Impact of Thin Film Thickness on the Structural, Energetic and Optoelectronic Properties of Two-Dimensional FPEA₂(MA_n–1)Pb_nI_3n+1 Perovskites

Perovskite solar cell devices, composed of solution-processed perovskite layers with thicknesses of a few hundred angstroms, represent a leading technology in thin-film photovoltaics. Here, we performed a theoretical investigation based on ab initio calculations to explore the role of perovskite thin film thickness, with the general formula FPEA₂(MA_n–1)Pb_nI_3n+1, where FPEA represents 4-fluorophenylethylammonium cations and n ranges from 1 to 4 layers. Our findings reveal that increasing the thickness of the inorganic layer significantly influences the structural, energetic, and optoelectronic properties. Enhanced charge transfer within the inorganic framework and stronger organic–inorganic interactions are observed as the effective charge distribution shifts with increasing thickness. Exothermic trends in adsorption and interaction energies highlight the stabilizing effects of van der Waals forces and hydrogen bonding. The PbI₆-octahedra play a critical role in determining the optical activity and the formation of valence and conduction bands. Thicker films exhibit more intense absorption, emphasizing the importance of PbI₆-octahedra in driving optical properties. Moreover, the work function (ϕ) decreases with increasing thickness due to reduced quantum confinement effects, while the nature of polar FPEA molecules induces deviations in ϕ, underscoring the interaction between molecular composition and thickness. Band alignment further reveals strong spin–orbit coupling effects on the conduction band minimum (CBM), influenced by charge-transfer variability from FPEA to halides. These findings provide insights into thickness-dependent properties that are essential for optimizing perovskite-based devices.