Light Harvesting using Metal-Organic and Organic Sensitizers in Hybrid Solar Cells: Synthesis, Characterisation and Application

This thesis addresses the question how to improve light harvesting with novel tailor-made metal-organic and organic sensitizers for solid-state hybrid solar cell applications. Two approaches are in the focus: 1) the design and synthesis of sensitizers featuring high extinction coefficients over a broad wavelength range and 2) modern device concepts to further enhance or extend the absorption by the combination of two sensitizers. In short: The primary goal was to broaden and boost the optical density of hybrid solar cells. To reach this, novel sensitizer with extended conjugated π-system providing excellent optical properties had to be designed and synthesised in complex multi-step reaction sequences. For ideal sensitizers, further aspects had to be taken into account such as structural demands, electronic properties, and the tendency towards aggregation. The first part of this thesis deals with the synthesis, characterisation and application of a series of metal-organic ruthenium(II) donor-antenna complexes. In addition to the typically broad absorption of Ru(II)bis(bipyridyl)(NCS)2 complexes in the blue-green region arising from MLCT, these dyes feature much higher extinction coefficients in comparison to a commercially available reference dye lacking any donor-antenna groups. By the application of these Ru(II) complexes in solid-state dye-sensitized solar cells, we found a clear structure-property relationship. The performance - especially the photocurrent density - was significantly improved with increasing extension of the delocalized system of the donor-antenna groups. To further boost the optical density in hybrid solar cells sensitized with a donor-antenna ruthenium dye (Ru-TPA-NCS), we developed an innovative and technically relevant concept for multichromophore sensitization involving a second sensitizer (TPD-dye). The latter absorbs in a region where Ru-TPA-NCS absorbs weakly. The solar cells were fabricated according to a novel method developed by us. However, the power conversion efficiencies of multichromophore hybrid blend solar cells were initially low due the weak interconnectivity of the TiO2 particles. This issue was addressed by an optimization of the TiO2:spiro-OMeTAD ratio and the addition of PCBM A further concept dealing with the combination of two sensitizers in solid-state dye-sensitized solar cell was accomplished by co-sensitization of a triphenyldiamine-based dye (TPD-dye) absorbing in the blue region and squaraine dye (SQ-dye) mainly absorbing the red part of the visible spectrum. In this way, the optical response of the device was extended up to 700 nm. Under optimized conditions, a conversion efficiency of 2.41 % could be reached. To accomplish the desired panchromaticity or even an extension of the absorption up to the NIR region with a single sensitizer, novel BODIPY dyes with excellent optical properties were designed and synthesised. We prepared BODIPYs with donor-groups to extend the delocalized system and integrated a meso-ethynyl bridge between the BODIPY core and the anchoring group to improve the electronic connection between them. For comparison, we synthesised the corresponding BODIPYs without donor-moieties and without ethynyl bridge. The multi-step synthetic routes were optimized, the mechanism of the donor-attachment was clarified and the introduction of the ethynylphenyl group in the meso-position was accomplished for the first time. The optical characterisation of the compounds disclosed an impressively broad and intensive spectral response, especially for a meso-ethynylphenyl BODIPY with donor-groups. This dye absorbs up to 1030 nm with high extinction coefficients. This makes suitable functionalised BODIPYs promising candidates for solar cell applications. The next part took advantage of the excellent optical properties of BODIPYs and expanded the topic towards the concept of energy transfer. Here, an unattached energy donor dye provides additional adsorption and transfers the energy to a sensitizing acceptor dye. Indeed, for appropriate combinations an additional contribution to the external quantum efficiency was found in the absorption region of the energy donor dye. Furthermore, a review chapter covering all aspects of dye-sensitized solar cells and the sensitizers is added as appendix. In summary, this thesis presents the successful design, synthesis and characterisation of both metal-organic and organic sensitizers including ruthenium complexes, triphenyldiamine-based dyes, a squaraine sensitizer and BODIPY dyes. The sensitizers (either individually or in combination with complementary sensitizers) provide excellent optical properties for the application in solar cells. The applicability of these sensitizers was successfully demonstrated in standard solid-state dye-sensitized solar cells, in newly developed multichromophore hybrid blend solar cells, co-sensitized solar cells and in solid-state dye-sensitized solar cells taking advantage of energy transfer.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2012

Author dc:creator

• Gräf, Katja

Contributors dc:contributor

• Thelakkat, Mukundan

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/183/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:183