Low Band Gap Polymers for Roll-to-Roll Coated Polymer Solar Cells

We present the synthesis of a low band gap copolymer based on dithienothiophene and dialkoxybenzothiadiazole (poly(dithienothiophene-co-dialkoxybenzothiadiazole), PDTTDABT). The optical properties of the polymer showed a band gap of 1.6 eV and a sky-blue color in solid films. The polymer was explored in roll-to-roll coating experiments and was optimized with respect to the manufacturing process and mixing ratio with [60]PCBM through a series of experiments with variation of the composition in steps of 1% w/w of respectively PDTTDABT and [60]PCBM and a relatively broad optimimum was found around a 1:2 mixing ratio. Roll-to-roll coated polymer solar cell devices were prepared under ambient conditions employing solution processing in all steps including the metallic back electrode that was printed as a grid giving semitransparent solar cell devices. Solar cell modules comprising 16 serially connected cells were prepared with a total module active area of 96 cm2. The devices were tested for operational stability under simulated sunlight (AM1.5G) and natural sunlight, and the photochemical stability of the polymer was examined using a combination of UV−vis and IR spectroscopy.