Passivation and interlayer engineering are important approaches to increase the efficiency and stability of perovskite solar cells. Thin insulating dielectric films at the interface between the perovskite and the charge carrier transport layers have been suggested to passivate surface defects. Here, we analyze the effect of depositing poly(methyl methacrylate) (PMMA) from a very low-concentration solution. Spatial- and time-resolved photoluminescence and atomic force microscopy analyses of samples with diverse morphologies demonstrate the preferential deposition of PMMA in topographic depressions of the perovskite layer, such as grain and domain boundaries. This treatment results in an increase in the fill factor of more than 4% and an absolute efficiency boost exceeding 1%, with a maximum efficiency of 20.4%. Based on these results, we propose a physical isolation mechanism rather than a chemical passivation of perovskite defects, which explains not only the data of this study but also most results found in earlier works. This work was partially funded by a Swiss Government Excellence Scholarship (2017.1080) and by the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie project PerSisTanCe with grant agreement No. 841005 (E.O.M.). It also received financial support from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is cofinanced by the Participating States and by the European Union’s Horizon 2020 research and innovation programme. E.O.M., R.D.O., P.F., and U.S. acknowledge financial support by the Adolphe Merkle Foundation. E.O.M.would like to thank Dr. Silver Hamill Turren-Cruz from Helmholtz-Zentrum Berlin for fruitful discussions and Dr. Jovana Milic from the University of Fribourg and Brian Carlsen from the Laboratory of Photomolecular Science (EPFL) for facilitating and carrying out stability measurements.