Conjugated polymers with functional side chains for photovoltaics
Polymer solar cells are quickly approaching viability as a large scale electrical power source. To increase their stability and processability in green solvents, we want to modify their solubilizing side chains with different functionalities. Therefore, in this thesis, a several series of conjugated polymers are presented along with their characterization and utilization. A specific focus is put on the morphology characterization of polymer:PCBM-blends with the use of transmission electron microscopy, which can be a powerful tool to illuminate the bulk of materials where the morphology is of utmost importance. Three case studies are presented where polymer solar cells were characterized with this instrument along with the different conclusions that can be drawn from the data. Afterwards a series of conjugated random copolymers bearing thermocleavable side chains is presented with the purpose of introducing solvent processability which can be removed, leaving insoluble polymer films. They are characterized extensively and their thermal degradation is studied in detail. A morphological study is performed in order to analyze the effect the thermal stabilization has on the morphology and the pure thermocleavable polymer completely stabilized the film. In the following section, a series of fluorene based polymers with polar pendant groups were presented. The polymers were used as cathode interfacial material in all-polymer solar cells and increased the performance from 2.7% to a maximum of 5.4%, a level comparable to that of the inorganic lithium fluoride material. This method thus shows a good alternative for future high throughput production of solar cells. In a related project, similar polymers with varied energy levels were produced with the aim of studying their performance as active layer materials. Polymers bearing tertiary amines are common in interlayers, but not in active layers, due to hole trapping when tried in solar cells, one polymer showed a performance of above 1%. These results are a step towards polymer solar cell active layers processable in alcohol/water.
side chain engineering
polymer solar cells
green solvent solubility
transmission electron microscopy