The experimental spectral response and reflectance of high-efficiency a-Si solar cells are systematically investigated by using an optical simulation based on realistic optical properties of the transparent conducting oxide (TCO), a-Si, and metal electrode, in order to improve the spectral response. It is shown that a practically important optical loss results from absorption by the TCO, which is enhanced by the optical confinement effect. This suggests that improvement in the spectral response is possible by suppressing the optical confinement in the TCO.