Organometallic perovskites have recently emerged as excellent solar cell materials for high power conversion efficiencies owed to their remarkable absorber characteristics, easy processing and low cost. Many efforts have been made by choosing appropriate additives to improve the power conversion efficiency and the stability. In this work, guanidine thiocyanate as the additive is introduced into the lead precursor solution to prepare perovskite films via two-step solution method. It is found that guanidine thiocyanate additive can notably improve the power conversion efficiency and the stability of MAPbI(3) perovskite solar cells due to the improvement of quality and crystal structure of perovskite films. After adding guanidine thiocyanate into lead precursor to prepare perovskite films, the high power conversion efficiency of solar cell is increased from 15.24% to 16.92%. The improvement mechanisms of power conversion efficiency and thermal stability of perovskite solar cells have been studied. Thiocyanate ion additive largely affects the immediate nucleation process of perovskite film in which provides the improved morphology of the perovskite film, while thiocyanate ions are absent from the resulting perovskite film. However, guanidine ion can substitute methylammonium ion in lattices of perovskite. The guanidine-ion additive is mainly responsible for the improvement of thermal stability of perovskite film and can increase the power conversion efficiency of solar cells. (c) 2018 Elsevier Ltd. All rights reserved.