This paper describes the influence of acid pretreatment of TiO2 mesoporous films prior to dye sensitization on the performance of dye-sensitized solar cells based on [(C4H9)(4)N](3)[Ru(Htcterpy)(NCS)(3)] (teterpy = 4,4',4&DPRIME;-tricarboxy-2,2',2&DPRIME;-terpyridine), the so-called black dye. The HCl pretreatment caused an increase in overall efficiency by 8%, with a major contribution from photocurrent improvement. It is speculated, from the analysis of incident photon-to-electron conversion efficiency, UV-vis absorption spectra, redox properties of the dye and TiO2, and the impedance spectra of the dye-sensitized solar cells, that photocurrent enhancement is attributed to the increases in electron injection and/or charge collection efficiency besides the improvement of light harvesting efficiency upon HCl pretreatment. Open-circuit photovoltage (V-oc) remained almost unchanged in the case of significant positive shift of flat band potential for TiO2 upon HCl pretreatment. The suppression of electron transfer from conduction band electrons to the I-3(-) ions in the electrolyte upon HCl pretreatment, reflected by the increased resistance at the TiO2/dye/electrolyte interface and reduced dark current, resulted in a V-oc gain, which compensated the V-oc loss due to the positive shift of the flat band. Using the HCl pretreatment approach, 10.5% of overall efficiency with the black dye was obtained under illumination of simulated AM 1.5 solar light (100 mW cm(-2)) using an antireflection film on the cell surface.