The manuscript reports on the influence of 40 keV Ag –ve ion bombardment with different fluences on the microstructural and optical properties of thermally evaporated Bi/GeSe 2 bilayer thin films. Two different fluences (5 × 10 14 ions cm −2 and 1 × 10 15 ions cm −2 ) of Ag –ve ions were used to irradiate the thin films that changed the microstructure and optical properties as studied by different spectroscopic methods like X-ray diffraction method (XRD), Energy dispersive X-ray spectroscopy (EDS), Field emission scanning electron microscopy (FESEM), Atomic force microscopy (AFM), Raman spectroscopy, and UV–Vis spectroscopy. The evolution of topological Bi 2 Se 3 phase occurs after ion irradiated diffusion of Bi into GeSe 2 matrix. The optical parameters as calculated from the transmission spectra infers the indirect allowed transition with reduction of E g on ion irradiation. The various optical parameters like absorption coefficient ( α ), optical energy gap ( E g ), Tauc parameter ( B 1/2 ), Urbach energy ( E e ), extinction coefficient ( k ), refractive index ( n ) were modified with ion irradiation. The surface morphology is being changed after irradiation as probed by AFM and FESEM. The Raman spectra support the formation of Bi 2 Se 3 phase with irradiation. The obtained results have been explained on the basis of increase in band tailing of valence band due to defect states caused by the irradiation.