Interfacial engineering of the meso-TiO2 surface through a modified sequential deposition procedure involving a novel PbI2–HMPA complex pretreatment is conducted as a reproducible method for preparing MAPbI3 based perovskite solar cells providing the highest efficiencies yet reported with the polymer HTM layer. Grazing-incidence X-ray diffraction depth profiling confirms the formation of a perovskite film with a PbI2-rich region close to the electron transport layer (ETL) due to the strong interaction of HMPA with PbI2, which successfully retarded the dissolution of the PbI2 phase when depositing the perovskite layer on top. These results are further confirmed by energy-dispersive X-ray spectroscopy performed in a scanning transmission electron microscope, which reveals that the I/Pb ratio in samples treated with the complex is indeed reduced in the vicinity of the ETL contact when compared to samples without the treatment. The engineered interface leads to an average power conversion efficiency of 19.2% (reverse scan, standard deviation SD < 0.2) over 30 cells (best cell at 19.5% with high FF of 0.80).