Synthesis and properties of pi-conjugated polymers for organic photovoltaics

Doctoral thesis, 2013

Organic photovoltaics is a renewable energy technology able to solve global warming and the upcoming energy gap, issues that both originate from fossil fuel consumption. Out of all renewable energy sources, the Sun is the only source that produces enough energy to fulfill all our energy needs, now and in the future. Photovoltaics based on -conjugated polymers are envisioned to offer a low cost alternative to the present technology, but optimization of the polymer structure is needed to achieve efficiencies high enough to make this technology economically viable. This thesis deals with both the optimization of several parent structures via the process of energy level engineering and establishing structure-property relationships upon alteration of these parent structures. The initial work explored the effect of carbon-silicon exchange on various physical, optical and photovoltaic properties of fluorene/silafluorene-based copolymers. The optical, redox and photovoltaic properties of these polymers remained virtually unchanged except for the thermal behavior. The work was continued by optimizing the energy levels and bandgap of TQ1 with the aim to surpass its already high power conversion efficiency of 6%. Aside from improved spectral coverage and energy level optimization, several interesting structure-property relationships were found. Finally, another well-performing structure, PDPPTPT, was modified with alkoxy sidechains to investigate the effect on various polymer properties. Aside from a redshifted absorption, additional flexibility in the polymer backbone was obtained with concomitant changes in polymer properties. By comparing polymer and oligomer properties, methoxy substitution seems to initially increase melting and crystallization temperatures, but this is then supposedly counteracted due to increased irregularity in the polymer backbone.