We report the fabrication of highly crystalline, large-grained organic-inorganic perovskite [CH (NH2)(2)](0.8)Cs0.2PbI3 thin films on TiO2 layers by a one-step method using 1-Cyclohexyl-2-pyrrolidone and thiosemicarbazide as additives without using an antisolvent. The perovskite thin films have no pinhole, grain size of micrometer order, and high crystallinity, with a crystallite size greater than 200 nm. An n-i-p structured [CH(NH2)(2)](0.8)Cs0.2PbI3 thin film solar cell exhibits power conversion efficiencies of 17.8% and 13.8% during reverse and forward scanning, respectively.