Conjugated π‑Extension of Small Molecular Nonfullerene Acceptor for Efficient Ternary Organic Solar Cells with an Efficiency of 19.10%

In this study, we have successfully incorporated a small molecular acceptor, Y-LC, with conjugated π-extension as a secondary acceptor in the PM6:BTP-eC9-based organic photovoltaics. The performance of the device was significantly promoted from 18.45% in the binary system of PM6:BTP-eC9 to over 19% in the ternary system with a minimal Y-LC loading. This enhancement in performance can be attributed to the alloy-like structures of acceptors and the optimized active layer morphology, which leads to improved hole and electron mobilities, thereby suppressing charge recombination, and finally resulting in a higher photocurrent in the solar cells. Furthermore, a complementary absorption of Y-LC is observed with PM6 and BTP-eC9, which can broaden the absorption spectrum of the photoactive layer and enable more photons from sunlight to be absorbed. Additionally, Y-LC facilitates efficient charge transfer from the donor to the acceptor by forming cascade energy levels between PM6 and BTP-eC9. These advantages collectively contribute to the superior performance obtained in the ternary solar cells. This work also highlights that the adoption of a nonfullerene acceptor with suitable conjugated π-extensions as a minor additive in ternary photovoltaics is a powerful approach for achieving the state-of-the-art organic solar cells.