Emerging Energy Possibilities

Doktorsavhandling, 2010

This Thesis presents a thorough study of the electron transfer (ET) processes, both inter-molecular - between dyes for dye-sensitized solar cells (DSSCs), and intra-molecular - in porphyrin based electron donor-bridge-electron acceptor model systems. The focus has been on charge separation and subsequent re-combination in the femtosecond-nanosecond range in both systems. Studying these processes on nanocrystalline mesoporous films in rather heterogeneous systems for solar cell application is quite a challenge. Therefore, the knowledge gained by studying the same type of processes in designed model systems is in-valuable. To this end, ET has been verified and studied in a series of donor-bridge-acceptor (D-B-A) model systems using femtosecond transient absorption. The D-B-A series show photo-induced ET with exponential distance dependence via superexchange interactions. The attenuation factor of the ET rate, beta, was shown to be direction specific, which is in accordance with the McConnell model. A parallel study of the influence of the barrier height showed that the decrease in electronic coupling for the charge recombination (CR) reaction could be correlated to the impact of charge location. The importance of charge location was also used to explain the large differences in energy conversion efficiency found for two solar cell dyes with differently conjugated anchoring groups adsorbed on TiO2. The disparity in energy conversion efficiency for these two dyes was ascribed to the effect of differences in electron density on the binding oxygens, rather than efficiency variations in the injection process. An exponential distance dependence for CR was found also for dyes adsorbed on TiO2, with attenuation factors similar to those found for the D-B-A system. In addition, the hole transfer to and from NiO films was studied for two p-type dyes that had shown great differences in current generation. The CR was found to be slower for the dye with the strongest charge transfer character which was, surprisingly, the dye with the poorest solar cell performance. The reason for the meagre energy conversion efficiency of this dye was suggested to be a lack of driving force for regeneration by the redox couple.