The understanding of defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on the device efficiency and stability is currently a challenge. Here, we report that adding the organic molecule 3- phosphonopropionic acid (H3pp) to the halide perovskite precursor solution results in unchanged overall optoelectronic performance while having a tremendous impact on the stability of the halide perovskite by means of ion immobilization. As a result, we obtained PSCs with ~21 % efficiency and outstanding operational stability, retaining nearly 100 % of the initial efficiency after 1000 h at the maximum power point under simulated AM1.5 illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H… I and O…H) from the phosphonate group allows immobilization of ions that leads to remarkable device stability. This binding mode results in shallow point defect passivation that has a significant impact on the device stability but not on the nonradiative recombination and device efficiency. We expect that decoupling the effect of defects on the efficiency and stability will have important implications for the current understanding and advancement of operational PSCs.