Periodic gratings on solar cell back reflectors are an alternative to randomly textured surfaces to provide absorption enhancement. Theoretically, it is impossible to excite quasi-guided modes at every wavelength for a given grating geometry and the broad band enhancement can only be achieved by strong absorption peaks at several wavelengths. Therefore, the critical issue is how to maximize the short circuit current using a limited number of discrete quasi-guided modes. In this work, a common dielectric-semiconductor-dielectric- metal solar cell structure is investigated. It is found that although the number of guided mode peaks has pronounced effect on the solar cell short circuit current, the geometry resulting in the highest short circuit current does not coincide with the geometry leading to the most supported modes. It is also found that high-Q modes are always resulted from global optimization for TE incidence, while low-Q modes are resulted for TM incidence on one-dimensional gratings without a dielectric spacer. Besides, a properly designed and configured dielectric spacer can provide >40% improvement in short circuit current. It is therefore suggested for solar cells with metallic back reflectors, dielectric spacer should be included, and the texture should be formed on the dielectric spacer itself rather than on the metal. Finally, the optimization of the mode quality is proved to be critical in all cases, in addition to the number of supported modes.