Instrumentation: 1 H NMR and 13 C NMR spectra were measured in deuterated chloroform using a Bruker DRX 400 and a Bruker AC 250. Chemical shifts (δ values) are given in part per million with tetramethylsilane as an internal standard. Elemental analysis was performed on a CHNS-932 Automat Leco. Differential scanning calorimetry (DSC) measurements were carried out on a Mettler DSC 30 with a cell purged with nitrogen. Calibration for temperature and enthalpy changes was performed using an Indium standard. The temperature was changed between 0 and 250 o C with a heating/cooling rate of 10 K/min. In total two heating-cooling cycles were performed on each sample. Thermogravimetric analysis (TGA) was performed on a Mettler TA-300-thermal analyzer operating under air atmosphere. The samples were heated from 0 to 700 °C with a heating rate of 10 K/min. Gel permeation chromatography was performed on a set of Knauer using THF as eluent and polystyrene as standard. The absorption spectra were recorded in dilute chloroform solution (c ≈ 10-6 mol l-1) on a Perkin-Elmer UV/VIS-NIR Spectrometer Lambda 19. Quantum-corrected emission spectra were measured in dilute chloroform solution with a LS 50 luminescence spectrometer (Perkin-Elmer). The solution photoluminescence quantum yields were calculated either according to Demas and Crosby against quinine sulfate in 0.1 N sulfuric acid as a standard (Φ f = 55%) 1 or in ethanol against rhodamine 6G standard (Φ f = 95%). Thin film (from CHCl 3 solution) absorption and emission spectra were measured with a Hitachi F-4500 Fluorescence Spectrophotometer. Their absolute photoluminescence quantum yields were measured in an intergrating sphere. The absorption spectra of thin film from chlorobenzene solution were recorded on Varian UV/Vis spectrophotometer and the corresponding emission spectra were recorded on a home-built photoluminescence setup. Thin films were spin coated on glass substrates using chlorobenzene based solutions (0.6-0.8 wt %). Infrared spectroscopy was recorded on a Nicolet Impact 400. Cyclic voltammetry (CV) was performed with a PA4 polarographic analyzer (Laboratory Instruments, Prague, CZ) with a

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High‐molecular weight anthracene containing poly(p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene) (PPE‐PPV) copolymers bearing linear and/or branched alkyl side chains have been synthesized and investigated for use in polymer–fullerene bulk heterojunction solar cells. By variation of the nature and length of alkyl side chains the ordering in thin films could be tuned. Besides the basic photophysical characterization, small‐angle X‐ray scattering on extruded fibers and cyclovoltammetry measurements are reported. The photovoltaic characterization indicates that improved ordering due to linear side chains on the arylene–ethynylene segment as well as lower side‐chain volume fraction yield higher photocurrents. Power conversion efficiencies of nearly 2.5% are reported for the best devices with a polymer:fullerene composition of 1:1 by weight. IV‐characteristics of partly optimized photovoltaic devices based on [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) blends with a blending ratio of 1:1 under solar simulator illumination.

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