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We report on the fabrication and properties of the semiconducting 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n–1Pb_nI_3n+1 (n = 1, 2, 3, and 4) perovskite thin films. The band gaps of the series decrease with increasing n values, from 2.24 eV (CH₃(CH₂)₃NH₃)₂PbI₄ (n = 1) to 1.52 eV CH₃NH₃PbI₃ (n = ∞). The compounds exhibit strong light absorption in the visible region, accompanied by strong photoluminescence at room temperature, rendering them promising light absorbers for photovoltaic applications. Moreover, we find that thin films of the semi-2D perovskites display an ultrahigh surface coverage as a result of the unusual film self-assembly that orients the [Pb_nI_3n+1]⁻ layers perpendicular to the substrates. We have successfully implemented this 2D perovskite family in solid-state solar cells, and obtained an initial power conversion efficiency of 4.02%, featuring an open-circuit voltage (V_oc) of 929 mV and a short …