Tin–zinc-oxide nanocomposites (SZO) with various Sn : Zn ratios were successfully fabricated and tested as electron transport layers (ETLs) in perovskite solar cells (PVSCs). The fabricated nanocomposites showed good crystallinity, good contact between layers, good electrical conductivity, and favorable light absorption, resulting in an enhancement in the net efficiency of CH₃NH₃PbI₃ (MAPI)-based perovskite solar cells. The device made of SZO–Sn_0.05 as an ETL showed a maximum power conversion efficiency (PCE) of 17.81% with a short-circuit current density (J_sc) of 23.59 mA cm⁻², an open-circuit voltage (V_oc) of 1 V, and a fill factor (FF) of 0.754. However, the ETL containing lower Sn ratios showed PCEs of 12.02, 13.80 and 15.86% for pure ZnO, SZO–Sn_0.2 and SZO–Sn_0.1, respectively. Meanwhile, the reproducibility of 30 fabricated devices proved the outstanding long-term stability of the cells based on SZO nanocomposites, retaining ≈85% of their PCE over 1200 h of operation. In addition, the incident-photon-to-current efficiency (IPCE) exceeded 90% over the entire wavelength range from 400 to 800 nm. The enhancement in the PCE of the fabricated PVSCs can be ascribed to the large surface area of the SZO nanoparticles, high charge extraction efficiency, and suppression of charge recombination provided by SnO_x. The current results suggest that our synthesized tin–zinc-oxide nanocomposite is an effective electron transport layer for efficient and stable perovskite solar cells.