The study introduces a synergistic redox modulation strategy using potassium borohydride (KBH4) to enhance the performance of nickel oxide (NiO$_x$)-based inverted perovskite solar cells (PSCs). The approach aims to address the detrimental interfacial redox reactions between high-valence Ni$^{4+}$ and perovskite, as well as the iodide-to-I$_2$ oxidation within the perovskite film. By treating the NiO$_x$ surface with KBH4, the Ni$^{4+}$ species are reduced, and residual KBH4 further reduces I$_2$ within the perovskite precursor. This results in a significant suppression of non-radiative recombination, an increase in carrier lifetime, and improved device stability. The power conversion efficiency (PCE) of NiO$_x$-based PSCs reaches 24.17%, and the efficiency of NiO$_x$-based perovskite solar modules under ambient conditions achieves a record 20.2%. Additionally, the modules retain 94% of their initial efficiency after 2000 hours of continuous illumination at 65 °C in ambient air. The study demonstrates the effectiveness of KBH4 in enhancing the performance and stability of NiO$_x$-based perovskite solar devices.The study introduces a synergistic redox modulation strategy using potassium borohydride (KBH4) to enhance the performance of nickel oxide (NiO$_x$)-based inverted perovskite solar cells (PSCs). The approach aims to address the detrimental interfacial redox reactions between high-valence Ni$^{4+}$ and perovskite, as well as the iodide-to-I$_2$ oxidation within the perovskite film. By treating the NiO$_x$ surface with KBH4, the Ni$^{4+}$ species are reduced, and residual KBH4 further reduces I$_2$ within the perovskite precursor. This results in a significant suppression of non-radiative recombination, an increase in carrier lifetime, and improved device stability. The power conversion efficiency (PCE) of NiO$_x$-based PSCs reaches 24.17%, and the efficiency of NiO$_x$-based perovskite solar modules under ambient conditions achieves a record 20.2%. Additionally, the modules retain 94% of their initial efficiency after 2000 hours of continuous illumination at 65 °C in ambient air. The study demonstrates the effectiveness of KBH4 in enhancing the performance and stability of NiO$_x$-based perovskite solar devices.