Optimization of Narrow Band-Gap Propylenedioxythiophene:Cyanovinylene Copolymers for Optoelectronic Applications
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Four analogues of the narrow band gap poly(3,4-propylenedioxythiophenedialkyl)cyano-p-phenylenevinylene (PProDOT−R2:CNPPV) polymer family have been designed with the goal of improving the film forming ability and utility of this family of polymers when applied in redox switchable and optoelectronic devices. These polymers were synthesized via Knoevenagel condensation with yields ranging from 40 to 80%. Number-average molecular weights between 9000 and 24 000 g mol-1 were estimated using size exclusion chromatography, and the polymer repeat unit structures and end groups were confirmed by MALDI mass spectrometry. Linear and branched alkoxy substituents were introduced along the polymer backbone to yield materials highly soluble in chloroform, THF, and toluene, and homogeneous films were prepared by spin-coating or spray-casting from organic solutions. The identity of the substituents has a significant effect on the optical properties of the polymer solutions. A blue-shift in the absorption was observed by replacing linear alkoxy substituents with branched substituents suggesting poorer packing. The HOMO and LUMO energy levels were studied by thin film electrochemistry, and optical band gaps of 1.70−1.75 eV have been determined spectroelectrochemically from the onset of the π to π* transition, importantly near the wavelength of maximum solar photon flux. These narrow band gap polymers have shown significant photovoltaic performance as electron donors when combined with the electron acceptor [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) in bulk heterojunction photovoltaic devices. AM1.5 efficiencies up to 0.4% were attained with short circuit current densities >1.0 mA cm-2, an open circuit voltage of 0.7 V, and a fill factor of 40%. The identity of the substituents did not have a significant influence on the solubility of the polymers (the linear and branched polymers exhibit a solubility of about 15 mg mL-1 in chloroform) and on the films homogeneity. The solubility and film properties of the polymers along with their electrochromic behavior, switching from a neutral blue/purple state to highly transmissive gray in the oxidized and reduced states, are potentially useful in large area electrochromic displays. Polymer light-emitting diodes have been prepared with one of the polymers, exhibiting a bright red emission with λmax = 704 nm.