ZnO quantum dots as an electron-transport layer for highly efficient and stable organic solar cells

An advanced protocol for the mild synthesis of stable and concentrated ZnO quantum dots (QDs) yields colloidal inks suitable for applications in electron-transport layers (ETLs) of organic solar cells, delivering superior power conversion efficiency (PCE) and photodegradation stability as compared to bulk-like commercially available ZnO inks. The champion ZnO QDs-based devices with a quaternary PM6 : L8BO : BTP-eC9 : PC₇₀BM absorber exhibit a PCE of 18.86%, surpassing similar cells with bulk-like ZnO ETL (18.15%). The ZnO QDs exhibited size-dependent electron-transport efficiency, with the highest performance achieved for QDs of 4.4-4.5 nm, decreasing for larger QDs down to the level of the bulk-like ZnO reference. A correlation between the photoluminescence and electron-transport efficiencies of ZnO quantum dots (QDs) was observed and interpreted in terms of an interplay between the defect state density and exciton confinement in size-selected ZnO QDs.