A key problem for materials limitations lead-based perovskite solar cells is the oxidation of iodide species within the precursor solution, which produces molecular iodine (I₂).¹,² This process leads to defect formation, reduced film quality, and ultimately poorer device efficiency and stability.³ In this work, we investigate the incorporation of two ionic borohydride salts such as sodium borohydride (NaBH₄) and potassium borohydride (KBH₄) into CsFAPbI₃ perovskite precursor solutions as a strategy to mitigate these degradation pathways and enhance solar cell performance.

Both borohydride salts act as effective reducing agents, converting iodine (I₂) back into iodide (I⁻) and suppressing oxidation processes in the precursor solution. By limiting the formation of triiodide (I₃⁻), which strongly interacts with formamidinium (FA⁺) and promotes deprotonation and decomposition of the perovskite structure, these additives significantly improve the chemical stability of the solution.⁴ In addition to stabilizing the solution precursor and borohydride salts contribute to improved solid-state properties of the resulting films.

Structural and optoelectronic characterization reveals that the additives enhance crystallinity, increase grain size, and passivate surface defects, which reduces non-radiative recombination losses. These effects suggest directly improved photovoltaic properties, including higher photoconversion efficiencies and better operational stability. In this work, two additives studied like NaBH₄ show superior performance compared to KBH₄, primarily due to its stronger reducing capability and more favorable ionic interactions within the precursor. As a result, NaBH₄ more effectively suppresses iodide oxidation, minimizes defect formation, and improves film morphology.

Here, solar cells fabricated with borohydride additives achieve efficiencies close to 20%, together with markedly improved durability. A particularly notable result is the long-term stability of unencapsulated CsFAPbI₃ devices incorporating NaBH₄ When stored strong under ambient conditions with 60% relative humidity at 25 °C. These devices retain stability for more than 900 hours after fabrication over five times longer than reference devices without additives. Furthermore, this demonstrates the strong protective effect of the reducing agent against moisture-induced degradation. These findings provide a promising pathway toward more reliable perovskite photovoltaics and contribute to overcoming one of the key challenges for their future large-scale commercialization.