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All-inorganic perovskite CsPbI2Br usually requires a high fabrication temperature (higher than 200 °C) and suffers from an unwanted phase transition from a photo-active cubic phase to a photo-inactive orthorhombic phase. Here, we demonstrate an effective anti-solvent-free route to simultaneously stabilize the cubic phase and lower the fabrication temperature of perovskite solar cells with guanidinium (GA) cation doping. It is suggested that a trace amount of GA in the precursor could enter the substitutional sites to stabilize the cubic phase by relaxing the lattice strain and forming strong hydrogen bonds. State-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements reveal that GA could advance the crystallization of the cubic phase, indicating that the formation of the perovskite cubic phase experiences a lowered energy barrier with the assistance of GA, thereby resulting in a significant decrease of the fabrication temperature. An efficiency as high as 14.34% was achieved at a low fabrication temperature of 140 °C. With the help of GA, the device maintains ∼94% of its initial efficiency after being stored for 1000 h.

Vapor reaction is one of the major methods used for large-scale fabrication of high-quality organic–inorganic halide perovskite solar cells, but its detailed process is rarely investigated. In this study, the thermal and illumination effects on PbI2 and its transformation to CH3NH3PbI3 (MAPbI3) under CH3NH3I (MAI) vapor were investigated. Atomic shifting of the PbI2 layer was observed, which caused the generation of defects and distortion in PbI2 without breaking the phase structure. The reaction with MAI started with decoupling of PbI2 layers and formation of nanoislands and then, the transformation of PbI2 to MAPbI3 proceeded randomly on the surface from top to bottom. Additional illumination of the reaction suppressed the transformation, causing the generation of PbI2 nano-walls along [100] and [010] directions, leading to decrease in power conversion efficiency.

High quality polycrystalline CH3NH3PbI3 was prepared by dripping the γ-butyrolactone (GBL) solution of CH3NH3PbI3 into toluene, dimethylformamide (DMF) and dimethylsilicone oil. In toluene, a compact CH3NH3PbI3 thin film was observed but with grain size smaller than 1 μm because of the rapid extraction of GBL. As CH3NH3PbI3 is soluble in DMF, the crystallization in DMF is understood by interdiffusion induced local and temporal supersaturated droplets, which allow for growth of irregular, separated single crystals with size of more than 0.1 mm. In dimethylsilicone oil, compact, yolk-like polycrystalline CH3NH3PbI3 was observed. Unlike the round grains observed in toluene, it is composed of single crystal grains with clear angular surfaces. The average crystal size is tens of μm and can be modulated by the solvent temperature. We suggest that the poor miscibility between GBL and dimethylsilicone oil, and the poor solubility of CH3NH3I, PbI2 and CH3NH3PbI3 in the dimethylsilicone oil, allow slow extraction of GBL while keeping the stoichiometry of CH3NH3PbI3. The results offer a feasible and high-yielding method for growing uniform CH3NH3PbI3 crystals in a few seconds.