NOVELTY - Polycyclic dithiophene compounds (I) are new.    USE - (I) and/or an oligomer or polymer are useful: in an organic semiconductor material, layer or component; as a charge-transport material, electrically conducting material, photoconducting material, light emitting material, surface-modifying material, electrode material in a battery, alignment layer, or in an organic field effect transistor, integrated circuit, thin film transistor, display, radiofrequency identification tag, electro- or photoluminescent device, backlight of a display, photovoltaic or sensor device, charge injection layer, photodiode, Schottky diode, memory device (e.g. ferroelectric field-effect transistor), planarizing layer, antistatics, conductive substrate or pattern, photoconductor, or electrophotographic application or recording material. (I) are useful in semiconductor device, especially a diode, a photodiode, an organic photovoltaic device (solar cell), an organic field effect transistor, or a device containing a diode and/or a photodiode and/or an organic field effect transistor, and/or a solar cell (all claimed). (I) are useful in backplanes for flexible displays (e.g. personal computers, cell phones, or handheld devices), smart cards, sensors (e.g. light-, image-, bio-, chemo-, mechanical- or temperature sensors), security devices and organic light emitting transistors.    ADVANTAGE - (I) have excellent solubility in organic solvents and excellent film-forming properties. (I) provides semiconductor device with high efficiency of energy conversion, excellent field-effect mobility, good on/off current ratios and/or excellent stability.    DETAILED DESCRIPTION - Polycyclic dithiophene compounds of formula (I) are new.    R1, R1a = H or a substituent, halo or SiR6R4R5;    R2, R2a = e.g. 1-25C alkyl, 3-12C cycloalkyl or 2-25C alkenyl;    X = divalent linking group e.g. -C(R3)=C(R3a)- or -C(=O)-N(R8)-C(=O)-;    R3, R3a = H or a substituent, or amino;    Y1, Y1a = e.g. phenyl or thiophenyl;    n, p = 0-6; and    R4-R6 = e.g. 1-25C alkyl, 3-12C cycloalkyl or 4-25C aryl.    Full Definitions are given in the DEFINITIONS (Full Definitions) Field. INDEPENDENT CLAIMS are included for:    (1) oligomer, polymer or copolymer comprising at least 2 fused thiophene structural units of the formula (II) or (III);    (2) polymer obtainable by homopolymerization of fused thiophene compounds of formula (V) or by copolymerization of (V) with a suitable further monomer;    (3) polymer obtainable by homopolymerization of fused thiophene compounds of formula (XIV) or by copolymerization of (XIV) with a suitable further monomer such as dihalogenated or diboronated, optionally substituted 1-19C heteroaryls, preferably dihalogenated or diboronated mono- or oligothiophene compounds of formula (a1) or benzothiadiazole compounds of formula (b1);    (4) a semiconductor device, comprising (I), especially e.g. photodiode and solar cell; and    (5) the preparation of an organic semiconductor device, comprising applying a solution and/or dispersion of (I), and/or an oligomer or polymer, in an organic solvent to a substrate and removing the solvent.    m = number of structural units (III) in the oligomer or (co)polymer, preferably 2-50000;    X = a divalent linking group comprising -C(R3)=C(R3a)- or -C(=O)-N(R8)-C(=O)-;    X1a = a trivalent linking group which is derived, together with the moiety integrated into the chain from -C(=N(R18))-, -C(R12)(R13)-, -M(R4)(R5)-, -B1(R20)-, -P(R20)-, -C(=O)-N(R18)-C(=O)-, -C(=N-R19)-C(=N-R19a)- or -C(R15)=C(R15a)-;    M = Si or Ge; either    R3, R3a, R15, R15a = H or a substituent; or    CR3R3a, CR15R15 = bridging group (a), 5- or 6-membered optionally substituted hydrocarbon ring, or a 5-membered optionally substituted heterocyclic ring comprising at least one heteroatom of N, O or S;    R4, R5 = 1-25C alkyl, 3-12C cycloalkyl, 4-25C aryl or 5-25C aralkyl, which may further be interlinked to form a divalent hydrocarbon residue of 4-25C, which may be substituted and/or interrupted;    R8 = substituted 4-10C aryl or 1-19C heteroaryl; either    R9, R9a, R10, R10a = H or a substituent; or    CR9R9a, CR9R10, CR9aR10a = 5-membered optionally substituted heterocyclic ring comprising at least one heteroatom of N, O or S;    R11 = H or CH3; either    one of R12, R13 = H while the other, or both R12, R13, are substituents; or    R12R13 = divalent hydrocarbon residue of 2-25C, which may be substituted and/or interrupted;    q = 0-4;    R14 = substituent, or 2 or 3 neighboring residues R14 may be interlinked to form a divalent or trivalent hydrocarbon residue of 4-25C, which may be substituted and/or interrupted;    R16, R16a = H or a substituent;    R18 = substituent, or substituted 4-10C aryl or 1-19C heteroaryl;    R19, R19a = bridging group comprising -C(R16)=C(R16a)- or phen-1,2-yl (substituted by (R14)q);    R20 = 1-25C alkyl, 3-12C cycloalkyl, 4-25C aryl, 5-25C alkylaryl or 5-25C aralkyl (all optionally substituted), where X1a contains one of R4, R5, R9, R9a, R10, R10a, R12, R13, R14, R14a, R18, R19, R19a, R20 comprising a substituent, which contains a polymerizable ethylenic double bond;    Hal = halo, preferably Br;    R1p = a residue of a boronic acid or its ester or Hal; and    s = 1-6.