Synthesis and Characterization of Infrared-Absorbing Osmium-Polypyridyl Dyes for Use in Dye-Sensitized Solar Cells
- Author(s): Cardon, Joseph Michael
- Advisor(s): Ardo, Shane
- et al.
Dye-sensitized solar cells (DSSCs) are a potentially low-cost alternative to silicon solar cells, however DSSCs have not been largely commercialized due to their poor solar to electric energy conversion efficiency, ~14%, compared to silicon, ~25%. One reason for their poor efficiency is that current record holding DSSCs employ dyes that absorb a less-than-ideal portion of the solar spectrum as dictated by the Shockley-Queisser Limit. To remedy this, infrared-absorbing osmium polypyridyl dyes were synthesized and their basic parameters characterized by ultraviolet-visible absorption spectroscopy, stepwise potential step spectroelectrochemistry, and cyclic voltammetry. The dyes appeared to have close-to-ideal optical pseudo-bandgaps and excited state energy levels, however, their performance in DSSCs employing the commonly used iodide-triodide redox shuttle was very poor. To better understand the behavior of these dyes in a working environment apparent diffusion coefficients of later self-exchange electron transfer between oxidized and reduced dyes bound to mesoporous titanium dioxide films was studied using several different techniques which produced different results. The different techniques were able to be harmonized by correct use of mathematical assumptions and measurement of non-ideal Nernstian behavior of bound dyes. Foot-of-the-wave analysis was modified for use on dyes bound to mesoporous films with substrate in solution. This revealed that several infrared absorbing osmium polypyridyl dyes performed very slow electron transfer from iodide with maximum second order electron transfer rate constants of 78 M-1 s-1, which could explain their poor performance in DSSCs using the iodide triiodide redox shuttle. Rates with 1,1’-dimethyl ferrocene were much faster, with rate constants as high as 74,000 M-1s-1. Nanosecond transient absorption spectroscopy and nanosecond transient microwave conductivity measurements were performed on DSSCs containing working benchmark dye N3 and infrared-absorbing osmium polypyridyl dyes. Osmium polypyidyl dyes were observed to successfully inject electrons into the TiO2 film with electron in TiO2 to oxidized dye recombination rates comparable to N3. Oxidized N3 rapidly regenerated in the presence of iodide, resulting in charge separated states that lasted for almost a second, but the infrared-absorbing osmium dyed did not. Several catalysts were fabricated to speed up iodide oxidation, but none worked in a solar cell.