We demonstrate a facile and well controlled heat assisted solvent vapour treatment (HASVT) method for the growth of compact perovskite layers with good surface coverage areas in ambient atmosphere. The structural and optical properties of the as deposited and solvent vapour treated films have been investigated using SEM, XRD, UV-Vis and PL spectroscopy. Furthermore, photovoltaic devices in layered configuration (FTO/TiO₂/CH₃NH₃PbI_3−XCl_X/P3HT/Ag) are fabricated using as deposited and solvent vapour treated CH₃NH₃PbI_3−XCl_X films. The photoactive perovskite films were treated with chlorobenzene solvent vapour using the HASVT method. The comparative photovoltaic performance of devices fabricated using as deposited and solvent vapour treated CH₃NH₃PbI_3−XCl_X films was assessed. The maximum power conversion efficiency in the case of vapour treatment was estimated to be ∼8.05% as compared to the devices fabricated using as deposited films (∼5.51%). Our systematic investigations reveal that the enhancement in power conversion efficiency is due to modification in the structural and surface morphology of CH₃NH₃PbI_3−XCl_X films.