To achieve the full potential of monolithic perovskite/silicon tandem solar cells, crystal defects and film inhomogeneities in the perovskite top cell must be minimized. We discuss the use of methylenediammonium dichloride as an additive to the perovskite precursor solution, resulting in the incorporation of in situ–formed tetrahydrotriazinium (THTZ-H+) into the perovskite lattice upon film crystallization. The cyclic nature of the THTZ-H+ cation enables a strong interaction with the lead octahedra of the perovskite lattice through the formation of hydrogen bonds with iodide in multiple directions. This structure improves the device power conversion efficiency (PCE) and phase stability of 1.68 electron volts perovskites under prolonged light and heat exposure under 1-sun illumination at 85°C. Monolithic perovskite/silicon tandems incorporating THTZ-H+ in the perovskite photo absorber reached a 33.7% independently certified PCE for a device area of 1 square centimeter.