•Electron injection and transport properties of mesoporous layer containing TiO2 nanoparticles are investigated.•The photovoltaic properties are poor with small TiO2 particles and thick mesoporous layers.•This poor performance is due to the increase of the area of the TiO2/TiO2 interface.•The TiO2/TiO2 interfacial resistance largely influences the photovoltaic parameters. Mesoporous TiO2 (mp-TiO2) layers are commonly used as electron transport layers in perovskite solar cells, which help to extract electrons from the perovskite light-absorbing layer and transport them to the electrodes. We investigated the effects of the layer thickness of mp-TiO2 and particle size of TiO2 on photovoltaic properties, in terms of the surface area of the mp-layer and the interfacial areas of the TiO2 nanoparticles in the mp-layer. Various mp-TiO2 layers with thicknesses of 150, 250, and 400 nm and particle sizes of 25 nm and 41 nm were prepared to compare the photovoltaic properties of such layer-containing perovskite solar cells. Time-resolved photoluminescence decay and impedance studies showed that interfacial resistance as well as perovskite-to-TiO2 charge injection are important factors affecting photovoltaic performance. The deterioration of the photovoltaic parameters with increasing TiO2/TiO2 interfacial area also confirms that the interfacial series resistance that arises from these connections should be reduced to enhance the performance of mesoscopic perovskite solar cells.