Solution-processed bulk heterojunction solar cells have experienced a remarkable acceleration in performances in the last two decades, reaching power conversion efficiencies above 10%. This impressive progress is the outcome of a simultaneous development of more advanced device architectures and of optimized semiconducting polymers. Several chemical approaches have been developed to fine-tune the optoelectronics and structural polymer parameters required to reach high efficiencies. Fluorination of the conjugated polymer backbone has appeared recently to be an especially promising approach for the development of efficient semiconducting polymers. As a matter of fact, most currently best-performing semiconducting polymers are using fluorine atoms in their conjugated backbone. In this review, we attempt to give an up-to-date overview of the latest results achieved on fluorinated polymers for solar cells and to highlight general polymer properties’ evolution trends related to the fluorination of their conjugated backbone.

Efficient ternary organic photovoltaics (OPVs) are fabricated with two polymer materials PF2 and J71 as donors and Y6 as the acceptor. The PF2 and J71 based binary OPVs exhibit 10.26% and 9.56% power conversion efficiency (PCE), respectively. Even with similar highest occupied molecular orbital (HOMO) energy levels of PF2 and J71, the J71 based binary OPVs show a relatively high open circuit voltage (V_OC) of 0.86 V compared with 0.71 V for PF2 based binary OPVs. The V_OCs of ternary OPVs monotonously increased with the increase of J71 content in donors, which should be mainly ascribed to the reduced energy loss according to the V_OCs of two binary OPVs and the HOMO energy levels of two donors. The short circuit current density (J_SC) and fill factor (FF) of ternary OPVs can also be simultaneously enhanced with the J71 content up to 30 wt% in donors, leading to 12.12% PCE of the optimized ternary OPVs with a J_SC of 24.97 mA cm⁻², a FF of 64.70% and a V_OC of 0.75 V. Reducing energy loss by the ternary strategy is rarely reported in the literature, which should also be an efficient method to realize performance improvement of OPVs.

The recently reported high power conversion efficiencies achieved in fluorinated polymer:fullerene solar cells have been accounted for by the presence of face-on oriented polymer backbones that enable charge transport towards the collecting electrodes. In this work, we demonstrate that, in contrast to the results of a number of reports, the face-on polymer orientation is due to the bulky side chains, rather than to aggregation in solution. This conclusion is supported by a comparative study of polymers having similar conjugated backbones but different number of fluorine atoms and different number and type of alkyl side chains. While the latter are primarily introduced to tune polymer solubility, the present in-depth thin-film morphology investigation shows that increasing the chain bulkiness favors formation of crystalline lamellae with face-on oriented backbones, independently of the degree of fluorination. By contrast, introduction of fluorine atoms is found to substantially enhance the π-stacking interactions that remain invariably strong upon blending of the polymer with fullerene. Our results demonstrate that, for the polymer family under investigation, fluorination and functionalization by bulky alkyl side chains are both needed for reaching power conversion efficiencies above 10%.