Importance of recombination at the TCO/electrolyte interface for high efficiency quantum dot sensitized solar cells

Here, we show that, in CdSe quantum dot sensitized solar cells (QDSCs), recombination of electrons from the transparent conducting oxide (TCO) front electrode with oxidized species of the polysulfide redox electrolyte cannot be neglected like in dye-sensitized solar cells (DSCs). We demonstrate that light to electric power conversion efficiencies up to 4% can be achieved when recombination at the front electrode is suppressed by a compact TiO ₂ layer deposited in between the TCO substrate and the QD sensitized porous TiO ₂ film. Numerical simulations based on a simple equivalent circuit suggest that, over a wide potential range, electron transfer into the electrolyte at the TCO substrate is the dominant recombination path, which is usually not considered, suggesting that the current understanding of QDSCs has to be revised.