Though bismuth vanadate (BiVO4) is extensively used as a photoactive material, its performance in harnessing solar energy is limited by ineffective separation of photo-excited charge carriers. We demonstrate here a concept of n-n(+) homojunction of BiVO4/BiV1-xMnxO4-y, which improves its charge separation efficiency. Using first-principles theoretical calculations, we determine the effect of Mn substitution on oxygen vacancy formation energies and associated changes in the electronic structure of BiVO4. Showing that Mn substitution pushes the Fermi level of BiVO4 towards its conduction band, we predict that the associated enhanced bending of bands at the homojunction (BiVO4/BiV1-xMnxO4-y) facilitates efficient separation of charge carriers. With Mott-Schottky experiments, we verify the increased band bending at the n-n(+) homojunction, and show that the maximum photocurrent density measured in a sample with n-n(+) homojunction is ten times higher than that obtained of the pristine sample. Secondly, Mn substitution in BiVO4 also reduces the oxygen vacancy formation energy, promoting higher concentration of O-vacancies, further enhancing the photoelectrochemical response. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.