Restricting δ-phase transformation of HC(NH2)2PbI3via iodine-vacancy filling for efficient perovskite solar cells

Formamidinium lead halide (alpha-FAPbI(3)) with a broad light absorption spectrum, has recently received considerable attention in optoelectronic applications. However, the FAI-PbI2-DMSO (DMSO: dimethyl sulfoxide) intermediate anisotropic fibers readily form a non-perovskite phase (delta-FAPbI(3)) and uncontrolled excess PbI2, which hinders the further increase in the efficiencies of solar cells. Caculations indicate that iodine defects in polycrystalline films would enlarge the perovskite tolerance factor and result in the formation of iodide Frenkel defects. Herein, we introduce a post-treatment technique to heal the as-prepared FAPbI(3) thin layer and restrain the notorious S-FAPbI(3) through vacancy filling. Furthermore, a new intermediate phase of FAI-PbI2-DMSO-FACl led to a high-quality perovskite layer with an enlarged average grain size that exceeded 2 mu m. Consequently, the power conversion efficiencies of FAPbI(3) solar cells were significantly enhanced due to the high crystallity of the pure alpha-phase perovskite. Therefore, this method can be used to obtain high pure-black perovskite films and efficient solar cells.