We formed phosphorous(P)-ion-implanted n-BaSi₂ films on p-Si(111) substrates and demonstrated solar-cell functionality of the n-BaSi₂/p-Si heterojunction under AM1.5 illumination. The BaSi₂ films were grown by molecular beam epitaxy, followed by implantation of P ions to the BaSi₂ films using PF₃ gas at an energy of 10 keV and a dose of 1 × 10¹⁴ cm⁻². Subsequent postannealing was conducted at 500°C in Ar for different durations (t = 30–480 s) to activate the P atoms. The diffusion coefficient for P atoms in BaSi₂ was evaluated from the depth profiles of P atoms by secondary-ion mass spectrometry. The activation energies of lattice and grain boundary diffusion were found to be 1.1 ± 0.6 and 2.5 ± 0.6 eV, respectively. From the analysis of Raman and photoluminescence spectra, the ion implantation damage was recovered by the postannealing. For one treated sample with t = 120 s, the internal quantum efficiency reached 67% at a wavelength of 870 nm. This is the highest ever achieved for n-BaSi₂/p-Si heterojunction solar cells. Ion implantation is thus applicable to BaSi₂ films grown by any other method. This achievement thereby opens a new route for the formation of BaSi₂ solar cells.