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.