PbS Quantum Dots Capped with Amorphous ZnS for Bulk Heterojunction Solar Cells: The Solvent Effect

In this study, two distinct structures of PbS quantum dots (QDs) are produced with amorphous ZnS as the capping material by successive ionic layer adsorption and reaction. With methanolic solution, spherical PbS QDs (∼5 nm) are embedded in the ZnS matrix (i.e., embedding structure), exhibiting relatively large distance between the QDs. With aqueous solution, irregularly shaped PbS QDs (<3 nm) blend intimately with the ZnS medium (i.e., blending structure), showing indiscernible QD spacing. This is attributed to a relatively low reactivity of Pb(2+) ions in water, suppressing quantum dot growth in the mesopores. Bulk heterojunction of mesoporous TiO2 substrate filled up with the blending configuration shows superior photovoltaic performance to the embedding architecture, because of the small QD size and close distance between the QDs.