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Achieving efficient and stable tin-based perovskite solar cells remains challenging. In this work, we incorporate the ethylenediammonium diiodide (EDAI 2 ) additive into a cesium–guanidinium doped formamidinium tin triiodide perovskite with the composition of (CsGA) x FA 1−2x SnI 3 + y % EDAI 2 . This new perovskite utilizes the strong hydrogen bonding of the guanidinium cation and the lattice strain relaxation of the small cesium cation as well as the hollowing and passivation effects of the EDAI 2 additive. The EDAI 2 additive not only yields pinhole-free cubic phase perovskite films but also decreases both shallow and deep trap states in the perovskite films. These effects are pronounced with the increase of substitution of the pair of GA + and Cs + . The new perovskites are deployed in inverted planar solar cells. A maximum power conversion efficiency (PCE) of 5.01% is achieved with the (CsGA) 0.15 FA 0.70 SnI 3 + 0% EDAI 2 device but the device degrades after storage in a nitrogen-filled glove box for 30 days. Both performance and stability are improved with the addition of EDAI 2 . A maximum PCE of 5.72% is achieved with the (CsGA) 0.15 FA 0.70 SnI 3 + 1.0% EDAI 2 device. The (CsGA) 0.15 FA 0.70 SnI 3 + 1.5% EDAI 2 devices exhibit a maximum PCE of 5.69% and the performance is further increased to 6.39% after storage in a nitrogen-filled glove box for 4 days; 70% of the initial PCE is retained after 45 days. This study demonstrates the benefit of tuning cation sizes and introducing divalent cations to integrate stabilizing factors into pure Sn perovskites, creating new routes for efficient and stable lead-free perovskite solar cells.