NOVELTY - Substituted 6-benzo(1,2-b;4,5-b')dithiophen-2-yl-thieno(3,4-d)thiazole polymer (I), are new.    USE - (I) are useful as charge transport, semiconducting, electrically conducting, photoconducting or light emitting material in optical, electro-optical, electronic, electroluminescent or photoluminescent components or devices, where the component or device is organic field effect transistors (OFET), thinfilm transistors (TFT), integrated circuits (IC), logic circuits, capacitors, radio frequency identification (RFID) tags, devices or components, organic light emitting diodes (OLED), organic light emitting transistors (OLET), flat panel displays, backlights of displays, organic photovoltaic devices (OPV), organic solar cells (OSC), photodiodes, laser diodes, photoconductors, photodetectors, electrophotographic devices, electrophotographic recording devices, organic memory devices, sensor devices, charge injection layers, charge transport layers or interlayers in polymer light emitting diodes (PLEDs), Schottky diodes, planarising layers, antistatic films, polymer electrolyte membranes (PEM), conducting substrates, conducting patterns, electrode materials in batteries, alignment layers, biosensors, biochips, security markings, security devices, and components or devices for detecting and discriminating DNA sequences, preferably OFET, bulk heterojunction OPV device, inverted BHJ OPV device and organic photodetector (all claimed).    ADVANTAGE - (I): have improved semiconducting, charge transport, hole/electron transport, hole/electron blocking, electrically conducting, photoconducting or light emitting properties, good morphology, good processability, high solubility in organic solvents, and high oxidative stability; is suitable for large scale production; shows a low bandgap, high charge carrier mobility, and high external quantum efficiency in bulk heterojunction solar cells.    DETAILED DESCRIPTION - Substituted 6-benzo(1,2-b;4,5-b')dithiophen-2-yl-thieno(3,4-d)thiazole polymer of formula (I), are new.R1-R5 = H, halo, or an optionally substituted carbyl or hydrocarbyl group, where one or more C atoms are optionally replaced by a hetero atom.    INDEPENDENT CLAIMS are included for: substituted 6-benzo(1,2-b;4,5-b')dithiophen-2-yl-thieno(3,4-d)thiazole monomer of formula (II); #a mixture or polymer blend comprising (I) and one or more compounds or polymers having semiconducting, charge transport, hole/electron transport, hole/electron blocking, electrically conducting, photoconducting or light emitting properties; #a formulation comprising (I), mixtures or polymer blends, and one or more solvents, preferably organic solvents; and #a process of preparing (I).    R8, R9 = Cl, Br, I, O-tosylate, O-triflate, O-mesylate, O-nonaflate, -SiMe2F, -SiMeF2, -O-SO2Z1, -B(OZ2)2 , -CZ3=C(Z3)2, -C=CH, - C=CSi(Z1)3, -ZnX0 or -Sn(Z4)3; either    Z1-Z4 = alkyl and aryl (both optionally substituted); or    Z2Z2 = cyclic group; and    X0 = halo, preferably Cl, Br or I.