The study of organic dyes for p-type dye-sensitized solar cells

Abstract: This thesis concerns the study of D–?–A type dyes as sensitizers for NiO-based p-type dye-sensitized solar cells. The focus has been on the design and synthesis of efficient dyes and the identification of parameters limiting the solar cell performance.We have developed a new design strategy for the dyes: upon photoexcitation of the dye, the electron density is moving from the part that is attached to the semiconductor towards the part which is pointing away. This intramolecular charge transfer provides an efficient pathway for the following charge transfer processes. The first organic dye, composed of a triphenylamine (TPA) moiety as the electron-donor, dicyanovinyl groups as the electron-acceptors and linked by thiophene units, showed much better photovoltaic performance than other dyes reported at the same time, turning it into a model for future dye design.A series of dyes with different energy levels were synthesized and characterized on NiO-based devices using iodide/triiodide as redox couple. Lower photovoltaic performance was obtained for the dye with less negative reduction potential due to the insufficient driving force for dye regeneration. We have investigated the symmetric and unsymmetric structures of the dyes. The breaking of molecular symmetry did not significantly broaden the absorption spectrum, or improve the efficiency. In addition, we have tuned the molecular structure to prevent charge recombination. Increasing the distance between the anchoring group and the electron-acceptor was an effective way to improve the device efficiency. Besides TPA-based compounds, a zinc porphyrin dye was also synthesized and tested in p-type solar cells. However, the solar cell performed less well due to its narrow absorption band and the tendency for aggregation. Co-sensitization of the TPA-based dye with the porphyrin dye did not result in higher photovoltaic performance.After optimization of the dye structure, the highest overall conversion efficiency was achieved for the P5-sensitized solar cell, based on 1.5 ?m NiO film prepared from NiCl2 and the F108 template precursor, and an acetonitrile-based electrolyte.