In order to improve the energy conversion efficiency of dye-sensitized solar cells (DSSCs), deposition of anatase titanium dioxide (TiO2) composite thin films as the negative electrode was attempted using constant-current electrophoresis with a colloidal mixture of two types of TiO2 nanoparticles with different sizes and surface properties. The first type was synthesized in the present study and had a size of about 5 nm; these are referred to as TNPs. The second type was commercially obtained TiO2 nano particles (P25) with a size of about 20 nm. The P25-to-TNP mass ratio was changed during electrophoresis. Because the TNPs were small and the film had a high specific surface area, the film was deposited to minimize the P25-to-TNP mass ratio in the region close to the entrance window of the DSSC. The upper limit of the film thickness was markedly increased compared to deposition in a colloid consisting of one or two kinds of TiO2 nanoparticles, but without a particle size gradient. A method for calculating the P25-to-TNP mass ratio, the gradient of the number average particle size, and the specific surface area of thin films with a particle size gradient was proposed. The harvesting efficiency for incident light was improved over a wide wavelength range, and as a result, the energy conversion efficiency of DSSCs fabricated using these thin films was increased. (C) 2015 Elsevier B.V. All rights reserved.