NOVELTY - Sulfone group-containing benzo-dithiophene polymer (I) is new.    USE - (I) is useful in photovoltaic field i.e. photoactive material, lithium-ion battery, electrochemical devices, organic photovoltaic device or field-effect transistor (FET),preferably solar cell device or photodetector device (claimed).    DETAILED DESCRIPTION - Sulfone group-containing benzo-dithiophene polymer of formula (I) is new.    R1=4-20C alkyl;    G=phenylene, naphthylene or at least one oxygen, selenium or sulfur atom-containing monocyclic heteroaryl;    Ar'=conjugated unit (optionally substituted); and    n=5-1000.    INDEPENDENT CLAIMS are also included for:    (1) preparation of (I);    (2) semiconductor mixture comprising (I) blended with a doping agent, where weight ratio between (I) and the doping agent is 1:1-5 (preferably 1:1.5 or 1:2), the doping agent is a compound, which is excited by light or electric fields and exhibit charge transfer and/or energy transfer with (I), preferably the doping agent is fullerene, fullerene derivative, perylenediimide, naphthalene diimide, (6,6)-phenyl C61 butyric acid methyl ester, (6,6)-phenyl C71 butyric acid methyl ester or indene-containing fullerene;    (3) use of semiconductor mixture in photovoltaic field i.e. photoactive material, lithium-ion battery, electrochemical devices, organic photovoltaic device or field-effect transistor (FET), preferably solar cell device or photodetector device; and    (4) energy device i.e. either photovoltaic device (100) comprising a first electrode(102), a second electrode(104) spaced apart with the first electrode, and at least one semiconductor layer between the first and the second electrodes, where the semiconductor layer (106) is composed of (I) or semiconductor mixture, or single-heterojunction photovoltaic device comprising a substrate/first electrode/modification layer/photovoltaic active layer/modified layer/second electrode, where the modified layer and the substrate may be present or absent, the photovoltaic active layer is composed (I) or semiconductor mixture, and the single heterojunction photovoltaic device is a polymer solar cell device, preferably forward or inverted structured bulk-heterojunction polymer solar cell device, or multi-heterojunction photovoltaic device comprising a substrate /first electrode/modified layer/photovoltaic active layer/modified layer/active layer/modified layer/second electrode, where the substrate may or may not be present, the modified layer may be modified with N- or P- type semiconductor or a metal, the active layer is composed of (I) or semiconductor mixture, and the multi-heterojunction photovoltaic device is series or parallel structured polymer solar cell device.