Unraveling the hidden function of a stabilizer in a precursor in improving hybrid perovskite film morphology for high efficiency solar cells

The morphology of the organometal trihalide perovskite (OTP) plays a critical role in the performance of solar cell devices. Nevertheless it has been frequently reported that the morphology of OTP films tends to be different in different laboratories even with the same film preparation procedure, which makes it very difficult to compare and understand the material and device physics. Here, we unravel a critical role of the H₃PO₂ stabilizer in HI, which has been largely ignored, in controlling the morphology of the perovskite films. The H₃PO₂ stabilizer in HI solution introduces MAH₂PO₂ impurities into the synthesized MAI (non-purified MAI) by reacting with methylamine (MA) aqueous solution. MAH₂PO₂ impurities can slow down the overall crystallization process of perovskite by forming an intermediate phase of Pb(H₂PO₂)₂. Both MAH₂PO₂ and Pb(H₂PO₂)₂ impede the fast reaction of PbI₂ and MAI, resulting in highly uniform and smooth perovskite films with larger grain sizes. The recrystallization of non-purified MAI can remove the MAH₂PO₂ impurity and form purified MAI, which however results in rough and non-uniform perovskite films. Uniform and smooth perovskite films can also be obtained by directly adding artificially synthesized MAH₂PO₂ into the purified MAI precursor. This study also suggests Pb(H₂PO₂)₂ to be a new precursor to form high quality perovskite films.