The invention relates to an
alkali metal doping method for preparing a CIGS absorbing layer on a flexible substrate, and belongs to the technical field of
copper indium gallium selenide (CIGS) thin film solar cells. The
alkali metal doping method comprises that a CIGS absorbing layer is deposited by using a co-
evaporation process; and with the increase of Cu content in the absorbing layer, the thin film growth experiences a
copper-poor to
copper-rich process, in the copper-rich process, when Cu(In+Ga)>1 in a CIGS thin film, the
evaporation of the
Cu element is stopped so that the slightly copper-rich CIGS thin film can finally become copper-poor, then the In and Ga atoms are evaporated until the deposition thickness is 1 / 10-3 / 10 of the thickness of the absorbing layer, in this process, an
alkali metal compound is co-evaporated, the
doping amount is 0.08-0.12% of the
atomic ratio with respect to the CIGS thin film, the temperature of the substrate is reduced to the
room temperature, and the CIGS thin film having a thickness of 1-3 <mu>m is obtained. The invention has the advantages of having a simplified process, a high production efficiency and thin film
crystal high-quality, increasing the carrier concentration of the absorbing layer, lowering the resistivity, improving the electrical properties of the thin film
cell, thus improving the
photoelectric conversion efficiency of the CIGS
thin film solar cell and the like.