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Fundamental aspects of light-driven water splitting are reviewed with emphasis on the kinetics and mechanism of the oxygen and hydrogen evolution reactions at semiconductor photoelectrodes. Information provided by in operando techniques is discussed and related to the mechanisms of light-driven reactions and catalysis.

© 2018 Elsevier B.V. Electrolyte contacts were used to characterize cadmium sulfide (CdS) films grown by chemical bath deposition on fluorine-doped tin oxide glass (FTO). Capacitance measurements were made in a buffered electrolyte free of redox species, and cyclic sweep voltammetry was carried out using an electrolyte containing Fe(CN)64− ions. A theoretical model was developed to describe the influence of pinholes on Mott Schottky plots of CdS-coated FTO. The model allows estimation of the pinhole coverage by fitting the Mott Schottky plot over an extended voltage range. Cyclic voltammetry in a hexacyanoferrate(II) electrolyte also allows detection of pinholes, but quantitative estimation of pinhole coverage is complicated by the fact that diffusion of Fe(CN)64− to pinholes occurs by hemispherical diffusion, which can substantially enhance the current response. Comparison of the results of capacitance and voltammetry measurements provides insights into average pore size and number density. Results presented for CdS films grown using two different chemical bath compositions reveal substantial differences in pinhole numbers and doping density.

To reduce the amount of chalcogen needed in the post-annealing process, we demonstrate significantly increased sulfur incorporation into pure sulfide CZTS films achieved by increasing the thiourea content of DMSO-based precursor solution. The increase of sulfur content was confirmed by thermogravimetric analyses (TGA). To understand how the elemental distribution across the CZTS layer is affected by extra thiourea, a systematic compositional study was carried out using X-ray photoelectron spectroscopy (XPS). XPS depth profiling reveals increased sulfur incorporation in the final CZTS films when more thiourea is added to the solution. The grain size was reduced slightly with increased sulfur content and the surface morphology was changed significantly. The effect on the surface of the CZTS film has been investigated using scanning electron microscopy (SEM), Raman spectroscopy, and XPS. External-quantum-efficiency (EQE) measurements with an electrolyte contact were used to investigate the optoelectronic properties of the deposited CZTS films.

The transient photocurrent response of semiconductor electrodes to chopped illumination often shows spikes and overshoots that are usually interpreted as evidence that surface recombination is occurring. In the case of the high intensities used for light-driven water splitting, the interpretation is less straightforward since the electron transfer reactions are so slow that the minority carrier concentration at or near the surface increases to high values that modify the potential drop across the Helmholtz layer in the electrolyte, leading to ‘band edge unpinning’. In addition, changes in chemical composition of the surface or local changes in pH may also alter the potential distribution across the semiconductor/electrolyte junction. A quantitative theory of band edge unpinning due to minority carrier build up is presented, and numerical calculations of transient photocurrent responses are compared with experimental examples for n-type Fe2O3 and p-type lithium-doped CuO electrodes. It is shown that the apparently high reaction orders (up to third order) with respect to hole concentration reported for hematite photoanodes can be explained as arising from an acceleration of hole transfer by the increased voltage drop across the Helmholtz layer associated with band edge unpinning. The limitations of the band edge unpinning model are discussed considering additional effects associated with modification of the potential distribution brought about by light-induced changes in surface composition, surface dipoles and surface ionic charge.

Two-dimensional spectral response distribution maps in dye-sensitized solar cells (DSSC) are reported using 1 × 1 mm pixels for 0.5 × 2 cm DSSC devices by coupling external quantum efficiency (EQE) measurements with an X-Y scanning stage. The technique has been applied to singly dyed (D149 and N719) and co-sensitized (N719/SQ1 and D149/SQ1) devices. Dye desorption and re-dyeing processes have also been successfully mapped using this technique to help understand dye loading and dye location in multiply-sensitized cells. Keywords: Solar energy, Manufacturing, Spectral response maps, Selective dyeing

Photo-catalytic performance for oxide films, here for inkjet-printed TiO2 (ca. 1 μm thickness on FTO) and for spray-pyrolysis-coated Nb2O5 (ca. 1 μm thickness on FTO), is affected by oxygen vacancies that form during vacuum-heat treatment at 550 °C. The effects of the oxygen vacancies are associated with formation of Ti(III) and Nb(IV) sites, respectively, and therefore optically visible as “grey” coloration. Photo-electrochemical light-on-off transient experiments are performed in the limit of thin film photoanodes, where front and back illumination result in the same photo-current responses (i.e. with negligible effects from internal light absorption gradients). It is shown that generally the magnitude of photo-currents correlates linearly with light intensity, which is indicative of dominant “photo-capacitive” behaviour. At an applied voltage of 0.4 V vs. SCE (in the plateau region of the photo-current responses) the potential and also the pH (going from 1.0 M KOH to 0.1 M HClO4 in the presence of methanol quencher) have no significant effect on photo-currents; that is, surface chemical/kinetic effects appear to be unimportant and interfacial hole transfer may be rate limiting. Under these conditions (and based on a simplistic mechanistic model) changes in photo-currents introduced by oxygen vacancy doping (detrimental for TiO2 and beneficial for Nb2O5) are assigned primarily to changes in electron mobility.

Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaic absorber layers are fabricated by selenizing Cu2ZnSnS4 (CZTS) nanoparticle thin films in a selenium rich atmosphere. The selenium vapor pressure is controlled to optimize the morphology and quality of the CZTSSe thin film. The largest grains are formed at the highest selenium vapor pressure of 226mbar. Integrating this photovoltaic absorber layer in a conventional thin film solar cell structure yields a champion short circuit current of 37.9mA/cm2 without an antireflection coating. This stems from an improved external quantum efficiency characteristic in the visible and near-infrared part of the solar spectrum. The physical basis of this improvement is qualitatively attributed to a substantial increase in the minority carrier diffusion length.

The analysis of capacitance data for regular nanostructured photoelectrodes is revisited using a hematite nanorod array as an example. The effects of the cylindrical nanorod geometry on the capacitance-voltage behaviour are outlined, and the limiting case of complete depletion is discussed in terms of the residual geometric capacity at the base of the nanorods. Since nanorod arrays generally leave areas of the substrate exposed, it is necessary to consider the parallel capacitance associated with the fraction of uncovered surface. The sensitivity of the capacitance fitting to parameter variation is explored. The enhancement of external quantum efficiency (EQE) by nanostructuring is also discussed using hematite nanorod arrays as experimental examples. It is shown that, although very substantial EQE enhancement should be achieved by simple geometric effects, the performance of nanostructured hematite electrodes in the visible region of the spectrum is considerably lower than predicted if all charge carriers generated in the space charge region (SCR) were collected. Further analysis reveals that the internal quantum efficiency increases with photon energy, suggesting that the probability of generating free, rather than bound, electron-hole pairs in hematite depends on the excess energy hν - E gap .

Metal chalcogenide thin films have a wide variety of applications and potential uses. Tin(II) Sulfide, is one such materi-al which presents a significant challenge with the need for high quality SnS, free of oxide materials (e.g. SnO2) oxides and higher tin sulfides (e.g. Sn2S3 and SnS2). This problem is compounded further when the target material exhibits a number of polymorphic forms with different optoelectronic properties. Unlike conventional chemical vapor deposition (CVD) and atomic layer deposition (ALD), which relies heavily on having precursors that are volatile, stable and reac-tive, the use of aerosol assisted CVD (AA-CVD) negates the need for volatile precursors. We report here, for the first time, the novel and structurally characterized single source precursor (1), Dimethylamido-(N-Phenyl-N’,N’-Dimethyl-Thiouriate)Sn(II) dimer, and its application in the deposition, by AA-CVD, of phase-pure films of SnS. A mechanism for the oxidatively controlled formation of SnS from precursor (1) is also reported. Significantly, thermal control of the deposition process allows for the unprecedented selective and exclusive formation of either orthorhombic-SnS (α-SnS) or zinc blende-SnS (ZB-SnS) polymorphs. Thin films of α-SnS or ZB-SnS have been deposited onto Mo, FTO, Si and glass substrates at the optimized deposition temperatures of 375 oC and 300 oC, respectively. The densely packed poly-crystalline thin films have been characterized by XRD, SEM, AFM, Raman spectroscopy, EDS and XPS analysis. These data confirmed the phase purity of the SnS formed. Optical analysis of the α-SnS and ZB-SnS films show distinctly dif-ferent optical properties with direct band gaps of 1.34 eV and 1.78 eV, respectively. Furthermore photoelectrochemical and external quantum efficiency (EQE) measurements were undertaken to assess the optoelectronic properties of the deposited samples. We also report for the first time the ambipolar properties of the ZB-SnS phase.

The basic concepts of light-driven water splitting using semiconductor-based photoelectrochemical devices are reviewed with the emphasis on the prospects of achieving solar to hydrogen (STH) efficiencies that will enable the development of competitive hydrogen generation technologies to begin to replace those based on fossil fuels. Examples of the calculation of conversion efficiencies are given for different approaches to solar photoelectrolysis, illustrating the need to use well-defined figures of merit in all publications dealing with the photoelectrochemical generation of solar fuels.

Electrodeposition has considerable potential for application in the fabrication of low cost thin film solar cells. The expected demand for earth-abundant replacements for the rare, toxic or expensive materials currently used in thin film solar cells has stimulated research into electrochemical preparation of the different component layers (absorber, buffer, window) in the emerging area of earth-abundant photovoltaics. Recent progress in this rapidly developing area is reviewed with particular emphasis on the kesterites Cu2ZnSnS4 and Cu2ZnSnSe4. Keywords: CIGS, CZTS, Electrodeposition, Kesterites, Photovoltaics, Zinc oxide

Photoelectrochemical (PEC) water oxidation is considered to be the rate-limiting step of the two half-reactions in light-driven water splitting. Consequently, considerable effort has focused on improving the performance of photoanodes for water oxidation. While these efforts have met with some success, the mechanisms responsible for improvements resulting from photoanode modifications are often difficult to determine. This is mainly caused by the entanglement of numerous properties that influence the PEC performance, particularly processes that occur at the photoanode/electrolyte interface. In this study, we set out to elucidate the effects on the surface carrier dynamics of hematite photoanodes of introducing manganese (Mn) into hematite nanorods and of creating a core-shell structure. Intensity-modulated photocurrent spectroscopy (IMPS) measurements reveal that the introduction of Mn into hematite not only increases the rate constant for hole transfer but also reduces the rate constant for surface recombination. In contrast, the core-shell architecture evidently passivates the surface states where recombination occurs; no change is observed for the charge transfer rate constant, whereas the surface recombination rate constant is suppressed by ∼1 order of magnitude.

Transparent nano-structured hematite (α-Fe2O3) films of approximately 550 nm thickness on tin-doped indium oxide (ITO) have been obtained conveniently by ink-jet printing of a Fe(NO3)3/Brij® O10 precursor ink and subsequent annealing at 500 °C in air. When illuminated with a blue LED (λ = 455 nm, ca. 100 mW cm−2), the hematite films exhibited photocurrents of up to 70 μA cm−2 at 0.4 V vs. SCE in 0.1 M NaOH electrolyte. Thermal annealing in vacuum at 500 °C for 2 h increased photocurrents more than three times to 230 μA cm−2 in agreement with previous literature reports for pure hematite materials. These results suggest that a simple ink-jetting process with surfactants is viable. The effects of vacuum-annealing on the photoelectrical properties of α-Fe2O3 films are discussed in terms of a sub-surface state templating hypothesis based on data gathered from photo-transients, field emission scanning electron microscopy, X-ray photoelectron spectroscopy analysis, X-ray diffraction, photocurrent spectra, and cyclic voltammetry.

Differences in the solar cell performance of solid-state PbS quantum-dot-sensitized solar cells fabricated with ex situ and in situ grown PbS quantum dots were investigated. The PbS quantum dots were either anchored on mesoporous TiO2 via l-glutathione (GSH) linker exchange or prepared in situ by the successive ionic layer adsorption/reaction (SILAR) method to create quantum-dot-sensitized solar cells. Spiro-OMeTAD was used as the organic p-type hole transporting material (HTM). The performance of the cells was evaluated with current–voltage, external quantum efficiency, and impedance spectroscopy (IS) measurements, and electron lifetimes were measured with open-circuit voltage decay (OCVD), intensity-modulated photovoltage spectroscopy (IMVS) techniques as well as with IS measurements. Analysis of the experimental data indicates that the SILAR route provides more intimate contacts at both TiO2/PbS and PbS/HTM interfaces, which results in more efficient charge injection and separation and thus higher phot...

Some fundamental aspects of light-driven water splitting at semiconductor electrodes are reviewed along with recent experimental and theoretical progress. The roles of thermodynamics and kinetics in defining criteria for successful water-splitting systems are examined. An overview of recent research is given that places emphasis on new electrode materials, theoretical advances and the development of semi-quantitative experimental methods to study the dynamics of light-driven water-splitting reactions. Key areas are identified that will need particular attention as the search continues for stable, efficient and cost-effective light-driven photoelectrolysis systems that exploit electron/hole separation in semiconductor/electrolyte junctions.

The sensitization of dispersed P25 TiO2 nanoparticles (NPs) and macroporous TiO2 films with water-soluble and air-stable PbS quantum dots (QDs) capped with l-glutathione (GSH) ligands was investigated. Optimum sensitization was achieved by careful adjustment of the surface charges of TiO2 and PbS QDs by controlling the pH of the QD solution. Efficient electron transfer from photoexcited PbS QDs via the GSH ligands into the conduction band of TiO2 was demonstrated by photoluminescence (PL) spectroscopy of PbS-sensitized P25 nanoparticles. The PbS QD-sensitized porous TiO2 electrodes were used to prepare quantum-dot-sensitized solar cells (QDSSCs) utilizing a CuxSy counter electrode and aqueous polysulfide electrolyte. Cells with up to 64% injection efficiency, 1.1% AM 1.5 conversion efficiency, and short circuit current density of 7.4 mA cm–2 were obtained. The physical parameters of the cells were investigated using impedance spectroscopy.

Numerous studies have shown that the performance of hematite photoanodes for light-driven water splitting is improved substantially by doping with various metals, including tin. Although the enhanced performance has commonly been attributed to bulk effects such as increased conductivity, recent studies have noted an impact of doping on the efficiency of the interfacial transfer of holes involved in the oxygen evolution reaction. However, the methods used were not able to elucidate the origin of this improved efficiency, which could originate from passivation of surface electron-hole recombination or catalysis of the oxygen evolution reaction. The present study used intensity-modulated photocurrent spectroscopy (IMPS), which is a powerful small amplitude perturbation technique that can de-convolute the rate constants for charge transfer and recombination at illuminated semiconductor electrodes. The method was applied to examine the kinetics of water oxidation on thin solution-processed hematite model photoanodes, which can be Sn-doped without morphological change. We observed a significant increase in photocurrent upon Sn-doping, which is attributed to a higher transfer efficiency. The kinetic data obtained using IMPS show that Sn-doping brings about a more than tenfold increase in the rate constant for water oxidation by photogenerated holes. This result provides the first demonstration that Sn-doping speeds up water oxidation on hematite by increasing the rate constant for hole transfer.

A novel optoelectroactive system based on an oriented periodic mesoporous organosilica (PMO) film has been developed. A tetra-substituted porphyrin silsesquioxane was designed as a precursor, and the porphyrin macrocycles were covalently incorporated into the organosilica framework without adding additional silica sources, using an evaporation-induced self-assembly process. The synthesized PMO film has a face-centered orthorhombic porous structure with a 15 nm pore diameter. This large pore size enables the inclusion of electron-conducting species such as [6,6]-phenyl C61 butyric acid methyl ester in the periodic mesopores. Optoelectronic measurements on the resulting interpenetrating donor-acceptor systems demonstrate the light-induced charge generation capability and hole-conducting property of the novel porphyrin-based PMO film, indicating the potential of PMO materials as a basis for optoelectroactive systems.

Single crystals of Cu2ZnSnS4 have been produced within sealed quartz ampoules via the chemical vapour transport technique using I2 as the transporting agent. The effects of temperature gradient and I2 load on the crystal habit and composition are considered. Crystals have been analyzed with XRD, SEM, and TEM for compositional and structural uniformities at both microscopic and nanoscopic levels. The synthesized crystals have suitable (I2-load dependent) properties and are useful for further solar absorber structural and physical characterizations. A new chemical vapour transport method based on longitudinally isothermal treatments is attempted. Based on a proposed simplistic mechanism of crystal growth, conditions for crystal enlargement with the new method are envisaged.

Single crystals of Cu2ZnSnS4 (CZTS) have been grown by iodine vapor transport with and without addition of NaI. Crystals with tin-rich copper-poor and with zinc-rich copper-poor stoichiometries were obtained. The crystals were characterized by single crystal X-ray diffraction, energy-dispersive X-ray spectroscopy, photocurrent spectroscopy and electroreflectance spectroscopy using electrolyte contacts as well as by spectroscopic ellipsometry, Raman spectroscopy and photoluminescence spectroscopy (PL)/decay. Near-resonance Raman spectra indicate that the CZTS crystals adopt the kesterite structure with near-equilibrium residual disorder. The corrected external quantum efficiency of the p-type crystals measured by photocurrent spectroscopy approaches 100% close to the bandgap energy, indicating efficient carrier collection. The bandgap of the CZTS crystals estimated from the external quantum efficiency spectrum measured using an electrolyte contact was found to be 1.64-1.68 eV. An additional sub-bandgap photocurrent response (Urbach tail) was attributed to sub bandgap defect states. The room temperature PL of the crystals was attributed to radiative recombination via tail states, with lifetimes in the nanosecond range. At high excitation intensities, the PL spectrum also showed evidence of direct band to band transitions at ∼1.6 eV with a shorter decay time. Electrolyte electroreflectance spectra and spectra of the third derivative of the optical dielectric constant in the bandgap region were fitted to two optical transitions at 1.71 and 1.81 eV suggesting a larger valence band splitting than predicted theoretically. The high values of the EER broadening parameters (192 meV) indicate residual disorder consistent with the existence of tail states.

Perovskite solar cells (PSC) are shown to behave as coupled ionic-electronic conductors with strong evidence that the ionic environment moderates both the rate of electron-hole recombination and the band offsets in planar PSC. Numerous models have been presented to explain the behavior of perovskite solar cells, but to date no single model has emerged that can explain both the frequency and time dependent response of the devices. Here we present a straightforward coupled ionic-electronic model that can be used to explain the large amplitude transient behavior and the impedance response of PSC., 16 pages, 7 figures

The solar cell sensitizer cis -bis(isothiocyanato)-bis(2,2′-bipyridyl-4,4′dicarboxylato)-ruthenium(II) (N719) is adsorbed and investigated at two electrode surfaces: (i) at a bare fluorine-doped tin oxide (FTO) and (ii) at a nano-particulate anatase (TiO 2 ) film in contact with FTO. N719 is adsorbed from acetonitrile onto FTO surfaces giving poor quality partial or multi-layer coverage commencing at 10 −7 M concentration. In contrast, from 50% acetonitrile 50% t BuOH solution of N719 Langmuirian adsorption occurs with well-defined mono-layer coverage and a binding constant ca. 2 × 10 5 mol −1 dm 3 . The adsorbed N719 exhibits voltammetric oxidation/back-reduction responses with E mid ≈ 0.56 (weaker) and 0.68 (dominant) V vs. Ag/AgCl (3 M KCl) and with chemically reversible characteristics at sufficiently high scan rates (ca. 16 V s −1 ). A chemical reaction step involving oxidised N719 at the electrode surface leads to the loss of electrochemical activity at slower scan rates with a first order chemical rate constant of ca. 2.4 s −1 . The electro-catalytic oxidation of iodide is demonstrated for both the intact metal complex N719 and the reaction product formed after oxidation. When adsorbed onto TiO 2 (porous films made from approximately 9 nm diameter TiO 2 particles, Langmuirian binding constant ca. 10 5 mol −1 dm 3 ), immersed in acetonitrile (0.1 M NBu 4 PF 6 ), and at sufficiently fast scan rates (ca. 16 V s −1 ), the N719 metal complex exhibits reversible voltammetric responses (with E mid ≈ 0.68 V vs. Ag/AgCl (3 M KCl)). At slower scan rates, the voltammetric response again appears irreversible, however, this time without significant degradation of the N719 metal complex at the TiO 2 surface. It is shown that the conduction mechanism via electron hopping becomes ineffective due to degradation of FTO-adsorbed N719. In the presence of iodide, the electro-catalytic iodide oxidation process (dark electro-catalysis) is shown to occur predominantly at the N719-modified FTO electrode surface. Implications of this dark-reactivity for the solar cell performance are discussed.

Light-driven water-splitting (photoelectrolysis) at semiconductor electrodes continues to excite interest as a potential route to produce hydrogen as a sustainable fuel, but surprisingly little is known about the kinetics and mechanisms of the reactions involved. Here, some basic principles of semiconductor photoelectrochemistry are reviewed with particular emphasis on the effects of slow interfacial electron transfer at n-type semiconductors in the case of light-driven oxygen evolution. A simple kinetic model is outlined that considers the competition between interfacial transfer of photogenerated holes and surface recombination. The model shows that, if interfacial charge transfer is very slow, the build-up of holes at the surface will lead to substantial changes in the potential drop across the Helmholtz layer, leading to non-ideal behavior (Fermi level pinning). The kinetic model is also used to predict the response of photoanodes to chopped illumination and to periodic perturbations of illumination and potential. Recent experimental results obtained for α-Fe 2O 3 (hematite) photoanodes are reviewed and interpreted within the framework of the model.

In order to study the behavior of photoinjected electrons in dye-sensitized solar cells (DSC), steady-state microwave reflectance measurements (33 GHz, Ka band) have been carried out on a working cell filled with electrolyte. The experimental arrangement allowed simultaneous measurement of the light-induced changes in microwave reflectance and open circuit voltage as a function of illumination intensity. In addition, frequency-resolved intensity-modulated microwave reflectance measurements were used to characterize the relaxation of the electron concentration at open circuit by interfacial transfer to tri-iodide ions in the electrolyte. The dependence of the free and trapped electron concentrations on open circuit voltage were derived, respectively, from conductivity data (obtained by impedance spectroscopy) and from light-induced near IR transmittance changes. These electron concentrations were used in the fitting of the microwave reflectivity response, with electron mobility as the main variable. Changes in the complex permittivity of the mesoporous films were calculated using Drude-Zener theory for free electrons and a simple harmonic oscillator model for trapped electrons. Comparison of the calculated microwave reflectance changes with the experimental data showed that the experimental response arises primarily from the perturbation of the real component of the complex permittivity by the high concentration of trapped electrons present in the DSC under illumination. The results suggest that caution is needed when interpreting the results of microwave reflectance measurements on materials with high concentrations of electron (or hole) traps, since an a priori assumption that the microwave response is solely determined by changes in conductivity (i.e., by free electrons) may be incorrect. The intensity-modulated microwave reflectance measurements showed that relaxation of the free and trapped electron concentrations occurs on a similar time scale, confirming that the free and trapped electron populations remain in quasi-equilibrium during the decay of the electron concentration.

Eight new N-arylstilbazolium chromophores with electron donating –NR2 (R = Me or Ph) substituents have been synthesized via Knoevenagel condensations and isolated as their PF6− salts. These compounds have been characterized by using various techniques including 1H NMR and IR spectroscopies and electrospray mass spectrometry. UV–vis absorption spectra recorded in acetonitrile are dominated by intense, low energy π → π* intramolecular charge-transfer (ICT) bands, and replacing Me with Ph increases the ICT energies. Cyclic voltammetric studies show irreversible reduction processes, together with oxidation waves that are irreversible for R = Me, but reversible for R = Ph. Single crystal X-ray structures have been determined for three of the methyl ester-substituted stilbazolium salts and for the Cl− salts of their picolinium precursors. Time-dependent density functional theory calculations afford reasonable predictions of ICT energies, but greater rigour is necessary for –NPh2 derivatives. The four new acid-functionalized dyes give moderate sensitization efficiencies (ca. 0.2%) when using TiO2-based photoanodes, with relatively higher values for R = Ph vs Me, while larger efficiencies (up to 0.8%) are achieved with ZnO substrates.

The photovoltaic performance of ZnO-based dye-sensitized solar cells (DSCs) has been studied for three different configurations involving two dyes and two types of electrolytes with the iodide/iodine as redox mediator: ZnO/N719/organic solvent electrolyte (C1), ZnO/D149/organic solvent electrolyte (C2), and ZnO/N719/ionic liquid electrolyte (C3). The DSCs were characterized by measuring current–voltage curves and photovoltage as a function of light intensity and by electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), intensity-modulated photovoltage spectroscopy (IMVS), and open circuit photovoltage decay (OCVD). The results demonstrate the good light harvesting properties of the D149 dye and highlight the photovoltage limitation of the solvent-free (ionic liquid) electrolyte. The intensity dependence of the photovoltage and the OCVD, EIS, and IMVS results provide evidence of the nonlinear character of the recombination kinetics. It has been found that by com...

The opportunities for photovoltaic (PV) solar energy conversion are reviewed in the context of projected world energy demands for the twenty-first century. Conventional single-crystal silicon solar cells are facing increasingly strong competition from thin-film solar cells based primarily on polycrystalline absorber materials, such as cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). However, if PVs are to make a significant contribution to satisfy global energy requirements, issues of sustainability and cost will need to be addressed with increased urgency. There is a clear need to expand the range of materials and processes that is available for thin-film solar cell manufacture, placing particular emphasis on low-energy processing and sustainable non-toxic raw materials. The potential of new materials is exemplified by copper zinc tin sulphide, which is emerging as a viable alternative to the more toxic CdTe and the more expensive CIGS absorber materials.

Potentiostatic deposition of Cu2O thin films on glass substrates coated with F-doped SnO2 from an alkaline electrolyte solution (pH 12.5) containing copper (II) sulfate and lactic acid was studied for fabrication of a Cu2O/Al-doped ZnO (AZO) heterojunction solar cell. The band gap of the electrodeposited Cu2O films was determined by photoelectrochemical measurements to be around 1.9 eV irrespective of the applied potentials. The solar cells with a glass/FTO/Cu2O/AZO structure were fabricated by sputtering an AZO film onto the Cu2O film followed by deposition of an Al contact by vacuum evaporation. The highest efficiency of 0.603% was obtained with a Cu2O film deposited at −0.6 V (vs. Ag/AgCl). This was attributed to better compactness and purity of the Cu2O film than those of the Cu2O films deposited at other potentials.

Three dimensional ferromagnet–superconductor core–shell structures are realized by electrocrystallisation and their magnetic properties investigated. We observe fully re-entrant core superconductivity in increasing fields that survives well above the bulk critical field due to compensation effects. The net measured magnetization of optimized structures could be switched from absolute para(ferro-)magnetic to diamagnetic by tuning the external magnetic field. Micromagnetic simulations of our structures are in good qualitative agreement with our results

In this paper we demonstrate the electrodeposition of nickel, a common ferromagnetic material, in various magnetically desirable shapes including nanowires, nanoparticles and highly faceted shells. In order to obtain three dimensional mesostructures, the electrochemical deposition of nickel was performed on highly oriented pyrolytic graphite (HOPG) under different electrolyte composition and deposition potential conditions. Under potentiostatic deposition at one distinct potential negative with respect to the reversible potential of nickel, three stages of nucleation and growth take place leading to a complex morphology of deposits. However, dual-pulse potential deposition and electrodeposition in low pH solutions causing hydrogen evolution, lead to nickel deposits in the form of nanowires and nanoparticles with the complete absence of a faceted morphology. Highly faceted nickel shells were electrodeposited via a dual-bath method on prefabricated silver mesocrystals as 'template' electrodeposited on HOPG. Magnetic properties of faceted three dimensional nickel shells reveal clear signatures of facets of mesocrystals in the form of sharp steps in measured hysteresis loops and a strong magnetic anisotropy with respect to applied field direction.

Optical waveguide spectroscopy has been used to study the real-time adsorption of ruthenium 535-bisTBA (N-719) dye in mesoporous nanocrystalline titanium dioxide films of the type used in dye-sensitized solar cells (DSCs). Porous titania films were prepared on top of gold substrates, and prism coupling was used to create a guided wave in the nanocrystalline film. The conditions under which a guided mode can be excited are dependent on both the refractive index and the extinction coefficient of the mesoporous layer, where the mesoporous layer refers to both the nanocrystals of TiO2 and the composition of the pores. It was therefore possible to track changes in dye concentration in the pores in real time. The total concentration of dye in the film appeared to continue increasing even after 22 h, in contrast to the amount of dye in the pores that was able to absorb light at 632.8 nm, which saturated after ∼5 h. The total concentration of dye molecules was 2.47 × 10−4 mol cm−3 as a function of the total pore ...

We prepared the complexes [Ru{4,4`-(CO2R-bpy)2}{Cu(exoO2-cyclam)}][NO3]2 [R = Et (1), H (2)], which possess a CuII centre covalently linked to a (bipyridyl)RuII fragment. The complexes were characterised by cyclic voltammetry, UV/Vis spectroscopy, hybrid DFT and TD-DFT (time-dependent density-functional theory) calculations, EPR (electron paramagnetic resonance), emission spectroscopy and UV/Vis spectroelectrochemistry, with the latter showing reversible conversion to the mono- and di-oxidised and mono- and di-reduced species. The data suggest the first oxidation to be largely based on the Ru centre but with little energetic difference between the highest occupied orbitals based on Ru and based on Cu. There was also evidence of solvent coordination in both the electrochemistry and spectroelectrochemistry experiments. The redox potentials and the strongvisible absorptions of 2 (λmax = 562 nm, ϵmax = 22200 Μ–1 cm–1)make it appropriate for study as a sensitiser in a dye-sensitised solar cell. Charge-separated lifetime of photoexcited 2 on TiO2 and incident photon-to-current conversion efficiency (IPCE) as a function of wavelength were studied and are discussed alongside the formation of dye-sensitised solar cells with the best efficiency achieved of η = 2.55 %, Voc = 608 mV, Isc = 5.84 mA cm–2 and ff = 0.72. Limitations to the maximum efficiency obtained were attributed to a mismatch between the bipyridyl-based unoccupied orbital of the dye and the TiO2 conduction band edge.

A limiting step to roll-to-roll production of dye-sensitized solar cells on metals is TiO2 sintering (10–30 min). Near infrared (NIR) heating is a novel process innovation which directly heats titanium substrates giving rapid binder removal and sintering. NIR heating (for 12.5 s) at varying power gave titanium temperatures of 545, 685 and 817°C yielding cells with efficiencies of 2.9, 2.8 and 2.5%. Identical cells prepared in a conventional oven (1800 s) at 500, 600 and 800°C gave 2.9, 2.6 and 0.2% efficiency. NIR sintering is ultrafast and has a wide process window making it ideal for rapid manufacturing on metals. Copyright © 2010 John Wiley & Sons, Ltd.

The electrochemical and spectroelectrochemical properties of the sensitizer dye Z907 (cis-RuLL'(SCN)2 with L=4,4′-dicarboxylic acid-2,2′-bipyridine and L'=4,4′-dinonyl-2,2′-bipyridine) adsorbed on fluorine-doped tin oxide (FTO) and TiO2 surfaces have been investigated. Langmuirian binding constants for FTO and TiO2 are estimated to be 3 × 106 M−1 and 4 × 104 M−1, respectively. The Ru(III/II) redox process is monitored by voltammetry and by spectroelectrochemistry. For Z907 adsorbed onto FTO, a slow EC-type electrochemical reaction is observed with a chemical rate constant of ca. k = 10−2 s−1 leading to Z907 dye degradation of a fraction of the FTO-adsorbed dye. The Z907 adsorption conditions affect the degradation process. No significant degradation was observed for TiO2-adsorbed dye. Degradation of the Z907 dye affects the electron hopping conduction at the FTO–TiO2 interface.

The electron collection efficiency in dye-sensitized solar cells (DSCs) is usually related to the electron diffusion length, L = (Dτ)1/2, where D is the diffusion coefficient of mobile electrons and τ is their lifetime, which is determined by electron transfer to the redox electrolyte. Analysis of incident photon-to-current efficiency (IPCE) spectra for front and rear illumination consistently gives smaller values of L than those derived from small amplitude methods. We show that the IPCE analysis is incorrect if recombination is not first-order in free electron concentration, and we demonstrate that the intensity dependence of the apparent L derived by first-order analysis of IPCE measurements and the voltage dependence of L derived from perturbation experiments can be fitted using the same reaction order, γ ≈ 0.8. The new analysis presented in this letter resolves the controversy over why L values derived from small amplitude methods are larger than those obtained from IPCE data.

Two [Pt(II)(substituted-pyridyl)2(dithiolate)] dyes with the formulas [Pt(4-CO2CH3-py)2(dmit)] and [Pt(4-NO2-py)2(mnt)] (where py = pyridyl, dmit = 1,3-dithiol-2-thione-4,5-dithiolate and mnt = maleonitriledithiolate) and their dichloride precursors [PtCl2(4-R-py)2] have been synthesized and compared to a previously-reported dye [Pt(4-CO2CH3-py)2(mnt)]. Variation of either the pyridyl ligands or the ditholate ligand showed tuning of the electrochemical and spectroscopic characteristics of the dyes as evidenced by cyclic and differential pulse voltammetry, hybrid DFT calculations, UV/Vis spectroelectrochemistry and in situ EPR spectroelectrochemistry. The HOMO was shown to be mostly dithiolate based and the LUMO pyridyl based allowing absorption characteristics to be predictably tuned to longer wavelengths, which is important for optimization of such dyes in applications such as solar energy conversion.

A new steady-state method for determination of the electron diffusion length in dye-sensitized solar cells (DSCs) is described and illustrated with data obtained using cells containing three different types of electrolyte. The method is based on using near-IR absorbance methods to establish pairs of illumination intensity for which the total number of trapped electrons is the same at open circuit (where all electrons are lost by interfacial electron transfer) as at short circuit (where the majority of electrons are collected at the contact). Electron diffusion length values obtained by this method are compared with values derived by intensity-modulated methods and by impedance measurements under illumination. The results indicate that the values of electron diffusion length derived from the steady-state measurements are consistently lower than the values obtained by the non-steady-state methods. For all three electrolytes used in the study, the electron diffusion length was sufficiently high to guarantee ...

Polycrystalline thin films of Cu2ZnSnSe4 (CZTSe) were produced by selenisation of Cu(Zn,Sn) magnetron sputtered metallic precursors for solar cell applications. The p-type CZTSe absorber films were found to crystallize in the stannite structure (a = 5·684 A and c = 11·353 A) with an electronic bandgap of 0·9 eV. Solar cells with the indium tin oxide structure (ITO)/ZnO/CdS/CZTSe/Mo were fabricated with device efficiencies up to 3·2% measured under standard AM1·5 illumination. Copyright © 2009 John Wiley & Sons, Ltd.

Dye-sensitized solar cells (DSCs, also known as Gratzel cells) mimic the photosynthetic process by using a sensitizer dye to harvest light energy to generate electrical power. Several functional features of these photochemical devices are unusual, and DSC research offers a rewarding arena in which to test new ideas, new materials, and new methodologies. Indeed, one of the most attractive chemical features of the DSC is that the basic concept can be used to construct a range of devices, replacing individual components with alternative materials. Despite two decades of increasing research activity, however, many aspects of the behavior of electrons in the DSC remain puzzling. In this Account, we highlight current understanding of the processes involved in the functioning of the DSC, with particular emphasis on what happens to the electrons in the mesoporous film following the injection step. The collection of photoinjected electrons appears to involve a random walk process in which electrons move through the network of interconnected titanium dioxide nanoparticles while undergoing frequent trapping and detrapping. During their passage to the cell contact, electrons may be lost by transfer to tri-iodide species in the redox electrolyte that permeates the mesoporous film. Competition between electron collection and back electron transfer determines the performance of a DSC: ideally, all injected electrons should be collected without loss. This Account then goes on to survey recent experimental and theoretical progress in the field, placing particular emphasis on issues that need to be resolved before we can gain a clear picture of how the DSC works. Several important questions about the behavior of "sticky" electrons, those that undergo multiple trapping and detrapping, in the DSC remain unanswered. The most fundamental of these concerns is the nature of the electron traps that appear to dominate the time-dependent photocurrent and photovoltage response of DSCs. The origin of the nonideality factor in the relationship between the intensity and the DSC photovoltage is also unclear, as is the discrepancy in electron diffusion length values determined by steady-state and non-steady-state methods. With these unanswered questions, DSC research is likely to remain an active and fruitful area for some years to come.

In situ near-IR transmittance measurements have been used to characterize the density of trapped electrons in dye-sensitized solar cells (DSCs). Measurements have been made under a range experimental conditions including during open-circuit photovoltage decay and during recording of the IV characteristic. The optical cross section of electrons at 940 nm was determined by relating the IR absorbance to the density of trapped electrons measured by charge extraction. The value, σn = 5.4 × 10−18 cm2, was used to compare the trapped electron densities in illuminated DSCs at open and short circuit in order to quantify the difference in the quasi Fermi level, ΔnEF under the two conditions. It was found that ΔnEF for the cells studied was 250 meV over wide range of illumination intensities. IR transmittance measurements have also been used to quantify shifts in conduction band energy associated with dye adsorption.

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...

An electrodeposition-annealing route to films of the promising p-type absorber material Cu2ZnSnS4 (CZTS) using layered metal precursors is studied. The dependence of device performance on composition is investigated, and it is shown that a considerable Cu-deficiency is desirable to produce effective material, as measured by photoelectrochemical measurements employing the Eu3+/2+ redox couple. The differing effects of using elemental sulphur and H2S as sulphur sources during annealing are also studied, and it is demonstrated that H2S annealing results in films with improved crystallinity.

The feasibility of a new fabrication route for films of the attractive solar absorber Cu2ZnSnS4 (CZTS) has been studied, consisting of electrodeposition of metallic precursors followed by annealing in sulfur vapour. Photoelectrochemical measurements using a Eu3+ contact have been used to establish that the polycrystalline CZTS films are p-type with doping densities in the range (0.5–5) × 1016 cm−3 and band gaps of 1.49 ± 0.01 eV, making them suitable for terrestrial solar energy conversion. It has been shown that a somewhat Cu-poor composition favours good optoelectronic properties. Keywords: Photoelectrochemistry, Photovoltaics, Cu2ZnSnS4, CZTS, Solar cell, Semiconductor

Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2‘7,7‘-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9‘-spirobifluorene (spiro-MeOTAD), and three organic indoline-based sensitizer dyes with high molar extinction coefficients. The cells were characterized by several techniques, including spectral response measurements, photovoltage decay transients, intensity modulated photovoltage spectroscopy (IMVS), and charge extraction. The differences in apparent electron lifetime observed for cells fabricated using the three dyes are attributed in part to changes in the surface dipole potential at the TiO2/spiro-MeOTAD interface, which shift the TiO2 conduction band energy relative to the Fermi level of the HTM. These energy shifts influence both the open circuit voltage (as a result of changes in free electron density) and the short circuit current (as a consequence of changes in the overlap between the dye LUMO level and the conduction band). A self-consistent ...