Suppressing the crystallographic disorders induced by excess PbI₂ to achieve trade-off between efficiency and stability for PbI₂-rich perovskite solar cells

Excess PbI₂ can improve the efficiency of perovskite solar cells (PSCs). However, it usually leads to poor cell stability due to the chemically active nature of PbI₂. Improving the stability of PbI₂-rich perovskite solar cells without scarifying the efficiency remains challenging. In this work, it is found that the non-uniform and multiple nucleation of perovskite induces crystallographic disorders at grain boundaries (GBs) of PbI₂-rich perovskite. These disorders working as the n-type shallow dopant reduces the energy barrier for photogenerated electrons to reach GBs, triggering the decomposition of excess PbI₂ there. Therefore, the instability of PbI₂-rich PSCs is related with the remote coupling of excess PbI₂ with the disorders at GBs. In light of this finding, we introduced a small amount (0.5 mol%) of CdI₂ into precursor solution to optimize the crystallization of PbI₂-rich perovskite, and finally achieved a trade-off between efficiency and stability in PbI₂-rich PSCs. The optimized PSCs with 10 mol% excess PbI₂ (PC-Device) achieved a champion efficiency of 24.26% at the bandgap 1.55 eV with robust stability. PC-Devices maintain more than 90% of the initial efficiency after being stressed under UV light in ambient with a relative humidity of 50% for 500 hours, and 70% under heat at 85˚C for 400 hours, both of which are far superior to the pristine PbI₂-rich solar cells.