This study presents the fabrication and properties of 2D perovskite thin films of the series $(\mathrm{CH}_{3}(\mathrm{CH}_{2})_{3}\mathrm{NH}_{3})_{2}(\mathrm{CH}_{3}\mathrm{NH}_{3})_{n-1}\mathrm{Pb}_{n}\mathrm{I}_{3n+1}$ (n = 1, 2, 3, and 4). The band gaps of the series decrease with increasing n values, from 2.24 eV (n = 1) to 1.52 eV (n = ∞). These materials exhibit strong light absorption in the visible region and strong photoluminescence at room temperature, making them promising light absorbers for photovoltaic applications. The 2D perovskite thin films show an ultrahigh surface coverage due to their unique self-assembly, with the [Pb$_n$I$_{3n+1}$]$^- layers oriented perpendicular to the substrates. The 2D perovskite family was successfully implemented in solid-state solar cells, achieving an initial power conversion efficiency of 4.02% with an open-circuit voltage (Voc) of 929 mV and a short-circuit current density (Jsc) of 9.42 mA/cm² from the n = 3 compound. The device retained its performance after long exposure to a high-humidity environment. The homologous 2D halide perovskites are promising for stable and efficient light-absorbing materials in solid-state photovoltaics and other applications. The 2D perovskite films show excellent moisture resistance, with the (BA)₂(MA)₂Pb₃I₁₀ film remaining unchanged after 2 months of exposure to 40% humidity. The high moisture stability is attributed to the hydrophobicity of the long BA cation chain and the highly oriented and dense nature of the perovskite films. The 2D perovskite films also exhibit strong photoluminescence at room temperature, indicating efficient carrier generation and access to the highest possible open-circuit voltage. The 2D perovskite films were fabricated using a one-step spin-coating method, resulting in smooth, ultrahigh surface coverage films. The 2D perovskite films show vertical growth, which is beneficial for charge transport in photovoltaic devices. The 2D perovskite films also show high charge transport mobility due to the evolution of continuous crystallites with few crystal boundaries. The 2D perovskite films were used in solid-state solar cells, achieving a power conversion efficiency of 4.02%. The study demonstrates the potential of 2D perovskite materials for photovoltaic applications due to their highThis study presents the fabrication and properties of 2D perovskite thin films of the series $(\mathrm{CH}_{3}(\mathrm{CH}_{2})_{3}\mathrm{NH}_{3})_{2}(\mathrm{CH}_{3}\mathrm{NH}_{3})_{n-1}\mathrm{Pb}_{n}\mathrm{I}_{3n+1}$ (n = 1, 2, 3, and 4). The band gaps of the series decrease with increasing n values, from 2.24 eV (n = 1) to 1.52 eV (n = ∞). These materials exhibit strong light absorption in the visible region and strong photoluminescence at room temperature, making them promising light absorbers for photovoltaic applications. The 2D perovskite thin films show an ultrahigh surface coverage due to their unique self-assembly, with the [Pb$_n$I$_{3n+1}$]$^- layers oriented perpendicular to the substrates. The 2D perovskite family was successfully implemented in solid-state solar cells, achieving an initial power conversion efficiency of 4.02% with an open-circuit voltage (Voc) of 929 mV and a short-circuit current density (Jsc) of 9.42 mA/cm² from the n = 3 compound. The device retained its performance after long exposure to a high-humidity environment. The homologous 2D halide perovskites are promising for stable and efficient light-absorbing materials in solid-state photovoltaics and other applications. The 2D perovskite films show excellent moisture resistance, with the (BA)₂(MA)₂Pb₃I₁₀ film remaining unchanged after 2 months of exposure to 40% humidity. The high moisture stability is attributed to the hydrophobicity of the long BA cation chain and the highly oriented and dense nature of the perovskite films. The 2D perovskite films also exhibit strong photoluminescence at room temperature, indicating efficient carrier generation and access to the highest possible open-circuit voltage. The 2D perovskite films were fabricated using a one-step spin-coating method, resulting in smooth, ultrahigh surface coverage films. The 2D perovskite films show vertical growth, which is beneficial for charge transport in photovoltaic devices. The 2D perovskite films also show high charge transport mobility due to the evolution of continuous crystallites with few crystal boundaries. The 2D perovskite films were used in solid-state solar cells, achieving a power conversion efficiency of 4.02%. The study demonstrates the potential of 2D perovskite materials for photovoltaic applications due to their high