Characterisation of materials for organic photovoltaics
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Organic solar cells offer the possibility for lightweight, flexible, and inexpensive photovoltaic devices. This thesis studies the physics of a wide range of materials designed for use in organic solar cells. The materials investigated include conjugated polymers, conjugated dendrimers, and inorganic nanocrystals. The materials studied in this thesis fall into five categories: conjugated polymers blended with a buckminsterfullerene derivative PCBM, nanocrystals synthesised in a conjugated polymer matrix, conjugated polymers designed for intramolecular charge separation, conjugated dendrimers blended with PCBM, and nanocrystals synthesised in a matrix of conjugated small molecules or dendrimers. Conjugated polymers blended with PCBM have been extensively studied for photovoltaic applications, and hence form an ideal test bed for new experiments. In this thesis this blend was used to achieve the first pulsed electrically detected magnetic resonance experiments on organic solar cells. Nanocrystals are attractive for photovoltaics because it is possible to tune their band gap across the solar spectrum. In this thesis a one-pot synthesis is used to grow PbS and CdS nanocrystals in conjugated polymers, soluble small molecules, and dendrimers, and characterisation is performed on these composites. Previous work on dendrimer: nanocrystal composites has been limited to non-conjugated molecules, and the synthesis developed in this thesis extends this work to a conjugated oligomer and a conjugated dendrimer. This synthesis can potentially be extended to a variety of conjugated soluble small molecule: nanocrystal and dendrimer: nanocrystal systems. Conjugated dendrimers have been successfully employed in organic light emitting diodes, and in this thesis they are applied to organic solar cells. Materials based on fluorene and cyanine dye cores show excellent absorption tunability across the solar spectrum. A set of electronically asymetric polymers designed for intramolecular charge separation were investigated. Quenching of the luminescence was observed, and light induced electron paramagnetic resonance measurements revealed that photoexcitation led to approximately equal numbers of positive polarons and nitro centred radical anions. This indicates that charge separation is occurring in these molecules.
Thesis, PhD Doctor of Philosophy
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