Engineered CuInSexS2-x Quantum Dots for Sensitized Solar Cells

Colloidal CuInSexS2-x quantum dots (QDs) are an attractive less-toxic alternative to PbX and CdX (X = S, Se, and Te) QDs for solution-processed semiconductor devices. This relatively new class of QD materials is particularly suited to serving as an absorber in photovoltaics, owing to its high absorption coefficient and near-optimal and finely tunable band gap. Here, we engineer CuInSexS2-x QD sensitizers for enhanced performance of QD-sensitized TiO2 solar cells (QDSSCs). Our QD synthesis employs 1-dodecanethiol (DDT) as a low-cost solvent, which also serves as a ligand, and a sulfur precursor; addition of triakylphosphine selenide leads to incorporation of controlled amounts of selenium, reducing the band gap compared to that of pure CuInS2 QDs. This enables significantly higher photocurrent in the near-infrared (IR) region of the solar spectrum without sacrificing photovoltage. In order to passivate QD surface recombination centers, we perform a surface-cation exchange with Cd prior to sensitization, which enhances chemical stability and leads to a further increase in photocurrent. We use the synthesized QDs to demonstrate proof-of-concept QDSSCs with up to 3.5% power conversion efficiency.