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From Femto- to Millisecond and From Ensemble to Single Molecule Photobehavior of Some Nanoconfined Organic Dyes for Solar Cells Improvement

Final Report Summary - NANOSOL (From Femto- to Millisecond and From Ensemble to Single Molecule Photobehavior of Some Nanoconfined Organic Dyes for Solar Cells Improvement)

The main objectives of the NANOSOL project were to study and develop a new generation of dye-sensitized solar cells systems based on confined structures, confined electron dynamics and charge transport. NANOSOL aims at a detailed study of the photobehaviour of a few recently introduced metal-free organic dyes (from the triphenylamine family) in solution and confined by Ti-doped mesoporous materials both in the absence and in the presence of TiO2 nanoparticles.
Modern time resolved absorption and emission techniques have been applied to study dyes in solution, suspension, on transparent films and in real solar cells, on the time scale from 50 fs to single ms. In addition to that, stationary absorption and emission spectroscopy have been also used. We have studied 9 different morphologies of titania material (nanoparticles of two different size, 3 different titanium-doped mesoporous silica materials, randomly distributed titania nanotubes and nanorods, and vertically oriented nanotube films of 2 different diameters), 3 different dyes and 4 different semiconductors (TiO2, ZnO, ZrO2 and Al2O3).
First, the studies of the free dyes in solution showed the importance of the charge transfer state properties in the dye's photobehavior, and the dominant role of the solvation in the initial, ultrafast dynamics. These results appeared late to have a large impact on the understanding of the interaction of the dye and titania material. Moreover, we found the existence of an equilibrium between neutral and anion structures, depending on solvent H-bond acceptor ability and concentration of the dye. This property explained the different solar cell efficiencies when different solvent were used as immersing bath during solar cell preparation.
Next, the studies of triphenylamine dyes interacting with titania nanoparticles in polar solvent suspension and on thin films brought the information about the rates of electron injection (from the dye excited state to the conduction band of the semiconductor) and the back electron transfer (recombination from the conduction band or trap states to the radical cation of the dye). In suspension, we found a multi-exponential electron injection process with time constants from 100 fs to tens of ps, and fast partial recombination on the time scale from single to tens of ps. The studies on nanoparticle films put more detailed light and revealed that the fast electron injection (~100 fs) originates from locally excited and high energy vibrational levels of the single excited state of the dye to the conduction band, and this process results in efficient charge separation. This charge separation recombines on a relatively long lifetime (stretched exponential decay with a characteristic time of 0.5 microseconds). On contrary, the electron injection from the relaxed charge transfer state takes place on 1-50 ps time scale and takes place directly to coupled trap states of semiconductor, from which a rapid recombination to the parent dye occurs. This process limits the efficiency of the charge separation, and therefore, the solar cell performance.
Moreover, we have also investigated the effect of different substituents in a dye's spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced solar cell conversion efficiencies. Furthermore, we studied titania nanotubes as alternative one-dimensional material for solar cells, and we compared the results with those of standard nanoparticles. Both the electron injection and back electron transfer dynamics were similar in titania nanoparticles and nanotubes. Variations between the two film types were only found in the time resolved emission transients, which can be explained in terms of the encapsulation effect and the difference in local electric fields, both affecting the position of the emission bands.
Finally, we studied different Ti-doped silica mesoporous sieves (MCM-41 type) in suspension, films and in complete photovoltaic cells. They are another, alternative one-dimensional materials of promising properties for solar cells, especially from the point of view of large specific surface area. We found that from the point of view of interfacial electron separation, the performance of Ti-doped MCM-41 material is comparable or even better than a typical titania nanoparticle one (similar electron injection rate, slower recombination and faster dye regeneration by electrolyte). However, the performance of complete solar cells was still worse in MCM-41 type of material due to smaller dye loading and limitation in an electron transport along MCM-41 channels. Therefore this kind of material needs further modification in terms of improving the total dye loading and the conductivity, in order to be used in photoviltaics.
In summary, we believe that the studies performed in NANOSOL project brought a lot of new light on the understanding of the interaction of large class of organic dyes with titania materials. We characterized the dynamics of its interaction with standard semiconductor nanoparticles and different one-dimensional morphologies to improve the solar cell efficiency. The information obtained in NANOSOL project should be very helpful in better optimization of the design of dye sensitized solar cells based on organic dyes as prospective alternative for renewable energy source. The materials used in the NANOSOL project were obtained in a collaboration with 4 different scientific groups (from Sweden, China, USA and Spain). As a result of the project, 7 papers have been already published in leading peer-reviewed journals of high impact factors, 2 more manuscripts have been submitted and one manuscript is under preparation. Moreover, the results have been shown on 4 international conferences as oral contributions or posters.