Impact of side chain placement on thermal stability of solar cells in thiophene-thiazolothiazole polymers

Stability of polymer-based bulk-heterojunction solar cells (PSCs) is an even more crucial factor than power conversion efficiency for commercialization. Here, the thermal stability of PSCs is studied using a thiophene-thiazolothiazole semiconducting polymer system. Interestingly, it was found that the cells that used polymers with a less regular side chain placement (iPTzBT) showed higher thermal stability than the cells that used polymers with a more regular placement (PTzBT). The difference can be correlated with the change in the polymer structural order upon thermally annealing the polymer/fullerene blend films. Whereas the structural order of iPTzBT is unchanged after annealing, that of PTzBT is decreased, although the crystallinity is lower in iPTzBT than in PTzBT. These results clearly demonstrate that careful design of the side chain placement in the semiconducting polymers is an important factor for improving the thermal stability of PSCs.