Additive effects of guanidinium [C(NH₂)₃, GA] iodide, formamidinium [CH(NH₂)₂, FA] iodide, and guanidinium chloride to CH₃NH₃PbI₃-based photovoltaic devices were investigated. Short-circuit current densities, open-circuit voltages, series resistances and shunt resistances were improved by the GA addition. The short-circuit current densities were increased by FA addition with GA, and the external quantum efficiencies increased, which results from the suppression of pin-holes in perovskite layers by GA addition. X-ray diffraction showed that the lattice constants of the perovskite crystals increased by the GA and FA addition, and that the GA substituted partially at the CH₃NH₃-site.