Metal Selenides as a New Class of Electrocatalysts for Quantum Dot-Sensitized Solar Cells: A Tale of Cu_1.8Se and PbSe

The development of a Pt-free, highly active electrocatalyst for a counter electrode (CE) is vital to the construction of highly efficient quantum dot-sensitized solar cells (QDSSCs). As an alternative to Pt, the use of various metal sulfides, such as Cu2S, CoS, and PbS, has been successfully demonstrated; however, the studies on the utilization of non-sulfide materials have been scarcely reported. In this regard, we examined eight different types of binary metal selenides as new candidate materials, and found that the electrocatalytic activity of Cu1.8Se and PbSe toward polysulfide reduction was superior to that of Pt. In depth investigation into these two materials further revealed that, while the electrocatalytic activity of both metal selenides surpasses that of Pt, the long-term utilization of the PbSe CE is hindered by the formation of PbO on the surface of PbSe, which is attributed to the instability of PbSe under air. Unlike PbSe, Cu1.8Se was found to be chemically stable with a polysulfide electrolyte and was even better than Cu2S, a commonly used CE material for QDSSCs. Using the Cu1.8Se CE, we obtained a power conversion efficiency of 5.0% for CdS/CdSe-sensitized solar cells, which was an efficiency almost twice that obtained from Pt CE. This work provides a new application for metal selenides, which have been traditionally utilized as sensitizers for QDSSCs.