The performances of thin film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass / back reflector / n-i-p type Si / ITO is well characterized by the structure of naturally _surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), the open circuit voltage of 0.539 V and the short current density of 25.8 mA/cm2 as independently confirmed by Japan Quality Assurance. The optical confinement effect explains the excellent spectral response at long wavelength for our cells through the PCID analysis. The higher sensitivity at long-wavelength of our cell appeared in quantum efficiency curves is well correlated to the result of reflectance measurement. The efficiency of 9.3% cell with a thickness of 1.5 pm was proved to be entirely stable with respect to the lightsoaking. Based on the result of various evaluation of diffusion length, it is postulated that the low temperature poly-Si prepared by plasma CVD gives a device quality of poly-Si film.