Undoped and lightly Si doped GaAs1-xSbx bulk layers have been grown on GaAs substrates with Sb composition as high as 0.5 by molecular beam epitaxy. Low temperature (4 K) photoluminescence (PL) and absorption measurements were carried out. For samples with x values smaller than 0.06, PL spectra were dominated by two bands, the emission associated with the bound excitons and with the electron (or donor) to acceptor recombinations. However, the free exciton peak was clearly observable in absorption spectra. For higher Sb compositions (x>0.06), only one peak associated with the impurity or defect related transitions dominates PL spectra. We found that the energy and the line shape of this main peak are closely related to the sample quality for the same x value. The lower peak energy than the band gap, the low signal intensity and the large line broadening are due to the emissions associated with the defect band below the ideal (perfect crystal) energy gap introduced by the lattice mismatch near the heterointerface of GaAs1-xSbx and GaAs. As expected, higher quality epilayers provide a stronger PL intensity, smaller line width, as well as a higher peak energy.