Doctor of Philosophy
Intelligent Polymer Research Institute
Griffith, Matthew James, Charge generation and recombination in porphyrin based dye sensitized solar cells, Doctor of Philosophy thesis, Intelligent Polymer Research Institute, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3661
This study has investigated the charge generation and recombination behaviour of porphyrin-based dye sensitized solar cells (DSSCs) using a range of time-resolved and steady state techniques. The enhanced understanding of these processes was then utilized to develop unique strategies to circumvent performance limitations associated with injection or recombination, thereby improving the power conversion efficiencies of these devices.
Injection behaviour has been studied using transient absorption with sub-ns time resolution and an optical model employed in conjunction with simple absorption measurements to calculate the absorbed photon-to-current conversion efficiency of devices. Recombination was monitored using stepped-light induced measurements of photocurrent and photovoltage to determine the lifetime of electrons in the semiconductor films. Furthermore, infrared spectroscopy has been used to probe the nature of the electronic linkage between the dye and semiconductor, and X-ray reflection spectroscopy has been adopted to analyze the orientation of the dyes adsorbed to the surface. The simultaneous deployment of this suite of tools provides unprecedented insight into the efficiency determining electron transfer steps in porphyrin DSSCs.
The major problems that limit performance in porphyrin DSSCs were determined to be low injection yields and an enhanced level of recombination in comparison to analogous sensitizers. The electron lifetime of porphyrin dyes was found to vary widely depending on different structural features, particularly the metallation of the macrocycle core. This was shown to result from the dye molecules attracting the electron acceptor species in the redox mediator towards the TiO2 surface and enhancing this recombination reaction.
Three specific strategies to improve charge injection and diminish recombination in porphyrin DSSCs were subsequently investigated. The first such strategy involved a simultaneous increase in the injection yield and decrease in the recombination rate of zinc porphyrin DSSCs after a 1 hour light exposure treatment. Measurements of the injection yield and electron lifetime in various electrolytes revealed that this phenomenon results from an exchange of electrolyte cations at the semiconductor surface. The second strategy involved co-sensitizing the same TiO2 surface with two different porphyrin dyes. This again resulted in improved charge generation and a decreased recombination rate, attributed to synergistic energy and charge transfer reactions between the dyes based upon transient absorption results. Finally, the effect of modifying the porphyrin dye structure was investigated, with studies of two different porphyrin dimers demonstrating that the higher extinction coefficients of the dimers can produce improved light harvesting and charge injection in devices. The dimers also provide a much bulkier sensitizer, thereby preventing the acceptor species from approaching the TiO2 surface and reducing the recombination rate.