High-performance perovskite solar cells (PSCs) typically require interfacial passivation, yet this is challenging for the buried interface, owing to the dissolution of passivation agents during the deposition of perovskites. Here, this limitation is overcome with in situ buried-interface passivation—achieved via directly adding a cyanoacrylic-acid-based molecular additive, namely BT-T, into the perovskite precursor solution. Classical and ab initio molecular dynamics simulations reveal that BT-T spontaneously may self-assemble at the buried interface during the formation of the perovskite layer on a nickel oxide hole-transporting layer. The preferential buried-interface passivation results in facilitated hole transfer and suppressed charge recombination. In addition, residual BT-T molecules in the perovskite layer enhance its stability and homogeneity. A power-conversion efficiency (PCE) of 23.48% for 1.0 cm2 inverted-structure PSCs is reported. The encapsulated PSC retains 95.4% of its initial PCE following 1960 h maximum-power-point tracking under continuous light illumination at 65 °C (i.e., ISOS-L-2I protocol). The demonstration of operating-stable PSCs under accelerated ageing conditions represents a step closer to the commercialization of this emerging technology.