Importance of the reorganization energy barrier in computational design of porphyrin-based solar cells with cobalt-based redox mediators

The shift from iodide based redox mediators in dye sensitized solar
cells towards octahedral cobalt complexes has lead to a significant increase in the efficiency. However, due to the nature of this type of complexes the driving force required for the regeneration of the dye is very high and this limits the achievable efficiency. Here we show that the large driving force is a direct consequence of the large reorganization energy of the dye regeneration reaction. The reorganization energies for charge transfer between a simple zinc porphyrin dye and two popular cobalt based redox mediators is calculated using ab-initio molecular dynamics with explicit solvent. These results are then combined with a
Marcus based extrapolation scheme to obtain the reorganization energies of more than five thousand porphyrin based dyes. We propose a scheme for scoring the performance of the porphyrin dyes which is able to identify already known high-performance dyes in addition to a number of even better candidates. Our analysis shows that large internal reorganization energy of the Co-based redox mediators is a main bottleneck for achieving higher efficiencies.