New Type of 2D Perovskites with Alternating Cations in the Interlayer Space, (C(NH₂)₃)(CH₃NH₃)_nPb_nI_3n+1: Structure, Properties, and Photovoltaic Performance

We present the new homologous series (C(NH₂)₃)(CH₃NH₃)_nPb_nI_3n+1 (n = 1, 2, 3) of layered 2D perovskites. Structural characterization by single-crystal X-ray diffraction reveals that these compounds adopt an unprecedented structure type, which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). Compared to the more common Ruddlesden-Popper (RP) 2D perovskites, the ACI perovskites have a different stacking motif and adopt a higher crystal symmetry. The higher symmetry of the ACI perovskites is expressed in their physical properties, which show a characteristic decrease of the bandgap with respect to their RP perovskite counterparts with the same perovskite layer thickness (n). The compounds show a monotonic decrease in the optical gap as n increases: E_g = 2.27 eV for n = 1 to E_g = 1.99 eV for n = 2 and E_g = 1.73 eV for n = 3, which show slightly narrower gaps compared to the corresponding RP perovskites. First-principles theoretical electronic structure calculations confirm the experimental optical gap trends suggesting that the ACI perovskites are direct bandgap semiconductors with wide valence and conduction bandwidths. To assess the potential of the ACI perovskites toward solar cell applications, we studied the (C(NH₂)₃)(CH₃NH₃)₃Pb₃I₁₀ (n = 3) compound. Compact thin films from the (C(NH₂)₃)(CH₃NH₃)₃Pb₃I₁₀ compound with excellent surface coverage can be obtained from the antisolvent dripping method. Planar photovoltaic devices from optimized ACI perovskite films yield a power-conversion-efficiency of 7.26% with a high open-circuit voltage of ∼1 V and a striking fill factor of ∼80%.