A method for obtaining a salt with a general formula: RxNI, wherein: RxN is an organic cation (RxN+), R represents substituents (R−) independently selected from a group consisting of organic substituents: R1−, R2—, R3— and hydrogen (H—), x is a number of the substituents R— directly linked with the nitrogen (N) atom in the organic cation RxN+, wherein x is 3 or 4, I is an iodide anion (I−). The method comprises: preparing a reaction mixture comprising the steps of: synthesizing hydrogen iodide (HI) in situ by mixing molecular iodine (I2) with formic acid (COOH) in a molar ratio of molecular iodine (I2): formic acid (COOH) of no less than 1.01:1, in a solvent medium, introducing into the solvent medium a compound being a donor of organic cation RxN+ in an amount providing the molar ratio of the donor of organic cation RxN+: molecular iodine (I2) of no less than 1.01:1, and maintaining the reaction mixture at a temperature of not less than 20° C. for the time necessary to obtain the reaction product being the salt with the general formula RxNI. The obtained product is a substrate for synthesis of perovskites.