A facile and new solution-based synthesis for the preparation of Nb2O5 nanoparticles (NPs) is firstly introduced. Chain-like particles of 30 to 50 nm are synthesized through a hydrolysis reaction using an ethanol-based NbCl5 precursor and H2O2 as a strong oxidizer by UV treatment. A uniform dispersion of ultrasmall Nb2O5 NPs with a quantum size of 2 to 5 nm is further obtained by modifying the procedure with acetic acid as a stabilizer. The resultant Nb2O5 NPs are controllably coated on fluorine-doped tin oxide (FTO) conductive glass through 2-volt electrophoresis. The Nb2O5-coated thin film functioned as an effective blocking layer between a mesoporous TiO2 and FTO; it thereby reduced the recombination at the TiO2/FTO/electrolyte interface in dye-sensitized solar cells. Furthermore, it serves as an ultrathin and compact hole blocking layer in perovskite solar cells, resulting in moderate hysteresis effect and improved photovoltaic efficiency from high-transmittance, low capacitance and low series resistance. (C) 2017 Elsevier Ltd. All rights reserved.