A rational comparison of the effects of halogen atoms incorporated into the polymer donors on the performance of polymer solar cells

Despite the high performance polymer solar cells obtained by the fluorinated or chlorinated polymer donors, there is no effective guideline for how to choose a reasonable halogen atom incorporated into copolymer donors when designing organic photovoltaic materials. Two polymers of PBDT-F-3T2C containing F atom and PBDT-F-3T2C containing Cl atom are designed and synthesized. A direct comparison of the effects of fluorine and chlorine atom on the properties of the copolymer and resulting device performance is systematically studied. It can be found that both of the chlorine and fluorine atoms can effectively reduce the HOMO and LUMO levels, but chlorine atom is revealed to possess stronger downshifting ability for energy levels than fluorine atom in the copolymers. On the other hand, compared with the chlorine atom with larger atomic radius, the smaller fluorine atom possess strong self-assembly feature and negligible steric hindrance, enable the PBDT-F-3T2C with higher crystallinity and solution temperature-dependent aggregation behavior than PBDT-Cl-3T2C. The higher absorption coefficient, more favorable morphology, higher and more balanced charge mobilies endow the PBDT-F-3T2C with higher device performance than PBDT-Cl-3T2C (11.67% vs 10.19%). In addition, the performance of the 1,2,4-trimethylbenzene (TMB) solvent processed devices is also sensitive to halogenate atoms. An expressive PCE of 10.04% is obtained for F-containing devices, which is almost double higher than the Cl-containing devices. These results would provide a guideline for rational incorporation of halogen atoms into the polymer donors for high performance polymer solar cells.