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We numerically investigated the effect of an optically ultrathin top metal grating in an absorber structure composed of an amorphous silicon (a-Si) thin film and a metal reflector for efficient solar cells. The ultrathin metal grating-based structure shows nearly polarization-insensitive net absorption due to a great improvement for TE polarized wave, which is about 2.5 times enhanced compared with a relatively thick metal grating case. In particular, it is also shown that the absorption angle of the proposed solar cell is as wide as 80^° owing to light coupling into an active layer through a leaky mode of flat dispersion characteristic for TM polarization.