Differentiating the 2D Passivation from Amorphous Passivation in Perovskite Solar Cells.

The introduction of two-dimensional (2D) perovskite layers on top of three-dimensional (3D) perovskite films enhances the performance and stability of perovskite solar cells (PSCs). However, the electronic effect of the spacer cation and the quality of the 2D capping layer are critical factors in achieving the required results. In this study, we compared two fluorinated salts: 4-(trifluoromethyl) benzamidine hydrochloride (4TF-BA·HCl) and 4-fluorobenzamidine hydrochloride (4F-BA·HCl) to engineer the 3D/2D perovskite films. Surprisingly, 4F-BA formed a high-performance 3D/2D heterojunction, while 4TF-BA produced an amorphous layer on the perovskite films. Our findings indicate that the balanced intramolecular charge polarization, which leads to effective hydrogen bonding, is more favorable in 4F-BA than in 4TF-BA, promoting the formation of a crystalline 2D perovskite. Nevertheless, 4TF-BA managed to improve efficiency to 24%, surpassing the control device, primarily due to the natural passivation capabilities of benzamidine. Interestingly, the devices based on 4F-BA demonstrated an efficiency exceeding 25% with greater longevity under various storage conditions compared to 4TF-BA-based and the control devices.