Minimizing the Reorganization Energy of Cobalt Redox Mediators Maximizes Charge Transfer Rates from Quantum Dots

Light-induced charge separation is at the very heart of many solar harvesting technologies. The reduction of energetic barriers to charge separation and transfer increases the rate of separation and the overall efficiency of these technologies. Here we report that the internal reorganization energy of the redox acceptor, the movement of the atoms with changing charge, has a profound effect on the charge transfer rates from donor quantum dots. We experimentally studied and modelled with Marcus Theory charge transfer to cobalt complexes that have similar redox potentials covering 350 mV, but vastly different reorganization energies spanning 2 eV. While the driving force does influence the electron transfer rates, the reorganization energies had a far more profound effect, increasing charge transfer rates by several orders of magnitude. Our studies suggest that careful design of redox mediators to minimize reorganization energy is an untapped route to drastically increase the efficiency of quantum dot applications that feature charge transfer.