We present a first-principles study on the electronic properties of TiO2 containing both dopants and defects on the basis of density-functional theory. We show that the formation energies of defects can be reduced by up to 80% in N-doped TiO2 (N substitutes O), as compared to those in perfect TiO2, but the H-doping (H substitutes O) has less effect on the defect formation. We predict that dopant-interstitial Ti cluster plays an important role in the narrowing of band gap of N-doped anatase and brookite TiO2, and H-doped rutile TiO2. Importantly, the defect bands within the band gaps can enhance the visible-light absorption and improve the photocatalytic performance of the systems due to the reduced gap between the bands. The dopant-defect cluster in the doped TiO2 may be responsible for the observed visible-light absorption in experiments. The present study provides a new route to enhance the performance of photocatalyst. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.