High efficiency stable planar perovskite solar cells via heavy water additive

Although great advances have been made in low-cost manufacturing of air-processed perovskite solar cells (PSCs), the moisture degradation of halide perovskite films remains a key problem to be urgently solved. High-quality perovskite films with large grains and uniform coverage are essential to enhance the performance of PSCs with high efficiency and moisture stability. Herein, heavy water (D₂O) is employed as an additive in PbI₂ precursor to manipulate the average grain size and enhance surface coverage of perovskite film, resulting in improved humidity stability of PSCs under ambient conditions. It is found that the introduction of D₂O effectively improves the quality of perovskite film and significantly reduces the grain boundaries for reduced carrier recombination. The optimal concentration of D₂O can boost the in-plane growth of perovskite films and increase the average grain size of perovskite from 494 nm to 717 nm. Consequently, the PSCs based on the perovskite film with 1% D₂O exhibits a superior PCE of 21.66% (0.055 cm²) in small area, which is 9.78% higher than that of the control device (19.73%). In addition, the large-area (1 cm²) PSCs with 1% D₂O show an efficiency of 20.02%. The unencapsulated PSCs made with 1% D₂O can remain 95% of its initial PCE after aging 60 days in ambient air under dark with 20–25% relative humidity.