TiO2/CH3NH3PbI3-based photovoltaic devices were fabricated by a spin-coating method using a mixture solution. Photovoltaic properties of the solar cells were investigated by optical absorption, current density-voltage characteristics, and incident photon to current conversion efficiency. Effects of TiO2 addition and hole-transport layers were also investigated. Microstructures of the cells were investigated by transmission electron microscopy, which indicated a nanoparticle structure of TiO2, and atomic arrangements of Pb atoms in the CH3NH3PbI3 crystal were also directly observed.