TECHNOLOGY FOCUS - ELECTRONICS - Preferred Device: The polymer solar cell device has a bulk heterojunction structure, where one of ingredients of the bulk heterojunction structure is fullerene or a fullerene derivative.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The fullerene derivatives is (6,6)-phenyl 61C butyric acid methyl ester or (6,6)-phenyl 71C butyric acid methyl ester.    TECHNOLOGY FOCUS - POLYMERS - Preparation: No general method for the preparation of the conjugated polymer (P1) is given. Preferred Definitions:    R5, R6, R'3, and R'6=protons;    R'1, R3, R4, R'1a, R'4 and R'5=proton, optionally substituted alkyls or optionally aryls;    R'2 and R'2a=proton, optionally substituted alkyls or halo.    EXAMPLE - 2-Ethyl-1-thieno(3,4-b)thiophen-2-yl-hexan-1-one (2.66 g) was dissolved into DMF (30 ml) under protect of argon. NBS (4.45 g) was added in one portion. After 20 minutes of stirring, the reactant was poured into a cold solution of sodium thiosulfate and stirred for several minutes. Then, the mixture was extracted by ethyl ether and purified by silica gel column using hexane as eluent to obtain 1-(4,6-dibromo-thieno(3,4-b)thiophen-2-yl)-2-ethyl-hexan-1-one. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexoxy)-benzo(1,2-b;3,4-b)-dithiophene (1 mmol), 1-(4,6-dibromo-thieno(3,4-b)thiophen-2-yl)-2-ethyl-hexan-1-one (20 ml) of toluene and DMF (2 ml) were put into a two-necked flask with oil bath. The solution was flushed with argon for 10 minutes, then Pd(PPh3)4 (30 mg) were added into the flask. The solution was flushed again for 20 minutes. The oil bath was heated to 110 degrees C carefully, and the reactant was stirred for 16 hours at this temperature under argon atmosphere. Then, the reactant was cooled to room temperature and the polymer was precipitated by addition of 100 ml methanol, and filtered, which was then subjected to extraction with methanol, hexane, and chloroform. The polymer was recovered as a solid sample from the chloroform fraction by rotary evaporation. The solid was dried under vacuum for 1 day to obtain poly(4,8-bis(2-ethylhexoxy)-benzo(1,2-b:4,5-b')dithiophene-2,6-diyl-alt-2-(2-ethylhexanoyl)thieno(3,4-b)thiophene-4,6-diyl).