A research team led by Prof. XU Jixian from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) synthesized a novel stabilizer, dimethylammonium formate (DMAFo), for air-processed p-i-n perovskite solar cells, inhibiting the oxidization of iodide ions and the deprotonation of organic cations. The study was published in Nature Energy. 

Recent years have seen rapid progress in the power conversion efficiency (PCE) of metal halide perovskite solar cells. Up to now, the certified steady-state efficiency of the perovskite solar cells has exceeded 26.1%. However, these highly efficient solar cells have to be fabricated in an inert atmosphere. If fabricated in ambient atmosphere, the efficiency and stability of the solar cells would be greatly degraded, which severely hinders mass production and applications of the perovskite solar cells. 

It has been found that the preparation of perovskite films in ambient air undergoes a "whole-process" degradation, including rapid degradation of the precursor solution when exposed to moisture, oxygen and heating. The degradation is mainly driven by the halide oxidation and the deprotonation of organic cations. The destructive phase transitions induced by the hydration during the crystallization in air, as well as the substantial p-type defects generated at the perovskite/air interface, make it more challenging to fabricate efficient and stable p-i-n devices in ambient air. 

To address these issues, the team synthesized a "whole-process" ionic liquid stabilizer, DMAFo. The reducing effect and hydrogen bonding with the perovskite precursor of DMAFo inhibited the deprotonation of organic cations and the oxidation of halide ions, enabling the long-term storage of the perovskite solution under ambient air and heating condition. The protective effect of DMAFo can extend to the crystallization of perovskite in air, improving the crystallinity of perovskite films and reducing the energetic disorder and non-radiative recombination induced by atomic defects. 

Moreover, the comprehensive defect investigations and device simulations verified that the proliferation of p-type defects within the bulk rather than surface defects is the main reason for the degradation of air-processed perovskite, which means that conventional surface passivation alone is insufficient for the fabrication of perovskite solar cells in air. 

Based on the above findings, the team developed a 1.53-eV p-i-n perovskite solar cells in ambient air, which achieved a maximum PCE of 25.4% and a certified stabilized efficiency of 24.7%, close to that of the best-performing perovskite fabricated in nitrogen. The team also confirmed the applicability of DMAFo in the wide bandgap perovskite, which boost the fabrication of stacked devices in ambient air.