Overcoming the energy loss in asymmetrical non-fullerene acceptor-based polymer solar cells by halogenation of polymer donors

Abstract

To achieve efficient polymer solar cells (PSCs), it is important to overcome the energy loss (E_loss) from the optical bandgap to the open-circuit voltage (V_oc) of the device by properly matching donor/acceptor (D/A) photovoltaic materials. Here, we develop efficient PSCs based on the absorption complementary pairs of halogenated polymer donors (PM6 and PM7) and an asymmetrical non-fullerene acceptor (IDT6CN-M). The D/A pairs of PM6:IDT6CN-M and PM7:IDT6CN-M show very small highest occupied molecular orbital offsets (ΔE_HOMO) of 0.10 and 0.08 eV due to the fact that the halogenation decreases the HOMO levels of polymers, which helps the PSCs achieve a high V_oc of 1.04 and 1.05 V, and a small E_loss of 0.56 and 0.55 eV, respectively. Our studies show that the D/A pairs have good morphology compatibility, and the related devices have efficient photon capture and exciton dissociation, balanced and efficient charge transfer and extraction processes. As a result, the PSCs based on PM6:IDT6CN-M and PM7:IDT6VN-M achieved the same high device efficiency of 13.3%, which is one of the highest values for single-junction PSCs with a V_oc of over 1.0 V.