Bismuth (Bi)-doped flexible Cu(In,Ga)Se-2 (CIGS) films on stainless steel (SUS) substrates with different Bi contents are prepared, obtained by covering Bi thin layers (as Bi source) on Mo layers with Bi thickness from 0 (without Bi-doping) to 100 nm. It is disclosed that [Ga]/([Ga] thorn [In]) profiles are almost identical when Bi content is increased by enhancing the Bi thickness. The CIGS grain size and carrier lifetimes are improved through the proper Bi-doping (Bi thickness of 20-65 nm) under low deposition temperature of 457 degrees C. The 12.6%-efficient flexible Bi-doped CIGS solar cell under the deposition temperature of 457 degrees C is consequently obtained. Conversion efficiency of the flexible Bi-doped CIGS solar cell under the low deposition temperature of 486 degrees C is further enhanced to 14.8%, very close to that (15.1%) of the solar cell under convention deposition temperature (543 degrees C) without Bi-doping. Furthermore, the Bi-doping is effective for the enhancement of the photovoltaic performances under the deposition temperature range of 457-486 degrees C, whereas the Bi-doping under high CIGS deposition temperature range of 543-572 degrees C has the detrimental impact on the cell parameters, resulting from the sever Bi diffusion into the CIGS film.