Four Orders of Magnitude Acceleration of Electron Recombination at the Dye-TiO2/Electrolyte Interface Severely Limiting Photocurrent with High-Oxidation-Potential Cu^2+/1+ Complexes

High-oxidation-potential Cu1+ complexes have been claimed to exhibit fast electron transfer toward oxidized dyes (dye)•+ on electrode surfaces (regeneration) at a low driving force. Herein, we show that the regeneration of the surface-bound (dye)•+ with heteroleptic Cu1+ complex electrolytes with a high oxidation potential, defined by the lowest oxidation potential redox peak, with a dye regeneration driving force of 170 meV is slow. On the other hand, the recombination reaction between the electrons in the semiconductor and the (dye)•+ is accelerated by 4 orders of magnitude in the presence of a Cu2+ complex. Thus, using the Cu2+/1+ complex mixture in an electrolyte solution, such previously unknown acceleration of recombination could easily be misinterpreted as dye regeneration. This significant finding helps to explain the previously unknown origin of the photocurrent drop of solar cells using high-redox-potential Cu2+/1+ complexes with high apparent regeneration yield. Although the origin of such unprecedented acceleration of kinetics is not yet fully identified, partial oxidation of the dye layer by the Cu2+ complexes is one of the key sources. The study is expected to influence the design of new redox mediators at a low driving force and specifically how their regeneration efficiency is evaluated. The huge acceleration of kinetics could provide a new avenue to control electron transfer rates at semiconductor/organic molecule interfaces relevant to a number of electrochemical applications involving electron transfer at semiconductor/electrolyte junctions, including photocatalysis, photosynthetic systems, and sensing.