The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71 BM Bulk Heterojunction Systems

The best performing low bandgap copolymers PTB series to date which is based on thieno[3,4-b]thiophene-alt-benzodithiophene units blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), have been the focus of polymer-based solar cells. Here, novel fluorinated polymers PTB7-Fx (fluorine units coupled with submonomer thieno[3,4-b]thiophene) with varied degree of fluorination are used as electron donor materials. The PTB7-Fx:PC71BM bulk heterojunction (BHJ) films spin-coated from the host solvent chlorobenzene without and with solvent additive 1,8-diiodooctane (DIO) and the corresponding solar cell devices are systematically investigated to address the morphology-efficiency relationship. Self-assembled BHJ morphology is already observed for as-spun blend films. After adding the solvent additive DIO, the pronounced ordered structures are suppressed and better intermixed films with much smaller domain sizes result. Full fluorination of the third C-atom of thienothiophene gives rise to the highest power conversion efficiency. As the absorption properties, film morphology and crystallinity remain similar for different degrees of fluorination, the main influence of the photovoltaic performance is ascribed to the different lowest unoccupied molecular orbital (LUMO) of each polymer instead of the film morphology. Thus the device performance can be efficiently improved by tuning the energy level of the polymer without necessarily changing either the film nanomorphology or crystallinity dramatically.