Long-term stability remains a key issue impeding the commercialization of halide perovskite solar cells (HPVKSCs). The diffusion of molecules and ions causes irreversible degradation to photovoltaic device performance. Here, we demonstrate a facile strategy for producing highly stable HPVKSCs by using a thin but compact semimetal Bismuth interlayer. The Bismuth film acts as a robust permeation barrier that both insulates the perovskite from intrusion by undesirable external moisture and protects the metal electrode from iodine corrosion. The Bismuth-interlayer-based devices exhibit greatly improved stability when subjected to humidity, thermal and light stresses. The unencapsulated device retains 88% of its initial efficiency in ambient air in the dark for over 6000 h; the devices maintain 95% and 97% of their initial efficiencies after 85 °C thermal aging and light soaking in nitrogen atmosphere for 500 h, respectively. These sound stability parameters are among the best for planar structured HPVKSCs reported to date.

A power conversion efficiency of 17.9% has been obtained for the device with a critical BCP thickness of 5 nm. While if the BCP layer is too thin or too thick, charge accumulation will emerge and lead to device performance degradation.

Much less additive content of 0.5% CaI2 instead of 5% PbI2 was incorporated into the CH3NH3PbI3 film and a dense and surface-smooth morphology was obtained with much enlarged crystal grains. The champion PSC based on MAPbI3(CaI2)0.005 layer demonstrated a very high PCE of 19.3% with superior long-term stability.