Metal halide perovskites have thepotential to revolutionizethefield of photovoltaics, though limited stability has impeded commercialexploitation. The soft heterointerface between the perovskite andcharge-transporting layer is one of the major bottlenecks that limitsoperational stability. Here, we present rationally designed molecularmodulators that synergistically improve the stability of the & alpha;-FAPbI(3)-based perovskite solar cells while retaining power conversionefficiency (PCE) of 24.0% with a high open-circuit voltage (V (OC)) of & SIM;1.195 V. The interfacially modifiedphotovoltaic cells exhibit high operational stability, whereby thechampion device retains & SIM;88% of initial performance after 2000h of maximum power point tracking at 40 & DEG;C and 1 sun illumination.The molecular origins of such enhanced stability and device performanceare corroborated by multiscale characterization techniques and modeling,providing insights into the origins of performance and stability enhancements.