Surface passivation by thin-slab 2D perovskites is an effective method to suppress interfacial carrier recombination of 3D metal halide perovskite photoabsorbers. However, improving device performance by integrating such 2D/3D perovskite heterojunctions in the sunward, transparent contacts of perovskite solar cells is not evident due to the high exciton binding energies of phase-pure 2D perovskites, which may result in inefficient free-carrier generation and collection. Here, we overcome this challenge by tuning the dimensionality of 2D perovskites via structural isomers of butylammonium (BA) as a small organic cation, occupying the A-site of the 2D perovskite lattice. The discontinuous iso-BA-based 2D crystals on the 3D perovskite surface yield improved interfacial passivation and enhanced hole extraction. Besides an increased open circuit voltage, this remarkably leads to an enhanced photocurrent (
1 mA cm2 ) compared with control perovskite solar cells, resulting in bifacial perovskite/silicon tandem solar cells with power generation densities >27 mW cm2 .