NOVELTY - Preparation of flexible polyimide (PI) substrate doped with sodium (Na) absorption layer includes placing two rollers parallel with absorbent layer of deposited film inside vacuum chamber, rolling flexible substrate with molybdenum (Mo) back electrode oriented parallel to horizontal plane, setting three process chambers provided with evaporation sources underneath thin film chamber; heating first chamber, heating evaporation sources; moving substrate to second chamber, heating evaporation sources; and moving PI substrate to third chamber, heating evaporation sources and cooling.    USE - Method for preparing flexible polyimide (PI) substrate doped with sodium (Na) absorption layer (claimed) is used in battery.    ADVANTAGE - The flexible polyimide substrate improves electrical performance of battery.    DETAILED DESCRIPTION - Preparation of flexible polyimide (PI) substrate doped with sodium (Na) absorption layer comprises placing two rollers parallel with absorbent layer of deposited film inside vacuum chamber, rolling flexible substrate with molybdenum (Mo) back electrode oriented parallel to horizontal plane, setting three process chambers provided with evaporation sources underneath thin film chamber; evacuating first chamber to less than 2asterisk103 Pa, setting temperature at 400-450 degrees C, heating sodium fluoride (NaF) source at 600-700 degrees C, gallium (Ga) source at 930-980 degrees C, indium (In) source at 890-940 degrees C and selenium (Se) source at 260-300 degrees C, evaporating at 4-6asterisk106 cps to get Mo back electrode layered with 100-150 nm Na-doped indium, gallium and sulfur (IGS) thin film; preparing copper (Cu)-rich IGS film (CIGS film) by moving PI substrate via terminal roller to second chamber, controlling chamber temperature at 400-450 degrees C, heating NaF evaporation source at 600-700 degrees C, Cu source at 1200-1300 degrees C, Ga source at 960-1010 degrees C, In source at 900-950 degrees C and Se source at 240-280 degrees C, maintaining substrate speed at 10-20 mm/minute to get substrate with 1.3-1.8 mu m CIGS film; and moving PI substrate with CIGS film to third chamber, controlling chamber temperature at 400-450 degrees C, heating NaF evaporation source at 600-700 degrees C, Cu source at 1200-1250 degrees C, Ga source at 960-1010 degrees C, In source at 900-950 degrees C and Se source at 240-280 degrees C, keeping substrate speed at 10-20 mm/minute to get substrate with 0.5-0.87 mu m copper depleted CIGS film and cooling. The first chamber is provided with evaporation sources of NaF, gallium, In and Se. The second chamber is provided with evaporation sources of NaF, Cu, Ga, In and Se. The third chamber is provided with evaporation sources of NaF, Cu, Ga, In and Se. All evaporation sources are set 300-400 mm away from flexible PI substrate. Each chamber and PI substrate has transporting speed of 10-20 mm/minute.