Molecular Engineering, Photophysical and Electrochemical Characterizations of Novel Ru(II) and BODIPY Sensitizers for Mesoporous TiO2 Solar Cells

To realize the dream of a low carbon society and ensure the wide spread application of renewable energy sources such as solar energy, photovoltaic devices should be highly efficient, cost-effective and stable for at least 20 years. Dye sensitized solar cells (DSCs) are photovoltaic cells that mimic the natural photosynthesis. In a DSC, the dye absorbs photons from incident light and converts those photons to electric charges, which are then extracted to the outer circuit through semiconductor TiO2, whereas the mediator regenerates the oxidized dye. A sensitizer is the pivotal component in the device in terms of determining the spectral response, color, photocurrent density, long term stability, and thickness of a DSC. The breakthrough report by O'Regan and Gratzel in 1991 has garnered more than 18,673 citations (as of October 9, 2014), which indicates the immense scientific interest to better understand and improve the fundamental science of this technology.;With the aforementioned in mind, this study has focused on the molecular engineering of novel sensitizers to provide a better understanding of structure-property relationships of novel sensitizers for DSCs. The characterization of sensitizers (HD-1-mono, HD-2-mono and HD-2) for photovoltaic applications showed that the photocurrent response of DSCs can be increased by using mono-ancillary ligand instead of bis-ancillary ligands, which is of great commercial value considering the difference in the molecular weights of both dyes. The results of this work were published in Journal of Materials Chemistry A (doi:10.1039/c4ta01942c) and ACS Applied Materials and Interfaces (doi: 10.1021/am502400b).;Furthermore, structure-property relationships were investigated in Ru (II) sensitizers HL-41 and HL-42 in order to elucidate the steric effects of electron donating ancillary ligands on photocurrent and photovoltage, as discussed in Chapter 4. It was found that the electron donating group (ethoxy) ortho to the CH=CH spacer precludes coplanarity of the naphthalene moiety, thus decreasing the extracted photocurrent response from solar device. The findings were published in Dyes and Pigments (doi:10.1016/j.dyepig.2014.08.005). For HD-7 and HD-8, intriguing difference caused by structural isomerization based on anthracene and phenanthrene stilbazole type ancillary ligands, respectively in Ru (II) sensitizers was investigated using femtosecond transient absorption spectroscopy. It was found that the excited electrons in HD-7 are prone to ISC (intersystem crossing) much more than that in HD-8 and those triplet electrons are not being injected in TiO2 efficiently as discussed in Chapter 5. To achieve long term stability, we combined the strong electron donor characteristics of carbazole and the hydrophobic nature of long alkyl chains, C7 (HD-14 ), C18 (HD-15) and C2 (NCSU-10), tethered to N-carbazole. HD-15 showed strikingly good long term light soaking stability and maintained up to 98% of initial efficiency value compared to 92% for HD-14 and 78% for NCSU-10, as discussed in Chapter 6.;Boron dipyromethene (BODIPY) dyes HB-1, HB-2 and HB-3 were synthesized and fully characterized for dye solar cells. It was found that having long alkyl chains tethered to the donor groups alone are not sufficient for achieving highly efficient photovoltaic response from BODIPY dyes (Chapter 7). Thus, replacement of fluorines from BODIPY core with long alkoxy chains has been suggested for future work.