Impact of side-chain fluorination on photovoltaic properties: fine tuning of the microstructure and energy levels of 2D-conjugated copolymers

Abstract

The control of the molecular energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is crucial to the design of highly efficient polymer solar cells (PSCs). In particular, the fine control of the HOMO energy level is vital because it can ensure that polymer solar cell devices have a high open circuit voltage (V_oc) and a high short circuit current density (J_sc). We systematically synthesized a series of polymers substituted with different numbers of fluorine (F) atoms in conjugated side chains to fine-tune the HOMO energy levels of the donor polymer, and then optimized the associated solar cell devices by varying the length of the alkyl chain in the conjugated side chains. A series of conjugated side chains with non-fluorinated (0F), mono-fluorinated (1F), and di-fluorinated (2F) alkoxyphenyl groups at the meta position were synthesized and introduced into the donor polymer. The strong electron-withdrawing properties of fluorine were found to reduce the HOMO energy level and yield high V_oc values up to 1.01 V. The substitution of the conjugated side chain with fluorine affects not only the energy level of the polymer but also its intermolecular packing and crystallinity due to the resulting intermolecular interactions, and was found to be effective in the control of J_sc and FF. A maximum efficiency of 7.64% was achieved for the polymer with the mono-fluorinated conjugated side chain.