High Electronic Coupling between Copper Complexes and Dyes
Copper complexes are promising redox mediators for dye-sensitized solar cells (DSSCs), achieving the highest energy conversion efficiency to date. When light is irradiated onto the DSSCs, electrons are injected into the oxide semiconductor from the dyes, leading to oxidized dyes on the surface. To increase the short-circuit current density of DSSCs, the oxidized dyes should be reduced as fast as possible. Copper complexes are known to reduce oxidized dyes more effectively than any other redox mediators. According to Marcus theory, there are three electron transfer rate-determining factors for interfacial electron transfer: the driving force, the reorganization energy, and the electronic coupling. The faster electron transfer (ET) kinetics of copper complexes was thought to originate from the lower internal reorganization energy accompanied by subtle structural changes from Cu+ to Cu2+. However, this assumption had not been experimentally verified.
A recent study demonstrated that the dominant factor of the faster electron transfer (ET) kinetics arises from higher electronic coupling, with only a minor contribution from reorganization energy. In a collaborative work between Shinshu University and University of Wollongong, the authors compared the driving force dependence of the dye reduction rates among a series of copper and cobalt complexes. The copper complexes have shown a two orders of magnitude higher reduction rate at matched driving forces. The Marcus parabola for the copper complexes was upshifted compared to that of the cobalt-mediator based dataset. This was interpreted by the increased electronic coupling by copper complexes.
An important step towards understanding the origin of increased coupling, the electronic coupling values between the oxidized dye and two copper and cobalt complexes have been calculated using density functional theory. First, the geometry of the oxidized dye and the reduced form of the complexes were optimized (PW91/TZ2P). Then, the complexes were placed at various distances away from the dye backbone ranging from 6 to 14 Å. The electron coupling values between the dye and the complex fragments were calculated using the Electrontransfer keyword within the Frozen Density Embedding formalism, as implemented in ADF. Two cycle of freeze and thaw were implemented manually, and the calculations were performed at the PW91/TZP level. As shown in the figure, the calculated electronic coupling decreases almost logarithmically with increasing distance between the molecules, as expected for intermolecular electron transfer. At the same position, the calculations showed that Cu complexes had two to three times higher electronic coupling than Co complexes. These results were obtained at the orientation yielding maximum electronic coupling, as shown in the inset.
Some of the Cu complexes studied did not follow the trend of the other complexes, suggesting these complexes have even higher electronic coupling. Further work should focus on understanding the reason, to design complexes with even higher coupling. The ultimate aim is fast reduction (microsecond timescale) at a driving force of less than 0.1 eV.
