In the past decade, the energy conversion efficiency has exceeded 25%, making low-cost solution-fabricated metal halide perovskite solar cells (PSCs) promising for commercialization in the future. However, due to the presence of various defects, grain boundaries (GBs) have been shown to induce metal halide perovskite degradation under light, moisture and/or thermal stress, which makes them extremely vulnerable in perovskite solar cells (PSCs).

   There are many optimization strategies for grain boundaries, such as composition adjustment, additive passivation, and low-dimensional material modification. It is worth noting that these methods all work after the defects are formed, that is, the defects that have formed at the perovskite grain boundaries are processed.

       Although various post-treatments have been shown to be effective in repairing GBs, preventing the degradation of perovskite GBs remains an extremely difficult challenge. In a report in JOUEL in October 2022, the research team employed a general pre-annealing treatment method to reconstruct sequentially deposited perovskite crystallites and GBs. With this strategy, robust GBs are created upon defect suppression with preferred oriented large perovskite grains.

      This method addresses the growth behavior and degradation mechanism of perovskite thin film materials under thermal stress and water vapor, that is, using a small amount of oleylamine chloride (OACl) before annealing the perovskite film under thermal stress and water vapor. Titanite films are pretreated, an action that fundamentally changes their crystallization and decomposition patterns during annealing. Early intervention allowed the perovskite and its grain boundaries to be reconstituted during the annealing process. This not only improves the crystal quality, but also effectively suppresses the formation of defects and residual PbI2 at the grain boundaries, thereby greatly improving the efficiency and stability of the device.

      As a result, the resultant n-i-p-structured PSCs deliver an impressive power conversion efficiency of nearly 25% and an extremely high fill factor of 85.5%. The unencapsulated cells maintain 81% of their initial efficiencies after 1,000 h of continuous operation at maximum power point at about 55°C in an inert atmosphere, showing greatly improved light stability.

      This study is the first in this field to present the process of distributing Pbl₂ to the surface of perovskite grains instead of grain boundaries, indicating that the regulation of residual Pbl₂ and the improvement of perovskite stability are positive. This pre-intervention treatment method is not only important for improving the photostability of perovskite solar cells, but also for enhancing the commercialization of perovskite solar cells in the field of perovskite photovoltaics.