Jesús Idígoras

ZnO-based dye solar cell with pure ionic-liquid electrolyte and organic sensitizer: the relevance of the dye–oxide interaction in an ionic-liquid medium

The use of non-volatile electrolytes and fully organic dyes are key issues in the development of stable dye-sensitized solar cells (DSCs). In this work we explore the performance of ZnO-based DSCs sensitized with an indoline derivative coded D149 in the presence of a pure ionic-liquid electrolyte. Commercial nanostructured zinc oxide and an electrolyte composed of iodine plus (1) pure 1-propyl-3-methyl imidazolium iodide (PMII) and (2) a blend of PMII with low-viscosity ionic liquids were employed to construct the devices. Without further additives, the fabricated devices exhibit remarkable short-circuit photocurrents and efficiencies under AM1.5 simulated sunlight (up to 10.6 mA cm−2, 2.9% efficiency, 1 sun, active area = 0.64 cm2) due to the high surface area of the ZnO film and the high absorptivity of the D149 dye. Impedance spectroscopy is used to characterize the devices. It is found that the addition of the low-viscosity ionic-liquid improves the transport features (leading to a better photocurrent) but it does not alter the recombination rate. The robustness of the dye–oxide interaction is tested by applying continuous illumination with a Xenon-lamp. It is observed that the photocurrent is reduced at a slow rate due to desorption of the D149 sensitizer in the presence of the ionic liquid. Exploration of alternative ionic-liquid compositions or modification of the ZnO surface is therefore required to make stable devices based on ZnO and fully organic dyes.

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.

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.

ZnO–ionic liquid hybrid films: electrochemical synthesis and application in dye-sensitized solar cells

An innovative ionic liquid (IL)-based synthesis route was developed to obtain ZnO-based hybrid nanostructured films with a modified surface. In particular, ZnO–IL hybrid films, with thickness from 1.5 to 4.5 μm and an appealing sponge-like morphology, were obtained from the electrochemical reduction of NO3− in ionic liquid-based electrolytes containing Zn2+. The presence of the ionic liquid moieties and/or derivatives in the as-deposited films was demonstrated by Fourier transform infra-red spectroscopy and energy dispersive X-ray spectroscopy. However, a relatively soft thermal annealing (i.e. 1 hour in air at 350 °C) was proved to be an effective way to remove the ionic liquid content from the samples, leading to porous ZnO films with high specific surface area. In comparison to ionic liquid-free samples, a blue shift of ∼40 meV in the absorption onset is detected for the ZnO-PYR14TFSI hybrid films, which suggests a modification of the bandgap. Both kind of films (i.e. ZnO–IL and ZnO) were sensitized with an indoline dye, coded D358, and evaluated as photoanodes in Dye-sensitized Solar Cells (DSCs). Although less D358 was adsorbed onto ZnO–IL samples (suggesting co-adsorption of the IL and D358), a clear enhancement (by a factor of ca. 2) of the power conversion efficiency was detected in DSC based on ZnO–IL hybrid films. This improvement was mainly due to a huge increase (∼180 mV) in the photovoltage, which reaches values of up to 780 mV. The DSC characterization, by electrochemical impedance spectroscopy and open circuit voltage decay techniques, indicates that the photovoltage improvement is likely due to a negative displacement of the conduction band in hybrid films. Therefore, the use of metal oxide–IL hybrid anodes appears to be a promising strategy to increase the open circuit voltage of the DSCs.

Highly efficient flexible cathodes for dye sensitized solar cells to complement Pt@TCO coatings

Solution-processed, semi-transparent, conductive electrodes based on PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)] using silver grids were developed on a plastic support. The flexible electrode (flextrode) was employed as a cathode in dye sensitized solar cells (DSSCs) and acts in two-fold manner: the PEDOT layer will yield a high surface area for effective electrocatalysis, while the silver grid will decrease the series resistance to rival transparent conducting oxide (TCO) coatings. DSSCs fabricated with Y123 dye and the developed cathodes show power conversion efficiencies of 7%, which are comparable to the reference device (6.9%) using Pt@FTO (F-doped SnO2) coatings on glass under the same conditions. A flexible and cost effective electrode will help to penetrate the market for DSSCs and hybrid-organic–inorganic photovoltaics. Electrochemical impedance spectroscopy and modelling results confirm similar values of the series resistance and charge transfer resistance of the cathodes both in Pt-based and PEDOT-based cells, as well as the limited impact on the I–V curve from the modification of the charge transfer parameters in the PEDOT-based cells.

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.

Impact of moisture on efficiency-determining electronic processes in perovskite solar cells

Moisture-induced degradation in perovskite solar cells was thoroughly investigated by structural (SEM, EDS, XRD and XPS) and device characterization (impedance and intensity modulated photocurrent spectroscopy) techniques. Both the influence of the perovskite composition and the nature of the hole selective material were analyzed. The degradation rate was found to be significantly slower for mixed perovskites and P3HT-based devices. However, for a fixed degradation degree (defined as a 50% drop from the initial photocurrent), all configurations show similar features in small-perturbation analysis. Thus, a new mid-frequency signal appears in the impedance response, which seems to be related to charge accumulation at the interfaces. In addition, faster recombination, with a more important surface contribution, and slower transport were clearly inferred from our results. Both features can be associated with the deterioration of the contacts and the formation of a higher number of grain boundaries.

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.

Organic dyes for the sensitization of nanostructured ZnO photoanodes: effect of the anchoring functions

Among all n-type metal oxide semiconductors that can be used in solar cells as photoanode, ZnO is one of the most appealing alternatives to the ubiquitous TiO2. This material offers some potentially favourable characteristics with respect to TiO2, such as higher electron mobility in the bulk and a rich variety of nanostructures. However, ZnO has certain drawbacks as photoanode material, for example, a poor chemical stability and a slower charge separation process at the ZnO interface that reduces the electron injection rate. Therefore, in the case of dye-sensitized solar cells, the search of new dyes with a higher light harvesting efficiency and specifically designed to bind to ZnO can be considered as a possible strategy to improve performance in systems characterized by a low electron injection rate. In this work the optical, electrochemical and photovoltaic properties of a family of purely organic sensitizers with various anchoring groups have been investigated and compared with the most commonly used ruthenium dye N719. In particular, we have shown that the structurally simple organic dye coded RK1 is an excellent sensitizer for ZnO photoanodes. Thanks to this molecule, the energy conversion efficiency under standard conditions (1 sun AM 1.5 illumination) employing ZnO-based photoanodes reached 3.7%, which is more than two times higher than obtained with the N719 dye studied under the same conditions.

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.