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The recent emergence of lead-halide perovskites as active layer materials for thin film semiconductor devices including solar cells, light emitting diodes, and memristors has motivated the development of several new drift-diffusion models that include the effects of both mobile electronic and ionic charge carriers. The aim of this work is to provide a comprehensive guide to Driftfusion, a versatile simulation tool built for simulating one-dimensional ordered semiconductor devices with mixed ionic-electronic conducting layers. Driftfusion enables users to model devices with multiple, distinct, material layers and up to four charge carrier species: electrons and holes by default plus up to two ionic species. The time-dependent carrier continuity equations are fully-coupled to Poisson's equation enabling transient optoelectronic device measurement protocols to be simulated. In addition to material and device-wide properties, users have direct access to adapt the physical models for carrier transport, generation and recombination. Furthermore, a graded-interface approach circumvents the requirement for boundary conditions at material interfaces and enables interface-specific properties, such as high rates of interfacial recombination, to be introduced., Comment: Main text: 30 pages, 17 figures, Supplemental information: 29 pages, 20 figures

Nowadays, perovskite Photovoltaic Solar Cells (PSCs) have attracted scientists’ attention due to the high- power conversion efficiency (PCE) and potentially low cost of manufacturing. However, the charge trapping and associated recombination processes negatively affect perovskite device performance. Here, we are using a new optical Pump-Push-Photocurrent (PPP) technique to study the carrier dynamic in perovskite devices. Particularly we are interested in the kinetics of charge trapping and nonradiative recombination in the device active layer. We have performed quasi-steady-state measurements of device performance under simultaneous illumination by visible and NIR laser diode. The preliminary results show that the effect of NIR push light scales linearly with NIR light intensity but depends in a more complex way from the intensity of visible light. By measuring the induced photocurrent as a function of light-modulation frequency we have performed an estimation of carrier lifetime. Moreover, the temperature-dependent current density measurements show that the effect of NIR light on the device performance does not originate from the sample heating but likely leads to the carrier detrapping.

The four-flux radiative transfer method describes light propagation through a light scattering media for a low computational cost by using experimentally extracted input parameters. The method can be used to aid the optimal design of the light harvesting layers in solar technologies such as the porous TiO2 films used dye-sensitized solar cells (DSSCs). The results of these light propagation simulations are sensitive to the input parameters, particularly the simulation results of systems involving mesoporous scattering centres. Here we present the four-flux parameters to simulate common mesoporous TiO2 film architectures. The parameters are highly interdependent, and there is no linear relationship between the independent scattering centres used in the films and experimentally extracted parameters. The results suggest that theoretically evaluated four flux parameters cannot be used in place of experimentally extracted parameters for four-flux simulation of DSSCs.

Bifacial dye sensitized solar cells (DSSCs) show potential for use in diffuse and low light environments, but their photoanode light scattering properties have not been optimised. We examine DSSC photoanodes composed of a mixed scattering layer (made by blend of 18NR-T and WER2-O pastes), sandwiched between nanostructured TiO2 layers (made by 18NR-T paste). WER2-O paste was chossen after accessing scattering properties: scattering coefficient (S), forward scattering ratio (FSR) and forward path length enhancement (FPLE) of the solid TiO2 particles of different shapes and sizes. Monte Carlo simulations of light harvesting indicate the optimal volume fraction of scattering particles ( f vsca ) in the sandwiched layer to be 5–30%. The proposed photoanode absorbs light effectively and the DSSC with scattering layer made by blend of 60% of 18NR-T and 40% of WER2-O paste showed the power conversion efficiency (PEC) of 8.54% and 5.26%, when illuminated from photoanode (PA) and counter electrode (CE) side, respectively. The effective sorting criteria and optimization routine used in this work can also be used for other devices like perovskite solar cells.

Two-dimensional (2D) hybrid perovskites have attracted considerable attention due to their enormous structural and electronical variability, making this class of semiconductors interesting for photovoltaics, light-emitting diodes and lasers. 2D perovskites consist of sheets of bulky organic cations alternately sandwiched by layers of lead halide octahedra. The properties of these materials strongly depend on the thickness of the octahedra layers, defined by the number of octahedra sheets in a layer, n. Consequently, controlling the layer thickness purity (i.e. minimizing the spread in n) is important for any 2D perovskite thin film application. Here, we show that using rationally chosen solvent additives in the precursor solution offers a facile way to control the crystal disorder in 2D perovskites films. Our method leads to significantly reduced variation in n around the target value relative to films obtained by conventional fast-crystallization methods without solvent additives. The improved phase purity in optimized n = 2 and n = 3 films is verified by X-ray diffraction, UV-vis absorption, and photoluminescence measurements. Additionally, we find that 2D perovskite films with n ≥ 2 arising from additive-assisted growth exhibit an unusual crystal orientation with the perovskite interlayers predominantly aligned parallel to the substrate, which we assign to the slow crystallization process induced by the lead-complexing solvent additives. Improved control over the phase purity translates into a better control of the optoelectronic properties of 2D perovskite films. Furthermore, the unusual horizontal crystal orientation of n = 2 and n = 3 films makes this family of tunable organic–inorganic perovskites promising for applications where lateral charge transport is desired, thus enlarging the potential for thin film-based applications of the 2D perovskites.

Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for molecular sensitized semiconductors in electrolytes must account for the restricted geometry of these systems compared to “free” molecules in solution. Here, we explore the extent to which dielectric properties of the surrounding medium can explain the rates of charge transfer processes, measured using transient absorption spectroscopy, involving photo-oxidized thiophene–carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes. We observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. The activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which we attribute to energetic disorder in the molecular monolayer. The recombination rate in different solvents scales with the hole hopping rate. It can also be varied by adding inert salts in the electrolyte and by controlling the access of cations in solution to the oxide surface. Finally, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules in solvents with low polarity is verified, but the kinetics are limited by the ionic dissociation. Our study highlights the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes.

Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin-film solar cells by probing the decay of a small optically-induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in reported carrier lifetime values for the same systems, and the reporting of unrealistic reaction order values. In this work, we explore the validity of TPV to extract reliable charge carrier lifetimes in thin-film solar cells through the use of time-dependent drift diffusion simulations and measurements. We find that in low mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics, and that the extracted time constant is highly mobility dependent. To address this shortcoming, we develop Transient Photo-Charge (TPQ) a new technique to measure the charge carrier density during the photo-voltage decay and apply it to study the recombination dynamics in a series of (fullerene and non-fullerene) organic solar cell systems. We show that using this technique the charge carrier recombination lifetime in the active layer can be more accurately determined.

In Methyl Ammonium Lead Iodide (MAPI) perovskite solar cells, screening of the built in field by mobile ions has been proposed as part of the cause of the large hysteresis observed in the current/voltage scans in many cells. We show that photocurrent transients measured immediately (e.g. 100 μs) after a voltage step can provide direct evidence that this field screening exists. Just after a step to forward bias, the photocurrent transients are reversed in sign (i.e. inverted), and the magnitude of the inverted transients can be used to find an upper bound on the width of the space charge layers adjacent to the electrodes. This in turn provides a lower bound on the mobile charge concentration, which we find to be 1 x 10 17 /cm 3 . Using a new photocurrent transient experiment, we show that the space charge layer thickness remains approximately constant as a function of bias, as expected for mobile ions in a solid electrolyte. We also discuss additional characteristics of the inverted photocurrent transients that imply either an unusually stable deep trapping, or a photo effect on the mobile ion conductivity.

Solid electrolyte interphase (SEI) layers form on sensitized-TiO2 photoanodes and platinum counter electrodes when dye-sensitized solar cells (DSSCs) are subjected to an accelerated aging protocol (e.g., heating at 85 °C in the dark for 500 h). To understand how this impacts device operation, we conducted an electrochemical impedance spectroscopy study and found that the SEI induces an additional electron-transfer process from the TiO2 to the electrolyte. This is materialized by the onset of a new charge-transfer semicircle at higher frequencies, predominantly visible under bias voltages similar to and greater than the open-circuit voltage. Our results emphasize the detrimental role of SEI formation on device performance and lifetime. Additionally, nanosecond transient absorption spectroscopy showed that SEI formation reduced the rate of oxidized dye regeneration. We also found that a proportion of the photogenerated holes on the dyes were transferred to the SEI itself. A prolonged aging duration led to the electrode’s mesoporosity network being entirely clogged by the SEI, thus impeding efficient transport of the electrolyte redox couple and being responsible for a further decline in photovoltaic performances.

We report the development of redox-active conjugated polymers with potential application to electrochemical energy storage. Side chain engineering enables processing of the polymer electrodes from solution, stability in aqueous electrolytes and efficient transport of ionic and electronic charge carriers. We synthesized a 3,3' dialkoxybithiophene homo polymer (p type polymer) with glycol side chains and prepared naphthalene 1,4,5,8-tetracarboxylic-diimide-dialkoxybithiophene (NDI gT2) copolymers (n type polymer) with either a glycol or zwitterionic side chain on the NDI unit. For the latter, we developed a post-functionalization synthesis to attach the polar zwitterion side chains to the polymer backbone to avoid challenges of purifying polar intermediates. We demonstrate fast and reversible charging of solution processed electrodes for both the p- and n type polymers in aqueous electrolytes, without using additives or porous scaffolds and for films up to micrometers thick. We apply spectroelectrochemistry as an in operando technique to probe the state of charge of the electrodes. This reveals that thin films of the p-type polymer and zwitterion n-type polymer can be charged reversibly with up to two electronic charges per repeat unit (bipolaron formation). We combine thin films of these polymers in a two-electrode cell and demonstrate output voltages of up to 1.4 V with high redox stability. Our findings demonstrate the potential of functionalizing conjugated polymers with appropriate polar side chains to improve specific capacity, reversibility and rate capabilities of polymer electrodes in aqueous electrolytes., Main text 16 pages including 6 figures + 49 pages of supporting information. arXiv admin note: substantial text overlap with arXiv:1711.10457

Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings suggest a strategy to design thin film electronic components with large, tuneable, capacitor-like and inductor-like characteristics. The resulting simple equivalent circuit model, which we verified with time-dependent drift-diffusion simulations of measured impedance spectra, allows a general description and interpretation of perovskite solar cell behaviour., Comment: 12 pages main text, total 51 including supplementary information

We report a design strategy that allows the preparation of solution processable n type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol based side chain. A series of random copolymers are prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90 and 100 with respect to the alkyl side chains. These are characterized in order to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions and performance in OECTs when operated in aqueous electrolytes. We observe that glycol side chain percentages of >50 % are required to achieve volumetric charging while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.

In dye-sensitized solar cells (DSSCs) photogenerated positive charges are normally considered to be carried away from the dyes by a separate phase of hole-transporting material (HTM). We show that there can also be significant transport within the dye monolayer itself before the hole reaches the HTM. We quantify the fraction of dye regeneration in solid-state DSSCs that can be attributed to this process. By using cyclic voltammetry and transient anisotropy spectroscopy, we demonstrate that the rate of interdye hole transport is prevented both on micrometer and nanometer length scales by reducing the dye loading on the TiO2 surface. The dye regeneration yield is quantified for films with high and low dye loadings (with and without hole percolation in the dye monolayer) infiltrated with varying levels of HTM. Interdye hole transport can account for >50% of the overall dye regeneration with low HTM pore filling. This is reduced to about 5% when the infiltration of the HTM in the pores is optimized in 2 μm th...

Solar cells composed of methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that (i) hydrated crystal phases are formed when MAPI is exposed to water vapor at room temperature and (ii) these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH3NH3PbI3·H2O followed by (CH3NH3)4PbI6·2H2O (upon long exposure times) was observed using time-resolved XRD and ellipsometry of thin films prepared using “solvent engineering”, single crystals, and state-of-the-art solar cells. In contrast to water vapor, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI2. MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH3NH3PbI3·H2O formed on single crystals were determined, with a bandgap at 3.1 eV. Using the single-crystal optical constants and thin-film ellipsometry measurements, the time-dependent changes to MAPI films exposed to moisture were modeled. The results suggest that the monohydrate phase forms independent of the depth in the film, suggesting rapid transport of water molecules along grain boundaries. Vapor-phase hydration of an unencapsulated solar cell (initially Jsc ≈ 19 mA cm–2 and Voc ≈ 1.05 V at 1 sun) resulted in more than a 90% drop in short-circuit photocurrent and ∼200 mV loss in open-circuit potential; however, these losses were fully reversed after the device was exposed to dry nitrogen for 6 h. Hysteresis in the current–voltage characteristics was significantly increased after this dehydration, which may be related to changes in the defect density and morphology of MAPI following recrystallization from the hydrate. Based on our observations, we suggest that irreversible decomposition of MAPI in the presence of water vapor only occurs significantly once a grain has been fully converted to the monohydrate phase.

Three organic or hybrid photovoltaic technologies are compared with respect to performance and stability under the harsh regime of concentrated light. Although all three technologies show surprisingly high (and linear) photocurrents, and better than expected stability, no golden apples are awarded.

We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I-2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO(3) can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.

Solid-state semiconductor-sensitized solar cells require a thin, dense hole-blocking layer at the conducting glass substrate (F-doped tin oxide (FTO)) to prevent shorting beween the FTO and hole conductor. We found that by adding a small amount of Sb ions to a ZnO chemical deposition bath a thin (few tens of nanometers thick) dense and uniform layer of Sb-incorporated ZnO forms. Here we investigate the electronic properties of this layer in comparison to the continuous ZnO layer at the base of the ZnO rods formed in the standard preparation. Devices incorporating the Sb-incorporated dense layer followed by a standard ZnO nanorod growth, onto which CdS or CdSe was grown followed by a CuSCN hole conductor, showed 100–200 mV higher photovoltage together with occasional improvement in the short-circuit current. Electrochemical and electrical measurements indicated complete coverage of the FTO substrate by both preparations; however, the shunt resistance (resistance to a reverse leakage current) in the cells (...

DSSCs incorporating Co(II/III) tris(bipyridine) redox couple in acetonitrile and the Z907 dye were subjected to ≥2000 hours of light soaking at 20 °C and ∼1 sun light intensity from white LEDs (∼no UV component). Initial energy efficiencies were near 6.3%. After 2000 hours, the best acetonitrile based cells maintained ∼66% of the initial efficiency. Both JSC and fill factor (FF) declined, while the VOC remained highly stable. In comparison, the best Z907/cobalt cells with 3-methoxypropionitrile (MPN) as a solvent, maintained 91% of the initial efficiency after 2000 hour light soaking. Only FF declined in MPN based cells. In follow up testing of similar ACN cells at the maximum power point, at 30 °C, the best cells maintained ∼100% of the initial efficiency after 1000 hours. Impedance, JSCvs. intensity and charge extraction data are consistent with a decrease in the Co(III) concentration, or a restriction in Co(III) diffusion, during light soaking.

We present femtosecond to nanosecond transient absorption (TA) data on electron injection in dye-sensitized solar cells (DSSCs) fabricated with low volatility, commercially relevant electrolytes, with and without added lithium. Results are shown over an extended time range (300 fs–6.3 ns) and extended wavelength range (800–1400 nm) for both N719 and C106 dyes. Kinetics were measured on both TiO2 and noninjecting ZrO2. Using the latter, we have determined the spectra and absorption coefficient of N719* across the wavelength range. We find an isosbestic point in the TA spectra on TiO2 near 900 nm for all cells, existing from 1 ns. We show how measurements near this isosbestic point can give a false impression of uniformly femtosecond injection dynamics in DSSCs. Comparison of dynamics measured at 1200 nm with mid-IR transient absorption measured at 5100 nm confirms a majority proportion of slow (>10 ps) electron injection in these commercially relevant cells. We also comment on a recent publicatio...

With the application of near-infrared radiation (NIR), TiO2 films for dye-sensitized solar cells (DSCs) on metallic substrates can be sintered in just 12.5 seconds. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 seconds. Here we characterise the electron transport, electron lifetime and phase-morphological properties of ultrafast NIR sintered films, using impedance spectroscopy, transient photovoltage decay and X-ray diffraction measurements. An important factor in NIR processing of TiO2 films is the peak metal temperature (PMT) and we show that during the 12.5 second heat treatment, a PMT of around 635 °C gives near identical electron transport, electron lifetime and morphological properties, as well comparable photovoltaic performance to a conventionally sintered (500 °C, 30 min) film. We demonstrate that the rapid heating of the TiO2 (to temperatures of up to 785 °C) does not lead to a large scale rutile phase transition. As such photovoltaic performance of resultant DSC devices is maintained since the heating period is insufficient to induce a significant transition from anatase to rutile or morphology changes which result in a loss of photocurrent.

Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results.

Charge recombination between oxidized dyes attached to mesoporous TiO2 and electrons in the TiO2 was studied in inert electrolytes using transient absorption spectroscopy. Simultaneously, hole transport within the dye monolayers was monitored by transient absorption anisotropy. The rate of recombination decreased when hole transport was inhibited selectively, either by decreasing the dye surface coverage or by changing the electrolyte environment. From Monte Carlo simulations of electron and hole diffusion in a particle, modeled as a cubic structure, we identify the conditions under which hole lifetime depends on the hole diffusion coefficient for the case of normal (disorder free) diffusion. From simulations of transient absorption and transient absorption anisotropy, we find that the rate and the dispersive character of hole transport in the dye monolayer observed spectroscopically can be explained by incomplete coverage and disorder in the monolayer. We show that dispersive transport in the dye monolayer combined with inhomogeneity in the TiO2 surface reactivity can contribute to the observed stretched electron–hole recombination dynamics and electron density dependence of hole lifetimes. Our experimental and computational analysis of lateral processes at interfaces can be applied to investigate and optimize charge transport and recombination in solar energy conversion devices using electrodes functionalized with molecular light absorbers and catalysts.

We investigated the influence of moisture on methylammonium lead iodide perovskite (MAPbI3) films and solar cells derived from non-stoichiometric precursor mixtures. We followed both the structural changes under controlled air humidity through in situ X-ray diffraction, and the electronic behavior of devices prepared from these films. A small PbI2 excess in the films improved the stability of the perovskite compared to stoichiometric samples. We assign this to excess PbI2 layers at the perovskite grain boundaries or to the termination of the perovskite crystals with Pb and I. In contrast, the MAI-excess films composed of smaller perovskite crystals showed increased electronic disorder and reduced device performance owing to poor charge collection. Upon exposure to moisture followed by dehydration (so-called solvent annealing), these films recrystallized to form larger, highly oriented crystals with fewer electronic defects and a remarkable improvement in photocurrent and photovoltaic efficiency.

Leguy, Aurélien M. A. et al., We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations.1 Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamics effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity and supressed electron-phonon scattering events., The authors thank Juraj Sibik and Axel Zeitler for their contribution in the measurement and interpretation of the terahertz absorption spectra, and Philip Calado and Davide Moia for their helpful discussions regarding the TOC graphic. PB and AL are grateful to the EPSRC (EP/J002305/1, EP/M014797/1 and EP/M023532/1) for financial support. The work at Bath was supported by the EPSRC (EP/K016288/1, EP/K004956/1, EP/L000202, and EP/M009580/1) and the ERC (Grant 277757). ARG, MIA and MCQ thank the Spanish Ministry of Economy and Competitiveness (MINECO) for its support through Grant No. CSD2010-00044 (Consolider NANOTHERM) and MAT2015-70850-P (HIBRI2). The work at ICMAB was carried out under the auspices of the Spanish Severo Ochoa Centre of Excellence program (grant SEV-2015-0496). F. B. is funded through the EU DESTINY Network (Grant No. 316494). JN acknowledges the EPSRC for founding (EP/J017361). AP would like to thank the Royal Irish Academy for the Charlemont grant for funding the research visit that made the terahertz work possible.

Yes. Testing 7 different metals as a substrate for a counter electrode in dye sensitized solar cells (DSSC) showed that some metals can be a good option for use with cobalt electrolyte. It was found that Stainless steels 304 and 321 as well as Ni and Ti suit well to the counter electrodes in DSSCs with cobalt electrolyte. In these 4 cases both the efficiency and the lifetime were similar to the reference cells on conducting glass substrates. In contrast, the cells with Al, Cu and Zn substrates suffered from both a low efficiency and a poor stability. These three metals had clear marks of corrosion such as apparent corrosion products in the aged cells. Additionally, we also investigated how the different types of catalyst materials perform in the case of a metal counter electrode (stainless steel 304) with cobalt electrolyte in comparison to reference glass cells. Among the 5 different catalyst layers the best results for stainless steel electrode were achieved with low temperature platinization whereas polymer catalysts poly(3,4-ethylenedioxythiophene)-p-toluenesulfone and poly(3,4-ethylenedioxythiophene)-polystyrenesulfone that worked well on the glass worked very poorly on the metal.

The effect of electrolyte filling method on the performance of the dye-sensitized solar cells is investigated with the segmented cell method, a recent technique which is very simple but effective as it can be used to examine all the photovoltaic characteristics. The electrolyte filling techniques compared were single injection, which is typically used in small laboratory cells, and pumping the electrolyte through the cell several times, which is often used for larger cells and modules. Significant photovoltage and photocurrent variations occur with the repeated pumping of the electrolyte in the cell preparation. Transient and charge extraction measurements confirmed that the differences in open circuit voltage were due to the shifts of the TiO 2 conduction band and time correlated single photon counting confirmed that the reduction of short circuit current was largely due to reduced electron injection correlated with the increasing conduction band edge in the studied cases. This was interpreted as an effect of molecular filtering by the TiO 2 causing an accumulation of electrolyte additives (4- tert -butylpyridine and benzimidazole) near the electrolyte filling hole, the concentration of which increased with repeated pumping of the electrolyte. Interestingly, spatial variations were seen not only in the relative TiO 2 conduction band energy but also in the density of trap states. In this contribution it is demonstrated how the changes in the conduction band can be separated from the changes in the density of trap states which is an essential for the correct interpretation of the data.

We propose and apply a new experimental pro-tocol for determining the kinetics of the oxidation of iodide (aka dye regeneration) in dye sensitized solar cells (DSSCs) using measurements on full cells under operating conditions. Transient absorption (TA) decays of the oxidized dye after a laser pulse are correlated with the short circuit current (Jsc) and electron concentration in the TiO2, measured during the TA experiment. Results for a series of cells with N719 [(Bu4N)2][Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl), a standard electrolyte, and varying iodide concentration allow us to fit for the fundamental regeneration rate constant (krg = 7.8 × 105 M−1 s−1) and to determine that the order in iodide, in these cells, is near 1 (0.98 ± 0.16) and clearly not 2. The rate and order allow us to rank various reaction mechanisms and to discuss possible rate-limiting steps that could be catalyzed to improve regeneration. The method can also give estimates of the fundamental rate constant for elect...

A significant difference exists between two measured estimates of the electron diffusion length in dye sensitized solar cells. One method is based on spectral response at short circuit (L1) and the other on small perturbations (Ln). Following a recent theoretical treatment of the diffusion length for “non first order recombination”, one can propose an “effective diffusion length” at short circuit (λn). Using simulation and measured values we show that L1 ≈ λn, for a wide range of recombination orders, despite the first order assumption made when deriving L1. We also show Ln ≈ λn, when Ln is derived from a time dependent full cell model incorporating non first order recombination. Thus the deviation of experimentally measured Ln from simulated Ln and L1 cannot be explained simply by the incorporation of nonlinear recombination. Our results give support to the spectral response method, but do not give a theoretical basis for rejecting the small perturbation method.

A simple analytical function based on the multiple trapping model, is used to describe the biomolecular recombination of charge carriers in a bulk heterojunction (BHJ) film in the presence of an exponential energetic tail of localized hole "trap" states. The function is used to fit charge carrier decay data from an unannealed P3HT/PCBM film measured by transient absorption. The analysis assumes that only free holes participate in recombination and transport. This implies an effective recombination rate coefficient which varies with the ratio of free to trapped holes. The fit parameters yield a bimolecular recombination constant for free holes with free electrons (k(o) = 3.4 x 10(-12) cm(3) s(-1)) and information about the distribution of trap states (trap distribution parameter beta = 0.29) Assuming the Langevin recombination limit, the analysis yields a concentration dependent effective hole mobility saturating at mu(o) approximate to 7 x 10(-2) cm(2) V-1 s(-1). This approach should be useful to compare BHJs in a consistent and meaningful manner.

Recombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, the impact of active layer crystallinity on the accumulated charge and open‐circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI) is demonstrated. It is shown that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high‐performance devices.

There is great interest in synthesizing organic dyes to replace organometallic dyes as sensitizers in nanocrystalline TiO2 solar cells. We present a detailed comparison of interfacial electron transfer kinetics in dye-sensitized solar cells containing the coumarin based organic dye (NKX2677) against those observed for the ruthenium based organometallic dye, N719, including analysis of excited state lifetimes, injection kinetics, regeneration kinetics, and recombination to both oxidized dyes and electrolyte. We find three primary differences in behavior between these two dyes which limit the performance of NKX2677-sensitized solar cells: a shorter lifetime of the electron injecting state for NKX2677 versus N719 (primarily due to it being a singlet rather than triplet sensitizer); a faster rate constant for recombination to the electrolyte (RCE) for NKX2677-sensitized solar cells; and the greater tendency for NKX2677 to aggregate, reducing the electron injection efficiency. The shorter excited state lifetim...

A long electron diffusion length (L) relative to film thickness is required for efficient collection of charge in photoelectrodes. L was measured in a nanocrystalline, mesoporous TiO2 electrode during photochemical water splitting by two independent methods: (1) analyzing the ratio of incident photon conversion efficiency (IPCE) measured under back and front side illumination, and (2) analyzing transient photovoltage rise and decay measurements. These gave values of L that agreed (L = 8.5−12.5 μm between −0.15 and 0.1 V vs Ag/AgCl 3 M KCl at pH 2). The transient measurements were consistent with trap limited transport and recombination of electrons as previously observed in dye-sensitized solar cells. L ∼ 10 μm is sufficient to collect separated electrons with minor electrolyte recombination losses in thin electrodes. However, charge collection may limit performance in doped mesoporous electrodes with weak visible light absorption where thicker films are required.

Simultaneously acquired transient absorption, photocurrent, and photovoltage signals are used to characterize operational dye-sensitized solar cells (DSSCs) under 1 sun illumination. At open circuit, the biphasic decay of both photovoltage and 980 nm absorption have identical decay profiles. The data show that, in devices incorporating N719 [(Bu4N)2[Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl) as the sensitizer, the 950−1020 nm absorption can be assigned to electrons alone. This differs from some previous assignments. At short circuit, the comparison of the integrated photocurrent transient, electron absorption, and the absorption of the oxidized dye (830 nm) can be used to show relative loss of photocurrent to oxidized dyes. In addition, we show that, under operational conditions, there is no evidence for long-lived di-iodine radicals (I2•−). Further, we show that the decay of the transient bleach at 560 nm, previously assigned to the bleach of the ground-state dye, contains additional long...

Photocurrents generated by thick, strongly absorbing, dye-sensitized cells were reduced when the electrolyte iodine concentration was increased. Electron diffusion lengths measured using common transient techniques (L(n)) were at least two times higher than diffusion lengths measured at steady state (L(IPCE)). Charge collection efficiency calculated using L(n) seriously overpredicted photocurrent, while L(IPCE) correctly predicted photocurrent. This has implications for optimizing cell design.

In this paper we focus upon the electron injection dynamics in complete nanocrystalline titanium dioxide dye-sensitized solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time-resolved single photon counting, and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with a half-time of 200 +/- 60 ps and quantum yield of 84 +/- 5%. We find that these injection dynamics are independent of presence of iodide/triiodide redox couple and of the pH of the peptization step used in the synthesis of the TiO(2) nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of tert-butylpyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO(2) conduction band energetics as a function of tBP and Li(+) concentration, from which we conclude that a 100 meV shift in band edge results in an approximately 2-fold retardation of injection dynamics. We find that the electron injection quantum yield determined from these transient emission data as a function of tBP and Li(+) concentration shows a linear correlation with device short circuit density J(sc). We thus conclude that the relative energetics of the dye excited state versus the titanium dioxide acceptor state is a key determinant of the dynamics of electron injection in DSSC, and that variations in these energetics, and therefore in the kinetics and efficiency of electron injection, impact directly upon device photovoltaic efficiency. Finally, we discuss these results in terms of singlet versus triplet electron injection pathways and the concept of minimization of kinetic redundancy.

Injection efficiency, ηinj, and diffusion length, L, in dye-sensitized solar cells have been derived from the spectral response (incident photon to current efficiency, IPCE) of the cells under front side or backside illumination. Values of L from IPCE are found to be ∼2 times shorter than the values of L derived from the normal small perturbation transient method. IPCE-derived values of L (2 to more than 40 μm) and ηinj (63−90%) are found to correlate with the photocurrent (and indirectly with the photovoltage) of the different cells indicating the extent to which each factor limits the cell efficiency. IPCE spectra varied with light intensity, so that diffusion lengths derived from both methods show similar trends, e.g., L from IPCE is found to increase 3 times when the light intensity is increased 10 times up to approximately 0.1 sun where L tends to plateau or peak. The values for ηinj derived from the spectral response are shown to be in quantitative agreement with those determined from picosecond tra...

Thin films of α -Fe 2 O 3 (hematite) were deposited using filtered arc deposition. The structural, optical and electrical properties of the films have been characterized. High-purity hematite films were produced, free from other iron oxide phases and impurities. The films exhibit preferred orientation, with the c-axis of the hexagonal structure aligned perpendicular to the substrate. The films have an upper uncertainty bound of the porosity of 15%, with a microindentation hardness of 17.5 ± 1 GPa and elastic modulus of 1235 ± 5 GPa. The indirect and direct band gap energies were found to be approximately 1.9 eV and 2.7 eV, respectively. The refractive index, and the extinction and absorption coefficients were determined from total reflectance and direct transmittance measurements. The thin films exhibit a high resistivity (≥ 10 5 Ω cm) which indicates pure α -Fe 2 O 3 . An activation energy of 0.7 eV was calculated from an Arrhenius plot of the conductivity.

Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood. Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase composition. By exploiting the different signatures in the photoinduced absorption of trapped charges in anatase and rutile, we were able to separately track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photogenerated holes transfer from rutile to anatase on submicrosecond time scales. This hole transfer can significantly increase the anatase hole yield, with a 20:80 anatase:rutile composite showing a 5-fold increase in anatase holes observed from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irrespective of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for determining the direction of photogenerated charge separation across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.

Transient photovoltage and photocurrent measurements coupled show that the migration of ionic defects can explain hysteresis in perovskite solar cells. We further show that the ionic migration appears present even in cells with minimal hysteresis.

Multilayer structures of alternating thin titanium and tungsten oxide layers having dimensions of ∼20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique. Single, double, and triple layer titanate and tungstate thin films were deposited on silicon substrates, and these films were initially annealed at 400 °C. Structural and microstructural aspects of the films were investigated using a variety of techniques, including X-ray reflectometry, grazing incidence X-ray absorption spectroscopy (GIXAS), cross-sectional transmission electron microscopy (TEM), and secondary ion mass spectrometry. The dimensions of the films and the character of the interfaces were principally gauged by cross-sectional TEM and X-ray reflectometery. All films were continuous on a local scale and had relatively low surface roughness. At the treatment temperature of 400 °C, only the tungsten oxide component showed appreciable crystallinity. The multilayer films had rel...

Doped and undoped hematite (α-Fe2O3) thin film electrodes for photoelectrochemical hydrogen production have been prepared using reactive magnetron sputtering. We present an extensive characterization of the photoelectrochemical, structural, electrical, and optical properties of the α-Fe2O3 films to assess the mechanisms by which the dopants influence photoelectrode performance. The Si-doped and Ti-doped α-Fe2O3 exhibited much higher photoelectrochemical activity than the undoped material. The Ti-doped α-Fe2O3 showed the largest photocurrent as well as the lowest onset potential, the highest electrical conductivity, the lowest activation energy, and the highest charge carrier density. The crystallographic orientation of the films does not appear to be a dominant factor affecting the photocurrent. The changes in conductivity with doping are insufficient to explain the observed changes in the photoelectrochemical activity. The proposed mechanism for the enhanced photocurrent is reduction of recombination due...

Novel nanostructured tungsten trioxide (WO3) films with strong photocatalytic activity, have been prepared by a simple sol–gel route from aqueous peroxopolytungstic acid (PTA) precursor solutions. We demonstrate that films with different morphologies can be synthesized by simply adjusting the pH of this precursor solution using different mineral acids such as HCl and HC1O4, and that this control of film texture represents a way of optimizing photocurrent yield. The best films produced using these methods generated anodic photocurrents of 3.8 mA/cm2 for oxidiation of methanol and 2.2 mA/cm 2 for water splitting, under AM1.5 simulated solar illumination.

A simple route for synthesizing nanostructured tungsten trioxide (WO3) films on conducting glass substrates has been developed. Films with different morphologies were synthesized by the addition of mineral acids (such as HCl) to an aqueous peroxopolytungstic acid precursor solution. The films were characterized by SEM, SAXS, in-situ X-ray diffraction, cyclic voltammetry, optical absorption and photoelectrochemical measurements. The best of these films generated anodic photocurrents as high as 4.0 mA cm−2 for oxidization of methanol and 2.3 mA cm−2 for water splitting, under Xe lamp illumination at 86 mW cm−2. The improvement of photocurrent can be attributed to enhanced light harvesting and charge transport arising due to the nanostructure.

Leguy, Aurélien M. A. et al., The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS)., We thank Jarvist M. Frost and Paul N. Stavrinou for useful discussions, Xiao Li for assistance with sample preparation and Stefan A. Maier for access to equipment. PB and AL are grateful to the EPSRC (EP/J002305/1, EP/M014797/1 and EP/M023532/1) for financial support. JN thanks the EPSRC for funding (EP/J017361 and EP/K030671/1). MCQ and MIA acknowledge financial support from MINECO through project MAT2012- 37776. The theory portion of this work was supported by EPSRC (EP/M009602/1, EP/K016288/1 and EP/M009580/1).