Multi-junction solar cells containing hybrid halide perovskites are often considered to be the next generation photovoltaic (PV) systems, due to the strongly enhanced power conversion efficiency (PCE) and, thus, reduced area-related module costs. Moreover, tandem devices based on combination of perovskite materials with different constituents allow fabrication of ultra-thin and lightweight perovskite-perovskite tandem modules, which could be produced at even lower cost. In either case of tandem applications, the wide energy band gap (>1.7 eV) perovskite absorbers inherently suffer from poor stability mainly due to halide segregation phenomenon in which perovskite halides de-mix and create I-rich and Br-rich domains, leading to spatial variations in energy bandgap and inferior performance.                                                                                                                                      To solve this notorious issue, we have added an alkylammonium molecule to the perovskite precursor solution, which tailors perovskite crystallization, passivates surface states and inhibits halide segregation. From ¹H nuclear magnetic resonance (NMR) spectroscopy we show that nucleophilic alkylammonium interacts with Formamidinium (FA) to form a bulky cation releasing ammonia. From grazing incidence X-ray diffraction (GIXRD) measurements we clearly identify the creation of low dimensional perovskite enabled by the long aliphatic part of the created bulky cation. Combining top-view images from scanning electron microscopy (SEM) and microscopically resolved photoluminescence (µ-PL), we demonstrate that addition of the alkylammonium-based additive has a direct impact on the perovskite surface morphology and opto-electronic heterogeneity. From absolute PL quantum yield measurements, we show that moderate quantities of the alkylammonium-based additive passivate surface defects and increase the quasi-Fermi level splitting. Remarkably, perovskite samples produced via the alkylammonium doped solution, which are placed under continuous illumination exhibit exceptional photostability. In stark contrast, the PL peak of perovskite without an addition of the alkylammonium passivant splits into two distinct peaks which is a common signature of halide segregated regions with different bandgaps.  Lastly, we show that the addition of the alkylammonium additive results not only in the enhancement of photostability but also in the PCE by 2%. We believe the proposed stabilization method is a prominent and reproducible technique which brings perovskite-based PV closer to commercialization.