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Organic-inorganic hybrid solar cell (HSC) combines both the unique properties of the conjugated polymers and inorganic nanocrystals. It has shown great potential in realizing low-cost solar cells. One of the key challenges in the advancement for the performance of hybrid photovoltaics is the incompatible interfaces between the organic and inorganic components. Post-synthesis ligand exchange of the nanocrystals is one of the viable approaches to improve the donor-acceptor interfaces. The selection of the replacing surface ligands are based on several considerations such as the size, structure and end groups of the ligand molecules and its affinity for nanocrystals surface and polymers. In this thesis, surface chemistry of cadmium selenide (CdSe) nanocrystals is investigated to improve the performance of the solar cells. CdSe nanorods are synthesized using hot-coordinating solvent method and are used as the electron acceptor in the HSCs. The ligand used to control growth during the synthesis is phosphonic acid with relatively long alkyl chain. To improve the electrical properties of the blend films, ligand exchange of CdSe nanorods using thiophenealkylamines is first demonstrated. Surface analysis of the CdSe nanorods showed that the original bulky surface ligands are partially replaced by the replacing ligands. The short-circuit current density (JSC) of the HSCs made of ligand exchanged CdSe nanorods is found to be improved by at least three times due to the reduced distance between polymer and nanocrystals and higher conductivity across the new ligands. Atomic force microscopy (AFM) is used to determine the surface morphology of the films. It is found that a combination of surface ligands is a better option to obtain optimal film morphology for photovoltaics applications. Carboxylic acid-based molecules have the potential for both controlling the anisotropic growth and providing the surface chemistry for good electrical transport of the CdSe nanocrystals. Therefore, the effect of several thiophene-carboxylic acid molecules as surface ligands for CdSe nanorods are studied and the HSCs made of these nanorods was able to achieve reasonably good performance. Pyridine is added into the host solvent as solvent additive to aid the dispersion of the nanorods. The effect of solvent additive is observed to be two-fold. It aids in dispersing the nanocrystals but at the same time induces severe aggregation of the semi-crystalline poly(3-hexylthiophene) (P3HT). Doctor of Philosophy (MSE) |
