Build upon our experience on the study of interfacial charge transfer processes at the molecular level, we are already leading studies in the control of the charge recombination processes that limit the performance of molecular photovoltaic devices.
The first example has been the design of a squarine molecule and its properties as a light harvesting material in Dye Sensitised Solar Cells (DSSC). We have demonstrated that the control at the nanoscale of the electron transfer reactions, between the photo-injected electrons and the oxidised electrolyte in the device under illumination, is controlled also by the hydrophobic barrier, which is provided by the molecule when achieves a dense monolayer of molecular aggregates (Figure 1). In this case, the results achieved represent a significant advance on the understanding of the processes that limit the efficiencies of these new photovoltaic devices. These studies clearly demonstrate the ability of our group to design, fabricate and characterise such innovative systems.
Furthermore, within the framework of a European project (FP7-ROBUST DSSC-212792), we have also established that it is possible to combine different dyes at the surface of transparent mesoporous nanocrystalline semiconductor films and achieve a panchromatic sensitisation, which improves further the device photoresponse. This work combines the use of organic dyes that absorb light in different regions of the solar spectrum. In fact, Zinc-phthalocyanines have been utilized for the light harvesting at the near IR part of the solar spectrum. For the first time, efficiencies for the solar energy conversion into electricity as a high as 4% have been demonstrated for DSSC using phthalocyanine as a sensitiser. Focussed on those near IR dyes we have carried extensive work related to the phthalocyanine structure-device efficiency relationship.
We found that those aromatic systems, needed to absorb light at low energies, do have in several cases a negative effect over the efficiency of the devices. In fact, we have confirmed that in the case of Ruthenium based phthalocyanines the dye accelerates the recombination reactions that limit the device performance.
Figure 1. Graphical representation of the HOMO (a) and the LUMO orbitals for a squiarine dye.
Finally, it was possible to prove that slow electron injection processes are possible in dye sensitised TiO2 mesoporous thin films, which has been a matter of controversy since the publication of the first ultra-fast electron injection measurements in the field. This work was also funded by an European project (FP6-HETEROMOLMAT- STRP 516982) coordinated by Dr Palomares, ICREA Research Professor at ICIQ.
During 2007, the group has been working on the synthesis and characterisation of new heteroleptic Ruthenium(II) dyes as a target molecules for : (a) the achievement of higher photocurrents on DSSC and (b) to study the role of the electro-donating and electro-withdrawing groups present at the bipyridine moieties coordinated to the central Ru(II) atom (Figure 2). This work is part of the objectives to develop within the Ministerio de Educación y Ciencia project (CTQ-2007-60746: Estudio de moleculas óptica y electroquímicamente activas y su aplicación en dispositivos fotovoltaicos moleculares).
The first results have been very much promising and we obtained photovoltaic devices that show enhanced photocurrent when the complex cis-(4,4’-dicarboxy-2,2’-bipyridine)(5,6-dimethyl-1,10-phenantroline)dithiocyanate Ruthenium (II) was used as a sensitiser (Figure 2).
Figure 2. Graphical representation of AR25 [cis-(4,4’-dicarboxy-2,2’-bipyridine) (5,6-dimethyl-1,10 phenantroline) dithiocyanate ruthenium (II)].
The studies have been focussed on the dye molecular structure-device efficiency relationship and illustrate that there is an important influence of the substituent moieties at the bipyridin rings that affects dramatically the cell photovoltage and photocurrent. Further work has also been carried out during the framework of the European Network of Excellence (FP6-OrgaPVNet) where Dr. Palomares is the national representative. The network, that coordinates more than 10 European countries ( http://www.orgapvnet.eu), is the European platform for future projects for the development of the molecular photovoltaic technology. During September 2008 a conference meeting organised by OrgaPVNet will take place in Warwick (UK) and the results on hybrid CdSe/P3HT blend devices obtained in our group will be presented.
The work carried out in our group ha attracted the attention of several national and international companies working on the area of renewable energy. For example, ACCIONA SOLAR has signed contracts with the ICIQ for the study and development of this new technology. Furthermore, other companies, ARTERSA and CETEMSA, have established contacts with the ICIQ to expand the fundamental knowledge on the processing of organic materials in flexible substrates.
As a part of our commitment to the CONSOLIDER project awarded in 2007 (HOPE-HOPE-CSD-000HOPE-CSD-0007-2007) for the development of Hibrid Organic Photo-Electrochemical Devices we have also participate on the characterization of ionic liquid based electrolytes for efficient DSSC[4]. Moreover, a systematic study of organic dyes is been carried out in our group to find the pathways for a rational design of efficient sensitiser that improve the photocurrent and photovoltage parameters of DSSC based on low cost sensitisers.
On the other hand, as mentioned in the Abstract, our group also focuses on the development of low-cost/ easy to use environmental molecular probes for toxic substances. During 2007 we have developed a sensor based on our previous work on colorimetric determination of mercury(II) using Ruthenium bis-thiocyanate molecules. This work is mainly the core of a Ph.D. Thesis. We have demonstrated that it was possible to scavenge mercury(II) selectively from water solutions even in the presence of other metals in a very efficiently way. Moreover, the work carried out on this matter has been protected with an international patent (PCT Int. Appl. (2008), 37pp. CODEN: PIXXD2 WO 2008025977). The multidisciplinary nature of this project has evolved towards the use of mathematical algorithms, which provides higher degree of control over the sensitivity measurements for mercury (II) in water solution using dip and read test procedures.
We have developed transparent films with high surface area and high stability that contain the molecular receptor which, upon mercury binding, changes its colour from deep red-purple to orange. Those films can be fully regenerated and re-used again for the analysis of mercury (II) in aqueous solution. The colour change is monitored using a modified optical fiber and the different spectrums obtained are processes by using genetic algorithms and statistical home-made software allowing to improve the limit of sensitivity of our films for mercury changes its colour from deep red-purple to orange. The colour change is monitored using a modified optical fibre and the different spectrums obtained are processes by using genetic algorithms and statistical home- made software allowing the improvement of the sensitivity limit of our films for mercury.
Figure 3. Schematic representation of the optical probe used for the determination of mercury(II) in water (top) and cross-section of the optical fiber probe tip (a) and the Atomic Force Microscopy (AFM) image of the mesoporous Al2O3 film deposited ontot the optical fiber silica core (b).