Perovskites have emerged as a promising candidate for low-cost production of solar cells. However, the most critical barrier for commercialization of perovskite solar cells (PSCs) is the inadequate stability of the organic metal halide perovskites (OMHPs). The degradation of OMHPs is induced by light, heat, air, and electrical bias. The known degradation pathways involve the oxidation of I- and Sn2+, dissolution of perovskites by moisture, irreversible reactions with water, ion migration, and ion segregation. To improve the stability of OMHPs various methods are adopted, such as additive engineering, perovskite surface treatment, and composition engineering. Surface ligands are used on top of perovskite thin films to passivate the undercoordinated ions leading to improved charge collection efficiency and stability of PSCs. However, not all surface ligands stay at the surface of the perovskite. Some of them penetrate the perovskite layer forming reduced dimensional phases at the surface. This kind of behavior not only alters the electronic nature at the interface, but also negatively affects the stability of the OMHPs compared to surface ligands that remain only at the surface. On the other hand, additives are commonly used to reduce defects in bulk of the perovskites and thus improve their stability. They improve the stability of OMHPs by controlling the morphology of OMHP thin films, improving the thermodynamic stability of Sn2+ and I-, and lowering the ion migration and ion segregation. The stability of OMHPs is also significantly improved by incorporating bulky organic cations into the perovskite composition. Although these routes for improving the stability are optimistic, it is not clear how the surface chemistry of OMHPs and chemical nature of additives or organic cations affects stability.
Surface chemistry of OMHPs can be tuned to control the extent of ligand penetration by changing the composition and processing conditions of OMHPs. To this end, it is important to find out what affects the extent of ligand penetration. We find that the perovskite compositions used in this study have little or no effect on ligand penetration. However, the perovskite film processing conditions have a greater effect on ligand penetration. Using a family of phenethylammonium iodide (PEAI) with different substituents on the benzene ring, we show that the ligand penetration can be affected by type of substituents as well. Stabilizing the perovskite precursors is also important as degraded precursors lead to defective perovskites with poor stability. Here, we show that additives influence the thermodynamic stability of Sn2+ and I- by changing the acidity of the precursor solutions. Using additives with a range of pKa we find that additives with higher pKa provide a more stabilizing chemical environment for Sn2+ and I-.
It is known that bulky organic cations improve the stability of OMHPs by shielding the metal-halide octahedra from air. However, how the structure of the organic cations affect the air, oxygen, and moisture stability of the OMHPs is not well understood. Using twelve different organic cations we show that the stronger the attractive interactions between the organic cations in two dimensional (2D) OMHPs the higher is the stability. The stability of 2D-OMHP thin films decreases as the orientation of the 2D sheets deviates from planarity with respect to the substrate plane.