Since the inception of organic metal halide perovskite (OHP) solar cells, the scientific community has shown strong interest in improving their precursor materials through simple and easily accessible polymer additives, leading to enhanced performance of photovoltaic devices. Currently, most research focuses on the application of hydrophilic polymers. Moreover, the operational stability of OHPs bound with polymers under extreme environmental conditions has become a hot topic of research. However, the specific mechanisms behind these performance enhancements remain largely unclear.

In our study, we focus on the role of polymer sodium salts in enhancing the stability of methylammonium lead iodide (MAPI). We employ a series of sophisticated photoelectron spectroscopy techniques, including in situ X-ray photoelectron spectroscopy (XPS) and hard X-ray photoelectron spectroscopy (HAX-PES), to reveal the stability mechanisms of perovskite-sodium salt composites under high humidity and temperature. Additionally, our research is dedicated to exploring the contribution of sodium salts in passivating defect sites and enhancing the stability of MAPI under actual operation pressures.

 

Optical performance tests show that the addition of sodium salts to the precursor solution during traditional one-step MAPI preparation effectively reduces the density of defect sites in MAPI. The cross-linking action of polymer sodium salts not only fills the grain boundaries of MAPI, reducing pinhole phenomena but also encapsulates MAPI microcrystals. This is proven further by XPS quantitative analysis and angle-resolved HAXPES depth profiling using a non-destructive Ga Kα source, which confirms that polymer sodium salts are primarily distributed within the crystal, achieving internal encapsulation of MAPI devices. In situ XPS results indicate that the addition of sodium salts significantly slows the thermal-induced degradation process of MAPI in a vacuum.

Notably, devices modified with sodium salts exhibited a maximum power conversion efficiency (PCE) of 18.6%, in contrast, the unmodified reference devices had a maximum PCE of 17.6%. More importantly, after nearly 500 hours of storage at 80% relative humidity, devices with sodium salts maintained 89.1% of their optimal efficiency, while the efficiency of reference devices dropped to 61.7%. In a 400-second high-temperature exposure test, sodium salt-modified devices retained 81.0% of their initial efficiency, whereas reference devices only retained 51.6% efficiency, further validating the significant role of sodium salt additives in enhancing the stability of perovskite solar cells.