Alkylated Y-series acceptors for ternary organic solar cells with improved open-circuit voltage processed from non-halogenated solvents

The efficiencies of organic solar cells have steadily increased over the last few decades. Recently, other crucial factors have moved into focus, such as the removal of environmentally harmful halogenated solvents. The polymer:acceptor combination PM6:DTY6 has already achieved exceptional efficiencies processed from o-xylene. In this work, we focus on improving the efficiency of the promising PM6:DTY6 setup by increasing the photovoltage in a ternary solar cell setup. For this, we synthesized four Y-series acceptors (Y-Me, Y-Pr, Y-Bu, and Y-Hex), in which the electron withdrawing fluorine atoms in the end groups were substituted with alkyl chains of increasing length (methyl, propyl, butyl, and hexyl). All four acceptors exhibit upshifted energy levels compared to DTY6 as well as improved solubility in o-xylene with increasing alkyl chain length. The third component successfully increased the open-circuit voltage (VOC) from 0.81 V for the binary reference setup (PM6:DTY6) to 0.87 V for the ternary setups (PM6:DTY6:A, using 15–20 wt.% of Y-Me, Y-Pr, Y-Bu, and Y-Hex, respectively). This resulted in an increase in efficiency from 13.3 ± 0.2% for the binary to 14.1 ± 0.2% for the ternary PM6:DTY6:Y-Me devices. Morphological investigations revealed that the alkylated acceptors facilitate the formation of favourable fibrillar crystals and in addition, they distinctly influence the packing behaviour of the DTY6 acceptor.