Toward Efficient and Stable Perovskite Photovoltaics with Fluorinated Phosphonate Salt Surface Passivation

Surface passivation has been proven to be an effective strategy to improve power conversion efficiency and stability of perovskite solar cells. However, the rationale for choosing an appropriate passivator, in terms of the type of interaction with the underlying perovskite layer, is still not clear yet. Here, we purposively choose two molecules as passivators for perovskites, fluorinated phosphonic acid and its corresponding phosphonate salt, to monitor the extent of interaction between these passivators and the perovskite surface. The effect of passivation on film stability and device performance is also determined. Higher photoluminescence intensity and longer carrier lifetime are observed in perovskite films that treated with phosphonium salt passivation because of the stronger interaction with perovskites. The corresponding device shows enhancement in power conversion efficiency from 18.27 to 19.44%. Furthermore, the water contact angle of passivated perovskite films exceeds 110.9° as compared to the pristine, untreated perovskite (74.5°). This superhydrophobic nature imparted by fluorinated phosphonium salt passivation enables excellent long-term stability of devices, allowing over 90% of their initial efficiency to be retained even after 90 days' storage under ambient conditions with 30% relative humidity.