This study presents a series of ultrastable Dion-Jacobson (DJ) perovskite solar cells with high efficiency and long-term stability. The DJ perovskites, (1,4-cyclohexanedimethanammonium)(methylammonium) $ _{n-1} $ Pb $ _{n} $ I $ _{3n+1} $ ( $ n \geq 1 $ ), are designed for photovoltaic applications. Using a scalable blade-coating method, the cells achieve a maximum stabilized power conversion efficiency (PCE) of 19.11% under atmospheric conditions. Unencapsulated cells retain 92% of their initial efficiency after 4000 hours of aging at 90% relative humidity. These cells also exhibit excellent thermal (85°C) and operational stability, with negligible efficiency loss after 5000 hours of heat treatment or 6000 hours of operation at 45°C under 100 mW cm $ ^{-2} $ light. The stability of these cells is attributed to their unique slight-interlayer-displacement quantum-well configuration, which reduces interlayer spacing and enhances charge transport and structural stability. The use of cycloalkyl organic cations helps maintain molecular flexibility and electronegativity, reducing lattice stress and improving stability. The DJ perovskites show remarkable moisture, thermal, and operational stability, with degradation ratios of only 8% for moisture and negligible efficiency loss for thermal and operational stability. The study also highlights the importance of material design and dimensional regulation in improving the long-term stability of perovskite solar cells. The DJ perovskites demonstrate superior performance compared to other 2D and 3D perovskite solar cells, with high efficiency and stability under various environmental conditions. The results suggest that the DJ perovskite series has great potential for scalable and stable photovoltaic applications.This study presents a series of ultrastable Dion-Jacobson (DJ) perovskite solar cells with high efficiency and long-term stability. The DJ perovskites, (1,4-cyclohexanedimethanammonium)(methylammonium) $ _{n-1} $ Pb $ _{n} $ I $ _{3n+1} $ ( $ n \geq 1 $ ), are designed for photovoltaic applications. Using a scalable blade-coating method, the cells achieve a maximum stabilized power conversion efficiency (PCE) of 19.11% under atmospheric conditions. Unencapsulated cells retain 92% of their initial efficiency after 4000 hours of aging at 90% relative humidity. These cells also exhibit excellent thermal (85°C) and operational stability, with negligible efficiency loss after 5000 hours of heat treatment or 6000 hours of operation at 45°C under 100 mW cm $ ^{-2} $ light. The stability of these cells is attributed to their unique slight-interlayer-displacement quantum-well configuration, which reduces interlayer spacing and enhances charge transport and structural stability. The use of cycloalkyl organic cations helps maintain molecular flexibility and electronegativity, reducing lattice stress and improving stability. The DJ perovskites show remarkable moisture, thermal, and operational stability, with degradation ratios of only 8% for moisture and negligible efficiency loss for thermal and operational stability. The study also highlights the importance of material design and dimensional regulation in improving the long-term stability of perovskite solar cells. The DJ perovskites demonstrate superior performance compared to other 2D and 3D perovskite solar cells, with high efficiency and stability under various environmental conditions. The results suggest that the DJ perovskite series has great potential for scalable and stable photovoltaic applications.