In planar perovskite solar cells (PSCs), the electron transport layer (ETL), with the function of transporting electrons and blocking holes, plays a significant role in determining the photoelectric conversion properties. Thus, it is crucial to exploit potential ETL materials for improved performance. Herein, we demonstrate efficient planar PSCs employing ultrathin Nb2O5 films as ETLs fabricated by a facile spin-coating method. The effects of the precursor concentration and annealing time of Nb2O5 on the photovoltaic performance are investigated in detail. The planar PSCs constructed with 0.02 M-prepared Nb2O5 films sintered for 1 h possess an optimized performance, giving rise to a champion efficiency of 14.82% with high reproducibility. The superior performance is ascribed to the appropriate conduction band position of Nb2O5, high transmittance, increased charge transport, and diminished charge recombination of the Nb2O5 film. The results demonstrate that the utilization of spin-coated Nb2O5 as the ETL provides a new platform to construct highly-efficient planar PSCs.