Morphological, chemical, and electronic changes of the conjugated polymer PTB7 with high temperature annealing*

We study the effect of high temperature annealing on the properties of the semiconducting polymer PTB7, widely used for efficient polymer solar cells. Annealing to moderate temperatures (<260°C) progressively increases morphological order and mobility of the polymer. Annealing to a temperature of 290°C results in a previously unseen polymorph where the π-π stacking distance is decreased by 11%. However, despite the closer packing motif, there is an increase in electronic disorder, decrease in mobility, and decrease in exciton diffusion length. Since the morphology change is accompanied by a mass loss of 10% and changes in FT-IR and UV-Vis , we conclude that thermal cleavage of polymer sidechains leads to the closer packing of molecules, but the chemical reaction may create by-products that act as trap states which decrease mobility. Changes in FT-IR and UV-Vis are similar between 290°C annealing and solar irradiation in air, implying that sidechain cleavage may be an important mechanism in OPV device burn-in.