| Summary: | Using reduced-dimensional halide perovskites is emerging as a promising strategy for enhancing the stability of optoelectronic devices such as solar cells, even if their performances remain a step below those of the 3D halide perovskites. Two-dimensional Ruddlesden–Popper (2D-RP) structures are characterized by the <i>n</i> parameter that represents the number of PbI<sub>6</sub> layers in the spacer-separated perovskite slabs. The present study focuses on formamidinium (FA)-based 2D-RP type perovskites denoted as PMA<sub>2</sub>FA<sub><i>n</i>−1</sub>Pb<sub><i>n</i></sub>I<sub>3<i>n</i>+1</sub> (PMA = Phenylmethylammonium or benzylammonium). We investigate the effect of <i>n</i> on the one step growth mechanism and the film morphology, microstructure, phase purity, and optoelectronic properties. Our findings demonstrate that the average <i>n</i> is not only determined by the initial spacer content in the precursor solution but also by the thermal annealing process that leads to a partial spacer loss. Depending on <i>n</i>, perovskite solar cells achieving a power conversion efficiency up to 21%, coupled with enhanced film stability compared to 3D perovskites have been prepared. By using MACl additive and an excess of PbI<sub>2</sub> in the perovskite precursor solution, we have been able to achieve high efficiency and to stabilize the <i>n</i> = 5 perovskite solar cells. This research represents a significant stride in comprehending the formation of FA-based layered perovskites through one-step sequential deposition, enabling control over their phase distribution, composition, and orientation.
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