NOVELTY - Preparing organic optoelectronic device cathode interface layer using conjugated polyelectrolyte, comprises e.g. (i) preparing an anionic and a cationic conjugated polyelectrolyte solutions, (ii) cleaning surface of a conductive substrate, (iii) immersing cleaned substrate in conjugated polyelectrolyte solution (having electric charge opposite to the substrate) to adsorb a layer of conjugated polyelectrolyte, (iv) immersing obtained substrate in conjugated polyelectrolyte solution (electrically opposite to the polyelectrolyte solution of step (iii)), and (v) repeating the steps (iii) and (iv).    USE - The cathode interface layer is useful for preparing polymer solar cells and polymer light-emitting diodes (claimed).    ADVANTAGE - The method: has simple operation, and easily controllable production conditions; and produces cathode interface layer having high-efficient cathode interface modification capability.    DETAILED DESCRIPTION - Preparing organic optoelectronic device cathode interface layer using conjugated polyelectrolyte, comprises (i) preparing an anionic conjugated polyelectrolyte solution and a cationic conjugated polyelectrolyte solution, where main chains of the anionic conjugated polyelectrolyte and cationic conjugated polyelectrolyte contain of substituted fluorene of formula (I), substituted 5H-dibenzo(b,d)silole of formula (II), substituted carbazole of formula (III), substituted 4H-thieno(3',2':4,5)cyclopenta(1,2-b)thiophene of formula (IV), substituted 7H-3,4-dithia-7-sila-cyclopenta(a)pentalene of formula (V), substituted 4H-dithieno(3,2-b:2',3'-d)pyrrole of formula (VI), substituted benzene of formula (VII), substituted thieno(2,3-f)(1)benzothiophene of formula (VIII), substituted 6,12-dihydroindeno(1,2-b)fluorene of formula (IX), substituted 5,11-dihydroindolo(3,2-b)carbazole of formula (X), substituted 2,2'-bithiophene of formula (XI) and/or substituted thieno(3,2-b)thiophene of formula (XII) homopolymer/copolymer units, and their side chains contain carboxylic acid salt and ammonium salt respectively, (ii) cleaning surface of a conductive substrate to obtain a charged substrate, (iii) immersing the charged substrate in conjugated polyelectrolyte solution having electric charge electrically opposite to the substrate, where the charged substrate adsorbs a layer of conjugated polyelectrolyte by electrostatic interaction, then taking out the substrate, washing repeatedly with deionized water, and gas drying, (iv) immersing the obtained dried substrate in conjugated polyelectrolyte solution, which is electrically opposite to the polyelectrolyte solution used in the step (iii), where the charged substrate adsorbs a layer of conjugated polyelectrolyte by electrostatic interaction, then taking out the substrate, washing repeatedly with deionized water, and gas drying, and (v) repeating the steps (iii) and (iv) to obtain organic optoelectronic device cathode interface layer having multilayered structure.    R, R1-R4 = alkyl chain (at least one carbon atom is replaced by oxygen atom, alkenyl group, alkynyl group, aryl group, hydroxy group, amino group, carbonyl group, carboxy group, ester group, cyano group or nitro group, and substituted with halo, alkenyl, alkynyl, aryl, OH, amino, carbonyl group, carboxy, ester group, cyano or nitro), H or 1-22C straight-chain, branched-chain or cyclic alkyl.