TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: The solar cell comprises electrode (120), electrode (160), and photoelectric converting layer comprising organic material containing polymer arranged between electrode (120) and electrode (160). The photoelectric converting layer of solar cell has active layer (140) containing the polymer and buffer layer (130,150) containing the polymer.    TECHNOLOGY FOCUS - POLYMERS - preparation (disclosed): The polymer is prepared by dissolving monomer comprising polymerizable functional group e.g. monomer comprising boronic acid residue or borate residue, monomer comprising halogen atoms, or trifluoromethanesulfonate group, and p-toluene sulfonate group, in organic solvent, polymerizing by aryl coupling reaction using alkali, in presence of catalyst e.g. palladium complex or nickel complex, and ligand. Preferred Composition: The polymer further comprises bivalent group-containing repeating unit of formulae (II)-(VI). The polymer comprises crosslinkable group as terminal group.A=bivalent conjugated coupling group;R2,R3=H, fluoro, cyano, nitro, or monovalent group chosen from alkoxycarbonyl, alkyl, alkanoyl, aryl or heteroaryl (all optionally substituted);R4,R5=R1;andL=O, S, or imino, methylene, silylene or germylene (all substituted). EXAMPLE - 1.091 g 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo(1,2-b:4,5-b')dithiophene and 0.055 g tetrakis(triphenylphosphine)palladium catalyst were provided in a three-necked flask under nitrogen, flask was evacuated, argon was introduced, and process was repeated 3 times. To the resultant product, solution obtained by dissolving 0.538 g 4,6-dibromo-N-octyl-2,3-dihydro-3-oxo-thieno(3,4-d)isothiazole-1,1-dione in 24 mL anhydrous toluene was added under nitrogen, and 6 mL anhydrous dimethylformamide was added. The three-necked flask was heated in an oil bath, reacted at reflux temperature for 12 hours and cooled to room temperature. Then, 0.109 g trimethyl phenyltin was measured, and dissolved in 4 mL deaerated anhydrous toluene, the resultant product was added to flask under nitrogen, heated and refluxed with injection device for 2 hours. The resultant product was cooled to room temperature, solution obtained by dissolving 0.097 g bromobenzene in 4 mL deaerated anhydrous toluene under nitrogen was added to flask, heated and refluxed with injection device for 2 hours. The resultant product was cooled to room temperature, stirred in 500 mL methanol, and polymer was precipitated. The resultant product was filtered, dissolved in chloroform and catalyst was removed in Cerite (RTM: diatomaceous earth) column. To the resultant product, methanol was added, solvent was evaporated, stirred, and filtered using glass filter, to obtain solid. 1.08 g Resultant product was vacuum-dried at 80 degrees C for 4 hours, extracted and purified using ethyl acetate, hexane, toluene, and chlorobenzene, sequentially, to obtain poly(4,8-bis(5-(2-ethylhexyl)thienyl)benzo(1,2-b:4,5-b')dithiophene-2,6-diyl-alt-(N- octyl-2,3-dihydro-3-oxo-thieno(3,4-d)isothiazole-4,6-diyl-1,1-dione)).