Ultrafast electron transfer from potential organic and metal containing solar cell sensitizers

Abstract

In this Thesis ultrafast spectroscopic techniques were used to study electron transfer processes in dye sensitized nanocrystalline semiconductor films as well as excitation relaxation in metal complexes in solution. The main interest was to clarify the mechanisms of heterogenous electron transfer in Ru(dcbpy)2(NCS)2 sensitized nanocrystalline TiO2 films, the active electrode of one of the most promising dye sensitized solar cells. It was established that the 'slow' picosecond injection occurs from a triplet states that could be directly populated by excitation of the red side of the MLCT band of the sensitizer. Heterogeneity of the injection was shown to originate from an inter-ligand electron transfer and to be dependent not only on relative positions of the excited triplet states of the sensitizer and the conduction band edge, but also on the coupling strength between the sensitizer and the semiconductor states. New results were obtained on electron injection from three aminophenyl dyes to nanocrystalline TiO2 films. All of these dyes showed sub 300 fs electron injection rates. It was found that one of the dyes containing flexible substituents in the terminal amino group underwent an extremely fast excited state isomerization that took place in parallel with the electron transfer, reducing the over all efficiency of injection. For RuN3 sensitizer in solution detailed studies of dynamics were carried out. Finally excited state dynamics of a metal complex Fe(aceylacetonato)3 in solution was studied by using UV excitation and mid-infrared probing. It was observed that after excitation the system underwent electronic relaxation from the initially excited state to a vibrationally hot state from where after cooling the system relaxed back to the ground state. Some evidence was obtained for mode selective transfer of vibronic excess energy to the solvent bath. A substantial effort was put in to build experimental setups and data collection and analysis programs. Several designs utilizing non-linear optical amplifiers; one stage and two stage versions for visible spectral region as well as an optical parametric amplifier for mid-infrared region were realized. Data acquisition and data analysis software packages were developed for a number of measuring schemes. These included a user friendly interface to control and record data from pump and probe experiments either by reading individual photodiodes or diode arrays at 1 kHz repetition rate of the laser. Data analysis packages included software for fitting of kinetic traces and global analysis software.