Perovskite/perovskite/silicon triple-junction solar cells offer notable potential for high power output at low cost, yet their development is hindered by the phase instability of perovskites, which limits both device reproducibility and performance. The ~1.50-eV formamidinium lead triiodide (FAPbI3)-based middle layer degrades during subsequent fabrication steps, and the ~2.0-eV bromide-rich top layer suffers from light-induced phase segregation. Here we address these challenges by introducing ammonium propionic acid to enhance the phase stability in both perovskite layers. This strategy raises the phase transition energy barrier and suppresses vacancy defect formation through additional bonding with lattice cations. These improvements mitigate phase instabilities and enhance the power conversion efficiency of devices based on the modified perovskite films. As a result, perovskite/perovskite/silicon triple-junction solar cells achieve a power conversion efficiency of 28.7% on a 1-cm2 aperture area, with substantially improved reproducibility.