TECHNOLOGY FOCUS - POLYMERS - Preparation (Claimed): Preparing the polymer (P1) involves: coupling monomer (VIa) or (VIb) (where R7 and R8 are selected from Cl, Br, I, -B(OZ2)2 and -Sn(Z4)3) with each other and/or with monomers selected from R7-(Ar1)a-A'c-(Ar2)c-R8 (VIII), R7-Ar1-R8 (IX) or R7-Ar3-R8 (where R7 and R8 are selected from Cl, Br, I, B(OZ2)2 and -Sn(Z4)3) in an aryl-aryl coupling reaction.Ar3=Ar1;A'c=aryl or heteroaryl group that is different from U' and Ar1-Ar3, has 5 to 30 ring atoms, is optionally substituted by R's and is selected from aryl or heteroaryl groups having electron acceptor properties.Preferred Compound: The polymer (P1) comprising polycyclic repeating units selected from 11 compounds as given in the specification e.g. compound of formula (Ia). The polymer (P1) is selected from 16 compounds as given in the specification e.g. -((Ar1)a-(U')b-(Ar2)c-(Ar3)d)- (IIa) or -((U')b-(Ar1)a-(U')b-(Ar2)c-(Ar3)d) (IIb).b=0-2 (preferably 1);d=0 or 1-10.Preferred Mixture: The mixture or polymer blend comprises the polymer (P1) and n-type organic semiconductor compound (preferably optionally substituted fullerene). Preferred Definitions:    V'=CH.    EXAMPLE - Preparation of poly(2,27-(12,12,17,17-tetraoctyl-12,17-dihydro-s-indaceno(1,2-b:5.6-b')di(4,4-bis-(4-dodecyl-phenyl)-4H-indenon(1,2-b)-thiophene))-alt-(2,5-thieno(3,2-b)thiophene)) was carried as follows: nitrogen gas was bubbled through a mixture of 253.8 mg of 2,27-dibromo(12,12,17,17-tetraoctyl-12,17-dihydro-s-indaceno(1,2-b:5,6-b')di(4,4-bis-(4-dodecyl-phenyl)-4H-indeno(1,2-b)thiophene)), 58.3 mg of 2,5-bis-trimethylstannanyl-thieno(3,2-b)thiophene, 1.8 mg of tris(dibenzylideneacetone)dipalladium(0), 3 mg of tri-o-tolyl-phosphine, 4 cm3 of anhydrous toluene and anhydrous 0.8 cm3 of N,N-dimethylformamide for one hour. The reaction mixture was then heated in a pre-heated oil bath at 100 degrees C for 60 minutes before further 5 cm3 of toluene was added. Then 0.03 cm3 of bromobenzene was added and the mixture heated at 100 degrees C for 10 minutes. Then 0.12 cm3 of tributyl-phenyl-stannane was added and the mixture heated at 100 degrees C for 20 minutes. The mixture allowed to cool slightly and poured into stirred 100 cm3 of methanol and the polymer precipitate collected by filtration. The crude polymer was subjected to sequential Soxhlet extraction by using acetone, petrol and chloroform. The chloroform extract was poured into 400 cm3 of methanol and the polymer precipitate collected by filtration to form a yellow solid of poly(2,27-(12,12,17,17-tetraoctyl-12,17-dihydro-s-indaceno(1,2-b:5.6-b')di(4,4-bis-(4-dodecyl-phenyl)-4H-indenon(1,2-b)-thiophene))-alt-(2,5-thieno(3,2-b)thiophene)) (210 mg, yield 84%) (Mn=217000 g/mol and Mw=586000 g/mol).