The effect of photocrosslinkable groups on thermal stability of bulk heterojunction solar cells based on donor-acceptor-conjugated polymers

Three different types of photocrosslinkable groups into a low band-gap donor-acceptor-conjugated polymer, namely poly{benzo[1,2-b:4,5-b]dithiophene-alt- thieno[3,4-b]thiophene} (PBT), were developed to comparatively investigate the effect of the photocrosslinkable groups on the thermal stability of bulk heterojunction solar cells. Compared with vinyl groups, bromine- and azide- photocrosslinkable groups are more prompt for photocrosslinking to yield a denser crosslinking network, probably due to the different crosslinking mechanisms and reaction rates. In contrast to the reference device decreasing to less than 10% of its initial efficiency value after 80 h of annealing at 150 degrees C, a great improvement in the thermal stability of performance of all these crosslinked functional copolymers devices demonstrates that photocrosslinking can effectively improve the thermal stability of the active layer by suppressing [6,6]-phenyl-C-61-butyric acid methyl diffusion and phase separation. Furthermore, the solar cells with crosslinked bromine- and azide-functionalized PBT polymers showed very thermally stable photovoltaic device performance by retaining 78 and 66% of their initial device efficiency, respectively, whereas vinyl-functionalized PBT devices retained only 51% of its initial value after long-time thermal annealing. This suggests that an appropriate crosslinking network with homogenous active morphology could dramatically enhance the device stability without sacrificing the performance. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4156-4166