**3. Film quality control of 2D perovskite**

Consistent with the situation in the 3D PSCs, it is of vital importance to control the film quality of absorption layer for a high-performance 2D PSC. For the synthesis of the light-absorption layer, the most-commonly used spin-coating method is applied. However, things become more difficult when it comes to the crystallization process of the 2D perovskite films. To improve the film quality of the 2D peroskite, many strategies are used to optimize the film quality, mainly focusing on adjusting the crystal orientation and the phase distribution, as summarized below.
