Doctoral thesis is dedicated to the modification of tin disulphide thin films obtained by closed spaced sublimation (CSS) method with the aim to obtain phase transitions in SnxSy thin films. The work includes description and analysis of SnS2 thin films structure properties and chemical composition changes before and after thermal annealing in vacuum furnace, and modification by second harmonic Nd:YAG laser irradiation with different laser parameters. The structural investigations of the SnS2 thin films annealed in vacuum oven at different temperatures and with different exposure time were carried out. As a result, optimal parameters for phase transition initiation were determined, with the aim to obtain single-phase SnS thin film from initial SnS2 thin film. A new technique to initiate phase transition in Sn-S binary compounds using laser irradiation was proposed. For the first time, possibility to locally initiate phase transitions in polycrystalline SnxSy thin films using second harmonic Nd: YAG laser was shown. In this respect, three intensity thresholds (Ith) were determined, which results in initiation of phase transition from hexagonal SnS2 phase to orthorhombic SnS phase, formation of a metallic phase of Sn on the surface of SnS, as well as an ablation of SnS2 material. The mechanism of initiation of phase transition SnS2-->SnS un SnS-->Sn has been determined which was related to the drift of S atoms towards the surface of thin film due to the temperature gradient effect and subsequent evaporation of S. The possibility to induce phase transitions in SnxSy material opens a potential of this material to be used in optoelectronic devices, such as solar cells, because it is possible to obtain p-n heterojunction with Sn metallic contact using one technological task. Experimental results are discussed and explained in context of scientific literature. A new method of initiation of phase transition in tin sulphides has been developed and a patent of the Republic of Latvia LV15214 “Initiation method of phase transitions in tin sulphides” (L.Grase, A.Medvids, P.Onufrijevs) has been received.