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Control of the alkali doping level in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) films was demonstrated using alkali-silicate thin layers of various thickness deposited on substrates prior to the sputtering of the Mo back contact layer. Not only the alkali density in the CIGS film, but also the Ga composition distribution in CIGS films, CIGS grain size, and consequent photovoltaic performance showed variations with the silicate layer thickness. Using alkali-silicate thin layers as an alkali source material, 17.4% and 17.7% efficiency flexible CIGS solar cells have been demonstrated on Ti and zirconia substrates, respectively.