Improving the Efficiency of the Mn2+/3+ Couple in Quantum Dot Solar Cells: The Role of Spin Crossover

In this study, we present the synthesis of a family of first-row transition metal redox mediators based on the bis­[hydrotris­(pyrazolyl)]­borate manganese­(II/III) (MnTp2) redox couple. Using cyclic voltammetry, the electrochemical properties and characteristic spin crossover inherent in this class of metal complexes were analyzed. From the electrochemical analysis the standard heterogeneous rate constant (k s,h) was estimated. These constants were 1–2 orders of magnitude lower than other outer-sphere redox couples, such as Co­(bpy)3 2+/3+ with k s,h values decreasing from 6.39 (± 0.7) × 10–3 cm/s in Co­(bpy)3 to 1.60 (± 0.3) × 10–5 cm/s in bis­[hydrotris­(4-ethylpyrazolyl)]­borate manganese­(II/III). It was theorized that the drastic reduction in the rate of electron transfer could be used to increase the lifetimes of the injected electrons in quantum dot sensitized solar cells (QDSSCs). Indeed, this was found to be the case with the slope of open-circuit voltage decay measurements being an order of magnitude lower in the Mn-based redox couples, compared to Co­(bpy)3 when using cells prepared under the same conditions. This increase led us to then focus on optimizing the electrolyte solvent to assess the current–voltage characteristics of cells prepared using the MnTp2 family of redox mediators. These cells displayed enhanced power conversion efficiencies when compared to Co­(bpy)3 despite poor diffusion throughout the nanostructured TiO2 film. Analysis of the quenching rate constant via Stern–Volmer quenching analysis suggested that the MnTp2 family of redox mediators possesses an adequate ability to regenerate the quantum dot sensitizer, with values of k q being on similar orders of magnitude as other Co- and Cu-based redox couples employed in dye-sensitized solar cells. Ultimately, it was concluded that the increase in the lifetime of the injected electron, working in concert with increased open-circuit voltage potentials, was the source of the significantly improved power conversion efficiencies.