Infoscience

Thesis

Solid-state dye-sensitized mesoporous TiO2 solar cells

The objective of this work was to develop a solid-state dye-sensitized solar cell, in which the liquid electrolyte, commonly applied in photoelectrochemical cells, is replaced by a solid organic charge transport material (spiro-OMeTAD). The dye-sensitized nanostructured solid-state junction realized in this work shows monochromatic photon-to-electron conversion efficiencies (IPCE) as high as 50 % (uncorrected for transmission losses) compared to 70 % IPCE for similar photoelectrochemical cells. Maximum white light conversion efficiencies reach 1.8 % (AM1.5; 10 mW/cm2). This is the first time that dye-sensitized hybrid organic/inorganic solar cells with such high conversion yields are obtained exceeding previously reported performances by about two orders of magnitude. Transient laser spectroscopy showed that dye regeneration in the solid junction proceeds at least one order of magnitude faster than in comparable photoelectrochemical systems, however the analysis of photocurrent action spectra revealed interfacial charge recombination during charge collection as the cells main loss mechanism. Blocking of the SnO2-glass window material with a dense TiO2 layer proved to be vital in avoiding internal short circuits in the cell, while gold, platinum, graphite and conducting polythiophene polymers were found to be suitable materials to establish ohmic contacts to spiro-OMeTAD. The conductivity of thin spiro-OMeTAD films was adjusted by the addition of appropriate dopants like the diradical cation spiro-OMeTAD++(PF6-)2, which was synthesized and isolated in crystalline form. The electronic properties of organic/inorganic photovoltaic junctions could be controlled by means of a selfassembled organic layer located at the heterojunction. The dipole moment of the self-assembling molecules was found to be the chief parameter, controlling the rectifying junction properties. Finally, mobility studies were conducted on charge transport materials containing spiro-centers. High charge mobilities were measured, comparable to those of non-spiro-analogues. The "spiro-concept" therefore proved to be a powerful strategy to confer high morphologic stability to a material, while maintaining its electronic properties.

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