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.