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.
