PbS as a Highly Catalytic Counter Electrode for Polysulfide-Based Quantum Dot Solar Cells

We show that a significant improvement of the light to electric power conversion efficiency of quantum dot sensitized solar cells with a polysulfide electrolyte can be achieved when a PbS counter electrode is used. An acid pretreatment of a Pb metal foil followed by dipping into polysulfide solution provides a counter electrode which improves the short circuit current, open circuit voltage, and fill factor compared to commonly used Pt electrodes. Electrochemical impedance spectroscopy reveals a low charge transfer resistance between polysulfide and PbS, indicating the high catalytic activity of PbS. Moreover, a comparison with reported charge transfer resistance values at open circuit potential for alternative counter electrodes for polysulfide shows that PbS is the most catalytic of all. For a CdSe quantum dot sensitized mesoporous TiO₂ electrode, we achieved a conversion efficiency of 3% using PbS as a counter electrode.