High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)₄Pb₅I₁₆

Here, we present the fifth member (n = 5) of the Ruddlesden-Popper (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 family, which we successfully synthesized in high yield and purity. Phase purity could be clearly determined from its X-ray powder diffraction patterns, which feature the (0k0) Bragg reflections at low 2θ angles. The obtained pure n = 5 compound was confirmed to be a direct band-gap semiconductor with E_g = 1.83 eV. The direct nature of the band gap is supported by density functional theory calculations. Intense photoluminescence was observed at room temperature at 678 nm arising from the band edge of the material. High-quality thin films can be prepared by the hot-casting method from solutions with a pure-phase compound as a precursor. The planar solar cells fabricated with n = 5 thin films demonstrate excellent power-conversion efficiency of 8.71% with an impressive open-circuit voltage of ∼1 V. Our results point to the use of layered perovskites with higher n numbers and pure chemical composition.