Type-II colloidal quantum dot sensitized solar cells with a thiourea based organic redox couple

In this paper, one kind of organic electrolyte based on tetramethylthiourea is employed for quantum dot sensitized solar cells (QDSCs). By reducing the impedance between the electrolyte and the counter electrode, the fill factor of such organic electrolyte based QDSCs is significantly improved. It is possible to substantially increase the photovoltage and to reach an efficiency three times higher than that of a commonly used inorganic electrolyte. The light harvesting ability of the organic electrolyte based QDSCs is successfully extended by using type-II QDs, where the adsorption of ZnS gives an additional advantage in further enhancing the stability of the cells. It is observed that core/shell ZnSe/CdS type-II QDs give higher electron injection than CdS/ZnSe QDs, proving that the electron distribution in the QDs is important for the electron extraction. A full working mechanism of the organic redox couple for the QDSCs is proposed.