The work documented in this thesis concerns the fundamental understanding of the intrinsic stabilization mechanisms afforded by A-site cation compositional tuning in state-of-the-art formamidinium (FA)-rich perovskite absorbers and devices. Photovoltaic (PV) devices fabricated from FA-rich perovskite films exhibit certified power conversion efficiencies (PCEs) exceeding 26% in single-junction devices and 34% in tandem configurations. Guaranteeing minimal performance losses for these highly efficient devices under real-world operational conditions is paramount to the successful deployment of this emerging technology. A nanoscopic fundamental understanding of the local structural features that seed instabilities in these perovskite absorbers is essential for mitigating device degradation. This thesis presents advanced, low-dose electron microscopy investigations coupled with sophisticated optoelectronic characterizations in different leading FA-rich perovskite formulations. In doing so, the goal is to study the structure-property relationships in these compositions and elucidate their impact on the resulting device performance and long-term operational stability.