NOVELTY - Polymer (I) comprising first, second and third comonomer repeat units is new, where the: first comonomer repeat unit comprises a silacyclopenta-dithiophene moiety; second comonomer repeat unit comprises cyclopentadithiophene moiety; and third comonomer repeat unit comprises a benzothiadiazole moiety.    USE - (I) Is useful in an article (preferably a photovoltaic cell) and in a system (preferably photovoltaic system comprising a tandem photovoltaic cell) (all claimed). (I) is useful in organic semiconductive devices (field effect transistors, photodetectors (e.g. infra red (IR) detectors), photovoltaic detectors, imaging devices (e.g. red-green-blue imaging devices for cameras and medical imaging systems), light emitting diodes (LED) (e.g. organic light emitting diodes and IR or near IR LEDs), lasing devices, conversion layers (e.g. layers that convert visible emission into IR emission), amplifiers and emitters for telecommunication (e.g. dopants for fibers), storage elements (e.g. holographic storage elements), and electrochromic devices (e.g. electrochromic displays).    ADVANTAGE - The article has an efficiency of at least 4, preferably at least 5 under air mass 1.5 conditions. The silacyclopentadithiophene moiety: significantly improves the solubility and processibility of (I) and the morphology of a photoactive layer prepared from (I), thus increasing the efficiency of a photovoltaic cell; can shift the absorption wavelength of (I) toward the red and near IR portion (e.g. 650-900 nm) of the electromagnetic spectrum, which is not accessible by most other polymers, hence enabling the cell to absorb the light in this region of the spectrum and increasing the current and efficiency of the cell. The comonomer repeat units have improved solubility in an organic solvent (e.g. chlorobenzene) and provide an organic photovoltaic cell with increased fill factors and photocurrent than a polymer containing only first and third comonomer repeat units or second and third comonomer repeat units. (I) has suitable band gap (e.g. 1.4-1.6 eV), which can improve photocurrent and cell voltage, and high positive charge mobility (e.g. 10-4-10-1 cm2/Vs), which facilitates charge separation in a photoactive layer.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) an article, comprising: a first electrode; a second electrode; and a photoactive material between the first and second electrodes, where the photoactive material comprises (I) and the article is configured as a photovoltaic cell (420); and    (2) a system, comprising: first and second electrodes; and first and second photoactive layers between the first and second electrodes, where at least one of the first and second photoactive layers comprise (I) and the system is configured as a photovoltaic system (400).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic of a system containing multiple photovoltaic cells electrically connected in parallel.    Photovoltaic system (400)    Module (410)    Photovoltaic cell (420)    Load (430)