive by varying solution conditions and have been quantified using small angle x-ray scattering techniques. The effective DNA interaction charge was extracted by fitting the scattering profiles with the generalized one-component method and inter-DNA Yukawa pair potentials. A significant charge is measured at low to moderate monovalent counterion concentrations, resulting in strong inter-DNA repulsion. The charge and repulsion diminish rapidly upon the addition of divalent counterions. An intriguing short range attraction is observed at surprisingly low divalent cation concentrations, approximately 16 mM Mg2+. Quantitative measurements of inter-DNA potentials are essential for improving models of fundamental interactions in biological systems.     

Effects of Co-Solvent Nature and Acid Concentration in the Size and Morphology of Wrinkled Mesoporous Silica Nanoparticles for Drug Delivery Applications

Hierarchically porous materials, such as wrinkled mesoporous silica (WMS), have gained interest in the last couple of decades, because of their wide range of applications in fields such as nanomedicine, energy, and catalysis. The mechanism of formation of these nanostructures is not fully understood, despite various groups reporting very comprehensive studies. Furthermore, achieving particle diameters of 100 nm or less has proven difficult. In this study, the effects on particle size, pore size, and particle morphology of several co-solvents were evaluated. Additionally, varying concentrations of acid during synthesis affected the particle sizes, yielding particles smaller than 100 nm. The morphology and physical properties of the nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and dynamic light scattering (DLS). Homogeneous and spherical WMS, with the desired radial wrinkle morphology and particle sizes smaller than 100 nm, were obtained. The effect of the nature of the co-solvents and the concentration of acid are explained within the frame of previously reported mechanisms of formation, to further elucidate this intricate process.     

X-ray absorption spectroscopy study of the electronic structure and local coordination of 1st row transition metal-promoted Chevrel-phase sulfides

Abstract

The electronic structures of S and Mo as well as the local coordination of Mo are investigated as a function of metal promotion Chevrel-phase (CP) sulfides. We observe the effect of metal promoter-induced electron donation into the stoichiometric range M_xMo₆S₈ (M?=?Fe, Ni, Cu; x?=?0–2) through analysis of X-ray absorption near-edge structure regions. We further observe the effect of this promotion on the bonding environment of Mo₆ metal centers through extended X-ray absorption fine structure analysis. We monitor expansion and contraction of Mo₆ octahedra with and without metal promotion, as has been predicted by Hückel molecular orbital theory. We further observe a marked tunability in the electronic structure of sulfur upon charge transfer between promoting species and Mo₆S₈ units. Average Mo₆ octahedron Mo–Mo bond contraction from 2.76 Å to as short as 2.69 Å was observed upon incorporation of metal promoters, while intercluster separation displays a pronounced increase for promoter-host lattices compared to un-promoted Mo₆S₈. To corroborate spectroscopically observed phenomena, we performed computational analyses of spin-polarized densities of state for the CP materials investigated herein, where a detectable increase in sulfur-based frontier orbital population is observed in accordance with experimentally validated orbital filling.