[en] The aim of this paper is to study the effect of graded bandgap Al_xGa_1-xAs front layers on the optical properties and cell performance of a GaAs solar cell (SC) grown by molecular beam epitaxy on (111)A GaAs substrate. For comparison, a GaAs SC with the same structure but without graded bandgap Al_xGa_1-xAs layers was also grown. The two SCs were investigated by spectroscopic ellipsometry and photo-absorption techniques. The extracted optical constants and the calculated energy loss functions are interpreted as a function of photon energy in the photon energy range of 1-6 eV. The critical point energies are calculated and identified. SE results show that Al_xGa_1-xAs-graded layers have a major influence on conversion efficiency of the front-graded SC. Photocurrent results show that SC grown with graded bandgap Al_xGa_1-xAs front layers has much larger responsivity than that of the reference SC. Our results show that graded bandgap Al_xGa_1-xAs front layers can ameliorate performance of GaAs SCs grown on (111)A GaAs substrates due to the built-in electric field induced by the bandgap gradation. (author)