Thermally-stable hydroxyl radicals implanted on TiO2 electron transport layer for efficient carrier extraction in PbS quantum dot photovoltaics

The electron-transport layer (ETL) between the PbS colloidal quantum dots (CQDs) active layer and the FTO substrate plays a crucial role in the associated heterojunction photovoltaics. However, the presence of surface defects in the ETL limits the performance of the photovoltaics. Herein, we report the improved carrier extraction at the TiO2/PbS interface of planar PbS CQDs photovoltaics, which can be easily accomplished through pre-implanting thermally stable hydroxyl radicals on the surface of TiO2 nanoparticles (NPs) for constructing ETL. The pre-implantations on the surface of TiO2 NPs realized using HNO3 aqueous solution treatment under the hydrothermal condition surprisingly wouldn't be completely removed after the annealing procedure necessary for preparing the conventional Ell, and effectively eliminate the interband trap sites of the TiO2 NPs by passivating intrinsic oxygen-deficient defects for achieving well-matched electron affinity and work function. As expected, the interfacial bimolecular recombination events are obviously suppressed thanks to the well-acceptable trap densities existing in TiO2 ETL as well as favorable interfacial band alignment for carrier extraction. Consequently, the power conversion efficiency (PCE) is pushed up to 9.31%, much better than the control competitors. Undoubtedly, this work provides another opportunity for optimizing performance of the relevant PbS CQDs photovoltaics resorting to interface engineering guideline.