The performance of perovskite solar cells (PSCs) with magnetron-sputtered tin oxide (SnO<sub><i>x</i></sub>) electron transport layers (ETLs) is strongly influenced by the optical and electrical characteristics of the SnO<sub><i>x</i></sub>. However, magnetron-sputtered SnO<sub><i>x</i></sub> typically exhibits oxygen-vacancy (V<sub>O</sub>)-related point defects. This leads to significant interface charge recombination, which restricts both the open-circuit voltage (<i>V</i><sub>OC</sub>) and fill factor (FF) of PSCs using SnO<sub><i>x</i></sub> ETLs. In this study, a Hf-doping strategy is proposed to enhance the transmittance of SnO<sub><i>x</i></sub> ETLs, reduce V<sub>O</sub> defects, and modulate the carrier density. The introduction of Hf dopants into SnO<sub><i>x</i></sub> successfully minimized V<sub>O</sub>-defect formation, as confirmed by Hall-effect measurements, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy, leading to a reduced carrier density in SnO<sub><i>x</i></sub>. Density functional theory simulations corroborated these experimental findings, revealing the mechanism behind V<sub>O</sub> suppression. PSCs incorporating HTO ETLs demonstrated marked improvements in key performance parameters, including short-circuit current density, <i>V</i><sub>OC</sub>, and FF. Optimized HTO-based PSCs achieved an average power-conversion efficiency (PCE) of 18.23%, exhibiting a 14.2% increase compared with undoped SnO<sub><i>x</i></sub>-based devices. Additionally, the best-performing PSCs utilizing HTO ETLs achieved an optimal PCE of 21.2% under reverse scan and 19.9% under forward scan.