Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 mu m. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H (0.45 mu m)/poly-Si (5 mu m) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (J(sc) above 27 mA/cm(2) at the 4.7 mu m thin-film poly-Si solar cell.