To elucidate the influence of A-site cations on the photovoltaic properties of two-dimensional Ruddlesden-Popper-type lead iodide perovskites, this work investigates (PA)₂APb₂I₇ using first-principles calculations, where PA denotes n-pentylammonium cation ([CH₃(CH₂)₄NH₃]⁺, PA) and A denotes guanidinium ([C(NH₂)₃]⁺, Gua) or acetamidinium ([C₂H₃(NH₂)₂]⁺, Ace). The calculations were performed using the projector augmented-wave method and the Perdew-Burke-Ernzerhof exchange-correlation functional, and included geometry optimization, structural distortion analysis, electronic structure analysis, defect property calculations, and optical absorption analysis. The results show that the A-site organic cations mainly alter the local Pb-I framework structure through N-H···I hydrogen bonds, while contributing little to the band edge electronic states. In the Gua-based material, the Pb-I-Pb angle distortion is more pronounced, whereas the Ace-based material maintains a more linear in-plane Pb-I-Pb connection, which is beneficial for Pb/I orbital coupling within the inorganic layer. Both materials are direct band gap semiconductors. The valence band maximum is mainly derived from antibonding hybridization between I-5p and Pb-6s orbitals, while the conduction band minimum primarily originates from Pb-6p orbitals. Compared to Gua-based material, the Ace cation-regulated system exhibits a narrower band-gap, lower in-plane effective masses, and more pronounced band edge localization, which favors efficient carrier transport within the inorganic layer. In the Gua-based material, the Gua vacancy (V_Gua), Pb vacancy (V_Pb), and interstitial defects introduce deep transition levels, which may act as carrier trapping or recombination centers. In contrast, low-formation-energy defects in the Ace-based material generally show shallow-level characteristics. For the interstitial iodine defect (I_i), which appears as a deep-level defect in both systems, the Ace-based material shows a higher formation energy and migration barrier than the Gua-based material. Both materials exhibit strong absorption in the visible and near-ultraviolet regions, with absorption coefficients on the order of 10⁵cm^-1. The Ace-based material shows a red-shift absorption edge and reaches a maximum spectroscopic limited maximum efficiency (SLME) of 22.79%. In summary, A-site cation regulation alters the Pb-I framework connectivity and defect characteristics. Compared with the Gua-based material, the Ace-based material shows advantages in carrier transport, defect tolerance, and theoretical photovoltaic performance.

Key words: two-dimensional perovskite, first-principles, electronic structure, defect properties