Sn-based perovskites offer lower lead content but face a major challenge: Sn²⁺ oxidizes readily, which has led most research groups to use gloveboxes and chemical additives during processing. Here, we investigate whether precursor molarity alone can mitigate this oxidation problem in ambient air. MAPb_0.75Sn_0.25I₃ solar cells with mesoporous N–i–P architecture were prepared from 1.0 M and 0.9 M solutions by spin-coating with ethyl acetate antisolvent, under standard lab conditions (28–34 °C, 30–45% RH). The characterization included SEM, XRD, XPS, profilometry, and J–V measurements. The 0.9 M concentration produced thinner films (275 nm vs. 474 nm), better Sn²⁺/Sn⁴⁺ ratios (16.5%/83.5% vs. 77.6%/22.4% by XPS), lower band gaps (1.51–1.52 vs. 1.55–1.56 eV), and larger grains. Device efficiency increased from 1.61 ± 0.68% (1.0 M) to 4.53 ± 0.91% (0.9 M), with the best cell reaching 5.91%—about 85% of our MAPbI₃ control (6.96%). After one month of storage, 0.9 M cells retained 61% efficiency compared to 37% for 1.0 M devices. These findings demonstrate that a simple reduction in precursor molarity can substantially suppress Sn⁴⁺ formation during ambient fabrication, providing a practical route for laboratories without controlled atmospheres.