Thermophotovoltaic (TPV) cells are fabricated based on lattice-mismatched In0.68Ga0.32As with bandgaps of 0.6 eV grown on InP substrates. The performace of TPV cell is significantly improved for the optimized material quality owing to the strain relaxation of the InAsxP1-x buffer layer, and therefore to suppress the dislocation. Under a standard AM1.5G spectra, the open circuit voltage of TPV cell increases from 0.19 V to 0.21 V, the long wavelength external quantum efficiency reaches 85%, and the conversion efficiency is increased by 30%.