We developed a new perovskite film forming process based on solvent bathing method. The preheating of substrate before spin-coating of precursor solution drastically changed the quality of the obtained perovskite films. The perovskite films formed by modified solvent bathing method were much dense with large grains. The improvement of open circuit voltage implied that the modification succeeded to form the better interface between TiO2 and perovskite layer and to grow the higher quality perovskite layer. Finally, we obtained the best conversion efficiency of 12.6%. Although the conversion efficiency of this work is not sufficient compared with other reports, our modified solvent bathing method has a wider process window comparing to the commonly used anti-solvent dipping methods. We expect that our modified method can also applied to monolithic tandem solar cells. •The solvent bathing method with preheating of substrate was developed.•Obtained perovskite films were much dense with large grains.•The improvement of open circuit voltage due to a better interface between TiO2 and perovskite layers.•The modified solvent bathing method has a wider process window.