Synergistic effect of guanidine thiocyanate additive and dimethyl sulfoxide post-treatment towards efficient and stable perovskite solar cell

•A synergistic method is used to prepare high quality perovskite film.•The guanidine thiocyanate additive is introduced in lead iodide precursor solution.•The mixed perovskite intermediate product can enhance properties of perovskite film.•The solvent post-treatment can make the perovskite film re-crystallizing.•The device by the synergistic method yields an enhanced efficiency of 18.04%.