Influence of chemical structure on molecular interactions and microstructure of organic photovoltaic materials

The performance of organic solar cells based on blend films of a conjugated polymer and a fullerene derivative is strongly related to the blend microstructure. Therefore, in selecting new materials for improved light harvesting or higher photovoltage, it is essential to understand the effect of chemical structure on molecular interactions in order to find ways to control the final blend microstructure. In this thesis, we first address the influence of the number of side chains attached to the fullerene on the microstructure of a regioregular poly(3-hexylthiophene)(P3HT):fullerene blend using transmission electron microscopy, steady-state and ultra-fast photoluminescence spectroscopy and grazing incidence X-Ray diffraction (GIXRD). We investigate the influence of fullerene side chains on the P3HT:fullerene binary phase behaviour using differential scanning calorimetry (DSC). We relate our findings to the observed optoelectronic properties of the same binary systems. This study highlights the importance of miscibility and crystallisation in microstructure development. Secondly, we seek a spectroscopic probe of the onset of aggregation (or solubility limit) of fullerenes in polymer. We study regiorandom P3HT:fullerene blend filmsas a function of fullerene type and content using steady-state and ultra-fast photoluminescence spectroscopy. We relate the onset of fullerene aggregation with the appearance of charge transfer (CT) state emission. We correlate the CT state emission energy with fullerene crystallisation, probed by DSC. Thirdly, we address the influence on polymer packing of the bridging atom between the backbone and the side chains in poly[2,1,3-benzothiadiazole-4,7-diyl[4,4- bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl]] (PCPDTBT) using GIXRD and molecular modelling. We find that chemical substitution of the carbon bridging atom by silicon increases side chain flexibility, leading to different crystal structures. Finally, we use quasi-elastic neutron scattering and molecular dynamics to probe the side chain dynamics of polythiophenes with different side chain lengths.Open Acces