Small Energetic Disorder Enables Ultralow Energy Losses in Non-Fullerene Organic Solar Cells

The relatively large non-radiative recombination energy loss (Delta E-3) is the main source of energy losses in organic solar cells (OSCs). The energetic disorder plays a crucial role in non-radiative energy losses; however, reducing the energetic disorder by modifying terminal groups has rarely been investigated. Herein, four acceptors, BTP-ICB1F, BTP-ICB2F, BTP-ICB3F, and BTP-ICBCF3, with fluorinated phenyl terminal groups are reported at identified substitution sites. The theoretical and experimental results show that this system possesses smaller energetic disorder than the generally-used Y6 acceptor due to the strong electron polarization effect arising from tight, 3D molecular packing. Therefore, the PM6:BTP-ICBCF3 combination achieves high efficiency of 17.8% with high open circuit voltage (V-OC) of 0.93 V and ultralow Delta E-3 of 0.18 eV, which is the smallest Delta E-3 for the binary OSCs with power conversion efficiency (PCEs) over 17% reported to date. Lastly, using the ternary strategy by incorporating the BTP-ICBCF3 acceptor into PM6:BTP-eC9, a higher PCE of 18.2% is achieved with enhanced V-OC. The results imply that introducing new terminal groups in acceptors is promising for reducing energetic disorder and energy losses.