Elena Guillén

Metal free sensitizer and catalyst for dye sensitized solar cells

The carbon neutral process for energy conversion is of utmost importance, in this context photovoltaics, especially dye sensitized solar cells (DSSCs) are a viable alternative. Few terawatts of carbon free renewable energy can be produced by DSSCs. However the judicious use of platinum group free metals may further enhance the limit of energy production by making it further cost effective. In this context a metal free sensitizer and electrocatalyst related to DSSCs are reviewed for their merits. The current state-of-the-art sensitizer as well as carbon based materials for its chemistry and photovoltaic characterization are discussed. The present article combines recent progress and its emerging behavior from our laboratories and from other groups working on this perennial topic.

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.

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.

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.

Improving photoresponse characterization of dye-sensitized solar cells: application to the laser beam-induced current technique

The photocurrent response of dye-sensitized solar cells (DSSCs) to light excitation from focused and non-focused laser beams is investigated. We observe that part of the photocurrent is produced by the activation of the irradiated area, whereas another part is generated by the previously photoexcited area. A mathematical algorithm has been devised to describe the rise and decay processes. The application of this algorithm leads to a significant improvement in the surface photoresponse and quantum yield measurements in DSSCs by means of the laser beam-induced current (LBIC) technique. This algorithm enhances the quality and definition of the LBIC images and opens the way to use this technique to cope with the biphasic features of these photovoltaic devices and extracting key properties for device performance such as internal quantum efficiencies and electron diffusion lengths.