SnS44– Metal Chalcogenide Ligand, S2– Metal Free Ligand, and Organic Surface Ligand Toward Efficient CdSe Quantum Dot- Sensitized Solar Cells

Abstract

Inorganic surface ligands such as metal chalcogenides ligand SnS44- and metal free ligand S2- were introduced for CdSe quantum dot sensitized solar cell (QDSSC) applications. The SnS44- ligand QDs were successfully deposited onto TiO2 photoanode through metal ion coordination. In solution, metal-ammonia complexes of Zn2+, Cd2+, and Cu2+ can be coordinated by the SnS44- ligands and reverse the zeta potential. Similarly, the metal ions can be sandwiched by SnS44- ligands and the photoanode. Using the metal ion bridged SnS44- ligand QDs as the sensitizer, photovoltaic properties of the QDSSCs have been studied. Cd2+ mediated deposition case showed better photovoltaic performance than the cases of Zn2+ or Cu2+. To further investigate the surface ligand effect on QDSSC, organic/inorganic mixed surface CdSe QDs were introduced using partial ligand exchange after the deposition onto the TiO2 photoanode. The postdeposition surface ligand exchange with inorganic ligands such as SnS44- and S2- is thought to retain the initial organic ligands between QDs and TiO2 photoanode and selectively replace the QD ligands that would contact the electrolytes. Metal free S2- ligand QDSSC showed the best photovoltaic performance recording 1.4 times enhanced photocurrent and 1.5 times enhanced photoconversion efficiency when compared with the initial organic surface ligand QDSSC. Comparison studies on the photovoltaic properties of QDSSCs with different surfaces suggest that (i) the Cd-SnS44- complex and SnS44- surface ligand act as efficient electron traps hurdling the photovoltaic performance severely, (ii) S2- surface ligand works as an efficient hole trap only at the interface between the QD and TiO2, and (iii) S2- surface ligand blocks back electron transfers better than the initial organic ligand.