Pseudohalogen-Based 2D Perovskite: A More Complex Thermal Degradation Mechanism Than 3D Perovskite

(MA)₂Pb(SCN)₂I₂, a new pseudohalogen-based 2D perovskite material, was reported as a very stable and promising photo-absorber in PSCs previously. However, the later researchers found that MA₂Pb(SCN)₂I₂ was not as stable as claimed. Thus, it is very critical to clarify the controversy and reveal the degradation mechanism of MA₂Pb(SCN)₂I₂. On the other hand, a large number of studies have indicated that adding a small amount of SCN^- improves surface topography and crystallinity. However, whether SCN^- ions can be incorporated into a 3D perovskite film remains debatable. In this work, the thermal degradation pathway of (MA)₂Pb(SCN)₂I₂ is revealed by thermal gravimetric and differential thermal analysis coupled with quadrupole mass spectrometry and density functional theory calculations. The decomposition of (MA)₂Pb(SCN)₂I₂ has been proved experimentally to be more complex than that of MAPbI₃, involving four stages and multi-reactions from room temperature to above 500 °C. By combining the experimental results and theoretical calculations, it is found that 2D (MA)₂Pb(SCN)₂I₂ actually is unstable when serving as photo-absorber in PSCs. Moreover, the role of SCN^- in improving the crystallinity of 3D perovskite has also been discussed in detail.