Photophysical and Photoelectrochemical Properties of CsPbBr3 Films Grown by Electrochemically Assisted Deposition.

Perovskite have had a great impact on the solid-state physics world in the last decade not only achieving great success in photovoltaics but, more recently, also in the implementation of other optoelectronic devices. One of the main obstacles for the adoption of Pb-based perovskite technologies are the high amounts of Pb needed in the conventional preparation methods. Here we present for the first time a detailed analysis of the photophysical and photoelectrochemical properties of CsPbBr3 films directly grown on fluorine-doped tin oxide (FTO) coated glass through a novel technique based in the electrodeposition of PbO2 as CsPbBr3 precursor. This technique allows to save up to 90 % of the Pb used compared to traditional methods and can be scalable compared with the commonly used spin-coating process. The low temperature analysis of their photoluminescence spectra, performed in both steady state and time dependence, revealed a strong interaction between electrons and longitudinal optical (LO) phonons dominant at high temperatures. On the other hand, the electrochemical and photoelectrochemical analysis proves that CsPbBr3 prepared using this new method has state-of-the-art features, showing a p-type behavior under depletion regime. This is also confirmed by photoelectrochemical measurements using p-benzoquinone as target molecule. These results prove that the proposed method can be used to produce excellent CsPbBr3 films, saving much of the lead waste.

Understanding the Interfaces between Triple-Cation Perovskite and Electron or Hole Transporting Material.

The properties of efficient solar cells fabricated with triple-cation perovskite placed between a mesoporous titania layer and a spiro-OMeTAD layer are studied by using devices either prepared under water-free drybox conditions or fabricated under ambient room humidity. The morphological studies indicate that the content of unreacted PbI2 phase in the perovskite structure is much higher near the interface with titania than near the interface with spiro-OMeTAD. The stationary emission spectra and transient bleach peaks of perovskites show additional long-wavelength features close to the titania side. Time-resolved techniques ranging from femtoseconds to seconds reveal further differences in charge dynamics at both interfaces. The population decay is significantly faster at the titania side than at the spiro-OMeTAD side for the cells prepared under ambient conditions. An increased hole injection rate correlates with higher photocurrent seen in the devices prepared under drybox conditions. The charge recombination loss on the millisecond time scale is found to be slower at the interface with titania than at the interface with spiro-OMeTAD. The ideality factor of the cells is found to increase with increasing DMSO content in the precursor solution, indicating a change in recombination mechanism from bulk to surface recombination. We also found that the charge dynamics are not uniform within the whole perovskite layer. This feature has significant implications for understanding the operation and optimizing the performance of solar devices based on mixed cation perovskites.

Correlation between the Effectiveness of the Electron-Selective Contact and Photovoltaic Performance of Perovskite Solar Cells.

Metal halide perovskites (MHPs) are mixed electronic-ionic semiconductors with a remarkable photovoltaic potential that has led to a current world record efficiency surpassing 23%. This good performance stems from the combination of excellent light harvesting and relatively slow nonradiative recombination, which are characteristic of MHPs. However, taking advantage of these properties requires electron and hole transport materials that can efficiently extract charge with minimal photovoltage losses and recombination. It is well-known that n-type anatase TiO2 is a good electron-selective contact (ESC), although the fundamental reasons for its functioning are not completely clear to date. In this Letter, we investigate this issue by preparing perovskite-based solar cells with various n-type metal-oxide electron-selective contacts of different chemical nature and crystal structure. Our main finding is that the open-circuit photovoltage remains essentially independent of the nature of the contact for highly selective electron contacts, a fact that we attribute to a recombination rate that is mainly governed by the bulk of the MHPs. In contrast, replacement of the "standard" TiO2 contact by alternative contacts leads to lower short-circuit photocurrents and more pronounced hysteresis, related to enhanced surface recombination at less effective electron-selective contacts.

Understanding the Influence of Interface Morphology on the Performance of Perovskite Solar Cells.

In recent years, organo-halide perovskite solar cells have garnered a surge of interest due to their high performance and low-cost fabrication processing. Owing to the multilayer architecture of perovskite solar cells, interface not only has a pivotal role to play in performance, but also influences long-term stability. Here we have employed diverse morphologies of electron selective layer (ESL) to elucidate charge extraction behavior in perovskite solar cells. The TiO2 mesoporous structure (three-dimensional) having varied thickness, and nanocolumns (1-dimensional) with tunable length were employed. We found that a TiO2 electron selective layer with thickness of about c.a. 100 nm, irrespective of its microstructure, was optimal for efficient charge extraction. Furthermore, by employing impedance spectroscopy at different excitation wavelengths, we studied the nature of recombination and its dependence on the charge generation profile, and results showed that, irrespective of the wavelength region, the fresh devices do not possess any preferential recombination site, and recombination process is governed by the bulk of the perovskite layer. Moreover, depending on the type of ESL, a different recombination mechanism was observed that influences the final behavior of the devices.

Enhancing Moisture and Water Resistance in Perovskite Solar Cells by Encapsulation with Ultrathin Plasma Polymers.

A compromise between high power conversion efficiency and long-term stability of hybrid organic-inorganic metal halide perovskite solar cells is necessary for their outdoor photovoltaic application and commercialization. Herein, a method to improve the stability of perovskite solar cells under water and moisture exposure consisting of the encapsulation of the cell with an ultrathin plasma polymer is reported. The deposition of the polymer is carried out at room temperature by the remote plasma vacuum deposition of adamantane powder. This encapsulation method does not affect the photovoltaic performance of the tested devices and is virtually compatible with any device configuration independent of the chemical composition. After 30 days under ambient conditions with a relative humidity (RH) in the range of 35-60%, the absorbance of encapsulated perovskite films remains practically unaltered. The deterioration in the photovoltaic performance of the corresponding encapsulated devices also becomes significantly delayed with respect to devices without encapsulation when vented continuously with very humid air (RH > 85%). More impressively, when encapsulated solar devices were immersed in liquid water, the photovoltaic performance was not affected at least within the first 60 s. In fact, it has been possible to measure the power conversion efficiency of encapsulated devices under operation in water. The proposed method opens up a new promising strategy to develop stable photovoltaic and photocatalytic perovskite devices.

One-reactor plasma assisted fabrication of ZnO@TiO 2 multishell nanotubes: assessing the impact of a full coverage on the photovoltaic performance.

This paper addresses the fabrication of vertically aligned ZnO@TiO2 multishell nanotubes by a combined full vacuum-plasma approach at mild temperatures. The growth is carried out within the premises of a one-reactor approach, i.e. minimizing the number of vacuum chambers and sample transferences. In this way, the interface between ZnO and TiO2 is fully preserved from humidity thus increasing ZnO durability and stability. These nanostructures are studied by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy in STEM (EDX-STEM). High density one-dimensional arrays of these nanotubes formed on FTO substrates are applied as photoanode in a dye-sensitized solar cell (DSC). The evolution of the dye adsorption capacity and solar cells parameters are explored as a function of the crystallinity and thickness of the TiO2 shell. The results show the critical effect of a full coverage by TiO2 of ZnO core to explain the mixed results found in the literature.

Plasma assisted deposition of single and multistacked TiO2 hierarchical nanotube photoanodes.

We present herein an evolved methodology for the growth of nanocrystalline hierarchical nanotubes combining physical vapor deposition of organic nanowires (ONWs) and plasma enhanced chemical vacuum deposition of anatase TiO2 layers. The ONWs act as vacuum removable 1D and 3D templates, with the whole process occurring at temperatures ranging from RT to 250 °C. As a result, a high density of hierarchical nanotubes with tunable diameter, length and tailored wall microstructures are formed on a variety of processable substrates as metal and metal oxide films or nanoparticles including transparent conductive oxides. The reiteration of the process leads to the development of an unprecedented 3D nanoarchitecture formed by stacking the layers of hierarchical TiO2 nanotubes. As a proof of concept, we present the superior performance of the 3D nanoarchitecture as a photoanode within an excitonic solar cell with efficiencies as high as 4.69% for a nominal thickness of the anatase layer below 2.75 µm. Mechanical stability and straightforward implementation in devices are demonstrated at the same time. The process is extendable to other functional oxides fabricated by plasma-assisted methods with readily available applications in energy harvesting and storage, catalysis and nanosensing.

Interface Play between Perovskite and Hole Selective Layer on the Performance and Stability of Perovskite Solar Cells.

Perovskite solar cells with variety of hole selective contacts such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl), poly[bis(4-phenyl)(2,5,6-trimentlyphenyl)amine], 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (HMPDI), and 2',7'-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene] were employed to elucidate its role at the interface of perovskite and metallic cathode. Microscopy images revealed Spiro-OMeTAD and HMPDI produce smoother and intimate contact between perovskite/hole transporting materials (HTM) interfaces among others evaluated here. This morphological feature appears to be connected with three fundamental facts: (1) hole injection to the HTM is much more efficient as evidenced by photoluminescence measurements, (2) recombination losses are less important as evidenced by intensity-modulated photovoltage spectroscopy and impedance spectroscopy measurements, and (3) fabricated solar cells are much more robust against degradation by moisture. Devices with higher open-circuit photovoltages are characterized by higher values of the recombination resistance extracted from the impedance data. The variation in device hysteresis behavior can be ascribed mainly due to the molecular interaction and the core of HTM employed. In all cases, this fact is related with a larger value of the low-frequency capacitance, which indicates that the HTM can induce specific slow processes of ion accumulation at the interface. Notably, these processes tend to slowly relax in time, as hysteresis is substantially reduced for aged devices.

Specific cation interactions as the cause of slow dynamics and hysteresis in dye and perovskite solar cells: a small-perturbation study.

Hysteresis is one of the most remarkable features of perovskite solar cells; however, it is also present in other kinds of devices such as dye-sensitized solar cells. Hysteresis is due to underlying slow dynamic processes that interfere with the process of charge separation which depends critically on the selective contacts used. In this work we focus on the low-frequency (0.1-10 Hz) dynamics using impedance and intensity-modulated photocurrent spectroscopy and found that both perovskite solar cells (PSCs) and "viscous electrolyte containing" dye-sensitized solar cells (DSSCs) can be described on the same fundamental grounds. By comparing different electrolyte compositions in DSSCs and both methylammonium and formamidinium-based PSCs, we find a connection between the polar nature of the cations and the low-frequency component of these solar cells. There is evidence that in both cases ion transport and specific chemical interactions with the TiO2 surface give rise to the slow dynamics and the hysteresis. This is mainly inferred from the slope of the capacitance vs. applied voltage which shows accumulation behavior for the formulations with higher dipole moments only.

Determination of Interfacial Charge-Transfer Rate Constants in Perovskite Solar Cells.

A simple protocol to study the dynamics of charge transfer to selective contacts in perovskite solar cells, based on time-resolved laser spectroscopy studies, in which the effect of bimolecular electron-hole recombination has been eliminated, is proposed. Through the proposed procedure, the interfacial charge-transfer rate constants from methylammonium lead iodide perovskite to different contact materials can be determined. Hole transfer is faster for CuSCN (rate constant 0.20 ns(-1) ) than that for 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD; 0.06 ns(-1) ), and electron transfer is faster for mesoporous (0.11 ns(-1) ) than that for compact (0.02 ns(-1) ) TiO2 layers. Despite more rapid charge separation, the photovoltaic performance of CuSCN cells is worse than that of spiro-OMeTAD cells; this is explained by faster charge recombination in CuSCN cells, as revealed by impedance spectroscopy. The proposed direction of studies should be one of the key strategies to explore efficient hole-selective contacts as an alternative to spiro-OMeTAD.

Vacuum template synthesis of multifunctional nanotubes with tailored nanostructured walls.

A three-step vacuum procedure for the fabrication of vertical TiO2 and ZnO nanotubes with three dimensional walls is presented. The method combines physical vapor deposition of small-molecules, plasma enhanced chemical vapor deposition of inorganic functional thin films and layers and a post-annealing process in vacuum in order to remove the organic template. As a result, an ample variety of inorganic nanotubes are made with tunable length, hole dimensions and shapes and tailored wall composition, microstructure, porosity and structure. The fabrication of multishell nanotubes combining different semiconducting oxides and metal nanoparticles is as well explored. This method provides a feasible and reproducible route for the fabrication of high density arrays of vertically alligned nanotubes on processable substrates. The emptying mechanism and microstructure of the nanotubes have been elucidated through SEM, STEM, HAADF-STEM tomography and energy dispersive X-ray spectroscopy. In this article, as a proof of concept, it is presented the straightforward integration of ZnO nanotubes as photoanode in a photovoltaic cell and as a photonic oxygen gas sensor.

Charge Transfer Reductive in situ Doping of Mesoporous TiO2 Photoelectrodes - Impact of Electrolyte Composition and Film Morphology.

Some material properties depend not only on synthesis and processing parameters, but may furthermore significantly change during operation. This is particularly true for high surface area materials. We used a combined electrochemical and spectroscopic approach to follow the changes of the photoelectrocatalytic activity and of the electronic semiconductor properties of mesoporous TiO2 films upon charge transfer reductive doping. Shallow donors (i.e. electron/proton pairs) were introduced into the semiconductor by the application of an external potential or, alternatively, by band gap excitation at open circuit conditions. In the latter case the effective open circuit doping potential depends critically on electrolyte composition (e.g. the presence of electron or hole acceptors). Transient charge accumulation (electrons and protons) in nanoparticle electrodes results in a photocurrent enhancement which is attributed to the deactivation of recombination centers. In nanotube electrodes the formation of a space charge layer results in an additional decrease of charge recombination at positive potentials. Doping is transient in nanoparticle films, but turns out to be stable for nanotube arrays.

Universal Features of Electron Dynamics in Solar Cells with TiO2 Contact: From Dye Solar Cells to Perovskite Solar Cells.

The electron dynamics of solar cells with mesoporous TiO2 contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current-voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 µs, 1 order of magnitude longer, is required to reproduce the current-voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO2 contact with electrons.

Control of the recombination rate by changing the polarity of the electrolyte in dye-sensitized solar cells.

Recombination in Dye-sensitized Solar Cells (DSCs) is an electron transfer process critical for high efficiency. The chemical nature of the electron acceptor is known to have an important impact on recombination and, hence, limits the choice of hole conductors in DSCs and related solar cells. In this respect, Room Temperature Ionic liquids (RTILs) have been recognized as an alternative to volatile organic solvents due to their negligible vapor pressure, which offers the chance for long-term stability. However, RTIL-based electrolytes lead to lower performance, a feature that has been attributed to the high viscosity of ionic liquids and the mass-transport limitation associated with it. In this work we show that the origin of the lower performance is also related to an increase in the recombination loss due to the polar nature of the RTIL and the influence of the reorganization energy of the electron acceptor in a polar environment. To investigate this chemical effect, different mixing ratios of RTILs and an organic solvent (acetonitrile) have been considered. The fabricated devices have been characterized by small-perturbation techniques (Impedance Electrochemical Spectroscopy and Intensity-Modulated Photovoltage and Photocurrent Spectroscopies) and Open-Circuit Voltage Decay measurements, which have been used to extract electron lifetimes at different applied voltages. Two different ruthenium dyes (hydrophilic N719 and hydrophobic Z907) and two different cations in the RTIL (imidazolium- and pyrrolidinium-based) have been considered. The results obtained show that for pure ionic liquids the lifetime-voltage curve is exponential, which is a signature of large reorganization energies for electron transfer. In contrast, pure acetonitrile exhibits a non-exponential behavior, which is consistent with relatively low reorganization energy. Interestingly, and as a general rule, we find that recombination is faster in systems with higher reorganization energies. This is interpreted as a consequence of the availability of more acceptor states for electron transfer. In addition, it is found that mixing RTILs and acetonitrile is an interesting strategy to increase the stability of DSCs without significant recombination losses, provided that the right dye and RTIL, in particular, a pyrrolidinium component, are used.

Mechanisms of electron transport and recombination in ZnO nanostructures for dye-sensitized solar cells.

ZnO is an attractive material for applications in dye-sensitized solar cells and related devices. This material has excellent electron-transport properties in the bulk but its electron diffusion coefficient is much smaller in mesoporous films. In this work the electron-transport properties of two different kinds of dye-sensitized ZnO nanostructures are investigated by small-perturbation electrochemical techniques. For nanoparticulate ZnO photoanodes prepared via a wet-chemistry technique, the diffusion coefficient is found to reproduce the typical behavior predicted by the multiple-trapping and the hopping models, with an exponential increase with respect to the applied bias. In contrast, in ZnO nanostructured thin films of controlled texture and crystallinity prepared via a plasma chemical vapor deposition method, the diffusion coefficient is found to be independent of the electrochemical bias. This observation suggests a different transport mechanism not controlled by trapping and electron accumulation. In spite of the quite different transport features, the recombination kinetics, the electron-collection efficiency and the photoconversion efficiency are very similar for both kinds of photoanodes, an observation that indicates that surface properties rather than electron transport is the main efficiency-determining factor in solar cells based on ZnO nanostructured photoanodes.

Comparison of TiO2 and ZnO solar cells sensitized with an indoline dye: time-resolved laser spectroscopy studies of partial charge separation processes.

Time-resolved laser spectroscopy techniques in the time range from femtoseconds to seconds were applied to investigate the charge separation processes in complete dye-sensitized solar cells (DSC) made with iodide/iodine liquid electrolyte and indoline dye D149 interacting with TiO2 or ZnO nanoparticles. The aim of the studies was to explain the differences in the photocurrents of the cells (3-4 times higher for TiO2 than for ZnO ones). Electrochemical impedance spectroscopy and nanosecond flash photolysis studies revealed that the better performance of TiO2 samples is not due to the charge collection and dye regeneration processes. Femtosecond transient absorption results indicated that after first 100 ps the number of photoinduced electrons in the semiconductor is 3 times higher for TiO2 than for ZnO solar cells. Picosecond emission studies showed that the lifetime of the D149 excited state is about 3 times longer for ZnO than for TiO2 samples. Therefore, the results indicate that lower performance of ZnO solar cells is likely due to slower electron injection. The studies show how to correlate the laser spectroscopy methodology with global parameters of the solar cells and should help in better understanding of the behavior of alternative materials for porous electrodes for DSC and related devices.

A continuity equation for the simulation of the current-voltage curve and the time-dependent properties of dye-sensitized solar cells.

A numerical model that simulates the steady-state current-voltage curve and the time-dependent response of a dye-sensitized solar cell with a single continuity equation is derived. It is shown that the inclusion of the multiple-trapping model, the quasi-static approximation and non-linear recombination kinetics leads to a continuity equation for the total electron density in the photoanode with an electron density-dependent diffusion coefficient and a density-dependent pseudo-first order recombination constant. All parameters in the model can be related to quantities accessible experimentally. The required power exponents are taken from impedance spectroscopy measurements at different voltages. The model provides new insights into the physical interpretation of the power exponents. Modeling examples involving a high-efficiency TiO(2)-based dye solar cell and a ZnO-based dye solar cell are presented. It is demonstrated that the model reproduces the transient behavior of the cell under small perturbations. The spatial dependence of the recombination rate and the influence of film thickness and of voltage dependent injection efficiency on cell performance are studied. The implications of the model are discussed in terms of efficiencies potentially attainable in dye-sensitized solar cells and other kinds of solar cells with a diffusional mechanism of charge transport.