Quantum dot sensitized solar cells are primarily composed of mesoporous nanocrystalline titanium dioxide and cadmium chalcogenide quantum dots. With the report of multiple exciton generation, these solar cell constructs have the potential to reach efficiencies of 40% (8% over other single junction architectures). However, to date the highest efficiency demonstrated with a QDSSC is ~7%. The reported efficiencies are low due to the inability to extract charge from the photoanode. There exist no suitable redox mediators to complete the circuit in these liquid junction devices. To address this critical need, presented herein is a study of a series of manganese complexes for use as charge transporters. These mediators realized a 50% average increase in efficiency over cobalt trisbipyridine, a commonly used small molecule mediator. This increase in efficiency is due to (1) the more positive reduction potentials of the complexes leading to increase V<sub>OC</sub> and (2) higher fill factors indicative of decreased parasitic back electron transfer. The decreased rate of back electron transport is most likely due to the barrier imposed by the charge transfer induced spin crossover exhibited by the manganese complexes. Systematic manipulation of the ligand environment will be discussed as a method to increase efficiency beyond the current set-point. Also, a detailed study of the recombination kinetics between electrons in TiO<sub>2</sub> and the redox mediator will be presented.

Exploring charge transfer induced spin cross-over redox mediators in quantum dot sensitized solar cells

249th National Meeting (2015)

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