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The parameters controlling the kinetics of intermolecular charge transfer are traditionally estimated from electronic structure calculations on the charge donor and charge acceptor in isolation. Here, we show that this procedure results in inaccuracies for hole transfer between a pair of organic dye molecules by comparing charge-constrained density functional theory (DFT) calculations on a dye cation/neutral dye pair to the conventional DFT calculations on the isolated molecules. We quantify the error made in the reorganization energy of hole exchange between dye molecules (λi). We choose three indolene-based organic dyes with application to dye-sensitized solar cells, namely, D149, D102, and D131, for which experimental values of λ are available. We find that, although highly system dependent, the intermolecular interaction between the charge donor and acceptor can lead to a 0.25 eV change in λi, illustrating the limitations of the widely used original method in predicting the rate of charge transfer.

Corrigendum: The dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cells

Solar cells based on organic–inorganic halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behaviour such as current–voltage hysteresis and a low-frequency giant dielectric response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current–voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and experimental study suggest that hybrid halide perovskites are mixed ionic–electronic conductors, a finding that has major implications for solar cell device architectures., Understanding the mechanism of ionic transport in organic–inorganic halide perovskites is crucial for the design of future solar cells. Here, Eames et al. undertake a combined experimental and computational study to elucidate the ion conducting species and help rationalize the unusual behaviour observed in these perovskite-based devices.

Dye-sensitized TiO2 can be used as the active layer of solar-cell devices without an additional hole-transporting material. In this architecture, holes are transported through the dye monolayer.

Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (∼0.2 μC/cm2) and another, larger charge (40 μC/cm2), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of ∼5, independent of bias light intensity. The fast decay (∼1 μs at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J–V hysteresis. Capa...

Reliable measurement of the photoconversion efficiency for semiconductor electrodes is essential to the assessment of electrode performance. In this paper, the influence of the choice of light source on measured photoconversion efficiencies for semiconductor photoelectrodes is examined. Measurements of efficiency performed under xenon lamp and solar illumination are compared with efficiencies calculated by integrating the incident photon conversion efficiency (IPCE) over the lamp and solar spectra. It is shown that use of a xenon lamp as the light source can lead to a large overestimate of the photoconversion efficiency, relative to that obtained under standard AM1.5 solar illumination. The overestimate is greater when a water filter is fitted to the xenon lamp, and when a wide-band gap semiconductor such as TiO2 is used as the photoelectrode. Achievable photoconversion efficiencies using rutile TiO2 are calculated taking into account the losses due to imperfect absorption, reflection and charge-carrier recombination; these calculated efficiencies agree with the measurements to within experimental uncertainties. It is demonstrated that many photoconversion efficiencies presented in the literature are overestimated. It is concluded that reliable estimation of efficiency under standard conditions is best obtained by measuring the IPCE as a function of wavelength, and integrating over the AM1.5 solar spectrum, or by measuring under sunlight with a similar zenith angle to that of the AM1.5 spectrum.

Replicate ice cores have been drilled about 10 m apart for the top 790 m of the ice sheet at Dome C, Antarctica. This provides an opportunity to examine inter-core variation of the signal for identical events, based on dielectric profile (DEP) comparisons. Comparison of the signal from the same core (a section 48 m long), measured 1 year apart, showed good reproducibility, with peak heights varying by around 10% between the two measurements. For the two replicate cores, identical peaks were matched and showed variability between cores of typically a factor 1.5. This can be explained based on the likelihood of significant time periods of missing accumulation in any single core at sites with such low snow accumulation rate. To synchronize core depths by matching peaks, it is essential to use the pattern of peaks, rather than just widely spaced individual strong peaks. To derive a quantitative volcanic index from these low-accumulation rate sites, it will be necessary to combine or average the results from several closely spaced parallel cores.

Understanding the microstructure of ice underpins the interpretation of ice-core measurements and many ice-sheet properties. A detailed study of polar snow and ice using scanning electron microscope (SEM) and X-ray analysis revealed the micro-structural distribution of soluble impurities. Sublimation under vacuum (etching) concentrated impurity from both the bulk and grain boundaries on to the specimen surfaces in detectable quantities. Sublimation in the cold room before examination (pre-etching) collected previously unobservable quantities of impurity at triple junctions. A heterogeneous distribution of impurities was observed. Chloride was frequently found to originate from the lattice, but not usually at triple junctions. Other impurities, particularly sodium chloride, were detected at grain boundaries and bubble surfaces. Sulphate was often found at triple junctions in specimens containing a high bulk concentration of the acid, frequently in conjunction with cations. This suggests the possibility that veins were only filled with significant amounts of impurity when the surrounding grain boundaries were saturated. The model of impurity arrangement inferred from the results reconciles differences between previous SEM studies; additionally it is consistent with and explains recent electrical conduction observations. The disconnected arrangement of impurity-filled grain boundaries and veins limits opportunities for significant post-depositional solute movement.

Determining the microphysical location of impurities in natural ice from the polar regions is necessary for understanding the physical properties of ice and for assuring the integrity of ice core records. SEM, using a cold stage and X-ray microanalytical techniques, has proved to be the most powerful method so far for undertaking such work. Methods are adapted from those used to study frozen hydrated biological material. Sublimation within the cryo-chamber is often needed in order to concentrate impurities onto a plane, but this can lead to artifacts that must be recognized. Over 100 samples from different depths and sites in Greenland and Antarctica have been examined. Typical physical features, including air bubbles, clathrate hydrates of air, and dust particles are identified. The dust is found preferentially at grain boundaries in some samples; by pinning the boundaries, it can slow grain growth. Of the soluble material, chloride seems to be found most frequently in the ice lattice. Other impurities are found at grain boundaries, and only when the bulk concentration exceeds a threshold, at triple junctions. These findings give new insights into processes determining the physical properties of ice samples and of ice sheets, and new impetus for theoretical studies of the energetics that lead to this distribution.

Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (∼0.2 μC/cm(2)) and another, larger charge (40 μC/cm(2)), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of ∼5, independent of bias light intensity. The fast decay (∼1 μs at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J-V hysteresis. Capacitance vs open circuit voltage (V(oc)) data indicate that the hysteresis involves a change in internal potential gradients, likely a shift in band offset at the TiO2/MAPI interface. The TPV results show that the V(oc) hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at V(oc) suggests that the hysteresis is also not due to an increase in charge separation efficiency and that charge generation is not a function of applied bias. We also show that the J-V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.

Through materials and device developments and by using measurements such as transient photovoltage decay and impedance spectroscopy we have begun to identify recombination mechanisms in perovskite solar cells. At the forefront of our developments is a transparent, indium free, cathode which allows measurements to be made whilst illuminating from both the photoanode side and the cathode side of the device. Recombination is consistently faster when illuminated from the cathode side and we conclude that in this case, as charge carriers are generated closer to the perovskite/SPIRO-OMeTAD interface, interfacial recombination is a significant contributor to voltage losses within the device.

In this paper we address the JV hysteresis behavior of planar organic–inorganic lead halide perovskite solar cells fabricated using PC60BM as the cathode. At room temperature, these devices exhibit apparently hysteresis free JV scans. We observe that cooling the temperature to 175 K results in the appearance of substantial JV hysteresis. Employing chronoamperometric measurements, we demonstrate that the half-time for the relaxation process underlying this hysteresis slows from 0.6 s at 298 K to 15.5 s at 175 K, yielding an activation energy of 0.12 eV. We further demonstrate that by cooling a cell to 77 K while held under positive bias, we are able to “freeze” the cell into the most favorable condition for efficient photovoltaic performance. We thus conclude that changes to device architecture that appear to remove room temperature JV hysteresis may not remove the underlying process(es), but rather shift them to time scales not readily observable in typical room temperature JV scans.

A numerical method is presented to estimate the influence of a nearby substrate on the polarization energy and outer sphere reorganization energy (λo) for intermolecular hole transfer for a series of dye molecules. The calculation considers the net charge distribution of the oxidised molecule (determined from quantum chemical calculation of the highest occupied molecular orbital of the neutral molecule within the frozen orbital approximation) encapsulated within a conformal cavity, by the molecules total electron density. An analytical point charge approximation was used at longer range. The molecular cavity was either surrounded by a single polarizable continuum, or, to simulate a nearby substrate, embedded at different positions relative to the interface between two semi-infinite slabs with different dielectric constants. The calculated λo values in the single dielectric medium were linearly related to the outer-sphere reorganisation energy calculated from DFT with a polarizable continuum model, validating the approach. In the two phase system, variations in λo was sensitive to the position of the substrate relative to the molecule and differences in the Pekar factor (1/εo - 1/εr) for the media. For dye molecules in ACN positioned touching a TiO2 substrate λo was typically about 20% lower than in pure ACN depending on the molecular configuration. Our approach can be adapted to systems of more than two media.

Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/ mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (∼0.2 μC/ cm2) and another, larger charge (40 μC/cm2), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of ∼5, independent of bias light intensity. The fast decay (∼1 μs at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J−V hysteresis. Capacitance vs open circuit voltage (Voc) data indicate that the hysteresis involves a change in internal potential gradients, likely a shift in band offset at the TiO2/MAPI interface. The TPV results show that the Voc hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at Voc suggests that the hysteresis is also not due to an increase in charge separation efficiency and that charge generation is not a function of applied bias. We also show that the J−V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.

Samples taken from the Dome C ice core, Antarctica, and the GRIP ice core, Greenland, are examined using the scanning electron microscope to determine their microstructure. In both cores, samples are taken from two differing climatic periods: the Holocene and the last glacial period. Many of the usual features observed in similar samples under the light microscope are observed, including: bubbles, grain boundaries and clathrate hydrates. Features not resolvable using the light microscope are also found. Dust particles are found in situ. Eighty-five per cent of those observed contained silicon, which was generally associated with aluminium and magnesium. An estimation is made of the relative proportions of dust particles located at grain boundaries and in the bulk of the ice grain. At Dome C a higher proportion than expected from a random distribution of particles was found located at grain boundaries, although in Greenland this was not found to be the case for most samples. Direct evidence is also presented indicating the role of dust particles and microscopical inclusions in impeding or ``pinning’’ grain-boundary migration. Soluble impurities are also detected at some triple junctions and grain boundaries.

We measured the rate of hole hopping between dye molecules on titanium dioxide nanocrystals using cyclic voltammetry. Dyes commonly used in the field of dye sensitized solar cells exhibited efficient intermolecular charge transport, showing apparent diffusion coefficient values between 10−8 up to over 10−7 cm2 s−1 at room temperature. From temperature dependent measurements, we observed that hole transport across dye monolayers is a thermally activated process with Arrhenius activation energies between about 170 and 370 meV depending on the dye. Analysis of the data in terms of non-adiabatic Marcus theory of charge transfer enabled the estimation of the reorganization energy (740–1540 meV) and of an effective electronic coupling for the different systems. The measured reorganization energies show reasonable agreement with values obtained from density functional theory based calculations, validating our computational approach. Finally, we interpret the experimental and calculated data with reference to the chemical structure of the dyes and to the packing of the dyes on the surface of the TiO2 and suggest that delocalization of the HOMO and rigidity of the conjugated molecular structure result respectively in lower outer and inner sphere reorganization energies.

In order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize the conversion efficiency of electrodes of different thicknesses containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thicknesses can be understood as the result of the randomization of the light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparable to the thickness of the film. This simple relationship could also be used as a design rule to attain the optimum optical design in other photovoltaic materials. This journal is © The Royal Society of Chemistry.

Measured hole diffusion coefficients in dye monolayers are larger than can be explained by a charge hopping model with a static distribution of parameters describing intermolecular hole transfer. We show that large amplitude fluctuations of the tethered dye configurations on the surface could explain the observed diffusion rates by enabling charges trapped in particular configurations to escape as the dye orientation changes. We present a multiscale model of hole transport that includes the effect of dynamic rearrangement of the monolayer of anchored dyes. Conformations of pairs of indolene dye molecules (both D102 and D149) were generated by a rigid molecular packing algorithm and Car−Parrinello molecular dynamics to mimic the conformational and configurational disorder of a dye monolayer adsorbed to an anatase (101) titanium dioxide surface. The electronic coupling (Jij) for each pair of neighboring dyes was calculated to build distributions representing the disorder in a real system. These values were used s inputs to Marcus’ non-adiabatic equation for charge transfer to calculate the rate of hole hopping for each pair. Hole diffusion was simulated with a continuous time random walk, accounting for different time scales of molecular rearrangement (changes in the dye geometry). The dynamic nature of configurational disorder was captured by reassigning the values of Jij, drawn from the aforementioned distributions, after a fixed renewal time. We found hole diffusion coefficients of 3.3 × 10−8 and 9.2 × 10−8 cm2 s−1 for D102 and D149, respectively, for a renewal time of 10−7 s. This is in good agreement with the corresponding measured coefficients for D102 and D149 of 9.6 × 10−8 and 2.5 × 10−7 cm2 s−1, whereas the diffusion coefficients are underestimated by at least a factor of 15 if the dynamics are ignored. Fast rearrangement of dye monolayer configuration may explain the high lateral hole diffusion coefficients determined experimentally. Our results indicate that both chemical structure and the availability o different packing configurations must be considered when designing conductive molecular monolayers.&nbsp

We study the kinetics of the lateral hole transfer occurring between dye molecules anchored at the surface of the metal oxide in Dye Sensitized Solar Cells (DSSC). We use Marcus' charge transfer rate equation for which we need the electronic coupling between two molecules (J) and the reorganization energy (λtot). In DSSC the medium surrounding the dyes is highly polar. This means that the contribution of the solvent to the reorganization energy cannot be neglected. Here we elaborate a method to calculate, from first principles, the total (i.e., inner- and outer-sphere) reorganization energy of hole exchange between ruthenium dyes. The influence of the solvent and of the ions in the solvent is incorporated. The inner-sphere reorganization energy depends on the nature of the dye, 0.1 eV for ruthenium dyes with CN ligands, 0.2 eV for ruthenium dyes with NCS ligands. In acetonitrile, the solvent reorganization energy contributes for at least 80% of the total giving a total reorganization energy of around 0.86 eV for ruthenium dyes with CN ligands and 0.95 eV for ruthenium dyes with NCS ligands. We use these results to estimate the rate of hole transfer within Marcus theory. We suggest that low diffusion coefficients observed experimentally may arise from the high polarity of the medium rather than by the chemical structure of the dye.

Ridges have been observed at the bonds between sintering snow grains. Images collected using the scanning electron microscope (SEM) and x-ray analysis of Antarctic snow are presented. These indicate the ridges are composed from soluble impurity and not ice or water as previously suggested. The ridges can be explained by the coagulation of soluble impurities distributed either at the grain boundaries or on the grain surface. Coagulation is most likely to occur as a result of sublimation during the preparation of the snow specimens for SEM examination, however, they could also form under specific temperature conditions in the snow pack. Consequently, a theory of grain boundary diffusion is not required to explain the presence of ridges at snow bonds.

Tools that assess the limitations of dye sensitized solar cells (DSSCs) made with new materials are critical for progress. Measuring the transient electrical signals (voltage or current) after optically perturbing a DSSC is an approach which can give information about electron concentration, transport and recombination. Here we describe the theory and practice of this class of optoelectronic measurements, illustrated with numerous examples. The measurements are interpreted with the multiple trapping continuum model which describes electrons in a semiconductor with an exponential distribution of trapping states. We review standard small perturbation photocurrent and photovoltage transients, and introduce the photovoltage time of flight measurement which allows the simultaneous derivation of both effective diffusion and recombination coefficients. We then consider the utility of large perturbation measurements such as charge extraction and the current interrupt technique for finding the internal charge and voltage within a device. Combining these measurements allows differences between DSSCs to be understood in terms such as electron collection efficiency, semiconductor conduction band edge shifts and recombination kinetics.

It is demonstrated that the molecular filtering effect of TiO2 has a significant influence on dye solar cell (DSC) performance. As electrolyte is injected to a DSC, some of the electrolyte components adsorb to the surface TiO2 (here 4-tert-butylpyridine and 1-methyl-benzimidazole) and accumulate near the electrolyte filling hole resulting in varying electrolyte composition and performance across the cell. The spatial performance distribution was investigated with a new method, the segment cell method. Not only is the segmented cell method simple and cheap when compared to the only other method for examining spatial variation (photocurrent mapping), it also has the major advantage of allowing the spatial variation in all other operating parameters to be assessed. Here the molecular filtering effect was to influence the cell performance in case of all the five studied electrolytes causing up to 35% losses in efficiency. Raman spectra indicated that the loss in photocurrent in the electrolyte filling was in correlation with the loss of thiocyanate ligands suggesting that dye regeneration may also be a significant factor in addition to electron injection in some of the cells. There were also shifts in the absorption spectra the photoelectrodes which further supported changes in the thiocyanate ligands. Besides absorption changes, there were additional shifts in the IPCE spectra which may relate to deprotonation of the dye. The efficiency losses were reduced to ∼10% with contemporary electrolyte compositions.

Efficiencies of 12 % have recently been achieved using dye sensitised and semiconductor sensitised solar cells. I will describe the developments as well as the transient absorption and optoelectronic techniques that are used to understand and improve the performance of these devices.

A numerical model of the dye sensitised solar cell (DSSC) is used to assess the importance of different loss pathways under various operational conditions. Based on our current understanding, the simulation describes the processes of injection, regeneration, recombination and transport of electrons, oxidised dye molecules and electrolyte within complete devices to give both time dependent and independent descriptions of performance. The results indicate that the flux of electrons lost from the nanocrystalline TiO(2) film is typically at least twice as large under conditions equivalent to 1 sun relative to dark conditions at matched TiO(2) charge concentration. This is in agreement with experimental observations (Barnes et al. Phys. Chem. Chem. Phys. [DOI: 10.1039/c0cp01855d]). The simulated difference in recombination flux is shown to be due to variation in the concentration profile of electron accepting species in the TiO(2) pores between light and dark conditions and to recombination to oxidised dyes in the light. The model is able to easily incorporate non-ideal behaviour of a cell such as the variation of open circuit potential with light intensity and non-first order recombination of conduction band electrons. The time dependent simulations, described by the multiple trapping model of electron transport and recombination, show good agreement with both small and large transient photocurrent and photovoltage measurements at open circuit, including photovoltage rise measurements. The simulation of photovoltage rise also suggests the possibility of assessing the interfacial resistance between the TiO(2) and substrate. When cells with a short diffusion length relative to film thickness were modelled, the simulated small perturbation photocurrent transients at short circuit (but not open circuit) yielded significantly higher effective diffusion coefficients than expected from the mean concentration of electrons and the electrolyte in the cell. This implies that transient measurements can overestimate the electron diffusion length in cells which have a low collection efficiency. The model should provide a useful general framework for exploring new cell descriptions, architectures and other factors influencing device performance.

Electrical and luminescence characterization was performed on 16 dye sensitized solar cells with different formulations, from different industrial and academic sources. Most of the cells were fabricated in pre-industrial pilot lines. The cells were put through a light soaking period up to 150 hours and then re-characterized. The results show the commonly observed increase in Jsc with light soaking is due to a decrease in the conduction band energy (with respect to the electrolyte) and an increase in the injection rate and efficiency. The strong correlation between the luminescence decay lifetime (

A simple and powerful approach for assessing the recombination losses in dye sensitised solar cells (DSSCs) across the current voltage curve (j-V) as a function of TiO(2) electron concentration (n) is demonstrated. The total flux of electrons recombining with iodine species in the electrolyte and oxidised dye molecules can be thought of as a recombination current density, defined as j(rec) = j(inj)-j where j(inj) is the current of electrons injected from optically excited dye states and j is the current density collected at cell voltage (V). The electron concentration at any given operating conditions is determined by charge extraction. This allows comparison of factors influencing electron recombination rates at matched n. We show that j(rec) is typically 2-3 times higher under 1 sun equivalent illumination (j(inj)0) relative to dark (j(inj) = 0) conditions. This difference was increased by increasing light intensity, electrolyte iodine concentration and electrolyte solvent viscosity. The difference was reduced by increasing the electrolyte iodide concentration and increasing the temperature. These results allowed us to verify a numerical model of complete operational cells (Barnes et al., Phys. Chem. Chem. Phys., DOI: 10.1039/c0cp01554g) and to relate the differences in j(rec) to physical processes in the devices. The difference between j(rec) in the light and dark can be explained by two factors: (1) an increase in the concentration of electron acceptor species (I(3)(-) and/or I(2)) when current is flowing under illumination relative to dark conditions where the current is flowing in the opposite direction, and (2) a non-trivial contribution from electron recombination to oxidised dye molecules under light conditions. More generally, the technique helps to assign the observed relationship between the components, processing and performance of DSSCs to more fundamental physical processes.

We report the application of spectroelectrochemical techniques to compare the hole percolation dynamics of molecular networks of two ruthenium bipyridyl complexes adsorbed onto mesoporous, nanocrystalline TiO(2) films. The percolation dynamics of the ruthenium complex cis-di(thiocyanato)(2,2'-bipyridyl-4,4'-dicarboxylic acid)-(2,2'-bipyridyl-4,4'-tridecyl) ruthenium(II), N621, is compared with those observed for an analogous dye with an additional tri-phenyl amine (TPA) donor moiety, cis-di(thiocyanato)(2,2'-bipyridyl-4,4'-dicarboxylic acid)-(2,2'-bipyridyl-4,4'-bis(vinyltriphenylamine)) ruthenium(II), HW456. The in situ oxidation of these ruthenium complexes adsorbed to the TiO(2) films is monitored by cyclic voltammetry and voltabsorptometry, whilst the dynamics of hole (cation) percolation between adsorbed ruthenium complexes is monitored by potentiometric spectroelectrochemistry and chronoabsorptometry. The hole diffusion coefficient, D(eff), is shown to be dependent on the dye loading on the nanocrystalline TiO(2) film, with a threshold observed at ∼60% monolayer surface coverage for both dyes. The hole diffusion coefficient of HW456 is estimated to be 2.6 × 10(-8) cm(2)/s, 20-fold higher than that obtained for the control N621, attributed to stronger electronic coupling between the TPA moieties of HW456 accelerating the hole percolation dynamics. The presence of mercuric ions, previously shown to bind to the thiocyanates of analogous ruthenium complexes, resulted in a quenching of the hole percolation for N621/TiO(2) films and an enhancement for HW456/TiO(2) films. These results strongly suggest that the hole percolation pathway is along the overlapped neighbouring -NCS groups for the N621 molecules, whereas in HW456 molecules cation percolation proceeds between intermolecular TPA ligands. These results are discussed in the context of their relevance to the process of dye regeneration in dye sensitised solar cells, and to the molecular wiring of wide bandgap inorganic materials for battery and sensing applications.

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The short diffusion length of photo-excited charge carriers in Fe 2 O 3 is one of the factors limiting the water splitting efficiency of iron oxide based materials. To overcome this problem we are engineering transparent arrays of nanowires to act as conducting substrates for the Fe 2 O 3 . To help understand the charge transport characteristics of the Fe 2 O 3 component we report transient photocurrent measurements performed on an absorbing thin film of Fe 2 O 3 deposited by filtered arc deposition on conducting glass with a semi-transparent silver Schottky top contact. Ultraviolet laser pulses were used to generate charge carriers near the surface and the resulting current transients were measured. A simulation of this charge transport has also been developed. The sign of the observed transients was independent of applied bias, consistent with a fully depleted film. The measurements also suggest that recombination may play a significant factor in determining the transient shape. Further investigation is required to confidently predict mobilities.

Two 788 m conductivity records from ice cores drilled at Dome C, Antarctica, provide an unprecedented opportunity to examine the past roughness of the Antarctic surface. By measuring the distribution of depth differences between synchronous events in the cores, the surface height distribution can be estimated during time intervals in the past. For the first time we publish a record of this type and consider its paleoclimatic implications. The paleoroughness, originating from sastrugi and dunes on the ice sheet surface, is hypothesized to be related to the wind speed, temperature, and accumulation rate during the period of preservation. The roughness record indicates only a slight decrease in the preserved surface roughness from the last glacial period (0.031 m root mean square (r.m.s.) of surface deviation) to the present (0.029 m r.m.s.). This result is surprising given the large change in temperature and accumulation rate that occurred during the last climatic transition. However, it could be consistent with modeling results suggesting low wind speeds on the East Antarctic Plateau during the last ice age if the possible influences of the accumulation and temperature changes are ignored. Additionally, the reliability of the Dome C ice core record is assessed, and the probability of short-term events being missing from the profile is determined. These results are of particular interest when constraining the proportion of events that may be synchronously matched with other ice cores. The principles and results present here allow inferences about the natural variation in ice core records generally.

The microstructure and location of impurities in polar ice are of great relevance to ice core studies. We describe a reliable method to examine ice in the scanning electron microscope (SEM). Specimens were cut in a cold room and could have their surfaces altered by sublimation either before (pre-etching) or after (etching) introduction to the cryo-chamber of the SEM. Pre-etching was used to smooth surfaces, whilst etching stripped away layers from the specimen surface, aiding the location of particles in situ, and allowing embedded structures to be revealed. X-ray analysis was used to determine the composition of localized impurities, which in some cases had been concentrated on the surface by etching. Examining uncoated surfaces was found to be advantageous and did not detract from qualitative X-ray analysis. Imaging uncoated was performed at low accelerating voltages and probe currents to avoid problems of surface charging.

[1] Electrical conductivity measurements made using the dielectric profiling technique (DEP) are compared to chemical data from the top 350 m of the Dome C ice core in Antarctica. The chemical data are used to calculate the concentration of the major acidic impurities in the core: sulphuric acid and hydrochloric acid. The conductivity coefficients in solid ice for sulphuric acid (βH2SO4) and hydrochloric acid (βHCl) are found to be 4.9 and 4.5 S m−1M−1. These are consistent with previously found values for the acid conductivity coefficient at different sites and suggest that the same conductivity mechanisms are important in all polar ice. A method of rolling regression analysis is used to find the variation of the pure ice conductivity (σ∞ pure) and the conductivity coefficient of sulphuric acid, βH2SO4, with depth. Then σ∞ pure and βH2SO4are assessed against changes in core density and hence volume fraction of ice, v, due to the inclusion of air bubbles in the firn. Looyenga's model for dielectric mixtures applied to conduction in firn broadly predicts the variation observed in σ∞ purebut does not fit well for ice above 110 m. A previous application of the theory of percolation in random lattices is used to model the conductivity coefficient in firn. The coefficient βH2SO4 is linked to vby the power law: βH2SO4(v) ∝ βH2SO4(1) (v − vc)t; where vc is a threshold volume fraction below which no conduction can take place and is related to the geometry of the conducting lattice being modeled. The value of the exponent tis also dependent on the structure of the lattice and is here found to be t = 2.5, which is slightly lower than the previously obtained value of t = 2.7 for a structure where each grain has between 14 and 16 nearest neighbors. This model is consistent with the concept of conduction, via liquid H2SO4, taking place at two grain boundaries for firn. The model does not, however, preclude conduction taking place via acid situated at three grain boundaries or in an interconnected vein network at densities above 640 kg m−3.

There is intense interest in developing new novel nanostructured photoanodes for water splitting. It is therefore important that methods to analyze the effect of nanostructuring on water splitting yields are developed in order to rationalize the relative merits of this approach for different materials. In this study the dependence of charge separation efficiency (η(sep)) on potential during photoelectrochemical water splitting at pH 2 has been quantified in a model electrode system (nanocrystalline, mesoporous TiO2) using two independent methods. These are (i) analysis of incident photon conversion efficiency (IPCE) measurements and (ii) transient absorption (TA) spectroscopy measurements. The techniques provide good agreement with each other and show that a low maximum value of η(sep) (~0.18) is the primary cause of the low IPCE for water oxidation on these nc-TiO2 electrodes.

We have measured the binding coefficients of iodine to three dyes used in Dye Sensitised Solar Cells (DSSCs). Binding coefficients are quantified via the effect of iodine binding on the UV-vis spectrum of the dye. From iodine titration curves of dye sensitised TiO(2) films we find that the binding coefficients of iodine to the dyes C101, N719 and AR24 (vide infra) are in the range of 2000-4000 M(-1). From FTIR results and molecular modelling we show the iodine binds to the thiocyanate group in all these dyes. For the AR24 dye we present evidence that iodine also binds to the amine moiety on this dye. With these binding coefficients we show that the dye-iodine complex will be present at much higher concentrations than free iodine in the pore structure of a DSSC. As we have recently shown that iodine (rather than tri-iodide) is the dominant acceptor in electron recombination, the concentration dye-iodine complexes could influence recombination rates and thus V(oc). By comparison of recombination data on full cells, we show that AR24 accelerates recombination by a factor of 7 over N719, presumably due to the iodine binding to the amine group. We leave open the question why iodine binding to the amine group seems to have a stronger effect on the recombination than does binding to the thiocyanate.

In this study, the injection efficiencies and photovoltaic device performances for two different phthalocyanine sensitisers—a Zn(II)Pc (TT-1) and a Ru(II)Pc (TT-35) in dye sensitized photoelectrochemical solar cells were compared. These dyes have similar structures and energetics, but differ significantly in their photophysics, with TT-1 exhibiting a reasonably long lived singlet state, whilst TT-35 exhibits rapid intersystem crossing to a long lived triplet state. Time correlated single photon counting (TCSPC) approach and incident photon conversion efficiency (IPCE) measurements were applied to study the injection efficiency of these two Pc dyes. A comparison of the injection efficiency determined by the two independent techniques, TCSPC and IPCE analysis, shows a good agreement. TT-35 shows higher injection efficiency in comparison to TT-1. This result is consistent with the relative energy and lifetime of the TT-35 triplet state compared to the TT-1 singlet excited state. The high injection efficiency and the long electron diffusion length shown by TT-35 make this dye an interesting red absorbing sensitizer for dye solar cells.

Thermally oxidized TiO 2 (rutile) films on titanium showed about an order of magnitude higher photoelectrochemical energy conversion efficiency for water splitting (∼0.2%, simulated AM1.5 sunlight) when the substrate had been etched in acid prior to oxidation. We investigated the cause of the improved photocurrents. Film morphology is insufficient to explain the variation in photoelectrochemical response. Charge-carrier profiles from capacitance-voltage measurements using Schottky contacts indicated a gradual transition from near-stoichiometric oxide at the surface to highly substoichiometric deeper in the film approaching the titanium substrate for oxide grown on etched substrates. In contrast, the oxide layer on unetched samples showed higher carrier densities and a more abrupt transition to substoichiometric or metallic. This interpretation is supported by varying incidence-angle X-ray diffraction measurements suggesting more fully oxidized surface regions with a more gradual transition to suboxides for etched samples. Impedance measurements also suggested that the surface region of oxide layers grown on acid-etched substrates have fewer short-time-constant electronic defects (intraband gap states) than the surface of layers grown on more disordered metallic substrates. The improved photoelectrochemical performance probably results from higher charge-carrier mobilities, reduced recombination, and higher photoconductivity due to long time constants of trapping states.

The work described in this paper falls into two parts. The first is concerned with a study of the deformation of polycrystalline ice (crystal size ca. 1 mm) in uniaxial compression and when subjected to indentation. The uniaxial compression experiments covered strain ratesfrom 10-9to 10-2s-1and temperatures from 0 to —48 °C. It is shown that over the whole range of experimental conditions the secondary creep behaviour can be described by a single relation of the type where o is the applied stress, Q an activation energy an d A, a and n are suitable constants. This reduces to the more familiar power law over more restricted portions of the experimental curve. Over the whole range n has a value close to 3 but Q has two distinct values: 120 J mol-1above — 8 °C: 78 J mol-1below —8 °C. The in dentation hardness experiments cover loading-times from 10-4to 104 s and a temperature range of 0 to — 25 °C. The hardness behaviour may be linked with the creep properties using the analysis of Atkins, Silverio & Tabor (1966) which assumes that the rate-determining process is the diffusion of the hemispherical elastic-plastic zone surrounding the indenter into the undeformed material ahead. There is very good agreement between the hardness data and the creep parameters determined in the uniaxial compression experiments. In addition the hardness experiments enable experiments to be carried out at much higher compressive stresses.

Titanium dioxide is often used as a photoelectrode material for water splitting, but its band gap is too wide to achieve high conversion efficiencies in sunlight. Flame oxidation (pyrolysis) of titanium sheet has been suggested as a method to produce carbon doped TiO2 with a reduced band gap. In the present study, films were prepared using this and other thermal oxidation techniques and the photoelectrochemical water splitting efficiency of the resultant materials was examined using a xenon lamp. The oxide films were characterised physically, optically and electrochemically. Incident photon conversion efficiency measurements indicated no significant reduction in the band gap offilms prepared using a hydrocarbon flame compared to rutile TiO2. Furthermore, these films were found to have an efficiency comparable to that offilms produced in a carbon free flame (hydrogen/oxygen). The results suggest that flame pyrolysis may not ‘dope’ TiO, with carbon in the manner previously proposed. Any enhancement in photocatalytic eficiency resulting from the technique is likely to be due to the formation of non-stoichiometric TiO2 or titanium sub-oxides.

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3+ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1–1 ms, faster than most observed hysteresis., Hysteresis often exists in the characterization of methylammonium lead halide-based solar cells, but is not well understood. Here, the authors use quasielastic neutron scattering to study the dynamics of dipolar organic cations and shed light on the hysteresis behaviour.