Stability of Precursor Solution for Perovskite Solar Cell: Mixture (FAI + PbI) versus Synthetic FAPbI Crystal.

We report here on the stability of a precursor solution for perovskite solar cells. Solution was aged at ambient conditions for 4 weeks, where two different precursor solutions were prepared by dissolving FAI and PbI in DMSO/DMF solvent (precursor mixture solution) and the synthesized single crystalline α-FAPbI in the same solvent (single crystal solution). Perovskite films were prepared by depositing fresh or aged solutions at weekly intervals. Photovoltaic parameters were hardly altered by aging the single crystal solution, while power conversion efficiency was gradually decreased with aging time for the precursor mixture solution mainly due to the decreased photocurrent density and fill factor. Solution pH was changed from basic to acidic due to HI formed by aging the precursor mixture solution, which prevents the formation of α-phase of FAPbI. For the single crystal solution, basic conditions remained unchanged by aging. In addition, the presence of δ-phase in the annealed perovskite films was found to have negative influence on the long-term stability. It is thus important to maintain the pH of the precursor solution to avoid aging effects and remove δ-phase in the annealed film for device stability.