Mn<sup>II/III</sup> Complexes as Promising Redox Mediators in Quantum-Dot-Sensitized
Solar Cells
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Abstract
The
advancement of quantum dot sensitized solar cell (QDSSC) technology
depends on optimizing directional charge transfer between light absorbing
quantum dots, TiO<sub>2</sub>, and a redox mediator. The nature of
the redox mediator plays a pivotal role in determining the photocurrent
and photovoltage from the solar cell. Kinetically, reduction of oxidized
quantum dots by the redox mediator should be rapid and faster than
the back electron transfer between TiO<sub>2</sub> and oxidized quantum
dots to maintain photocurrent. Thermodynamically, the reduction potential
of the redox mediator should be sufficiently positive to provide high
photovoltages. To satisfy both criteria and enhance power conversion
efficiencies, we introduced charge transfer spin-crossover Mn<sup>II/III</sup> complexes as promising redox mediator alternatives in
QDSSCs. High photovoltages ∼1 V were achieved by a series of
Mn poly(pyrazolyl)borates, with reduction potentials ∼0.51
V vs Ag/AgCl. Back electron transfer (recombination) rates were slower
than Co(bpy)<sub>3</sub>, where bpy = 2,2′-bipyridine, evidenced
by electron lifetimes up to 4 orders of magnitude longer. This is
indicative of a large barrier to electron transport imposed by spin-crossover
in these complexes. Low solubility prevented the redox mediators from
sustaining high photocurrent due to mass transport limits. However,
with high fill factors (∼0.6) and photovoltages, they demonstrate
competitive efficiencies with Co(bpy)<sub>3</sub> redox mediator at
the same concentration. More positive reduction potentials and slower
recombination rates compared to current redox mediators establish
the viability of Mn poly(pyrazolyl)borates as promising redox mediators.
By capitalizing on these characteristics, efficient Mn<sup>II/III</sup>-based QDSSCs can be achieved with more soluble Mn-complexes