Dye-sensitized solar cells (DSSCs) based on ZnO photoanodes have, despite extensive research, lacked behind cells based on TiO2, which is due to generally lower open-circuit voltages VOC and fill factors....

No abstract

Surface reactions of electrolyte additives and consequences for cell properties are studied and assigned to characteristics specific for both semiconductors.

The power conversion efficiency in established dye-sensitized solar cells (DSSCs) suffers from high overpotentials needed because of slow electron transfer kinetics. If redox couples are used that have a low reorganization energy, fast dye regeneration can be achieved, but fast recombination reactions can barely be suppressed. If they become competitive to electron transport to the back electrode, solar cell efficiencies drastically drop. In this work, it is shown that electron transport is facilitated by substituting the commonly used photoanode material, nanoparticulate TiO2, by electrodeposited ZnO, which, albeit more complex surface reactions, provides electron transport by orders of magnitude faster than nanoparticulate TiO2. With TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as the redox mediator, the dye is efficiently regenerated with overpotentials well below 0.2 V. We demonstrate that the external quantum efficiency with TiO2-based photoanodes is significantly limited by recombination, while it is maintained at high values for electrodeposited ZnO. It is thereby shown that redox couples with fast kinetics can be employed in DSSCs without drawbacks in quantum efficiency if sufficient fast electron transport in the porous semiconductor network is provided.