TECHNOLOGY FOCUS - POLYMERS - Preparation: No preparation method is given. Preferred Components: (I) has a: number-average molecular weight of 8-50 kDa; and polydispersity index of 1.5-4. The electron-withdrawing fullerene derivative is (6,6)-phenyl-C61-butyric acid methyl ester. The polymer (P1) includes: only one of the compositions (I)-(IV); or at least two of the compositions (I)-(IV). Preferred Definitions:    Y1-Y6 = S;    Z1 = 3,7-dimethyloctyl carbonate;    W1 = F;    R1, R2, R4, R5, R7, R8 = H; and    R3, R6 = 3,7-dimethyloctyl.    EXAMPLE - 4,8-Bis-(5-(3,7-dimethyl-octyl)-thiophen-2-yl)-benzo(1,2-b;4,5-b')dithiophene (2.03 g) was dissolved in 30 ml anhydrous tetrahydrofuran and cooled to -78 degrees C under nitrogen protection. Then n-butyllithium (2.95 ml) was added dropwise and the mixture was kept at -78 degrees C for 1 hour and then slowly heated up to room temperature for another 2 hours. Then the mixture was cooled to -78 degrees C again, and 9.25 ml of trimethyltin chloride solution (1 M in hexane) was added and slowly heated up to room temperature to keep stirring for another 12 hours. Then the reaction was quenched with 50 ml of water and extracted with ether. The organic extraction was dried with anhydrous sodium sulfate and the solvent was evaporated to give 2,9-bis(trimethyltin)-4,8-bis(5-(3,7-dimethyloctyl)-thiophen-2-yl)benzo(1,2-b:4,5-b')dithiophene (A1). 4,6-Dibromo-3-fluoro-thieno(3,4-b)thiophene-2-carboxylic acid 2-ethyl-hexyl ester (227.1 mg), A1 (480.3 mg) and tetrakis(triphenylphosphine)palladium(0) (24.5 mg) were weighed into a 25 ml one-neck round-bottom flask. The flask was subjected to three successive cycles of vacuum followed by refilling with argon. Then anhydrous dimethylformamide (2 ml) and anhydrous toluene (8 ml) were added. The polymerization was carried out at 120 degrees C for 1 day under nitrogen protection. The raw product was precipitated into methanol and collected by filtration. The precipitate was dissolved in chloroform and filtered. After precipitated from methanol again, the collected polymer was washed with methanol (8 hours), hexanes (12 hours) and acetone (5 hours). Finally the polymer was extracted with chloroform. The polymer solution was condensed and precipitated in methanol to give poly(6-(4,8-Bis-(5-(3,7-dimethyl-octyl)-thiophen-2-yl)-benzo(1,2-b;4,5-b')dithiophen-2-yl)-3-fluoro-thieno(3,4-b)thiophene-2-carboxylic acid 2-ethyl-hexyl ester) (A2). 4,6-Dibromo-3-fluoro-thieno(3,4-b)thiophene-2-carboxylic acid 3,7-dimethyl-octyl ester (250.1 mg), (A1) (480.3 mg) and tetrakis(triphenylphosphine)palladium(0) (24.5 mg) were weighed into a 25 ml one-neck round-bottom flask. The flask was subjected to 3 successive cycles of vacuum followed by refilling with argon. Then anhydrous dimethylformamide (2 ml) and anhydrous toluene (8 ml) were added. The polymerization was carried out at 120 degrees C for 1 day under nitrogen protection. The raw product was precipitated into methanol and collected by filtration. The precipitate was dissolved in chloroform and filtered. After precipitated from methanol again, the collected polymer was washed with methanol (8 hours), hexanes (12 hours) and acetone (5 hours). Finally the polymer was extracted with chloroform. The polymer solution was condensed and precipitated in methanol. The precipitate was collected and dried by vacuum overnight to give poly(6-(4,8-Bis-(5-(3,7-dimethyl-octyl)-thiophen-2-yl)-benzo(1,2-b;4,5-b')dithiophen-2-yl)-3-fluoro-thieno(3,4-b)thiophene-2-carboxylic acid 3,7-dimethyl-octyl ester) (yield 82%).