Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (O V ) leads to large scale enhancement of photoconductivity in BiFeO 3 (BFO). The O V concentration is increased by substituting an aliovalent cation Ca 2+ at Bi 3+ sites in the BFO structure. Furthermore, the O V concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi 1-x Ca x FeO 3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca 2+ concentration which signifies the role of O V . A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that O V at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2-4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk O V are nearly flat, degenerate, and discrete, the defect states due to surface O V are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells. © 2018 Author(s).