TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The active layer has a bulk hetero junction structure. The electron acceptor in the active layer comprises a fullerene or a fullerene derivative.    TECHNOLOGY FOCUS - POLYMERS - Preparation: Preparation of the polymer comprises: dissolving the monomer in an organic solvent as necessary and polymerizing by using aryl coupling reaction in which alkali, a catalyst or a ligand is used; and performing e.g. a Stille coupling reaction and a Suzuki coupling reaction to obtain the polymer. Preferred Components: The rate of the repeating unit (I) in relation to a total number of moles of all repeating units in the polymer is greater than or equal to 50 mol.%. The polymer comprises sulfur containing heterocyclic structure of formula (II), (III), or (IV).R5, R6, R2a, R3a, R4a = monovalent group comprising alkyl, alkoxy, aromatic group, hetero-aromatic group (all optionally substituted), H or halo, where at least one of R5, R6 is aromatic group having electron withdrawing group or electron releasing group or a crosslinking group, and at least one of R2a, R3a, R4a is crosslinking group;R1a = R1;A1 = divalent group comprising aromatic group or hetero-aromatic group (both optionally substituted); andn/(n+m) = 0.01-0.5. Preferred Definitions:    at least one of R2, R3, R4 = crosslinking group.    EXAMPLE - Under nitrogen, 1.811 g of 2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo(1,2-b:4,5-b')dithiophene, 0.078 g of tetrakis(triphenylphosphine)palladium (catalyst) and 0.96 g of 1,3-dibromo-5-octyl-2,7-dithia-5-azacyclopenta(a)pentalene-4,6-dione were fed into a three-neck flask with a three-way cock. Argon was made to flow into the three neck flask via the three-way cock, where a reflux tube with an argon conduit was attached to the three-neck flask with argon made to flow. Then, the three-way cock was closed, the inside of the three-neck flask was vacuumized, and argon was introduced again. The above operation was repeated three times. From one end of the three-way cock of the three-neck flask, 24 ml of degassed toluene anhydride was added. One of the three-way cock of the three-neck flask was opened and a toluene solution of the compound DM in the syringe was added. Further, 8 ml of degassed dimethylformamide anhydride was added into the three-neck flask from one end of the three-way cock and the three-way cock was closed. This three-neck flask was heated in an oil bath, made to react at a reflux temperature for 12 hours, and then cooled to a room temperature. Under nitrogen, 0.161 g of trimethylphenyltin was weighed and dissolved into 4 ml of toluene anhydride having been degassed, added into the three-neck flask by a syringe similarly to the above, and heat refluxing was carried out for 2 hours. After cooling down to the room temperature, under nitrogen, 0.157 g of bromobenzene was weighed and dissolved into 4 ml of degassed toluene anhydride, and heat refluxing was carried out for 2 hours. After cooling down to room temperature, the above reaction solution was dropped into 1 l of methanol while being stirred, to precipitate the polymer. The precipitate was filtered by a glass filter, and dissolved into chloroform.