NOVELTY - Preparing polymer solar cell comprises: (a) adhering a layer of adhesive tape on the glass with indium tin oxide (ITO); (b) confecting electron donor polymer and electron acceptor 1-(trimethoxyhydroxyl)-propyl-1-phenyl-(6,6)-C61 (PCBM); (c) drying conductive glass coated with poly(3,4-dioxyethyl thiophene):poly(p-styrene sulfonic acid) water solution film; and (d) placing the treated and heated polymer solar cell to vacuum coating machine for steaming.    USE - The method is useful for preparing polymer solar cell (claimed).    ADVANTAGE - The metal employs composite cathode LiF/Aluminum as cathode, that is, a thin layer LiF is inserted between photo-active layer and Aluminum to form excellent ohm contact between metal electrode and cathode, to reduce impedance of system so as to effectively improve the properties of polymer solar cell.    DETAILED DESCRIPTION - Preparing polymer solar cell comprises: (a) adhering a layer of adhesive tape on the glass with indium tin oxide (ITO), cutting the to-be-etched adhesive tape with knife blade, etching the electrode with width of 4 mm and length of 30 mm by concentrated hydrochloric acid and zinc powder, washing ITO conductive glass etched by concentrated hydrochloric acid and zinc powder cleanly and drying, placing the processed conductive glass on the pasting machine, uniformly coating the poly(3,4-dioxyethyl thiophene):poly(p-styrene sulfonic acid) water solution (PEDOT:PSS) with mass percent of 1.5% on the whole conductive glass substrate, using the pasting machine to rotate for 60 seconds with 2500 rotations/minute so that PEDOT:PSS forms a layer of 30 nm on conductive glass surface, then placing it to the dry oven of 120 degrees C to heat for 20-30 minutes for future use; (b) confecting electron donor polymer poly(2,5-bis(2'-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-co-MEH-PPV) of formula (1) and electron acceptor 1-(trimethoxyhydroxyl)-propyl-1-phenyl-(6,6)-C61 (PCBM) with mass ratio of 1:3 to the solution of electron donor polymer BEH-co-MEH-PPV with concentration of 5 mg/ml by using organic solvent toluene in a sleeve box having nitrogen gas, heating and stirring for 3 hours at 40-50 degrees C to obtain polymer solution stirred uniformly; the molecular structural formula of electron donor polymer BEH-co-MEH-PPV is as follows: in the formula m:n=1:1; (c) drying conductive glass coated with poly(3,4-dioxyethyl thiophene):poly(p-styrene sulfonic acid) water solution film, then transferring it to a sleeve box having nitrogen gas, placing on the pasting machine, coating the PEDOT:PSS with mass percent of 1.5% on the whole surface of film, controlling the spin coating speed and time, firstly rotating for 3 seconds with 500 rotations/minute, then 3 minutes with 2000 rotations/minute, so that the mixture forms a layer of polymer film with 200 nm thickness of electron donor polymer BEH-co-MEH-PPV: PCBM on ITO surface, then placing the conductive glass on the heating table, heating and annealing for 30 minutes at 80 degrees C; and (d) placing the treated and heated polymer solar cell to vacuum coating machine for steaming, when the vacuum degree is higher than 1/10000, lithium fluoride (LiF) is begun to steam; the LiF steaming ratio is controlled by heating current; the LiF steaming ratio is 1-2 A/S; the thickness is 1-1.5 nm; finally steaming the aluminum electrode with 150 nm thickness to obtain polymer solar cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a side view of the polymer solar cell.