Aggregation and photophysics of merocyanine dyes for photovoltaic application The supramolecular structure essentially determines properties and function of organic solar cells and other organic optoelectronic devices. Aggregation in organic bulk heterojunction (BHJ) solar cells is usually beneficial and a lack of aggregation is one of the main factors limiting the efficiency. To enhance the knowledge about aggregation of dyes and their effect on electro optical parameters it is very useful to have a model system of similar molecules which can be forced into different intermolecular arrangements in thin films. With these model systems it is possible to investigate the influence of different aggregate structures on the optical absorption emission and recombination kinetics. Another point that turned out to be crucial is that especially dipolar molecules tend to build dimers micelles etc. in solution. Furthermore it turned out that the dimerization can be enhanced by the illumination with light. Finally in the thesis at hand I could highlight the impact of supramolecular structure on absorption and emission spectra as well as on the excited state kinetics. For this research we have focused on merocyanine dyes which were well investigated in solution but lacking a comprehensive understanding on how to broadly tuning the supramolecular structure and their influence on optical and electronical properties as well as excited state dynamics. I succeeded to systematically tune the supramolecular structure towards H aggregates by means of depositing Langmuir layers at increasing surface pressures. The accompanied increase in supramolecular order leads to creation of efficient dark deactivation channels what explains the lower power conversion efficiency by dye aggregation in a DSSC device.