Thermally reactive Thiazolo[5,4-d]thiazole based copolymers for high photochemical stability in polymer solar cells

New thermally reactive copolymers based on dithienylthiazolo[5,4-d]thiazole (DTZ) and silolodithiophene (SDT) have been synthesized and explored in bulk heterojunction solar cells as mixtures with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In thin films the polymers had optical band gaps in the range of 1.64-1.80 eV. For solubility the polymers have incorporated alkyl groups on the SDT unit and thermally removable ester groups on the DTZ unit that can be eliminated around 200 degrees C for improved photochemical stability in thin films. The bulkiness of the alkyl chains on the SDT unit proved to be very significant in terms of photovoltaic performance of the polymer: PCBM solar cells. Polymers based on 4,4-dihexyl-4H-silolo[3,2-b:4,5-b']dithiophene reached power conversion efficiencies (PCEs) up to 1.45% but changing the alkyl groups to more bulky ethylhexyl chains reduced the PCE to 1.17%. More noteworthy is that the photovoltaic performance improves for the polymers based on 4,4-dihexyl-4H-silolo[3,2-b:4,5-b']dithiophene after the ester groups has been eliminated from the DTZ unit by a thermal treatment around 200 degrees C. To confirm that elimination of the solubilizing groups improve the long term durability of the materials the photochemical stability was estimated by a novel accelerated degradation method by which the photobleaching of the polymer was followed during degradation at 100 solar intensities. This clearly shows that the pristine polymer films are by far the most unstable under the given conditions emphasizing the unfavorable effect of solubilizing groups on the photochemical stability of conjugated polymers.