As for the solution-processed MAPbI(3) film, the pinholes and gaps are an inevitable and undesirable phenomenon, which can usually lead to serious interface charge recombination due to the penetration of hole transport materials in perovskite solar cells (PSCs). Herein, a hydrophobic beta-NaYF4 :Yb,Er up-conversion ultrathin layer with particle size ca. 6 nm was first applied to modify the MAPbI(3) film for optimizing the surface of perovskite film. Thereby, the corresponding PSCs yield a significantly enhanced power conversion efficiency of 15.17% compared with that (13.32%) of the pristine PSCs. Furthermore, the PSCs also have a good stability in air atmosphere (relative humidity <= 45%) because of the introduction of this hydrophobic material. After a series of detailed investigations and comparison, it can be easily concluded that the filled pinholes and gaps in perovskite films via beta-NaYF4:Yb,Er nanoparticles is the main reason for the performance enhancement of the beta-NaYF4:Yb,Er based PSCs, which can result in an enhanced hole extraction efficiency and reduced interface charge recombination. As a conclusion, it is a feasible and effective approach for improving the photovoltaic performance of PSCs by filling up the pinholes and gaps in perovskite films with up-conversion ultrathin layers. (C) 2018 Elsevier B.V. All rights reserved.