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 h, and 70% under heat at 85˚C for 400 h, both of which are far superior to the pristine PbI2-rich solar cells.