Biuret was introduced to regulate the crystallization process and passivate the defects of perovskite films, leading to MAPbI3 solar cells with efficiency over 21% and enhanced thermal stability.

The performance of perovskite solar cells (PSCs) is sensitive to the quality of perovskite films. Sequential deposition enabling the controlled growth of film is promising to obtain high-quality perovskite films. However, it is challenging to fabricate formamidinium-based planar PSCs via sequential deposition, mainly due to the small penetration depth of the large size formamidinium cation into a compact PbI2 film during perovskite formation. Here, CaI2 is introduced into PbI2 precursor solution, which brings two benefits. First, the porous low-crystalline CaI2–PbI2 film results in reduced local nucleation sites and improved perovskite conversion, leading to perovskite films with high crystallinity and larger grains. Second, the residual CaI2 existing at the surface and grain boundaries of the perovskite films can passivate the detrimental crystal defects. Solar cells based on the CaI2–PbI2 precursor demonstrate high open-circuit voltage (V oc) up to 1.14 V and improved power conversion efficiency (PCE) of more than 20%, as compared to the control cells with PCEs of about 18% and V oc of 1.10 V. Moreover, the CaI2–PbI2 based devices show enhanced stability that maintains 98% of the initial PCE after 12 days under the natural environment.