A visible-near-infrared absorbing A–π2–D–π1–D–π2–A type dimeric-porphyrin donor for high-performance organic solar cells

Abstract

Most of the currently available small molecule bulk heterojunction organic solar cells (BHJ OSCs) only utilize visible light and, to further increase the efficiency, the development of new organic materials that harvest near-infrared (NIR) light to produce an electric current is essential. Herein, a new A–π₂–D–π₁–D–π₂–A type dimeric porphyrin-cored small molecule (CS-DP) is designed, synthesized and characterized. The use of CS-DP with a narrow bandgap (E_g) (1.22 eV) and the deep energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) affords the highest power conversion efficiency of 8.29% in BHJ OSCs with PC₇₁BM as an acceptor, corresponding to a short circuit current of 15.19 mA cm⁻², an open circuit voltage of 0.796 V and a fill factor of 70% under AM 1.5G solar irradiation. The high device performance is attributed to the visible-near-infrared light-harvesting capability of CS-DP, and the super low energy loss feature. The energy loss (E_loss) lies between 0.43 and 0.51 eV in the system, which is related to the very small energy offset of the LUMOs between the CS-DP donor and PC₇₁BM (ΔE_LUMO = 0.06 eV). The value of ΔE_LUMO, which is considered as a driving force for the photoinduced charge separation, is much smaller than the empirical threshold of 0.3 eV, but would not be a limiting factor in the charge separation process. The results indicate that there may be room for further improving the PCE for low bandgap dimeric porphyrin systems.