TECHNOLOGY FOCUS - POLYMERS - Preparation (Claimed): The preparation of (I), comprises carrying out redox reaction by reacting benzo-dithiophene polymer of formula (II) with an oxidizing agent in an ice-water bath, and stirring continuously, where the oxidizing agent is inorganic oxidants including hydrogen peroxide, chromium trioxide, halogen compounds (sodium hypochlorite, sodium periodate) and organic oxidizing agent including organic peroxyacids (peracetic acid, trifluoroperacetic acid, m-chloroperbenzoic acid), and/or dimethyldioxirane, and then purifying by adding sodium thiosulfate after the completion of the reaction, washing with deionized water, extracting, then separating by column chromatography, precipitating with methanol, filtering and drying to obtain (I), where molar ratio between (II) and oxidizing agent is 1:5-30, and the oxidation reaction time is 10 minutes-72 hours. Preferred Components: The oxidation reaction is carried out in a solvent including chlorobenzene, ortho-dichlorobenzene, N,N-dimethylformamide and chloroform. (I) is poly(6-(5-(4,8-Bis-(5-(2-ethyl-hexane-1-sulfonyl)-thiophen-2-yl)-benzo(1,2-b;4,5-b')dithiophen-2-yl)-thiophen-2-yl)-4-thiophen-2-yl-thieno(3,4-b)thiophene-2-carboxylic acid 2-octyl-dodecyl ester) of formula (P1), poly(1-(6-(4,8-Bis-(5-(2-ethyl-hexane-1-sulfonyl)-thiophen-2-yl)-benzo(1,2-b;4,5-b')dithiophen-2-yl)-thieno(3,4-b)thiophen-2-yl)-2-ethyl-hexan-1-one) of formula (P2) and poly(6-(4,8-Bis-(5-(2-ethyl-hexane-1-sulfonyl)-thiophen-2-yl)-benzo(1,2-b;4,5-b')dithiophen-2-yl)-thieno(3,4-b)thiophene-2-carboxylic acid 2-ethyl-hexyl ester) of formula (P3). Preferred Definitions:    Ar'=vinylene, ethynyl, pyrido(3,4-d)pyridazine-1,4-diyl, thieno(3,2-b)thiophene-2,5-diyl, thiophene-2,5-diyl, thiazolo(5,4-d)thiazole-2,5-diyl, thiazole-2,5-diyl, pyrazine-2,5-diyl, 1,2,4,5-tetrazine-3,6-diyl, benzo(c)thiophene-1,3-diyl, thieno(3,4-b)pyrazine-5,7-diyl, 5,6-diphenyl-benzo(c)thiophene-1,3-diyl, 2-thia-4-aza-indene-1,3-diyl or thiazolo(5,4-d)thiazole-2,5-diyldithiophene5,5'-diyl.