UBC Theses and Dissertations
Cobalt tris(2,2'-bipyrimidine) redox mediators generate high dye-sensitized solar cell photovoltages Chen, Yingqian Kitty
Dye-sensitized solar cells (DSSCs) are promising, cost-effective technologies used to harness solar energy for electricity. Previous efforts to improve the solar-to-electricity conversion efficiency have primarily focused on sensitizer engineering and photocurrent generation. Alternatively, the efficiency can be increased by tuning the redox potential of the charge mediator to maximize the photovoltage in the device. This work describes the implementation of a new cobalt mediator (Co-bpm) with an exceptionally positively shifted redox potential of 1.07 V vs NHE in the DSSC. The best-performing device showed one of the highest reported DSSC photovoltages. The poor solubility of Co-bpm in MeCN was a major obstacle that was overcome by testing a variety of electrolyte solvent systems and counterions. Notwithstanding, Co-bpm mediator-based devices exhibited low photocurrents and low power conversion efficiencies despite the high voltages. A comparative study was then performed to elucidate how the positively shifted redox potential affect the photocurrent in Co-bpm mediator-based devices. Three cobalt analogs [Co-(bpm-DTB), Co-bpy and Co-(bpy-DTB)] of varying redox potentials were studied alongside Co-bpm to determine the trend between redox potential, device performance, and recombination lifetime. The redox potentials of the cobalt analogs were tuned by installing tert-butyl substituents and varying the number of nitrogen atoms in the ligand. A positive shift in the redox potentials correlated to a linear increase in photovoltage and non-linear decrease in photocurrent in DSSCs. A low quasi-Fermi level (EF,n) at open-circuit conditions and a short electron lifetime (Tn) in device containing Co-bpm indicate that a significant loss of electrons from TiO₂ via recombination pathways is one key factor that contribute to the poor photocurrent and overall device performance.
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