Pressure dependence of band gap of (0 4 0) oriented TlI films

•Structural and optical properties of orthorhombic TlI films are investigated using DFT calculations and experiment.•Films show a perfect (040) oriented growth with strain without exception for any growth condition.•Hill’s value of Young’s modulus (16.72 GPa) and Poisson ratio (0.329) were calculated for strain to stress conversion.•Direct Band gap of films linearly correlate with their residual stress with pressure coefficient of band gap −1eV/GPa.•Stress free band gap of 2.92 eV from experiment tallies very well with calculated 2.88 eV from band structure.