Future alternative and promising energy sources involve photoelectrochemical (PEC) devices that can convert sunlight and abundant resources such as water and CO2 into chemical fuels and value-added products. However, identifying suitable photoabsorber semiconductor materials that fulfill all the stringent requirements of photoelectrodes in PEC devices remains a significant challenge. A key factor for tailoring and optimizing existing and novel photoabsorbers is understanding the processes occurring at the semiconductor/liquid electrolyte interface under working conditions. This perspective focuses on the application of operando Raman spectroscopy (RS) in synergy with (photo)electrochemical techniques. Despite being a relatively new field of application, when applied to photoelectrochemistry, operando RS offers insights into the evolution of photoelectrode structure (i.e. phase purity and degree of crystallinity) and surface defects under working conditions. The challenges associated with operando RS for (photo)electrochemical applications, including the low quantum efficiency of inelastic scattering and fluorescence, and possible mitigation strategies are discussed. Furthermore, practical aspects such as sample/reactor geometry requirements and the surrounding environment of the photoelectrode sample during operando RS under PEC conditions are reviewed. We demonstrate that operando RS can be used to perform product analysis of solar-driven biomass reforming reactions, showing the approach's limitations and discussing possible solutions to overcome them. This work concludes with a discussion on the current state of operando RS of semiconducting photoelectrodes and devices for photoelectrochemistry. We show a new methodology for performing operando RS with illumination resembling AM1.5 conditions and with time resolution spanning from tens to hundreds of milliseconds, suitable timescales for real-time monitoring of chemical reactions and degradation mechanisms occurring at the photoelectrode under investigation.

Metal oxides are considered as stable and low-cost photoelectrode candidates for hydrogen production by photoelectrochemical solar water splitting. However, their power conversion efficiencies usually suffer from poor transport of photogenerated charge carriers, which has been attributed previously to a variety of effects occurring on different time and length scales. In search for common understanding and for a better photo-conducting metal oxide photoabsorber, CuFeO2, alpha-SnWO4, BaSnO3, FeVO4, CuBi2O4, alpha-Fe2O3, and BiVO4 are compared. Their kinetics of thermalization, trapping, localization, and recombination are monitored continuously 100 fs-100 mu s and mobilities are determined for different probing lengths by combined time-resolved terahertz and microwave spectroscopy. As common issue, we find small mobilities < 3 cm(2)V(-1)s(-1). Partial carrier localization further slows carrier diffusion beyond localization lengths of 1-6 nm and explains the extraordinarily long conductivity tails, which should not be taken as a sign of long diffusion lengths. For CuFeO2, the localization is attributed to electrostatic barriers that enclose the crystallographic domains. The most promising novel material is BaSnO3, which exhibits the highest mobility after reducing carrier localization by annealing in H-2. Such overcoming of carrier localization should be an objective of future efforts to enhance charge transport in metal oxides.

The unique possibilities of rapid thermal processing (RTP) for overcoming two significant challenges in the development of oxide thin-film photoelectrodes are demonstrated. The first is the need to exceed normal temperature limits for glass-based F:SnO2 substrates (FTO, similar to 550 degrees C) to achieve the desired density, crystallinity, and low defect concentrations in metal oxides. Flash-heating of Ta2O5, TiO2, and WO3 photoelectrodes to 850 degrees C is possible without damaging the FTO. RTP heating-rate dependencies suggest that the emission spectrum of the RTP lamp, which blue-shifts with increasing heating power, can significantly influence the crystallization behavior of wide-bandgap photoelectrodes (>= 1.8 eV). The second challenge is avoiding the formation of structural defects, trap states, grain boundaries, and phase impurities, which can be particularly difficult in multinary metal oxides. RTP treatment of alpha-SnWO4, a promising photoanode material, resulted in an increase in grain size and favorable crystallographic reorientation, culminating in a new performance record.

The widespread application of solar-water-splitting for energy conversion depends on the progress of photoelectrodes that uphold stringent criteria from photoabsorber materials. After investigating almost all possible elemental and binary semiconductors, the search must be expanded to complex materials. Yet, high structural control of these materials will become more challenging with an increasing number of elements. Complex metal-oxides offer unique advantages as photoabsorbers. However, practical fabrication conditions when using glass-based transparent conductive-substrates with low thermal-stability impedes the use of common synthesis routes of high-quality metal-oxide thin-film photoelectrodes. Nevertheless, rapid thermal processing (RTP) enables heating at higher temperatures than the thermal stabilities of the substrates, circumventing this bottleneck. Reported here is an approach to overcome phase-purity challenges in complex metal-oxides, showing the importance of attaining a single-phase multinary compound by exploring large growth parameter spaces, achieved by employing a combinatorial approach to study CuBi2O4, a prime candidate photoabsorber. Pure CuBi2O4 photoelectrodes are synthesized after studying the relationship between the crystal-structures, synthesis conditions, RTP, and properties over a range of thicknesses. Single-phase photoelectrodes exhibit higher fill-factors, photoconversion efficiencies, longer carrier lifetimes, and increased stability than nonpure photoelectrodes. These findings show the impact of combinatorial approaches alongside radiative heating techniques toward discovering highly efficient multinary photoabsorbers.

A new method for enhancing the charge separation and photo-electrochemical stability of CuBi2O4 photoelectrodes by sequentially depositing Bi2O3 and CuO layers on fluorine-doped tin oxide substrates with pulsed laser deposition (PLD), followed by rapid thermal processing (RTP), resulting in phase-pure, highly crystalline films after 10 min at 650 degrees C, is reported. Conventional furnace annealing of similar films for 72 h at 500 degrees C do not result in phase-pure CuBi2O4. The combined PLD and RTP approach allow excellent control of the Bi:Cu stoichiometry and results in photoelectrodes with superior electronic properties compared to photoelectrodes fabricated through spray pyrolysis. The low photocurrents of the CuBi2O4 photocathodes fabricated through PLD/RTP in this study are primarily attributed to their low specific surface area, lack of CuO impurities, and limited, slow charge transport in the undoped films. Bare (without protection layers) CuBi2O4 photoelectrodes made with PLD/RTP shows a photocurrent decrease of only 26% after 5 h, which represents the highest stability reported to date for this material. The PLD/RTP fabrication approach offers new possibilities of fabricating complex metal oxides photoelectrodes with a high degree of crystallinity and good electronic properties at higher temperatures than the thermal stability of glass-based transparent conductive substrates would allow.

The steady-state photoluminescence (PL) of CsPbBr3 films with varying thicknesses was studied under light-soaking on semiconductive and insulating contacts, showing reversible changes in PL, entirely dependent on the nature of the contact and film thicknesses. The PL at 50-100 nm CsPbBr3 on TiO2 increased, and decreased in thicker layers, with no thickness-dependent PL in CsPbBr3 on glass/Al2O3. These observations are described using a spatial charge distribution model which interprets the migration of Br- and V-Br(+) under light-soaking as suppressing electron injection into the TiO2 due to upward band bending, enhancing PL at the interface and nonradiative recombination further away from it.

In this Letter, we systematically explore the influence of TiO2 thickness with nanometric variations over a range of 20-600 nm on the photovoltaic parameters (open-circuit voltage, short circuit current, fill-factor, and power conversion efficiency) of CH3NH3PbI3-based solar cells. We fabricate several sample libraries of 13 x 13 solar cells on large substrates with spatial variations in the thickness of the TiO2 layers while maintaining similar properties for the other layers. We show that the optimal thickness is similar to 50 nm for maximum performance; thinner layers typically resulted in short-circuited cells, whereas increasing the thickness led to a monotonic decrease in performance. Furthermore, by assuming a fixed bulk resistivity of TiO2, we were able to correlate the TiO2 thickness to the series and shunt resistances of the devices and model the variation in the photovoltaic parameters with thickness using the diode equation to gain quantitative insights.

The photovoltage of perovskite solar cells (PSCs) was studied over a wide range of light intensities, showing changes from pristine to light-soaking (LS) conditions, explained using a specific model of spatial charge distribution. Migration of ions and vacancies under photovoltage conditions results in localized charge redistribution manifested as positive charge accumulation at the TiO2 or TiO2 MgO interlayer perovskite interface, signifying photoinduced interfacial upward band bending. Consequentially, generation of an electrostatic potential (V-elec) and an increase in interfacial recombination rate are confirmed. The magnitude and effect of V-elec and interfacial recombination on the photovoltage depend on the illumination intensity and on the LS duration. PSCs with mesoporous Al2O3 showed similar changes, validating the role of the compact TiO2. Faster generation and a gradual increase of Velec are apparent under LS, which expresses the constant migration of ions and vacancies toward the interface. The nonrigid TiO2 perovskite interface calls for a vital perspective change of PSCs.

In this communication, we describe the synthesis of new chiral alumina nanofilms and surfaces. Our method is based on chiral templating of alumina nanofilms by cellulose microfibers using the atomic layer deposition process. The chiral nature of the alumina nanofilms was characterized by a variety of techniques, such as quartz crystal microbalance, chiral circular-dichroism adsorption, chiral high-performance liquid chromatography and cyclic voltammetry measurements.

In this study we systematically explored the mixed cation perovskite Cs-x(CH3NH3)(1-x)PbI3. We exchanged the A-site cation by dipping MAPbI(3) films into a CsI solution, thereby incrementally replacing the MA(+) in a time-resolved dipping process and analysed the resulting thin-films with UV-Vis, XRD, EDAX, SEM and optical depth-analysis in a high-throughput fashion. Additional in situ UV-Vis and time-resolved XRD measurements allowed us to look at the kinetics of the formation process. The results showed a discontinuity during the conversion. Firstly, small amounts of Cs+ are incorporated into the structure. After a few minutes, the Cs content approaches a limit and grains of delta-CsPbI3 occur, indicating a substitution limit. We compared this cation exchange to a one-step crystallisation approach and found the same effect of phase segregation, which shows that the substitution limit is an intrinsic feature rather than a kinetic effect. Optical and structural properties changed continuously for small Cs incorporations. Larger amounts of Cs result in phase segregation. We estimate the substitution limit of Cs(x)MA(1-x)PbI(3) to start at a Cs ratio x = 0.13, based on combined measurements of EDAX, UV-Vis and XRD. The photovoltaic performance of the mixed cation perovskite shows a large increase in device stability from days to weeks. The initial efficiency of mixed Cs(x)MA(1-x)PbI(3) devices decreases slightly, which is compensated by stability after a few days.

Drastic changes in open-circuit voltage decay (OCVD) response time in CH3NH3PbX3 perovskites have been systematically investigated in order to elucidate the dynamic properties of the interface. Under pre-illumination treatment, the decay times are reduced by orders of magnitude, but if left to rest for sufficient time, the solar cell evolves to its original decay kinetics. In order to explain these observations, we developed advanced modeling of the perovskite solar cell to obtain a realistic description of the immediate vicinity of the interface, including ionic variable concentration and accumulation of holes via degenerate statistics in the space charge region. The results reveal a large amount of majority carriers at the minority carrier extraction contact, assisted by additional ionic charge. The surface band bending related to accumulation gives an electrostatic contribution to the photovoltage in a manner governed by slow dynamics of cations at the electron-selective contact. The modeling of the interface allows us to describe the dynamics of the contact region dominated by surface charging and recombination. These phenomena also play an important role in operation conditions and current-voltage scans of the solar cell.

CONSPECTUS: Organic inorganic halide perovskites are in consensus to revolutionize the field of photovoltaics and optoelectronic devices due to their superior optical and electronic properties which are unprecedented in comparison to those of other solution processed semiconductors. These hybrid materials are used as light absorbers and also as charge carriers which makes them very versatile to be implemented and studied in a multitude of fields. Traditionally, the working paradigm in solar cells and optoelectronic devices' characterization has been that the properties of photovoltaic materials remain stable following illumination of varying times and intensities. However, recently there has been a growing number of reports on prolonged illumination-dependent physical changes in perovskite films and perovskite based devices. The changes are reversible and range from structural transformations and differences in optical characteristics, to an increase in optoelectronic properties and physical parameters. In this Account, we review the physical changes in three reported model systems which display changes under prolonged illumination of light intensities of similar to 0.01-1 sun. The three systems are (i) a free-standing perovskite film on a glass substrate, (ii) a symmetrical system with nonselective electrical contacts, and (iii) a working perovskite solar cell (either a planar or a porous structure). We examine each model system and discuss its photoinduced physical changes and conclude with the implications on future experimentation design, data analysis, and characterization that involve organic inorganic halide perovskites illumination. Since hybrid perovskites are considered to be mixed ionic electronic conductors in nature, ions that migrate in the perovskite under electrical fields can influence its properties. Therefore, an important distinction is made between photoinduced effects and photo and electric field induced effects. Thus, photoinduced effects are designated as observed effects in illuminated free-standing films or symmetrical devices without selective contacts. In contrast, photo- and electric field induced effects are designated as observed effects under open-circuit potential or during voltage scanning (internal electrical field exists across, the device). In the latter case, the two effects are superimposed and it is difficult to evaluate the relative influence of each one (light or electric field). However, we show that the magnitude and the importance of the photoinduced effect are substantial.

With solar conversion efficiencies surpassing 20%, organometallic perovskites show tremendous promise for solar cell technology. Their high brightness has also led to demonstrations of lasing and power-efficient electroluminescence. Here we show that thin films of methylammonium lead iodide, prepared by solution processing at temperatures not exceeding 100 degrees C, exhibit a highly nonlinear intensity-dependent refractive index due to changes in the free-carrier concentration and for femtosecond excitation at higher intensities undergo saturation that can be attributed to the Pauli blocking effect. Nonlinear refractive index and nonlinear absorption coefficients were obtained by the Z-scan technique, performed simultaneously in open- and closed-aperture configurations. Both nanosecond- and femtosecond-pulsed lasers at multiple wavelengths were used in order to distinguish between the mechanisms inducing the nonlinearities. The magnitude and sign of the nonlinear refractive index n(2) were determined. For resonant excitation, free carrier generation is the dominant contribution to the nonlinear refractive index, with a large nonlinear refractive index of n(2) = 69 X 10(-12) cm(2)/W being observed for resonant femtosecond pumping and n(2) = 34.4 X 10(-9) cm(2)/W for resonant nanosecond pumping. For nonresonant femtosecond excitation, bound-charge-induced nonlinearity leads to n(2) = 36 x 10(-12) cm(2)/W. These values are equivalent to the best reported metrics for conventional semiconductors, suggesting that organometallic perovskites are promising materials for optical switching and bistability applications.

Hybrid methyl-ammonium lead trihalide perovskites are promising low-cost materials for use in solar cells and other optoelectronic applications. With a certified photovoltaic conversion efficiency record of 20.1%, scale-up for commercial purposes is already underway. However, preparation of large-area perovskite films remains a challenge, and films of perovskites on large electrodes suffer from non-uniform performance. Thus, production and characterization of the lateral uniformity of large-area films is a crucial step towards scale-up of devices. In this paper, we present a reproducible method for improving the lateral uniformity and performance of large-area perovskite solar cells (32 cm(2)). The method is based on methyl-ammonium iodide (MAI) vapor treatment as a new step in the sequential deposition of perovskite films. Following the MAI vapor treatment, we used high throughput techniques to map the photovoltaic performance throughout the large-area device. The lateral uniformity and performance of all photovoltaic parameters (V-oc, J(sc), Fill Factor, Photo-conversion efficiency) increased, with an overall improved photo-conversion efficiency of similar to 100% following a vapor treatment at 140 degrees C. Based on XRD and photoluminescence measurements, We propose that the MAI treatment promotes a ``healing effect'' to the perovskite film which increases the lateral uniformity across the large-area solar cell. Thus, the straightforward MAI vapor treatment is highly beneficial for large scale commercialization of perovskite solar cells, regardless of the specific deposition method.

The high open-circuit potential (V-oc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The V-oc analysis is essential to understand their working mechanisms. A large number of CH3NH3PbI3-xClx PSCs were fabricated on single large-area substrates and their V-oc dependencies on illumination intensity, I-0, were measured showing three distinctive regions. Similar results obtained in Al2O3 based PSCs relate the effect to the compact TiO2 rather than the mesoporous oxide. We propose that two working mechanisms control the V-oc in PSCs. The rise of V-oc, at low I-0 is determined by the employed semiconductor n-type contact (TiO2 or MgO coated TiO2). In contrast, at I-0 close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated E-Fn (as in the dye sensitized solar cells) to an E-Fn, directly determined by the CH3NH3PbI3-xClx absorber is suggested.

In the pursuit to better understand the mechanisms of perovskite solar cells we performed Raman and photoluminescence measurements of free-standing CH3NH3PbI3 films, comparing dark with working conditions. The films, grown on a glass substrate and sealed by a thin glass coverslip, were measured subsequent to dark and white-light pretreatments. The extremely slow changes we observe in both the Raman and photoluminescence cannot be regarded as electronic processes, which are much faster. Thus, the most probable explanation is of slow photoinduced structural changes. The CH3NH3PbI3 transformation between the dark and the light structures is reversible, with faster rates for the changes under illumination. The results seem to clarify several common observations associated with solar cell mechanisms, like performance improvement under light soaking. More important is the call for solar-cell-related investigation of CH3NH3PbI3 to take the photoinduced structural changes into consideration when measuring and interpreting the results.

Photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes show extremely slow response. The CH3NH3PbI3, bridging a gap of similar to 2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

Band gap localized states and surface states play a dominant role in the application of nanocrystalline metal oxides to photovoltaics and solar fuel production. Electrons injected in nanocrystalline TiO2 by voltage or photogeneration are mainly located in band gap states. Therefore, charging a nanoparticulate semiconductor network allows one to recover the density of states (DOS) in the energy axis. However, shallow traps remain in equilibrium with the conduction band electrons, while deep traps do not. We show that the characteristic peak of the apparent DOS mixes an exponential DOS and a monoenergetic surface state. A model that incorporates the trap's kinetics proves to be very efficient to assess the important parameters that determine both contributions via variation of charging rate. Contrary to the common theory, we demonstrate that the peculiar capacitance peak of nanocrystalline TiO2 can be mainly attributed, in some cases, to deep traps in the exponential distribution.

In photoelectrochemical cells, one major recombination pathway involves a reaction between the photogenerated electrons that diffuse inside the semiconductor electrode and holes, in the form of oxidized ions, which travel in the electrolyte to the counter electrode. Here we present direct imaging of the recombination/reduction sites in two types of porous TiO2 electrodes, P25 and submicrometer particles, chosen for studying the influence of the TiO2 particles' sizes and shapes on the recombination sites. The sites were labeled with 2-5 nm silver particles, electrodeposited on the TiO2 surface using chronoamperometry. The model assumes that reduction and recombination are similar with respect to the electron transfer from the TiO2 surface to an ionic electron acceptor in the electrolyte redox mediator/Ag+ ion. Consequently the metal deposit marks the reaction locations. This first high-resolution view clearly identifies the connecting points between TiO2 particles and then the {101} facets as the sites of recombination.

Silver nanowires and nanoparticles are synthesized by a polyol method in a millifluidic reactor. We have been able to optimize the flow chemistry reaction conditions to get a high yield of nanowires in a continuous flow. By changing reaction parameters we have demonstrated the synthesis of single crystalline silver nanoparticles in a rapid reaction time of only 3 minutes. All results are compared with standard batch and microwave reactions. An example of application is provided through the silver nanowire assisted etching of silicon wafers. This colloidal approach of metal assisted silicon etching allows transferring of the nanowire shape to silicon.