NOVELTY - A carbazole derivative copolymer (I) is new.    USE - Carbazole derivative copolymer used in polymer solar energy battery device, organic electroluminescent device, organic field effect transistor, organic optical storage and organic laser application (claimed).    ADVANTAGE - The copolymer has strong donor-acceptor structure, increased material stability, reduced energy band gap, expanded light absorption range, and improved photoelectric conversion efficiency.    DETAILED DESCRIPTION - A carbazole derivative copolymer of formula (I) is new.    n=1-100; and    R1-R3=1-20C alkyl.    INDEPENDENT CLAIMS are included for:    (1) preparation of compound (I);    (2) preparation of benzothiadiazole derivative of formula (II) comprising taking brominated benzothiadiazole derivative of formula (IV), dissolving in N,N-dimethylformamide, adding n-butyllithium at molar ratio of 1:2-1:4, stirring to react for 2 hours, injecting dichlorosilane derivative of formula (V) at - 90 degrees C, reacting at room temperature for 10 hours, washing, extracting with anhydrous diethyl ether, taking organic phase, drying over anhydrous sodium sulfate, filtering, vacuum-distilling, and separating by silica gel column chromatography;    (3) preparation of carbazole derivative of formula (III) comprising taking brominated carbazole derivative of formula (VI), dissolving in tetrahydrofuran under inert gas protection, slowly adding n-butyllithium at - 78 degrees C, stirring to react for 1-2 hours, adding 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, reacting at - 78 degrees C for 1 hour, warming to room temperature, reacting for 6 hours, adding saturated aqueous sodium chloride solution to stop reaction, extracting with diethyl ether, drying over anhydrous sodium sulfate, filtering, rotary evaporating, and separating by silica gel column chromatography;    (4) preparation of compound (IV) comprising dissolving brominated benzothiadiazole derivative of formula (VII) in tetrahydrofuran, adding tin(II) chloride, refluxing to react at 100 degrees C for 10 hours, cooling, adjusting pH value to 8.0 using sodium hydroxide, extracting with anhydrous diethyl ether, distilling, adding hydrochloric acid, adding sodium nitrite solution, stirring at 5 degrees C for 30 minutes, reacting for 30 minutes, adding potassium iodide, stirring to react for 12 hours, adjusting pH value to 7.0 using sodium hydroxide, extracting, washing organic layer with water, drying over anhydrous magnesium sulfate, filtering, drying, separating in silica gel chromatographic column, and recrystallizing in methanol;    (5) preparation of compound (VII) comprising taking 4,7-dibromo-5-nitro-2,1,3-benzothiadiazole (DBNBTD), copper powder and N,N-dimethylformamide, mixing, reacting at 120 degrees C for 3 hours, cooling, adding toluene, stirring for 30 minutes, filtering, washing with water and saturated brine, combining organic layers, drying over anhydrous magnesium sulfate, filtering, rotary evaporating, and recrystallizing in anhydrous ethanol; and    (6) preparation of DBNBTD comprising mixing 2-amino-5-nitroaniline and thionyl chloride, slowly adding pyridine, heating to 80-90 degrees C, refluxing to react for 24 hours, rotary evaporating at 80 degrees C, cooling, adding water, stirring, filtering, washing with water, and vacuum-drying.