We propose a realistic topological p?n junction (TPNJ) by matching two Bi₂Se₃ (0001) slabs with opposite arrangements of planar twin boundary defects. The atomistic modeling of such a device leads to dislocation defects in the hexagonal lattice in several quintuple layers. Nevertheless, total energy calculations reveal that the interface relaxes, yielding a smooth geometrical transition that preserves the nearest-neighbors fcc-type geometry throughout these defect layers. The electronic, magnetic, and transport properties of the junction have then been calculated at the ab initio level under open boundary conditions, i.e., employing a thin-film geometry that is infinite along the electron transport direction. Indeed, a p?n junction is obtained with a built-in potential as large as 350 meV. The calculations further reveal the spin texture across the interface with unprecedented detail. As the main result, we obtain non-negligible transmission probabilities around the Γ point, which involve an electron spin-flip process while crossing the interface.