To address the issue of instability and surface defects in three-dimensional lead halide perovskite solar cells (PSCs), two-dimensional (2D) pseudo-(quasi) perovskites, such as Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) perovskites, have garnered significant attention. Controlling the orientation of their layered crystallites is key to vertical charge transport in 2D PSCs; however, their detailed electronic properties are yet to be fully understood. Herein, we investigated the effect of processing additives (ammonium salts: NH4Cl, NH4Br, NH4Cl, NH4SCN, and CH3NH3SCN with/without alkali metal salts (NaCl, KCl, RbCl, and CsCl) in water) on the crystallographic nature and anisotropic microwave photoconductivity of RP and DJ perovskites. Importantly, a strong correlation (correlation coefficient, r = 0.882) was observed between the power conversion efficiencies (1–11%) of inverted PSCs and anisotropic (both out-of-plane and in-plane to the substrate) photoconductivities for the 34 compositions of additives analyzed herein. In contrast, the one-dimensional photoconductivity and crystallographic degree of orientation exhibited significantly weaker correlations. These results underscore the importance of anisotropic mobility in 2D PSCs and could serve as guidelines for their further development.

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