Electron Diffusion Coefficient Research Articles

Influence of the electrolyte cation in organic dye-sensitized solar cells: lithium versus dimethylimidazolium

We investigate the influence of electrolyte cations such as lithium and dimethylimidazolium on the spectroscopic and electrical characteristics of dye-sensitized solar cells based on a metal-free chromophore C218. An evident bathochromic effect of lithium with respect to dimethylimidazolium is noticed for the C218 dye-coated nanocrystalline titania film via measuring electronic absorption and photocurrent action spectra. In comparison with dimethylimidazolium, the use of lithium as the electrolyte cation evokes a downward shift of the excited-state redox potential of the C218 sensitizer by 120 mV, and that of the conduction band edge of the nanocrystalline TiO2 film by 390 meV. The resultant remarkable variation in the energy alignment at the titania/dye interface brings on dissimilar electron injection yields as revealed by transient emission measurements. Modulating the titania surface states distribution with lithium relative to dimethylimidazolium induces a slightly declining electron diffusion coefficient in the mesoporous titania film. However, the adsorption of lithium cations on titania strongly retards the interfacial charge recombination compared to dimethylimidazolium, contributing to an over one order of magnitude of enhancement of electron diffusion length. The improvement of electron diffusion length has a noticeable effect on the charge collection yield, which can also be observed by measuring monochromatic incident photon-to-electron conversion efficiencies. Further interface engineering is needed to enhance both the photovoltage and photocurrent, making use of the full power of metal-free organic dyes.

Charge Transport Characteristics of High Efficiency Dye-Sensitized Solar Cells Based on Electrospun TiO2 Nanorod Photoelectrodes

In this report, dye-sensitized solar cells (DSSCs) with high energy conversion efficiencies were fabricated using TiO2 nanorods electrospun from a solution mixture of titanium n-propoxide and poly(vinyl acetate) in dimethyl formamide. Investigation of the charge transport characteristics of this unique type of DSSC disclosed that the efficiency of the DSSCs was enhanced by optimizing the nanorod morphology to facilitate charge transport. Our TiO2 nanorods have an intrinsically higher sensitizer loading capability than conventional TiO2 nanoparticles and have much slower recombination lifetimes compared to conventional nanoparticles. Long electron lifetime in nanorod electrode contributes to the enhanced effective photocarrier collection as well as the conversion efficiency. The electron transport behavior of nanorod photoelectrodes was further improved by TiCl4 post-treatment. The post-treatment reduces the pore volume of nanorod photoelectrodes while improving inter-rod connectivity and enhancing electron diffusion. The electron diffusion coefficient of post-treated nanorod was ∼51% higher than that of an untreated one, leading to a charge collection efficiency that was 19% higher at a incident photonflux of 8.1 × 1016 cm−2 s−1. Finally, the efficiency of nanorod-based DSSCs was optimized at a photoelectrode thickness of 14 μm to achieve 9.52% under masked illumination of simulated solar light, AM 1.5 Global (Voc = 761 mV, Jsc = 17.6 mA cm−2, fill factor = 70.0%).

Iodine/Iodide-Free Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are built from nanocrystalline anatase TiO(2) with a 101 crystal face (nc-TiO(2)) onto which a dye is absorbed, ruthenium complex sensitizers, fluid I(-)/I(3)(-) redox couples with electrolytes, and a Pt-coated counter electrode. DSSCs have now reached efficiencies as high as 11%, and G24 Innovation (Cardiff, U.K.) is currently manufacturing them for commercial use. These devices offer several distinct advantages. On the basis of the electron lifetime and diffusion coefficient in the nc-TiO(2) layer, DSSCs maintain a diffusion length on the order of several micrometers when the dyed-nc-TiO(2) porous layer is covered by redox electrolytes of lithium and/or imidazolium iodide and their polyiodide salts. The fluid iodide/iodine (I(-)/I(3)(-)) redox electrolytes can infiltrate deep inside the intertwined nc-TiO(2) layers, promoting the mobility of the nc-TiO(2) layers and serving as a hole-transport material of DSSCs. As a result, these materials eventually give a respectable photovoltaic performance. On the other hand, fluid I(-)/I(3)(-) redox shuttles have certain disadvantages: reduced performance control and long-term stability and incompatibility with some metallic component materials. The I(-)/I(3)(-) redox shuttle shows a significant loss in short circuit current density and a slight loss in open circuit voltage, particularly in highly viscous electrolyte-based DSSC systems. Iodine can also act as an oxidizing agent, corroding metals, such as the grid metal Ag and the Pt mediator on the cathode, especially in the presence of water and oxygen. In addition, the electrolytes (I(-)/I(3)(-)) can absorb visible light (lambda = approximately 430 nm), leading to photocurrent loss in the DSSC. Therefore, the introduction of iodide/iodine-free electrolytes or hole-transport materials (HTMs) could lead to cost-effective alternatives to TiO(2) DSSCs. In this Account, we discuss the iodide/iodine-free redox couple as a substitute for the fluid I(-)/I(3)(-) redox shuttle. We also review the adaptation of solid-state HTMs to the iodide/iodine-free solid-state DSSCs with an emphasis on their pore filling and charge mobility in devices and the relationship of those values to the performance of the resulting iodide/iodine-free DSSCs. We demonstrate how the structures of the sensitizing dye molecules and additives of lithium or imidazolium salts influence device performance. In addition, the self-organizing molecular interaction for electronic contact of HTMs to dye molecules plays an important role in unidirectional charge diffusion at interfaces. The poly(3,4-ethylenedioxythiophene) (PEDOT)-based DSSCs, which we obtain through photoelectrochemical polymerization (PEP) using 3-alkylthiophen-bearing ruthenium dye, HRS-1, and bis-EDOT, demonstrates the importance of nonbonding interface contact (e.g., pi-pi-stacking) for the successful inclusion of HTMs.

Improved Electron Diffusion Coefficient in Electrospun TiO2 Nanowires

TiO2 nanowires with a diameter of ∼150 nm, length of ∼3−4 μm, and aspect ratio of 10:1, were prepared by ultrasonically dispersing electrospun continuous nanofibers in monocarboxylic acids. The resulting pastes were used for making nanowire films on conducting glass substrates with thicknesses in the range of 500 nm to 100 μm, good adhesion, and high nanowire packing. These films were used to fabricate dye-sensitized solar cells using the D131 dye and the iodide/triiodide electrolyte. Transient photocurrent measurements showed a high electron diffusion coefficient in those nanowire films. The measured diffusion coefficient in those TiO2 nanowires was orders of magnitude higher than that observed in nanoparticles under similar experimental conditions. The charge-transport mechanism in the nanowire sample is discussed in support with the measured open-circuit voltage decay curves.

Characterization and sensing properties of a carbon nanotube paste electrode for acetaminophen

A carbon nanotube paste electrode was prepared by dispersion of multi-wall carbon nanotubes in mineral oil. An electrochemical study on acetaminophen showed that the peak current had increased and that the peak potential was negatively shifted compared to a carbon paste electrode without nanotubes, thus resulting in an electrode for acetaminophen. The linear range for the response ranges from 0.15 µM to 126 µM, and the detection limit is 43 nM (at S/N = 3). Electron transfer number, diffusion coefficient and proton number have been calculated. The relative standard deviation (n = 10; at 20 µM of acetaminophen) is 1.9%. The electrode was applied to determine acetaminophen in effervescent dosage samples.

Influence of the degree of crystal perfection and deviation from the stoichiometric composition on the diffusion processes in samarium sulfide

The processes of diffusion of samarium and europium in nonstoichiometric samarium sulfide (SmS) at temperatures in the range 950–1600°C have been investigated by the radioactive isotope method and the method of weight loss upon evaporation of excess samarium. It has been found that there is a correlation between the diffusion coefficient D and the size of coherent X-ray scattering regions in SmS samples, as well as between the diffusion coefficient and the degree of deviation from the stoichiometric composition: the diffusion coefficient of impurities decreases as the size of coherent X-ray scattering regions increases and the stoichiometric composition is approached. The calculation of the diffusion coefficient of electrons in samarium sulfide at T = 77–300 K has demonstrated that the value of D increases with increasing temperature and increasing size of coherent scattering regions.

Modeling of dye-sensitized titanium dioxide solar cells

The system TiO2–photo sensitive dye of a dye-sensitized TiO2 solar cell is numerically simulated. The steady-state numerical model used is based on the continuity and transport equations for all charge species involved in the system, in connection to Poisson’s equation. The dependence of both electron diffusion coefficient and light absorbance upon TiO2 porosity is taken into consideration. The resulting electron density after illumination is also set as a function of the illuminating beam characteristics and material properties. Furthermore, an effective dielectric constant dependent upon the porosity of TiO2 is used in the model. Results for different values of pore size and thickness of the TiO2 electrode are found in accordance with results reported in the literature.

Electron transport in electrospun TiO2 nanofiber dye-sensitized solar cells

In this study dye-sensitized solar cells (DSCs) were fabricated using electrospun TiO2 nanofibers. Electrochemical impedance spectroscopy and open-circuit voltage decay were used to analyze electron transport in electrospun nanofibers and determine their applicability in DSCs. The polycrystalline nature and random web structure of the nanofibers exhibit effective electron diffusion coefficients close to that of high efficiency nanoparticle cells. The electron lifetime was, however, low due to the presence of a large number of surface traps despite the presence of a space-charge recombination barrier.

Electrochemistry of Molecule-like Au25Nanoclusters Protected by Hexanethiolate

Near-monodisperse fractions of hexanethiol-protected gold nanoclusters (Au(25) and Au(144)) have been obtained by using a variation of the two-phase Brust method based on constant low temperature (4 °C) and control of the stirring conditions during the synthesis procedure. The size and dispersity of the Au25(SC6)18 and Au144(SC6)59 nanoclusters are determined by electrochemical means. The two first oxidation steps for the molecule-like Au25(SC6)18 clusters are quasi-reversible, and the electron transfer rate constants and diffusion coefficients have been determined by cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical behavior of these nanoclusters in different solvents can be explained by using the model described by Girault et al. (J. Phys. Chem. B 2006, 110, 21460; 2005, 109, 23925.) that considers the contribution of the bulk solvent relative permittivity on the charging energy values. Electrolyte ions and solvent molecule penetration must be taken into account to explain the deviation of the charging energy value in solvents such as toluene, whose relative permittivity is close to that of the protecting monolayer. The HOMO−LUMO energy gap measured by electrochemical and optical spectroscopy means does not show any influence of the solvent nature as observed for Au25(PhC2S)18.

Stabilization of a Bilirubin Oxidase-Wiring Redox Polymer by Quaternization and Characteristics of the Resulting O2 Cathode

Quaternization of poly-N-vinylimidazole partly complexed with produced, after cross-linking with polyethylene glycol diglycidyl ether, a redox hydrogel with a redox potential of vs and an apparent electron diffusion coefficient of at . Electrostatic binding and cross-linking with bilirubin oxidase from Trachyderma tsunodae provided a mechanically robust -electroreduction catalyst withstanding a shear stress of associated with the rotation of a radius electrode at . Unlike other catalysts made with osmium complexes, comprising in their inner coordination sphere imidazole-exchangeable chloride, it did not appreciably deteriorate by cross-linking through ligand exchange, leading to dehydration. The threshold potential of -electroreduction at pH 7.4, NaCl, and was near that calculated for the reversible half-cell. The -electroreduction current reached half of its maximum at vs , more positive than the potential for bilirubin oxidase wired with PAA-PVI- (, -N-vinylimidazole).

Improvement of electron transport by low-temperature chemically assisted sintering in dye-sensitized solar cell

We have succeeded in preparing TiO 2 film at low-temperature using a binder-free paste and a process of low-temperature crystallization of anatase formed from hydrolysis of TiCl 4 at 150 °C. The electron diffusion coefficient of the chemically assisted sintered dye-sensitized solar cell (DSSC) was 3 times larger at the 1 × 10 17 photons flux than that of non-sintered one. This is thought to be ascribed from the formation of inter-connecting TiO 2 between the TiO 2 nanoparticles by chemically assisted sintering process.

How Efficient Is Electron Collection in Dye-Sensitized Solar Cells? Comparison of Different Dynamic Methods for the Determination of the Electron Diffusion Length

The diffusion length of electrons in high efficiency liquid electrolyte dye-sensitized nanocrystalline solar cells has been investigated using two different approaches. The first method is based on measuring the rise and decay times of the small amplitude photovoltage increment generated by a short laser pulse superimposed on a range of steady-state illumination levels. The advantage of this technique is that it allows the simultaneous measurement of the diffusion coefficient and electron lifetime under identical conditions. In addition to transport-controlled substrate charging, direct injection of electrons into the substrate from dye adsorbed at the contact interface was observed at the high laser pulse energies required for measurements at high dc photovoltages. The second method involves using intensity-modulated photocurrent and photovoltage spectroscopies (IMPS and IMVS, respectively) to measure the electron diffusion coefficient and electron lifetime at short circuit and open circuit, respectively...

Electron Transport in Dye-Sensitized Solar Cells Based on ZnO Nanotubes: Evidence for Highly Efficient Charge Collection and Exceptionally Rapid Dynamics

Dye-sensitized solar cells based on ordered arrays of polycrystalline ZnO nanotubes, 64 mum in length, are shown to exhibit efficient electron collection over the entire photoanode array length. Electrochemical impedance spectroscopy, open-circuit photovoltage decay analysis, and incident-photon-to-current efficiency spectra are used to quantify charge transport and lifetimes. Despite the relatively thick photoanode, the charge extraction time is found to be faster than observed in traditional TiO(2) nanoparticle photoanodes. If the extraction dynamics are interpreted as diffusive, effective electron diffusion coefficients of up to 0.4 cm(2) s(-1) are obtained, making these pseudo-1D photoanodes the fastest reported for an operating DSC to date. Rapid electron collection is of practical significance because it should enable alternative redox shuttles, which display relatively fast electron-interception dynamics, to be employed without significant loss of photocurrent.

Modelling of Reaction and Diffusion Processes in a High‐surface‐area Biofuel Cell Electrode Made of Redox Polymer‐grafted Carbon

AbstractA mathematical model considering reaction and diffusion processes in biofuel cells, such as enzyme reactions, apparent electron diffusion in the redox polymer, and diffusion of a substrate in a redox polymer film, verified the effectiveness of a high‐surface‐area biofuel cell electrode with a thin, grafted redox polymer layer. The model calculation shows that the rate‐limiting step of apparent electron diffusion can be overcome by using the electrode with a thin redox polymer, even when the redox polymer used has a low apparent electron diffusion coefficient. In the electrode, increase in the second‐order rate constant for the reaction between an enzyme and a mediator above a certain value does not increase the current density, and thus the mediator used in the electrode should have an adequate second‐order rate constant and a more negative redox potential. In addition, increase in the turnover rate of the enzyme and the surface coverage of the enzyme allows an increase in the current density; the surface coverage of the enzyme has the most influence. Based on these calculations, it is important to realise high‐surface‐area electrodes with a thin redox polymer, and with high enzyme loading.

The Influence of Charge Transport and Recombination on the Performance of Dye‐Sensitized Solar Cells

Electrochemical impedance spectroscopy (EIS) and transient voltage decay measurements are applied to compare the performance of dye sensitized solar cells (DSCs) using organic electrolytes, ionic liquids and organic-hole conductors as hole transport materials (HTM). Nano-crystalline titania films sensitized by the same heteroleptic ruthenium complex NaRu(4-carboxylic acid-4'-carboxylate) (4,4'-dinonyl-2,2'-bipyridyl)(NCS)(2) , coded Z-907Na are employed as working electrodes. The influence of the nature of the HTM on the photovoltaic figures of merit, that is, the open circuit voltage, short circuit photocurrent and fill factor is evaluated. In order to derive the electron lifetime, as well as the electron diffusion coefficient and charge collection efficiency, EIS measurements are performed in the dark and under illumination corresponding to realistic photovoltaic operating conditions of these mesoscopic solar cells. A theoretical model is established to interpret the frequency response off the impedance under open circuit conditions, which is conceptually similar to photovoltage transient decay measurements. Important information on factors that govern the dynamics of electron transport within the nanocrystalline TiO(2) film and charge recombination across the dye sensitized heterojunction is obtained.

Electron Injection Efficiency and Diffusion Length in Dye-Sensitized Solar Cells Derived from Incident Photon Conversion Efficiency Measurements

Injection efficiency, ηinj, and diffusion length, L, in dye-sensitized solar cells have been derived from the spectral response (incident photon to current efficiency, IPCE) of the cells under front side or backside illumination. Values of L from IPCE are found to be ∼2 times shorter than the values of L derived from the normal small perturbation transient method. IPCE-derived values of L (2 to more than 40 μm) and ηinj (63−90%) are found to correlate with the photocurrent (and indirectly with the photovoltage) of the different cells indicating the extent to which each factor limits the cell efficiency. IPCE spectra varied with light intensity, so that diffusion lengths derived from both methods show similar trends, e.g., L from IPCE is found to increase 3 times when the light intensity is increased 10 times up to approximately 0.1 sun where L tends to plateau or peak. The values for ηinj derived from the spectral response are shown to be in quantitative agreement with those determined from picosecond transient emission spectroscopy. To illustrate the utility of this method, L and ηinj were measured on cells with and without the TiCl4 chemical bath treatment. The results show that the increase in photocurrent after the TiCl4 treatment is due to around a 2-fold increase in L despite a 3-fold reduction in the electron diffusion coefficient. The increased L can be explained by a factor of 10 decrease in electron recombination rate.

Interfacial Electron-Transfer Kinetics in Metal-Free Organic Dye-Sensitized Solar Cells: Combined Effects of Molecular Structure of Dyes and Electrolytes

Electron diffusion coefficient, lifetime, and density in the TiO(2) electrode of dye-sensitized TiO(2) solar cells (DSCs) employing I(-)/I(3)(-) redox couples were measured with eight different metal-free organic dyes and three Ru complex dyes. At matched electron density, all DSCs using organic dyes (ODSCs) showed shorter electron lifetime with comparable or larger diffusion coefficients in comparison to the DSCs using the Ru dyes (RuDSC). The shorter lifetime was attributed partially to the slower dye cation reduction rate of the organic dyes by I(-), faster electron diffusion coefficient in the TiO(2), and mostly higher I(3)(-) concentration in the vicinity of the TiO(2) surface. Whereas a slight shift of the conduction band edge potential (E(cb)) of the TiO(2) was seen with a few organic dyes, no correlation was found with the dipole moment of the adsorbed dyes. This implies that the adsorbed dyes interact with cations in the electrolyte, so the direction of the dipole is altered or simply screened. The increase of [I(3)(-)] in the vicinity of the TiO(2) surface was interpreted with partial charge distribution of the dyes. Under one-sun conditions, less electron density due to shorter electron lifetime was found to be the main reason for the lower values of V(oc) for all ODSCs in comparison to that of RuDSCs. Among the organic dyes, having larger molecular size and alkyl chains showed longer electron lifetime, and thus higher V(oc). Toward higher open circuit voltage, a design guide of organic dyes controlling the electron lifetime is discussed.

Effect of Nonuniform Generation and Inefficient Collection of Electrons on the Dynamic Photocurrent and Photovoltage Response of Nanostructured Photoelectrodes

This paper investigates how nonuniform generation and inefficient collection of electrons influence the dynamic photocurrent and photovoltage response of nanostructured photoelectrodes. The standard diffusion model theory of small amplitude light intensity modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopy is refined and generalized to an arbitrary electron generation profile, allowing straightforward coupling to any optical model. Expressions are derived for the local electron concentration and IMPS and IMVS transfer functions, for localized, uniform, and exponential generation profiles. Both limited collection and nonuniform generation of electrons modify the photoelectrode thickness (d) dependence of the characteristic IMPS and IMVS time constants and complicate their interpretation. This can lead to significant overestimation of the electron diffusion coefficient, diffusion length, and collection efficiency when using common approximate relations. With near contact electron generation, the IMPS response exhibits two time constants, only the slower one of which corresponds to electron transport across the film and scales with d. In the presence of this effect it is possible that in case of two equally thick samples, the one with smaller electron diffusion coefficient displays apparently faster electron transport. These errors demonstrated by experimental IMPS data of pressed TiO2 photoelectrodes can be minimized by using modulated light incident from the counter electrode side and avoided when analyzing the ratio of IMPS at opposite directions of illumination.

Electron Transport and Recombination in Dye-Sensitized TiO2 Solar Cells Fabricated without Sintering Process

Nanoporous TiO2 electrodes were prepared from a colloidal suspension of TiO2 nanoparticles with heating processes at 80 °C for 12 h (regarded as nonsintering process) or 450 °C for 30 min (as sintering process) in air, and dye-sensitized solar cells (DSCs) were prepared from them. Electron diffusion coefficients (D) and lifetime (τ) in the DSCs were measured under various light intensities. The DSCs with nonsintered TiO2 showed lower values of D and τ than those in the sintered TiO2 at the matched electron density. A plot of open-circuit voltage (Voc) vs electron density of the DSCs showed that the nonsintered TiO2 gave higher Voc at the matched electron density under high light intensity and lower Voc under low light intensity. These results suggest that nonsintered TiO2 has less density of shallow trap sites and sintering process will form the traps at the grain boundary during the growth of the necks between TiO2 particles.

Energy-Level and Molecular Engineering of Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells

A series of organic D-π-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length.