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We present a high-throughput (combinatorial) method to screen thin ceramic films as Li-ion conductors by mapping an optical effect of Li-ion conduction. The method, while qualitative, is fast and simple to implement, provides a planar (XY) map of Li-ion conductivity through different parts of the film. The effect, FTO darkening, is an optoelectrochemical one that relies on darkening of the FTO (F-doped Tin Oxide) substrate, onto which the investigated film is deposited. The rate of color change of the FTO reflects the rate of Li-ion migration through the film. The method is validated by testing two model systems, a Li-La-S-O film with uniform composition and varying thickness, and a Li-La-P-O film with varying thickness and lateral composition. The darkening rate, obtained from optical transmission, correlates linearly with inverse film thickness. The darkening rate map can be compared with a resistance map obtained by impedance measurements, showing that only Li conduction is measured. We discuss the con...

Among the commercially common transparent conducting oxides (TCOs) are fluorine-doped tin oxide (FTO) and indium-doped tin oxide (ITO), neither of which meets all criteria for the optimal TCO. Despite its superior chemical stability and being composed of abundant elements, FTO suffers from high surface roughness compared to ITO. Here, we introduce a path to substantially decrease the surface roughness of FTO, while preserving most of its original advantages, by depositing an SnO2 coating on top of the FTO layer using pulsed laser deposition. Such an enhancement may allow future use of FTO in devices that use now the more expensive, less stable ITO, which contains relatively rare indium.

Cadmium telluride (CdTe) quantum dots are integrated with mesoporous titanium dioxide “TiO2” electrode using an in-situ sensitization approach for the first time, where water soluble N-Acetyl Cysteine capped CdTe quantum dots (diameter 4–5 nm) are grown hydrothermally inside mesoporous TiO2 electrode matrix. This in-situ sensitization approach has shown the effective sensitization over the conventional reported sensitization processes. The in-situ CdTe sensitized TiO2 electrodes are further unified with PbS counter electrode to realize the quantum dot sensitized solar cell (QDSSC). The fabricated CdTe QDSSC has shown the optimal short circuit current density 3.35 ± 0.21 mA/cm2 and open circuit voltage 0.58 ± 0.01 V. The observed current density is the highest among such cell configurations, reported till date. The in-situ sensitized CdTe QDs are further treated with zinc sulfide for surface passivation to realize the reduced back recombination, if any. These ZnS passivated CdTe QDs sensitized solar cells in similar device configurations resulted into the significant reduction in the short circuit current density (20%) with the minimal change in open circuit voltage (5%). The impedance spectroscopy measurements suggest that the transmission resistance “Rtr” has increased after ZnS treatment for these electrodes, however, the recombination resistance Rrec has not changed much. The observed high Rtr might be responsible for the observed current deterioration after ZnS surface passivation. These studies suggest that in-situ QD sensitization may provide a new method of integrating QDs into mesoporous TiO2 matrix for effective QDSSC device performance without any additional surface passivation.

In this work, we describe the formation of a reduced bandgap CeNiO3 phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO3 phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of CexNi1–xOy is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce–Ni–O materials. The library displays a reduced bandgap throughout, calculated to be 1.48–1.77 eV, compared to the starting materials, CeO2 and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO3 phase is realized. The CeNiO3 reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured ...

Combinatorial materials science is a powerful approach to discover new materials, especially by using the continuous compositional spread (CCS) method, which forms spatially varying stoichiometry across a sample. Though the chemical composition of the candidate materials is typically the primary parameter studied, in the case of metal oxides CCS the oxygen concentration is usually either neglected or studied in a discrete and non-combinatorial manner. The present work reports the use of oxygen concentration as a combinatorial parameter that varies continuously across a sample, using a pulsed laser deposited (PLD) SnO2 film as a model system. As the oxygen concentration decreases, the SnO2 crystal lattice expands, the number of defects is increased, and the electrical conductivity rises exponentially. A relatively low electrical resistivity of 8.16∙10−4 Ω cm is achieved. The sample also showed superior infrared transparency, 67% at 2000 nm, compared to commercial F:SnO2 (FTO) which is only 12% transparent at this wavelength. The improved transparency and conductivity were achieved within a single experiment, without any additional optimization steps, and with further improvement may allow reconsideration of SnO2 as a transparent conductive oxide. Our findings serve as a demonstration for the importance of oxygen concentration as a combinatorial parameter.

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 × 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 ∼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 (Velec) and an increase in interfacial recombination rate are confirmed. The magnitude and effect of Velec 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.

The dynamic hysteresis of perovskite solar cells consists of the occurrence of significant deviations of the current density-voltage curve shapes depending on the specific conditions of measurement such as starting voltage, waiting time, scan rate, and other factors. Dynamic hysteresis is a serious impediment to stabilized and reliable measurement and operation of the perovskite solar cells. In this Letter, we formulate a model for the dynamic hysteresis based on the idea that the cell accumulates a huge quantity of surface electronic charge at forward bias that is released on voltage sweeping, causing extra current over the normal response. The charge shows a retarded dynamics due to the slow relaxation of the accompanying ionic charge, that produces variable shapes depending on scan rate or poling value and time. We show that the quantitative model provides a consistent description of experimental results and allows us to determine significant parameters of the perovskite solar cell for both the transient and steady-state performance.

We investigate the significance of indium (In) doping of Co3O4 in the operation of TiO2|Co–In–O|RuO2 all-oxide solar cells by employing combinatorial experiments and density functional theory (DFT) calculations. We observed an increase in the open-circuit voltage, Voc, of more than 240 mV with an enhancement by a factor of 4 in the short-circuit current, Jsc, in the low-doping range. This constitutes a maximum power that is five times greater than that of pure Co3O4-based photovoltaic (PV) devices. Surprisingly, a concurrent marginal change in the band gap and a decrease in the optical absorption coefficient as a function of indium concentration was observed, contrary to what has been assumed previously. Using DFT in conjunction with joint density of states calculations, we show that with increasing amounts of In, there is a reduction in the low-energy photon absorption due to disallowed electronic transitions. Moreover, we show that emergence of In 5s states results in a free-electron-like band in the co...

Summary Drastic changes in open-circuit voltage decay (OCVD) response time in CH 3 NH 3 PbX 3 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.

Recently, metal oxides have been gaining large interest in the field of renewable energy. This fact is mostly related to their properties which make them suitable for long lasting, cost efficient and environmentally friendly devices. Dimanganese trioxide (Mn2O3) was investigated in the past in different fields, yet, the fabrication of the film in most cases is not straightforward and requires several steps, consequently there is insufficient data provided in literature regarding this material. This paper presents a single step deposition of dimanganese trioxide (Mn2O3) thin films by spray pyrolysis on glass substrates at temperatures lower than 450 °C. The process was conducted under atmospheric conditions, making it an ideal and cost effective fabrication technique for large scale manufacture. The obtained Mn2O3 film was characterized chemically and physically. The film X-ray diffraction analysis reveals a crystalline phase of α-Mn2O3 in a bixbyite-like structure. Physical characterization included optical absorptance, band gap analysis, energy band diagram, and four-point probe resistivity measurement. The film exhibits a band gap of 1.4 eV making it a good candidate for applications such as solar cells.

Here we investigate the impact of four different metal back contacts on the photovoltaic (PV) performance of Co3O4 thin film all-oxide photovoltaic cells. A combinatorial TiO2|Co3O4 heterojunction thin film device library was made with thickness gradients for both metal oxide layers. Grids of four different metal back contacts were then deposited on top of these layers. A significant effect of the metal back contacts on the final photoconversion performance has been observed by combinatorial PV measurements. We analyze these results via advanced numerical simulations and different scenarios in order to explain the recombination mechanisms at the different back contacts. We conclude that the nature of the back contact material controls the density of surface states and, therefore, the undesirable surface recombination at the absorber–back contact interface. This project has received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No 309018. Funding from the European Union Seventh Framework Program [FP7/2007-2013] under grant agreement 316494 is also acknowledged.

The present work shows the effect of magnesium doping on structural, optoelectrical and electrical properties of Cu2O thin films prepared by spray pyrolysis. The variation in the concentration of Mg shows significant impact on the final thin film properties, whereas the film doped with 0.5 at% of Mg exhibited major property improvements in comparison with the undoped thin film and among the other concentrations tested. This condition was further applied for the deposition of an absorber layer in a heterojunction solar cell array with a gradient in thicknesses of active layers to investigate the impact of changing thicknesses on the PV parameters of the solar cell. TiO2 was used as a window layer and the 0.5 at% Cu2O doped film as an absorber layer. The produced heterojunction solar cell array was further exposed to a rapid thermal annealing treatment. The I–V measurements show an open circuit voltage of up to 365 mV and a short circuit current density, which is dependent on absorber layer thickness, and reaches to a maximum value of 0.9 mA/cm2.

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 °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 n2 were determined. For resonan...

Oxide materials have been widely investigated for use as absorber layers in high-performance solar cells, and in this study combinatorial methods were used to specifically investigate cobalt–iron (Co–Fe) oxide composites. Structural inversion of Co–Fe oxides from a normal to an inverse spinel structure occurs at the critical composition of approximately 33 % Fe added to Co, causing changes in the crystallinity, symmetry, sub-lattice vibrational modes, optical bandgap, and electrical resistivity. When used as an absorber layer in all-oxide solar cells with the multi-layered geometry of glass|FTO|TiO2|Co–Fe–O|Au, enhanced photovoltaic performance was observed, with a maximum Voc of 534 mV at a composition of approximately 45 % Fe and a 200 % improvement in Pmax compared to cobalt-rich devices. Using combinatorial data maps of the various material properties, a significant correlation between the solar cell properties and the chemical composition of the Co–Fe–O layer was revealed. This correlation allows for a better understanding of the Co–Fe–O system, which is a necessary step towards the development of Co–Fe–O-based all-oxide solar cells.

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 ∼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.

In this study we systematically explored the mixed cation perovskite Csx(CH3NH3)1-xPbI3. We exchanged the A-site cation by dipping MAPbI3 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 δ-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 CsxMA1-xPbI3 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 CsxMA1-xPbI3 devices decreases slightly, which is compensated by stability after a few days.

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 ∼100% following a vapor treatment at 140 °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.

In this work, we describe the formation of a reduced bandgap CeNiO

The high open-circuit potential (Voc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The Voc analysis is essential to understand their working mechanisms. A large number of CH3NH3PbI3-xClx PSCs were fabricated on single large-area substrates and their Voc dependencies on illumination intensity, I0, 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 Voc in PSCs. The rise of Voc at low I0 is determined by the employed semiconductor n-type contact (TiO2 or MgO coated TiO2). In contrast, at I0 close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated EFn (as in the dye sensitized solar cells) to an EFn, directly determined by the CH3NH3PbI3-xClx absorber is suggested.

Growth in energy demands, coupled with the need for clean energy, are likely to make solar cells an important part of future energy resources. In particular, cells entirely made of metal oxides (MOs) have the potential to provide clean and affordable energy if their power conversion efficiencies are improved. Such improvements require the development of new MOs which could benefit from combining combinatorial material sciences for producing solar cells libraries with data mining tools to direct synthesis efforts. In this work we developed a data mining workflow and applied it to the analysis of two recently reported solar cell libraries based on Titanium and Copper oxides. Our results demonstrate that QSAR models with good prediction statistics for multiple solar cells properties could be developed and that these models highlight important factors affecting these properties in accord with experimental findings. The resulting models are therefore suitable for designing better solar cells.

Data mining tools have been known to be useful for analyzing large material data sets generated by high-throughput methods. Typically, the descriptors used for the analysis are structural descriptors, which can be difficult to obtain and to tune according to the results of the analysis. In this Research Article, we show the use of deposition process parameters as descriptors for analysis of a photovoltaics data set. To create a data set, solar cell libraries were fabricated using iron oxide as the absorber layer deposited using different deposition parameters, and the photovoltaic performance was measured. The data was then used to build models using genetic programing and stepwise regression. These models showed which deposition parameters should be used to get photovoltaic cells with higher performance. The iron oxide library fabricated based on the model predictions showed a higher performance than any of the previous libraries, which demonstrates that deposition process parameters can be used to model photovoltaic performance and lead to higher performing cells. This is a promising technique toward using data mining tools for discovery and fabrication of high performance photovoltaic materials.

Significant overpotentials between the sensitizer and both the electron and hole conductors hamper the performance of sensitized solar cells, leading to a reduced photovoltage. We show that by using properly designed type-II quantum dots (QDs) between the sensitizer and the hole conductor in thin absorber cells, it is possible to increase the open circuit voltage (Voc) by more than 100 mV. This increase is due to the formation of a photoinduced dipole (PID) layer. Photogenerated holes in the type-II QDs are retained in the core for a relatively long time, allowing for the accumulation of a positively charged layer. Negative charges are, in turn, injected and accumulated in the TiO2 anode, creating a dipole moment, which negatively shifts the TiO2 conduction band relative to the electrolyte. We study this phenomenon using a unique TiO2/CdSe/(ZnSe:Te/CdS)/polysulfide system, where the formation of a PID depends on the color of the illumination. The PID concept thus introduces a new design strategy, where the operating parameters of the solar cell can be manipulated separately.

Here we present a comprehensive investigation of TiO2–Cu2O hetero-junction solar cells with different back contacts (Au, ITO, Cu or Ag). Combinatorial hetero-junction libraries consisting of a linear TiO2 thickness gradient produced by spray pyrolysis and a bell shaped Cu2O profile synthesized by pulsed laser deposition were chosen to investigate the impact of the two metal oxide layer thicknesses. The back contacts were deposited as round patches onto a grid of 13 × 13 points, 169 contacts for each contact material, forming a library containing 4 × 13 × 13 = 676 back contacts. Each back contact represented a solar cell with an individual TiO2 and Cu2O thickness. I–V measurements show that all four materials provide an ohmic contact and that the open circuit voltage of ∼300 mV is rather independent of both layer thicknesses and contact material. The size of the Cu2O crystals drastically decreases with distance from the center of deposition, which leads to a drastic increase of series resistance when the crystal size is

All-oxide-based photovoltaics (PVs) encompass the potential for extremely low cost solar cells, provided they can obtain an order of magnitude improvement in their power conversion efficiencies. To achieve this goal, we perform a combinatorial materials study of metal oxide based light absorbers, charge transporters, junctions between them, and PV devices. Here we report the development of a combinatorial internal quantum efficiency (IQE) method. IQE measures the efficiency associated with the charge separation and collection processes, and thus is a proxy for PV activity of materials once placed into devices, discarding optical properties that cause uncontrolled light harvesting. The IQE is supported by high-throughput techniques for bandgap fitting, composition analysis, and thickness mapping, which are also crucial parameters for the combinatorial investigation cycle of photovoltaics. As a model system we use a library of 169 solar cells with a varying thickness of sprayed titanium dioxide (TiO2) as the window layer, and covarying thickness and composition of binary compounds of copper oxides (Cu-O) as the light absorber, fabricated by Pulsed Laser Deposition (PLD). The analysis on the combinatorial devices shows the correlation between compositions and bandgap, and their effect on PV activity within several device configurations. The analysis suggests that the presence of Cu4O3 plays a significant role in the PV activity of binary Cu-O compounds.

We present combinatorial method for high-throughput screening and synthesis of libraries of solid lithium-ion conductors. The method has the advantages of speed, simplicity and the capability to produce and control simultaneously a variety of compositions by the co-deposition on a substrate of several different materials to create “materials library”. In our research we fabricate materials libraries by two methods. The first one is RF-magnetron sputtering, where Ar plasma sputters a target material that is being deposited on a substrate. The effects of RF power applied to Li2O and LiAlSiO2 targets, the deposition time, rotation of the substrate and heat treatment on Li-ion conductivity of 169 samples of one library have been studied. The XPS tests showed that RF coating contains Li2O, Li2CO3, LiAl0.18Si0.13O1.22, LiAl0.67Si0.41O2.49 and LiAl1.17Si0.71O3.93 compounds. The non-stoichiometric lithium aluminum silicates are amorphous as displayed in the XRD patterns. The room-temperature conductivity of sputtered about 350nm-thick LiAlxSiyOz film varies from 10-8 to 10-7S/cm depending on the stoichiometry. The conductivity vs. temperature plots show Arrhenius behavior. The activation energy found for the bulk conductivity is 22kJ/mol and for the grain boundaries- 55kJ/mol. The substrate morphology influences the fabricated film morphology and grain boundaries conductivity. The second method is spray pyrolysis, based on simultaneous spraying of different precursors solutions from a multi-head nozzle on a substrate. This is followed by pyrolysis and oxidation. The preparation of multi-component solid electrolytes library by spray pyrolysis is a novel approach. Despite relatively low density of the LixLayPO4 ceramics, preliminary ionic conductivities, were found to vary from 2*10-7 to 4*10-5 S cm−1.

In recent years, quantum dot-sensitized solar cells (QDSSCs) have emerged as attractive candidates for constructing efficient third-generation photoelectrochemical solar cells. Despite a starting point of relatively low performing solar cells, we have been witnessing a boost in scientific research conducted both from the material and the physical points of view, leading to a huge leap in our understanding of the operational mechanisms of QDSSCs followed by a significant improvement of their conversion efficiencies to about 7%. In this feature article, we give an overview of the four main materials and interfaces constructing the QDSSC: (1) sensitizer materials, (2) TiO2/QDs/electrolyte interface, (3) redox electrolyte, and (4) counter electrode. We focus on the scientific challenges associated with each one of the materials/interfaces while highlighting the recent advances achieved in overcoming those obstacles. Finally, we discuss possible future directions for this field of research with an aim toward highly efficient QD-sensitized solar cells.

Using photoelectron spectroscopy, the interface formation of anatase and rutile TiO2 with RuO2 and tin-doped indium oxide (ITO) is studied. It is consistently found that the valence band maximum of rutile is 0.7 ± 0.1 eV above that of anatase. The alignment is confirmed by electronic structure calculations, which further show that the alignment is related to the splitting of the energy bands formed by the O 2pz lone-pair orbitals. The alignment can explain the different electron concentrations in doped anatase and rutile and the enhanced photocatalytic activity of mixed phase particles.

Recently, a new field in photovoltaics (PV) has emerged, focusing on solar cells that are entirely based on metal oxide semiconductors. The all-oxide PV approach is very attractive due to the chemical stability, nontoxicity, and abundance of many metal oxides that potentially allow manufacturing under ambient conditions. Already today, metal oxides (MOs) are widely used as components in PV cells such as transparent conducting front electrodes or electron-transport layers, while only very few MOs have been used as light absorbers. In this Perspective, we review recent developments of all-oxide PV systems, which until today were mostly based on Cu2O as an absorber. Furthermore, ferroelectric BiFeO3-based PV systems are discussed, which have recently attracted considerable attention. The performance of all-oxide PV cells is discussed in terms of general PV principles, and directions for progress are proposed, pointing toward the development of novel metal oxide semiconductors using combinatorial methods.

Combinatorial quest for cost effective - environment friendly all oxide photovoltaics raised several outliers that didn't match the anticipated p-n junction mechanisms. Further investigation showed that spontaneous nanostructures forming at the back contacts are the source of Surface Plasmon Resonance, causing Hot Electron injection and PV activity.

In this paper we present the design proposal, fabrication procedure as well as the numerical simulation of a new type of device which is capable of converting an incoming electrical field into a radially polarized field. The device consists from a tapered optical fiber while on top of its external surface, dielectric nanorods are radially disposed. The tails of the propagating optical mode are interacting with the radially disposed nanorods. This interaction enhances the generated radially polarized field which propagates along the optical fiber.

Here, we show that, in CdSe quantum dot sensitized solar cells (QDSCs), recombination of electrons from the transparent conducting oxide (TCO) front electrode with oxidized species of the polysulfide redox electrolyte cannot be neglected like in dye-sensitized solar cells (DSCs). We demonstrate that light to electric power conversion efficiencies up to 4% can be achieved when recombination at the front electrode is suppressed by a compact TiO2 layer deposited in between the TCO substrate and the QD sensitized porous TiO2 film. Numerical simulations based on a simple equivalent circuit suggest that, over a wide potential range, electron transfer into the electrolyte at the TCO substrate is the dominant recombination path, which is usually not considered, suggesting that the current understanding of QDSCs has to be revised.

In this paper we present a new type of optical fiber aiming to radially polarize the electrical field. This special device is composed of a tapered fiber having a classical core while on top of its external surface, radially oriented nanorods are disposed. The tails of the propagated wave are interacting with those radially oriented nanorods. This interaction performs the polarization of the propagated mode into a radial polarization.

In this work we present three novel viologen-type electrochromic compounds, which turn into red, green and blue (RGB) upon electrochemical reduction on porous titania surface. The synthesis of the three compounds involved attaching benzyl group with phosphonic acid residue as an anchor to 4,4′-bipyridine, 3,3′-dimethyl-4,4′-bipyridine and 4,4′,4″,4′″-thieno-2,3,4,5-tetrayltetrapyridine. The sterically demanding benzyl groups help to decrease the strong propensity of the colored radical cations to aggregation. The electrochromic derivatives were adsorbed on the surface of porous titania layers prepared by electrophoretic deposition onto a fluorine-doped tin oxide (FTO) glass followed by mechanical compression. Finally the electrodes with the grafted electrochromic derivatives were glued to a second FTO glass to form a closed cell, which operates with two electrodes. The electrochromic windows exhibited high contrast, fast response and low current consumption (∼1 mA) when relatively low voltage (∼5 V) was applied.

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.

In the current work, pristine α-Fe2O3 metal oxide was doped with Mg in an attempt to modulate its electronic properties. To this end, we employed an experimental high throughput strategy, including scanning XRD and optical spectroscopy, which were complimented by atomistic density functional theory (DFT) calculations. The combined study reveals that at Mg/Fe atomic ratios up to ∼1/3, the bandgaps of the hematite-Mg composite materials are similar to that of the pure material. The observed bandgaps are rationalized by electronic band structure and density of states calculations. Additional rationale for the similar bandgaps in pure and doped hematite is provided by topological Bader charge analyses, which indicate that the Mg and Fe ions in the hematite matrix have similar partial atomic charges. Nonetheless, the small charge density difference between the Mg and Fe ions induces a slight spin polarization on both oxygen and Fe ions, resulting in changes in the band edges. Further charge density analyses, using charge density maps and chemical-bonding analyses with the crystal orbital Hamiltonian population scheme, indicate that Mg forms ionic bonds with the neighboring oxygen atoms. This change from iron-oxygen covalent bonds to a more ionic nature for magnesium-oxygen bonds is probably responsible for the reduction observed in the computed bulk modulus of α-Mg(0.17)Fe(1.83)O3 (193 GPa) compared to α-Fe2O3 (202 GPa).

Inorganic quantum dots (QDs) show great potential as absorbers in sensitized solar cell, but there are open questions about the role of quantum dots, where primary electron and hole generation occurs, in the recombination process in this kind of solar cells. In opposition to the conventional dye-sensitized solar cell, here we show that inorganic QDs play a direct role in the recombination process. This fact has been determined by a fingerprint of QDs in the capacitance of the device, where the QDs surface states affect the density of states (DOS) distribution. It indicates that now surface states of QD contribute to the common DOS distribution of TiO2/QDs/ZnS, which behaves as a single entity, being impossible to distinguish between TiO2 and QDs. This result highlights the necessity of treating (and optimizing) QD-sensitized solar cells from another perspective than dye-sensitized solar cells, considering the fundamental differences in their behavior.

Here we show that in dye-sensitized solar cells a lanthanum post treatment of mesoporous TiO2 electrodes increases either the photovoltage or the photocurrent, depending on the solution pH. With an electrode treatment at pH 7, the photovoltage increase is compensated by a decrease in the photocurrent, leading to slightly reduced conversion efficiency. With an acidic post treatment, the photocurrent increases by 20–25% while the photovoltage remains unchanged. As a result, an improvement of the light to electric power conversion efficiency from 5.6 to 7.0% is achieved. The same behavior is observed when lanthanum acetate is added to the commonly used TiCl4 post-treatment, where the observed efficiency increase is significantly higher compared with TiCl4-treated electrodes without lanthanum.

The use of Forster resonant energy transfer (FRET) has recently shown promise for significant improvement in various aspects of photoelectrochemical cells. Considering the particular case of semiconductor quantum dot donors, we show that they can enable broadening of the spectral response and increased optical density of the cell, thus increasing the current while potentially decreasing the electrode thickness. Moreover, the use of FRET and the separation of optical and electrical function within the cell provide flexibility in the choice of materials for both the antenna and sensitizer, opening new paths for performance optimization and achievement of long-term cell stability.

In this article, the physics of FRET is demonstrated for an architecture of dye-sensitized solar cells, in which the quantum dot “antennas” that serve as donors are incorporated into the solid titania electrode, providing isolation from electrolyte quenching, and potentially increased photostability. The energy transferred to the dye acceptor from the quantum dot donor, in addition to the direct light absorption by the dye, finally induce dye excitation and electron injection to the metal oxide semiconductor electrode. We use time-resolved photoluminescence measurements to directly show achievement of FRET efficiencies of up to 70%, corresponding to over 80% internal quantum efficiency when considering radiative energy transfer as well. The various parameters governing the FRET efficiency and the requirements for high efficiency FRET-based cells are discussed. Since both buried donors inside the electrode and donors solubilized in the electrolyte have both been shown to achieve high energy transfer efficiencies, and as the two methods take advantage of different available volumes of the electrode to introduce donors providing the excess absorption, synergy of the two methods is highly promising for achieving panchromatic absorption within a thin electrode.

A unique concept of photoreference electrode developed for thin photoelectrochemical solar cells enables three-electrode characterization of quantum dot sensitized solar cells (QDSSCs) under operat...

We show that a significant improvement of the light to electric power conversion efficiency of quantum dot sensitized solar cells with a polysulfide electrolyte can be achieved when a PbS counter electrode is used. An acid pretreatment of a Pb metal foil followed by dipping into polysulfide solution provides a counter electrode which improves the short circuit current, open circuit voltage, and fill factor compared to commonly used Pt electrodes. Electrochemical impedance spectroscopy reveals a low charge transfer resistance between polysulfide and PbS, indicating the high catalytic activity of PbS. Moreover, a comparison with reported charge transfer resistance values at open circuit potential for alternative counter electrodes for polysulfide shows that PbS is the most catalytic of all. For a CdSe quantum dot sensitized mesoporous TiO2 electrode, we achieved a conversion efficiency of 3% using PbS as a counter electrode.

Quantum dot sensitized solar cells (QDSSC) may benefit from the ability to tune the quantum dot optical properties and band gap through the manipulation of their size and composition. Moreover, the inorganic nanocrystals may provide increased stability compared to organic sensitizers. We report the facile fabrication of QDSSC by electrophoretic deposition of CdSe QDs onto conducting electrodes coated with mesoporous TiO(2). Unlike prior chemical linker-based methods, no pretreatment of the TiO(2) was needed, and deposition times as short as 2 h were sufficient for effective coating. Cross-sectional chemical analysis shows that the Cd content is nearly constant across the entire TiO(2) layer. The dependence of the deposition on size was studied and successfully applied to CdSe dots with diameters between 2.5 and 5.5 nm as well as larger CdSe quantum rods. The photovoltaic characteristics of the devices are greatly improved compared with those achieved for cells prepared with a linker approach, reaching efficiencies as high as 1.7%, under 1 sun illumination conditions, after treating the coated electrodes with ZnS. Notably, the absorbed photon to electron conversion efficiencies did not show a clear size-dependence indicating efficient electron injection even for the larger QD sizes. The electrophoretic deposition method can be easily expanded and applied for preparations of QDSSCs using diverse colloidal quantum dot and quantum rod materials for sensitization.

Type II dye-sensitized solar cells (DSSCs) differ from conventional DSSCs by their mechanism of light absorption and electron injection. Instead of photoexcitation of the dye, followed by electron injection to the semiconductor conduction band, in type II DSSCs, there is a direct electron injection from the HOMO level of the sensitizer into the conduction band of the semiconductor. The main drawback of such cells is their extremely rapid back-electron-transfer rate. Herein, we present a new approach for inhibiting back electron transfer in a catechol-sensitized type II DSSC using a thin layer barrier coating of SrTiO3 between the semiconductor and the sensitizer. A 70% improvement in charge collection efficiency is reported. A proposed mechanism for the operation of the SrTiO3 barrier layer is presented.

A new design of dye-sensitized solar cells involves colloidal semiconductor quantum dots that serve as antennas, funneling absorbed light to the charge separating dye molecules via nonradiative energy transfer. The colloidal quantum dot donors are incorporated into the solid titania electrode resulting in high energy transfer efficiency and significant improvement of the cell stability. This design practically separates the processes of light absorption and charge carrier injection, enabling us to optimize each of these separately. Incident photon-to-current efficiency measurements show a full coverage of the visible spectrum despite the use of a red absorbing dye, limited only by the efficiency of charge injection from the dye to the titania electrode. Time resolved luminescence measurements clearly relate this to Forster resonance energy transfer from the quantum dots to the dye. The presented design introduces new degrees of freedom in the utilization of quantum dot sensitizers for photovoltaic cells. In particular, it opens the way toward the utilization of new materials whose band offsets do not allow direct charge injection.

A dye-sensitized solar cell (DSSC) fabricated inside a glass tube to form a dye-sensitized solar tube (DSST) is presented. We developed the synthesis of Fluorine-doped Tin oxide (FTO) with a high optical transmittance and low sheet resistance, which was deposited inside a glass tube by spray pyrolysis. The FTO was covered with a mesoporous TiO 2 film using electrophoretic deposition (EPD). Subsequently the tube was sintered, sensitized with a Ru-dye (N3) and immersed into redox electrolyte while a Pt-coated rod closed the electrical circuit. Sealing of DSSCs is still a major problem. The new design reduces significantly the area that requires sealing compared to conventional flat cells, moreover sealing of a tube is significantly simpler than two flat plates. A huge advantage of the tube design is the possibility to incorporate a current collector without blocking direct sunlight from entering the cell. Panels consisting of DSSTs allow air flow in between the tubes, thus decreasing the wind resistivity while the cylindrical shape of the DSST can be used to collect diffuse light more effectively. Initial results on a prototype show a light to electric power conversion efficiency of 2.8%, demonstrating the feasibility of the new tube concept.

Here, a new method based on sol–gel electrophoretic deposition to produce uniform high-quality inorganic conformal coatings on mesoporous nanoparticulate films is presented. This novel sol preparation method allows for very fine control of the coating properties, thus inducing new adjustable functionalities to these electrodes. It is shown that the deposition of an amorphous TiO2 and/or MgO shell onto photoanodes used in dye-sensitized solar cells (DSSCs) improves their light-to-electric-power conversion efficiency without the need for sintering. It is proposed that the amorphous TiO2 coating improves the electronic inter-particle connection and passivates the surface states. The insulating MgO coating further reduces the electron transfer from the conduction band into the electrolyte while the electron injection from the excited dye state remains unperturbed for thin coatings. Using a low-temperature method for DSSC production on plastic substrates, a maximum efficiency of 6.2% applying pressure together with an optimized TiO2 coating is achieved. For systems that cannot be pressed a conversion efficiency of 5.1% is achieved using a double shell TiO2/MgO coating.

We demonstrate herein the coating of dielectric surfaces with metallic thin films using the plasma-reduction of metallic ions such as HAuCl 4 , AgNO 3 , H 2 PtCl 6 , PdCl 2 , Ni(NO 3 ) 2 and Co(NO 3 ) 2 in solutions. Stable plasma was formed using 12 MHz/18 W and 2.45 GHz/900 W plasma generators and was confined in a Pyrex vacuum chamber with an inert gas environment. Depending on the controlled parameters, the fabricated films are either continuous or granular with grains of nano-size dimensions. All samples were chemically analyzed using X-ray diffraction analysis, and were morphologically observed using High Resolution Electron Beam microscopy. As an example of their electrical conductivity, Van der Pauw resistivity measurements were carried out for a gold layer at a wide temperature range. The smooth resistivity signal spreading at this range indicates that the conductive layer is stable and electrically continuous. An estimation of the adhesion of the deposited films was conducted by resistance measurements, followed by an ultrasonic bath treatment. Our results indicate no change in the resistance after this treatment, namely, a good adhesion of metallic particles to the glass surface has been achieved. The fact that this method is fast, simple, and low in cost (compared to other methods like sputtering or cvd), and can be applied for coating flat, rough, and porous surfaces can indicate its importance in future applications.