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3 October 2005 Design and applications of polymer solar cells with lifetimes longer than 10000 hours
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The sidechains that are added to conjugated polymer materials convey desirable solubility and film forming properties but they generally lead to a low glass transition temperature that is comparable or slightly higher than the operational temperature for the devices. The use of thermo cleavable sidechains allow for solution processing and film forming while a subsequent thermal treatment leads to efficient removal of the sidechains yielding a dense insoluble film with in principle a much higher glass transition temperature making diffusion phenomena much slower. This approach has been shown to improve the operational lifetime using accelerated lifetime testing with an incident light intensity of 1000 W m-2 (AM1.5) at 72oC and opens up the possibility for the formation of multilayer structures by sequential film forming and thermal cycles. The synthesis of regiorandom poly(2,5-thienyl-co-3'-(1''-valeryloxy-1''-ethyl)-2',5'-thienyl) (4) a polythiophene based on a copolymer of thiophene and the valeric acid ester of 3-hydroxyethylthiophene was demonstrated to thermo cleave valeric acid efficiently at temperatures above 200oC leaving vinyl groups on the polythiophene backbone giving regiorandom poly(2,5-thienyl-3'-vinyl-2',5'-thienyl) (5). The thermocleaved film was insoluble in common organic solvents. Thermocleavage experiments using spincoated films of a 1:1 (w/w) mixture of 4 and the soluble fullerene derivative 6,6-phenyl-C61-butyric acid methylester (PCBM) gave films of 5 and PCBM in a ratio of 2:3 (w/w). Photovoltaic devices were prepared and devices based on 5 and PCBM gave significantly improved lifetimes for devices operated in the atmosphere while the efficiency for the devices was lowered by a factor of 20 upon thermo cleavage for this system.
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Frederik C. Krebs "Design and applications of polymer solar cells with lifetimes longer than 10000 hours", Proc. SPIE 5938, Organic Photovoltaics VI, 59380Y (3 October 2005);


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