Juan A. Anta

ZnO solar cells with an indoline sensitizer: a comparison between nanoparticulate films and electrodeposited nanowire arrays

The photovoltaic properties of nanostructured ZnO films sensitized with the indoline derivative dye D149 were studied. The performance of dye-sensitized solar cells built from ZnO building blocks with different morphology, (a) randomly oriented nanoparticle network and (b) nanowire arrays, was compared. The nanoparticle networks were prepared by the standard doctor blade technique from commercial ZnO powders and the nanowire arrays were electrodeposited in aqueous media. Two different lengths for the nanowire arrays (2.5 and 5 µm) were considered. The characterization included electron microscopy, adsorption measurements, optical spectroscopy, current–voltage characteristics, open-circuit voltage versus light intensity, incident-photon-to-current efficiency, open circuit voltage decay and impedance spectroscopy under illumination. In spite of the smaller dye loadings of the nanowires with respect to the nanoparticles, the former showed a remarkably effective integrated optical absorption (in the range from 370 to 700 nm, 57% versus 69% for the latter). However the photocurrents for nanowires were lower than expected from this good absorption, which suggests that recombination rather than solar light harvesting can be the limiting factor in these nanowire-based solar cells. The impedance analysis and the open-circuit voltage decays showed smaller recombination resistances and shorter lifetimes for the nanowire-based solar cells. However, the interpretation of the recombination resistances, capacitances and lifetimes in the case of the nanowires is likely affected by space-charge effects and back-reaction through the substrate. An understanding of the effects discovered in this study is very valuable for the development of strategies to enhance the energy conversion efficiency for the ZnO nanowire array based solar cells.

How Important is Working with an Ordered Electrode to Improve the Charge Collection Efficiency in Nanostructured Solar Cells

The collection efficiency of carriers in solar cells based on nanostructured electrodes is determined for different degrees or morphological one-dimensional order. The transport process is modeled by random walk numerical simulation in a mesoporous electrode that resembles the morphology of nanostructured TiO2 electrodes typically used in dye-sensitized solar cells and related systems. By applying an energy relaxation procedure in the presence of an external potential, a preferential direction is induced in the system. It is found that the partially ordered electrode can almost double the collection efficiency with respect to the disordered electrode. However, this improvement depends strongly on the probability of recombination. For too rapid or too slow recombination, working with partially ordered electrodes will not be beneficial. The computational method utilized here makes it possible to relate the charge collection efficiency with morphology. The collection efficiency is found to reach very rapidly a saturation value, meaning that, in the region of interest, a slight degree of ordering might be sufficient to induce a large improvement in collection efficiency.

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.

Random walk numerical simulation for solar cell applications

Numerical modelling based on random walk methods provides a very versatile tool to describe transport and recombination processes in solar cells made with disordered materials and nanostructured devices. These methods are especially suited to describe these systems since they are characterized by a wide dispersion of spatial and energy states.

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.

Self-consistent effective interactions in charged colloidal suspensions

We use an integral equation scheme to obtain self-consistently the effective interaction between colloids in salt-free charged colloidal suspensions. The colloid–counterion direct correlation function (DCF) is obtained for the fixed colloid–colloid pair structure by solving the corresponding hypernetted-chain equation (HNC). This DCF is then used to formulate an effective colloid–colloid pair potential for which the one-component reference hypernetted-chain equation is solved. Both processes are iterated until self-consistency is achieved. Counterion–counterion correlations are considered linear and uncoupled from the rest of the correlations. The method is based on a similar treatment utilized in liquid metals [Phys. Rev. B 61, 11400 (2000)] and provides equivalent results to those obtained using the standard multicomponent HNC equation for mixtures of charged hard spheres. The theory proves rather accurate when compared with molecular dynamic simulations of charged hard and soft spheres for colloidal ch...

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.

IR-spectrophotoelectrochemical characterization of mesoporous semiconductor films.

A combined IR-spectroscopic and electrochemical approach for the study of photo- and bias-induced reactions at the semiconductor/electrolyte interface is presented. Information on the electronic structure of a mesoporous semiconductor nanoparticle network, concretely the density and distribution of band gap states, as well as the nature of solution species are analyzed in situ. It has been shown that under appropriate conditions the electrode potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation, i.e., application of a bias voltage or electrode exposure to photons exceeding the semiconductor band gap at open circuit. Importantly, electronic properties of the semiconductor and vibrational properties of solution species can be addressed simultaneously by IR-spectroscopy. In addition, electrochemical methods provide a means for the active manipulation (in potentiostatic measurements) or the passive tracking (during open circuit potential decay) of the quasi-Fermi level in the mesoporous film together with the possibility of electron quantification (by charge extraction experiments).

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.