At both micro- and electronic levels, this paper investigates the effects of Ru-doping on the efficiency of TiO2-based dye-sensitized solar cells (DSSCs). A two-layered working electrode design, one for TiO2 (P-25) and the other for Ru-doped TiO2, respectively with and without a p-n junction, is applied. Two calculations based on first principles thermodynamics using the computer software (FactSage) are performed to simulate the chemical equilibria of Ru-TiO2 particles at 600 degrees C as well as to demonstrate the ability of Ti and Ru in taking oxygen atoms to form TiO2 or RuO2 at 600 degrees C. Our results reveal a 9.5% increase in open-circuit photovoltage (V-oc) as well as a 83.4% increase in short-circuit photocurrent density (J(sc)), compared with the DSSC fabricated with a TiO2 electrode. The observed enhancement in both Voc and J(sc) can be explained by Ru-modified defect levels in the TiO2 band gap, which may be the origin of the improved performance. Due to the formation of a p-n junction (p-type Li-doped NiO and n-type Ru-doped TiO2) in the working electrode, the value of Voc has been further increased while maintaining a high J. This study advances our fundamental understanding and methodology on metal-doping in general photo-electrodes. (C) 2016 Elsevier Ltd. All rights reserved.